In this paper, a numerical study of buoyancy-aided steady convection heat transfer from a horizontal cylinder placed in free stream is presented. Solutions for cases with Re=10, 20, 40 and Ri=0, 0.1, 0.25, 0.5, 1.0 were obtained using finite difference scheme. The results shows that the buoyancy force assists the forced convection flow by delaying the flow separation thus improves the convective heat transfer compared to corresponding pure forced convection. Such assisted flow significantly improves the heat transfer characteristics from the heated cylinder.

Combined forced and natural (mixed) convection in which neither the forced nor the natural convection effects are dominant and both modes are in a comparable level arise in many natural and technological processes. The present study was conducted to numerically investigate the transport mechanism of laminar combined convection in a shear- and buoyancy-driven cavity. The focus was on the interaction of the forced convection induced by the moving wall with the natural convection induced by the buoyancy. Two orientations of thermal boundary conditions at the cavity walls are considered in order to simulate the aiding and opposing buoyancy mechanisms. Velocity and temperature distributions of the flow are carried out through a stream function-vorticity transformation with a finite difference scheme. Parametric studies of the effect of the mixed convection parameter, GR/RE{sup 2}, on the fluid flow and heat transfer have been performed. Calculations cover Re=100, Pr=0.71 and Gr/Re{sup 2} the range of 0.01--100. Three different regimes are observed with increasing Fr/Re{sup 2}. Forced convection (with negligible natural convection), mixed convection (comparable forced and natural convection) and natural convection (with negligible forced convection). The code developed was carefully tested for the two limiting cases, the pure forced convection and the pure natural convection, for which experimental and numerical data are available.

The mixed convection regime is a transitional heat transfer regime between forced convection and natural convection, where both the forced component of flow, and the buoyancy induced component are important. Aiding flow is when buoyancy forces act in the same direction as the forced flow (heated upflow or cooled downflow), while opposing flow is when the buoyancy force is in the opposite direction of the forced flow (cooled upflow or heated downflow). For opposing flow the buoyancy always increases the rate of heat transfer over the forced convection value. For aiding flow, as the heat flux increased, a reduction in heat transfer is encountered until a condition known as laminarization occurs, where the heat transfer is at a minimum value. Further increases in the wall heat flux causes re-transition to turbulence, and increased heat transfer. In this paper, for the first time, experiments were performed to characterize the effect of surface roughness on heat transfer in mixed convection, for the case of aiding flow. A correlation was developed to allow calculation of mixed convection heat transfer coefficients for rough or smooth tubes.

Loop type LMFBR heat transport system dynamics after reactor shutdown and during subsequent decay heat removal are considered with emphasis on steam generator dynamics including the development and evaluation of various post-scram steam generator control systems, and natural circulation of the sodium coolant, including the influence of superimposed free convection on forced convection heat transfer and pressure drop. The normal operating and decay heat removal functions of the overall heat transport system are described.

In this study, we conduct a two dimensional numerical analysis of double diffusive natural convection in an emplacement drift for a nuclear waste repository. In-drift heat and moisture transport is driven by combined thermal- and compositional-induced buoyancy forces. Numerical results demonstrate buoyancy-driven convective flow patterns and configurations during both repository heat-up and cool-down phases. It is also shown that boundary conditions, particularly on the drip-shield surface, have a strong impact on in-drift convective flow and transport.

In this study, we conduct a two-dimensional numerical analysis of double diffusive natural convection in an emplacement drift for a nuclear waste repository. In-drift heat and moisture transport is driven by combined thermal- and compositional-induced buoyancy forces. Numerical results demonstrate buoyancy-driven convective flow patterns and configurations during both repository heat-up and cool-down phases. It is also shown that boundary conditions, particularly on the drip-shield surface, have strong impacts on the in-drift convective flow and transport.

The paper gives a brief description of some of the better understood aspects of condensation heat transfer and includes discussion of the liquid-vapour interface, natural and forced convection laminar film condensation and dropwise condensation. (orig.)

A forced convection gas-fired oven has been designed which will sterilize a very heavy load of syringes and instruments and containers. It is superior to a gas-fired, infrared, radiant-heated oven which was designed for sterilizing syringes.

The knowledge of forced convection transient heat transfer at various periods of exponential increase of heat input to a heater is important as a database for understanding the transient heat transfer process in a high temperature gas cooled reactor (HTGR) due to an accident in excess reactivity. The transient heat transfer coefficients for forced convection flow of helium gas over a horizontal cylinder were measured using a forced convection test loop. The platinum heater with a diameter of 1.0mm was heated by electric current with an exponential increase of Q0exp(t/?). It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period ? over 1 s, and it becomes higher for the period of ? shorter than 1s. The transient heat transfer shows less dependent on the gas flowing velocity when the period becomes very shorter. Semi-empirical correlations for quasi-steady-state and transient heat transfer were developed based on the experimental data.   

A computational study of laminar mixed convection over a shrouded vertical rectangular fin array attached to a vertical base has been performed. Maintaining the base-fin system above the surrounding temperature, a fan velocity is imposed to enhance the heat transfer through the mixed convection process. Present study finds the effects of clearance spacing, fin spacing, fin length, Reynolds number and Grashof number on the thermal performance of the base-fin system. Mixed convection inlet velocity is decoupled to forced and natural convection velocity components and the resulting pressure drop across the duct length arises purely due to the forced convection velocity component. Thus, Reynolds number is estimated based on forced convection velocity component as the inlet velocity does not va...

A novel approach to the numerical simulation of conjugate heat transfer is presented. Heat conduction in a solid is implicitly coupled with heat convection in viscous fluid flow. The frame of the solution is the Navier-Stokes equations set for viscous Newtonian fluid. A formulation for planar geometry is described in detail. The main advantage of the presented approach is implicit handling of the heat transfer conditions at the solid-fluid interface. Computed test examples include conjugate forced convection in a channel and conjugate natural convection in a cavity. (Author)

Experimental discussions have been given on heat transfer of combined forced and natural convection when the forced convection flows crossing the natural convection generated around a horizontal heated cylinder. Local heat transfer around a cylinder using water as a testing fluid has decreased in front of the cylinder, especially noticeably at a place beneath the cylinder, as the buoyancy has increased, but has increased monotonously on the rear side of the cylinder. Flow separation around the cylinder has affected strongly these changes in the local heat transfer. Because the amount of heat transfer accelerated in the rear side of the cylinder exceeds the amount of heat transfer degraded in the front of the cylinder, the average heat conductivity in the cylinder has always been higher than the values evaluated with the pure forced and pure natural convections. The average heat conductivity around the cylinder with the combined convections cross-flowing can be put into order by the non-dimensional parameter [zeta] to express buoyancy and inertia (= Gr[sub d] (corrected Grashof number)/Nu[sub d] (average Nusselt number) [center dot] Re[sub d][sup 2] (Reynolds number)) as in the cases of parallel flows and opposed flows. It was found that the combined convection region of the cross flow corresponds to the range of 0.1 < [zeta] < 20. 8 refs., 9 figs.

Coolant flows in the cores of current gas-cooled nuclear reactors consist of ascending vertical flows in a large number of parallel passages. Under post-trip conditions such heated turbulent flows may be significantly modified from the forced convection condition by the action of buoyancy, and the thermal-hydraulic regime is no longer one of pure forced convection. These modifications are associated primarily with changes to the turbulence structure. Flow laminarization may occur, and in that event heat transfer rates may be as low as 40% of those in the corresponding forced convection case. The present work is concerned with the modelling of such ?mixed? convection flows in a vertical heated pipe. All fluid properties are assumed to be constant and buoyancy is accounted for within the Bou...

This report surveys near-critical heat transfer, including the following areas: thermal ... itions and suggests procedures for approaching a critical-point heat- transfer problem. ... Heat Transfer in Forced Convection Systems ..... Calculated ... In order to use pseudoboiling models, certain two-phase quantities in addition to the ...

Papers are presented on heat conduction, natural and forced heat convection, two-phase flows and visualization, boiling heat transfer, condensation heat transfer, thermal radiation, heat and mass transfer in porous media, and heat exchangers. Also considered are nuclear reaction heat transfer, combustion heat transfer, high-temperature heat transfer, enhanced heat transfer, and industrial heat transfer. Topics considered include an enthalpy method for the solution of the temperature field during the alloy solidification process, laminar heat transfer and flowfield downstream of backward-facing steps, thermal analysis and optimum design for radiating spines of various geometries, and thermal radiation properties of refractory metals and electrically conductive ceramics at high temperatures.

Heat transfer data were obtained from low flow steam cooling experiments in a partially uncovered 64-rod bundle. These tests indicated that free convection effects were superimposed on the laminar and turbulent forced convection heat transfer. This paper describes the influence of buoyancy on laminar and turbulent forced convection heat transfer coefficients. Mechanisms due to buoyancy which alter the local heat transfer are summarized. Criteria indicating the importance of buoyancy on laminar and turbulent upflow in a vertical pipe were developed and compared to other criteria found in the literature. These criteria were used to determine the steam cooling data with significant buoyancy influence. Data with buoyancy influence were compared to mixed convection correlations and to a numerical study for rod bundles.

Convective heat and mass transfer coefficients are used to calculate the rate at which convection sweeps heat and mass away from the interface. Convection in melt growth is driven by various forces, and the resulting convoluted flows are laminar or turbulent. Furthermore, cross-flow through the “porous” interface has a profound effect on convection. Thus, a general effective coefficient (heff ), which accounts for: (i) uniform flow “suction” through the porous interface, (ii) forced and/or natural convection, (iii) laminar or turbulent flow, and (iv) finite Schmidt numbers, is derived. Focusing next on solute segregation, mass conservation is used to derive a simple equation for keff (effective segregation coefficient) as a function of heff. Here, heff is an...

Two superposed liquid layers display a variety of convective phenomena that are inaccessible in the traditional system where the upper layer is a gas. We consider several pairs of immiscible liquids. Once the liquids have been selected, the applied temperature difference and the depths of the layers are the only independent control parameters. Using a perfluorinated hydrocarbon and silicone oil system, we have made the first experimental observation of convection with the top plate hotter than the lower plate. Since the system is stably stratified, this convective flow is solely due to thermocapillary forces. We also have found oscillatory convection at onset in an acetonitrile and n-hexane system heated from below.

In construction, the use of Phase Change Materials (PCM) allows the storage/release of energy from solar radiation and internal loads. The application of such materials for lightweight construction (e.g., a wood house) makes it possible to improve thermal comfort and reduce energy consumption. The heat transfer process between the wall and the indoor air is convection. In this paper, we have developed a numerical model to evaluate several convective heat transfer correlations from the literature for natural, mixed and forced convection flows. The results show that the convective heat transfer highly influences the storage/release process in case of PCM walls. For the natural convection, the numerical results are highly dependent on the correlation used and the results may vary up to 200%. ...

Experiments have been conducted in a low speed horizontal wind tunnel to study the interaction of radiation and conduction on mixed convective heat transfer from an upward facing horizontal flat plate in air. Differential interferometer has been used to measure local convective heat fluxes. It has been observed that interaction between surface radiation and convection is significant for a low thermal conductivity plate material. On the basis of the previous and current studies, it can be stated that the multi-mode interaction problem is an outcome of the nature of convective boundary layer. The interaction between different modes of heat transfer would remain similar irrespective of the nature of convection (free/mixed or forced).

The invention described here, consists of a thermovalve using natural convection for the regulation of the heat flow from an intermittent heat source (solar collector for example) to the heat storage element. This thermovalve allows the circulation of the heated fluid to the storage when the temperature is superior to the tank base temperature. The thermovalve is composed of a small reservoir where two outlet tubes are possible: if temperature is sufficiently high, the convection effect will force the heat fluid in one of the 2 tubes, conducting to the storage tank.

Conjugate heat transfer by forced convection through an externally heated pipe has many important engineering applications. In the present work, the radial and axial heat conductions and thermal stresses in a pipe with uniform or non-uniform wall heat flux of fully developed laminar forced convective conjugate heat transfer have been considered for analysis. The analysis is based on the two-dimensional steady-state heat conduction equation and laminar boundary layer equation for the flowing fluid by using a finite difference scheme. Water has been used as a fluid. Numerical calculations have been performed by using the FLUENT 4.5 and HEATING7 computer codes. The temperature and stress ratio distributions inside the pipe wall, heated from the outer surface by applying uniform and non-uniform heat fluxes, have been presented for two different mean flow velocities. The temperature distributions of the flowing fluid inside the pipe have also been presented for all investigated cases.

An analysis is made for the conjugate heat transfer problem of natural convection on side of a vertical wall and forced convection on the other side. The natural convection mode is treated analytically by employing the Oseen linearization approach. The forced convection boundary layer is analyzed on the basis of the integral technique. The two solutions are matched on the wall so as to satisfy the continuity of the heat flux across the wall between the two fluids. The analysis shows that the complexion of this two-fluid problem is governed by a dimensionless conjugate parameter which relates the heat transfer effective of the forced convection to that of the free convection. The boundary conditions at the wall are not prescribed in the analysis in advance, rather, determined among the results. The heat transfer and flow characteristics in the two counter-flowing boundary layers are presented in graphs. Heat transfer results of engineering importance are determined as a function of the conjugation parameter. (author)

Forced convective heat transfer of non-Newtonian fluids on a flat plate has been investigated using a recently proposed modified power-law model. For a shear-thinning fluid, non-Newtonian effects are illustrated via local temperature distributions, heat transfer rate, and surface temperature distribution. Most significant effects occur near the leading edge, gradu-ally tailing off far downstream.

Conjugate convective-conductive heat transfer in a rectangular region with forced flow and a heat source is simulated numerically. Distributions of the thermal and hydrodynamic characteristics of the flow regimes studied are obtained. The evolution of the process analyzed is shown.

Pipe-cooling has been widely used for reducing hydration heat and controlling cracking in massive concrete structures. Therefore, the heat transfer coefficients in flow convections, which represent the thermal transfer between the inner stream of the pipe and the concrete, must be estimated accurately. In this paper, a device measuring the heat transfer coefficient is developed based on the concept of internal forced convection. The main influencing factors on the heat transfer coefficient in the flow convection are the flow velocity, pipe diameter and thickness, and the pipe material. Using experimental results obtained from the developed device, a general prediction model for heat transfer coefficients is suggested. The proposed prediction model was found to estimate the heat transfer coefficient correctly with respect to the properties of the flow and pipe in comparisons of measured data and the numerical results of a heat transfer analysis conducted on an actual massive concrete structure.   

Good knowledge of the convective boundary condition is necessary for finite element analysis of thermal deformation behavior in machine tools. There are a number of correlation equations for natural and forced convection and several correlations for mixed convection. Due to a relatively wide range of dimensions, temperatures and speeds, all regimes of convective heat transfer can be observed in machine tools, including the transition region between laminar and turbulent free convection, characterized by Rayleigh number values ranging between Ra = 108 - 109. Since convection in machine tools is highly influenced by external and internal factors, the heat transfer coefficient characterizing convective heat transfer and its changes has to be evaluated experimentally. An experimental technique for evaluating the heat transfer coefficient on the wall and its changes between the wall and the ambient air, based on an active sensor, is being developed. Since the probe dimensions are not negligible, given the fluid motion structures near the wall which are induced by buoyancy or by forced flow, the influence of the probe has to be considered. Paper deals with latest experimental results and summarizes previous work.

The use of computational fluid dynamics (CFD) to evaluate the climate distribution in agricultural buildings has grown in importance in recent years. Convection and radiation are the dominant forms of heat transfer from an animal's body, and accurately accounting for animal heat flux during CFD simulations is necessary to achieving a good understanding of the livestock's thermal environment. Of the total heat flux leaving the animal, the convective part is regularly calculated using a predetermined ratio between the convective to radiative parts (C-R ratio). However, by employing this ratio the CFD modeller is essentially forcing the simulated animal to lose a certain amount of convective heat, irrespective of the environmental conditions at their location. Therefore, unless the indoor env...

The study is conducted to evaluate the heat transfer characteristics of two new and versatile enhancement configurations in a double tube heat exchanger annulus. The novelty is that they are usable in single phase forced convection, evaporation and condensation. Heat transfer coefficients are determined by the Wilson Plot technique in laminar and turbulent flow and correlations are proposed for Nusselt numbers. Comparisons are then made between heat transfer and flow friction.

In MTR research reactors, heat removal is, safely performed by forced convection during normal operation and by natural convection after reactor shutdown for residual decay heat removal. However, according to the duration time of operation at full power, it may be required to maintain the forced convection, for a certain period of time after the reactor shutdown. This is among the general requirements for the overall safety engineering features of MTR research reactors to ensure a safe residual heat removal. For instance, in safety analysis of research reactors, initiating events that may challenge the safe removal of residual heat must be identified and analyzed.In the present work, it was assumed a total loss of coolant accident in a typical MTR nuclear research reactor with the objectiv...

Apparatus and methods for cooling high temperature superconducting materials (HTSC) to superconductive temperatures within the range of 27.degree. K. to 77.degree. K. using a mixed refrigerant consisting of liquefied neon and nitrogen containing up to about ten mole percent neon by contacting and surrounding the HTSC material with the mixed refrigerant so that free convection or forced flow convection heat transfer can be effected.

Heat losses of different origins are largely distributed inside the structure of electrical rotating machines but also inside the cooling fluid itself. The thermal dimensioning of an electrical machine (i.e. the calculation of the temperature field and the determination of heat evacuation ways) requires general laws and particular relations which are synthesised in this paper: general heat transfer laws (conduction, convection, radiant heat transfers); heat conduction inside the structure (simple examples of application, radial heat transfers in stationary regime inside a simplified stator, representation of heterogenous elements, interfaces and contacts between parts, some data concerning materials); convective transfers (characteristic parameters, forced convection inside a fixed channel, inside a narrow annular space, inside an axial rotor channel and near coil heads, relations and complementary remarks, some data concerning fluids). (J.S.) 61 refs.

Dynamic modeling of LMFBR heat transport system is discussed. Uncontrolled transient behavior of individual components and of the integrated heat transport system are considered. For each component, results showing specific dynamic features of the component and/or model capability were generated. Controlled dynamic behavior for alternative steam generator control systems during forced and natural sodium coolant circulation was analyzed. Combined free and forced convection of laminar and turbulent vertical pipe flow of liquid metals was investigated.

An experimental and numerical investigation of mixed convection phenomena about a finite-length, vertical, cylindrical heat source in a unfiorm, liquid-saturated, porous medium was conducted. Bouyancy-induced upflow about the heat source was systematically altered by the superposition of vertical, pressure-driven flows which opposed the bouyancy-induced fluid motion. The evolution of the mixed convection velocity and thermal fields with increasing magnitude of the imposed-flow Peclet number are reported. The ratio of the natural convection Rayleigh number Ra to the imposed-flow Peclet Number Pa is shown to be the nondimensional parameter that characterizes the relative influence of buoyancy-induced to pressure-driven fluid motion. Using total disappearance of buoyancy-induced upflow as the criterion, the transition from mixed to forced convection, for opposing flows, is numerically predicted to occur for )vertreverse arrowbar) Ra/Pe )vertreverse arrowbar)approx. =1/2, independent of the heat source length or power input.

Abstract in english This work presents a study related to conductive/convective drying of paper (cellulose) sheets over heated surfaces, under natural and forced air conditions. The experimental apparatus consists in a metallic box heated by a thermostatic bath containing an upper surface on which the paper samples (about 1 mm thick) are placed. The system is submitted to ambient air under two different conditions: natural convection and forced convection provide by an adjustable blower. The (more) influence of initial paper moisture content, drying (heated surface) temperature and air velocity on drying curves behavior is observed under different drying conditions. Hence, these influence is studied through the proposal of generalized drying curves. Those curves are analyzed individually for each air condition exposed above and for both together. A set of equations to fit them is proposed and discussed.

From 4th international seminar on heat and mass transfer in liquid metals; Trojir, Yugoslavia (6 Sep 1971). See CONF-710942-. Experimental results obtained on a Na-to-NaK heat exchanger operated in the 88-to-1500 Peclet number range are presented. For the present test section, operating in the horizontal position (equivalent diameter D/sub H/ =7.24 x 10/sup -2/ m), an important influence of free convection at Peclet numbers lower than 800 was observed, The effect is characterizcd by a noticeable sodium stratification, with temperature differences on the shell in some cases of nearly90 deg C, and by a decrcase in the average heat-exchange rate, although locally, for a value of L/D/sub H/ = 68.5, Nusselt numbers are higher than forced-convection values. The temperature distributions of the sodium on the shell-side and the shell-side mixedconvection Nusselt numbers compared with those for forced convection are shown. (auth)

Highlights: > We investigate the magneto-convection of cold water in an open cavity. > Temperature of maximum density leaves strong effects on flow and heat transfer. > Convection is enhanced by thermocapillary force when buoyancy force is weakened. > Heat transfer behaves nonlinearly with density inversion parameter and Marangoni number. > Heat transfer rate decreases on increasing the Hartmann number. - Abstract: The aim of the present study is to understand the problem of buoyancy and thermocapillary induced convection of cold water near its density maximum in an open cavity with temperature dependent properties in the presence of uniform external magnetic field. The governing equations are solved by the finite volume method. The results are discussed for various values of reference temperature parameter, density inversion parameter, Rayleigh, Hartmann and Marangoni numbers. It is observed that the temperature of maximum density leaves strong effects on fluid flow and heat transfer due to the formation of bi-cellular structure. Convection heat transfer is enhanced by thermocapillary force when buoyancy force is weakened.

The forced convection heat transfer from a vibrating sphere, a cylinder and a square-section tube to water was experimentally investigated. The obtained heat transfer data with the vibration effect is well correlated in terms of the energy dissipation calculated from the fluid drag acting on the vibrating bodies. Through the use of the energy dissipation, the heat transfer from vibrating bodies to a fluid flow can be discussed analogously with the mass transfer.   

The scheme variants of implementing the thermal protection against heat flows being generated by the body of a high-temperature stationary gas turbine engine (GTE) are presented. The scheme of the experimental bench with a working section is given. Methodical approaches to the heat transfer calculation at different variants of forced and natural convection organization and under various operating conditions are described. The generalized results of the experiments carried out using a heat curtain being generated by porous injection are presented.

This paper describes an experimental study of heat transfer in an axially rotating tube fitted with twin twisted tapes. The manner in which rotation modifies the forced heat convection is considered for the case where the tube rotates about an axis in parallel to the tube's axis of symmetry with particular reference to the design of enhanced cooling channels for rotor windings in a rotating electro-machinery. A selection of experimental results illustrates the individual and interactive effects of Coriolis and centripetal buoyancy forces on heat transfer along the radially outer edge of rotating tube. With the prevailing swirl-flow structures generated by twin twisted tapes, the isolated Coriolis force effect plays a dominant role to initiate the heat transfer reduction form the static-tube scenario that is followed by a subsequent recovery which could lead to heat transfer improvement as the relative strength of Coriolis force increases. The reversed buoyancy impact from improving to impeding heat transfer develops at the higher level of Coriolis force. An empirical correlation, which is physically consistent, has been developed to permit the evaluation of interactive effects of swirling-flows, convective inertial force, Coriolis force and centripetal buoyancy on heat transfer.

In this article nonsimilarity solution for mixed convection from a horizontal surface in a saturated porous medium was obtained for the case of variable surface heat flux. The entire mixed convection regime, ranging from pure forced convection to pure free convection, is considered by introducing a single nonsimilarity parameter. Heat transfer results are predicted by employing four different flow models, namely, Darcy's law, the Ergun model, and the Brinkman-Forchheimer-extended Darcy model with constant and variable porosity. The variable porosity effect is approximated by an exponential function. Effects of transverse thermal dispersion are taken into consideration in the energy equation, along with variable stagnant thermal conductivities. The formulation of the present problem shows that the flow and heat transfer characteristics depend on five parameters, that is, the power in the variation of surface heat flux, the nonsimilarity mixed-convection parameter, the inertia effect parameter, the boundary effect parameter, and the ratio of thermal conductivity of the fluid phase to that of the solid phase. Numerical results for the local Nusselt number variations, based on the various flow models, are presented for the entire mixed convection regime. The impacts of different governing parameters on the heat transfer results are thoroughly investigated.

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In this article, developing turbulent forced convection flow of a water-Al2O3 nanofluid in a square tube, subjected to constant and uniform wall heat flux, is numerically investigated. The mixture model is employed to simulate the nanofluid flow and the investigation is accomplished for particles si...

Report (Aero jet Liquid Rocket Co.) 228 p RC ..... engine data and descriptions necessary for mixed-3'1ode orbit-transfer- vehicle studies. 8. ...... Pbtn Pwr. Outlet Flow Rate. lb/sec. Volmetrlc Flow Rate, GPM. NPSH. f t. Suctton Speclfic Spted ...... Hines, W.S., Turbulent Forced Convection Heat Transfer to Liquids at Very High ...

Abstract In refrigerated spaces, the inside air is cooled by a heat sink operating either by forced or natural convection. The last situation is more frequently used in small apparatus, such as domestic household refrigerators. The inside air temperature is not usually monitored in these ref...

Within the project "Convection in a Cylinder" (CiC) heat transfer enhancement is studied for the case of two concentric, vertically aligned cylinders. The cylindrical gap is filled with a dielectric liquid, which viscosity is just few times higher than that of water. The inner cylinder is heated and the outer one is cooled. This setup in a gravitational buoyancy field leads to a fluid movement in a single convective cell with hot fluid rising at the inner boundary and cold fluid sinking at the outer boundary. The top and bottom part of the system shows horizontal movement, again in boundary layers. The strengthening of temperature gradient induces instabilities of that convective motion. If we vary the buoyancy force by means of electro-hydrodynamic effects, the patterns of convection differ from those instabilities rising only from variation of the temperature gradient.

The natural circulation tests of the Fast Flux Test Facility (FFTF) demonstrated a safe and stable transition from forced convection to natural convection and showed that natural convection may adequately remove decay heat from the reactor core. The COBRA-WC computer code was developed by the Pacific Northwest laboratory (PNL) to account for buoyancy-induced coolant flow redistribution and interassembly heat transfer, effects that become important in mitigating temperature gradients and reducing reactor core temperatures when coolant flow rate in the core is low. This report presents work sponsored by the US Department of Energy (DOE) with the objective of checking the validity of COBRA-WC during the first 220 seconds (sec) of the FFTF natural-circulation (plant-startup) tests using recorded data from two instrumented Fuel Open Test Assemblies (FOTAs). Comparison of COBRA-WC predictions of the FOTA data is a part of the final confirmation of the COBRA-WC methodology for core natural-convection analysis.

Two-phase forced-convective fouling can occur in adiabatic two-phase flow and in diabatic two-phase flow, where it can be a significant contributor to fouling under flow-boiling conditions. For recirculating steam generators (SGs), it is, therefore, of significance to steam separators, tube support plates, tubesheet and the tube bundle. Loop test data are presented on forced-convective fouling rate of iron corrosion products under a range of conditions relevant to the secondary-side of recirculating SGs. The measurements were performed using a number of corrosion products (magnetite, hematite and lepidocrocite) under a range of water chemistry conditions, with several different amines. The measurements were limited to the straight-tube geometry. Comparable fouling data are given for flow-boiling conditions. A SG artefact was examined to corroborate the loop data. The rate constants for the forced-convective fouling measurements are compared with those for flow-boiling fouling. Their relative magnitudes can vary greatly, depending on the chemistry and thermohydraulic conditions. Boiling fouling dominated over forced-convection fouling for hematite and lepidocrocite particles, likely because of particle-bubble interactions. Forced-convective fouling rate was only slightly lower than boiling fouling for magnetite. For the region of cross-flow (upper tube bundle), deposits show significant thickness variation. Four or five deposit thickness peaks are noted, approximately equally spaced circumferentially. It is hypothesized that the fouling pattern is developed due to the cross-flow pattern present in the tube bundle. The possible interactions between the force-convective and nucleate-boiling fouling streams are briefly discussed. A method is presented for the superposition of the forced-convective and nucleate boiling fouling components. This method is based on the Chen heat transfer correlation. (author)

A model is developed for the study of mixed- convection film condensation from downward flowing vapors onto a sphere with variable wall temperature. The model combined natural convection dominated and forced convection dominated film condensation, concerning effects of pressure gradient (P), interfacial vapor shear drag and non-uniform wall temperature variation (A), has been investigated and solved numerically. The effect of pressure gradient on the dimensionless mean heat transfer, NuRe{sup -1/2} will remain almost uniform with increasing P until P=2/9F for various corresponding available values of F. Meanwhile, the dimensionless mean heat transfer, NuRe{sup -1/2} is increasing significantly with F for its corresponding available values of P. Although the non-uniform wall temperature variation has an appreciable influence on the local film flow and heat transfer; however, the dependence of mean heat transfer on A can be almost negligible. (orig.). With 7 figs.

The laminar combined convection heat transfer of the liquid sodium which flows through a single horizontal row of cooling tubes in the direction of gravity are studied using numerical analysis. The heat transfer characteristics at large Reynolds numbers are improved when Richardson numbers (=Gr/Re{sup 2}) are increased and the improvement rate is enlarged with an increase in p/d value. The temperature field at small Reynolds numbers does not exhibit much change even when the Richardson number reaches a high value. Consequently the Nusselt numbers do not differ from those of forced convection. In other words, in a decay heat removal system at a low velocity, there is a possibility that an improvement in the heat transfer characteristics by combined convection cannot be expected even in a system with a large Richardson number. (orig.).

A computational methodology for simulating mixed convection melting of a pure substance is presented. The mathematical model employs a stream function-vorticity-temperature formulation in conjunction with a time-variant mesh. Unlike most transformed grid techniques, the position of the phase front is determined implicitly on application of the Stefan condition. The model is then applied to the melting of a pure metal, gallium, for forced to free convection-dominated heat transfer. Results indicate that according to the relative intensity of buoyancy and inertia forces, the movement and the shape of the solid-liquid interface are considerably perturbed.

ABSTRACT: Convective heat transfer using different nanofluid types is investigated. The domain is differentially heated and nanofluids are treated as heterogeneous mixtures with weak solutal diffusivity and possible Soret separation. Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection. A modified formulation taking into account the thermal conductivity, viscosity versus nanofluids type and concentration and the spatial heterogeneous concentration induced by the Soret effect is presented. The obtained results, by solving numerically the full governing equations, are found to be in good agreement with the developed solution based on the scale analysis approach. The resulting convective flows are found to be dependent on the local particle concentration ? and the corresponding solutal to thermal buoyancy ratio N. The induced nanofluid heterogeneity showed a significant heat transfer modification. The heat transfer in natural convection increases with nanoparticle concentration but remains less than the enhancement previously underlined in forced convection case. PMID:21711755

Engineering heat transfer problems are very often of a complex nature and most often no analytical solutions exist. One way to create solutions to such problems is to apply numerical methods. This study concerns heat transfer problems with coupled conduction, convection and thermal radiation. Five important but different engineering problems are considered. (1) The transient temperature distribution in a rotating cylinder which is exposed to a time varying incident heat flux, e.g. a nuclear burst, is determined. The cylinder is cooled by mixed convection and thermal radiation. The effects of the leading parameters, such as rotation speed, the cooling parameters and the physical properties of the shell are studied. (2) The cooling of a roll system which is transporting/casting a thin hot plastic film. The leading roll is heated by the hot film, cooled at the interior by forced convection and on the outside by forced convection, thermal radiation and contact with a support roll. The influence of the cooling parameters and the rotation are studied. (3) The heat and mass diffusion in pre-insulated district heating/cooling pipes. The task is to determine the effects of the gas mass transport through the casing of the pipes on the thermal behaviour and effects of condensed water due to the mass diffusion of water vapour. The importance of the density of the casing, the wall thickness of the casing, the thickness of the insulation and the surrounding temperature is revealed. (4) The development of a cooling system for an electrical unit in which a time dependent heat is generated due to the Joule effect. (5) The heat transfer from a rectangular fin in a confined space. The fin is cooled by turbulent forced convection. The turbulence model applied is a low Reynolds k-{epsilon}-model. Predicted results are compared with experimental ones, and a correlation for the Nusselt number is proposed. The effects of thermal radiation for non-participating as well as participating media are considered 49 refs, 10 figs

An investigation of the thermal hydraulic characteristics in the passive residual heat removal system of the System integrated Modular Advanced ReacTor-P (SMART-P) has been carried out using the MARS code, which is a best estimate system analysis code. The SMART-P is designed to cool the system during accidental conditions by a natural convection. The dominant heat transfer in the steam generator is a boiling mode under a forced convection condition, and it is a single-phase liquid and a boiling heat transfer under a natural convection condition. Most of the heat is removed in the heat exchanger of the passive residual heat removal system by a condensation heat transfer. The passive residual heat removal system can remove the energy from the primary side as long as the heat exchanger is submerged in the refueling water tank. The mass flow is stable under a natural circulation condition though it oscillates periodically with a small amplitude. The parameter study is performed by considering the effects of an effective height between the steam generator and the heat exchanger, a hydraulic resistance, an initial pressure, a non-condensable gas fraction in the compensating tank, and a valve actuation time, which are useful for the design of the passive residual heat removal system. The mass flow in the passive residual heat removal system has been affected by the height between the steam generator and the heat exchanger, and the hydraulic resistance of the loop.

Collective interactions of a beam of neutrinos/antineutrinos traversing a dense plasma of electrons/positrons, protons and neutrons are studied with particular reference to the case of a Supernova. We find that the ponderomotive force exerted by neutrinos gives an important contribution to the revival of the shock for a successful Supernova explosion. The role of the magnetic field in the new heating mechanism is pointed out, as well as new types of convection induced by the ponderomotive force itself.

Highlights: > Superposition of forced and thermal convection is studied in a rectangular cavity. > For pure forced convection the mean wind exhibits a solid body rotation. > Four buoyancy induced convection rolls are formed for mixed convection at Ar {approx} 3.3. > The enthalpy flux difference between out- and inflowing air has a maximum at Ar {approx} 0.6. - Abstract: Results of an experimental study of flow structure formation and heat transport in turbulent forced and mixed convection are presented. The experiments were conducted in a rectangular cavity with a square cross section, which has an aspect ratio between length and height of {Gamma}{sub xz} = 5. Air at atmospheric pressure was used as working fluid. The air inflow was supplied through a slot below the ceiling, while exhausting was provided by another slot, which is located directly above the floor. Both vents extend over the whole length of the cell. In order to induce thermal convection the bottom of the cell is heated while the ceiling is maintained at a constant temperature. This configuration allows to generate and study mixed convection under well defined conditions. Results of forced convection at Re = 1.07 x 10{sup 4} as well as mixed convection at 1.01 x 10{sup 4} {<=} Re {<=} 3.4 x 10{sup 4} and Ra = 2.4 x 10{sup 8} (3.3 {>=} Ar {>=} 0.3), which were obtained by means of Particle Image Velocimetry and local temperature measurements, are presented. For purely forced convection a 2D mean wind, which can be approximated by a solid body rotation, is found. With increasing Archimedes number this structure becomes unstable, leading to a transition of the solid body rotation into additional smaller convection rolls. Proper orthogonal decomposition of the instantaneous velocity fields has been performed for further analysis of these coherent large-scale structures. Their fingerprint is found in the spatial temperature distribution of the out flowing air at the end of the outlet channel, which reveals a temporally stable profile with two maxima over the length of the outlet. Moreover a maximum in the global enthalpy transport by the fluid is found at Ar {approx} 0.6.

The Next Generation Nuclear Plant (NGNP) will most likely produce electricity and process heat, with both being considered for hydrogen production. To capture nuclear process heat, and transport it to a distant industrial facility requires a high temperature system of heat exchangers, pumps and/or compressors. The heat transfer system is particularly challenging not only due to the elevated temperatures (up to ~ 1300K) and industrial scale power transport (=50 MW), but also due to a potentially large separation distance between the nuclear and industrial plants (100+m) dictated by safety and licensing mandates. The work reported here is the preliminary analysis of two-phase thermosyphon heat transfer performance with alkali metals. A thermosyphon is a device for transporting heat from one point to another with quite extraordinary properties. In contrast to single-phased forced convective heat transfer via ‘pumping a fluid’, a thermosyphon (also called a wickless heat pipe) transfers heat through the vaporization / condensing process. The condensate is further returned to the hot source by gravity, i.e. without any requirement of pumps or compressors. With this mode of heat transfer, the thermosyphon has the capability to transport heat at high rates over appreciable distances, virtually isothermally and without any requirement for external pumping devices. Two-phase heat transfer by a thermosyphon has the advantage of high enthalpy transport that includes the sensible heat of the liquid, the latent heat of vaporization, and vapor superheat. In contrast, single-phase forced convection transports only the sensible heat of the fluid. Additionally, vapor-phase velocities within a thermosyphon are much greater than single-phase liquid velocities within a forced convective loop. Thermosyphon performance can be limited by the sonic limit (choking) or vapor flow and/or by condensate entrainment. Proper thermosyphon requires analysis of both.

Forced convective heat transfer and friction factor for air flowing inside rectangular horizontal duct over a set of pin-fins, is experimentally studied under uniform heat flux. Experiments are conducted with air subjected to a magnetic body force through a magnetic gradient field. Experiments are carried out for two different fin geometries. An experimental test loop equipped with the required measuring instruments is designed and constructed to assess the effects of magnetic field, mass flow rate, and applied heat flux on convection heat transfer process and pressure drop. The measurements of temperature, flow rate, applied voltage to the heater and pressure drop in the duct are recorded and manipulated to evaluate Nusselt number and friction factor. The obtained experimental results sho...

As part of the ongoing research on finned U-shape heat pipes for CPU cooling, the present work focuses on the characterization of working fluid in vertically oriented twin U-shape heat pipe, by taking into account the gravity of flow. Two-dimensional FE simulation is performed under natural and forced convection modes, by using ansys-flotran. The best heat input and coolant velocity for the simulations are determined experimentally, corresponding to the least thermal resistance. The wall temperatures at the evaporator, adiabatic and condenser sections, and the velocity and pressure distributions of vapor and liquid, are analyzed. The total heat input for minimum thermal resistance in both natural and forced convection is found to be 50W, and the coolant velocity is 3m/s. The predicted and ...

This paper is one in a series of reports on the experimental and numerical study of heat transfer from a module mounted on the floor of a parallel-plate channel to forced convective air flow. In this study a numerical analysis is performed to predict conjugate heat transfer in paths from the module through the module support and the floor to the air flow. The effect of the thermal wake originating from the module on heat transfer from the floor surface is captured by the adiabatic wall temperature function. Also serving as the basis of the analysis is the local heat transfer coefficient obtained and reported in the previous report. The two-dimensional heat conduction analysis with convective boundary condition provides temperature distributions of the floor and macroscopic thermal conductance values for heat transfer through the floor. The predictions are compared with the experimental data, and their good agreement is confirmed. 17 refs., 10 figs.

The study is conducted to evaluate the flow characteristics in a double tube heat exchanger using two new and versatile enhancement configurations. The novelty is that they are usable in single phase forced convection, evaporation and condensation. Correlations are proposed for flow development length and friction factor for use in predicting fluid pumping power in thermal equipment as well as in subsequent heat transfer characterization of the surface.

A Nusselt number, appropriate for forced convection in a channel bounded by two parallel plate walls heated asymmetrically, is introduced and evaluated for various velocity profiles, for either uniform heat flux or uniform temperature boundary conditions. It is shown that the value of this new Nusselt number is independent of the asymmetry if and only if the velocity profile is symmetric with respect to the midline of the channel. (author)

The effect of thermal storage mass on the performance of an attached sunspace is investigated for a particular design in Boston. Mass in the sunspace and in the adjoining building are compared. Performance is evaluated in terms of temperature conditions in the sunspace and delivery of useful solar heat to the adjoining building. The dependence of the results on the manner of heat delivery is studied. Both natural convection and fan-forced air flow are included.

Sep 1, 1981 ... Title: Topographic forcing of supercritical convection in a porous medium such ... that brackets pi, the natural wavenumber for convection in a porous slab with planar, ... Subject Terms: CONVECTIVE FLOW; EARTH CRUST; ...

A numerical study from the viewpoint of the relative magnitude of inertial to viscous effects have been carried out for laminar natural/mixed convections in a two-dimensional differentially heated square cavity. Results indicate that the natural/mixed convection can be divided into three flow regimes according to Rayleigh number: the boundary layer, the transition and the creeping motion regimes. For the boundary layer regime, the large Prandtl number leads to a reduction in both boundary layer thickness and characteristic velocity. The characteristic velocity is therefore proportional to {radical}(Gr/Pr). For the creeping motion regime, the characteristic velocity is proportional to Gr and independent on Pr, and the Nusselt number approaches 1. For the laminar mixed convection, the quantity measuring the relative importance of natural to forced convection is Gr/(Re{sup 2}Pr) for the boundary regime and Gr/Re for the creeping motion regime.

Heat and mass transfer associated with a binary liquid droplet immersed in another liquid undergoing rectilinear motion including gravity-and surface tension-induced flows is numerically studied. A full spectral numerical scheme that combines the Galerkin and collocation techniques with orthogonal basis functions is used to solve the governing partial differential equations based on the stream function-vorticity formulation. A liquid-liquid system having similar properties is used as a model problem to illustrate the temporal behaviors of the flow, temperature, and concentration fields. The results indicate that significant differences could exist when the drop is subjected to simultaneous forced convection, free convection, and surface tension-induced convection. Parameters that have been found to control the convective processes are the Reynolds number, the Grashof number, and the Marangoni numbers.

Claims have been made that infrasound can be used to significantly reduce electricity consumption in a convection heating process (up to 94%) in a convection heating process by enhancing the rate of heat transfer. In response to these claims, preliminary laboratory tests were performed to evaluate the electricity savings potential of infrasound. The results from the tests indicate that when infrasound is applied to steady air flow, the convection heat transfer is significantly increased. In certain conditions, the rate of heat transfer was double the rate in steady air flow. However, as the air flow rate was increased, the degree of heat transfer enhancement decreased; under steady flow conditions, infrasound can increase the rate of heat transfer by a factor of 2. The efficiency of the infrasound unit tested was very low (aproximately 0.04%), therefore it could not be used to reduce the total electricity consumption in a forced convection process. However, a more efficient method for generating the infrasound, if available, could achieve a net reduction in the electricity consumption. Recommendations are made for future research. 8 refs., 12 figs., 3 tabs.

In this article, forced convection heat transfer with laminar and developed flow for water-Al"2O"3 nanofluid inside a circular tube under constant heat flux from the wall was numerically investigated using computational fluid dynamics method. Both single and two-phase models are accomplished for either constant or temperature dependent properties. For this study nanofluids with size particles equal to 100nm and particle concentrations of 1 and 4wt% were used. It is observed that the nanoparticles when dispersed in base fluid such as water enhance the convective heat transfer coefficient. The Nusselt number and heat transfer coefficient of nanofluids were obtained for different nanoparticle concentrations and various Reynolds numbers. Heat transfer was enhanced by increasing the concentrati...

A thermal manikin headform was used to examine the effect of full-face motorcycle helmets on (primarily forced) convective heat loss under the following interventions: (i) a 30degree forward head tilt angle (six helmets), (ii) a wig installed between the headform and helmet (six helmets), and (iii) applied wind speeds ranging from 0-80kmh-1 (three helmets). In all interventions measurements were carried out for open and closed vents. The average convective heat loss was obtained from a steady state period under controlled ambient conditions. It was found that: (i) for many helmets a reduction in heat loss in the face section is found when tilting the head forward, (ii) the wig reduced the heat loss by a factor of 2, and (iii) heat loss is approximately linearly dependent on wind speed (0-8...

In micro pumps, dosing systems, heat exchanger and transfer devices the flow control is realized by means of external impressed force fields. Here we focus on the enhancement of heat transfer in an annular cavity, if an electrohydrodynamic force field is set up. This synthetic force field is established with a high voltage potential between differentially heated inner and outer cylinders, filled with a dielectric insulating fluid. It acts comparable to thermal buoyancy forces induced by gravity. Sitte et al. (2001) performed quantitative parabolic flight experiments without determining critical values and finally reported a broken azimuthally symmetry due to the instability in a recent parabolic flight experiment (Sitte et al., 2003). With the experiment accomplishment in the 14th parabolic flight, first scenarios are realized in order to weigh the different influences of natural buoyancy coming from g and electro-hydrodynamic buoyancy coming from synthetic force fields, which were studied with numerical simulations by Smieszek et al. (2008). Specific experiment objective was the convection in an annular cavity with differentially heated inner and outer cylinders under the influence of the both buoyancy driven forces. By scaling the annulus width to approximate 5mm the initial outer cell radius for a first parabolic flight campaign was set to 10mm. The inner cylinder is made of aluminum and is heated with heating cartridges. The outer cylinder is made of glass. The gap in between is the experimental volume, which is filled with silicone oil and particles. With this a Laser light sheet illumination was set up. The inner cylinder, made of aluminum, is connected to a high-tension up to 10kV. The glass cylinder is coated with Indium-Tin-Oxide (ITO) inside, to make the glass conductive and is connected to ground. The central force field is introduced by applying a high voltage difference between the two cylinders. Convection was observed during the whole parabolic flight. Starting with convection modes in normal g, the boost into the parabola is coupled with increase up to 1.8g. Here the global fluid flow in boundary layers is amplified with a reduction of movement in the centre of the research cavity. Then during the µg period, where minor acceleration due to gravity leads to collapse of convection, it is the electro-hydrodynamic force which offers buoyancy. As the microgravity is a short term one, convection mode remains in transient states. Nevertheless during the successive slowing down of the aeroplane, which involves again the 1.8g period boundary layered convection mode arises again. It is planned to refly the experiment again, in order to trace the effective magnitude of synthetic force balancing the natural convection under microgravity. References B. Sitte, J. Immohr, O. Hinrichs, R. Maier, C. Egbers, H. Rath (2001), Rayleigh-Bénard Con-e vection in dielectrophoretic force field, 12th International Couette-Taylor Workshop, September 6-8, 2001, Evanston, IL USA B. Sitte, H.J. Rath (2003), Influence of the dielectrophoretic force on thermal convection, Experiments in Fluids 34, 24-27 M. Smieszek, O. Crumeyrolle, I. Mutabazi, C. Egbers (2008), Numerical simulation of thermo-convective instabilities of a dielectric liquid in a cylindrical annulus, 59th Int. Astronautical Congress (IAC) 29.09.-03.10., 2008, Glasgow, UK

The thermal convective and magnetorotational instability is investigated by means of magnetohydrodynamic equations including anisotropic viscosity and resistivity dissipative effects. Magnetic force lines are assumed to be initially isothermal and the heat is restricted to being primarily transported along the magnetic force lines. To obtain the analytic expressions for the growth rate and instability criteria, we neglect the cross-field resistivity by applying our result to the weakly ionized environment. Under this assumption, the general dispersion relation describing the local thermal convective and magnetorotational instability is derived. The effects on the dispersion relation due to anisotropic resistivity and viscosity are discussed. Both the resistivity and viscosity show stabilizing effect on the thermal convective and rotational instability but do not affect the instability criterion. The analytic expression governing the growth rate is presented for Prandtl number P{sub m}=1 case.

Designs are described for implementing models for calculating the heat transfer and flow losses in porous debris in the lower head of a reactor vessel. The COUPLE model in SCDAP/RELAP5 represents both the porous and nonporous debris that results from core material slumping into the lower head. Currently, the COUPLE model has the capability to model convective and radiative heat transfer from the surfaces of nonporous debris in a detailed manner and to model only in a simplistic manner the heat transfer from porous debris. In order to advance beyond the simplistic modeling for porous debris, designs are developed for detailed calculations of heat transfer and flow losses in porous debris. Correlations are identified for convective heat transfer in porous debris for the following modes of heat transfer; (1) forced convection to liquid, (2) forced convection to gas, (3) nucleate boiling, (4) transition boiling, and (5) film boiling. Interphase heat transfer is modeled in an approximate manner. A design is also described for implementing a model of heat transfer by radiation from debris to the interstitial fluid. A design is described for implementation of models for flow losses and interphase drag in porous debris. Since the models for heat transfer and flow losses in porous debris in the lower head are designed for general application, a design is also described for implementation of these models to the analysis of porous debris in the core region. A test matrix is proposed for assessing the capability of the implemented models to calculate the heat transfer and flow losses in porous debris. The implementation of the models described in this report is expected to improve the COUPLE code calculation of the temperature distribution in porous debris and in the lower head that supports the debris. The implementation of these models is also expected to improve the calculation of the temperature and flow distribution in porous debris in the core region.

Because of the complexity of phenomena governing boiling heat transfer, the approach to solve practical problems has traditionally been based on experimental correlations rather than mechanistic models. The recent progress in computational fluid dynamics (CFD), combined with improved experimental techniques in two-phase flow and heat transfer, makes the use of rigorous physically-based models a realistic alternative to the current simplistic phenomenological approach. The objective of this paper is to present a new CFD model for critical heat flux (CHF) in low quality (in particular, in subcooled boiling) forced-convection flows in heated channels.

In this article, numerical investigation is carried out for mixed convection heat transfer within square cavities for various thermal boundary conditions on bottom and side walls based on thermal aspect ratio (A). A penalty finite element analysis with bi-quadratic elements has been used to investigate the results in terms of isotherms, streamlines, heatlines and average Nusselt numbers for a wide range of parameters (1 @? Re@?100, 0.015 @? Pr@?10, 10^3 @? Gr@?10^5). A detailed analysis of flow pattern shows that natural convection or forced convection depends on both parameters: Ri (Ri = Gr/Re^2) and Pe (Pe = Re.Pr). Results indicate that, at low Pr (Pr = 0.015) with low Gr (Gr = 10^3), isotherms are decoupled with flow profile and conduction dominant heat transfer is observed irrespectiv...

A space-marching boundary-layer program has been extensively modified to model conjugate conduction-convection heat transfer for the case of co-flowing high-speed gas and liquid coolant. Solid body conduction is modeled as one-dimensional, constant property heat transfer. The coolant is modeled empirically as a bulk fluid with combined forced convection and subcooled nucleate boiling. The flow solver was modified to solve the group of conjugate boundary equations simultaneously and implicitly with the existing momentum and energy equations for the gas. The resulting conjugate conduction-convection program has been applied to analysis of failure of a backside water-cooled nozzle for a high enthalpy, supersonic wind tunnel. The computational results have been used to establish that the primary failure mode is nucleate-boiling burnout and to propose a numerical burnout limit applicable to the specific nozzle configuration. 22 refs.

The 'universal climate machine' is able to air-condition a building endlessly by itself through self-regulatory control. This is a PCM climate ceiling (PCM refers to Phase Change Materials) in which heat exchange takes place through radiation and free convection, but also through forced convection. [Dutch] De 'universele klimaatmachine' kan zelfregelend een gebouw eindeloos autarkisch klimatiseren. Het gaat hier om een PCM-klimaatplafond (PCM staat voor Phase Change Materials) waarbij de warmteoverdracht plaatsvindt door straling en vrije convectie, maar ook door gedwongen convectie.

The computer code THERM3D implements the direct boundary element method (BEM) to solve transient heat conduction problems in arbitrary three-dimensional domains. This particular implementation of the BEM avoids performing time-consuming domain integrations by approximating a ``generalized forcing function`` in the interior of the domain with the use of radial basis functions. An approximate particular solution is then constructed, and the original problem is transformed into a sequence of Laplace problems. The code is capable of handling a large variety of boundary conditions including isothermal, specified flux, convection, radiation, and combined convection and radiation conditions. The computer code is benchmarked by comparisons with analytic and finite element results.

A micromachined convective accelerometer is a recently developed device. Typical micromachined accelerometers use a solid proof mass for measuring acceleration. But a micromachined convective accelerometer does not use a solid proof mass. A micromachined convective accelerometer is composed of a heating resistor and temperature sensors. This device measures acceleration by using convective heat transfer phenomenon. Therefore characteristics of a micromachined convective accelerometer are different as compared with typical micromachined accelerometer. In this research, we analyze the convective accelerometer by using transient convective heat transfer analysis. Based on the results of a convective accelerometer, we propose a new model which has improved performance.

Supercritical fuel reforming is being studied as a technology for reducing emissions of industrial gas turbine engines. In this study, experiments were performed in a 2.67-mm-inside-diameter stainless steel tube with a heated length of 0.610 m for the purpose of investigating the characteristics of supercritical heat transfer with endothermic fuel reforming. Thermocouples were positioned along the tube both in the fluid stream and on the heated wall for local heat transfer measurements. Both heat transfer coefficients and endotherms were calculated from the measured results. State-of-the-art correlations for heat transfer were evaluated, and a correlation for supercritical heat transfer to hydrocarbon fuel has been developed. The results provide a basis for supercritical fuel heat-exchanger/reactor design and its practical applications, in an area that has received relatively little attention in the engineering literature, viz., supercritical forced convection heat transfer with endothermic chemical reaction.

Designs are described for implementing models for calculating the heat transfer and flow losses in porous debris in the lower head of a reactor vessel. The COUPLE model in SCDAP/RELAP5 represents both the porous and nonporous debris that results from core material slumping into the lower head. Currently, the COUPLE model has the capability to model convective and radiative heat transfer from the surfaces of nonporous debris in a detailed manner and to model only in a simplistic manner the heat transfer from porous debris. In order to advance beyond the simplistic modeling for porous debris, designs are developed for detailed calculations of heat transfer and flow losses in porous debris. Correlations are identified for convective heat transfer in porous debris for the following modes of heat transfer; (1) forced convection to liquid, (2) forced convection to gas, (3) nucleate boiling, (4) transition boiling, and (5) film boiling. Interphase heat transfer is modeled in an approximate ma nner. Designs are described for models to calculate the flow losses and interphase drag of fluid flowing through the interstices of the porous debris, and to apply these variables in the momentum equations in the RELAP5 part of the code. Since the models for heat transfer and flow losses in porous debris in the lower head are designed for general application, a design is also described for implementation of these models to the analysis of porous debris in the core region. A test matrix is proposed for assessing the capability of the implemented models to calculate the heat transfer and flow losses in porous debris. The implementation of the models described in this report is expected to improve the COUPLE code calculation of the temperature distribution in porous debris and in the lower head that supports the debris. The implementation of these models is also expected to improve the calculation of the temperature and flow distribution in porous debris in the core region.

A numerical simulation based on a combined Euler and Lagrange method is investigated in this work to simulate the flow and migration of nanoparticles in a single channel. The motion of discrete nanoparticles is determined by the Lagrangian trajectory method based on the Newton’s second law that includes the influence of the body force, various hydrodynamic forces, the Brownian motion and the thermophoresis force. The coupling of discrete particles with continuous flow is realized through the modification of the source term of the continuous equation. The results reveal the two-phase flow nature of nanoparticle suspensions and their implications to the convective heat transfer of nanofluids.

Experiments to investigate the sensitivity of global ocean circulation and its associated transports of heat and salt to proposed changes in high-latitude buoyancy forcing and wind forcing. An experiment was designed to improve the representation of water-mass production in areas of known deep-water formation. At the same time, almost all of the deep and abyssal regions of the world ocean were freed from earlier restoring to observed values of temperature and salinity. This convective forcing experiment has been run on the NCAR Y-MP/864 for three years of a planned five-year sensitivity study.

A statistical procedure has been used to evaluate several alternative descriptions of pressurized water reactor (PWR) cladding corrosion behavior, using an extensive database of Improved (low tin) Zr-4 cladding corrosion measurements from fuel irradiated in commercial PWRs. The in-reactor corrosion enhancement factors considered in the model development are based on a comprehensive review of the current literature for PWR cladding corrosion phenomenology and models. In addition, because prediction of PWR cladding corrosion behavior is very sensitive to the values used for the oxide surface temperatures, several models for the forced convection and sub-cooled nucleate boiling (SNB) coolant heat transfer under PWR conditions have also been evaluated. This evaluation determined that the choice of the forced convection heat transfer has the greatest impact on the ability to fit the data. In addition, the SNB heat transfer model used must account for a continuous transition from forced convection conditions to fully developed SNB conditions. With these choices for the heat transfer models, the evaluation determined that the significant in-reactor corrosion enhancement factors are related to the formation of a hydride rim at the cladding outer diameter, the coolant lithium concentration, and the fast neutron fluence (author) (ml)

Thermal designs for a nozzle based on back-side water-cooling (BSWC) for a 200-atm and 2,800 Btu/lbm arc facility are presented. The designs must survive heat flux above 10,000 Btu/sq ft-sec. A critical heat flux (CHF) model was developed and accounts for velocity and curved-channel acceleration effects on subcooled forced-convection nucleate-boiling heat transfer. The CHF model is implemented in the AEDC Conjugate Conduction-Convection Program (CCCP) which solves simultaneously the gas and coolant-side momentum or energy equations along with the one-dimensional Fourier equation for solid body conduction. The CHF model is calibrated against AEDC HEAT-H1 nozzle failures and survivals, used to establish phenomenological trends, and then applied to identifying candidate 200-atm designs. The proposed designs are derivatives of an existing successful 100-atm design. It is suggested that additional technologies may have to be invoked.

Abstract in english This work studies the forced convection problem in internal flow between concentric annular ducts, with radial fins at the internal tube surface. The finned surface heat transfer is analyzed by two different approaches. In the first one, it is assumed one-dimensional heat conduction along the internal tube wall and fins, with the convection heat transfer coefficient being a known parameter, determined by an uncoupled solution. In the other way, named conjugated approach, (more) the mathematical model (continuity, momentum, energy and K-epsilon equations) applied to tube annuli problem was numerically solved using finite element technique in a coupled formulation. At first time, a comparison was made between results obtained for the conjugated problem and experimental data, showing good agreement. Then, the temperature profiles under these two approaches were compared to each other to analyze the validity of the one-dimensional classical formulation that has been utilized in the heat exchanger design.

Natural convection heat transfer in rectangular enclosures is an active research area, due to its significance for both fundamental interest and engineering applications such as thermal management of electronic components. In this present numerical study, combined conduction and natural convection have been analysed for one or two heat sources mounted on substrates in an enclosure. For one heat source, the substrate is mounted on a vertical wall, and for the case of two heat sources, the substrates are mounted vertically and horizontally. Buoyancy forces drive the fluid flow. The Picard method with a hybrid grid system is utilized to decouple between pressure and velocity components, resulting in componentwise splitting. Linear systems obtained via the Picard method have been solved by the...

The knowledge of convective heat transfer coefficient hcpf (absorber plate to flowing air) is necessary to predict or evaluate thermal performance of any solar dryer. In order to determine hcpf, laboratory models of direct (cabinet), indirect and mixed mode solar dryer are designed and constructed to perform no-load steady state experiments for natural and forced air circulation. The dryers are operated under indoor simulation conditions for absorbed thermal energy and air flow rate for the range of 300-800W/m2 and 1-3m/s, respectively. Separate methods depending on mode of heat utilisation are proposed for determination of hcpf for different dryers. Correlations of hcpf in terms of dimensionless numbers are developed for each dryer operating under natural and forced convection. Levenberg-...

The cooling requirements for an average car sized engine (spark-ignition, V-6, four-stroke, naturally aspirated, about 200 kg, about 100 kW) were looked at for Mars. Several modes of cooling were considered, including forced convection, exhaust, radiation and closed loop systems. The primary goal was to determine the effect of the thinner Martian atmosphere on the cooling system. The results show that there was only a 6-percent difference in the cooling requirements. This difference was due mostly to the thinner atmosphere during forced convection and the heat capacity of the exhaust. A method using a single pass counter-flow heat exchanger is suggested to offset this difference in cooling requirements.

A thermally insulated chimney attached to a vertical heated section induces an increase in the natural convection flow in the heated vertical cylinder and leads to a higher heat transfer rate. The flows in the chimney are originally driven by the natural convection. However its behavior is similar to the forced convective flows as the heated vertical cylinder is located in a duct or chimney and then the mass flow rate at every elevation should be the same. Heat transfer in the chimney depends on the dimensionless geometrical parameters (Fig. 1), such as the extension ratio (the total length of chimney system, L{sub t}, over the heated section length, L{sub h}), the expansion ratio (the diameter of chimney, D{sub t}, over the diameter of heated vertical cylinder, D{sub h}), the aspect ratio of cylinder (the length of cylinder, L{sub h}, over the diameter of cylinder, D{sub h}), and the location of cylinder in a duct (the top or bottom of chimney). Although some works have been done on the heat transfer in the chimney, arrangements detailed experimental investigations on the determination of the optimal location of the heated cylinder in the chimney are rare. And previous studies have been performed for extension ratio 1.0-5.0. This work investigated the influence on the chimney dimensions (entrance and exit length, and diameter) on the heat transfer of a vertical cylinder in a duct. The measured mass transfer rates for the natural convection of vertical cylinder in a duct were presented for Prandlt number 2,094, Rayleigh number 4.55x10{sup 9}, 5.79x10{sup 10}, and 1.69x10{sup 11}. Experiments were performed using a copper sulfate electroplating system to simulate heat transfer based upon the analogy concept

Papers are presented on buoyant plumes in the environment, environmental heat transfer, measurements of heat transfer in biological tissue, and the use of heat transfer in space manufacturing. Individual papers are also presented discussing effective thermal conductivity in spherical particle packed beds, natural convection, laminar convection between parallel vertical plates, double diffusive convection, heat transfer of porous media, heat transfer in thermosiphons, radiation and convection in gray fluids, and an analytical solution to the Graetz problem for Newtonian fluids in circular ducts.

A numerical experiment with the Los Alamos general circulation model reveals a marked sensitivity of stimulated climate to the treatment of cumulus convection. The diabatic heating produced by the cumulus parameterization not only directly affects the temperature field but also introduces a change of cloudiness which, in turn, alters radiative forcing to the temperature field. The wind fields are also affected through their thermodynamic relationship with temperature.

A Galerkin-based, finite element method is developed for numerically simulating nonisothermal flows in porous materials. The partial differential equations describing heat and mass transfer in a rigid porous matrix are described. Use of the finite element method to produce a computational procedure is outlined and solution procedures for the finite element equations are discussed. An experiment involving mixed free and forced convection is used to verify the accuracy of the numerical procedure.

In thin-layer electrodeposition the dissipated electrical energy leads to a substantial heating of the ion solution. We measured the resulting temperature field by means of an infrared camera. The properties of the temperature field correspond closely with the development of the concentration field. In particular we find, that the thermal gradients at the electrodes act like a weak additional driving force to the convection rolls driven by concentration gradients.

Abstract in english Research in convective heat transfer using suspensions of nanometer-sized solid particles in base liquids started only over the past decade. Recent investigations on nanofluids, as such suspensions are often called, indicate that the suspended nanoparticles markedly change the transport properties and heat transfer characteristics of the suspension. This second part of the review covers fluid flow and heat transfer characteristics of nanofluids in forced and free convecti (more) on flows and potential applications of nanofluids. Opportunities for future research are identified as well.

Abstract in portuguese O uso de coberturas em ambientes protegidos altera a movimentação de ar próximo do dossel da cultura, comparado com o ambiente externo, modificando os processos de trocas de calor e massa entre dossel e ar. Muitos trabalhos realizados em casas de vegetação têm estimado os fluxos de calor latente e sensível com emprego de números adimensionais caracterizadores do tipo de regime convectivo. O conhecimento do tipo de regime predominante (forçado, livre ou misto) per (more) mite simplificações e abordagens mais específicas para estimativa destes fluxos. No presente trabalho, foi determinado o tipo de regime convectivo predominante entre dossel e ar, em uma casa de vegetação em arco, com ventilação natural com a cultura do pimentão amarelo. Na maior parte do tempo (>70%) houve predominância da convecção forçada. Durante o dia, foi observado um pequeno aumento da convecção mista, sendo interessante para este período o uso de modelos que contemplem tanto a convecção mista quanto a forçada no estudo das trocas de calor e de massa entre dossel e ar. Abstract in english The use of covering materials in protected environments modifies the air movement close to the crop canopy compared to external environment, which changes the heat and mass transfer between canopy and air. Several researches have been made in greenhouses to estimate mass and heat flux using dimensionless numbers to characterize the type of convection (forced, free or mixed). The knowledge of which one is dominant allows simplifications and specific approaches. The dominan (more) t convection regime between canopy and air was determined in a naturally ventilated greenhouse cropped with sweet pepper. Forced convection was predominant, representing more than 70% of the time. During daytime, an increase of mixed convection was observed. It is thus appropriated the use of models that include both forced and mixed convection in the studies of mass and heat exchanges in canopy - air interface.

Experimental research has long shown that forced-convective heat transfer in wall-bounded turbulent flows of fluids in the supercritical thermodynamic state is not accurately predicted by correlations that have been developed for single-phase fluids in the subcritical thermodynamic state. In the present computational study, the statistical properties of turbulent flow as well as the development of coherent flow structures in a zero-pressure-gradient flat-plate boundary layer are investigated in the absence of body forces, where the working fluid is in the supercritical thermodynamic state. The simulated boundary layers are developed to a friction Reynolds number of 250 for two heat-flux to mass-flux ratios corresponding to cases where normal heat transfer and improved heat transfer are observed. In the case where improved heat transfer is observed, spanwise spacing of the near-wall coherent flow structures is reduced due to a relatively less stable flow environment resulting from the lower magnitudes of the wall-normal viscosity-gradient profile.

The concept of convection covers the description of the velocity and temperature fields of homogenous and isotropic fluids. This concept excludes: the two-phase transfers and flows, the fluidized beds, the transfers in complex fluids, and the transfers inside porous media. Basically, the equations of convection are the equations of fluid mechanics applied to a conduction problem in a deformable environment. The resolution of these equations is in general complex and requires the use of iterative calculus. This article treats of the resolution of these equations in different flow conditions: 1 - introduction; 2 - general considerations (equations, hypotheses); 3 - stationariness, boundary conditions, Newton's hypothesis and mixing temperature; 4 - implementation of Newton's hypothesis (characteristic temperature, non-dimensioning and characteristic numbers); 5 - similarity limits and Nusselt number (irreducible classes in forced and natural convection, expressions of the Nusselt number); 6 - numerical expressions of the Nusselt number (external and internal forced, natural and mixed convection). Three appendixes about the heat transfer equation in a deformable medium, about the non-dimensioning method and about the particular case of convection inside liquid metals complete this analysis. (J.S.)

This work consists of both experimental and theoretical parts. In the experimental part, an insulated solar pond with a surface area of 4m{sup 2} and a depth of 1.5m was built at Cukurova University in Adana, Turkey to conduct performance experiments. The system was filled with salty water of various densities to form three salty water zones (upper convective, non-convective and heat storage). During the months of January, May and August, a data acquisition device was used to measure and record the temperature readings at various locations in the pond (distributed vertically within and at the bottom of the pond, and horizontally and vertically within the insulated side-walls). In the theoretical part, we developed a performance model to determine the thermal efficiencies of the pond and its various zones. Temperature difference was seen to be the key driving force in heat transfer, particularly in heat rejection. As expected, the highest thermal efficiency was obtained for August as follows: 4.5% for the upper convective zone, 13.8% for the non-convective zone and 28.1% for the heat storage zone, respectively. (author)

BS>Research results on liquid metal heat and momentum transfer are presented. A review and summary of boiling heat transfer models found in the literature are presented; nucleate pool and film boiling models are examined, and a comparison of the various models is made. Several boiling heat transfer analyses are outlined. One analysis considers free convection film boiling from a vertical plate and the other describes forced convection film boiling in a pipe. Additional two-phase flow analyses conducted include turbulent-viscous pipe flow, turbulent-turbulent pipe flow, and viscous-viscous curvilinear channel flow. A number of preliminary two-phase, curvilinear flow experiments were completed in pipes containing helical guides. Information on flow patterns and phase separation was obtained using air and water. The results of studies on the Mollier Diagrams and physical properties of Hg and K are also included. (auth)

The evolution of temperature and velocity fields during laser spot welding of 304 stainless steel was studied using a transient, heat transfer and fluid flow model based on the solution of the equations of conservation of mass, momentum and energy in the weld pool. The weld pool geometry, weld thermal cycles and various solidification parameters were calculated. The fusion zone geometry, calculated from the transient heat transfer and fluid flow model, was in good agreement with the corresponding experimentally measured values for various welding conditions. Dimensional analysis was used to understand the importance of heat transfer by conduction and convection and the roles of various driving forces for convection in the weld pool. During solidification, the mushy zone grew at a rapid rate and the maximum size of the mushy zone was reached when the pure liquid region vanished. The solidification rate of the mushy zone/liquid interface was shown to increase while the temperature gradient in the liquid zone at...

Purpose - The purpose of this paper is to develop a thermal coupling method of a ball grid array (BGA) assembly during a forced convection reflow soldering process. Design/methodology/approach - The reflow oven was modeled in computational fluid dynamic (CFD) software (FLUENT 6.3.26) while the structural heating BGA package simulation was done using finite element method (FEM) software (ABAQUS 6.9). Both software applications were coupled bi-directionally using the code coupling software MpCCI. Findings - The convective heat transfer coefficient (h) simulated during the reflow process showed a sufficient view of the changing h in the BGA assembly of each reflow oven. The solder joints were found to experience phase change from solid to liquid during heating and liquid to solid during cooli...

Flow excursion induced dryout at low heat flux natural convection boiling, typical of liquid metal fast breeder reactor, is addressed. Steady state calculations indicate that low quality boiling is possible up to the point of Ledinegg instability leading to flow excursion and subsequent dryout in agreement with experimental data. A flow regime-dependent dryout heat flux relationship based upon saturated boiling criterion is also presented. Transient analysis indicates that premature flow excursion can not be ruled out and sodium boiling is highly transient dependent. Analysis of a high heat flux forced convection, loss-of-flow transient shows a significantly faster flow excursion leading to dryout in excellent agreement with parallel calculations using the two-dimensional THORAX code. 31 refs., 25 figs., 6 tabs.

A comparison is made between experimental data and analytical results for a single-phase natural convection test in an experimental sodium loop. The test was conducted in the Thermal-Hydraulic Out-of-Reactor Safety (THORS) facility, an engineering-scale high temperature sodium loop at the Oak Ridge National Laboratory (ORNL), used for thermal-hydraulic testing of simulated Liquid Metal Fast Breeder Reactor (LMFBR) subassemblies at normal and off-normal operating conditions. Electrical heating in the 19-pin assembly during the test was typical of decay heat levels. The test chosen for analysis in this paper was one of seven natural convection runs conducted in the facility. In this test the bypass line was open to simulate a parallel heated assembly and the test was begun with a pump coastdown from a small initial forced flow.

In the current study, after validating solution's accuracy, we are investigating Mixed Convection Heat Transfer in a square enclosure for drilling mud - as a Non Newtonian Fluid-and water-as a Newtonian Fluid-by finite volume method. The turbulence methods used in this study are the following: RNG k - ? Standard k - ? and RSM. The outcome of the investigation implies that under natural convection condition, velocity boundary layer would be somewhat asymmetrical on the cold wall and the fluid at the center of enclosure would remain stratified and still. The existing graphs also indicate that the turbulence intensity is higher for forced convection than natural convection. Also the maximum turbulence intensity for drilling mud is greater than for water. One of the most prominent outcomes to be named is that under similar circumstances, the Nusselt Number for water is far more than the one for drilling mud which itself is a prove that convection heat transfer is greater in the water than in the drilling mud.

This page discusses thermal convection as it applies to the Earth's mantle and includes three QuickTime movies for three different cases of convection: heating from below, heating from within, and a combination of the two.

This paper investigates the ability of two widely used evaporation models: Dalton based correlations and similarity theory results by comparing with experimental measurements. A series of experimental investigations are carried out over a wide range of water temperatures and air velocities for 0.01???Gr/Re 2???100 in a rectangular heated pool. The results show that for forced convection regime satisfactory results can be achieved by using the modified Dalton correlations, while, due to ripples appear on the water free surface, similarity theory under predicts the evaporation rate. In the free convection regime, Dalton based correlations even with modification are not able to predict acceptable results. For mixed convection regime, although both the similarity theory and Dalton based correl...

Large eddy simulation model is used to simulate the fluid flow and heat transfer in an industrial Czochralski crystal growth system. The influence of Marangoni convection on the growth process is discussed. The simulation results agree well with experiment, which indicates that large eddy simulation is capable of capturing the temperature fluctuations in the melt. As the Marangoni number increases, the radial velocity along the free surface is strengthened, which makes the flow pattern shift from circumferential to spiral. At the same time, the surface tension reinforces the natural convection and forces the isotherms to curve downwards. It can also be seen from the simulation that a secondary vortex and the Ekman layer are generated. All these physical phenomena induced by Marangoni convection have great impacts on the shape of the growth interface and thus the quality of the crystal. (copyright 2011 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

Various types of heat engine using shape memory alloy are proposed and studied. From the viewpoints of thermal efficiency and operational control of an engine an efficient heat exchanger is required for heating and cooling of the shape memory alloy. Furthermore a heat engine operated it a small range of strain is required because the fatigue life of shape memory alloy is strongly dependent on the strain of shape memory alloy. In this paper, the novel operation system of reciprocating type heat engine using shape memory alloy was proposed. This proposed engine could be operated continuously even within a small strain range. Shape memory alloys used were straight wires of Ti-Ni-Cu alloy which were heated and cooled by forced convection. Working characteristics of engines were investigated by experiments. Experiments were conducted under various heating temperatures and strains. Working characteristics such as the amount of work were discussed. 8 refs., 15 figs.

Current carrying equipment for power engineering is getting smaller and more compact in order to meet customer demands. In spite of small dimensions it should be able to carry still growing currents, which cause high current densities in the current carrying parts and therefore cause the high heating of these parts. Due to small dimensions the heat transfer from these devices is restricted too. To assure that the maximum temperature rise in the equipment parts doesn't exceed the allowed temperature rise fixed in the standards, temperatures in these parts should be calculated first, for instance with thermal networks. To achieve a good accuracy of the temperature calculation, the parameters as skin effect factor of the current path and of the enclosure, n{sub 1}-, c{sub 1}-factors of the affinity function Nu = c{sub 1}(GrPr){sup n1} for natural convection and n{sub 2}-, c{sub 2}-factors for the affinity function Nu = c{sub 2}Re{sup n2} for forced convection must be known. For this reason dependency investigations of the skin and proximity effect on the geometry of the current path and the enclosure of the Generator Circuit Breaker as well as on the geometry of the rectangular current bus bars have been carried out. In particular the influence of the enclosure, the dimensions of the current path, the phase shift between the current in the current path and in the enclosure, the influence of the different heat sink types on the current path, the temperature of the current paths and the current frequency on the skin effect of the current path and of the enclosure have been investigated. For rectangular bus bars of the three-phase current system the dimensions of the bus bar as well as its alignment and distance the other bus bars have been examined. The results of these investigations reveal papameters, which have the strongest influence on the skin effect in the current path. Another part of this work deals with heat transfer from the current path to the ambient air. The power losses produced in the current carrying parts are transferred through convection, radiation and conduction from these parts to the ambient. For better cooling effectiveness several heat sink types, for example on the parts of generator circuit breaker, can be used. To improve the accuracy of the temperature calculation with thermal networks, the n{sub 1}- and c{sub 1}-factors of the affinity function Nu = c{sub 1}(GrPr){sup n1} for natural convection, depending on the angle of the heat sink to the airflow have been determined experimentally. Another point of thermal investigation was the heat sink cooling with air, while the air was pre-warmed by other hot parts below the heat sink. This kind of interaction appears particularly in very compact devices and should therefore be investigated for the better accuracy of temperature calculation with thermal networks. Investigations have been carried out for different average temperatures of the heat plate, which was placed below the heat sink and for different heat sink positions to the airflow. Additionally the impact of the air stream detaining parts below the heat sink on the convective heat transfer from this heat sink by free convection was determined. To improve the convective heat transfer from the current path to the ambient forced cooling of this path can be used. It allows transferring much more heat power from the current path and therefore higher current loads without exceeding its maximal allowed temperature rise. The effectiveness of the heat sink by the forced convection depends similar to the free convection on its alignment in the cooling air stream and on its design. The velocity of the cooling air and the bypass above and besides of the heat sink are relevant too. To investigate all these parameters two heat sink types (pin fin heat sink and fin heat sink) were investigated in different wind tunnels, with different positions to the air stream as well as with different velocities of the cooling air. For each heat sink the convective heat transfer coefficient {alpha}{sub Ko} and the c{sub 2-}, n{sub 2-}factors of the affinity function Nu=c{sub 2}Re{sup n2} have been determined. The comparison between the values of the convective heat transfer coefficient {alpha}{sub Ko} shows which heat sink and which heat sinks positions in the air stream are particularly favorable for convective heat transfer from the heat sink. (orig.)

coupled buoyancy and thermo-capillary convection lead to a convective motion of the interface liquid/gas which drastically changes the heat and mass transfer across the liquid layer. Two experiments were considered, depending on the fluid: oil or mercury. The liquid is set in a cooled cylindrical vessel, and heated by a heat flux across the center of the free surface. The basic flow, in the case of oil, is a torus. When the heat parameter increases, a stationary flow appears as petals or rays when the aspect ratio. The lateral confinement selects the azimuthal wavelength. In the case of petals-like flow, a sub-critical Hopf bifurcation is underlined. The turbulence is found to be `weak`, even for the largest values of the Marangoni number (Ma = 1.3 10{sup 5}). In the case of mercury, the thermo-capillary effect is reduced to zero to impurities at the surface which have special trajectories we describe and compare to a simpler experiment. Only the buoyancy forces induce a unstationary, weakly turbulent flow as soon as the heating power exceeds 4W (Ra = 4.5 10{sup 3}, calculated with h = 1 mm). The past part concerns the analysis of the effect on the flow of the boundary conditions, the geometry, the Prandtl number and the buoyancy force with the help of the literature. Results concerning heat transfer, in particular the exponent of the law Nusselt number vs. heating power, were compared with available data. (author) 115 refs.

Despite numerous previous studies, two relationships between deep convection and the sea-surface temperature (SST) of the tropics remain unclear. The first is the cause for the sudden emergence of deep convection at about 28 deg SST, and the second is its proximity to the highest observed SST of about 30 C. Our analysis provides a rational explanation for both by utilizing the Improved Meteorological (IMET) buoy data together with radar rainfall retrievals and atmospheric soundings provided by the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE). The explanation relies on the basic principles of moist convection as enunciated in the Arakawa-Schubert cumulus parameterization. Our analysis shows that an SST range of 28-29 C is necessary for "charging" the atmospheric boundary layer with sufficient moist static energy that can enable the towering convection to reach up to the 200 hPa level. In the IMET buoy data, the changes in surface energy fluxes associated with different rainfall amounts show that the deep convection not only reduces the solar flux into the ocean with a thick cloud cover, but it also generates downdrafts which bring significantly cooler and drier air into the boundary-layer thereby augmenting oceanic cooling by increased sensible and latent heat fluxes. In this way, the ocean seasaws between a net energy absorber for non-raining and a net energy supplier for deep-convective raining conditions. These processes produce a thermostat-like control of the SST. The data also shows that convection over the warm pool is modulated by dynamical influences of large-scale circulation embodying tropical easterly waves (with a 5-day period) and MJOs (with 40-day period); however, the quasi-permanent feature of the vertical profile of moist static energy, which is primarily maintained by the large-scale circulation and thermodynamical forcings, is vital for both the 28 C SST for deep convection and its upper limit at about 30 C.

With respect to the transport of casks for radioactive material, the proof of the safe heat removal can be accomplished by validated calculation methods. The boundary conditions for thermal tests for type B packages are specified in the ADR based on the regulations defined by the International Atomic Energy Agency. The varying boundary conditions under transport or storage conditions are based on the varying thermal conditions true for different cask types. In most cases the cask will be transported in lying position under a cover (e.g. canopy or tarpaulin) and stored in standing position in an array with other casks. The main heat transport mechanisms are natural convection and thermal radiation. The cover or the storage building are furnished with vents that create an air flow, which will improve the natural convection. Depending on the thermal boundary conditions, the cask design and the heat power, about 50 - 95% of the heat power will be removed from the finned cask surface by natural convection. Consequently the convection by air flow is the main heat transport mechanism. The air flow can be approximated with analytical methods by solving the integral heat and flow balances for the domain. In a stationary state the overpressure due the buoyancy and the pressure loss in the flow resistances are equal. Based on the air flow, the relevant temperatures of the cask can be calculated in an iterative process. Due to the fast development of numerical calculation methods and computer hardware, the use of Computational- Fluid-Dynamics(CFD) calculations plays an important role. CFD-calculations are based on solving the equations of conservation (Navier-Stokes equations) using a finite element mesh or a finite volume mesh of the model. For a finned cask lying under a cover, where the main contributing element for heat removal is natural convection in combination with the thermal radiation, a CFD-calculation can be the most appropriate method. Common CFD-Codes are FLUENT or CFX using finite-volume solvers or ANSYS-FLOTRAN using finite-element solvers. The correct functioning of these codes is globally tested over a bread rage of industrial flow problems. The weakly forced natural convection coupled with convective and radiative heat transfer at a finned surface is a very special use of CFD, so that the numerical methods by using CFD have to be validated in detail for these applications.

Abstract in english This lecture addresses some recent developments in modelling of macroscopic thermodynamic and hydrodynamic non-equilibrium phenomena in convective phase change (boiling and condensation) of pure fluids and mixtures. Proper accounting of such phenomena may hold the key to explain and predict deviations from the classical (equilibrium) phase change convective heat transfer behaviour reported in the literature and yet not fully understood. In the first part of the paper, a d (more) etailed qualitative description of the classical heat transfer coefficient behaviour is presented together with two examples of departure from macroscopic equilibrium largely supported by experimental evidence. The second part of the paper reviews successful attempts to model the non-equilibrium phase change phenomena taking place in the two situations. The first example is a thermodynamic non-equilibrium slug flow model (one in which saturated Taylor bubbles become separated by slugs of subcooled liquid) that predicts the peaks in heat transfer coefficient at near-zero thermodynamic quality observed in forced convective boiling of some pure liquids. The occurrence of such peaks is typical of low latent heat, low thermal conductivity systems and of systems in which the vapour volume formation rate for a given heat flux is large. The second example is a comprehensive annular flow calculation methodology that predicts the decrease in the heat transfer coefficient with increasing quality observed in convective boiling of binary and multicomponent mixtures. In this case, as will be seen, coupled mass transfer resistance and hydrodynamic non-equilibrium effects generate concentration gradients between the liquid film and entrained droplets that are responsible for the heat transfer deterioration. In addition, it will be shown that for condensation of mixtures the methodology predicts a heat transfer intensification which has been subsequently confirmed by independent experimental results.

The aim of our study is to establish a reliable database for improving thermal hydraulic codes, in the field of turbulent flows with buoyancy forces. The flow considered is mixed convection in the Reynolds and Richardson number range: Re = 10{sup 3} to 6.10{sup 4} and Ri = 10{sup -4} to 1. Experiments are carried out in an upward turbulent flow between vertical parallel plates at different wall temperatures. Part 1 gives a detailed database of turbulent mixed flow of free and forced convection. Part 2 presents the installation and the calibration system intended for probes calibration. Part 3 describes the measurement technique (constant temperature probe and cold-wire probe) and the method for measuring the position of the hot-wire anemometer from the wall surface. The measurement accuracy is within 0.001 mm in the present system. Part 4 relates the development of a method for near wall measurements. This correction procedure for hot-wire anemometer close to wall has been derived on the basis of a two-dimensional numerical study. The method permits to obtain a quantitative correction of the wall influence on hot-wires and takes into account the velocity profile and the effects the wall material has on the heat loss. Part 5 presents the experimental data obtained in the channel in forced and mixed convection. Results obtained in the forced convection regime serve as a verification of the measurement technique close to the wall and give the conditions at the entrance of the test section. The effects of the buoyancy force on the mean velocity and temperature profiles are confirmed. The buoyancy strongly affects the fluid structure and deforms the distribution of mean velocity. The velocity profiles are asymmetric. The second section of part 5 gives an approach of analytical wall functions with buoyancy forces, on the basis of the experimental data obtained in the test section. (author)

Steady, laminar, mixed convection in the fully developed region of horizontal concentric annuli has been investigated numerically for the case of non-uniform circumferential heating. Two heating conditions were studied, one in which a 180 {sup circle} arc encompassing the top half of inner surface of the inner cylinder is uniformly heated while the bottom half is kept insulated, and the other in which the heated and the insulated surfaces were reversed. The fluid flow and heat transfer characteristics were found to be affected by the heating conditions. For the investigated range of the governing buoyancy parameter, the modified Grashof number (Gr{sup *}), it was found that bottom heating arrangement gives rise to a vigorous secondary flow, with the result that the average Nusselt numbers are much higher than those for pure forced convection. On the other hand, the local Nusselt numbers are nearly circumferentially uniform. In the case of top heating arrangement, a less vigorous secondary flow is induced because of temperature stratification, with average Nusselt numbers that are substantially lower than those for bottom heating and with large circumferential variation of the local Nusselt number. (orig.)

Nanofluids are suspensions of metallic or nonmetallic nanopowders in base liquid and can be employed to increase heat transfer rate in various applications. In this work laminar flow forced convection heat transfer of Al{sub 2}O{sub 3}/water nanofluid inside a circular tube with constant wall temperature was investigated experimentally. The Nusselt numbers of nanofluids were obtained for different nanoparticle concentrations as well as various Peclet and Reynolds numbers. Experimental results emphasize the enhancement of heat transfer due to the nanoparticles presence in the fluid. Heat transfer coefficient increases by increasing the concentration of nanoparticles in nanofluid. The increase in heat transfer coefficient due to presence of nanoparticles is much higher than the prediction of single phase heat transfer correlation used with nanofluid properties.

This paper discusses mechanisms in the post-CHF region which provide understanding and qualitative prediction capability for several current forced convective heat transfer problems. In the area of nuclear reactor safety, the mechanisms are important in the prediction of fuel rod quenches for the reflood phase, blowdown phase, and possibly some operational transients with dryout. Results using the mechanisms to investigate forced convective quenching are presented. Data reduction of quenching experiments is discussed, and the way in which the quenching transient may affect the results of different types of quenching experiments is investigated. This investigation provides an explanation of how minimum wall superheats greater than the homogeneous nucleation temperature result, as well as how these may appear to be either hydrodynamically or thermodynamically controlled. Finally, the results of a parametric study of the effects of the mechanisms upon the LOFT L2-3 hotpin calculation are presented.

The extratropical response to typhoon-related convective forcing over the western North Pacific in late summer is examined based on ECMWF global reanalysis (ERA-40) data during the 1958-2001 period. Typhoon activity is intimately associated with most of the major events in which an extratropical wavetrain structure prevails from the north of the Philippines through the central North Pacific. The vertical structure of the wavetrain pattern changes from baroclinic to a barotropic along the great circle. The analysis of the wave activity flux indicates that the extratropical wavetrain is stimulated by stationary Rossby waves. It was found that one or two typhoons, which are a synoptic-scale convective heat source over the western North Pacific, can induce the barotropic Rossby wavetrain and significantly influence the summer weather in the vicinity of Japan as remote forcing.   

A microchannel heat sink, integrated with pressure and temperature microsensors, is utilized to study single-phase liquid flow forced convection under a uniform heat flux boundary condition. Utilizing a wafer-bond-and-etch-back technology, the heat source, temperature and pressure sensors are encapsulated in a thin composite membrane capping the microchannels, thus allowing experimentally good control of the thermal boundary conditions. A three-dimensional physical model has been constructed to facilitate numerical simulations of the heat flux distribution. The results indicate that upstream the cold working fluid absorbs heat, while, within the current operating conditions, downstream the warmer working fluid releases heat. The Nusselt number is computed numerically and compared with experimental and analytical results. The wall Nusselt number in a microchannel can be estimated using classical analytical solutions only over a limited range of the Reynolds number, Re: both the top and bottom Nusselt numbers approach 4 for Re Nusselt numbers approach 0 and 5.3, respectively, for Re > 100. The experimentally estimated Nusselt number for forced convection is highly sensitive to the location of the temperature measurements used in calculating the Nusselt number.

Numerical simulation with the Forchheimer flow model and local thermal non-equilibrium model for porous region is performed on forced convective heat transfer in a tube partially filled with metallic foams. Flow and heat transfer of fluid in the hollow region and those of fluid in the porous region are conjugated together via the coupling conditions at porous-fluid interface. A heat flow model is proposed with special numerical treatments employed for non-equilibrium conjugated heat transfer in foam-fluid system. Velocity and temperature profiles in the flow direction are obtained and validated with analytical results. Effects of porosity, pore density, dimensionless interfacial radius and fluid-to-solid thermal conductivity ratio on flow characteristics and thermal performance are examined. Accordingly, the entrance effect is analyzed through the numerical simulation in terms of both flow and heat transfer. The present tube exhibits more excellent heat transfer performance at the expense of moderate pressure drop compared with the tube without porous material. The numerical work is not only developed for forced convection in metal-foam partially filled tube, but can also be extended to similar problem with porous-fluid interface for other porous media with significant thermal non-equilibrium effect.   

The basic, generic problem of a localized high-intensity heat source directed against one surface of a plate of finite thickness was investigated using the finite-element program ANSYS. After reviewing similar work in nuclear fuel and laser machining, ANSYS was verified against a known solution. ANSYS was used to create a model that yields minimum heat-transfer coefficients needed to prevent the initiation of melting in thin aluminum, titanium, and stainless steel (AISI 304) plates. These heat-transfer coefficients were converted into minimum local Nusselt-numbers and graphed against local Nusselt-number correlations for constant-temperature flat plates in forced- and free-convection regimes. A detailed listing of both laminar and turbulent correlations is presented along with references. The suitability of liquid sodium, air, and water (under high pressure) as coolants for a source intensity of 2.0 x 10/sup 7/ w/sq m was examined. For free convection, only liquid sodium cooling a titanium plate is feasible; for forced convection, liquid sodium is feasible in laminar flow for all three plates with velocities ranging from 0.28 to 1.09 m/s. Water is feasible for aluminum and titanium in turbulent flow at velocities of approximately 4 m/s.

An investigation was carried out to provide a detailed analysis of laminar fluid flow and heat transfer in internally finned pipes. Three mathematical models were formulated for this purpose, and shown to be capable of simulating the actual situation of pressure drop and heat transfer in such tubes. Steady, laminar forced convection heat transfer in the thermal entrance region of internally finned tubes was investigated numerically for the case of fully developed hydrodynamics using the H1 and T thermal boundary conditions. Steady, laminar fluid flow in the hydrodynamic entrance region of internally finned tubes was investigated numerically. Results are presented for the smooth tube geometry and sixteen geometries corresponding to various combinations of relative fin heights and number of fins. Steady, laminar mixed convection in the fully developed region of horizontal internally finned tubes was investigated for the case of uniform heat input axially and uniform wall temperature circumferentially. Fluid flow and heat transfer characteristics were found to be dependent on a modified Grashof number, Prandtl number, relative fin height, and number of fins. Internal finning was found to retard the onset of significant free convective effects and to suppress the enhancement in friction factor and Nusselt number compared to smooth tubes. 54 refs., 93 figs., 12 tabs.

Coarse debris is a characteristic ground material in high alpine environments. The special thermal properties of this ground material favour the existence of permafrost. However, the most important processes explaining the common thermal anomaly found within these materials are still not yet fully understood. Many different approaches try to explain these processes. The most common explanation is the heat transfer between atmosphere and ground, driven by heat convection in autumn and winter and stable stratification of the interstitial air in summer. These processes could be shown at the investigated site in an earlier study (Hanson and Hoelzle 2005). On the contrary, Gruber and Hoelzle (2008) tried to explain the observed measurements independent of convective processes, only based on model calculations, which were based on the interaction between winter snow cover and the very low thermal conductivity of the coarse debris layer. In the present study, we took the ground surface temperature data from the uppermost 90 cm of the active layer of the rock glacier Murtèl-Corvatsch in combination with meteorological data, such as air temperature, snow depth and radiation to analyze the dominant heat transfer mechanisms during the different seasons. The main focus was to assess the contribution of convective processes. The potential for free convection was estimated using the Rayleigh number. In addition, the air circulation within the uppermost active layer measured by three wind sensors was taken into consideration. These data were compared with the other climate variables of the nearby meteorological station. After analyzing the data, it can be concluded that the potential for free convection in the cavities of the upper blocky layer is high as soon as the stable thermal stratification during the summer month gets instable due to a cooling of the surface. Especially in the autumn and early winter months a strong ground cooling could be observed caused by the low air temperatures and the related vertical convective heat exchange. These processes are limited by the formation of a continuous snowpack. A thick snow cover limits to a large extent the exchange with the atmosphere. Within the blocky layer, however, the potential for convection is still high. During snowmelt in spring, the latent heat transfer is the dominant process. During the summer month, the snow-free period, air circulation in the blocky layer is mainly caused by forced convection. However, this effect decreases with increasing depth of the blocky layer. The analysis of the wind sensor data shows that the ventilation system in the blocky layer is very variable and complex. However, some interpretations could be done. A) it can be shown that the airflow velocities within the blocky layer in the snow-free season are influenced by the atmospheric wind speed. B) in summer and autumn the effect of free convection could be identified with help of the measured air flow velocities and a comparison with the temperature data. C) the formation of a continuous snow cover reduces the airflow velocities, and the influence of forced convection. This effect is intensified during spring by the formation of ground ice caused by percolating melt water. Through the comparison of air and ground temperatures with the airflow velocities convective processes could be detected during a prolonged period in autumn, winter and sometimes even in spring. This indicates that convective processes in the microclimate of the active layer at the rock glacier Murtèl-Corvatsch site play probably a larger role than previously expected.

In this study, thermally developing laminar forced convection in a pipe including viscous dissipation and wall conductance is investigated numerically. The constant heat flux is assumed to be imposed at the outer surface of the pipe wall. The finite volume method is used. The distributions for the developing temperature and local Nusselt number in the entrance region are obtained. The dependence of the results on the Brinkman number and the dimensionless thermal conductivity are shown. The viscous heating effect on the wall is shown. Significant viscous dissipation effects have been observed for large Br.

A numerical study is presented for the combined heat and mass transfer by natural convection adjacent to a vertical surface embedded in a stably thermally stratified, fluid-saturated, low-porosity medium. A wide range of thermal stratification levels is considered in flows with both aiding and opposing buoyant forces. The thermal stratification was shown to have a profound influence on the heat and mass transfer rates and on the direction of flow and transport. The underlying physical phenomena are explained while providing the Nusselt and Sherwood number data.

The effect of thermal and mass buoyancy forces on the development of laminar mixed convection between two vertical parallel plates with uniform heat and mass fluxes has been investigated numerically. Velocity, temperature and concentration profiles have been presented. The effect of the different parameters on heat and mass transfer between the plates has been discussed for positive and negative values of the buoyancy ratio, N. The results showed that positive N increases the Nusselt number and Sherwood number, while negative N decreases them. Schmidt number has the highest effect on the Nusselt number and Sherwood number. (authors)

A nonlinear sub-grid-scale (SGS) stress model based on a quadratic constitutive relation developed previously for the RANS approach is introduced in large eddy simulation (LES) of turbulent thermal flows. This model is combined with a linear and nonlinear SGS heat flux model, and applied to LES of combined forced and natural turbulent convection in a vertical slot with two differentially heated side walls. The prediction for the resolved-scale fields is compared to that based on a dynamic Smagorinsky model (DM), as well as DNS results. The combination of nonlinear SGS models yields improved predictions compared to those of conventional linear models. (authors)

Forced convection transient heat transfer for helium gas at various periods of exponentially increasing heat input (Q0exp(t/?)) to a horizontal plate (ribbon) was experimentally and theoretically studied. In the experimental studies, the authors measured heat flux, surface temperature, and transient heat transfer coefficients for forced convection flow of helium gas over the horizontal plate under wide experimental conditions. The platinum plate with a length of 50 mm was used as a test heater. The gas flow velocities ranged from 4 to 10 m/s, the gas temperatures ranged from 313 to 353 K, and the periods of heat generation rate, ?, ranged from 46 ms to 17 s. The pressures were from 400 to 800 kPa. It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period longer than about 1 s, and it becomes higher for the period shorter than around 1 s. Empirical correlations for quasi-steady state heat transfer and transient one were obtained based on the experimental data under various pressures. In the theoretical study, transient heat transfer was numerically solved based on a turbulent flow model. The values of numerical solutions for surface temperature and heat flux were compared and discussed with authors' experimental values. It was obtained that the surface temperature difference and heat flux increase exponentially as the heat generation rate increases with the exponential function. It is understood that the gradient of the temperature distribution near the heater surface is higher at a higher surface temperature difference. The values of numerical solutions for heat flux agree well with the experimental data, though surface temperatures show some differences.   

Numerous experimental and theoretical investigations on two-phase flow instability and burnout in heated microchannels have been reported in the literature. However none of these investigations deals with the possible effects of wall vibrations on such flow boiling processes within microchannels. Fluid-structure interaction in ultra high power density systems cooled by high velocity single phase forced convection in microchannels may result in vibration amplitudes that are a significant fraction of the diameter of the channel. Such vibrations may significantly impact vapor bubble dynamics at the wall and, hence, the limiting heat fluxes corresponding to the onset of flow instability and/or burnout. The primary purpose of this research was to experimentally quantify the effect of forced wall vibration on the onset of flow instability (OFI) and the critical heat flux (CHF) in uniformly-heated annular microchannels. The secondary interest of this investigation was to compare the experimental data collected in the single-phase regime to commonly used single-phase forced convection correlations. Experimental data acquired in the flow boiling regime were to be utilized to confirm the validity of common flow boiling correlations for microchannel flow. The influence of forced wall vibration on subcooled single-phase forced convection and flow boiling was examined. The Georgia Tech Microchannel Test Facility (GTMTF) was modified to allow such experiments to be conducted at controlled values of transverse wall vibration amplitudes and accelerations for a range of frequencies. The channel demand curves were obtained for various inner and outer surface heat fluxes. Experiments were conducted for broad ranges of transverse wall vibration amplitudes over a range of frequencies. The experiments conducted in this investigation provide designers of high power density systems cooled by forced convection in microchannels with the appropriate data and correlations to confidently design systems under realistic operational conditions, including the potentially significant effects of fluid-structure interactions. The data also provides the basis for development and validation of future models on the effect of wall vibrations on bubble dynamics in flow boiling systems. The observed enhancements in OFI and CHF resulting from wall vibration suggest that correlations for undisturbed channels can be conservatively used for system design calculations.

Drift-emplaced waste canisters are under consideration for the long-term storage of high-level spent fuel in the proposed underground repository at Yucca Mountain. These canisters will be placed on pedestals above the floor of the drifts and exchange heat with the walls of the drift and with air circulating through the repository. To assess the requirements of the repository ventilation system, values of the dimensionless convective heat transfer coefficient and the pressure drop across individual canisters were measured in a experimental model of a drift. The results were curvefitted as functions of the spacing between the canisters and the Reynolds number of the flow. Both natural and forced convection effects were investigated.

Wakes behind heated cylinders, circular, and square have been experimentally investigated at low-Reynolds numbers. The electrically heated cylinder is mounted in a vertical airflow facility such that buoyancy aids the inertia of main flow. The operating parameters, i.e., Reynolds number and Richardson number are varied to examine flow behavior over a range of experimental conditions from forced to mixed convection regime. Laser schlieren-interferometry has been used for visualization and analysis of flow structures. Complete vortex shedding sequence has been recorded using a high-speed camera. The results on detailed dynamical characteristics of vortical structures, i.e., their size, shape and phase, Strouhal number, power spectra, convection velocity, phase shift, vortex inception length,...

During the last few years, several models have been proposed for the calculation of green roof thermal behavior, but the validation studies of such models are lacking a comprehensive set of highly accurate data. In this study, an experimental laboratory setup was used to create different environmental conditions and to measure sensible heat fluxes to/from a vegetated roof assembly. This experimental setup has been successfully used for different wind velocities (0-3 m/s) to create free and forced convection conditions around green roof tested samples. Furthermore, our study proposed a "basic model" for calculations of the convective heat transfer at green roof assemblies, which is a modified version of the Newton’s cooling law, calibrated and then validated with different sets of da...

Numerical simulation of rotating convection in plane layers with free slip boundaries show that the convective flows can be classified according to a quantity constructed from the Reynolds, Prandtl and Ekman numbers. Three different flow regimes appear: Laminar flow close to the onset of convection, turbulent flow in which the heat flow approaches the heat flow of non-rotating convection, and an intermediate regime in which the heat flow scales according to a power law independent of thermal diffusivity and kinematic viscosity.

The formation and dynamics of large-scale circulations in forced and mixed convection has been studied at ambient and elevated fluid pressure by means of particle image velocimetry and temperature measurements. The study has been conducted in two rectangular containers of the same shape and aspect ratios of {Gamma}{sub xz} = 1 and {Gamma}{sub yz} = 5. For the measurements at high fluid pressure the dimensions of the cell have been scaled down by a factor of 5. Air with Pr = 0.7 has been used as fluid in both configurations. Forced convection has been investigated at Re = 1.01 x 10{sup 4} and mixed convection has been studied at Ar = 3.3, Re = 1.01 x 10{sup 4} and Ra = 2.4 x 10{sup 8}. In this configuration low-frequency oscillations in the heat transfer between the inlet and outlet have been found for mixed convection. Instantaneous velocity vector fields obtained from particle image velocimetry have been analysed using proper orthogonal decomposition and an algorithm to detect the core and the core centre position of large-scale circulations.

This paper presents results from 240-member ensemble simulations of aerosol indirect effects on tropical deep convection and its thermodynamic environment. Simulations using a two-dimensional cloud-system resolving model are run with pristine, polluted, or highly polluted aerosol conditions and large-scale forcing from a 6-day period of active monsoon conditions during the 2006 Tropical Warm Pool - International Cloud Experiment (TWP-ICE). Domain-mean surface precipitation is insensitive to aerosols primarily because the large-scale forcing is prescribed and dominates the water and static energy budgets. The spread of the top-of-atmosphere (TOA) shortwave and longwave radiative fluxes among different ensemble members for the same aerosol loading is surprisingly large, exceeding 25 W m-2 even when averaged over the 6-day period. This variability is caused by random fluctuations in the strength and timing of individual deep convective events. The ensemble approach demonstrates a small weakening of convection averaged over the 6-day period in the polluted simulations compared to pristine. Despite this weakening, the cloud top heights and anvil ice mixing ratios are higher in polluted conditions. This occurs because of the larger concentrations of cloud droplets that freeze, leading directly to higher ice particle concentrations, smaller ice particle sizes, and smaller fall velocities compared to simulations with pristine aerosols. Weaker convection in polluted conditions is a direct result of the changes in anvil ice characteristics and subsequent upper-tropospheric radiative heating and weaker tropospheric destabilization. Such a conclusion offers a different interpretation of recent satellite observations of tropical deep convection in pristine and polluted environments compared to the hypothesis of aerosol-induced convective invigoration. Sensitivity tests using the ensemble approach with modified microphysical parameters or domain configuration (horizontal gridlength, domain size) produce results that are similar to baseline, although there are quantitative differences in estimates of aerosol impacts on TOA radiative fluxes.

The physical phenomenon of forced convective boiling is probably one of the most interesting and complex transport phenomena. It has been under study for more than two centuries. Simply stated, forced convective subcooled boiling involves a locally boiling fluid: (1) whose mean temperature is below its saturation temperature, and (2) that flows over a surface exposed uniformly or non-uniformly to a high heat flux (HHF). The objective of this work is to assess and/or improve the present ability to predict local axial heat transfer distributions in the subcooled flow boiling regime for the case of uniformly heated coolant channels. This requires an accurate and complete representation of the boiling curve up to the CHF. The present. results will be useful for both heat transfer research and industrial design applications. Future refinements may result in the application of the results to non-uniformly heated channels or other geometries, and other fluids. Several existing heat transfer models for uniformly heated channels were examined for: (1) accurate representation of the boiling curve, and (2) characterizing the local heat transfer coefficient under high heat flux (HHF) conditions. Comparisons with HHF data showed that major correlation modifications were needed in the subcooled partial nucleate boiling (SPNB) region. Since the slope of boiling curve in this region is important to assure continuity of the HHF trends into the fully developed boiling region and up to the critical heat flux, accurate characterization in the SPNB region is essential. Approximations for the asymptotic limits for the SPNB region have been obtained and have been used to develop an improved composite correlation. The developed correlation has been compared with 363 water data points. For the local heat transfer coefficient and wall temperature, the over-all percent standard deviations with respect to the data were 19% and 3%, respectively, for the high velocity water data.

The Solar Dynamic Power Module being developed for Space Station Freedom uses a eutectic mixture of LiF-CaF2 phase change material (PCM) contained in toroidal canisters for thermal energy storage. Presented are the results from heat transfer analyses of a PCM containment canister. One and two dimensional finite difference computer models are developed to analyze heat transfer in the canister walls, PCM, void, and heat engine working fluid coolant. The modes of heat transfer considered include conduction in canister walls and solid PCM, conduction and pseudo-free convection in liquid PCM, conduction and radiation across PCM vapor filled void regions, and forced convection in the heat engine working fluid. Void shape, location, growth or shrinkage (due to density difference between the solid and liquid PCM phases) are prescribed based on engineering judgment. The PCM phase change process is analyzed using the enthalpy method. The discussion of the results focuses on how canister thermal performance is affected by free convection in the liquid PCM and void heat transfer. Characterizing these effects is important for interpreting the relationship between ground-based canister performance (in 1-g) and expected on-orbit performance (in micro-g). Void regions accentuate canister hot spots and temperature gradients due to their large thermal resistance. Free convection reduces the extent of PCM superheating and lowers canister temperatures during a portion of the PCM thermal charge period. Surprisingly small differences in canister thermal performance result from operation on the ground and operation on-orbit. This lack of a strong gravity dependency is attributed to the large contribution of container walls in overall canister energy redistribution by conduction.

The device for improving the efficiency of a Curie motor, according to the invention, consists of two vertically orientated, adjacent endless belts running over two reversing rollers each. One of these belts has permanent magnets fitted at regular intervals and the other belt made of ferro-magnetic material is situated in a closed container filled with a heat transfer medium (e.g. water). The upper roller of this belt has heat supplied to it, so that the ferro-magnetic material is heated above its Curie point. In the lower part, the permanent magnets exert a force on the belt consisting of ferro-magnetic material, which increases with decreasing distance. A tensile force is therefore exerted on both belts, which sets them into motion. The convection of the water ensures that the upper roller is at a temperature above the Curie temperature and the lower roller is at a temperature below the Curie temperature. (LU).

Loop heat pipe (LHP) is a promising means for electronics cooling since LHP is a exceptionally efficient heat transfer device. In this paper, a novel miniature LHP (mLHP) system is presented and optimal design of condenser is considered seeing that evaporators have been able to handle very high-heat fluxes with low-heat transfer resistances since most of the previous researchers focused on the evaporator of mLHP. The arrayed pins were designed and machined out on the bottom of condenser to enhance condensation heat transfer. The parameters of the arrayed pins, including layout, cross-section shape and area, were optimized by finite element analysis. Tests were carried out on the mLHP with a CPU thermal simulator using forced air convection condenser cooling to validate the optimization. Th...

A numerical investigation has been carried out for conjugate forced convection heat transfer in tubes with multiple obstructions. The investigation included the ranges of Reynolds number Re, thermal conductivity ratio k{sub r}, and the distance between two obstructions d of 50--600, 0.3--3.0, and 2.0--8.0, respectively. A fluid Prandtl number Pr of 7.56 is used. The obstructions create recirculation flow/back flow in the tube with increase and decrease of local heat transfer rates. Reversal in the direction of heat transfer occurs at certain locations of the tube due to the presence of the obstructions. At high values of Re and d, reversal in the direction of heat transfer occurs on the outer wall after the second obstruction. The overall heat transfer rate decreases with the decrease of k{sub r}.

An asymptotic method to account for variable property effects, recently described in this journal, is now applied to a complex benchmark geometry. It is a room which is ventilated by forced convection through inlet and outlet slit nozzles at the top and bottom of the side walls. Four heating elements standing on the ground floor add heat with constant heat flux density of varying strength. CFD solutions with the full coverage of all property temperature dependencies of air and SF6 are compared with asymptotic results (ACFD), applied for these fluids. ACFD results are given as systematic expansions with respect to a heat transfer parameter Formula Not Shown which serves as perturbation parameter. First and second order asymptotic results of the Nu?elt number at the surface of the heating el...

The present study concerns a stability analysis of a Soret convection within a porous layer subject to uniform fluxes of heat and solute on its horizontal boundaries. The solutal buoyancy forces are induced by the imposition of a solute gradient and by the thermal diffusion phenomenon. The Brinkman-extended Darcy model and the Boussinesq approximation are used to model the convective flow through the porous medium. Based on linear stability theory, the resulting linear perturbed equations are solved numerically using the finite element method. An analytical solution is derived on the basis of the parallel flow approximation, and validated against the numerical results obtained by solving the full governing equations using a finite difference method. The results corresponding to the cases of double-diffusive convection (without Soret effect) and Soret convection (with zero mass flux) are recovered by the present formulation as limiting cases. The two other limiting cases, namely the low porosity Darcy porous medium and the clear fluid medium emerge also from the present investigation. The critical Rayleigh numbers for the onset of subcritical and stationary convection are determined explicitly as functions of the governing parameters. The threshold of Hopf bifurcation is determined as function of the governing parameters by performing a linear stability analysis of the perturbed rest state. The existence of two codimension-2points (subcodimension-2 and Hopf-codimension-2) is proved and different flow regimes are delineated. The diagrams of stability show that there exists a range of Lewis number in which the subcritical convection disappears. It is shown that the thermal diffusion, has a strong effect on the instability thresholds and on heat and mass transfer characteristics. (author)

When modeling thermal performance of building components and envelopes, researchers have traditionally relied on average surface heat-transfer coefficients that often do not accurately represent surface heat-transfer phenomena at any specific point on the component being evaluated. The authors have developed new experimental techniques that measure localized surface heat-flow phenomena resulting from convection. The data gathered using these new experimental procedures can be used to calculate local film coefficients and validate complex models of room and building envelope heat flows. These new techniques use a computer-controlled traversing system to measure both temperatures and air velocities in the boundary layer near the surface of a building component, in conjunction with current methods that rely on infrared (IR) thermography to measure surface temperatures. Measured data gathered using these new experimental procedures are presented here for two specimens: (1) a Calibrated Transfer Standard (CTS) that approximates a constant-heat-flux, flat plate; and (2) a dual-glazed, low-emittance (low-e), wood-frame window. The specimens were tested under steady-state heat flow conditions in laboratory thermal chambers. Air temperature and mean velocity data are presented with high spatial resolution (0.25- to 25-mm density). Local surface heat-transfer film coefficients are derived from the experimental data by means of a method that calculates heat flux using a linear equation for air temperature in the inner region of the boundary layer. Local values for convection surface heat-transfer rate vary from 1 to 4.5 W/m{sup 2} {center_dot} K. Data for air velocity show that convection in the warm-side thermal chamber is mixed forced/natural, but local velocity maximums occur from 4 to 8 mm from the window glazing.

The dissertation contains 4 original chapters: 1) Parametric excitation of Soret-driven convection of binary mixture in a horizontal porous layer. 2) Soret-driven convection in a horizontal porous layer from a heat or concentration source. 3) Localization of convective flows under randomly inhomogeneous heating. 4) Synchrony of nonlinear systems driven by common noise.

Using an ocean general circulation model (OGCM) forced by daily mean wind stresses and heat fluxes derived from the bulk formulation with the NCEP/NCAR reanalysis of 1973-1995, we examined interannual atmosphere-ocean variations in the tropical western North Pacific related to the Asian summer monsoon-ENSO coupling. The OGCM simulation was successful in reproducing the east-west gradient of summertime SST anomalies between the South China Sea and the tropical western Pacific east of the Philippines that is linked with anomalous tropical convection in that vicinity. A heat budget analysis shows that a longitudinal asymmetry of surface latent heat flux anomalies is crucially responsible for the reinforcement and persistence of the east-west gradient of SST anomalies between the two regions. It is also found that the transition from premonsoon regime to monsoon regime in the tropical Indian Ocean affects the interannual atmosphere-ocean variations in the tropical western North Pacific during the period from the late 1970s to the early 1990s. In the spring before strong Asian summer monsoon, an equatorially asymmetric air-sea coupled mode tends to appear in the tropical Indian Ocean. Simultaneously with the beginning of the strong monsoon regime, the northern Indian Ocean and South China Sea are covered by cool SST anomalies due to enhanced wind speed and evaporation, whereas warm anomalies relevant to a cold episode of ENSO are still maintained in the warm pool region east of the Philippines. The latitudinally asymmetric anomalies of tropical convection and SST become dissipated and convection over the warm pool region of the western North Pacific becomes localized and enhanced with the establishment of east-west gradient of SST anomalies. Due to enhanced convective heating, equatorially asymmetric atmospheric Rossby waves are excited to the west of anomalous convection, which induce low-level westerly anomalies. This dynamic process further facilitates the localization of intense convection through the change in surface latent heat flux and SST. While such a positive atmosphere-ocean feedback system persists in boreal summer, the Pacific and Japan (PJ) teleconnection pattern in response to enhanced convection prevails and brings about unusually hot summers in recent years especially in the vicinity of Japan.   

Inverse Heat Convection Problems have received attention only recently. They usually involve the use of a high order model corresponding to the spatial discretization of the domain. In this numerical study, the possibility to quickly solve such a problem with a low order model is analysed. The proposed method can be applied to any forced convection problem, whatever the geometry, as far as it is linear. Starting from a Detailed Model (DM) of the system, the Modal Identification Method is applied to build a Reduced Model (RM), which can be used to solve the inverse problem. The inversion procedure is sequential and requires no iterations. The function specification method is used to stabilize the inverse problem. An illustrative application is given. Turbulent forced convection is considered, with a hydrodynamically fully developed, thermally developing, incompressible, turbulent flow of a newtonian and constant property fluid inside a parallel-plate duct. Axial conduction in the flow is neglected. Two wall heat flux densities, varying with time, are estimated from the knowledge of simulated transient temperature measurements inside the fluid. When solving the inverse problem with RM instead of DM, a drastic reduction of computing time is obtained (with a reduction factor up to 11,000 in the present study), without significant loss of accuracy. Effects of functional form of the unknowns, sensor numbers and position, measurement error, on the accuracy of estimates are examined.(Author)

The cloud radiative effects on responses of rainfall to the large-scale forcing during a landfall of severe tropical storm Bilis (2006) are investigated by analyzing sensitivity experiments imposed by large-scale forcing from NCEP/GDAS data in a two-dimensional cloud-resolving model. The daily average analysis is conducted on 15 and 16 July 2009, respectively, due to dominant stratiform and convective rainfall associated with different large-scale forcing. When cloud radiative effects are excluded, the increased mean rainfall is associated with the increased mean radiative cooling through the enhanced mean latent heat on 15 July. The reduction in mean rain rate is related to the slowdown in the mean net condensation while the enhanced mean radiative cooling from the removal of cloud radiative effects is balanced by the suppressed heat divergence on 16 July. The increased mean rainfall on 15 July and decreased mean rainfall on 16 July are mainly from raining stratiform regions. The enhanced stratiform rainfall is associated with the weakened local atmospheric moistening and strengthened local hydrometeor loss on 15 July, whereas the reduced stratiform rainfall is related to the weakened water vapor convergence on 16 July. When cloud-radiation interaction is excluded, the decreases in the mean rain rate are associated with the slowdown in the mean hydrometeor loss on 15 July and the suppression in the net condensation on 16 July. The decreased mean rainfall is mainly from convective regions on 15 July and raining stratiform regions on 16 July. The reduced convective rainfall is associated with strengthened transport of hydrometeor concentration from convective regions to raining stratiform regions on 15 July, whereas the decreased stratiform rainfall is related to the weakened water vapor convergence on 16 July.