Showing papers in "Journal of Heat Transfer-transactions of The Asme in 2003"

Investigation on Convective Heat Transfer and Flow Features of Nanofluids

Abstract: Low thermal conductivity is a primary limitation in the development of energy-efficient heat transfer fluids that are required in many industrial applications. In this paper we propose that an innovative new class of heat transfer fluids can be engineered by suspending metallic nanoparticles in conventional heat transfer fluids. The resulting {open_quotes}nanofluids{close_quotes} are expected to exhibit high thermal conductivities compared to those of currently used heat transfer fluids, and they represent the best hope for enhancement of heat transfer. The results of a theoretical study of the thermal conductivity of nanofluids with copper nanophase materials are presented, the potential benefits of the fluids are estimated, and it is shown that one of the benefits of nanofluids will be dramatic reductions in heat exchanger pumping power.

Temperature dependence of thermal conductivity enhancement for nanofluids

Abstract: Usual heat transfer fluids with suspended ultra fine particles of nanometer size are named as nanofluids, which have opened a new dimension in heat transfer processes. The recent investigations confirm the potential of nanofluids in enhancing heat transfer required for present age technology. The present investigation goes detailed into investigating the increase of thermal conductivity with temperature for nano fluids with water as base fluid and particles of Al 2 O 3 or CuO as suspension material. A temperature oscillation technique is utilized for the measurement of thermal diffusivity and thermal conductivity is calculated from it

Measuring Thermal and Thermoelectric Properties of One-Dimensional Nanostructures Using a Microfabricated Device

Abstract: We have batch-fabricated a microdevice consisting of two adjacent symmetric silicon nitride membranes suspended by long silicon nitride beams for measuring thermophysical properties of one-dimensional manostructures (nanotubes, nanowires, and mmobelts) bridging the two membranes. A platinum resistance heater/thermometer is fabricated on each membrane. One membrane can be Joule heated to cause heat conduction through the sample to the other membrane. Thermal conductance, electrical conductance, and Seebeck coefficient can be measured using this microdevice in the temperature range of 4-400 K of an evacuated Helium cryostat. Measurement sensitivity, errors, and uncertainty are discussed. Measurement results of a 148 nm and a 10 nm-diameter single wall carbon nanotube bundle are presented.

Laminar Natural Convection Heat Transfer in a Differentially Heated Square Cavity Due to a Thin Fin on the Hot Wall

Abstract: A finite-volume-based computational study of steady laminar natural convection (using Boussinesq approximation) within a differentially heated square cavity due to the presence of a single thin fin is presented. Attachment of highly conductive thin fins with lengths equal to 20, 35 and 50 percent of the side, positioned at 7 locations on the hot left wall were examined for Ra=10 4 , 10 5 , 10 6 , and 10 7 and Pr=0.707 (total of 84 cases). Placing a fin on the hot left wall generally alters the clockwise rotating vortex that is established due to buoyancy-induced convection. Two competing mechanisms that are responsible for flow and thermal modifications are identified. One is due to the blockage effect of the fin, whereas the other is due to extra heating of the fluid that is accommodated by the fin. The degree of flow modification due to blockage is enhanced by increasing the length of the fin. Under certain conditions, smaller vortices are formed between the fin and the top insulated wall

Evaporation Heat Transfer in Sintered Porous Media

Abstract: A two-dimensional model is presented to predict the overall heat transfer capability for a sintered wick structure. The model considers the absence of bulk fluid at the top surface of the wick, heat conduction resistance through the wick, capillary limitation, and the onset of nucleate boiling. The numerical results show that thin film evaporation occurring only at the top surface of a wick plays an important role in the enhancement of evaporating heat transfer and depends on the thin film evaporation, the particle size, the porosity, and the wick structure thickness. By decreasing the average particle radius, the evaporation heat transfer coefficient can be enhanced. Additionally, there exists an optimum characteristic thickness for maximum heat removal. The maximum superheat allowable for thin film evaporation at the top surface of a wick is presented to be a function of the particle radius, wick porosity, wick structure thickness, and effective thermal conductivity. In order to verify the theoretical analysis, an experimental system was established, and a comparison with the theoretical prediction conducted. Results of the investigation will assist in optimizing the heat transfer performance of sintered porous media in heat pipes and better understanding of thin film evaporation.

Thermal Resistance of Particle Laden Polymeric Thermal Interface Materials

Abstract: Particle laden polymers are one of the most prominent thermal interface materials (TIM) used in electronics cooling. Most of the research has primarily dealt with the understanding of the thermal conductivity of these types of TIMs. For thermal design, reduction of the thermal resistance is the end goal. Thermal resistance is not only dependent on the thermal conductivity, but also on the bond line thickness (BLT) of these TIMs. It is not clear which material property(s) of these particle laden TIMs affects the BLT and eventually the thermal resistance. This paper introduces a rheology based semiempirical model for the prediction of the BLT of these TIMs. BLT depends on the yield stress of the particle laden polymer and the applied pressure. The BLT model combined with the thermal conductivity model can he used for modeling the thermal resistance of these TIMs for factors such as particle volume faction, particle shape, base polymer viscosity, etc.

Backward Monte Carlo Simulations in Radiative Heat Transfer

Abstract: Standard Monte Carlo methods trace photon bundles in a forward direction, and may become extremely inefficient when radiation onto a small spot and/or onto a small direction cone is desired. Backward tracing of photon bundles is known to alleviate this problem if the source of radiation is large, but may also fail if the radiation source is collimated and/or very small. Various implementations of the backward Monte Carlo method are discussed, allowing efficient Monte Carlo simulations for problems with arbitrary radiation sources, including small collimated beams, point sources, etc., in media of arbitrary optical thickness

Importance of turbulence-radiation interactions in turbulent diffusion jet flames

Abstract: Traditional modeling of radiative transfer in reacting flows has ignored turbulence-radiation interactions (TRI). Radiative fluxes, flux divergences and radiative properties have been based on mean temperature and concentration fields. However, both experimental and theoretical work have suggested that mean radiative quantities may differ significantly from those predictions based on the mean parameters because of their strongly nonlinear dependence on the temperature and concentration fields. The composition PDF method is able to consider many nonlinear interactions rigorously, and the method is used here to study turbulence-radiation interactions. This paper tries to answer two basic questions: (1) whether turbulence-radiation interactions are important in turbulent flames or not; and (2) if they are important, then what correlations need to be considered in the simulation to capture them. After conducting many flame simulations, it was observed that, on average, TRI effects account for about 1/3 of the total drop in flame peak temperature caused by radiative heat losses. In addition, this study shows that consideration of the temperature self correlation alone is not sufficient to capture TRI, but that the complete absorption coefficient-Planck function correlation must be considered.

Nusselt Number Behavior on Deep Dimpled Surfaces Within a Channel

Abstract: Experimental results, measured on a dimpled teat surface placed on one wall of a channel, are given for a ratio of air inlet stagnation temperature to surface temperature of approximately 0.94, and Reynolds numbers R eH from 12,000 to 70,000. These data include friction factors, local Nusselt numbers, spatially-resolved local Nusselt numbers, and globally-averaged Nusselt numbers. The ratio of dimple depth to dimple print diameter δ/D is 0.3, and the ratio of channel height to dimple print diameter is 1.00. These results are compared to measurements from other investigations with different ratios of dimple depth to dimple print diameter δ/D to provide information on the influences of dimple depth. At all Reynolds numbers considered, local and spatially-resolved Nusselt number augmentations increase as dimple depth increases (and all other experimental and geometric parameters are held approximately constant). These are attributed to: (i) increases in the strengths and intensity of vortices and associated secondary flows ejected from the dimples, as well as (ii) increases in the magnitudes of three-dimensional turbulence production and turbulence transport

Effect of Pressure, Subcooling, and Dissolved Gas on Pool Boiling Heat Transfer From Microporous Surfaces in FC-72

Abstract: The present research is an experimental study of the effects of pressure, subcooling, and non-condensable gas (air) on the pool nucleate boiling heat transfer performance of a microporous enhanced and a plain (machine-roughened) reference surface. The test surfaces, 1-cm 2 flat copper blocks in the horizontal, upward facing orientation, were immersed in FC-72. The test conditions included an absolute pressure range of 30-150 kPa, a liquid subcooling range of 0 (saturation) to 50 K, and both gas-saturated and pure subcooling conditions. Effects of these parameters on nucleate boiling and critical heat flux (CHF) were investigated

Experimental Study on Frost Structure on Surfaces With Different Hydrophilicity: Density and Thermal Conductivity

Abstract: An experimental study has been conducted to investigate the effects of surface energy on frost formation. Test samples with three different surfaces of which dynamic contact angles (DCA) were 23, 55, and 88 deg were installed in a wind tunnel and exposed to a humid airflow. The airflow Reynolds number, humidity, the air and the cold plate temperatures were maintained at 9000, 0.0042 kg/kg', +12 and -22°C, respectively. The thickness and the mass of frost layer were measured and used to calculate frost density while heat flux and temperature profile were measured to obtain thermal conductivity. Exact positions of thermocouple junctions were verified by means of visualization system in order to increase accuracy. Results showed that frost density and thermal conductivity increase with time. The surface with a lower DCA showed a higher frost density and thermal conductivity during a two-hour test, but minor differences have been found after two hours of frost generation. Empirical correlations for thickness, mass deposition, density and thermal conductivity were proposed as the functions of test time and surface energy.

Effect of Tip Clearance on the Thermal and Hydrodynamic Performance of a Shrouded Pin Fin Array

Abstract: The effect of introducing tip clearance to a liquid cooled array of shrouded pins fins is examined Three arrays of height to diameter ratio ranging from 05 to 11 were evaluated experimentally The arrays were exposed to a uniform heal flux of 002 to 026 W/mm 2 and cooled with water through a nominal Reynolds number range of 200 to 10000 Tip clearance of 0 to 25% of pin height was assessed Mean heat transfer rates and adiribatic pressure drop across the array were determined and empirical correlations are proposed The introduction of clearance was seen to increase overall heat transfer in some cases

Theoretical Model for Nucleate Boiling Heat and Mass Transfer of Binary Mixtures

Abstract: A model is presented to calculate nucleate boiling heat transfer coefficients of binary mixtures. The model includes the governing physical phenomena, such as the variation of the phase interface curvature, the adhesion pressure between wall and liquid, the interfacial thermal resistance as well as the local variation of composition and liquid-vapor equilibrium. Marangoni convection is considered, too. The theoretical background of these phenomena is described and their implementation is explained. The model is verified by comparing calculated heat transfer coefficients of hydrocarbon mixtures with experimental data

Fully-Developed Thermal Transport in Combined Pressure and Electro-Osmotically Driven Flow in Microchannels

Abstract: Thermally fully-developed heat transfer has been analyzed for combined electro-osmotic and pressure driven flow in a circular microtube. The two classical thermal boundary conditions of constant wall heat flux and constant wall temperature were considered. Such a flow is established by the combination of an imposed pressure gradient and voltage potential gradient along the length of the tube. The induced flow rate and velocity profile are functions of the imposed potential gradient, electro-osmotic mobility of the fluid, the ratio of the duct radius to the Debye length, the established streamwise pressure gradient, and the fluid viscosity. The imposed voltage gradient neuritis in Joule heating in the fluid, with an associated distributed volumetric source of energy For this scenario, the solution for the fully developed, dimensionless temperature profile and corresponding Nusselt number have been determined

Evaporation Heat Transfer and Pressure Drop of Refrigerant R-410A Flow in a Vertical Plate Heat Exchanger

Abstract: Experiments are carried out hero to measure the evaporation heat transfer coefficient h r and associated frictional pressure drop ΔP f in a vertical plate heat exchanger for refrigerant R-410A. The heat exchanger consists of two vertical counterflow channels which are formed by three plates whose surface corrugations have a sine shape and a chevron angle of 60 deg. Upflow boiling of refrigerant R-410A receives heat from the hot downflow of water. In the experiments, the mean vapor quality in the refrigerant channel is varied from 0.10 to 0.80, the mass flux from 50 to 100 kg/m 2 s, and the imposed heat flux from 10 to 20 kW/m 2 for the system pressure fixed at 1.08 and 1.25 MPa. The measured data incinerate that both h r and AP, increase with the refrigerant mass flux except at low vapor quality. In addition, raising the imposed heat flux is found to significantly improve h r for the entire range of the mean vapor quality. However, the corresponding friction factor f is insensitive to the imposed heat flux and refrigerant pressure. Based on the present data, empirical correlations are provided for h r and f tp , for R-410A in the plate heat exchanger.

Heat transfer coefficients on the squealer tip and near squealer tip regions of a gas turbine blade

Abstract: Detailed heat transfer coefficient distributions on a squealer tip of a gas turbine blade were measured using a hue detection based transient liquid crystals technique. The heat transfer coefficients on the shroud and near tip regions of the pressure and suction sides of a blade were also measured. Tests were performed on a five-bladed linear cascade with a blow-down facility. The blade was a two-dimensional model of a first stage gas turbine rotor blade with a profile of a GE-E 3 aircraft gas turbine engine rotor blade. The Reynolds number based on the cascade exit velocity and axial chord length of a blade was 1.1 ×10 6 and the total turning angle of the blade was 97.7 deg. The overall pressure ratio was 1.2 and the inlet and exit Mach number were 0.25 and 0.59, respectively. The turbulence intensity level at the cascade inlet was 9. 7 percent. The heat transfer measurements were taken at the three different tip gap clearances of 1.0 percent, 1.5 percent, and 2.5 percent of blade span

Effects of Varying Geometrical Parameters on Boiling From Microfabricated Enhanced Structures

Abstract: The current study involves two-phase cooling from enhanced structures whose dimensions have been changed systematically using microfabrication techniques. The aim is to optimise the dimensions to maximize the heat transfer. The entranced structure used in this study consists of a stacked network of interconnecting channels making it highly porous. The effect of varying the pore size, pitch and height on the boiling performance was studied, with fluorocarbon FC-72 as the working fluid. While most of the previous studies on the mechanism of enhanced nucleate boiling have focused on a small range of wall superheats (0-4 K), the present study covers a wider range (as high as 30 K) A larger pore and smaller pitch resulted in higher heat dissipation at all heat fluxes. The effect of stacking multiple layers showed a proportional increase in heat dissipation (with additional layers) in a certain range of wall superheat values only. In the wall superheat range 8-13 K, no appreciable difference was observed between a single layer structure and a three layer structure. A fin effect combined with change in the boiling phenomenon within the sub-surface layers is proposed to explain this effect.

Thermal Processing of Materials: From Basic Research to Engineering

Abstract: We review the active and growing field of thermal processing of materials, with a particular emphasis on the linking of basic research with engineering aspects. In order to meet the challenges posed by new applications arising in electronics, telecommunications, aerospace, transportation, and other areas, extensive work has been done on the development of new materials and processing techniques in recent years. Among the materials that have seen intense interest and research activity over the last two decades are semiconductor and optical materials, composites, ceramics, biomarerials, advanced polymers, and specialized alloys. New processing techniques have been developed to improve product quality, reduce cost, and control material properties. However, it is necessary to couple research efforts directed at the fundamental mechanisms that govern materials processing with engineering issues that arise in the process, such as system design and optimisation, process feasibility, and selection of operating conditions to achieve desired product characteristics

Scalable Multi-Group Full-Spectrum Correlated-k Distributions for Radiative Transfer Calculations

Abstract: A new full-spectrum k-distribution method has been developed, in which spectral locations are sorted into M spectral groups, according to their absorption coefficient dependence on (partial) pressure and temperature. Calculating correlated-k full-spectrum k-distributions for each of the M groups, LBL accuracy can be obtained with M≤32. Database values have been assembled for CO 2 mixtures at atmospheric pressure. The method is fully scalable, i.e., spectral groups from the database can be combined to obtain coarser group models (M=1,2,4,...) for greater numerical efficiency (accompanied by slight loss in accuracy)

Direct Numerical Simulation of turbulent heat transfer across a mobile, sheared gas-liquid interface

Abstract: The impact of interfacial dynamics or turbulent heat transfer at a deformable, sheared gas-liquid interface is studied using Direct Numerical Simulation (DNS). The flow system comprises a gas and a liquid phase flowing in opposite directions. The governing equations for the two fluids are alternated solved in separate domains and then coupled at the interface by imposing continuity of velocity and stress. The deformations of the interface fall in the range of capillary waves of waveslope ak=0.01 (wave amplitude a times wavenumber k), and very small phase speed-to-friction velocity ratio, c/u*. The influence of low-to-moderate molecular Prandtl numbers (Pr) on the transport in the immediate vicinity of the interface is examined for the gas phase, and results are compared to existing will-bounded flow data. The shear-based Reynolds number Re* is 171 and Prandtl numbers of 1, 5, and 10 were studied. The effects induced by changes in Pr in both wall-bounded flow and over a gas-liquid interface were analyzed by comparing the relevant statistical flow properties, including the budgets for the temperature variance and the turbulent heat fluxes

A Parabolic Temperature Profile Model for Heating of Droplets

Abstract: A model for convective heating of droplets, which takes into account their finite thermal conductivity, is suggested. This model is based on the assumption of the parabolic temperature profile in the droplets. A rigorous numerical solution, without restrictions on temperature profiles inside droplets, is compared with predictions of the parabolic temperature profile and isothermal models

Heat Transfer Coefficients and Film-Cooling Effectiveness on a Gas Turbine Blade Tip

Abstract: The detailed distributions of heat transfer coefficient and film cooling effectiveness on a gas turbine blade tip were measured using a hue detection based transient liquid crystals technique. Tests were performed on a five-bladed linear cascade with blow-down facility. The Reynolds number based on cascade exit velocity and axial chord length was 1.1 × 10 6 and the total turning angle of the blade was 97. 7°. The overall pressure ratio was 1.2 and the inlet and exit Mach numbers were 0.25 and 0.59, respectively. The turbulence intensity level at the cascade inlet was 9. 7%. The blade model was equipped with a single row of film cooling holes at both the tip portion along the camber line and near the tip region of the pressure side. All measurements were made at the three different tip gap clearances of 1.0%, 1.5%, and 2.5% of blade span and the three blowing ratios of 0.5, 1, and 2

The Effect of Dissolving Salts in Water Sprays Used for Quenching a Hot Surface: Part 2—Spray Cooling

Abstract: The effect of adding one of three salts (NaCl, Na 2 SO 4 or MgSO 4 ) to water sprayed on a hot surface was studied experimentally. A copper test surface was heated to 240°C and quenched with a water spray. The variation of surface temperature during cooling was recorded, and the surface heat flux calculated from these measurements. Surface heat flux during cooling with pure water sprays was compared with that obtained using salt solutions. Dissolved NaCI or Na 2 SO 4 increased nucleate boiling heat transfer, but had little effect on transition boiling during spray cooling. MgSO 4 increased both nucleate and transition boiling heat flux. Enhanced nucleate boiling was attributed to foaming in the liquid film generated by the dissolved salts. MgSO 4 produced the largest increase in nucleate boiling heat transfer, Na 2 SO 4 somewhat less and NaCI the least. A concentration of 0.2 mol/l of MgSO 4 produced the greatest heat flux enhancement ; higher salt concentrations did not result in further improvements. During transition boiling particles of MgSO 4 adhered to the heated surface, raising .surface roughness and increasing heat transfer. Addition of MgSO 4 reduced the time required to cool a hot surface from 240°C to 120°C by an order of magnitude.

Reattachment of Three-Dimensional Flow Adjacent to Backward-Facing Step

Abstract: Numerical simulations for incompressible three-dimensional laminar forced convection flow adjacent to backward-facing step in rectangular duct are performed to examine the reattachment region of the separated flow on the stepped wall. The feasibility of utilizing the two-dimensional flow definition and the limiting streamline definition for identifying the reattachment line/region was examined. The downwash and the jet-like flow that develops near the sidewall creates significant spanwise flow adjacent to the stepped wall, making it difficult to identify a reattachment line/region both numerically and experimentally. The use of the line/region that identifies the location on a plane adjacent to the stepped wall where the gradient of the mean streamwise velocity component is zero (∂u/∂y| y=0 =0) is recommended for code and apparatus validation of three-dimensional separated flow

Simulation of Unsteady Small Heat Source Effects in Sub-Micron Heat Conduction

Abstract: In compact transistors, large electric fields near the drain side create hot spots whose dimensions are smaller them the phonon mean free path in the medium. In this paper we present a study of unsteady hot spot behavior. The unsteady gray phonon Boltzmann transport equation (BTE) is solved in the retardation fibre approximation using a finite volume method. Electron-phonon interaction is represented as a heat source term in the phonon BTE. The evolution of the temperature profile is governed by the interaction of four competing time scales: the phonon residence time in the hot spot and in the domain, the duration of the energy source, and the phonon relaxation time. The influence of these time scales on the temperature is investigated. Both boundary scattering and heat source localisation effects are observed to have considerable impact on the thermal predictions. Comparison of BTE solutions with conventional Fourier diffusion analysis reveals significant discrepancies.

Geometric Optimization of Radiant Enclosures Containing Specular Surfaces

Abstract: This paper presents an optimization methodology for designing radiant enclosures containing specularly-reflecting surfaces. The optimization process works by making intelligent perturbations to the enclosure geometry at each design iteration using specialized numerical algorithms. This procedure requires far less time than the forward ‘‘trial-anderror’’ design methodology, and the final solution is near optimal. The radiant enclosure is analyzed using a Monte Carlo technique based on exchange factors, and the design is optimized using the Kiefer-Wolfowitz method. The optimization design methodology is demonstrated by solving two industrially-relevant design problems involving twodimensional enclosures that contain specular surfaces. @DOI: 10.1115/1.1599369#

Observations of Early-Stage Frost Formation on a Cold Plate in Atmospheric Air Flow

Abstract: The present study is conducted to investigate the frost formation on a cold plate in atmospheric air flow by means of experimental and theoretical methods. In order to provide observations for the early stage of the frost growth process, a microscopic image system is used to record the pattern and the thickness of the frost layer per five seconds after the onset of frost formation. In this study, a multiple-step ascending frost growth pattern caused by melting of frost crystals at the frost surface has been observed. Effects of velocity, temperature and relative humidity of air (V, T a , and Φ) are studied, and the surface temperature of the cold plate (T w ) is also varied. The considered ranges of these dominant variables are: 2≤V≤13 m/s, 20≤T a ≤35°C, 40 percent≤Φ≤80 percent, and -13≤T w ≤-2°C. The theoretical model presented by Cheng and Cheng [22] for predicting the frost growth rate during the frost layer growth period is verified. Results show that the predictions of frost growth rate by the model agree with the experiment data, especially for the frost layer growth period.

Radiative Properties of Semitransparent Silicon Wafers With Rough Surfaces

Abstract: This paper presents a Monte Carlo model for prediction of the radiative properties of semitransparent silicon wafers with rough surfaces. This research was motivated by the need of accurate temperature measurement in rapid thermal processing (RTP) systems. The methods developed in this paper, however, can be applied to various semitransparent materials such as diamond films. The numerically obtained bidirectional reflectance distribution function (BRDF) showed a similar trend as the experimental data. Furthermore, a higher angular resolution can be achieved by simulation than by experiments. The bidirectional transmittance distribution function (BTDF) can also be computed in the same run for semitransparent wafers. Other spectral radiative properties (such as the directional-hemispherical transmittance and reflectance, the emittance and the absorptance) under various surface conditions were computed at various temperatures. The results can help gain a deeper understanding of the radiative behavior of semitransparent materials and may be applied to various fields.

Dissipation in Small Scale Gaseous Flows

Abstract: We discuss the effect of shear work at solid boundaries in small scale gaseous flows where slip effects are present. The effect of shear work at the boundary on convective heat transfer is illustrated through solution of the constant-wall-heat-flux problem in the slip-flow regime. We also present predictions for the dissipation in terms of the mean flow velocity in pressure-driven and gravity-driven Poiseuille flows for arbitrary Knudsen numbers

Heat Transfer Enhancement Caused by Sliding Bubbles

Abstract: Measurements that illustrate the enhancement of heat transfer caused by a bubble sliding under an inclined surface are reported. The data were obtained on an electrically heated thin-foil surface that was exposed on its lower side to FC-87 and displayed the output of a liquid crystal coating on the upper (dry) side. A sequence of digital images was obtained from two cameras: one that recorded the response of the liquid crystal and one that recorded images of the bubble as it moved along the heated surface. With this information, the thermal imprint of the bubble was correlated to its motion and position. A bubble generator that produced FC-87 bubbles of repeatable and controllable size was also developed for this study