Convective Thermal Resistance Research Articles

Thermal Performance of a Vapor Chamber-Based Plate of High-Power Light-Emitting Diodes Filled with Al<SUB>2</SUB>O<SUB>3</SUB> Nanofluid

Nanofluid that contains Al2O3 nanoparticles evenly dispersed in deionized water is considered to have superior heat transfer characteristics due to the significant increase in both collision frequency and contact surface area between the nanoparticles and the deionized water. The thermal performance of the vapor chamber-based plate of a high power light-emitting diode (LED) filled with Al2O3 nanofluid was experimentally investigated. The thermal characteristics of the vapor chamber were also compared with those of copper- and aluminum-based plates exposed to the heating source of four single-crystal LEDs. The experimental results show that the effective thermal conductivity of the vapor chamber increased with an increase in heat flux. A higher heating power caused an increase in the temperature variation in the vapor chamber, the illumination of the vapor chamber-, copper-, and aluminum-based plates, and the material resistance of the copper- and aluminum-based plates. In contrast, the spreading, convective, and total thermal resistance all decreased with an increase in heating power for all three base plates. The total thermal resistance of the vapor chamber is mostly due to its spreading thermal resistance, and appeared to be lower than that of the copper- and aluminum-based plates under a heating power of over 5 Watts.

Analytical Solutions of Heat Transfer and Film Thickness in Thin-Film Evaporation

A mathematical model predicting heat transfer and film thickness in thin-film region is developed herein. Utilizing dimensionless analysis, analytical solutions have been obtained for heat flux distribution, total heat transfer rate per unit length, location of the maximum heat flux and ratio of conduction thermal resistance to convection thermal resistance in the evaporating film region. These analytical solutions show that the maximum dimensionless heat flux is constant which is independent of the superheat. Maximum total heat transfer rate is determined for a given film region. The ratio of conduction thermal resistance to convection thermal resistance is a function of dimensionless film thickness. This work will lead to a better understanding of heat transfer and fluid flow occurring in the evaporating film region.

A Study on the Convective Heat Transfer Coefficient and Thermal Resistance of Clothing under Cross Ventilation

The convective heat transfer coefficient around the human body is important for evaluating the effects that airflows exert on thermal comfort. However measuring it is generally difficult. Therefore, in this research, a thermal manikin was set up inside a wind tunnel simulating a ventilation environment. This was used to measure the convective heat transfer coefficient around the human body and the relationship between the effects of airflows and the thermal resistance of clothing under various clothing conditions. The results showed that the convective heat transfer coefficient near the thermal manikin was somewhat lower than the value obtained with the calculation formula used in the PMV calculation. It was also confirmed that the thermal resistance of clothing decreases due to the effects of airflows in the case of clothing with high permeability, as has been described in previous research.

Multi‐objective optimization design of a micro‐channel heat sink using adaptive genetic algorithm

PurposeThe purpose of this paper is to show how, with a view to the shortcomings of traditional optimization methods, a multi‐objective optimization concerning the structure sizes of micro‐channel heat sink is performed by adaptive genetic algorithm. The optimized micro‐channel heat sink is simulated by computational fluid dynamics (CFD) method, and the total thermal resistance is calculated to compare with that of thermal resistance network model.Design/methodology/approachTaking the thermal resistance and the pressure drop as goal functions, a multi‐objective optimization model was proposed for the micro‐channel cooling heat sink based on the thermal resistance network model. The coupled solution of the flow and heat transfer is considered in the optimization process, and the aim of the procedure is to find the geometry most favorable to simultaneously maximize heat transfer while obtaining a minimum pressure drop. The optimized micro‐channel heat sink was numerically simulated by CFD software.FindingsThe results of optimization show that the base convection thermal resistance contributes to maximum the total thermal resistance, and base conduction thermal resistance contributes to least. The width of optimized micro‐channel and fin are 197 and 50 μm, respectively, and the corresponding total thermal resistance of the whole micro‐channel heat sink is 0.838 K/W, which agrees well with the analysis result of thermal resistance network model.Research limitations/implicationsThe convection heat transfer coefficient is calculated approximately here for convenience, and that may induce some errors.Originality/valueThe maximum difference in temperature of the optimized micro‐channel cooling heat sink is 84.706 K, which may satisfy the requirement for removal of high heat flux in new‐generation chips. The numerical simulation results are also presented, and the results of numerical simulation show that the optimized micro‐channel heat sink can enhance thermal transfer performance.

An improved porous medium model for microchannel heat sinks

In this paper, a new porous medium model for microchannel heat sinks is presented. The substrate is taken into account in the temperature equation. Thus, an approximate boundary condition at the position of the bottom of microchannel is avoided. For hydrodynamically and thermal fully developed flow in the microchannel, an analytical solution is derived. The normalized temperatures from the present analytical solution are compared with those from three-dimensional numerical simulation and the previous porous medium model. The present model and the numerical simulation correctly reproduce the temperature distribution while the previous porous medium model cannot describe the temperature distribution in the substrate and in the region near the channel bottom. The convective thermal resistance calculated from the present model shows good agreement with that from numerical simulation without any additional modification since the substrate has been taken into account. With the increase of the channel height, the discrepancy between the numerical simulation and the porous medium model increases. Much attention should be given to the cases with high aspect ratio.

Comparison of Micro-Pin-Fin and Microchannel Heat Sinks Considering Thermal-Hydraulic Performance and Manufacturability

This paper explores the potential of micro-pin-fin heat sinks as an effective alternative to microchannel heat sinks for dissipating high heat fluxes from small areas. The overall goal is to compare microchannel and micro-pin-fin heat sinks based on three metrics: thermal performance, hydraulic performance, and cost of manufacturing. The channels and pins of the microchannel and micro-pin-fin heat sinks, respectively, have a width of 200 ?m and a height of 670 ?m. A comparison of the thermal-hydraulic performance shows that the micro-pin-fin heat sink has a lower convection thermal resistance at liquid flow rates above approximately 60 g/min, though this is accompanied by a higher pressure drop. Methods that could feasibly fabricate the two heat sinks are reviewed, with references outlining current capabilities and limitations. A case study on micro-end-milling of the heat sinks is included. This paper includes equations that separate the fabrication cost into the independent variables that contribute to material cost, machining cost, and machining time. It is concluded that, with micro-end-milling, the machining time is the primary factor in determining cost, and, due to the additional machining time required, the micro-pin-fin heat sinks are roughly three times as expensive to make. It is also noted that improvements in the fabrication process, including spindle speed and tool coatings, will decrease the difference in cost.

J0601-2-2 熱回路網法による日射を受ける車内環境モデル化手法の検討([J0601-2]電子情報機器,電子デバイスの強度・信頼性評価と熱制御(2))

This paper describes the appropriate modeling of the temperature environment in the car under the insolation by using thermal network method. Considering new cooling technology for the car, heat storage material is applied to the cooler in the car. We measured the temperature inside the car under the insolation experimentally. Then we created the thermal network model corresponding to the experimental condition and compared the results from thermal network analysis with experimental results. The calculation results indicate as follows. In the case that enough nodal points in the heat storage material are modeled, transient temperature change shows good agreement with measured temperature on the surface of heat storage material. Further, in the case that natural convective thermal resistances, which are derived from natural convection on two-dimensional plate, are applied to the thermal resistance between the nodal points of air in the car, transient temperature change in the car can be expressed correctly by using thermal network model.

Analytical and numerical models for estimates of power loads from calorimetric measurements

The total power loads applied to inertially cooled components are commonly calculated starting from calorimetric measurements as cooling fluid temperature increase and flow rate during and after the power load application. In this paper, some generalizations to the standard models of measure interpretation are introduced. A method is presented to take into account the effect of thermal conduction and radiation between the component and the surrounding material. It consists in evaluating the conduction and convection thermal resistances and the radiation heat flux by Computational Fluid Dynamics (CFD) numerical models. An analytical model permits to use these evaluations to obtain, from the measured convection flux, the corresponding fluxes by conduction and radiation. Sometimes the durations of pulses and duty cycles are such that the calorimetric measurements after one pulse are actually influenced by the previously applied pulse. A correction factor is introduced to cancel this systematic error. The cooling down phase can be long compared to the duty cycles between the pulses, and consequently the signals cannot be completely recorded till the thermal equilibrium. In order to evaluate the total heat flux on a component also in this case, a model is presented which considers a proper extrapolation curve to estimate the integral of the energy exhausted by the cooling systems. As an example, the models introduced in the paper have been used to interpret experimental measurements carried out on the RADI experiment at IPP Garching and some results are presented in the paper.

Improved Cooling in the End Region of a Strip-Wound Totally Enclosed Fan-Cooled Induction Electric Machine

Computational fluid-dynamics modeling has been used to investigate the cooling of the end region of a two-pole strip-wound totally enclosed fan-cooled induction motor. The modeling is validated by experimental measurements. The changes in airflow and heat transfer that each configuration gives are discussed, and recommendations are made of features that can be used to achieve lower convective thermal resistance in the end region.

Three-dimensional laminar flow and heat transfer in a parallel array of microchannels etched on a substrate

Heat transfer and laminar fluid flow in an array of parallel microchannels etched on a silicon substrate with water as the circulating fluid was studied numerically. The fluid region consisted of a microchannel with a hydraulic diameter of 85.6 μm and aspect ratios ranging from 0.10 to 1.0. A constant heat flux of 90 W/cm 2 was applied to the y = H face of the computational domain, which simulates thermal energy generation from an integrated circuit. Generalized transport equations were discretized and solved in three dimensions for velocities, pressure, and temperature. The SIMPLE algorithm [S.V. Patankar, Numerical Heat Transfer and Fluid Flow, Hemisphere, New York, 1980] was used to link pressure and velocity fields, and a thermally repeated boundary condition was applied in the lateral direction to model the repeating nature of the geometry. The numerical results for apparent friction coefficient and convective thermal resistance at the channel inlet and exit closely matched the experimental data in the literature for the case of 0.32 aspect ratio. Apparent friction coefficients were found to increase linearly with Reynolds number. Inlet and outlet thermal resistance values monotonically decreased with increasing Reynolds number and increased with aspect ratio.

Performance analysis of nanofluid-cooled microchannel heat sinks

Microchannel heat sink (MCHS) performance using copper–water (Cu–H 2O) and carbon nanotube–water (CNT–H 2O) nanofluids as coolants is addressed analytically in this study. The velocity and temperature distributions in the MCHS were obtained by modeling the MCHS as a porous media. The resulting velocity and temperature were then used to evaluate the thermal resistance that characterizes MCHS performance. It was found that the nanofluid reduced the temperature difference between the MCHS bottom wall and bulk nanofluid compared with that from pure fluid. This temperature difference produces a reduction in conductive thermal resistance, which is one of the two sources contributing the total thermal resistance of the MCHS. The other source of thermal resistance, termed as convective thermal resistance, was found to increase when nanofluid is employed as the coolant due to the increase in viscosity and decrease in thermal capacity. Under the condition of a given pressure drop across the MCHS, optimum values of aspect ratio and porosity that producing the minimum thermal resistance can be found. It was found that using nanofluid can enhance the MCHS performance when the porosity and aspect ratio are less than the optimum porosity and aspect ratio. When the porosity and channel aspect ratio are higher than optimum porosity and aspect ratio, the nanofluid did not produce a significant change in MCHS thermal resistance.

Lumped thermal model for switched reluctance motor applied to mechanical design optimization

Switched reluctance motors (SRM) are attracting much attention because of their special advantages. Generated heat, due to losses, can reduce the life time of SRMs. Therefore, taking into account thermal modeling helps to improve their performance and increase their life time. In this paper, a lumped thermal model of SRM based on the analogue circuit of conductive and convective thermal resistances is proposed. First the heat transfer equations were applied for modeling of each motor part as a thermal equivalent circuit. Then, the thermal modeling of whole SRM was done via assembling all these sub-circuits. For validity of the obtained model on a wide range of geometrical and structural properties of motor, apt heat transfer relations have been used. This comprehensive model is used to investigate the influence of dimensions on heat transfer in SRM. Finally, an optimal design of the stator yoke dimensions has been reached concerning the mechanical aspects without degrading electromagnetic characteristics of SRM.

Analysis of Convective Thermal Resistance in Ducted Fan-Heat Sinks

The thermal performance of plate fin, round pin-fin, and offset strip-fin heat sinks with a duct-flow type fan arrangement was analytically evaluated. Heat sinks of 65mmtimes60 mm plan areatimes50 mm height with a 4300-RPM dc fan (60mmtimes15mm) were chosen for the performance comparison. A constant temperature, 6-mm thick heat sink base plate is assumed so that thermal spreading resistance is not involved. The operating point on the fan curve is based on the flow pressure drop impedance curve through a heat sink using the friction factor correlation for the chosen heat sink. The loss coefficients at both the entrance and the exit of the heat sink are included in the flow impedance curve. The operating point is defined by the balance point of the flow impedance curve and the fan performance curve. After determining the operating air velocity, the convective thermal resistance of heat sinks is evaluated from the Nusselt number correlation for the chosen heat sink. Results obtained show that optimized round pin-fin heat sinks provide 32.8%-46.4% higher convective thermal resistance compared to an optimized plate-fin heat sink. The optimized offset strip-fin heat sink shows a slightly lower convective thermal resistance than the plate-fin heat sink. As the offset strip length decreases, however, thermal performance seriously deteriorates

Optimum design parameters of Pin-Fin heat sink using the grey-fuzzy logic based on the orthogonal arrays

The effects of design parameters and the optimum design parameters for a Pin-Fin heat sink (PFHS) with the multiple thermal performance characteristics have been investigated by using the grey-fuzzy logic based on the orthogonal arrays. Various design parameters, such as height and diameter of pin-fin and width of pitch between fins are explored by experiment. The average convective heat transfer coefficient, thermal resistance and pressure drop are considered as the multiple thermal performance characteristics. Through the grey-fuzzy logic analysis, the optimization of complicated multiple performance characteristics can be converted into the optimization of a single grey-fuzzy reasoning grade. In addition, the analysis of variance (ANOVA) is applied to find the effect of each design parameter on the each or all thermal performance characteristics. Then the results of confirmation test with the optimal level constitution of design parameters have obviously shown that this logic approach can effective in optimizing the PFHS with the multiple thermal performance characteristics.

Forced convective heat transfer across a pin fin micro heat sink

This paper investigates heat transfer and pressure drop phenomena over a bank of micro pin fins. A simplified expression for the total thermal resistance has been derived, discussed and experimentally validated. Geometrical and thermo-hydraulic parameters affecting the total thermal resistance have been discussed. It has been found that very low thermal resistances are achievable using a pin fin heat sink. The thermal resistance values are comparable with the data obtained in microchannel convective flows. In many cases, the increase in the flow temperature results in a convection thermal resistance, which is considerably smaller than the total thermal resistance.

Heat transfer coefficients measurement in industrial freezing equipment by using heat flux sensors

Heat transfer coefficients, either for convection or conduction mechanisms were determined by using heat flux sensors coupled to temperature measurement devices. Three distinct pieces of equipment were assessed with respect to their heat transfer capabilities. The dynamic variability of heat transfer coefficients was determined along the processing length of a conduction–convection SuperContact ® tunnel and a fluidized bed freezer. The main sources of heat transfer inefficiency were uneven air speed profiles for convection and high thermal contact resistance for conduction mechanisms. Fluctuation of coolant temperature was determined to be the limiting factor in a plate freezer. Simulations either with Tylose ® gel (SuperContact ® tunnel) or with mashed carrot (plate freezer) were carried out to predict improvement in heat transfer coefficients.

Heat transfer from protein crystals: implications for flash-cooling and X-ray beam heating.

Three problems involving heat transfer from a protein crystal to a cooling agent are analyzed: flash-cooling in a cold nitrogen- or helium-gas stream, plunge-cooling into liquid nitrogen, propane or ethane and crystal heating in a cold gas stream owing to X-ray absorption. Heat transfer occurs by conduction inside the crystal and by convection from the crystal's outer surface to the cooling fluid. For flash-cooling in cold gas streams, heat transfer is limited by the rate of external convection; internal temperature gradients and crystal strains during cooling are very small. Helium gas provides only a threefold improvement in cooling rates relative to nitrogen because its much larger thermal conductivity is offset by its larger kinematic viscosity. Characteristic cooling times vary with crystal size L as L(3/2) and theoretical estimates of these times are consistent with experiments. Plunge-cooling into liquid cryogens, which can give much smaller convective thermal resistances provided that surface boiling is eliminated, can increase cooling rates by more than an order of magnitude. However, the internal conduction resistance is no longer negligible, producing much larger internal temperature gradients and strains that may damage larger crystals. Based on this analysis, factors affecting the success of flash-cooling experiments can be ordered from most to least important as follows: (1) crystal solvent content and solvent composition, (2) crystal size and shape, (3) amount of residual liquid around the crystal, (4) cooling method (liquid plunge versus gas stream), (5) choice of gas/liquid and (6) relative speed between cooling fluid and crystal. Crystal heating by X-ray absorption on present high-flux beamlines should be small. For a fixed flux and illuminated area, heating can be reduced by using crystals with areas normal to the beam that are much larger than the beam area.

Characterization of heat and mass transfer during deep-fat frying and its effect on cassava chip quality

Heat and mass transfer in thin slices of fresh cassava ( Manihot esculenta Crantz) were studied. The frying process was described as two stages involving inner vaporization of mainly high available water, which can be transported inside the material, at temperature around the boiling point and inner vaporization of “bound” water at higher temperature depending on water desorption equilibrium. During the first stage, drying rate was controlled by the serial association of both diffusive and convective thermal resistances. During the second stage, temperature gradients decreased and the core temperature followed the corresponding boiling curve (water desorption curve against boiling temperature). Oil uptake depended on the thermal history of the product, characterized by its final water content. Apparent elasticity modulus and color changes were similar for a same final water content. Boiling curves and core temperature measurement are therefore proposed as a means of predicting the development of chip quality during frying.

Thermal transmittance of multiple glazing: computational fluid dynamics prediction

Computational fluid dynamics (CFD) is applied for predicting the convective heat transfer coefficient, thermal resistance and thermal transmittance for a double glazing unit. The predicted thermal resistance of glazing is compared with reference data and good agreement is achieved. The convective heat transfer coefficient and thermal transmittance vary with the air space width and the temperature difference across glazing. The CFD technique can be used to gain insight into multiple glazing performance and also optimise the design and operation of novel multiple glazing systems such as air flow windows or double skin facades in terms of energy efficiency and thermal comfort.

Equilibrium temperature in a clump of bacteria heated in fluid

A theoretical model was developed and used to estimate quantitatively the "worst case", i.e., the longest, time to reach equilibrium temperature in the center of a clump of bacteria heated in fluid. For clumps with 10 to 10(6) cells heated in vapor, such as dry and moist air, and liquid fluids such as purees and juices, predictions show that temperature equilibrium will occur with sterilization temperatures up to 130 degrees C in under 0.02 s. Model development highlighted that the controlling influence on time for heating up the clump is the surface convection thermal resistance and that the internal conduction resistance of the clump mass is negligible by comparison. The time for a clump to reach equilibrium sterilization temperature was therefore decreased with relative turbulence (velocity) of the heating fluid, such as occurs in many process operations. These results confirm widely held suppositions that the heat-up time of bacteria in vapor or liquid is not significant with usual sterilization times.