大功率LED相变热沉的传热性能研究

研制了一种利用相变传热原理进行传热的大功率LED相变热沉.实验结果表明与传统实体热沉相比,相变热沉具有更快的启动性能,随着加热功率的增加,其热阻逐渐减小,并低于传统实体热沉,同时在高加热功率条件下,仍能保持良好的均热性能与轴向导热性能.     

Effect of chip spacing in a multichip module on the heat transfer for paraffin slurry flow

The experiments were conducted by using water and paraffin slurry to investigate the effect of a chip spacing in the multichip module on the cooling characteristics from an in-line 4 × 3 array of discrete heat sources which were flush mounted on the top wall of a channel, The experimental parameters were chip spacing in a multichip module, heat flux of simulated VLSI chip, mass fraction of paraffin slurry, and channel Reynolds number. The removable heat flux at the same chip surface temperature decreased as the chip spacing decreased at the first and fourth rows. The local heat transfer coefficients for the paraffin slurry were larger than those for water, and the chip spacing on the local heat transfer coefficients for paraffin slurry influenced less than that for water. The enhancement factor for paraffin slurry showed the largest value at a mass fraction of 5% regardless of the chip spacing, and the enhancement factors increased as the chip spacing decreased. This means that the paraffin slurry is more effective than water for cooling of the highly integrated multichip module.     

逆流冷凝塔热泵系统热质交换过程数值模拟研究

冷凝塔作为冷凝式热泵系统的重要组成部分,其热质交换过程对于降低能耗及热泵系统设计具有重要意义.以逆流冷凝塔热泵系统形式为基础,通过建立冷凝塔内热质交换过程的数学模型,推导出热质交换过程的通用计算方程组,并以贵阳地区的气象参数为参考,对不同工况下塔内热质交换情况进行模拟计算及具体分析.研究结果表明,不同入口空气参数对溶液...     

不同工质沟槽式微热管传热性能实验研究

为了研究工质对沟槽式微热管传热性能的影响,通过试验对不同充液率下,不同工质对沟槽式微热管的温差、热阻与极限功率等性能的影响进行研究.结果表明:充液率对丙酮热管的温差变化影响较大,在相同情况下,丙酮热管需要更高的充液率.乙醇热管的温差性能较差,但充液率小的乙醇热管其热阻较小.水热管的极限功率最高,且在100%的充液率时,有着最为良好的传热性能,较另两种工质更适合用于高功率场合.该研究对实际生产和应用中的工质选择有着重要的参考意义.     

Combined heat and mass transfer analysis during the dehumidification process within a plate type heat exchanger

Liquid desiccant systems have been paid attention because of its advantages in energy saving and an environmental friendliness. The use of liquid desiccant systems offers design and performance advantages over the solid desiccant systems, especially when solar energy is used for regeneration. The objective of this paper is to analyze the simultaneous heat and mass transfer characteristics of lithium chloride aqueous solution for the plate type dehumidification system. The effects of process air and solution inlet conditions on the dehumidification performance are studied in this study. It is found that the heat transfer coefficient of the air side gives much more significant effect on the absorption rate and dehumidification effectiveness than those of the solution and the coolant sides while the mass transfer coefficient of the solution side gives more significant effect than that of the air side. It is also found that the solution concentration is the most important factor for absorption performance improvement during the dehumidification process.     

Stability analysis of whirling composite shells partially filled with two liquid phases

In this paper, the stability of whirling composite cylindrical shells partially filled with two liquid phases is studied. Using the first-order shear shell theory, the structural dynamics of the shell is modeled and based on the Navier-Stokes equations for ideal liquid, a 2D model is developed for liquid motion at each section of the cylinder. In steady state condition, liquids are supposed to locate according to mass density. In this study, the thick shells are investigated. Using boundary conditions between liquids, the model of coupled fluid-structure system is obtained. This coupled fluid-structure model is employed to determine the critical speed of the system. The effects of the main variables on the stability of the shell are studied and the results are investigated.     

双效吸收式变热器热力性能分析

以Ｈ２Ｏ／ＬｉＢｒ和ＴＦＥ／ＮＭＰ为工质对分析了双效吸收式变热器的热力性能。结果表明双效吸收式变换器可达到两级吸收式变热器同样高的温度提升,但运行范围比两级吸收式变换器窄。以ＴＦＥ／ＮＭＰ为工质对的双效吸收式变热器比以Ｈ２Ｏ／ＬｉＢｒ为工质对的双效吸收式变热器得到的温升高,但系统性能降低。     

Numerical analysis of density wave instability and heat transfer deterioration in a supercritical water reactor

The present study numerically investigates Density wave oscillations (DWOs), a dynamic instability and captures Heat transfer deterioration (HTD) phenomenon for a Supercritical water reactor (SCWR) of the U. S. reference design. A Thermal-hydraulic (TH) model is employed to solve the nonlinearly coupled mass, axial momentum and energy conservation equations in a time domain using characteristics based implicit finite difference scheme accounting for the compressible flow dynamics of supercritical water. The analysis of DWOs is carried out to obtain the marginal stability thresholds. Next, the steady as well as transient safety analyses are carried out to determine the occurrence of HTD, access its severity and identify the safe operating conditions. Finally, the common operating conditions are predicted where the U. S. SCWR will be stable as well as safe.     

热锻过程中变形与热传导的耦合分析

热锻过程中温度对成型过程的影响是显著的,了解热锻过程中工件内温度场的分布对控制锻件的成型是有益的。由于热性能参数随温度变化或存在辐射换热,工模具的热传导是一个非线性瞬态问题,因而利用变形与热传导耦合的有限元法分析了发动机气门的热锻成型过程。     

热锻过程中变形与传热导的耦合分析

热锻过程中温度对成型过程的影响是显著的,了解热锻过程中工件内温度场的分布对控制锻件的成型是有益的。由于热性能参数随温度变化成或存在辐射换热,工模具的热传导是一个非线性瞬态问题,因而利用变形与热传导耦合的有限元法分析了发动机气门的热锻成型过程。     

Flow and convective heat transfer analysis using RANS for a wire-wrapped fuel assembly

This work presents the three-dimensional analysis of flow and heat transfer performed for a wire-wrapped fuel assembly of liquid metal reactor using Reynolds-averaged Navier-Stokes analysis in conjunction with SST model as a turbulence closure. The whole fuel assembly has been analyzed for one period of the wire-spacer using periodic boundary conditions at inlet and outlet of the calculation domain. Three different assemblies, two 7-pin wire-spacer fuel assemblies and one bare rod bundle, apart from the pressure drop calculations for a 19-pin case, have been analyzed. Individual as well as a comparative analysis of the flow field and heat transfer have been discussed. Also, discussed is the position of hot spots observed in the wire-spacer fuel assembly. The flow field in the subchannels of a bare rod bundle and a wire-spacer fuel assembly is found to be different. A directional temperature gradient is found to exist in the subchannels of a wire-spacer fuel assembly. Local Nusselt number in the subchannels of wirespacer fuel assemblies is found to vary according to the wire-wrap position while in case of bare rod bundle, it’s found to be constant.     

Exergo-economic analysis of finned tube for waste heat recovery including phase change heat transfer

In this paper, an exergo-economic criterion, i.e. the net profit per unit transferred heat load, is established from the perspective of exergy recovery to evaluate the performance of finned tube used in waste heat recovery. Also, the dimensionless exergy change number is introduced to investigate the effect of the flow (mechanical) exergy loss rate on the recovered thermal exergy. Selecting R245fa as a working fluid and exhaust flue gas as a heat source, the effects of the internal Reynolds number Re _i, the external Reynolds number Re _o, the unit cost of thermal exergy ? _q, the geometric parameter of finned tube η _o β and the phase change temperature T _v etc. on the performance of finned tube are discussed in detail. The results show that the higher T _v and η _o β, and lower Re _i may lead to the negligible flow (mechanical) exergy loss rate. There exists an optimal value of Re _i where the net profit per unit transferred heat load peaks, while the variations of Re _o, ? _q and T _v cause monotonic change of the net profit per unit transferred heat load. The phase change temperature exerts relatively greater influence on the exergo-economic performance of finned tube in comparison with other parameters. And there exists a critical phase change temperature, where the net profit per unit transferred heat load is equal to zero.     

Optimization of a high pressure turbine blade tip cavity with conjugate heat transfer analysis

The current study aims to understand the aero-thermal performance of a cooled cavity tip in a single stage transonic turbine. The squealer tip of the uncooled turbine blade was reduced to an aerodynamic loss with suppressing leakage flow. However, the aerodynamic loss study of the cooled turbine blade tip is rare. It is necessary to study the tip cavity of the cooled turbine blade. Depth, front blend radius and aft blend radius of the cavity were set as design variables, and 30 cases were chosen using design of experiments. These cases were calculated with conjugate heat transfer method. Approximation model was made using the Kriging method, and tip cavity shape was optimized with multidisciplinary design optimization. Average total pressure loss behind the trailing edge and cooling effectiveness of blade tip surface were set to the objective function. The aerodynamic optimization model decreased 1.6 % of total pressure loss, the heat transfer optimization model increased 1.3 % point of cooling effectiveness and aero-thermal optimization model were found. Volume of tip cavity becomes larger when three design variables are grown. Amount of tip leakage flow and its distribution over the tip region increases and total pressure loss and cooling effectiveness increase. In terms of heat transfer, blade tip without cavity is advantageous. Total pressure loss coefficient, however, also increases over 5 %. To improve both aero-thermal characteristics of cooled blade tip, the design using the multidisciplinary design optimization is recommended.

     

Wet surface heat transfer and pressure drop of aluminum parallel flow heat exchangers at different inclination angles

The effect of inclination angle on the heat transfer and pressure drop characteristics of brazed aluminum heat exchangers was experimentally investigated under wet conditions. Three samples having different fin pitches (1.25, 1.5 and 2.0 mm) were tested. Results show that heat transfer coefficients are not affected by the inclination angle. However, friction factors increase as the inclination angle increases with negligible difference between the forward and backward inclination. The effect of fin pitch on the heat transfer coefficient and on the pressure drop is also discussed. Comparison of the dry and wet surface heat transfer coefficients reveals that dry surface heat transfer coefficients are significantly larger than wet surface heat transfer coefficients. Possible explanation is provided by considering the condensate drainage pattern. The data are also compared with the existing correlation. This paper was recommended for publication in revised form by Associate Editor Man-Yeong Ha Nae-Hyun Kim is a Professor of Mechanical Engineering, University of Incheon. His area of interest spans boiling and condensation, heat transfer enhancement and heat exchanger design. He has been active in heat transfer community, and was a Chairman of Thermal Engineering Division of KSME. He holds several editorial position including Journal of Enhanced Heat Transfer. He is a recipient of Asian Academic Award awarded by SAREK and JSRAE.     

A numerica1 study of fluid flow and heat transfer in different microchannel heat sinks for electronic chip cooling

Four different microchannel heat sinks are designed to study the effects of structures in microchannel heat sinks for electronic chips cooling. Based on the theoretic analysis and numerical computation of flow and heat exchange characteristics, the electronic chip’s temperature and flow rate distributions are obtained. The correspondence between flow pressure drop and chip’s temperature in the four microchannel heat sinks is also studied and analyzed. Numerically analyzed results indicate that the topological structure in microchannel heat sink has a significant influence on electronic chips cooling. This study shows various thermal properties in the four microchannel heat sinks.     

A study on the heat transfer characteristics of a self-oscillating heat pipe

In this paper, the heat transfer characteristics of a self-oscillating heat pipe are experimentally investigated for the effect of various working fluid fill charge ratios and heat loads. The characteristics of temperature oscillations of the working fluid are also analysed based on chaotic dynamics. The heat pipe is composed of a heating section, a cooling section and an adiabatic section, and has a 0.002m internal diameter, a 0.34m length in each turn and consists of 19 turns. The heating and the cooling portion of each turn has a length of 70mm. A series of experiments was carried out to measure the temperature distributions and the pressure variations of the heat pipe. Furthermore, heat transfer performance, effective thermal conductivity, boiling heat transfer and condensation heat transfer coefficients are calculated for various operating conditions. Experimental results show the efficacy of this type of heat pipe.     

Sheathed probe thermal gas mass flow meter heat transfer analysis

The thermal gas mass flow meter is an important meter used in industrial measurement. When the environmental temperature changes, the measured gas physical parameters change correspondingly and the thermal gas mass flow meter output signal is affected, causing large measurement error. The influence of gas temperature on the sheathed probe measurements is analyzed in this paper based on experiments and heat transfer theory using a three dimensional probe and gas heat transfer mathematical model based on the heat conduction equation. The probe heat transfer process is analyzed under convection heat transfer coupling conditions. The experimental data were analyzed and compared against the theoretical results, with a maximum average relative error of only 4.56%. The rationality of the theoretical method is thus verified.     

Two-phase flow heat transfer of R-134a in microtubes

The characteristics of the two-phase flow heat transfer of R-134a in microtubes with inner diameters of 430 μm and 792 μm were experimentally investigated. The effect of the heat flux on the heat transfer coefficient for microtubes was significant before the transition quality. The boiling number expressed the interrelation between the heat flux and the mass about the heat transfer coefficients. The smaller microtube had greater heat transfer coefficients; the average heat transfer coefficient for the tube A (D _i = 430 μm) was 47.0% greater than that for the tube B (D _i = 792 μm) at G = 370 kg/m²·s and q″ = 20 kW·m². A new correlation for the evaporative heat transfer coefficients in microtubes was developed by considering the following factors: the laminar flow heat transfer coefficient of liquid-phase flow, the enhancement factor of the convective heat transfer, and the nucleate boiling correction factor. The correlation developed in present study predicted the experimental heat transfer coefficients within an absolute average deviation of 8.4%.     

Entropy analysis of flow and heat transfer caused by a moving plate with thermal radiation

This study examines the effects of thermal radiation on entropy generation in flow and heat transfer caused by a moving plate. The equations that govern the flow and heat transfer phenomenon are solved numerically. Velocity and temperature profiles are obtained for the parameters involved in the problem. The expressions for the entropy generation number and the Bejan number are obtained based on the profiles. Graphs for velocity, temperature, the entropy generation number, and the Bejan number are plotted and discussed qualitatively.     

Three-dimensional heat transfer analysis of LENSTM process using finite element method

Laser-engineered net shaping, referred to as LENS^TM process, is an additive manufacturing technique for building metallic parts, layer by layer, by direct deposition of metal powders in a melt pool created by a focused laser beam. The process involves rapid melting and solidification of a controlled amount of injected metal powders as a laser beam scans over each layer building the structure from the bottom to the top. Due to its unique capability to deposit precise amounts of powder material at a desired location, the LENS^TM process finds potential application in rapid tooling, prototyping, precision repair work, and manufacture of complex, intricate components with varying compositions. The peak temperature and thermal cycle experienced by each layer influence the final mechanical properties and dimensional accuracy of the part. An understanding and quantitative knowledge of the peak temperature, melt pool dimensions, and thermal cycles experienced in the deposited layers are essential for a priori selection of the process parameters in LENS^TM technique. It is important to ensure that the deposited layers have the desired dimensions, good interlayer bonding, and requisite mechanical properties. In an attempt to understand the process parameters to be used in achieving the desired nature of deposition, a three-dimensional model is developed based on finite element method to numerically simulate heat transfer phenomenon in LENS^TM process considering deposition of SS316 powders on a substrate of the same material. The computed temperature profiles are first validated with experimental results reported in the literature. The influence of process parameters on peak temperature, thermal cycles, and melt pool dimensions are studied subsequently. The continuous movement of laser and synchronized activation of elements depicting addition of powder particles are incorporated through an externally written user subroutine and using the element deactivation and activation features in the commercial finite element software ABAQUS 6.7. A unique non-dimensional parameter specific to LENS^TM process is defined considering the combined influence of process parameters and material properties. The non-dimensional parameter is further used to serve as a guideline for the selection of appropriate process parameters that can result in a steady melt pool dimension, thereby ensuring a target layer width with good interlayer bonding.