EXPERIMENTAL RESEARCH ON TRANSIENT HEAT TRANSFER IN SAND

In order to examine if the heat conduction in porous material behaves like a wave as claimed by other researchers, experiments on transient heat transfer is conducted in casting sand. The results show that the heat propagation can be described neither by wave nor by diffusion model. The CV-wave concept is discussed and challenged according to the experimental results in the entire transient stage, including the transient response of temperature and the response curve of penetration-depth versus penetration-time.

Thermo-structural analysis on evaluating effects of friction and transient heat transfer on performance of gears in high-precision assemblies

The high precision assemblies with considerable radial interference should be accompanied by heating and cooling processes.However,the mechanical properties of metals are greatly affected by thermal operations.So,for evaluating the stress distribution and distortion of teeth profiles in a gear/shaft assembly,a transient thermal analysis is necessary for finding the change in mechanical properties.The friction on the contact surface is another important parameter in interaction of the gear with the shaft.Evaluating the gear stress and deformation fields for several modes of heat transfer and friction coefficients showed that the maximum radial or tangential stresses on contact surface of the joint may have more than 8%increase by increasing friction coefficient;while the intensity of heat transfer at cooling stage has lower effect on stress distribution.

Transient Heat Transfer Study of Direct Contact Condensation of Steam in Spray Cooling Water

We conducted a transient experimental investigation of steam–water direct contact condensation in the absence of noncondensible gas in a laboratory-scale column with the inner diameter of 325 mm and the height of 1045 mm. We applied a new analysis method for the steam state equation to analyze the molar quantity change in steam over the course of the experiment and determined the transient steam variation. We also investigated the influence of flow rates and temperatures ofcooling water on the efficiency ofsteam condensation. Our experimental results show that appropriate increasing of the cooling water flow rate can significantly accelerate the steam condensation. We achieved a rapid increase in the total volumetric heat transfer coefficient by increasing the flow rate of cooling water, which indicated a higher thermal convection between the steam and the cooling water with higher flow rates. We found that the temperature ofcooling water did not play an important role on steam condensation. This method was confirmed to be effective for rapid recovering ofsteam.

Transient heat transfer characteristics of air-array-jet impingement with different nozzle arrangements on high-tempera-ture flat plate in small jet-to-plate distance

Numerical studies on transient heat transfer characteristics of air-array-jet impingement with a small jet-to-plate distance and a large temperature difference between nozzles and plate were presented.The dimensionless jet-to-plate distance(H/D)was 0.2,and non-dimensional nozzle-to-nozzle spacing(S/D)was 3,4,5 and 6,respectively.It is found that the quenching time is shortened at a constant total mass flow at air jet inlet m·(m·=218.21 kg/h),and the heat transfer uniformity is deterio-rated as S/D increases.However,the adding reversed-flow nozzles can shorten the quenching time of the glass plate considerably with a modest change in the heat transfer uniformity.The results at variable m·are the same as those at a fixed m·.Furthermore,the parity and arrangement of nozzles are also discussed,It is found that an odd number of nozzles is more beneficial for transient heat transfer.Based on these results,an appropriate proposal for ultra-thin glass tempering process is presented.

Correlation Analysis of Transient Heat Transfer Characteristics for Air Precooling Aggregate

In dam works, air precooling of aggregate is a common and effective method to avoid temperature cracks in concrete structure. In order to offer a reliable design theory for the air precooling process to avoid unreasonable energy consumption, the transient heat transfer characteristics of the aggregate are intensively analyzed. The combined structure of the aggregate and the interstitial space in the hopper is treated as a porous structure, and the space-average method is used to simulate the transient heat transfer process. Simulation results show that size of the hopper and the average air velocity in the cross section have great influence on the transient heat transfer process of the aggregate, while the porosity in the range of 0.4-0.5 has little influence. Nomograms are abstracted from simulation results, and then correlations of the compared excess temperature are precisely fitted to predict the air precooling transient heat transfer process of the aggregate.

Experimental Investigation of Transient Heat Transferin the Once-Through Boiler Water Separator

The transient heat transfer between working fluid and inner wall of 600 MW supercritical pressure boiler's water separator model during start-up was experimentally explored.The test conditioins for water steam two-phase were as follows:pressure P=3=11MPa,mass flow M=0.2-0.35kg/s,and mass quality x=0.2-0.8;for subcooled water (=3-11MPa,M=0.2-0.35kg/s,and subcooled degree Δθ=10℃.By using an accurate and tracable inverse heat conduction solution,the correlations of single-phase and two-phase transient heat transfer coefficients were obtained.The experimental results demonstrated the heat transfer enhanced for subcooled water and water steam mixture compared with fully developed turbulent flow in tube.The increasing heat transfer effect may be attributed to the deformation or destruction of the boundary layer.

Heat transfer enhancement in MOSFET mounted on different FR4 substrates by thermal transient measurement

Miniaturization of electronic package leads to high heat density and heat accumulation in electronics device, resulting in short life time and premature failure of the device. Junction temperature and thermal resistance are the critical parameters that determine the thermal management and reliability in electronics cooling. Metal oxide field effect transistor（MOSFET）is an important semiconductor device for light emitting diode-integrated circuit（LED IC） driver application, and thermal management in MOSFET is a major challenge. In this study, investigations on thermal performance of MOSFET are performed for evaluating the junction temperature and thermal resistance. Suitable modifications in FR4 substrates are proposed by introducing thermal vias and copper layer coating to improve the thermal performance of MOSFET. Experiments are conducted using thermal transient tester（T3ster） at 2.0 A input current and ambient temperature varying from25℃ to 75℃. The thermal parameters are measured for three proposed designs： FR4 with circular thermal vias, FR4 with single strip of copper layer and embedded vias, and FR4 with I-shaped copper layer, and compared with that of plain FR4 substrate. From the experimental results, FR4I-shaped shows promising results by 33.71% reduction in junction temperature and 54.19% reduction in thermal resistance. For elevated temperature, the relative increases in junction temperature and thermal resistance are lower for FR4I-shaped than those for other substrates considered. The introduction of thermal vias and copper layer plays a significant role in thermal performance.

Transient Response Characteristics of Separated Flow and Heat Transfer in Enlarged Rectangular Channel

Numerical results of three-dimensional separated flow and heat transfer in an enlarged rectangular channel are presented in this paper. The expansion ratio and aspect ratio of the channel are 2.0 and 8.0, respectively. Reynolds number of the flow is 200 and it is over the critical Reynolds number. Over the value, the flow in the symmetric channel becomes to deflect to one side of the walls. Transient response characteristics of the flow and heat transfer in the channel with the fully developed flow imposed one cycle of a pulsating fluctuation at the inlet are investigated. Vortex structure generated in the channel is visualized with a helicity isosurface. In the case of the fluctuation of Strouhal number 0.05, small streamwise vortices appear near the side walls and slightly upstream of the reattachment region of the short separation bubble. The vortices elongate and shed some vortices. These vortices attract some pairs of the streamwise vortices near the reattachment region quickly and they drift downstream along the side walls. They are inclined from the walls and are decaying gradually. It is clarified that high Nusselt number area appears and shifts downstream in accordance with the root of the vortices.

Consideration of transient heat conduction in a semi-infinite medium using homotopy analysis method

In the current work, transient heat conduction in a semi-infinite medium is considered for its many applications in various heat fields. Here, the homotopy analysis method （HAM） is applied to solve this problem and analytical results are compared with those of the exact and integral methods results. The results show that the HAM can give much better approximations than the other approximate methods： Changes in heat fluxes and profiles of temperature are obtained at different times and positions for copper, iron and aluminum.

Development of experimental study on coupled heat and moisture transfer in porous building envelope

A new facility was presented which can expediently and cheaply measure the transient moisture content profile in multi-layer porous building envelope.Then,a common multi-layer porous building envelope was provided,which was constructed by cement mortar-red brick-cement plaster.With this kind of building envelope installed in the south wall,a well-controlled air-conditioning room was set up in Changsha,which is one of typical zones of hot and humid climate in China.And experiments were carried out to investigate the temperature and moisture distribution in multi-layer building envelope in summer,both in sunny day and rainy day.The results show that,the temperature and humidity at the interface between the brick and cement mortar are seriously affected by the changes of outdoor temperature and humidity,and the relative humidity at this interface keeps more than 80% for a long-term,which can easily trigger the growth of mould.The temperature and humidity at the interface between the brick and cement plaster change a little,and they are affected by the changes of indoor temperature and humidity.The temperature and humidity at the interface of the wall whose interior surface is affixed with a foam plastic wallpaper are generally higher than those of the wall without wallpaper.The heat transfer and moisture transfer in the envelope are coupled strongly.

Analysis Process of Finite Element Method in Heat Transfer through Fabrics

According to heat transfer principle and the process of solving engineering problems by finite element method, examples were given to demonstrate how finite element analysis can be used to describe transient heat transfer through fabrics. Details were given to describe how conduction and convection affect temperature distribution and heat loss during heat transfer processes by taking advantage of the quick calculation of FEA software MSC.Marc. Experimental results show good agreement with the theoretical results.

THE NUMERICAL ANALYSIS OF UNSTEADY HEAT TRANSFER OF A PISTON-LINER COUPLED SYSTEM IN AN INTERNAL COMBUSTION ENGINE

This paper provides a numerical analysis model of unsteady heat trans-fer in piston-liner set of an internal combustion engine.The model simulates the un-steady heat transfer process among the combustion mixture,piston set,lubricantfilm,liner and coolant in a whole engine cycle,and can predict the temperature fluc-tuation and distribution on piston crown,inner surface of liner,piston ring and thelubricant film.A computer program is developed to calculate the unsteady heat trans-fer process of piston-liner system in a water-cooled diesel engine.

倾斜椭球凹陷阵列湍流强化传热实验与数值分析

为了提高涡轮叶片内部通道对流传热性能,本文基于瞬态液晶热像技术分别针对矩形通道表面排布的球形凹陷和斜椭球凹陷阵列的传热和压力损失特性进行了实验研究。在雷诺数为1×10^(4)~6×10^(4)内,与基准球形凹陷相比,平行排布和V形排布的斜椭球凹陷总体传热分别增强了23.8%~33.8%和104%~121%,V形排布的斜椭球凹陷综合传热性能显著提高了25%~68.3%。通过数值模拟发现,平行排布的斜椭球凹陷虽然可以诱发更强烈的二次流,但是平行的排布方式使下游凹陷受到来自上游凹陷升温后的流体冲刷,因此限制了传热的进一步增强。V形排布的斜椭球凹陷通过诱发大尺度的纵向涡对将来自主流的冷流体输运至壁面,冷流体直接冲击壁面并与近壁流体剪切,极大地增强了对流传热。

螺旋金属燃料多物理耦合分析方法与概念设计研究

螺旋金属燃料具有导热系数高、导热路径短、强制旋流交混的特点,可实现更高的堆芯功率密度,进而减小堆芯体积,提高反应堆的安全性和经济性。本文介绍了上海交通大学反应堆热工水力实验室建立的螺旋金属燃料热工水力、中子物理、力学特性分析方法及多物理耦合分析框架。在热工水力方面,基于自研仪器实现了交混及沸腾临界行为精细化测量,建立了三维及精细化子通道分析方法;在中子物理方面,建立了适用于特殊能谱、复杂几何的截面及稳瞬态中子物理特性的分析方法;在力学方面,基于分子动力学方法建立了U-Zr合金燃料基础热物性模型,并开展了辐照条件下螺旋棒宏观力学特性研究。基于热工-物理-力学多物理分析和优化,提出了螺旋金属燃料组件及堆芯设计,具有无硼化、堆芯功率密度高、体积小、换料周期长的特点。

多喷嘴喷雾冷却表面传热特性实验研究

低沸点制冷剂(R134a)喷雾冷却能快速降低表面温度,在高性能芯片等电子设备热管理、生物医学中均具有广泛应用,可实现喷雾表面的低温高效散热。多喷嘴喷雾冷却可实现较大冷却表面的均匀冷却及冷量的灵活调控。搭建了制冷剂喷雾冷却实验台,研究了多喷嘴喷雾冷却表面传热特性,并分析了喷嘴的孔径、间距、高度、喷雾时间和喷雾压力等因素对多喷嘴喷雾冷却表面传热特性的影响。结果表明:喷嘴孔径对冷却均匀性影响最大,其次是喷嘴间距,适当的喷嘴孔径(0.4 mm)和间距(11 mm)能使制冷剂喷雾在表面合理分布,最低温Tmin能达到最小值;10 mm喷嘴高度和1.2 MPa喷雾压力可以使液滴充分雾化并延长有效冷却时间。

飞秒激光脉冲作用熔石英的超快传热动力学研究

飞秒激光具有超短脉冲宽度、超高峰值功率的特点,飞秒激光与物质作用表现出的非线性吸收和低热扩散特性,使其在高精密微纳器件加工中有着重要的应用前景。建立了针对飞秒激光脉冲与熔石英作用的瞬态光电离以及非平衡传热的超快动力学模型。通过数值求解该模型获得了飞秒激光单脉冲作用熔石英的载流子密度和非平衡电子与声子温度的时空演化;得到了在非平衡态条件下,电声耦合时间随激光能量密度、脉冲宽度的近线性调控规律。进一步研究得到了瞬态电子热导、热容、电声耦合系数的变化规律,并对上述模拟现象进行了分析和探讨。

基于相变复合材料的新型液压油箱设计及研究

针对液压系统存在“低温启动困难”和“高温散热效率低”的矛盾问题,影响了液压系统设备性能的整体提升。设计一种采用相变复合材料构建的新型液压油箱结构,利用ANSYS软件对先后添加复合相变材料新型油箱进行建模与仿真,得出径向传热、瞬态传热等仿真云图并进行分析比较传热梯度的变化。仿真结果证实,相变复合材料能够快速吸收油液的热量,从而散热效果高于普通油箱。并且针对油箱传热的不同影响因素进行了仿真分析,得出油箱材料、环境温度、载荷温度仿真云图,仿真结果表明,利用相变复合材料的高储能及高导热特性,在低温条件下,液压油能够在油箱中快速均匀加热以及在高温条件下高效散热,提高油箱的换热效率,减小油箱体积,使其具备“容积小、导热快、换热面积大、油液加热均匀、对极端温度条件适应能力强”等优点,对延长液压油及液压元件的使用寿命,提高液压系统设备的工作可靠性和生存能力具有重要意义。

Analytical prediction and field validation of transient temperature field in asphalt pavements

This work presented the development and validation of an analytical method to predict the transient temperature field in the asphalt pavement.The governing equation for heat transfer was based on heat conduction radiation and convection.An innovative time-dependent function was proposed to predict the pavement surface temperature with solar radiation and air temperature using dimensional analysis in order to simplify the complex heat exchange on the pavement surface.The parameters for the time-dependent pavement surface temperature function were obtained through the regression analysis of field measurement data.Assuming that the initial pavement temperature distribution was linear and the influence of the base course materials on the temperature of the upper asphalt layers was negligible,a close-form analytical solution of the temperature in asphalt layers was derived using Green's function.Finally,two numerical examples were presented to validate the model solutions with field temperature measurements.Analysis results show that the solution accuracy is in agreement with field data and the relative errors at a shallower depth are greater than those at a deeper one.Although the model is not sensitive to dramatic changes in climatic factors near the pavement surface,it is applicable for predicting pavement temperature field in cloudless days.

Experimental study on transient behavior of semi-open two-phase thermosyphon

An experimental system was set up to measure the temperature, pressure, heat transfer rate and mass flow rate in a semi-open two-phase thermosyphon. The behaviors of a semi-open two-phase thermosyphon during startup, shutdown and lack of water were studied to get complete understanding of its thermal characteristics. The variation of wall temperature, heat-exchange condition and pressure fluctuations of semi-open two-phase thermosyphons showed that the startup of SOTPT needs about 60-70 min; the startup speed of SOTPT is determined by the startup speed of the condensation section; the average pressure in the heat pipe is equal to the environmental pressure usually; the shutdown of SOTPT needs about 30-50min; a semi-open two-phase thermosyphon has good response to lack of water accident.

Transient free convection flow of a viscoelastic fluid over vertical surface

In this paper, the viscoelsatic boundary layer flow and the heat transfer near a vertical isothermal impermeable surface and in a quiescent fluid are examined. The governing equations are formulated and solved numerically using MackCormak＇s technique. The results show excellent agreement with previously published results by a comparision. Representative results for the velocity and temperature profiles, boundary layer thicknesses, Nusselt numbers, and local skin friction coefficients are shown graphically for different values of viscoelsatic parameters. In general, it is found that the velocities increase inside the hydrodynamic boundary layers and the temperatures decrease inside the thermal boundary layers for the viscoelsatic fluid as compared with the Newtonian fluid due to favorable tensile stresses. Consequently, the coefficients of friction and heat transfer enhance for higher viscoelsatic parameters.