金属泡沫平板结构内对流换热的数值研究

基于Brinkman-Darcy模型和两方程模型,本文对流体在金属泡沫平板通道内的强制对流换热进行了自编程数值模拟,采用体积平均法对流体在金属泡沫内的流动和换热进行宏观处理。模拟结果表明：流体主流速度随孔密度增大而减小,随孔隙率增大而增大;流体相和固体相之间的局部对流换热系数随孔隙率和孔密度增加而增加,金属泡沫对流换热性能随孔隙率增大而减小,随孔密度增大而增大。金属泡沫强化换热的效果十分明显,可以应用于需要强化换热的紧凑式换热器和散热器。     

湿烟气在开孔翅片管外凝结换热数值模拟

为高效回收湿烟气全热并对烟气冷凝换热设备进行优化设计,以设计的紧凑式开孔翅片管换热器为对象,采用欧拉壁膜(EWF)模型与组分输运模型耦合研究低温烟气在翅片管换热器中的凝结换热规律。数值模拟得到的凝结速率及对流凝结换热系数与实验结果最大偏差分别为13.4%和10.9%。结果表明：对流凝结换热系数随入口水蒸气质量分数和烟气流速增大以及管壁温降低而增大,翅片开孔可以起到均压、破坏温度边界层、截断液膜、加快凝结液排出进而强化传热的作用,基于模拟拟合的关于改进雅各布数J的烟气对流凝结关联式与实验数据平均相对误差为13.1%,模拟关联式对于90%的实验结果预测误差在-20%～+20%以内。     

片式散热器冲压型涡发生器工程应用研究

在片式散热器上冲压出涡流发生器,在自然对流下,强化散热器散热。采用数值模拟的方法分析涡发生器长度、宽度、摆放角度、散热片间距及摆放位置对散热片散热量及对流换热系数的影响。由于散热片厚1mm,考虑到实际加工工艺,最后选择涡发生器长13.5mm、宽5mm、高3mm,与空气来流方向夹角为30°,两散热片对应位置涡发生器夹角60°时,两散热片同侧涡发生器向空气入口方向移动50mm时,可使散热片散热量提高15.71%。     

平流层电子设备温度数值模拟

分析了平流层电子设备内外部热环境,考虑平流层大气对流、设备内部自然对流、太阳直射辐射、大气辐射、地面反射太阳辐射、地球红外辐射以及设备自身辐射等因素的基础上,建立了计算电子设备温度分布特征的对流、辐射耦合模型,模拟了其在不同功率、不同对流换热、不同环境条件下的温度分布。结果表明:对于平流层电子设备散热,对流换热和辐射换热都会影响电子设备的温度分布,尽管由于平流层大气压力低、对流换热弱,但对流换热量占到散热总量的60%以上,是散热的主要方式。因此,在平流层电子设备热设计时,可以优先考虑采取开孔等强化对流散热方法来控制设备的温度。最后,开展了平流层模拟环境的实验验证,典型工况实验值与计算值吻合较好,验证了计算模型的正确性。对平流层电子设备热设计有重要的指导意义。     

电场作用下电对流强化换热现象研究

为了揭示电场对自然对流换热的影响,采用高速摄影技术对R113工质在自然对流区的电对流强化换热现象进行了实验研究,高压电源1～5 kV、热流密度4.46～17.80 kW/m~2,分析了电对流层高度、加热面温度、对流换热系数以及电场的强化换热系数的动态变化规律。实验发现:施加电场后,在加热面上方形成了稳定的电对流层,电对流层高度随着施加电压的升高而逐渐减小,由于电场增强流体的扰动,使得壁面温度降低,进而强化了换热,实验中强化换热系数可达1.2倍。     

换热管内插开圆孔扭带提高传热效率

文章简述传热设备的强化传热的节能技术;通过自制不同规格自旋开孔扭带置入换热管进行冷态实验和热态实验,分析换热管内插扭带后传热效果的改变;用实验数据解析扭带转速、压力降、传热系数与流速的内在关系,验证出开孔扭带比光滑扭带的强化传热效果更好,为企业在节能降耗改造现有换热器设备中提高其传热效率提供理论依据。     

管径对错列管束换热器对流冷凝换热特性影响研究

针对烟气横向冲刷错列管束换热方式进行了管径大小对于其纯对流换热以及对流冷凝换热影响规律研究。结果表明:当只有纯对流发生时,在保持换热面积以及入口工况不变情况下,随着管径的减小,出口烟温降低,传热量增加,换热得到明显增强,其强化传热的主要原因是管径减小强化了对流换热系数;当存在冷凝时,管径对总换热系数以及冷凝换热系数同样存在强化作用。     

低气压下计算机显卡芯片温升特性的试验研究

计算机显卡芯片的散热是通过与空气的对流换热来实现的.根据传热学原理,对流换热强度会由于气压的降低而减弱.青藏高原平均大气压力仅65.2 kPa,远低于海平面处.这将导致通过自然对流来散热的计算机显卡芯片的散热特性变差,工作温度升高,可能会影响其正常工作.通过试验研究了静音型(无风扇仅带散热片)计算机显卡芯片在大气压力降低时的温升特性.     

电场和螺旋线圈复合强化管内强制对流的实验

对管内油的层流流动换热采用内插螺旋线圈和外加高压电场两种强化技术进行了复合强化换热实验研究,实验结果发现,内插螺旋线圈强化管可以强化层流对流换热大约一倍左右;同时采用高压电场强化换热技术后,换热强化率可以再提高4倍左右;油温、流速对换热强化率没有显著影响,换热强化特性基本取决于外加高压电场。     

锅炉尾部烟气对流冷凝换热器结构优选分析

在分析比较了气体纯对流换热强化方式、纯净蒸汽凝结换热强化方式基础上,结合含不凝性气体蒸汽冷凝换热机理及纯净蒸汽凝结换热机理与气体纯对流换热机理的差异,对已有各类换热器结构自身在强化含不凝性气体蒸汽凝结换热程度进行了分析,得出了既能强化锅炉尾部烟气对流冷凝换热又能高效回收凝结水分的错列布置垂直光管管外凝结换热器与带有波纹槽道的板式换热器2种优选结构,进一步给出了根据烟气灰尘浓度高低选取2种冷凝换热器的原则。     

Development of a theoretical model for predicting the thermal performance characteristics of a vertical pin-fin array heat sink under combined forced and natural convection with impinging flow

A comprehensive theoretical and experimental study was carried out on the thermal performance of a pin-fin heat sink. A theoretical model was formulated that has the capability of predicting the influence of various geometrical, thermal, and flow parameters on the effective thermal resistance of the heat sink. An experimental technique was developed for measuring the thermal performance of the heat sink, and the overall convective heat transfer coefficient for the fin bundle. Experiments were carried out, and correlations obtained, for a wide range of parameters for pure natural convection and for combined forced and natural convection. The predictive capability of the theoretical model was verified by comparison with experimental data including the influence of various fin parameters and the existence of an optimum fin spacing.     

Role of Melt Convection on Optimization of PCM-Based Heat Sink Under Cyclic Heat Load

In this article, we study the thermal performance of phase-change material (PCM)-based heat sinks under cyclic heat load and subjected to melt convection. Plate fin type heat sinks made of aluminum and filled with PCM are considered in this study. The heat sink is heated from the bottom. For a prescribed value of heat flux, design of such a heat sink can be optimized with respect to its geometry, with the objective of minimizing the temperature rise during heating and ensuring complete solidification of PCM at the end of the cooling period for a given cycle. For given length and base plate thickness of a heat sink, a genetic algorithm (GA)-based optimization is carried out with respect to geometrical variables such as fin thickness, fin height, and the number of fins. The thermal performance of the heat sink for a given set of parameters is evaluated using an enthalpy-based heat transfer model, which provides the necessary data for the optimization algorithm. The effect of melt convection is studied by taking two cases, one without melt convection (conduction regime) and the other with convection. The results show that melt convection alters the results of geometrical optimization.     

加热方向对方腔内相变石蜡储热性能影响研究

为探究方腔内相变石蜡的储热性能,基于等效热容法和Boussinesq假设,建立相变石蜡融化储热计算模型,并针对加热方向及约束形式等因素对相变石蜡的储热性能的影响进行研究,并开展相变石蜡融化试验,验证计算模型的正确性。结果表明：相变石蜡融化储热过程是由热传导和自然对流传热综合决定的,其中自然对流传热在相变石蜡融化储热过程中起着极为显著的促进作用;不同加热方向下,相变石蜡表现出截然不同的融化储热效率,其中顶、底、侧边单独加热下的自然对流传热效应依次使储热效率提升了0.01,27.9和13.1倍,即底部热源的储热效率最高;在四面加热下,固相因无约束而下沉至底部,并抑制底部热壁面的自然对流传热效应,此时顶、底、侧热壁面的储热贡献率分别为17.3%,37.3%和22.7%;当固相运动被预埋热电偶等因素限制时,将形成钟型融化前缘,该形态包含了各热壁面单独加热下的融化储热特征,此时顶、底、侧热壁面的储热贡献率分别为19.2%,29.8%和25.5%。     

A study of an LED backlight thermal module

The present study uses a heat sink plate to conduct natural convection in order to examine different areas of the heat sink and the effects of mounting different quantities of LEDs on the same surface on the thermal mechanism performance. Based on the experimental results, when a heat sink plate is arranged vertically, the channel flow between the fins is parallel to gravity. The LED substrate plate temperature is different from that at the end of the fin, and rises with the increase of total power. The thermal resistance rises slowly and then declines with the increase of LED electric power. As for temperature change of the LED substrate and at the end of the fin, when the temperature difference is increased, it also increases the natural convection thrust. For thermal resistance, the environmental thermal resistance at the bottom of the heat sink plate is lower than at the middle and top sections. These LED power emissions will be changed synchronously. Regarding the LED quantity control, the rate of increase is the highest for the heat sink plate with 30 pcs LED, and the temperature is very high for the heat sink plate with 45 and 60 pcs LEDs when the power approaches 1 W. Moreover, the rising rate is the lowest for the heat sink plate with 60 pcs LEDs. Depending on the brightness requirement, the illuminant is provided by 60 pcs LEDs to obtain a lower temperature so that the system can reduce the thermal protective design. Evidence shows that a high conductivity heat pipe embedded in the channel can provide a more uniform temperature distribution. The present study provides a further understanding on the influence of different illuminant densities on the heat sink structure and the temperature difference in an LED heat transfer device, in order to provide a reference for heat sink design of a backlight module and LED illuminant module evaluation. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20321     

氧气转炉汽化冷却烟道传热计算

对氧气汽化冷却烟道传热的计算方法提出实用算法,而取代机械行业标准"烟道式余热锅炉设计导则"和"氧气转炉余热锅炉技术条件"中所提出的辐射传热算法。文中主要介绍了烟气成分计算、辐射传热计算和对流传热计算方法。其中烟气成分计算过程主要包括烟气的组成、焓值;辐射传热计算将放弃机械部标准所提出的辐射传热计算标准,而采用锅炉辐射室标准计算方法;增加对流传热部分,弥补机械部标准缺少对流换热计算的问题。综合整个过程中的辐射传热和对流传热与烟气焓值变化进行比较,进行综合试差运算,最终确定整个汽化冷却烟道的温度变化和热能回收状况。     

Effect of Convection Heat Transfer on Performance of Waste Heat Thermoelectric Generator

Efficiency of energy conversion processes can be improved if waste heat is converted to electricity. A thermoelectric generator (TEG) can directly convert waste heat to electricity. The TEG typically suffers from low efficiency due to various reasons, such as ohmic heating, surface-to-surrounding convection losses, and unfavorable material properties. In this work, the effect of surface-to-surrounding convection heat transfer losses on the performance of TEG is studied analytically and numerically. A one-dimensional (1-D) analytical model is developed that includes surface convection, conduction, ohmic heating, and Peltier, Seebeck, and Thomson effects with top and bottom surfaces of TEG exposed to convective boundary conditions. Using the analytical solutions, different performance parameters (e.g., heat input, power output, and efficiency) are calculated and expressed graphically as functions of thermal source and sink temperatures and convection heat transfer coefficient. Finally, a two-dimensional (2-D) mathematical model is solved numerically to observe qualitative results of thermal and electric fields inside the TEG. For all calculations, temperature-dependent thermal/electric properties are considered. Increase in thermal source temperature results in an increase in the power output with adiabatic side wall conditions. A change in boundary condition to convection heat transfer from adiabatic boundary has a large impact on thermal efficiency.     

Heat losses in Grätzel solar cells

The absorption and reflection spectra of a Grätzel-type solar cell were evaluated in terms of heat losses. Using these experimental data, the temperature distribution inside the solar cell was calculated. The temperature-controlling process in this cell is the heat transfer from the cell to the environment. Assuming the environment temperature of 25°C, the natural air convection leads to the temperature of 46.9°C inside the TiO₂ layer. Forced air convection causes the decrease, up to 30.1°C at the wind speed of 10 m s⁻¹. Variations in the cell dimensions, except the active layer length, have only a small effect on the temperature field, compared to the variations in the heat transfer coefficients.     

Effect of radiation in a radial heat sink under natural convection

A radial heat sink was investigated, considering both natural convection and radiation. Experiments were performed to validate numerical results and a close agreement was found. The effect of radiation on total heat transfer was examined by varying emissivity, and it was found that the maximum radiation contribution was 27%. The radial heat sink was optimized to maximize thermal performance. The thermal performance was enhanced by 12.3% while the mass increased by 20%. Radiation (emissivity) had a significant effect on the optimum long fin length when only thermal performance was considered. The average thermal resistance decreased by 8.7% for the optimized model with the same mass as the reference model, and the effect of radiation on total heat transfer was greater than the effect of natural convection.     

柴油机缸内传热计算

传热计算模型中考虑了柴油机气缸内对流、辐射和导热3个方面的传热,在计算缸内传热情况时,气缸内对流传热采用紊流模型,辐射传热采用火焰模型,导热采用周期性瞬态传热模型,并考虑三之间的耦合关系,得到了准确的计算结果。对150HP柴油机进行了计算和试验,取得了良好效果。