Skin friction and heat transfer of liquid jet over a continuous moving horizontal hot plate

The skin friction and heat transfer occurring in the laminar boundary layerwhich caused by a vertical liquid jet impinging on a continuously moving horizontal plate werestudied. Similarity solutions for shear stress and heat distribution were obtained by using thehooting technique. The results show that the skin friction decreases with an increase of velocityparameter, the evolving of thermal boundary decrease with increasing in Prandtl number, but increasewith increasing of velocity parameter.

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.

Numerical Solution of MHD Convection and Mass Transfer Flow of Viscous Incompressible Fluid about an Inclined Plate with Hall Current and Constant Heat Flux

The present numerically study investigates the influence of the Hall current and constant heat flux on the Magneto hydrodynamic (MHD) natural convection boundary layer viscous incompressible fluid flow in the manifestation of transverse magnetic field near an inclined vertical permeable flat plate. It is assumed that the induced magnetic field is negligible compared with the imposed magnetic field. The governing boundary layer equations have been transferred into non-similar model by implementing similarity approaches. The physical dimensionless parameter has been set up into the model as Prandtl number, Eckert number, Magnetic parameter, Schmidt number, local Grashof number and local modified Grashof number. The numerical method of Nactsheim-Swigert shooting iteration technique together with Runge-Kutta six order iteration scheme has been used to solve the system of governing non-similar equations. The physical effects of the various parameters on dimensionless primary velocity profile, secondary velocity profile, and temperature and concentration profile are discussed graphically. Moreover, the local skin friction coefficient, the local Nusselt number and Sherwood number are shown in tabular form for various values of the parameters.

Thermal Diffusion Effect on MHD Heat and Mass Transfer Flow past a Semi Infinite Moving Vertical Porous Plate with Heat Generation and Chemical Reaction

The objective of present work is to study the thermo diffusion effect on an unsteady simultaneous convective heat and mass transfer flow of an incompressible, electrically conducting, heat generating/absorbing fluid along a semi-infinite moving porous plate embedded in a porous medium with the presence of pressure gradient, thermal radiation field and chemical reaction. It is assumed that the permeable plate is embedded in a uniform porous medium and moves with a constant velocity in the flow direction in the presence of a transverse magnetic field. It is also assumed that the free stream consists of a mean velocity, temperature and concentration over which are super imposed an exponentially varying with time. The equations of continuity, momentum, energy and diffusion, which govern the flow field, are solved by using a regular perturbation method. The behavior of the velocity, temperature, concentration, Skin-friction, rate of heat transfer and rate of mass transfer has been discussed for variations in the physical parameters. An increase in both Pr and R results a decrease in thermal boundary layer thickness. However, concentration decreases as Kr, Sc increase but it increases with an increase in both So and δ.

Establishment and Application of UFC-ACC Heat Transfer Coefficient Model

Based on medium plate runout table ultra-fast cooling( UFC)-accelerated cooling equipment( ACC) system,a heat transfer coefficient model was constructed. Firstly,according to the measured data,heat transfer coefficients under different roll speed and water volume were calculated by using an inverse heat conduction method. Secondly,a monofactorial heat transfer coefficient calculation formula was obtained. Finally,the heat transfer coefficient model based on medium plate runout table UFC-ACC system was constructed by intercept function,slope function,interaction influence function and linear or nonlinear influencing factors. The precision of these models was validated by comparing model prediction value with measured data,and the results were in good agreement with practical needs,and the average deviation was less than 5%.

Numerical Modelling of Non-similar Mixed Convection Heat and Species Transfer along an Inclined Solar Energy Collector Surface with Cross Diffusion Effects

An analysis is performed to study thermo-diffusion and diffusion-thermo effects on mixed convection heat and mass transfer boundary layer flow along an inclined (solar collector) plate. The resulting governing equations are transformed and then solved numerically using the local nonsimilarity method and Runge-Kutta shooting quadrature. A parametric study illustrating the influence of thermal buoyancy parameter (&#950), Prandtl number (Pr), Schmidt number (Sc), Soret number (Sr), Dufour number (Du) and concentration-to- thermal-buoyancy ratio parameter, N, on the fluid velocity, temperature and concentration profiles as well as on local skin-friction, Nusselt and Sherwood numbers is conducted. For positive inclination angle of the plate (&#947 = 70 degrees), flow velocity (f') is strongly increased i.e. accelerated, with thermal buoyancy force parameter (&#950), in particular closer to the plate surface;further into the boundary layer, &#950 has a much reduced effect. Conversely temperature (&#952) and concentration (&#968) is decreased with increasing thermal buoyancy parameter, &#950. For negative plate inclination, the flow is accelerated whereas for positive inclination it is decelerated i.e. velocity is reduced. Conversely with negative plate inclination both the temperature and concentration in the boundary layer is reduced with the opposite apparent for positive inclination. Increasing Prandtl number strongly reduces temperature in the regime whereas an increase in Schmidt number boosts temperatures with temperature overshoots near the plate surface for Sc = 3 and 5 (i.e. for Sc > 1). Concentration is reduced continuously throughout the boundary layer, however, with increasing Schmidt number. A positive increase in concentration-to-thermal-buoyancy ratio parameter, N, significantly accelerates the flow in the domain, whereas negative N causes a deceleration. A velocity overshoot is also identified for N = 20, at intermediate distance from the plate surface. Negative N (thermal and concentration buoyancy forces oppose each other) induces a slight increase in both fluid temperature and concentration, with the reverse observed for positive N (thermal and concentration buoyancy forces assisting each other). Increasing Dufour number respectively causes a rise in temperature and a decrease in concentration, whereas an increase in Soret number cools the fluid i.e. reduces temperature and enhances concentration values. In the absence of Soret and Dufour effects, positive N causes a monotonic increase in local Nusselt number, NuxRex-1/2 with &#950 Cos &#947, for N = -1 the local Nusselt number remains constant for all values of parameter, &#950 Cos &#947. Local Sherwood number, ShxRex-1/2 is boosted considerably with higher Schmidt numbers and also with positive N values. The computations in the absence of Soret and Dufour effects correlate accurately with the earlier study by Chen et al. (1980).

Experimental Analysis of Performance of Heat Exchanger with Plate Fins and Parallel Flow of Working Fluids

Heat exchangers are devices in which heat is transferred from one fluid to another fluid as a result of temperature difference. Heat exchanger presented in the current paper in which inside the tubes flows water, but outside the tubes flows air aims to enable cooling of circulating water, which serves to cool the engine of a machine. Such exchangers find application in the automotive industry as well as heating and cooling equipment and HVAC systems etc. The surface of the heat exchanger by the air side always tends to be much larger using surface fins in order to facilitate equalization of thermal resistance for both sides of the heat exchanger, because the rate of transmission of heat from the water side is much greater. Furthermore, the paper will present analytical and experimental studies involved for determination of performance of plate-fin heat exchanger for various flows of working fluids in order to get the highest values of performances i.e.: overall heat transfer coefficient U, efficiency of heat exchanger ε, maximal and real heat transferred, pressure drop, air velocity and Reynolds number from the air side of heat exchanger etc. The present scientific paper is based on the fact that from the experimental model made for laboratory conditions, conclusions are derived that can be used during installation of such heat exchanger on certain machines in order to predict their performance.

不锈钢板式热沉传热特性的实验与模拟研究

目的 研究流量、胀高及焊点排布方式对不锈钢板式热沉传热特性的影响。方法 搭建实验平台,加工出结构参数不同的板式热沉实验件,同时采用静力隐式算法建立相应的几何模型,通过实验与数值模拟相结合的方法,研究流量、胀高和焊点排布方式的变化对板式热沉热阻的影响规律,并分析热沉流道内部流场状态。结果 首先计算了6个工况下的热阻,发现实际换热量为540 W时,数值模拟与实验结果的最大相对误差不大于25%。然后分析了误差来源,最后得到了热沉全流道速度场与温度场的分布情况。结论 实验与模拟结果有较好的符合程度,证明了数值模拟方法的可靠性。在换热量相同时,板式热沉的热阻随流量与胀高的增大而减小,焊点为菱形排布的热沉传热性能相对更好。板式热沉内部的流场具有周期性,焊点使流体扰动增强,并形成射流和旋涡,起到了强化传热的效果。

基于响应面法的矿用翅片管空冷器参数优化

矿用空冷器是矿井降温系统常用的末端设备,优化矿用空冷器换热性能对提升矿井降温系统能效具有重要意义。采用Fluent建立了平直翅片管矿用空冷器模型,研究了翅片间距、翅片厚度、横向管间距和纵向管间距对传热因子、阻力因子和综合性能评价指标的影响规律;采用Box-Behnken响应面法,分析了双参数协同作用下传热因子、阻力因子和综合性能评价指标变化规律。结果表明:随着翅片间距、横向管间距的减小,以及翅片厚度增加,传热因子、阻力因子和综合性能评价指标逐渐增加;综合性能评价指标则随纵向管间距的增加而先增加后降低;横向管间距、翅片厚度是影响空冷器传热及流动性能最显著的两个参数,而翅片间距、纵向管间距的影响最小;得到了平直翅片管矿用空冷器的最优参数,传热因子提升16.3%、阻力因子增加26.3%、综合性能评价指标提升8.3%。研究结果可为矿用翅片空冷器设计参数优化提供指导。

泵驱动的制冷剂相变冷板冷却系统实验研究

元器件的小型化和性能的提升使电子器件功率不断提高,优越的散热性能对于保证电子器件的可靠性具有重要意义。本文搭建了泵驱动的制冷剂相变冷板冷却实验系统,设计加工了4块不同材质(铜、铝)和流道高度(10、15 mm)的相变冷板,对散热功率为200~1000 W、热流密度为4.4~22.2 W/cm^(2)的集中热源在制冷能力为3~11 kW和不同热源位置时的冷板换热性能、阻力特性及泵功耗进行了研究。结果表明:相变冷板换热性能优越,在应对散热量为1 kW、热流密度为22.2 W/cm^(2)的集中热源时,传热系数最高可达26 kW/(m^(2)·℃),在系统总阻力小于20 kPa、制冷剂泵功耗小于20 W时可以实现模拟热源表面与制冷剂温差小于15℃,可以利用天然冷源进行散热,实现节能;相变冷板的换热特性可由翅片效率计算式和Kandlikar传热关联式进行描述,使用理论公式计算出的冷板温差与实际值偏差小于1℃,可以指导冷板流道设计。

气隙对燃料板矩形通道鼓泡工况流动传热行为的影响研究

利用Fluent软件针对核反应堆板状燃料组件出现的鼓泡现象进行了数值模拟研究,通过动网格技术模拟气体和固体鼓泡的换热流动特性,并对比了裂变气体鼓泡和以往研究中固体鼓泡的区别。研究发现:气体鼓泡会导致局部温度升高,并且相较于固体鼓泡,会显著提高局部热通量密度,其中鼓泡周围热通量密度提高3倍,但燃料板整体热通量变化较小;鼓泡的形成会使局部换热能力提高约10%,鼓泡侧热通量提高约4%;在高流速条件下,鼓泡的存在会导致燃料板两侧流体产生较大的压力差,使得燃料板发生变形,甚至堵塞流道。这些发现为核燃料板的设计和安全评估提供了重要依据。

涡旋压缩机浮动式静盘的性能优化研究

针对浮动式静盘涡旋压缩机静盘背部温度过高且集中于排气管处的问题,提出带散热结构的浮动式静盘背压机构,研究了新机构的质能交换,分析了热量交换过程,给出了各腔体传热的数学模型,并对新机构进行CFD模拟计算,得到了不同外界温度下的静盘背部温度分布情况。与原浮动式静盘背压机构对比分析可得,新机构有效地改善了静盘背部中心温度过高的问题,使静盘温度分布更加均匀,可以解决因温度过高而造成的变形、摩擦磨损以及泄漏等问题,提高了压缩机的可靠性。

某温泉庄园热源方案技术对比分析

随着热泵技术的快速发展和煤改电政策的推进,空气源热泵的应用优势越来越凸显。以大连某温泉庄园热源设计为例,结合业主意愿和要求,提出三种热源方案:温泉水经板换换热的直供方案;温泉水板换换热和水源热泵联合供暖;采用空气源热泵供暖。对三种设计方案进行了技术经济对比分析,提出空气源热泵供暖方案是最经济的。在保证电力可靠,安全运行的同时,合理利用能源,提高了经济效益。

锂电池肋片液冷板的传热与流动特性研究

对于锂离子电池热管理系统,在锂电池两侧插入带通道的冷板是一种有效的高温散热方法。针对电动汽车用方形锂电池,在传统并行流道冷板的基础上,设计了六种肋片强化换热冷板。通过数值模拟方法研究和比较了不同雷诺数下冷板的换热效果,同时计算了流场的压降、摩擦因子和速度等来分析其流动特性,最后采用综合考虑流体流动和传热特性的无量纲因子来判断冷板的综合性能。结果表明,不同雷诺数下六种肋片强化冷板散热性能均优于传统并行流道冷板,但压降也不可避免地增大,各个方案中矩形交错肋片液冷板的综合性能最优。

竖直多波纹板式换热器内R134a的流动沸腾实验研究

针对竖直多波纹板片内R134a的流动沸腾进行实验研究,采用高速相机捕捉并分析两相流型的衍变规律,分析干度对换热系数、压降的影响。结果表明:板片内部的汽液相于其所处平面沿水平方向存在交替分布特征;随着干度上升,板片左上角部分流域出现相对停滞的液膜区,其面积随着干度的增加而扩大;波纹板片内的主导换热机制由核态沸腾逐渐转变为对流沸腾。当干度x较低时(x<0.4),换热系数随干度的增加缓慢增加;随后换热系数随干度的上升速率加快(0.4<x<0.6),并于x=0.6时达到最大值;高干度时(x>0.7),板片内出现局部干涸现象,换热系数迅速下降。当质量通量相同时,板片的摩擦压降随干度的增加而增加,而饱和压力的上升会使得摩擦压降有所降低,最大降幅为20.4%。

基于“口琴板”强化280 Ah储能电池组热管理的实验研究

基于对风冷电池组热管理的研究,开发了一种新型“口琴板”风冷式储能电池组热管理系统。研究了不同放电倍率下,自然对流、入口风速、环境温度对热管理系统温度场和流场分布的影响。结果表明,在1.5 C放电倍率下,入口风速为10 m/s、环境温度为25℃时,储能电池组的最高温度为42.26℃、最大温差为3.2℃。此外,“口琴板”可有效提升储能电池组内部温度分布的均匀性,显著降低电池组的最高温度,为大容量方形储能电池风冷热管理组件的开发优化提供了一定的基础理论和实验数据支撑。

混合板换热器板片结构对流道内触点分布及流动换热性能的影响

混合板式换热器作为余热利用系统中高效节能设备,其换热性能对系统效率及稳定性有重要影响。该研究基于RNG k-?湍流模型,对M型和H型板式换热器流道内流动及传热过程展开数值模拟,研究板片结构对板间触点分布的影响,及雷诺数Re和板片组合对横向流道内流动及换热特性的影响规律。结果表明:M型板间触点呈“方格”,H型呈“菱形”,且H型流流体速度较大、温度场分布较均匀。随Re数增大,压降△P及平均努塞尔数■均增大;Re数较低时,H型的■明显提高,且△P增幅较小;而高Re数(Re=6000)时,与M型相比,H型■的增幅仅为△P的25%左右。随波纹间距s减小,触点数明显增多,△P和■均增大;s为12~16 mm较合适。研究结果为混合板式换热器的设计及优化提供理论依据。

基于机器学习的印刷电路板式换热器流动换热预测与仿真

基于Zigzag形通道印刷电路板式换热器内跨临界甲烷的流动换热数值模拟结果开展通道内局部对流传热系数与压降机器学习与预测。采用微元分段法提取各通道内局部多物理场参数构建数据库,通过互信息法筛选输入参数,并根据验证集预测效果确定最佳网络结构和超参数。预测结果表明,人工神经网络模型表现最佳,预测对流传热系数的平均绝对百分比误差为2.228%,预测压降则为5.009%。利用机器学习对流动换热参数的预测开发了一种Zigzag形通道印刷电路板式换热器一维仿真方法,实现了通道内流体温度、壁温、对流传热系数和压降的快速准确预测,为换热器设计提供了新的方法。

板式气冷器中超临界CO_(2)侧表面换热系数研究

为了探究板式气冷器中主导超临界CO_(2)侧表面换热系数h最值出现的因素及最值点的中移对平均换热系数大小的影响,采用基于MATLAB的换热器分布参数分析方法,研究了超临界CO_(2)侧表面换热系数h沿气冷器板片上的分布规律,并针对h最值出现的主导因素、改变板片尺寸和流量使最值点移动对平均换热系数的影响进行研究。结果表明,伪临界温度对板式气冷器中h最值的出现更具有决定性;一定压力下的C_(p)最大值会随着压力的增加不断增大,但伪临界温度逐渐降低;减小板片尺寸或者增大质量流量,可以使得平均换热系数增加,对应的换热量也会增大,而且由质量流量增加所带来的换热量增加幅度在3种工况下分别为12.72%,12.79%,11.75%,同时压降也会增加;当h最大值保持在6170 W/(m^(2)·K)或5120 W/(m^(2)·K)附近时,最值点中移不会使平均换热系数增加明显,而且对应的换热量也会减小。