Heat transfer to molten polymers flowing through tubes

A theoretical and experimental study of heat transfer to polymer melts flowing through circular tubes is presented. The mathematical model provides for shear heating and expansion cooling effects, and also heat of reaction during flow for various wall boundary conditions. Experimental results, obtained using low density polyethylene, show reproducible temperature and velocity profiles. The measured inlet melt temperature profile and the axial wall temperature profile provide the boundary conditions for the calculations. The experimental data confirm the predictions of the magnitude of the shear heating and expansion cooling effects during tube flow.     

系统压力影响下的喷雾冷却特性及温度均匀性

喷雾冷却是一种新型的高热通量换热方式。对以水为工质的封闭式喷雾冷却进行了实验研究,并结合实验现象对不同系统压力下喷雾冷却的换热特性及表面温度不均匀性进行了分析,并给出了相应的量纲1压力换热关联式。实验结果显示:由于系统压力较低,喷雾工质在较低的温度下进入两相区,其换热能力远远高于常压下的换热能力,且换热性能随系统压力减小呈指数性增大关系;在实验条件中,可以实现在85℃的平均壁面温度下,热通量达到360W.cm-2;低压下喷雾冷却被冷却壁面的温度不均匀性小于常压下温度不均匀性。     

微尺度通道内R134a的冷凝传热实验研究

建立采用射流冲击进行制冷剂冷却的冷凝传热实验系统,对当量直径为0.63 mm矩形微尺度通道内制冷剂R134a的冷凝传热特性进行研究。实验参数范围是制冷剂干度0~1,质量流率115~290 kg/(m²·s）,饱和压力0.35~0.5 MPa,实验获得了不同工况下微尺度通道的局部冷凝传热系数,并分析了制冷剂各参数对冷凝传热的影响。实验结果表明：冷凝过程中沿制冷剂流动方向,局部冷凝传热系数会随着干度减小而减小;在一定饱和压力下,局部冷凝传热系数与局部热通量相对应;冷凝传热系数随着饱和压力减小而增大。基于实验数据,整理出适用于本实验工况下微尺度通道内R134a的冷凝传热计算公式。     

真空闪蒸冷却换热机制及流量影响

通过对真空闪蒸冷却机理的分析,基于膜-渗透理论建立了以水为工质的真空闪蒸冷却换热模型,并利用实验研究和模型模拟相结合的方法,对闪蒸冷却的换热特性进行了研究。模型分析结果和实验结果误差小于10%,证明了所建模型的准确性。研究结果表明：液膜闪蒸换热在高热通量闪蒸冷却中占主导地位;随着工质流量的增加,闪蒸冷却换热性能提高;工质流量越小,液膜闪蒸换热所占份额越大,即工质闪蒸所带来的潜热利用率增加。研究结果为真空闪蒸冷却在航天热控方面的应用提供了理论基础。     

微小菱形离散肋通道中R134a的冷凝换热实验研究

建立了采用空气射流冲击冷却方法的冷凝换热实验系统,对R134a在铝质微小菱形离散肋通道中的冷凝换热特性进行了实验研究。实验工况范围为制冷剂干度0~1、饱和压力0.50~1.50 MPa、制冷剂质量流率160~380 kg/(m²·s)、热通量10.1~59.8 kW/m²。实验获得了不同工况下的通道局部冷凝传热系数,分析了干度、饱和压力、质量流率以及热通量对冷凝换热的影响规律。实验结果表明：局部冷凝传热系数随干度、质量流率和局部热通量的减小而减小,随饱和压力的降低而增大,其中在干度x>0.4的区域内质量流率对于冷凝传热系数的影响效果更为明显。基于实验数据,提出了一个适用于本实验中微小菱形离散肋通道的冷凝换热计算公式。     

EXPERIMENTAL INVESTIGATION OF HIGHLY EFFECTIVE COOLING WITH AN EVAPORATING ANNULAR FLOW IN A SMALL VERTICAL TUBE

An experimental study was performed to investigate the heat transfer characteristics of the non-boiling annular two-phase flow of nitrogen gas and subcooled water through a small vertical heated tube with uniform wall heat flux. The experimental results were compared with the numerical calculations based on the two-phase flow boundary layer model. The experimental data show that the annular two-phase flow of nitrogen gas and subcooled water in a small vertical tube can provide quite high heat transfer ability, while wall temperatures lie in a low range even if at quite high wall heat fluxes. In particular, the highest values of the wall temperatures are insensitive to the change of flowing and heating conditions. This characteristic is quite suitable for highly effective cooling of polymer electrolyte membrane fuel cells.     

EXPERIMENTAL INVESTIGATION OF HIGHLY EFFECTIVE COOLING WITH AN EVAPORATING ANNULAR FLOW IN A SMALL VERTICAL TUBE

An experimental study was performed to investigate the heat transfer characteristics of the non-boiling annular two-phase flow of nitrogen gas and subcooled water through a small vertical heated tube with uniform wall heat flux. The experimental results were compared with the numerical calculations based on the two-phase flow boundary layer model. The experimental data show that the annular two-phase flow of nitrogen gas and subcooled water in a small vertical tube can provide quite high heat transfer ability, while wall temperatures lie in a low range even if at quite high wall heat fluxes. In particular, the highest values of the wall temperatures are insensitive to the change of flowing and heating conditions. This characteristic is quite suitable for highly effective cooling of polymer electrolyte membrane fuel cells.     

Temperature gradients in molten polymers during the cooling process

Temperature gradients of various molten polymers during the cooling process were examined using a cooling jacket apparatus coupled with a novel temperature sensor. The temperature gradients appeared to be influenced by the heat transfer between the cooling medium and the polymer, the melt temperature, and crystallization temperature of the polymers. The results were then compared with theoretically generated data. The simple theoretical model yields a good comparison with the experimental data at the center of the duct but the accuracy is more limited at other radial positions. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3268–3274, 1999     

高湿度工业废气冷凝脱湿模型研究与数值模拟

针对高湿度工业废气冷凝脱湿进行模型研究和数值模拟,引入分配系数α表征雾状冷凝和膜状冷凝并存的权重.恒壁温冷凝管外混合气体在环形空间湍流冷却冷凝的温度分布、湿度分布及其梯度（传热传质推动力）分布的模拟结果显示,雾状冷凝的控制机理是冷壁面附近温度梯度与湿度梯度协同作用下传热传质产生的局部过饱和;膜状冷凝从冷壁面移出大量冷凝潜热,促使气相主体传热传质过程更迅速,脱湿效果更好.实际过程介于二者之间.DAP尾气冷却冷凝现场实验传热传质数据,在水汽冷凝减量34%～57%的范围内,与α＝0.2的数值模拟结果相当吻合,验证了本文的模型与数值模拟.     

搅拌塔式反应器传热特性研究

分析了搅拌塔式反应器（ＳＣＲ）中多级冷却盘管隔板的传热特性,用稳态和非稳态的传热方法进行了实验研究。提出了同时考察拌雷诺数和轴向液流雷诺数影响的ＳＣＲ中多级冷却盘管隔板表面传热膜系数的关联式。研究结果表明,对于ＳＣＲ中冷却盘管隔板表面的传热膜系数,轴向液轩诺系数的影响与搅拌雷诺数的影响相比具有同等的重要性。     

中空制品冷却过程的模拟分析

通过对中空制品的冷却过程进行分析,建立了冷却过程中的物理模型和传热方程,并以高密度聚乙烯为例,采用ANSYS有限元分析软件进行了模拟分析,得到冷却曲线,对中空制品的冷却过程提出了相应的观点。     

风洞真空排气系统直接接触传热过程

某风洞真空排气系统在运行过程中,会产生大量含水蒸气的混合气体。为了提高排气效率降低能耗,本文引入冷凝塔工艺,采用直接接触换热冷凝方式使来流含水蒸气的混合气体降温冷凝。基于微元塔高传质模型,对进入冷凝塔的热流气体与冷却水直接接触换热过程分析,推导出传质系数数学方程表达式。结合实验数据考察了进气压力与冷凝降温排出气体中水蒸气含量的影响,并拟合得到一定温度下进气压力与该气体水蒸气含量的数学表达式;也考察了冷却水质量通量和气液比变化对传质系数、体积传热系数、出气温度的影响,在此基础上拟合得到针对风洞气流直接接触换热气液比与体积传热系数数学关系式,并计算出最优气液比。实验得出的规律对风洞气流的直接接触换热优化设计和应用起到了指导作用。     

低熔点熔盐蓄热罐内温度分布与散热损失实验

为了研究熔盐蓄热罐内的温度分布变化与散热规律,采用课题组搭建的槽式聚光熔盐集热、传热与蓄热实验台中的熔盐罐作为实验装置,通过加热、内循环及冷却等实验,得到了罐体在各运行状态下的温度分布数据,并分析拟合得到了自然冷却降温阶段熔盐罐散热功率随时间和温度的变化曲线。实验结果表明：加热时,加热器下部熔融盐出现明显的温度分层现象。混合均匀后的冷却降温过程,熔融盐基本不出现温度分层现象。     

螺旋隔板套管换热器壳程传热与压降的数值模拟

以水-润滑油换热为对象,对螺旋隔板套管换热器的壳程传热与压降性能进行了实验研究与数值模拟。通过威尔逊图解法获得了管程的传热系数,并计算出了壳程的努塞尔特准数。采用Fluent软件模拟了润滑油在螺旋隔板套管换热器壳程层流流动时的流场、温度场以及传热与压降性能。结果表明,流体在螺旋隔板换热器的壳程流动均匀,在隔板附近没有返混和流动死区,但温度梯度最大。在相同雷诺常数下,壳程的努塞尔特准数和压降模拟值分别比实验值高1.3%～8%和4%～38.1%,模拟值与实验值吻合较好。     

水质参数对交叉缩放椭圆管污垢的影响

采用对比的实验方法对交叉缩放椭圆管与光管的污垢热阻进行了分析,并重点研究了冷却水（松花江水）水质参数对污垢产生的影响。实验结果表明：在低流速时,交叉缩放椭圆管的污垢热阻渐近值比光管低,且没有明显的诱导期;实验中冷却水水质参数的变化对污垢影响明显。并深度分析了水质参数随时间的变化及水质参数变化对强化换热表面污垢特性的影响,为换热器提高换热效率提供了参考。     

V型导液槽对HFC245fa水平管束外冷凝换热影响

试验研究了HFC245fa在水平光管与强化管管束外的冷凝换热特性,提出在管束中加装V型导液槽来消除管束效应的方法,并通过试验研究了V型导液槽对管束冷凝换热特性的影响。试验管束由4列排深为5排的列管构成,带导液槽管束的前4排管下方加装了两段长度为450 mm的导液槽;试验换热管公称外径为19.05 mm、换热长度为1000 mm。试验中,以Wilson描点法获得强化换热管水侧对流传热系数,以对比试验方法研究了V型导液槽对水平管束外冷凝换热性能的影响。结果表明：传统的管束效应模型仅在较小的热通量范围适用;凝液在管间的迁移形态与流态随管上作用凝液量的系列变化是导致管束效应变化的主要因素;加装导液槽可有效控制凝液在管间的迁移,控制管束效应;加装导液槽使光管单管冷凝传热能力下降10%左右,使光管管束总体换热能力提升4.5%相似文献   

低质量流速下超临界CO2在管内冷却换热特性

开展了低质量流速下超临界CO₂在水平直管内冷却过程的换热特性的实验研究。实验压力为p=7.5~9.0 MPa,质量流速为G=79.6~358.1 kg·m^-2·s^-1,流体温度为25.0~50.0℃。分析了质量流速、压力、流体温度对换热的影响,并引入Richardson数阐述浮升力对超临界CO₂在水平直管内冷却换热影响。实验结果表明: 传热系数随着质量流速的增加而增大。传热系数峰值点随压力的升高向高温区偏移。当质量流速较小时,传热系数峰值点出现在准临界温度之前,且浮升力作用加大,流体处于混合对流状态。将传热系数的实验值和已有的换热关联式计算值作对比后发现在低质量流速下误差较大,拟合了低质量流速工况的超临界CO₂在水平直管内冷却换热的关联式,94%的实验值和拟合关联式误差在±20%范围内。     

新型闭式冷却塔传热传质的实验研究

建立了椭圆管式闭式冷却塔的实验测试平台,通过改变管内水进口温度和流量、空气质量流量、空气干湿球温度、喷淋水流量等以测试其传热性能,采用Poppe和Dreybal的假设处理数据,得到了管外水膜对流传热系数和水膜与空气传质系数,实验结果表明:水膜传热系数是空气质量流量和喷淋水温度的函数,与已知文献中Mizushina,Ni...     

Simultaneous Aerodynamic and Thermal Analysis during Cooling of Stacked Spheres inside Ventilated Packages

Aerodynamic and thermal analysis during forced convection cooling of produce was conducted by modeling coupled airflow and heat transfer. Air velocities and heterogeneity indexes were predicted for different configurations of package openings at an airflow rate of 0.022 m³/s. Predicted temperature profiles were compared with experimental data for model validation. Good agreement between model prediction and measured data was obtained. The results showed that airflow distribution during the process was not homogeneous. More uniform airflow distribution was obtained by increasing the vent area from 2.4 to 12.1 %. The highest cooling heterogeneity index (108 %) was recorded at 2.4 % vent area whereas the lowest heterogeneity index (0 %) was detected in a package with 12.1 % vent area. Proper package vent design is necessary to provide more uniform cooling operation during the forced convection cooling process.