截面形状改进设计对缸盖鼻梁区沸腾传热的影响

针对高强化柴油机气缸盖排气门鼻梁区严重的热负荷问题,考虑到不同截面形状沸腾管冷却效果的差异性,在将气缸盖鼻梁区水腔截面结构分别简化为T形、矩形加V形及矩形加半圆形等基础上,对其截面进行了改进设计.采用气、液两相流沸腾传热计算模型,对截面形状改进前、后鼻梁区结构与冷却水腔所组成的流固耦合传热系统进行了仿真计算,计算结果与气缸盖温度场试验结果具有较好的一致性.在此基础上,采用试验设计方法,研究了截面形状改进前、后冷却水进口速度和温度对鼻梁区最高温度的影响.结果表明:截面形状改进后冷却水进口速度和温度对鼻梁区最高温度也具有重要影响;当冷却水进口速度最小或温度最大时,改进后3种截面形状都更有利于鼻梁区的沸腾冷却;改进后在相同的进口速度或温度条件下T形截面形状流道鼻梁区最高温度始终小于改进前.     

多孔微通道流动沸腾换热特性的实验研究

本文通过实验的方法对烧结的多孔微通道和铜基微通道的沸腾换热性能和流动不稳定进行研究.实验工质选用去离子水,采用的铜粉粒径分别为30μm、50μm、90 μm,烧结底厚为200 μm和400 μm.采取控制变量的方式,研究改变入口温度、铜粉粒径大小、入口流量对多孔微通道和铜基微通道换热性能的影响.研究表明:多孔微通道最优的厚度粒径比在2～5之间,在此区间的多孔微通道可以提高沸腾传热的性能.其中厚度粒径比为2和4的多孔微通道的最大换热系数是铜基微通道的换热系数的5倍.多孔微通道相对于铜基微通道有更好的换热能力,有着较低的壁面温度.     

窄通道内流动沸腾强化换热研究进展

窄通道具有结构紧凑、传热效率高等优点.随着科技发展,窄通道已经成为强化换热的常用结构形式之一,被广泛应用于各种换热设备.由于窄通道内间隙内气泡的尺寸受限,气泡在发展过程中会受到挤压而发生变形,带走大量的潜热,引起汽液界面的扰动,换热性能较常规通道有很大区别.本文综述了窄通道内的主要流型及转变准则;介绍了几何与工况参数变化对窄通道内换热效果影响的传热实验研究;分析了窄通道中传热机理以及两相摩擦压降机理,并对关联式进行了总结与评述;对窄通道内强化换热的机理与进一步强化换热的方法进行归纳总结;结合目前实验与理论研究总结了现存问题,为窄通道内流动沸腾强化换热的进一步研究提供了参考.     

气液两相流动及沸腾传热流体模化分析

基于制冷剂类物质热物性表与水和水蒸气热力性质图表,计算并比较了R12、R22、R123、R134a在相同气液密度比条件下模化水热力过程的能力;应用相似原理与量纲理论分析了流体模化方法在研究气液两相流动特性、高温高压传热过程、临界热流密度现象以及近临界和超临界传热方面的优势与方法.结果表明:制冷剂类物质具有良好的热力性质,其中R134a在相同气液密度比条件下的压力值仅约为水的1/5,而且其臭氧消耗潜能值为0,是环境友好的理想模化流体.提出并探讨了从本质上革新准则数以及基于模型探索新的模化条件和发展新的准则关系式的流体模化技术发展思路.     

沸腾对柴油机冷却系统数值模拟的影响

计算对比了不考虑沸腾和考虑沸腾2种冷却系统数值模拟计算,得出结论:考虑沸腾传热对内燃机冷却水腔内流动与压力的分布影响不明显,而对冷却水腔内传热过程的影响是很大的。若只考虑纯对流传热,计算结果可能与实际情况存在很大的差异。因此,在对强化内燃机进行流动与传热问题的研究时必须考虑沸腾传热的因素,以获得更为真实、准确的结果。     

汽（气）液两相流中雾滴尺寸的测量

本文介绍了西安交通大学开展两相流雾滴测量技术的研究工作,利用取样法与雾滴图象自动分析系统对两相流中的液滴尺寸及其分布进行了测定,提供了一组典型用例的分析结果,并与人工统计作了比较。图象分析系统以AppleⅡ计算机和普通NSX—20A摄象机为基础,从而避免了要求昂贵的,专门用途的设备。对自发凝结生成的小雾滴采用改进后的消光法测量装置来测定,试验结果表明,其测量稳定性和测量精度均较前有大的提高。     

一种适用于缸盖水腔沸腾传热计算的模型

基于VC 6.0开发了一种单相流沸腾传热模型,通过引入空泡份额的概念将沸腾发生时的流场看作一个气液均匀混合的单相流,从数学上对该模型进行了描述并介绍了模型的数值实现方法.通过与试验结果的对比,表明模型适用于缸盖冷却水腔内沸腾传热计算.试验和计算结果还表明,压力对沸腾传热的影响较为明显.最后以226B型发动机水腔为工程应用对象,计算出了水腔内的空泡份额分布和水腔内的流速分布情况.     

内燃机冷却水腔表面形貌对沸腾换热影响的研究

愈发严重的能源和环境问题要求内燃机的结构更加紧凑和复杂,导致缸盖等零部件所受的热负荷急剧增加,这对其内部冷却水腔提出了更高的换热要求。对流换热已不能满足苛刻的冷却要求,沸腾换热应运而生。据气泡动力学可知,汽化核心更倾向于分布在狭窄的凹坑处,因此在加热面上布置一定的表面形貌能够有效提高沸腾换热效果。文中设计4种形貌,并比较分析不同形貌的强化换热能力。结果表明:圆柱形凸起形貌强化沸腾换热和对流换热的效果最好,其次是圆柱形凹坑形貌,圆台形凹坑强化沸腾换热的效果好于半球形凹坑形貌,但强化对流换热的效果比半球形凹坑形貌差。不同凹坑形貌,汽化核心更倾向于分布在凹坑底部的狭窄区域,圆柱形凹坑底部狭窄区域的范围最广,有利于汽泡成核,圆台形凹坑次之,而半球形凹坑内部较为平整,不利于成核。     

竖直矩形狭缝通道内环状流沸腾换热分析模型

对竖直矩形狭缝通道内有液滴卷吸环状流阶段流动沸腾进行分析。以液膜紊流的动量方程和能量方程为基础,加上相应的边界条件和使控制方程组封闭的经验关系式,建立了环状流的教学模型并进行数值计算,得到了矩形狭缝通道内的液膜厚度分布、沸腾传热系数等结果;将模型预测的换热系数同实验关系式作比较,最大相对误差为17．8％。     

外加电场强化苯自然对流和沸腾换热的试验研究

对沉浸在非极性有机液体工质苯中的平板表面自然对流和沸腾换热的外加电场强化进行了试验，得出了自然对流和沸腾换热的换热系数、强化效果与电场电压、热流密度的关系。试验数据表明外加电场对平板表面苯的自然对流换热和沸腾换热都有一定的强化效果，但外加电场对平板表面苯自然对流换热的强化效果明显好于对沸腾换热的强化效果；且平板表面苯的自然对流换热的强化效果与试验所给定的热流密度无关，而外加电场对平板表面苯的沸腾换热的强化效果随热流密度的增大而减弱。     

考虑沸腾和缸内局部传热的缸盖流固耦合传热分析

以某商用车直列6缸柴油机作为研究对象,基于缸内传热模型获得内燃机缸盖和缸套的燃气侧局部传热边界条件;基于均相流沸腾传热模型获得水侧传热边界;实现水侧、燃气侧边界与结构温度场计算的耦合,并判断水腔内沸腾传热的状态。结果表明:缸盖温度计算值与实测值吻合,缸盖最高温度位于缸盖底面两个排气门之间;排气门之间的燃气传热系数和燃气温度均处于较高值,缸内局部传热显著;在缸盖底面中心和排气门附近水腔内的冷却水处于部分发展泡核沸腾状态。     

一种适用于缸盖水腔沸腾传热计算的模型

基于VC＋＋6．0开发了一种单相流沸腾传热模型,通过引入空泡份额的概念将沸腾发生时的流场看作一个气液均匀混合的单相流,从数学上对该模型进行了描述并介绍了模型的数值实现方法。通过与实验结果的对比,表明模型适用于缸盖冷却水腔内沸腾传热计算。实验和计算结果还表明,压力对沸腾传热的影响较为明显。最后以226B型发动机水腔为工程应用对象,计算出了水腔内的空泡份额分布和水腔内的流度分布情况。     

一种适用于缸盖水腔沸腾传热计算的模型

基于VC 6.0开发了一种单相流沸腾传热模型,通过引入空泡份额的概念将沸腾发生时的流场看作一个气液均匀混合的单相流,从数学上对该模型进行了描述并介绍了模型的数值实现方法。通过与实验结果的对比,表明模型适用于缸盖冷却水腔内沸腾传热计算。实验和计算结果还表明,压力对沸腾传热的影响较为明显。最后以226B型发动机水腔为工程应用对象,计算出了水腔内的空泡份额分布和水腔内的流度分布情况。     

Bubble Dynamics and Heat Transfer during Pool and Flow Boiling

A large number of studies of bubble growth rate and departure diameter have been reported in the literature. Because of uncertainty in defining the shape of an evolving interface, empirical constants are invariably used to match the model predictions with data. This is especially true when force balance is made on a vapor bubble to determine the departure diameter. In this paper, the results of an alternate approach based on a complete numerical simulation of the process are given. Single and multiple bubbles are considered for both pool and flow boiling. The simulations are based on the solution of the conservation equations of mass, momentum, and energy for both phases. Interface shape is captured through a level set function. A comparison of bubble shape during evolution, bubble diameter at departure, and bubble growth period is made with data from well-controlled experiments. Among other variables, the effect of magnitude of gravity and contact angle is explicitly investigated.     

Bubble Dynamics and Heat Transfer During Pool Boiling on Wettability Patterned Surfaces

Surfaces with spatial wettability patterns have been proven to enhance heat transfer coefficient and critical heat flux in pool boiling. To understand the physical mechanism behind this phenomenon and obtain the correlation among some critical parameters (bubble departure frequency, bubble size, nucleation site density, surface tension), pool boiling experiments were conducted. A Pyrex glass with a layer of indium-tin-oxide was used as the substrate. Hydrophobic patterns will serve as nucleation sites. Experiments were conducted in deionized water under atmospheric pressure at a relatively low heat flux. The processes of nucleation, growth, and departure of individual bubbles were visualized by using a high speed camera through the bottom of the heater surface. It has been found that the patterned surface performed the best in heat transfer for subcooled pool boiling when compared with hydrophilic and hydrophobic surfaces. The nucleation site density of the biphilic surface was much higher, when compared with that of the homogeneous surface. The individual bubbles always nucleate on the edge of the hydrophobic and hydrophilic area, and then move onto the hydrophobic pattern. Most of the individual bubbles detach from the wettability patterned surface in the diameter range from 300 µm to 450 µm (around 77.3%). The bubble departure periods scatter in the range from 80 ms to 1500 ms.     

Forced Convection Film Boiling Heat Transfer Over a Vertical Cylinder

The knowledge of subcooled film boiling heat transfer is important as the basis of understanding the reflooding phenomenon during emergency cooling in a nuclear reactor under a loss-of-coolant accident. In this study, forced convection film boiling heat transfer from a vertical cylinder in Freon-113 flowing upward along the cylinder was measured for the flow velocities ranging from 0 to 1.3 m/s, and liquid subcoolings ranging from 0 to 20 K at pressures near atmospheric. A platinum heater with a diameter of 3 mm was heated by electric current. The heat transfer coefficients obtained are almost independent of vertical positions on the cylinder. The heat transfer coefficients are almost independent of velocity for the velocities lower than about 1 m/s and become higher for the velocities higher than 1 m/s. The heat transfer coefficients at each velocity are higher for higher liquid subcoolings. Improvement of film boiling heat transfer from the vertical cylinder with the increase in flow velocity is much less than that of horizontal cylinder in cross flow previously reported by the authors. This is mainly due to the difference of heat transfer enhancement mechanism; the former is the drag force on vapor flow acted by a liquid flow, and the latter is the pressure gradient near the front stagnation point caused by external potential flow.     

A Study of Bubble Behavior and Boiling Heat Transfer Enhancement under Electric Field

The effect of a D.C. electric field on nucleate boiling heat transfer for refrigerants, R11, R113, and FC72, was investigated experimentally in a single-tube shell/tube heat exchanger by using the temperature control method of wall superheat. Also the behavior of bubble under nonuniform electric field produced by wire electrodes was studied by numerical calculation. For R11, the electrohydrodynamic (EHD) enhancement for boiling heat transfer was observed for all ranges of wall superheat tested. However, the enhancement in boiling heat transfer disappeared if the wall superheat exceeded 13°C for R113, and no electric field effect on the boiling heat transfer was observed for FC72. An application of approximately 5 kV was enough to eliminate the boiling hysteresis for R11 and R113. Numerical study of the electric field in a single medium has hinted that the bubbles are forced away from the heating surface and toward the electrostatic stagnation point by the dielectrophoretic force. Such modified bubble motion turns out to promote the boiling heat transfer if one uses proper electrode configuration.     

缸盖鼻梁区沸腾汽泡演化行为可视化试验研究

以某增压柴油机为研究对象,搭建了气缸盖鼻梁区内沸腾汽泡演化行为可视化试验平台,拍摄标定工况下鼻梁区域内汽泡成长、滑移、聚合等行为,开展了缸盖水腔内沸腾换热和汽泡演化行为特性的研究。研究结果表明,在成长开始阶段,汽泡成长平均直径的增长速度比较快,然后逐渐下降且有着稳定的趋势。在不同曲轴转角下,汽泡当量直径分布符合正态分布规律,且汽泡的聚合现象越多对整体汽泡行为的随机性影响就越强烈。     

基于Chen模型的缸盖冷却水腔内沸腾传热研究

基于Chen模型研究了柴油机模拟冷却水腔内的沸腾传热,并与试验数据进行对比,验证了模型的适用性,并将此模型应用于柴油机缸盖及冷却水套内的耦合传热计算。计算结果表明:沸腾传热可有效提高冷却水的换热能力,降低冷却水套壁面局部高温区域的温度,降低缸盖本体的温度梯度,从而降低缸盖热负荷及热应力;考虑沸腾时,缸盖局部温度点仿真计算结果与试验结果误差更小。