旋转射流冲击换热液晶显示实验研究

采用热色液晶测温技术对以二氧化碳为工质的稳态射流冲击换热和管内插入扭转带方式的旋转射流冲击换热进行了实验研究。与普通射流相比,旋转射流导致驻点附近区域的换热趋于均匀化。其换热系数在大于某一半径之后高于普通射流,但在驻点附近相对较低。旋转射流对换热的此种影响随雷诺数的增大而减弱。     

离散倾斜肋的传热强化及流动特性

本文以高湿透平叶片中腔内部冷却为应用背景，对两相对壁面具有三维离散斜肋的方形收敛通道内的传热进行了实验研究。本文获得的详尽的局部换热系数图谱、平均换热系数及复杂的近壁流场，可用于强化传热的设计。     

高温壁面液体射流冲击瞬态沸腾传热的实验研究

以水作为冷却介质,对高温壁面在射流冲击淬冷时的瞬态换热特性进行了实验研究,获得了介质在不同过冷度、不同射流速度下的完整沸腾曲线。实验结果表明。无论是增大工质过冷度还是提高射流速度,总会使得热壁面的冷却速率加大。在一定的过冷度和射流速度下壁温变化呈现快－慢－快的特点。临界热流密度随平均壁温变化率的增大而增大,二者之间存在线性关系．     

HEAT TRANSFER OF ORTHOGONAL-MODE RECIPROCATING TUBE FITTED WITH TWISTED-TAPE

This article describes the results of an experimental study aimed at revealing the heat transfer physics of a twisted-tape insert in a uniformly heated tube for transitional and turbulent flow situations when the test tube remains stationary and reciprocates. Dry air is used as the test fluid, and this study is motivated by the need to understand the effectiveness of twisted-tape inserts on the heat transfer augmentation when applied to the piston cooling of a compression-ignition engine. Prior to the experimental tests, a parametric analysis was performed for which the dimensionless groups that characterized heat convection were identified and adopted to formulate the experimental program and procedures of data reduction. The parametric test matrix involves Reynolds, pulsating, and buoyancy numbers in the ranges 3,000-12,000, 0-0.9, and 0.003-0.009, respectively, with five different reciprocating frequencies tested, namely, 0, 0.83, 1.25, 1.67, and 2 Hz. The nonreciprocating experimental data confirmed considerable heat transfer enhancement due to the twisted-tape insert. When the test tube was reciprocated orthogonally to the main flow direction, both pulsating and reciprocating buoyancy forces arose, which modified the heat transfer from nonreciprocating situations. Initially, a range of heat transfer impediments relative to the nonreciprocating results that could reduce local Nusselt numbers to levels about 75% of nonreciprocating values took place when these reciprocating forces were relatively weak. Further increases of relative strengths of reciprocating forces resulted in subsequent heat transfer recovery and could lead to about 60% heat transfer enhancement relative to nonreciprocating values.     

阵列射流冲击冷却局部对流换热特性的数值计算与试验验证

本文运用数值计算的方法对具有初始横流的阵列射流在不同的排列方式、冲击间距和横流/射流质量流量比下 的流动换热进行了三维数值研究,并采用热色液晶测试技术对阵列射流冲击的冷却表面温度分布进行了试验研究。获得了 每一股射流的冲击冷却局部对流换热系数分布的特征,研究结果表明本文的计算结果与实验特征是基本吻合的。     

旋转状态半受限冲射流流动和换热特性研究

利用数值模拟的方法,通过改变冲击雷诺数Rej(20000-35000)、旋转数Ro(0-0．0117)等参数,分析旋转条件下半受限单孔冲击射流的流动和换热特性,讨论了旋转对冲击射流的流场结构和换热特性的影响．研究结果表明:冲击射流在离心力和哥氏力的共同作用下发生弯曲,造成冲击靶面上的冲击斑随着旋转数Ro的增加而不断偏移,显著影响了冲击的冷却效果．冲击靶面的平均Nu数随着冲击雷诺数Rej的增加而增加;当旋转数Ro增加时,冲击靶面的平均Nu数先增加,后减小,然后再增加．计算结果和试验结论基本一致．     

粗糙表面对非定常冲击射流传热的影响

利用在射流驻点周围设置不同高度的圆环来模拟粗糙表面,进行非定常射流冲击换热的实验研究.非定常射流由一个特殊的质量流量控制装置产生,波形和频率可调.采用高度分别为1 mm,2 mm和3 mm的圆环,研究发现环高h=3 mm时,冲击射流流场发生了实质性的改变.对于流动形态没有发生实质改变的粗糙表面,在稳定射流冲击下,传热特性没有质的变化.然而,非定常射流冲击粗糙表面时,与其冲击光滑平板相比,传热特征有很大不同,传热强化系数总体上是削弱了,随着圆环高度的增加更为明显.     

ENHANCED POOL BOILING HEAT TRANSFER WITH SURFACE ATTACHMENT

Abstract

A pool boiling experiment with either water or Freon-113 was conducted to investigate nucleate boiling from a heated wall with either a spherical or a cylindrical attachment. The result revealed that nucleate boiling can be enhanced by applying a horizontal cylindrical attachment to a vertical heated wall, owing to the favorable thermal environment characterized by a small-gradient liquid temperature profile within the restricted regions between the attachment and the heated wait. Nucleate boiling is enhanced in terms of a lower wall superheat required for incipient boiling and more bubbles generated than from an open heated wall. As a result of the enhanced nucleate boiling, heal transfer of the vertical heated wall above the attachment was improved due to excessive bubbles moving upward along the heated wall, causing removal of the thermal layer near the wall and evaporation of the thin liquid film between the bubbles and the wall. The boiling curve hysteresis with Freon-113 was significantly reduced as a result. The effects of diameter, length and surface roughness of the cylindrical attachment were also investigated.     

高效微射流阵列冷却热沉传热特性

微射流阵列冷却热沉是利用射流冲击在驻点区能产生很薄的边界层来提高换热效率,本次研究设计的热沉是5层结构的模块式铜微射流阵列冷却热沉,以去离子水为工质对传热特性进行了实验研究.结果表明,采用微射流阵列冷却不仅能通过增加驻点数目来强化换热,而且能有效地降低换热表面的温差.热沉的热阻会随着泵功的增加而降低;随着泵功的不断提高,热阻变化趋于平缓.     

脉冲流冲击冷却换热的液晶显示实验研究

采用热色液晶定量测温技术对稳态及脉冲式空气-氮气圆形浸没射流冲击冷却换热进行了实验研究.脉冲射流的实验结果表明,对于喷嘴直径D=3 mm的情形,无论脉冲频率高低,均无强化换热效果;对于D=I0 mm的情形,在较高Re数时出现个别工况的驻点换热强化.脉冲射流表现出换热系数沿径向分布趋于均匀的特点.     

水平梯度表面能材料表面上的滴状凝结换热系数

本文在均质表面的单个球缺形液滴换热模型和液滴通用尺度分布规律的基础上,结合梯度表面能材料表面的液滴分布和凝结换热特性,得到了一维水平梯度表面能材料表面上的滴状凝结换热计算式。在此基础上,研究了壁面过冷度、接触角梯度、工质物性等参数对梯度表面能材料表面滴状凝结换热性能的影响。结果表明:随着过冷度的增加和凝结工质汽化潜热的增大和表面张力的减小和接触角梯度的增大,平均表面凝结换热系数会增大。     

横肋变截面U通道内换热特性的实验研究

本实验选取航空发动机涡轮叶片中段内部冷却通道为研究对象,对不同肋间距的横肋变截面U型通道的换热特性进行了研究。实验模型中,矩形肋对称布置在上下两个表面,肋高为1.5 mm,肋宽为2 mm,肋与流体流向夹角为90°。实验结果表明,当节距比为10时,通道的平均换热效果最佳,节距比为16.6时,通道的平均换热效果次之,节距比为13.3和20时,通道的平均换热效果相当,且较差。     

微射流阵列冲击恒热流壁面换热数值模拟

微射流冲击作为一种高热流冷却技术,在大功率激光器、微电子芯片等微型高热流器件冷却方面有广阔应用前景.本文对微射流阵列冲击恒热流表面的换热情况进行数值模拟,详细分析了微射流阵列的换热特点,对比了射流孔顺排和叉排方式的冷却性能,得出射流入口雷诺数、射流孔间距、射流高度等因素对冷却特性的影响规律.     

AN EXPERIMENTAL INVESTIGATION OF HEAT TRANSFER AND FLUID FLOW IN A RECTANGULAR DUCT WITH BROKEN V-SHAPED RIBS

An experimental investigation of heat transfer and fluid flow in a rectangular duct roughened by broken V-shaped ribs pointing upstream was carried out. The rectangular duct had an aspect ratio of 1/8, and the Reynolds number range was from 1000 to 6000. Liquid Crystal Thermography (LCT) was used to obtain the detailed heat transfer distributions on the ribbed wall. The main observed characteristics include spanwise variation, local maxima, and saw-tooth fashion along the streamwise direction. These features were correlated and explained by the detailed velocity structures, observed by Particle Image Velocimetry (PIV). The flow characteristics introduced by ribs include altered spanwise profile of the mean flow velocity, a complicated secondary flow over the cross section, and flow separation and reattachment along the streamwise direction. In addition, a comparison of overall thermal and hydraulic performance with previously tested continuous ribs was conducted. It was found that the broken ribs had better overall performance in the high Reynolds number range.     

天然气催化反应壁面传热传质的数值研究

应用二维数学模型研究天然气催化燃烧壁面反应及载体气固界面上热质传递与流动耦合的现象。通过改变载体通道入口混合气体(天然气与空气)的温度、质量分数及壁面上反应的条件来评价努塞尔数和舍伍德数。模拟结果表明,努塞尔数和舍伍德数并不单独取决于燃料质量分数及壁面条件,而取决于壁面的反应速率。模拟同时表明,进口的温度对通道壁面传质影响最大,其它影响较小。     

吸附作用在纳米颗粒悬浮液换热强化中的试验与机理研究

本文通过测量SiO_2纳米颗粒在不同工质、不同浓度的悬浮液在不同温度下的导热系数,结合了液氮冷冻、切割并在断面上复形的技术,并利用电镜观察了纳米颗粒在液体中的分布、团聚和液体与之的亲和性。着重分析了纳米颗粒悬浮液中SiO_2纳米颗粒与液体的亲和性对悬浮液导热性能影响的机理。文中提出了SiO_2纳米颗粒的表面吸附层和在作布朗运动时出现”微对流”而使悬浮液换热强化。     

旋转对带冲击孔的叶片前缘流动和换热的影响

本文采用数值模拟方法,对带有供气腔和冲击腔的旋转通道内的流动和换热进行了研究,获得了旋转时叶片前缘冲击孔流量系数以及冲击靶面平均Nu数的变化规律.计算结果表明,旋转作用所产生的哥氏力改变了冲击孔的流量系数值,转速越大,哥氏力作用就越明显.流量系数随着旋转雷诺数和出流比的增加而增加,随着入口雷诺数的增加而降低.旋转对冲击靶面的平均Nu数的影响不明显,只有在较高转速时, Nu数才随着转速的增加而稍有增加.     

不连续双斜内肋管的管外换热性能

实验和数值研究了不连续双斜内肋管管外的换热性能.在不连续双斜内肋管管外倾斜凹坑的作用下,不连续双斜内肋管和光滑圆管组成的套管内出现了纵向涡流动,雷诺数为30000~90000时比光滑套管换热增强50%,阻力增加60%～70%.     

凝结表面温差对水-酒精混合蒸气换热特性的影响

本文研究了温度梯度对不同浓度水-酒精混合蒸气的Marangoni凝结的影响.搭建了具有高气密性的实验台,设计了凝结表面存在温差的实验块.在低于大气压力(31.2 kPa,47.4 kPa,84.5 kPa)的条件下,对8个浓度(0%～50%)混合蒸气进行了实验研究并实现可视化.实验结果表明温度梯度在低过冷度(<10 K)下,提高了低浓度(0.5%,1%)的水-酒精混合蒸气的换热系数.分析认为是因为表面温差带来了表面张力的差异,促使液膜内扰动增大,减少了凝结液膜热阻的缘故.     

采用多孔介质和肋片复合传热强化的数值研究

本文采用数值计算方法分析了圆管中填充多孔介质和肋片时在充分发展段的流动与传热特性.结果表明,在管内部分填充多孔介质的情况下增加适量的不连续肋片能达到如下两个目的:(1)不连续的肋片能在壁面上扰动流体来强化换热而阻力增加不大;(2)不连续的肋片能在壁面上支撑多孔介质使其在径向固定.计算结果表明,这种复合强化方法的Nu和PEC值比只在管内填充多孔介质时平均高出54%和36%.