【作者】 焦安军；

【导师】 厉彦忠；

【作者基本信息】 西安交通大学 ， 动力工程及工程热物理， 2002， 博士

【摘要】 板翅式换热器在航空、化工、空气分离、HVAC等工业领域得到广泛应用,它属于紧凑式换热器并具有小温差、高Ntu等特点。引起该类换热器整体效能下降的主要因素包括换热器内部的温度场分布不均匀、流场分布不均匀以及纵向传热。这三大影响因素对换热器整体效能的影响存在着一定的耦合关系,这种耦合主要体现在物流分配的不均匀会加剧换热器内部温度场分布不均匀和纵向传热,从而加剧了换热器整体效能的下降,所以物流分配不均匀性对换热器效能的影响在三者中是最主要的。因此如何改善换热器内部的物流分配,从而改善换热器整体效能的研究,一直得到了国内、外专家的共同关注。引起板翅式换热器内部物流分配不均匀性的因素是多方面的,其中封头结构、导流片型式与导流角度、入口管管径等是引起其内部物流分配不均匀性的重要因素。因此对板翅式换热器入口管、封头结构、导流片结构进行研究,分析得到上述因素对换热器内部物流分配不均匀性影响的规律,进而对上述部件的结构型式进行优化设计,最后达到改善换热器内部物流分配的目的,这一研究具有重要学术价值与工程意义。本文建立了模化研究板束单元体物流分配实验台,对目前工业设计中广泛使用的板束单元体进行了结构模化,以便研究其内部物流的分配情况。研究结果表明:目前空分系统中广泛使用的导流片与封头结构对入口物流分配产生较大不均匀,板束单元体内部通道的最大流速与最小流速之比高达3～6倍。研究还表明流体雷诺数对板束单元体截面物流分配均匀性的影响是非常严重的。首次提出在导流区内部增设补液腔的设计思想,并定义了导流片结构参数h/H和物流分配不均性参数S。模化制作了6种不同导流角度、4种不同结构参数的导流片,实验研究了不同工况下,各种导流片结构对板束单元体截面物流分配不均匀性的影响,得到了不同导流角度、不同导流片结构参数h/H的物流分配不均性参数S与流体Re之间的关系曲线,并建立了拟合关系式。首次提出二次封头概念,定义了当量截面积和当量截面直径来描述其流通结构特征,模化设计制作了三种不同封头结构并进行了实验研究,研究了入口管管径D、一次封头出口当量截面直径Φ1、二次封头出口当量截面直径Φ2三者之间的关系以及它们对板束单元体内部物流分配不均匀性的影响。结果表明:当Φ1 /D=Φ2/Φ1时,板束单元体截面物流分配最为均匀。通过实验研究建立了不同封头结构条件下,物流分配不均性参数S与流体Re之间的关系曲线,并给出了拟合关联式。实验研究表明:通过对封头结构、导流片导流角度以及导流片结构参数等的优化组合设计,换热器入口物流分配特性得到了显著改善,板束单元体内部通道的最大流速与最小流速之比由原来的3～6倍降低到1.2～1.4倍。建立了物流分配不均匀性对换热器整体效能影响的理论分析模型。分析表明:物流分配的不均匀性引起板翅式换热器整体效能的下降量随换热器的Ntu值的不同而变化,Ntu值越大换热器效能下降量越大。在分析实验数据的基础上,根据该模型可方便的计算由于物流分配的不均匀性引起的换热器整体效能下降量。更多还原

【Abstract】 Plate-fin heat exchanger (PFHE) is widely used in industry, such as aerospace, chemical engineering, air separation plants and heating, ventilation, air condioning(HVAC), because of its small temperature difference, high NTU(Number of Transfer Units) and compactness. The main factors that can cause the deterioration of heat exchanger’s efficiency are longitudinal wall heat conduction, nonuniformity of temperature field and fluid flow. These factors could be coupled and it is especially true for the compact heat exchanger. The effect of fluid flow nonuniformity on heat exchanger’s efficiency is the most important one because it can intensify longitudinal wall heat conduction and the maldistribution of interior temperature. Therefore, how to improve the fluid flow distribution in order to enhance the efficiency of heat exchanger attracts lots of researchers’attention.Fluid flow maldistribution in PFHE may be caused by many factors, such as improper entrance configuration, various manufacturing tolerances and the heat transfer process. Therefore, it is important for both scientific research and engineering application to study the effects of these factors on fluid flow maldistribution so as to have optimum design of them and make the fluid flow more uniform in heat exchangers.The experimental system, which is simulated according to the configuration of PFHE unit, is set up. The experimental results show that the fluid flow maldistribution caused by the currently used header and distributor is very serious. The ratio of maximal and minimal velocity of channels of the PFHE is about 3～6. The results also display that the fluid Reynolds number has great effect on fluid flow nonuniformity.The original design idea that adds a complementary fluid cavity in the distributor is put forward for the first time. The definitions of distributor configuration parameter h/H and flow maldistribution parameter S are brought forward, too. Six different inlet angles and four types different h/H distributors are designed and manufactured. The experimental study of the effects of each distributor under different work conditions on fluid flow maldistribution in the PFHE has been completed. The correlations between the flow maldistribution parameter S and Re under different inlet angles and configuration parameter h/H have been obtained by the experimental study.It is for the first time that the author brings forward the concept of second header configuration. The definitions of the equivalent area and the diameter of it are defined in order to describe the characteristics of second header configuration. Three types of second header configuration are designed and manufactured. The experimental investigation of the effects of the inlet pipe diameter (D), the first header’s diameter of equivalent area (Φ1 ) and the second header’s diameter of equivalent area (Φ2) on the fluid flow maldistribution in the PFHE is performed. The results indicate that the fluid flow distribution in PHU is more uniform whenΦ1 /D equal toΦ2/Φ1 . The correlation of S and Re is obtained under different header configurations.The experimental studies prove that the performance of fluid flow distribution of the heat exchanger’s entrance can be improved greatly by the combined optimum design of header configuration, the distributor’s inlet angle and its configuration parameter h/H. The ratio of the maximal velocity and minimal velocity in the channels of PFHE drops from 3～6 to 1.2～1.4.A mathematical model describing the effect of fluid flow nonuniformity on the performance of compact heat exchanger is advanced. The computation results also show that the effects of fluid flow maldistribution on the deterioration of heat exchanger’s efficiency change with the value of NTU and the effect would be more serious when NTU becomes large. The model can be used to calculate the deterioration of efficiency of the heat exchanger due to flow maldistribution.更多还原