Fluid Flow Distribution Performance At Heat Exchanger's Entrance And Its Optimization

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.