The Investigation Of Flow Field Distribution In Plate-Fin Heat Exchangers Based On PIV Technology

Plate-fin heat exchanger (PFHE) is widely used in aerospace, chemical engineering, air separation plants and HVAC industries because of its small difference in temperature, high efficiency and compactness. The longitudinal wall heat conduction, the nonuniformity of temperature field and fluid flow are the main reason that causes the deterioration of heat exchanger's efficiency. The effect of fluid flow nonuniformity on heat exchanger's efficiency is the most important one among them because it can intensify longitudinal wall heat conduction and the maldistribution of interior temperature. Therefore, researchers still pay close attention to enhance the efficiency of heat transfer by improving the fluid flow distribution.The fluid flow maldistribution in PFHE may be caused by many factors, such as the improper entrance configuration, the manufacturing tolerance and the heat transfer process, especially the improper header configuration. In this paper, the study of header configuration on fluid flow maldistribution was done and the influencing rule was found for the optimum of configuration in order to make the fluid flow more uniform in heat exchangers. So this research is highly valuable of academics and engineering.The improved header configuration with punch baffle in staggered arrangement was proposed for the first time. The fluid flow distribution in conventional header and improved header were numerically investigated using CFD. The numerical results showed that the fluid flow maldistribution is very serious in conventional header and the improved header configuration can effectively improve the uniformity of fluid flow distribution and pressure distribution. The influence of baffle configuration on fluid distribution was also investigated. The uniformity of flow field distribution in the improved header can be obviously enhanced with the proper baffle configuration.The pattern of flow field distribution and mechanics of vortices formation, development, fracture and vanishing in conventional header and improved header were analyzed based on the investigation of vortex flow pattern perpendicular to the axial. The results showed that the fluid is distributed mainly by the pressure gradient in transversal direction and vortices are generated by the flow separation in the conventional header. The vortices consume mechanical energy and distort the flow field to maldistribution at the same time. For the improved header configuration with punched baffle, although the flow field is more complex, the vortices generate in front of the baffle and the vortices quantity in front of the header outlet decreases obviously. The negative effect of vortices on the flow distribution is reduced. It was shown that the uniformity of flow distribution is able to effectively enhance through the decrease of vortices quantity in the flow field, which supplies governing theory to the improvement of header configuration.Particle Image Velocimetry (PIV) was applied to the investigation of the flow field in the header of PFHE for the first time. A series of velocity vector and streamline graphs of different cross-sections at different Reynolds numbers were achieved. The experimental results indicated that flow maldistribution is very serious in conventional header. There was swirl and back flow in conventional header because of the unreasonable structure, and the flow velocity around the axis of the inlet tube is high while the velocity far from the axis is extremely low. However, the improved header configuration with punched baffle can effectively improve the uniformity of fluid flow distribution in both radical (y-direction) and axial direction (z-direction). The flow maldistribution parameter Sv in plate-fin heat exchanger reduced from 1.210 to 0.209, and the ratio of the maximum velocity to the minimum is reduced from 23.163 to 1.756 by installing the punched baffle. CFD results are in good agreement with PIV data. The results indicate that PIV and CFD are well suitable to investigate complex flow patterns and the conclusion is of great significance in the optimum design of plate-fin heat exchanger.The experiments were carried out to investigate the performance of the resistance and temperature distribution at the outlet of the PFHE with different fluids and different Reynolds number for conventional header and two improved headers. It shows that the temperature distribution is nonuniform at the outlet of conventional header and the maldistribution deteriorates with the increase of Reynolds number and the inlet temperature difference. While the improved headers with punched baffle can effectively improve the maldistribution of the outlet resistance and temperature distribution. The results show that the improved headers can effectively improve the resistance distribution under different Reynolds number. The heat transfer coefficient can improve 12.9%～17.4%. The performance of heat transfer can be effectively enhanced with the improved header with punched baffle especially at high Reynolds number. The resistance loss between the inlet and outlet is acceptable in the industrial application in the view of heat transfer enhancement.The theoretical and experimental results show that the uniformity of flow field and temperature field is enhanced obviously in the PFHE with the improved header configuration. The temperature difference between inlet and outlet of cold fluid increases and the heat transfer efficiency increase. The research production has been partly applied in Kaifeng Air Separation Ltd.