| Plate-fin heat exchanger is one of the core equipment of industrial thermal system.It has the advantages of compact structure,high thermal efficiency and small size.It is widely used in aerospace,vehicle manufacturing,resource exploitation,air separation and other fields.The air resistance value of a heat exchanger is one of the important parameters which reflects the heat transfer performance.Therefore,the high-precision measuring of the air resistance value is a key step in the development of the plate-fin heat exchanger.This project has developed a set resistance characteristic test system suitable for 400～12000m~3/h air volume.The air flow characteristics in this system and the structure characteristics of the measuring device are analyzed by using CFD software,and the correction equations for higg-precision measurement of the air resistance of heat excharger in open environment is given.The main research contents and conclusions are as follows:(1)A high-precision test system has been designed to test the air resistance characteristics of plate fin heat exchanger at 400～12000 m~3/h air volume.The mechanical structure has been designed and manufacted,and a control and test platform has been installed and debugged in Hangyang Ltd.(2)The structure and test principle of air resistance test subsystem and air volume control subsystem are introduced in detail,and the test principle and calculation method are studied according to the theory of fluid mechanics.The design requirements of the measure and control software are analyzed detailedly,and the software has been designed with C#.Aiming at the air volume control,a fuzzy PID control method is adopted to realize a faster,accurater and stabler control method.(3)According to the air resistance loss in the measuring pipeline will affect the accurate measurement of gas resistance,Fluent software is used to simulate and analyze the local resistance in the reducer,diffuser and straight pipe in the measuring system.The results show that the local resistance of the reducer can be neglected due to its large inlet diameter,slow wind speed and small vorticity;However,the local resistance coefficient of the diffuser varies with the air flow rate,so it needs to be corrected by fitting formula.The simulation results show that the fitting results of the diffuser’s resistance loss given in this paper are more accurate than those given in the existing literature under medium and high air flow.In addition,the difference between the simulation results of straight pipe resistance and the results given in the existing literature is very small,so the existing resistance formula is still used.(4)The rationality of measuring point distribution,multi nozzle combination mode and installation position of air flow measurement and control box is analyzed by using Fluent software.The results show that the best pressure measuring point for air flow measurement is within the range of 20～40mm from the nozzle outlet.When selecting nozzle for air volume measurement,single nozzle is more accurate than multi-nozzle,and the closer the upper limit of multi-nozzle measuring range is to measuring air volume,the more accurate it is.The nozzle layout in different directions on the baffle plate has little influence on the air flow measurement.The optimum distance between multiple nozzles is 2.5～3 times of the maximum nozzle diameter.(5)In this paper,the hardware and software of the heat exchanger resistance special test system are debugged,and the repeatability tests of different types of heat exchangers are carried out.The test results show that the system can efficiently and stably complete the task of high precision measurement of air resistance of plate-fin heat exchanger. |
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