Investigation On Pneumatic Expansion Technology Of Small Fin-Tube Heat Exchanger

With the implementation of China’s "carbon peak,carbon neutral" strategy,the development of high energy efficiency,small volume,low-cost heat exchanger is the main research direction of the current refrigeration industry.This project proposes the use of pneumatic expansion process for the small fin-tube heat exchanger with a tube diameter of5 mm,and carries out a numerical study of the pneumatic expansion forming mechanism of the small fin-tube heat exchanger.The mechanism of air pressure expansion and forming of small fin-tube heat exchanger and the influence of expansion process parameters on the heat exchanger expansion and forming were investigated to provide theoretical guidance for the air pressure expansion process of small fin-tube heat exchanger in engineering practice.The theoretical study,taking the small fin-tube heat exchanger as the object of analysis,reveals the mechanism of heat exchanger pneumatic expansion and forming,analyzes the stresses of the heat exchanger in the three major expansion processes,and derives the theoretical optimum value of heat exchanger pneumatic expansion pressure.In terms of numerical simulation,a three-dimensional flow-solid coupling model of small fin-tube heat exchanger is constructed,and a unidirectional flow-solid coupling method is used to study the small fin-tube heat exchanger.The heat exchanger was investigated in the one-stage air pressure path,three-stage air pressure loading path and pulsating air pressure loading path.The results show that:(1)the radial displacement of the heat exchanger tube after expansion is smaller in the horizontal direction and larger in the vertical direction.Increase the expansion pressure,the initial fin pitch of the heat exchanger,the initial gap between the tube fins,pulsating expansion amplitude or reduce the friction factor of the tube fins,heat exchanger tube wall thickness can be obtained after the expansion of greater radial displacement.In addition,the larger the heat exchanger tube outside diameter tolerance,the smaller the radial displacement of the expanded tube diameter.(2)In the process of changing the residual contact pressure between the tube and fin with the expansion pressure,it is found that the residual contact pressure is higher before the inner wall of the fin hole yields;when the expansion pressure makes the fin hole yields,continue to increase the expansion pressure will lead to incomplete expansion.At the same expansion pressure,the residual contact pressure is higher when the friction factor,pulsating expansion amplitude,initial fin pitch of heat exchanger or increasing the wall thickness of heat exchanger tube are properly reduced.Heat exchanger tube outside diameter tolerance change is small on the expansion of the residual contact pressure after the impact is not significant,but the tolerance of 0.04 mm,the residual contact pressure appears to rise abruptly.(3)heat exchanger expansion after the fin spacing and expansion pressure,pulsation amplitude is positively correlated with the tube and fin friction factor,the initial fin spacing,heat exchanger tube outside diameter and thickness is negatively correlated.(4)The overall heat exchanger post-expansion fin gap decreases as the expansion pressure,initial fin pitch,initial fin gap,heat exchanger tube thickness and pulsation amplitude increase,and the heat exchanger post-expansion fin gap increases when the heat exchanger tube outer diameter tolerance and friction factor increase.(5)In the numerical simulation,changing the holding time and pulsation frequency has no effect on the heat exchanger expansion and forming,but in the three-stage gradient path loading,the size of the last section of the expansion pressure plays a decisive role in the quality of the heat exchanger expansion and forming,and the gradient loading path has less effect on the quality of the heat exchanger expansion and forming than the expansion pressure.Experimentally,the use of self-developed air pressure expansion device for air pressure expansion heat exchanger,the heat exchanger obtained from air pressure expansion and mechanical expansion heat exchanger for heat transfer performance testing,the results of the study are as follows: air pressure expansion of the heat exchanger cooling air pressure expansion heat exchanger cooling capacity is better,the capacity deviation are less than 0.5%.