Investigation On Pull-out Strength Of Tubes And Fins In Air Heat Exchangers

The air heat exchanger is the core equipment in the entire air conditioning system,which is related to the performance of the heat exchange in the air conditioner.The mechanical roll expansion is widely applied in the production of heat exchangers for its high efficiency and stability,but certain damage to the inner grooved structure of the seamless copper tube and deterioration in the heat transfer performance of the air heat exchanger are caused by mechanical roll expansion.During the mechanical roll expansion,the uneven force on the inner surface of tubes brings problems such as the wrinkles and bending of the tube.Hence,the Tube Hydro-Forming technology is applied in the research of the expansion of heat exchange tubes and fins.From the previous researches on the hydraulic expansion,it is found that the problems caused by mechanical roll expansion in heat exchanger production are effectively solved by the application of hydraulic expansion.Therefore,the investigation on pull-out strength of tubes and fins is of great importance to the production of the air heat exchangers.This thesis mainly discusses the pull-out strength of the heat-exchanger tube and fin in the air-conditioning heat exchanger after hydraulic expansion.The research on pull-out strength of tubes and fins in air heat exchangers is conducted in this thesis.Inner grooved copper tubes and hydrophilic aluminum foil fins are applied in the research,and the pull-out strength of the tubes and fins after hydraulic expansion is studied through actual experiments.The main research contents are as follows:?1?A device was designed and developed for the hydraulic expansion on U-tubes and fins,which is reliable and adaptable to the tubes and fins of different numbers and lengths.The self-designed hydraulic expansion device was applied in the research instead of the traditional mechanical expansion device.Non-pulsating hydraulic expansion test was performed on the test samples,which is composed of copper tubes and fins.According to the test,the appropriate hydraulic pressure range was determined based on the results of the research.?2?Based on the non-pulsating hydraulic expansion test,the pull-out force of different bulging regions of the same test sample expanded under reasonable hydraulic pressure was researched for the pull-out force.Hence,the influence of different bulging areas on the pull-out force of heat transfer tubes was summarized.?3?The pulsating hydraulic expansion test was applied to the joint of tubes and fins,which is based on the parameters of pulsating amplitude and frequency.After the expansion,the tensile test was performed on the pulsating hydraulic expansion samples,and the maximum value of the pull-out force under different pulsation parameters was studied in the uniform speed tensile test.Hence,the influence of the pulsation parameters on the pull-out strength was summarized.The results of the research show that:?1?The self-designed device for the expansion of U-tubes and fins can meet the requirements of hydraulic expansion.This device has been applied and granted for the Patent for Utility Model,and the patent grant number is201920520872.9;?2?The self-designed device was used to expand?5mm heat exchange tubes and fins,and the experimental research showed that the proper hydraulic pressure range of the tubes and fins is from 16 MPa to 18 MPa;?3?According to the tensile test of the non-pulsating hydraulic expansion samples,the maximum pull-out force F_max that the test samples can withstand is 0.47 k N,and the optimal hydraulic expansion force is 18 MPa;?4?The pull-out force of different bulging areas of the same test sample expanded under reasonable hydraulic pressure showed that the middle part of the sample withstands a larger pull-out force than the two ends,which means that compared with the other bulging areas of the test samples,the bulging strength of the middle bulging area of the test piece is higher;?5?When the pulsation amplitude?P of the test samples is 2.83 MPa,the maximum pull-out force that the samples can withstand is 0.406 k N;?6?Under the same amplitude,the expansion of the tube and the fin becomes more uniform with the frequency becomes larger,which means that the fluctuation of the pull-out force loading curve is smaller,and the expansion joint of heat exchange tubes and fins is more reliable.