Study On Defrosting Characteristics Of Air Source Heat Pump Heat Bypass

Air source heat pump with the advantages of convenient installation,reliable operation is widely used in various industries,compared with the traditional single cooler system,the air source heat pump with both heating and refrigeration capacity,its function transformation of the reversing can be realized only through the four-way valve,because of the heat pump directly from the inexhaustible can take in the air,the unit has higher Energy Efficiency Ratio.But there are still some problems in the actual use,especially in the low temperature and high humidity air,outdoor unit in the heating operation is prone to frosting phenomenon,frost layer accumulation will lead to serious attenuation of the system heating performance,and even cause damage to the heat pump system,so the need for timely defrosting.In view of the above problems,this thesis takes the automobile air conditioning as the research object,and adopts the hot air bypass defrosting method to carry out the related experiment and simulation research.In this thesis,according to the working principle of air source heat pump heat bypass defrost system,the specific factors affecting the defrost energy are analyzed by using pressure enthalpy diagram.According to the principle of mass conservation,the model of hot gas bypass valve is established through the analysis and calculation of experimental data.Combined with the frost layer physical property and heat transfer model,the components were connected by the sequential module method,and a one-dimensional steady-state simulation model was established for the whole hot gas bypass defrost system through MATLAB software.Secondly,the frosting experiment was carried out in the heat bypass automobile heat pump air conditioning unit in the enthalpy difference laboratory.The system simulation heat production,evaporation pressure and condensation pressure were compared with the experimental data under the frosting condition,and the relative error of the system model was found to be less than 5%.The simulation results show that when the frost thickness reaches 0.6mm,the evaporation pressure and mass flow rate are the most seriously affected,which decrease by 12.6%and 13.1%respectively,while the heat production,Energy Efficiency Ratio and power consumption decrease by 9%,4.3%and 4.9%respectively.Finally,the steady-state model of hot gas bypass defrosting was used to study the defrosting characteristics of the system under different bypass opening degrees.The results show that the increase of bypass opening degree will lead to the increase of inspiratory enthalpy and the decrease of exhaust enthalpy,the increase of system defrosting energy and defrosting efficiency,and the faster defrosting speed.Combined with the energy efficiency analysis of the system in a defrosting cycle,the optimal bypass flow is 45%,and the COP_ave of the system at this time is 1.957.In addition,through the research of the compressor speed,it is found that adjusting the speed to 4000r/min and32%bypass flow during defrosting can make the system the best defrosting operation,the COP_ave at this time is 1.972,and the heat can reach 1.710kW,to meet the normal heating needs.