Research On Heat Transfer And Flow Characteristics Of Air-Water Heat Exchanger

The energy consumption of the building cooling and heating source system accounts for a large proportion of the total building energy consumption.How to improve efficiency and reduce energy consumption has always been a hot issue in the field of energy systems.The study on the heat exchange principle and pressure drop characteristics of air-cooled,water-cooled and air-water dual-purpose heat exchangers is of great significance to the practical application of heat exchangers and to decrease the energy consumption of building cooling and heating source systems.This study first takes the heat transfer performance of the air side of the heat exchanger as the research subject and uses the computational fluid dynamics(CFD)method to simulate its flow and heat transfer characteristics.the orthogonal test is conducted to determine a variety of model conditions.In this study,the numerical calculation method is employed to compare and analyze the Nusselt number and the resistance factor of the simulation results of the calculation condition of the heat transfer load of 15 kW,design air temperature 30℃,inlet wind speed 1-5m/s,tube wall temperature 40-60℃,fin spacing 1-5mm,fin thickness 0.5-4mm,tube longitudinal spacing 0.5-2.5 times the outer pipe diameter,the number of pipe rows is 1-5 rows.The parameter sensitivity analysis results show that:when 1mm≤fin spacing δ ≤ 5mm,444≤Reynolds number Re≤3405,the structural parameter fin spacing δ is the structural parameter that has the greatest influence on Nusselt number Nu and resistance factor f.Within this range,the correlation between the dimensionless parameters composed of fin pitch/characteristic length,Nusselt number Nu,and drag factor f is proposed.The correlation parameter diagram shows that the smaller the fin spacing δ and the larger the Reynolds number Re,the more beneficial it is to the heat transfer and resistance of straight fins.The conclusion can be used as reference for heat exchanger selection.In the second part,the current theoretical model of the two-phase flow heat transfer and pressure drop characteristics of the liquid side of the tube-type heat exchanger is calculated and analyzed,and the results are compared with the experimental data of more than 1,000 groups of literatures.Seven models of pressure drop resistance are checked and calculated one by one.The results are compared with experimental data in the literature.It is found that the Salimpour M R heat transfer model and the Lockhart&Martinelli pressure drop model have the best predictability,with average errors of 15.7%and 18.4%,respectively.In order to further improve the model,the regression analysis method is used to fit the calibration data.The results show that:the change of the Dean number Dn has the greatest effect on the heat transfer capacity;and the changes of the mass flow density G and the characteristic length De affect the pressure drop.Significantly.For these two sets of parameters,amendments are made to the above model.The new model has a more accurate prediction accuracy for the heat transfer and pressure drop characteristics of R404A refrigerant in the tube-type heat exchanger in the range of mass flow density G 150～500kg/(m2·s)and characteristic length De 2.5～20mm.The previous increase was 15.14%and 7.26%.This conclusion can provide a theoretical model for the evaluation of the heat transfer performance of the heat exchanger in the casing.Finally,the experimental system is introduced.The test device includes four cycles:the refrigerant main circuit,the refrigerant bypass circuit,the lubricant main circuit,and the lubricant bypass circuit.The article also proposes the corresponding experimental design plan.Including experiment purpose,test section,test conditions and parameters,main steps of experiment and error analysis,etc.The conclusions of this study can be used as a reference for the selection of air-water dual-purpose heat exchanger,and can also provide a theoretical model for the evaluation of heat transfer performance of finned tube heat exchanger and in-tube heat exchanger.