Study On Dynamic Heat Transfer Model And Sensitivity Analysis Of Helically Coiled Tube-in-tube Heat Exchanger With Corrugated Inner Tube

With the rapid development of economy,the demand for energy has also continued to grow.Energy shortage is a common concern of the world.Since heat exchangers play an important role in the industrial production,the development,use and optimization of high-performance heat exchangers have significant importance to solve energy problems.As a novel heat exchanger,the helically coiled tube-in-tube heat exchanger with corrugated inner tube which integrates two characteristics of corrugation and helix on the basis of common double-pipe heat exchanger,has reduced floor space and considerably enhanced heat transfer,and is very popular in air conditioning systems.In this paper,the following studies have been conducted for this novel heat exchanger:Firstly,a steady-state and dynamic numerical heat transfer model of helically coiled tube-in-tube heat exchanger with corrugated inner tube is established.In this model,the governing equations of the fluid and the inner tube wall are discretely solved.Taking into account the velocity slip between the gas and liquid in the two-phase region,a quasi-separated phase flow model is applied in refrigerant.As for the flow and heat transfer correlations of this novel heat exchanger,this paper comprehensively considers the influence of corrugation and helix on the basis of the empirical correlations of the horizontal smooth tube,and the linear superposition method and harmonic average superposition method are used to estimate them.Secondly,in order to verify the numerical heat transfer model,Ndiaye experimental data are recommended,and the steady-state condition and dynamic condition caused by the variation of the inlet refrigerant mass flow rate when the compressor start up under evaporation and condensation mode are validate respectively.The results show that in the steady-state condition,quasi-separated phase flow heat transfer model can accurately predict the outlet physical parameters of heat exchanger,and the results of the two superposition methods have little difference.The relative error of the pressure,enthalpy of refrigerant and water temperature at the outlet of the heat exchanger are less than 7%.Moreover,in the dynamic condition,quasi-separated phase flow heat transfer model can exactly predict the dynamic heat transfer process of the heat exchanger,and the accuracy of the harmonic average superposition method is obviously higher than that of the linear superposition method.Finally,according to the steady-state model of the heat exchanger,sensitivity analysis and structural optimization are performed.Through the principle of orthogonal experimental design,an orthogonal test of five factors and three levels of the heat exchanger is formulated with the pressure drop and heat transfer rate as the experimental indexes.The five factors are corrugation depth,pitch,helix angle,coil pitch and helical diameter.The heat exchangers with eighteen structural parameters are simulated respectively,and the results are dealt with range analysis and variance analysis.The orthogonal experimental design can get the trend,order and degree of the influence of each orthogonal factor on the test index,and also the optimal structural parameters combination is obtained.After all of the above done,the energy-saving evaluation criteria are employed to verify the orthogonal experimental results.The results show that in the evaporation mode,the influence of each factor on the heat transfer rate is not remarkable,but the corrugation depth,pitch and helix angle greatly affect the pressure drop.In the condensation mode,only the influence of the corrugation depth on heat transfer rate is significant,but each factor has a significant effect on the pressure drop,and the influences decreases from corrugation depth,pitch,helix angle,coil pitch to helical diameter.The research methods and results can provide some references for the numerical simulation and optimization design of helically coiled tube-in-tube heat exchanger with corrugated inner tube.