Simulational And Experimental Research On Two-phase Flow Boiling Heat Transfer In Tube Side Of The Spiral-wound Heat Exchanger

The spiral-wound heat exchanger（SWHE）is a kind of compact type shell and tube heat exchanger with high heat transfer efficiency,centrifugal force field and secondary flow.Its basic heat transfer unit is the helically-coiled tube which is widely used in heat and mass transfer,gas-liquid separation and other industries.At present,the research focus of fluid flow and heat transfer in helically-coiled tube is secondary flow,single phase flow of liquid,heat transfer and pressure drop analysis of gas-liquid two-phase flow of air and water.There are few studies on boiling heat transfer mechanism and gas-liquid two-phase distribution in helically-coiled tube.Therefore,this paper takes the SWHE as the research object and adopts the single-tube model to carry out theoretical and numerica simulation research on the boiling heat transfer in helically-coiled tube,and carries out experimental research on the bubble characteristics on the boiling condition.The main research contents and conclusions are as follows:（1）In order to solve the problem of pure liquid boiling heat transfer of the SWHE in the helically-coiled tube.The geometrical model and mathematical model of gas-liquid two-phase two-dimensional flow under boiling heat transfer in helically-coiled tube were established.Built on the theoretical model of the division phase flow and consider the influence of the viscous force,centrifugal force and gravity on the heat transfer.Relevant physical assumptions were made for the model,and then the two-dimensional flow control equations were simplified.At last,the solution method of the velocity distribution in the gas film and heat transfer coefficient was obtained.（2）The flow and heat transfer characteristics of gas-liquid two-phase flow in the helically-coiled tube under boiling conditions were tested by using numerical simulation method combined with the User-defined functions（UDF）,and the distribution law of gas-liquid phase was mainly studied.The results show that the gas phase is concentrated on the inner side of the helically-coiled tube（near the side of the central axis）and the upper wall surface.At the bottom end of the helically-coiled tube,liquid and gas stratification exists.With the increase of tube length,the volume fraction of the gas phase increases gradually and tends to be uniform.In addition,the influence of helically-coiled tube structure parameters on gas volume fraction,pressure drop and heat transfer characteristics was analyzed.Besides the effect of heat flux on wall temperature distribution,also be analyzed.（3）The characteristics of boiling heat transfer process in the helically-coiled tube and horizontal tube were analyzed under the same conditions.Considering the influence of the secondary flow,the De number need to be introduced to represent the heat transfer effect.The correlations of helically-coiled tube flow boiling heat transfer process were obtained through the regression method based on simulation results.Nu_tp（28）0.35De_Eq^0.7Pr_l^0.65X_tt^0.14P_i^-0.16（Bo′10⁴）^0.23,The results were compared with the modified Chen correlation and the mean absolute error was 4.5%.（4）Dealing with the SWHE in single tube model,in order to solve the helically-coiled tube boiling phase transformation condition,a segmented heat transfer calculation method which based on the heat exchange area was proposed.According to the phase change of working medium in the helically-coiled tube,the tube can be divided into three parts,like liquid phase period,gas-liquid two-phase flow period,gas phase period,the solution formulas of endpoint temperature in tube and shell side of each segment was given.The gas-liquid two-phase segment was further subdivided to solve the convection of heat transfer process,and the total heat transfer area was finally obtained.（5）The experimental platform for the boiling heat transfer of the SWHE in the tube was designed and set up in different mass velocity.Structure size of the helically-coiled tube is as follows:the inner diameter of the pipe is 4mm.The spiral diameter is 20mm,and the pitch is 7mm.The working medium in the tube is ethyl alcohol（C₂H₆O）There is two purpose of the boiling heat transfer experiment,visualization research was conducted on the bubble movement characteristics,and the flow characteristics of heat transfer were analyzed.Comparing with the simulation results in the same parameters of the helically-coiled tube,the results show that the relative error between the simulated value and the experimental value of heat transfer coefficient is 16¹9%,and the difference between the simulated value of inlet and outlet pressure drop and the experimental data is 0.4¹.2kPa.The accuracy of the simulation method was proved.