Experimental Investigation And Correlation Evaluation For Subcooled Flow Boiling Heat Transfer In Compound Heat Transfer Tubes

In the nuclear fusion reactor,the divertor is an important component directly facing the plasma.It needs to withstand the extremely high heat flux of the nuclear fusion reactor under normal operation and plasma rupture.It is necessary to investigate an efficient heat transfer enhancement technology to enhance the operation ability of the divertor under extremely high heat flux and the heat transfer ability of the working fluid.In the experiment,deionized water was used as the experimental medium.Under the condition of a one-sided heating flux,experimental research on the enhanced heat transfer characteristics of the subcooled flow boiling for the fusion reactor divertor was carried out.The compound heat transfer tube was designed by using the synergistic enhanced heat transfer effects of the internal thread tubes and twisted tape inserts.The heat transfer characteristics of the compound heat transfer tubes were studied by experiments,and the effects of various parameters on the heat transfer characteristics were investigated.According to the experimental data,a new heat transfer correlation under high heat flux is obtained by nonlinear fitting method.Finally,the heat transfer characteristics at the ONB point of subcooled boiling were studied and analyzed,and the combination with the pressure drop characteristics provided theoretical support for the stable operation of the divertor.（1）Experimental study on heat transfer and pressure drop performances of compound heat transfer tubesThe experimental parameters are as follows:mass fluxes from 992 to 4 960 kg/m² s,pressures from 0.4 to 2 MPa,inlet subcoolings from 87.01 to 119.21℃,and heat fluxes from 1 to 16.3 MW/m².The heat transfer characteristics and performance evaluation criteria（PEC）were compared and studied for the four kinds of enhanced heat transfer tubes（plain tubes,tubes with twisted tape inserts,internal thread tubes and compound heat transfer tubes）.The results showed that:compared with the other three kinds of tubes,the heat transfer coefficient and PEC of the compound heat transfer tubes were the highest,and the heat transfer characteristics were the best.Then,the influences of the twist ratios,pitches,pressures and mass fluxes on the heat transfer coefficient for the compound heat transfer tubes in the two-phase flow boiling were also conducted.It was found that the heat transfer coefficient was directly proportional to the pitches and mass fluxes,and inversely proportional to the twist ratios and the pressures.It is also found that the compound heat transfer tube has the largest pressure drop characteristics compared with the other three cooling tubes.With the increase of mass flow rate,the pressure drop characteristics show a gradually increasing trend;the influence of twist ratios and pitches on the pressure drop is opposite,and the pressure drop increases with the decrease of twist ratios and the increase of pitches.（2）Correlation evaluation of subcooled flow boiling heat transferBy collecting the existing four subcooled water flow boiling heat transfer models,a total of 13 existing heat transfer correlations are used to evaluate the heat transfer experimental data in this paper.Through the analysis,it is found that the enhancement model has the best prediction accuracy,and the model predictions of Baburajan and Moles and Shaw in the enhancement model are the best.Therefore,he Nusselt number correlation modified by adding the twist ratios and ratio of the pitches to the hydraulic diameter of the tube（t/D_h）for the compound heat transfer tubes in the subcooled flow boiling was proposed.It is found that the deviation between predicted values and experimental values is within±15%,the new correlation has a MAE of 5.69%and a RMSE of 7.55%,respectively.Therefore,it is indicated that the new developed correlation has the excellent agreement and effectiveness to predict the values for the compound heat transfer tubes.（3）Study on buoyancy force and Onset of Nucleate Boiling pointThrough the investigation of buoyancy force and Onset of Nucleate Boiling（ONB）point in the compound heat transfer tubes.The variation of Grashof number Gr with Nu under the same operating parameters was compared and analyzed,and the influences of twist ratio,pitch and mass flow rate were mainly studied.The results show that Nu decreases obviously with the increase of Gr.This trend decreases with the increase of Gr.The influence of each parameter on the ONB point is shown as follows:the wall superheat temperature and heat flux at ONB increases with increasing mass flux.The higher the pressure is,the lower the wall superheat temperatureΔT_sat is,but the higher the heat flux q is.The influences of pitches on the ONB point show the same trend compared to the mass flux,while the twist ratio only affects the heat flux at the ONB point.The larger the twist ratio is,the smaller the heat flux at the ONB is.Finally,a total of nine correlations of the two ONB heat flux models were compared and analyzed,and the results showed that the correlations of Jens and Lottes and Thom correlation have the best prediction accuracy.On this basis,a new correlation suitable for the wall superheat at the ONB point of the subcooled flow boiling of the compound heat transfer tubes was obtained by nonlinear fitting.The deviation between predicted values and experimental values is within±20%,the new correlation has a MAE of 13.49%and a RMSE of 16.8%,respectively,which could better predict the experimental results.Figure[37]Table[6]Reference[90]...