Studies On Heat Transfer And CHF Correlations For Subcooled Flow Boiling

The divertor is an essential component of International Thermonuclear Experimental Reactor(ITER).It can load the huge energy which is produced by ITER during normal operation and plasma bursts.So how to avoid divertor suffering from heat load becomes a key problem.(1)Experimental research of heat transfer and pressure drop in compound heat transfer tubesThe combination of multiple heat transfer enhancement methods can enhance the heat transfer.A compound heat transfer tube was designed by combining the internal screw rib tube and tube with twisted tape inserts.The coolant is deionized water.The influences of twist tape,tape width ratio,pitch of internal thread and mass flux on both the heat transfer coefficient and pressure drop have been investigated under one-sided high heat flux.Moreover,the comparative experimental researches of heat transfer coefficient,pressure drop and thermal performance factor in compound heat transfer tube,internal screw rib tube,tube with twisted tape inserts and plain tube were conducted.The experiment results showed that,the convection heat transfer coefficient and pressure drop of the compound heat transfer tube are the largest among the four cooling tubes,and have a high thermal performance factor.The compound heat transfer tube can enhance the heat transfer in the boundary layer and bulk fluid field;The heat transfer coefficient and pressure drop of compound heat transfer tube are assessed to directly proportional to tape width ratio,pitch of internal thread and mass flux,but inversely proportional to twist ratio.(2)Simulation analysis of heat transfer for tube with twisted tape inserts under non-uniform heatingTube with twisted tape inserts can enhance the heat transfer effectively.The heat transfer in subcooled boiling in tube with twisted tape inserts and plain tube was simulated numerically.When water was set as the working medium,the Euler multiphase flow model and non-equilibrium boiling model were adopted,the flow velocity was in the range of 0.3-0.7 m/s,the working pressure was 4.5 MPa,and the heat flux was 2 MW/m2.The heat transfer coefficient,void fraction,flow velocity,flow field streamline,solid component temperature and pressure drop of two kinds of cooling tubes were compared,and the comprehensive performance of the tube with twisted tape inserts was analyzed.The results showed that the heat transfer coefficient of the tube with twisted tape inserts increased by 6%to 90%,the flow speed increased by about 5%-15%,and the pressure drop increased by about 200%to 250%,compared with plain tubes.Compared with the plain tube,the tube with twisted tape inserts can delay the ONB,and the average void fraction on cross section was smaller than that of the plain tube.When the flow velocity was in the range of 0.4-0.6 m/s,the comprehensive performance evaluation factor(?)of the tube with twisted tape inserts was 1.1-1.3.(3)CHF correlations for subcooled water flow boiling at high pressure and high heat fluxThe paper compiled 1356 CHF experiment data points from 15 independent open literatures and evaluated 10 existing CHF correlations in subcooled water flow boiling.From the evaluation,the W-2 CHF correlation performs best for the experimental CHF data in all existing critical heat flux correlations.However,the predicted mean absolute error(MAE)of the W-2 correlation is not very ideal for all database and the MAE of the W-2 correlation is from 30%to 50%for the database of Celata et al.In order to enhance the prediction accuracy in subcooled water flow boiling CHF at high heat flux and high pressure,the present paper developed two new CHF correlations of inlet condition and outlet condition.The validation results showed that the new inlet condition correlation has a MAE of 10.05%and a root mean squared error(RMSE)of 16.61%,predicting 68.1%of the entire database within±10%and 81.5%within±15%.The MAE of the new inlet condition CHF correlation is 7.4%less than that of the best existing one(W-2 correlation)and 6.41%less than that of the outlet condition correlation,further confirming its superior prediction accuracy and reliability.Besides,the new inlet condition CHF correlation works well not only for a uniform high heat flux but also for a non-uniform heat flux for subcooled water flow boiling at high pressure and high heat flux.Experiments were performed to analyze the heat transfer performance of compound tubes and investigate the influence of each parameter on heat transfer performance.The methods of numerical simulation analysis were used to study the heat transfer performance,bubble distribution,temperature distribution and pressure drop of solid components in tube with twisted tape inserts.Finally,critical heat flux(CHF)experimental database under high pressure and high heat load was established and two new CHF prediction correlations with high precision were proposed.The study can provide some theoretical support for the thermo-hydraulic design in ITER divertor.Figure[30]table[8]reference[92]...