Experimental study of post-dryout with R-134a upward flow in vertical smooth tube and rifled tubes

Abstract

A study of post-dryout heat transfer was performed with a direct heated smooth tube and rifled tubes using vertical R-134a up-flow to investigate the heat transfer characteristics in the post-dryout region. Three types of rifled tube having different rib height and width were used to examine the effects of rib configuration and the experimental results of rifled tubes were compared with the results of smooth tube, using a mass flux of 70-800 kg/m$\^{2}$-sec and a pressure of 13-24 bar (corresponding to an approximate water pressure of 80-140 bar). Wall temperature distribution in all tubes was strongly dependent on pressure and mass flux. Wall temperatures of the rifled tubes in the post-dryout region were far lower than for the smooth tube at same conditions and the critical point at rifled tubes move to much higher qualities against the smooth tube. This was attributed to turbulence and swirl flow caused by the rib. The increasing of critical quality and the decreasing of wall temperature in the rifled tubes against the smooth tube are pronounced at higher pressure and higher mass flux. The thermal non-equilibrium, which is usually present in the post-dryout region, was also lowered at the rifled tube. The effects of rib configuration on decreasing of the wall temperature are present in the low mass flux and low pressure, but not in high mass flux and high pressure. Type B rifled tube was found to be most effective for decreasing of the wall temperature, and it suggested the wall temperature distribution is most dependent on the rib height. The empirical correlation for the post-dryout heat transfer in the smooth tube was developed. The predicted wall temperature by the empirical correlation for smooth tube agrees well with the experimental results of the present study and the prediction by CSO correlation. The heat transfer correlation for rifled tubes was also developed as a function of rib height and width with the modification of the smooth tube correl...