Experimental investigation of tube length effect on nucleate pool boiling heat transfer

The effect of a vertically installed tube length on the nucleate pool boiling heat transfer coefficient under atmospheric pressure has been empirically obtained using various combination of major parameters for application to advanced light water reactor design. The experimental data for q '' versus Delta T test are counted as 1,063 points and can cover the extent of D = 9.7 similar to 25.4 mm, epsilon = 15.1 similar to 60.9 nm, H = 5.25 similar to 30.93, and q '' less than or equal to 160 kW/m(2). The experimental results show that a shorter tube is more efficient to increase heat transfer rate due to smaller bubble slug formation on the tube surface. The effect of tube length is greatly observed before H (= L/D) gets 50. After that, the heat flux decreases linearly with H increase. To quantify tube length effect, a new empirical correlation has been developed based on the experimental data bank for pool boiling heat transfer and some parametric studies have been done using the newly developed empirical correlation to broaden its applicability. The newly developed empirical correlation has the form of q '' = 0.019 epsilon(0.570) Delta T-4.676/((DH0.072)-H-1.238) and can predict the experimental data within +/- 20% bound. Heat transfer characteristics can be changed with tube length variation and the transition point is H approximate to 50. Before the transition point, bubble coalescence is active and heat transfer rate gets rapidly decreased with increasing tube length. After that, heat transfer gets somewhat slowly decreased since bubble coalescence effect gets nearly equilibrium with liquid agitation effect. (C) 1998 Elsevier Science Ltd.