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 mor e efficient to increase heat transfer rate due to smaller bubble slug formation on the tube surface. The effect of tube length is greatly ob served before H (= L/D) gets 50. After that, the heat flux decreases l inearly with H increase. To quantify tube length effect, a new empiric al correlation has been developed based on the experimental data bank for pool boiling heat transfer and some parametric studies have been d one using the newly developed empirical correlation to broaden its app licability. 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 char acteristics can be changed with tube length variation and the transiti on point is H approximate to 50. Before the transition point, bubble c oalescence is active and heat transfer rate gets rapidly decreased wit h increasing tube length. After that, heat transfer gets somewhat slow ly decreased since bubble coalescence effect gets nearly equilibrium w ith liquid agitation effect. (C) 1998 Elsevier Science Ltd.