Study on the effect of tube gap on nucleate boiling heat transfer coefficient in horizontal tube bundle

Abstract

Boiling heat transfer characteristics of HFC-245fa in a horizontal tube bundle with a staggered arrangement were experimentally investigated. Thermal spray coated tubes as a boiling heat transfer enhancement tube were installed only at the first row of the tube bundle to form homogeneous bubble flows. In our previous results with the thermal spray coated tube, it was shown that uniform heat transfer enhancement factor to the single tube heat transfer could be obtained independent of the tube position. Since the effect will depend on the relative size of the tube gap to the size of vapor bubbles, the tube gap was reduced to 4.5 from 7.5 mm with the same tube pitch of 22.5 mm of the previous study. The experiments were carried out at the saturation temperature of 18 °C. Heat flux was varied from 1.33 to 50 kW/m2 with the mass flux at the minimum gap of 18.5 and 30.8 kg/(m2·s). As a result, it was confirmed that higher heat transfer enhancement factor could be obtained by the reduction of the tube gap in the case with the thermal pray coated tubes at the 1st row. The increase in heat transfer coefficient by installing the thermal spray coated tubes was larger under the lower mass flux condition of 18.5 kg/(m²·s).