Saturated flow boiling of water in a vertical small diameter tube

Abstract

The characteristics of flow boiling heat transfer of water in a vertical tube with a 1.45 mm diameter, which is less than the Laplace constant, are experimentally studied under atmospheric pressure and forced flow condition. Local heat transfer coefficients are measured in a range of mass fluxes from 23.4 to 152.7 kg/m 2 s, heat fluxes from 10 to 715 kW/m 2 and quality up to 0.8. The effects of mass flux, heat flux and quality on the boiling heat transfer coefficient are examined. Large heat transfer enhancement is observed and existing flow boiling correlations largely underpredict the heat transfer coefficient especially for a low heat flux condition. The underprediction gradually decreases with increasing heat flux. The dominant flow pattern in the tube is a slug-annular or an annular flow, and then liquid film evaporation is found to dominate the heat transfer. A heat transfer prediction method is proposed to reproduce the heat transfer coefficient in the flow pattern. The Katto’s critical heat flux (CHF) correlation consistently predicts the present CHF values but overpredicts by 20%.