The Study On Natural Convective Subcooled Boiling Heat Transfer In Vertical Narrow Annulars

In this paper, the experimental investigation of natural convection subcooled boiling heat transfer of water in vertical narrow annuli was carried out at atmospheric pressure. In the experiments, onset of the nucleate boiling (ONE), two-phase flow pattern, heat transfer performance, flow instability and critical heat flux were carefully observed and recorded. The effects of gap size, heat flux and inlet subcooling on the boiling incipience, flow pattern, heat transfer coefficient, flow instability and critical heat flux were analyzed and discussed. The experimental results could be summarized as follows:1. The position of ONE point increased with the rise of inlet subcooling. When inlet subcooling kept constant, the position of ONE point asymptotically decreased with the increasing of heat flux and the decrease of gap size.2. The heat transfer coefficient increased as the gap size get smaller. In the range of the experiments, the maximal local heat transfer coefficient of the smaller gap size( 8 =1.314mm) was five times larger than that of the larger one( 8 =3.14 mm).3. The qCHF under high inlet subcooling was higher than that under low inlet subcooling, but after inlet subcooling increased to a certain value, the critical heat flux approached to a constant.Based on the experimental and analytic results, applying multiple linear regressions to the experimental data, the empirical correlations for predicting the boiling incipience, heat transfer coefficient and critical heat flux were proposedrespectively, the calculative value agreed well with experimental data.