Forced Convective Boiling Heat Transfer Characteristics and Critical Heat Flux in Helically Coiled Tubes.

Abstract

Heat transfer and CHF (Critical heat flux) characteristics of flow boiling of R-113 in helically coiled tubes were experimentally investigated. Two coiled tubes with coil diameters of 0.165 and 0.32m, and 10mm I.D. were tested at a pressure of 0.39 MPa. In the nucleate boiling region, circumferential difference in heat transfer was not clarified qualitatively as well as quantitatively. The ratio of circumferential average boiling heat transfer coefficient to that of the single-phase flow in a curved tube, hTP/hLO, was found to be rather close to the correlations of Pujol-Stenning and Ueda-Kim for a straight tube flow. In the high quality region, the heat transfer coefficient was highest at the coil outside and lowest at the inside, and the average heat transfer coefficient ratio hTP/hLo was expressed in terms of Martinelli parameter, Xtt, only. A correlation applicable to both the nucleate boiling and forced convective evaporation regions was proposed. The CHF of the coiled tube was higher than that of the straight tube when the coil diameter was small, but it became considerably low at low mass velocity when the coil diameter was large.