Critical Heat Flux for Nearly Saturated Water Flowing Normal to a Cylinder

Abstract

Visual and photographic observations are used to construct a physical model of the mechanism of transition from nucleate to film boiling on a cylindrical heater. In this paper, interest is focused on forced-convection boiling of a liquid which is near its saturation temperature, while a companion paper deals with the effects of various degrees of liquid subcooling on the peak flux. An approximate analysis is presented of the saturated nucleate-boiling model which predicts the critical flux, and comparisons are made with experimental observations. Measurements of the peak nucleate-boiling heat flux are reported for water at atmospheric pressure over a velocity range of 1.2 to 9.5 feet per second. Resistively heated, stainless-steel wires and tubes of 0.010 to 0.189-inch diameter, the latter with wall thicknesses of 0.006 to 0.028 in., were used. Within these ranges of variables, the critical flux is found to increase with the square root of the velocity and to be independent of heater wall thickness. Only a weak dependence on the heater diameter is observable, but the tendency is for the peak flux to diminish for larger tubes.