Heat transfer to liquid drops from a small diameter channel at temperatures in the film boiling range

Abstract

An experimental study to determine the values of heat transfer per drop for small drops of water, methanol, and acetone falling down a heated channel of small diameter. Channel temperatures ranged from 118°K to 269°K above saturation, and channel diameters were smaller than drop diameters. Values of heat transfer per drop exhibited a maximum at a saturation excess of approximately 200°K for all three fluids. The occurrence of a maximum is assumed to be due to the effect of vapor viscosity on the growth of the vapor film. The occurrence of the maxima at a common value of saturation temperature excess is thought to occur because of the effect of vapor thermal conductivity, vapor density, modified heat of vaporization, and vapor viscosity on the rate of growth of the vapor film. It was also found that heat transfer per drop is independent of channel length. High-speed motion pictures and visual observations lead to the assumption that all appreciable heat transfer takes place in a very short period of time after the drop first contacts the channel. The maximum values of heat transfer per drop were found to be related to fluid properties, drop impact velocity, and geometry by the following relationship: (Q D) max(d/D) 3·088 d 3λ VFρ L = 0·232 (N sigma;) 0·256 (N μ) −1·632 (N ρ) −2·264 (We) −0·245 .