FILM BOILING AROUND A VERTICAL CYLINDER WITH TOP AND BOTTOM HORIZONTAL SURFACES

摘要

Saturated and subcooled film boiling heat transfer around a vertical finite-length silver cylinder with top and bottom horizontal surfaces has been investigated, experimentally and analytically, in terms of cooling curve, and the correlations of heat transfer were proposed in the present paper. Pool film boiling experiments were carried out by quenching method. Cooling curves are obtained for saturated water at atmospheric pressure. The heated cylinder is made of silver and 18 kinds of cylinder are tested in the ranges of the diameter from 8 to 100 mm and the length from 8 to 160 mm. For subcooled water, the experiments were carried out in the similar method to the case of saturated water. The ranges of the diameter and length of the cylinder are 32 to 50 mm and 16 to 64 mm, respectively. The degree of liquid subcooling ranges from 2 to 30 K. In order to predict the film boiling characteristics, the overall heat transfer rate from a cylinder with finite length was modelled by taking into account each convective heat transfer on the bottom, side and top surfaces of the vertical cylinder. Present correlation equations for heat transfer and the lower limit of film boiling are good agreement with the experimental data for saturated and subcooled water. The values of wall heat flux and temperature at the lower limit of film boiling are obtained as the point where the cooling rate has a minimum value on the cooling curve. For the case of saturated water, wall temperature at the lower limit of film boiling is about 136 K and irrespective of the configuration of a cylinder. For subcooled water, the correlation is proposed for the effect of liquid subcooling on wall temperature at the lower limit of film boiling.