Studies on film boiling

Abstract

A boundary layer analysis is made for the two-phase flow problem in laminar film boiling under various situations. The situations considered are:

Film boiling on horizontal cylinders and spheres Forced convection film boiling with radiation Film boiling under unsteady conditions

The analysis includes the effects of shear stress and velocity at the interface. In the case of forced convection film boiling with radiation, the interaction between radiation and convection is taken into account by including the radiation flux terms in the conservation equation for energy. Film boiling under unsteady conditions is investigated by assuming that the unsteadiness is caused by an arbitrary variation of either the wall temperature or the acceleration field with respect to time. The governing partial differential equations are reduced to ordinary differential equations by similarity transformations only for the case of a horizontal cylinder. Series expansions have to be used for all the other cases. The resulting ordinary differential equations are solved numerically on an electronic computer for various values of the governing parameters, namely:

Prandtl number (Pr) Gravity parameter (g') Other dimensionless groups

Numerical solutions are presented and the heat transfer results are discussed. It is found that the shear stress at the interface cannot be assumed to be zero in film boiling problems. For fluids with (??)(\rho \mu)(??) ratio less than 0.01, the assumption of zero velocity at the interface is a good approximation. The heat transfer rate increases with an increase in r1/2r^{1/2}r1/2 or with an increase in Prandtl number. The film thickness and heat transfer rate during film boiling on a sphere can be calculated from the tabulated universal functions. From the analysis of forced convection film boiling with radiation, it is found that the effect of radiation is to increase the vaporization rate and the film thickness. The heat conduction at the vapor-liquid interface and the vapor temperature are increased by radiation, while the heat conduction at the wall is decreased. The application of the results to solve forced convection film boiling problems with radiation is described. The unsteady behavior of the film during film boiling under unsteady conditions is described by a set of universal functions, which are tabulated for various values of the parameters. The unsteady effect can be evaluated by computing the deviation from the instantaneous steady-state heat transfer. The effect of varying the fluid properties on heat transfer is also discussed.