Natural convection evaporation from spherical particles in high‐temperature surroundings

Abstract

The prediction of the rate of evaporation of liquids in high-temperature surroundings is important in an increasing number of processes such as spray drying and flash drying, combustion of atomized liquid fuels, cyclone evaporation, and in operations carried out in gas-conveyed systems, such as in the atomized suspension technique (1 ) . Theoretically at least, the problem is capable of solution if sufficient knowledge exists concerning the boundary-layer flow pattern and the rate of heat transfer by radiation and convection at every point of the system.As pointed out by Hoffman ( 2 ) , most of the numerous investigations which have been reported in this field were concerned with low transfer rates and generally fall into two broad classes: measurement of the rate of heat and mass transfer from stationary liquid drops in surroundings at a temperature only moderately higher than that of the drop, and measurement of the rate of heat and mass transfer from liquid and solid spheres in a flowing medium under forced convection conditions.Virtually all of these studies, the most pertinent of which have been reviewed in reference 2 and will not be listed here, have assumed that the two transfer mechanisms did not affect one another. With work at high rates of mass transfer, Hoffman has, however, shown that the rate of heat transfer is governed by the effect of the evolved vapors on the boundary-layer flow.The present investigation was undertaken specifically to study the vaporization of liquids at high rates of mass transfer and to provide a better understanding of the complex interaction of the two transfer mechanisms. A theoretical analysis of this problem will first be presented, from which the various dimensionless groups governing the process will be deduced. This will be followed by the description of an experimental study on the rates of evaporation from stationary spheres in high-temperature surroundings. D. C. T. Pei is ANALYSIS OF PROBLEMIn an analysis of the convectivetransfer problem, the fundamental approach is to consider the equations which govern the transfer of momentum, heat, and mass in the boundary layer. To simplify matters the case of two-dimensional incompressible flow along a curved surface will be considered. With a curvilinear orthogonal system of coordinates whose x-axis will be in the direction of the wall, it can be shown that, in general, the boundary-layer equations for a flat wall may be applied to the case of a curved wall as well, provided that there are no large variations in curvature. When applied to a sphere, this approach is only an approximation which should indicate the nature of the dimensionless parameters on which the solution should depend. Thus, assuming constant physical properties, the expressions are Navier-Stokes momentum equation derived from a force balance