Assuming that 1) the droplet surface distortion can be neglected so that the droplet maintains its spherical shape, 2) there is no sudden break-up of the droplet due to internal phenomena such as micro-explosions, and 3) the droplet surface temperature remains at the boiling temperature as long as the assumption of equilibrium is valid, a similar formulation to Equation 6–83 is obtained for the boiling process by setting the surface temperature equal to the boiling temperature, T _b, and the surface mass fraction to unity. The boiling formulation is written as

(6–85)

where is the coefficient for the contribution of heat transfer by internal circulation at the saturation temperature, α _sh is the heat transfer enhancement through the effect of nucleation. The vaporization rate calculated based on Spalding’s mass transfer number (Equation 6–84 ) is no longer valid under these conditions.