Law 3 is applied to predict the convective boiling of a discrete phase droplet when the temperature of the droplet has reached the boiling temperature, , and while the particle still contains mass that can evaporate (the particle mass is larger than the non-volatile mass, ):

(12–99)

and

(12–100)

When the droplet temperature reaches the boiling point , the boiling rate is computed by solving equations Equation 12–93 and Equation 12–94 for the particle diameter to yield:

(12–101)

Note that pre-2019 R3 releases of Ansys Fluent used a simplified version of Equation 12–101 in which the Prandtl number was assumed constant and equal to 0.45. In this case, the term reduces to .

Ansys Fluent provides the option to set the heat capacity of the gas equal to the heat capacity of the evaporating species. See Description of the Properties in the Fluent User's Guide for more information about this option.

Equation 12–101 was derived assuming steady flow at constant pressure. Note that the model requires in order for boiling to occur and that the droplet remains at fixed temperature () throughout the boiling law.

When radiation heat transfer is active, Ansys Fluent uses a slight modification of Equation 12–101, derived by starting from Equation 12–93 and assuming that the droplet temperature is constant. This yields:

(12–102)

or

(12–103)

Using Equation 12–76 for the Nusselt number correlation and replacing the Prandtl number term with the empirical constant 0.23, Equation 12–103 becomes

(12–104)

In the absence of radiation, this result matches that of Equation 12–101 in the limit that the argument of the logarithm is close to unity. Ansys Fluent uses Equation 12–104 when radiation is active in your model and Equation 12–101 when radiation is not active. Radiation heat transfer to the particle is included only if you have enabled the P-1 or discrete ordinates radiation model and you have activated radiation heat transfer to particles using the Particle Radiation Interaction option in the Discrete Phase Model Dialog Box.

The droplet is assumed to stay at constant temperature while the boiling rate is applied. Once the boiling law is entered it is applied for the duration of the particle trajectory. The energy required for vaporization appears as a (negative) source term in the energy equation for the gas phase. The evaporated liquid enters the gas phase as species , as defined by your input for the destination species (see Setting Material Properties for the Discrete Phase in the User's Guide).