The Radial Model is used to determine the radial temperature distributions in the vapor core, wick and wall regions. In this model, a parabolic radial temperature distribution is assumed in each region, and the constants for the equations are related to the system variables in the Axial Model Equation Set given above. The Radial Model is then solved concurrently with the Axial Model.
Various boundary conditions and constraints on the average temperature are used in the model: Interphase mass and heat transfer between all of the phases is modeled. Melting and freezing are modeled with a discontinuous heat flux at the liquid-solid boundary. Evaporation and condensation are modeled using an evaporation rate derived from the kinetic theory of gases (modified to account for the presence of a noncondensible gas). Gas-solid mass transfer is modeled in a similar manner. Convective heat transfer at the liquid-vapor or solid-vapor interface is modeled. Through the solid and the heat pipe wall, heat transfer takes place by means of conduction. At the external wall boundary, a radiative heat flux condition is used. In the liquid, conduction and convection are both included in the model.