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Abstract
A two-dimensional transient model is developed to simulate the start-up of a radiatively cooled water heat pipe from a frozen state. The model incorporates sublimation and resolidification of working fluid; melting of working fluid in the porous wick structure; condensation of working fluid as a thin film on a frozen substrate, and drainage of the film; liquid flow and heat transfer in the porous wick; and thermal and hydrodynamic couplings of phases at their respective interfaces. The model handles the change of phase in the wick using a modified fixed-grid homogeneous enthalpy method, employing a mushy-cell temperature range of 2 × 10−8 K (limited by machine accuracy), without numerical instabilities. The simple method, based on the frozen fraction developed to calculate the heat fluxes at the boundaries of the mushy cell, improved accuracy and reduced oscillations in temperature time histories by 1–2 orders of magnitude.