Developing a Model of Condensation under Zero- and Nonzero-Gravity Conditions for Miniature Loop Heat Pipes

A physico-mathematical model of condensation in an annular flow regime, in which the gravity force influence is negligibly low, is developed. The condensation model is based on the calculation of frictional pressure drop using the technique developed by L. Sun and K.A. Mishima for minichannels. The model takes into account the pressure recovery due to a slowdown of vapor velocity. It is assumed that condensate moves in the form of laminar film with its velocity distributed according to the Hagen–Poiseuille law. Proceeding from this assumption, the film thickness and void fraction are calculated. For the Earth-normal gravity conditions, a method is developed for matching the annular flow model with the model of stratified flow, which can take place in the condensation end section if the vapor velocity slows down and the gravity force becomes dominating. Owing to this, the predicted values of the average heat transfer coefficient and condensation section length are in good agreement with the experimental results obtained in a miniature ammonia loop heat pipe in a climatic chamber.