Gravity scaling law of heat transfer in nucleate pool boiling

Abstract

Boiling phenomenon can realize high performance heat exchange due to latent heat transportation accompanying the liquid-vapor phase change, resulting in its wide applications for high heat flux transfer both in normal gravity on the ground and in space microgravity environment. It is well known that gravity strongly affects boiling phenomenon by creating forces in the systems that drive motions, shape boundaries, and compress fluids. The study of the influence of gravity on boiling heat transfer is of great significance for relevant space applications. Our current knowledge on boiling phenomenon, however, is often composed of a great amount of empirical correlations and semi-mechanistic models for engineering applications, which are mainly depended upon empirical data obtained from elaborately designed experiments on the ground. Although many empirical correlations and semi-mechanistic models include gravity as a parameter, they usually fail when extended beyond the empirical range they were based on. Microgravity experiment offers a unique opportunity to study the complex interactions in boiling phenomenon without gravity, and thus to reveal the mechanism of gravity, or gravity scaling law, underlying boiling phenomenon by comparing microgravity experimental data with those obtained in normal gravity. Review of the up-to-data progress on gravity scaling law of nucleate pool boiling heat transfer is made in the present paper, especially focusing on the RKM (Raj-Kim-McQuillen) gravity scaling law which presents a unified framework for scaling behaviours of nucleate pool boiling heat transfer relevant to gravity based on the quasi-steady state, ground-based short-term reduced gravity experimental results acquired during transition periods of approximately 3–5 s when the acceleration varied continuously from hypergravity to low- g , or vice versa , in parabolic flights. The gravity scaling parameter for heat flux was updated based on high quality microgravity data aboard ISS (the International Space Station), and its robustness in predicting low gravity heat transfer is further demonstrated by predicting many of the trends in the pool boiling literature that cannot be explained by any single model. There are, however, some deficiencies and/or unsolved problems in the RKM gravity scaling law. The definition of the dimensionless temperature involves the temperatures of the boiling incipiency and of the CHF (critical heat flux). It was implicitly assumed that these two characteristic temperatures are constants in different gravity conditions. These assumptions, however, have no theoretical or empirical basis in fact. Moreover, recent numerical studies utilizing the lattice Boltzmann method, as well as some experimental evidences, showed that the temperature of CHF increases with the gravity level. Furthermore, the hypotheses on the asymptotic behaviours near the boiling incipiency and the critical heat flux confused the meaning of different gravity scaling parameters, which are defined clearly in the present paper. An important reason for the deficiencies of the RKM gravity scaling law is the lack of empirical data, especially experimental results in long-term, steady state pool boiling in different levels of reduced gravity. Thus, a variable gravity pool boiling experiment project utilizing CSS (the Chinese Manned Space Station) under construction, as well as systematic numerical simulations, are suggested in order to promote the research in this field.