The Evolution of Temperature Disturbances During Boiling of Cryogenic Liquids on Heat-Releasing Surfaces

Abstract

This publication is devoted to studying the regularities of the dynamics of temperature disturbances on the heat-releasing surfaces during boiling of cryogenic liquids under conditions of free convection and in the flowing-down wave liquid films. Crisis phenomena and dynamics of transient processes at boiling are the subject of intense experimental and theoretical investigations. Nucleate boiling liquid is one of the most effective ways to remove heat from the heat-releasing surface. Temperature disturbances with different spatial-temporal scales occur on heat-releasing surface during boiling. Perturbations of the fluctuation character are the inner features of boiling. Crises of boiling liquids occur due to the change of boiling regimes with significantly different intensities of heat transfer. The transition from one regime to another occurs over a finite time, which is determined by the velocity of site propagation and the linear scale, which characterizes the average distance between the sites arising under the influence of different kinds of fluctuations. Knowledge of such characteristics as stability and speed of propagation of film boiling regime on heat-releasing surface is important in cryogenic fluids, for example, due to the necessity of cooling of superconducting devices. The emergence of film boiling and their expansion along the heat-transfer surface drastically affects the heat removal, leading to crash overheating of the heater surface and to its destruction. Dry spots formed in the pre-crisis mode in falling liquid films are the analogous sites of film boiling. Film flow of liquids (including cryogenic) are widely used in various technological processes for intensification of heat and mass transfer. Evaporation in the thin films of liquid provides high heat transfer rate at low cost and low temperature difference. The topicality of this subject matter is relates, in particular, to the development of efficient and compact systems for cooling of the elements of electronics and computers and high-productivity graphical processors whose response and lifetime are substantially dependent on the efficiency of dissipated-power removal. This raises the need for reliable prediction of the critical heat flux, whose excess leads to the complete draining of heat-releasing surface and uncontrolled heating. Study of heat transfer during boiling and evaporation of cryogenic fluids, a number of properties (purity, good wettability, near zero contact angles) differ essentially from properties of high-temperature liquids, is important to deepen understanding of the processes. This is a way to test the existing model descriptions of heat transfer and the development of transient processes and crisis phenomena.