TRANSITIONAL PROCESSES AND CRISIS PHENOMENA IN BOILING OF CRYOGENIC LIQUIDS

摘要

This paper presents the results of experimental study and physical modeling on transitional processes and crisis phenomena at boiling of cryogenic liquids. Fundamental regularities for crisis development at boiling under non-stationary heat release and alteration dynamics of boiling regimes were demonstrated using results of publication review and the current research for a wide range of key parameters. The studied processes were qualitatively described using physical models and generalized experimental data. In first part of this paper, characteristics of transient boiling at stepwise periodical pulsed and power laws of heat release have been investigated experimentally for helium and nitrogen over a wide range of relative pressures. It is shown that the effect of heat release increasing rate and the period between heat release pulses on the quantity of transient critical heat flux depends strongly on the presence of ready evaporation centers on the heat releasing surface. From consideration of thermal balance and phase transition conditions, expressions for calculation, which are in satisfactory agreement with experimental data, for minimal transient critical heat flux for different liquids are obtained. In the second part of this paper, the existing concepts on development dynamics and thermal stability of film boiling sites are presented. Results of numerical simulation for development of one- and two-dimensional sites of film boiling were analyzed. To present the valid boundary conditions at the transition front of boiling regimes, dimensionless parameter e which characterizes the ratio between the width of the temperature front along a heat-releasing surface and the linear scale of capillary forces' action was introduced. Calculations of stability zones from the film boiling sites depending on their initial sizes and dynamic characteristics of development are presented for different levels of heat flux. While describing dynamics of the front of boiling regime change in the simulation model, the non-stationary character of heat transfer within different zones at the front of regime change was taken into account. The approximated model for description of propagation of a self-maintaining evaporation front in a thermal layer near the heat-generating surface is considered under quasistationary and non-stationary heat release. Due to comparison of experimental data with calculation results, it is shown that arising instability of the interface leads to a drastic increase in the propagation velocity of the evaporation front. For low values of dimensionless heat flux through the interface, the velocity of boundary propagation can be approximately described by propagation of the undisturbed smooth evaporation front. For higher parameters, corresponding to high Jacobs numbers, a simple empirical dependence is proposed to consider the influence of quickly growing small-scale perturbations on the propagation velocity of the evaporation front.