Theoretical Critical Heat Flux Prediction Based on Non-Equilibrium Thermodynamics Considerations of the Subcooled Boiling Phenomenon

Abstract

Whenever power is to be transferred from a heated surface to a liquid coolant, it is usually desired to obtain high heat fluxes with low temperature differences to avoid excessive stress in mechanical components. For single-phase forced flows, there is a linear relationship between the heat flux and the temperature difference. If the heat flux is increased, some bubbles nucleate at the hot surface and then they depart to the subcooled fluid bulk where they collapse. This subcooled boiling regime enhances transference and tends to give higher heat fluxes for the same temperature difference than in pure singlephase convection. However, if the heat flux is further increased, at some point a vapor film is formed on the hot surface. The heat transfer rate is suddenly reduced and the wall temperature increases, usually up to prohibitive values. This dry-out phenomenon is known as departure of nucleate boiling (DNB), and the value of the heat flux at which it occurs is called the critical heat flux (CHF). As this value poses an upper bound to the rate at which heat can be extracted from a certain source, its prediction is of central importance in the design of heat removal systems. In this paper a theoretical derivation of the DNB wall temperature for low void fraction is proposed based on non-equilibrium thermodynamics considerations of the subcooled boiling phenomenon. Then, using two-phase heat-transfer correlations, values for the actual CHF are estimated. These predictions are applied and compared to empirical observations in a mathematical model of a nuclear reactor channel, as this subject is of special concern in the nuclear industry because although virtually an infinite power can be generated by fission of the uranium in the fuel, the actual thermal power is limited by the efficiency of the core cooling systems.