Thermal and Viscous Irreversibilities Analysis in a Liquid Metal Phase Change Counter Flow Heat Exchanger

Abstract

Sodium, a liquid metal, is utilized in a heat exchanger system coupled to a nuclear reactor to produce hydrogen. The subcooled liquid metal enters a counterflow heat exchanger, undergoes a phase change, and exits as superheated vapor. The sodium heating process occurs through heat exchange with superheated helium vapor in three phases: subcooled liquid, saturated vapor, and superheated vapor. This article analyzes the thermal and hydraulic performance of the three stages of the heat exchanger through thermal and viscous irreversibilities using analytical simulation. The solution obtained is based on applying the thermal efficiency method and the second law of thermodynamics. The thermodynamic Bejan number, the relationship between thermal irreversibility and total irreversibility, allows a cost-benefit analysis when determining thermal performance and viscous dissipation. The essential physical quantities used in the analysis are the lengths and internal diameters of the three segments of the heat exchanger. The results are analyzed and compared with previous analytical work published in the literature. Numerical and graphical results are obtained for temperature profiles, thermal effectiveness, heat transfer rate, thermal irreversibilities, pressure drops, viscous irreversibilities, entropy generation rate, and Bejan numbers. It is demonstrated that the cost-benefit is highly advantageous when the diameter used for the internal tubes of the three segments is equal to the diameter of the saturated sodium vapor section, corresponding to ¼ of the speed of sound in the tube.