Entropy generation analysis on a heat exchanger with different design and operation factors during transient processes

Abstract

Entropy generation analyses on the transient processes of heat exchangers can guide their designs and operations. A dynamic model of a typical recuperative heater is developed based on mass, energy, and momentum conservation equations. Dynamic behaviors of the heater during transient processes are analyzed based on the second law of thermodynamics. The real-time entropy generation rate due to the heat transfer between the work medium and metal surfaces, and the heat conduction in metals are presented and discussed. A cold fluid flow rate with 20% step increase is adopted as the boundary disturbance, and dynamic performances are obtained. Additional entropy is generated in the heater during the transient processes compared with stationary work conditions. Several design and operation factors of the heater are discussed. Calculation results show that the additional total entropy generation diminishes during the transient processes with the increase in the thermal diffusivity of metal. This rule is also suitable for the influence of thermal transfer resistance between the fluid and metal. By contrast, the metal thickness and specific heat capacity of hot work fluid have opposite influences on the total additional entropy generation of the heater during the transient processes.