Numerical analysis of CO₂ transient behavior in the gas cooler during variable start-up conditions of a transcritical refrigeration system

Transient behavior of CO₂ in the gas cooler during variable start-up conditions of a transcritical refrigeration system is numerically investigated. The analysis is based on three cases including cold and warm start-up with zero initial refrigerant mass flow rate, and warm start-up with nominal CO₂ mass flow rate. The numerical analysis is performed using a 3D model and simulations are carried out using CFD code FLUENT. Effects of the transient change in the gas cooler inlet boundary conditions on the CO₂ temperature, mass flow rate, thermophysical properties, heat transfer parameters and thermal effectiveness are analyzed. Results show that, when the refrigeration system starts at high temperature and nominal refrigerant mass flow rate, the maximum thermal effectiveness of the gas cooler is 85%. When it starts at high temperature, with zero initial mass flow rate, the thermal effectiveness decreases up to 60% and the transient period lengthens about 50 s.