Entropy generation analysis of a gas cooler for transcritical co2 systems

Abstract

The performance of a transcritical CO2 based refrigeration system is highly sensitive to the gas cooler pressure and exit temperature. This is due to the peculiar s-shape of the isotherm in the supercritical region. Hence proper design of the gas cooler is crucial to extract optimum performance from the CO2 based systems. The gas cooler considered for the current application is an air cooled, counter cross-flow spiral fin-andtube heat exchanger. A discretized node-by-node approach is adopted here to model the gas cooler. The governing equations for each node are formulated based on the NTU-ε method. Using the developed model, a study has been conducted to optimize the geometrical dimensions of the heat exchanger based on the minimization of the total entropy generation due to fluid friction and heat transfer. The total entropy generation is indicated in terms of a non-dimensional entropy generation number. The variations of this non-dimensional number with changes in face velocity, fin pitch, and total number of tubes in each row are investigated in the present study. It is observed that entropy generation number decreases with the increase in fin pitch for higher flow rate. However, at a lower flow rate, entropy generation number lines for different fin pitches overlap. On the other hand, entropy generation number decreases with increase in the total number of tubes. Finally, from the results, the mentioned parameters are optimized to achieve lower entropy generation number with lower fan power consumption.