Second law analysis on an elliptical twisted tube for a heat exchanger

This works presents a second law analysis on a heat exchanger tube configurated as circular smooth tube (CST), elliptical smooth tube (EST) and elliptical twisted tube (ETT) with different aspect ratios and pitch lengths. The ETTs were configurated with different aspect ratio (AR) of 1.5 and 2.0 and twist pitch length (PL) of 50, 100 and 200 mm. The hydraulic diameter was kept as constant for all cases, since the results are influenced by the change in hydraulic diameter. CFD analyzes were run to perform the second law analysis of the considered cases. The analyzes were carried out by considering that the thermo-physical properties of the water fluid change depending on the temperature. Besides, the analyzes were carried out under steady state condition and turbulent flow condition which corresponds to Reynolds number ranging from approximately 4000 to 27,000. The results are evaluated and discussed in terms of the thermal, the frictional and the total entropy generation, the Bejan number, the entropy generation number, the exergy destruction and the second law efficiency. It is resulted that the increase in AR and the decrease in PL for the ETTs show better the second law efficiency. As a result, in the case of ETT_AR=2.0_PL=50 at the minimum mass flow rate considered in the study, the highest second law efficiency is obtained as 0.45, which corresponds to a value greater than 80% in the case of CST.