Optimization of heat transfer performance of a micro-bare-tube heat exchanger using a genetic algorithm

This study presents the optimization of the heat transfer coefficient of a micro bare tube heat exchanger. A physical and mathematical model of a micro bare tube heat exchanger was built in Matlab using the simplified & epsilon;-number of transfer unit method. Using a temperature of minus 30 & DEG;C and a 0.5-1.0 mm tube outer diameter, a 1.7-8.0 mm longitudinal tube pitch, a 1.7-5 mm transverse tube pitch, and a 1.0-5.0 m/s velocity at minimum free flow area, with carbon dioxide as the refrigerant, a comparative variation analysis was performed to optimize the heat transfer coefficient of a micro-bare-tube heat exchanger. The results demonstrate that the heat transfer coefficient increases as the inlet air velocity is increased from 1.0 to 5.0 m/s, with a final gain of 93.17%. The growth rate of the heat transfer coefficient steadily decreases with increasing inlet air velocity and gradually approaches zero. The effect of the gradual decrease of the tube outer diameter from 1.0 to 0.5 mm on the heat transfer coefficient is 22.52% greater than that of the gradual decrease of the transverse tube pitch from 2.2 to 1.7 mm. The study also carried out an optimization analysis on the distribution of four different variables in the heat exchanger. With the use of a genetic algorithm, the study found an optimal distribution to maximize the heat transfer coefficient. The following parameter values resulted in the maximum heat transfer coefficient: a maximum inlet air velocity of 5.0 m/s, a minimum tube outer diameter of 0.5 mm, a 1.7 mm longitudinal tube pitch, and a 1.7 mm transverse tube pitch.