CALCULATION METHOD FOR FIN-AND-TUBE HEAT EXCHANGERS OPERATING WITH NONUNIFORM AIRFLOW

Abstract

This paper presents a new calculation method for fin-and-tube heat exchangers operating with nonuniform airflow. The calculation method consists of discretizing the heat exchanger tubes into tube elements and applying mass and energy conservation equations. The calculation method is capable of predicting local and overall values of thermal effectiveness, heat transfer rates and fluid outlet temperatures in fin-and-tube heat exchangers subject to airflow nonuniformity. The results of the tube element method are compared against experimental data, and good accordance has been achieved between predicted and measured values. Experimental data is collected for fin-and-tube heat exchangers placed inside an open wind tunnel, where airflow nonuniformity is generated by partially obstructing the entrance cross section. This study revealed that airflow nonuniformity causes effectiveness deterioration in fin-and-tube heat exchangers. The exact value of effectiveness deterioration depends on the heat exchanger dimensionless groups – the number of heat transfer units (NTU) and the heat capacity rate ratio ( C *) – as well as on the intensity and orientation of the airflow nonuniformity. The design of the tube-side fluid circuitry, which affects the heat exchanger flow arrangement, also plays a role in the effectiveness deterioration. The tube element method revealed that the effectiveness deterioration increases with the intensity of airflow nonuniformity. For an observed nonuniform airflow profile, the effectiveness deterioration is maximum in heat exchangers with a balanced heat capacity rate ratio ( C * = 1) and minimum in evaporators and condensers ( C * = 0).