Local and Average Transfer Coefficients for One-Row Plate Fin and Tube Heat Exchanger Configurations

Abstract

The analogy between heat and mass transfer has been used to obtain local and average transfer coefficients for a one-row plate fin and tube heat exchanger configuration. The mass transfer experiments were performed using the naphthalene sublimation technique. A heat exchanger passage was modeled using naphthalene plates; spacer disks simulated the tubes. Detailed measurements of the surface elevation of the naphthalene plates were made before and after a data run, and this enabled evaluation of the local transfer coefficients. Average transfer coefficients were determined both by surface integration of the local mass transfer or by weighing the test plate with a precision balance; the two methods gave results that agree within one or two percent. The local measurements revealed high values of the transfer coefficient on the forward part of the fin due to the presence of developing boundary layers. In addition, owing to a natural augmentation effect caused by a vortex system which develops in front of the tube, there are very high fin transfer coefficients in a U-shaped band that rings the tube. The effect of the vortex system is more pronounced at higher Reynolds numbers. Relatively low transfer coefficients are encountered on the portion of the fin that lies downstream of the minimum flow cross section. The coefficients are especially low in the region behind the tube.