Finite Difference Heat Exchanger Model: Flow Maldistribution with Thermal Coupling

Abstract

Channels not receiving the same amount of flow (flow maldistribution) is an important effectiveness loss for high effectiveness heat exchangers. This paper develops a finite difference model of a counter flow heat exchanger. It reproduced the simple NTU result for two channels exactly. For more than two channels, there is an edge effect, and the model agreed within 0.1% of the literature. Perfect agreement was achieved for a simple flow maldistribution case. For independent pairs of hot and cold channels, as flow rate is reduced, the effectiveness asymptotically approaches the irregularity parameter (a measure of channel flow maldistribution). With thermal coupling between pairs of channels (measured by an equivalent Peclet number), effectiveness continues to increase with decreasing flow rate. These results hold for uncorrelated flow maldistribution on both sides or no flow maldistribution on one side. However, when flow maldistribution is positively correlated, effectiveness is higher; and, when it is negatively correlated, effectiveness is lower. A number of resulting graphs illustrate the effectiveness for each channel with different flow rate and correlation. While one exemplary application is a polymer expanded microchannel heat exchanger, the model could be used for other heat exchangers.