Variable geometry microchannel heat exchanger modeling under dry, wet, and partially wet surface conditions accounting for tube-to-tube heat conduction

Abstract

Numerical models for microchannel heat exchangers are favored in the research and development process due to their cost effectiveness as opposed to prototype development and testing. A literature survey suggests that a unified air-to-surface heat and mass transfer modeling approach is lacking for microchannel heat exchangers, especially under dehumidifying conditions with tube-to-tube heat conduction. This research presents an air-to-fin heat and mass transfer model for a microchannel heat exchanger operating under dry, wet, and partially wet conditions. Typically, there are two boundary conditions for the fins in microchannel heat exchangers. The adiabatic fin tip boundary condition is applied to the extended fins on the top and bottom of the microchannel slab. The second boundary condition is the prescribed surface temperature, applicable to a fin bounded by two tubes. The proposed fin analysis method accounts for both boundary conditions and tube-to-tube conduction. The modeling approach is capable of locating the boundary between a dry and wet surface if a fin is partially wet. The model is verified against simulation results for air-to-surface heat transfer on a fin obtained using a commercially available computational fluid dynamics package. A new finite-volume microchannel heat exchanger model is developed using the proposed fin analysis method. The model is capable of predicting the performance of a variable geometry microchannel heat exchanger under both dry and dehumidifying conditions and is validated against experimental data. The average absolute capacity deviation between the predicted and measured values is 2.44% for condensers and 2.92% for evaporators. The absolute difference in sensible heat ratio for evaporators between predicted and measured values is 0.018. The proposed model allows for the most comprehensive and accurate analysis of microchannel evaporators and condensers.