A Generalized Effectiveness-NTU Based Variable Geometry Microchannel Heat Exchanger Model

Microchannel heat exchangers (MCHXs) are now widely used in the HVAC&R industry. Similar to tube-fin heat exchangers used mainly in the indoor units, the innovative variable geometry microchannel heat exchanger will further reduce its compact size and enhance its performance. Variable geometry refers to a heat exchanger with different tube and fin surfaces within the same core with one or more tube banks. A comprehensive literature review reveals that there is no modeling approach in the literature that can handle such arbitrary geometry. This paper introduces a new microchannel heat exchanger model which accounts for variable port, tube and fin geometry, variable tube and fin location and variable number of tubes per bank and variable fin density. This model adopts a port-by-port calculation approach on air side and the refrigerant side. The model is based on three-stream NTU method to account for variable fin type, height and air flow on top and bottom of a given tube. The heat exchanger location is based on a Cartesian grid which would account for the air propagation through multiple banks with variable geometry and fin location. Since the basic heat exchange calculation is performed at the port-level, the model lends itself very easily to account for port-level refrigerant flow maldistribution. In addition, the decrease in air-side heat transfer coefficient along the depth in the direction of the air flow can also be accounted for. Empirical correlations from literature are used for local heat transfer coefficient, pressure drop and void fraction calculations. The model is validated against experimental data for standard geometry microchannel condenser.