Two-phase thermofluidic modeling and validation of a multi-zone microchannel evaporator

In this work we present a numerical modeling framework to predict the thermofluidic performance of a multi-zone microchannel evaporator operating under forced-convection (pumped) and thermosyphon (gravity-driven) modes. The base model assumes uniform refrigerant flow and heater power to each evaporator zone and solves for coupled heat transfer and fluid flow within an individual microchannel evaporator zone, including two-dimensional conduction in the evaporator base plate. The approach utilizes established correlations for heat transfer and pressure drop within the microchannels and solves the resulting system of coupled differential equations numerically. Frictional pressure drops associated with the inlet and outlet plenums and the T-junction formed by the outlet plenum and riser tube are also estimated and included in the base model. The modeling results are validated with an experimental database for an 18-zone microchannel evaporator designed for cooling a telecommunications equipment shelf, with each zone targeting heat removal from a single ½ U circuit pack card and using R134a as the working fluid. The base model is then extended to account for zone-to-zone variations in refrigerant mass flow rate by developing a local pressure drop model that includes the effects of the inlet and outlet plenums that, respectively, supply refrigerant to and remove refrigerant from the individual evaporator zones. This model enables investigation of the thermofluidic characteristics of the multizone evaporator for uniform and spatially varying heat load profiles. We compare flow maldistribution effects of different evaporator designs under different operating conditions, and the corresponding predicted refrigerant flow and temperature distributions. The model provides insight into the design of the microchannel evaporator to prevent flow maldistribution.