Structural Design and Its Impact on Thermal Efficiency and Corrosion of All-Aluminum Microchannel Heat Exchangers

Abstract

In this study, high-fidelity conjugate heat transfer simulations are used to model a micro-channel heat exchanger (MCHE) in a crossflow to study its thermal-hydraulic performance. This study considers three different microchannels (internal flow) geometries (circular, triangular, and square) with louver-shaped fins. The local flow field showed a strong coupling between the microchannel flow, solid domain, and crossflow. The flow separation and wake regions formed near MCHE resulted in a large variation in the velocity field and temperature in the crossflow. The wake region had a significant spanwise variation due to its interaction with fins, which also causes variations in the thermal boundary layer. The heat conduction in the solid structure provided a non-uniform temperature field with a higher temperature near the microchannel and a slightly lower temperature near the surface exposed to the crossflow. The microchannel flow analysis showed that the internal geometry affects the pressure drop, which is highest for the triangular MCHE and lowest for the circular MCHE. However, the microchannel flow temperature change was relatively similar for all microchannels. Results showed that for the same volume of the microchannel, the circular shape microchannel has a higher performance index value than the triangular and square shapes. This study also considers three different fin (external flow/crossflow) geometries (louver, step, and saw) with the same tube and circular shape microchannel and identifies the corrosion hot spot. Crossflow shows higher temperatures near the boundary layer of the tube, which results in higher corrosion rates. A predicted flow field also identifies crevices between fins and tube surfaces as critical corrosion hot spots often associated with low-velocity regions. Electrochemical impedance spectroscopy (EIS) analysis was done on AA3102 (Alloy used in the circular channel and louver fin) alloy in corrosive environments containing low and high concentrations of the combination of sodium chloride and ammonium sulfate. Electrolytes used in this research have pH values ranging from 4.0 to 5.8, closer to nearly neutral environments encountered in many atmospheres. EIS results are presented, including Rsol, Rpore, and Rct of AA3102 with very thin arc evaporated porous Zinc film on AA 3102 along with their equivalent circuit.