Channel-to-channel heat flux variation in compact minichannel heat exchangers due to the effect of louvered fins

Abstract

A typical minichannel evaporator used in AC&R applications is made of stacked parallel aluminum extruded multiport plates and louvered fins placed between them. In this configuration cold refrigerant flows inside the ports, and hot air passes though the louvered fins. Heat flux variation between neighboring minichannels, in the direction from the leading to the trailing edge, may be expected since warmer air enters the louvered fins domain and its temperature is reduced at the outlet. The possible nonuniformity of the heat flux from channel to channel was studied numerically using ANSYS Fluent as a threedimensional time-dependent heat transfer problem of louvered fins bounded with multiport aluminum plates. While the fin geometry was kept constant in all simulations, two different multiport plate configurations (11 round ports, D= 1.2 mm; and 22 square ports, 0.54 0.54mm2) were analyzed at air face velocities from 1 m/s to 5 m/s. The wall temperature of all channels was set to be constant 10 oC, which corresponds to the typical saturation temperature of refrigerants used in AC&R applications. The incoming air flow temperature considered was 20 oC and 30 oC. Results illustrate that both air velocity and temperature play a profound role on heat flux variation from the leading to the trailing edge of the multichannel plate. The heat flux varies drastically in the case of the slower incoming air flow due to the significant change in the driving potential along the air flow, and it varies less at higher air velocities due to the heat transfer recovery effect behind the turning louver along with the smaller driving temperature difference between mixing cup and saturation temperature. The overall heat flux difference between the leading channel and the trailing one reaches almost 94% at free stream air velocity 1 m/s and 69% at air velocity of 5 m/s. This numerical modeling of the conjugate heat transfer problem proves the presence of heat flux difference among channels which was overlooked in the literature. Understanding of the channel-to-channel heat flux variation is valuable for understanding the flow boiling behavior in parallel non-uniformly heated minichannels and the two-phase flow maldistribution.