Transient Behavior and Maldistribution of Two-Phase Flow in Parallel Channels

Compared with a single channel of equivalent volume, parallel channels provide a larger surface area for heat transfer and are, therefore, a common design feature in two-phase heat exchangers. However, flows through parallel channels often encounter uneven flow distribution, resulting in the deterioration of thermal performance, leading to permanent device failure. Therefore, there is a need to understand the process and nature of flow maldistribution in parallel channels. This study investigates factors that influence the process of flow maldistribution in two parallel channels with the same inlet and outlet manifolds. Both model predictions and experimental characterization show significant flow maldistribution (flow ratio >10) when geometrically similar parallel channels (diameter ~1 mm, and length ~0.1 m) encounter disturbances such as sudden variations in heat load (e.g., 50% increase) and total flow rate (e.g., 50% decrease). Due to flow maldistribution, the resulting temperature distribution in the parallel channels can be drastically different. Besides changes in operating conditions, other factors influencing flow distribution include channel geometry and thermophysical properties of channel wall material. Although the wall properties do not affect the individual channel pressure drop curve at steady state, they influence the transient process that leads to flow maldistribution in parallel channels. Furthermore, we found that the predicted flow distribution assuming a radial lumped analysis can differ from models accounting for heat diffusion in the radial direction. We believe these observations are crucial in understanding the effect of flow maldistribution on parallel channel evaporators’ performance and can be used as a guideline to develop better heat sinks for electronics thermal management.