Thermodynamic Analysis and Experimental Characterization of Hysteresis Associated With Flow Maldistribution in Thermally Isolated Parallel Channels

Abstract

Flow boiling heat transfer in parallel channels has garnered significant attention due to its relevance in various engineering applications. Since two-phase flow behavior is highly nonlinear, the solution to the steady forms of mass, momentum, and energy balance equations applied to flow in heated parallel channels can be nonunique. With multiple solutions, flow in a heated parallel-channel system may also exhibit hysteresis, making the system's current state dependent on its history. Existing stability criteria offer no physical insight into the preference of one solution over the other possible flow solutions during the hysteresis process. The underlying physical mechanisms are yet to be investigated, which this study aims to address by adopting a thermodynamic perspective to understand flow hysteresis in parallel channels. By using experiments on two thermally isolated channels in a closed pumped loop cycle with water as the working fluid, we evaluate the entropy generated in the system for increasing and decreasing orders of flow and heating rates. The experiments revealed that flow maldistribution could be thermodynamically favored over a uniform flow state corresponding to the same system operational parameters, such as pump frequency, valve settings (percentage open or closed), and heating rates. We also conducted a parametric study to determine the effect of system parameters on flow distributions and hysteresis in parallel channels. In this regard, tunable system parameters, especially valves positioned before the evaporator, can affect hysteresis, supporting strategies like inlet restrictors to minimize flow maldistribution in parallel channels.