Parallel-channel flow instabilities and active control schemes in two-phase microchannel heat exchanger systems

Parallel-channel flow mal-distribution and pressure-drop flow oscillations are two of the most severe dynamic instabilities for boiling flow especially in microchannel systems. This paper presents a framework for the transient analysis and active control of microchannel flow instabilities at a system-level view. A lumped two-phase flow system model is derived from the momentum balance equation to capture the characteristics of the microchannel heat exchangers. Bifurcations of flow distribution and inlet pressure can arise in parallel-channel two-phase flow systems. This paper investigates the control-theoretic properties with different control devices, including inlet valves and supply pump. Individual control valves at the inlet of each channel can be used to suppress both flow mal-distribution and flow oscillations effectively, although this scheme is subject to higher pressure loss and potential higher supply pumping power. Using the pump alone can only suppress pressure-drop flow oscillations, but not for flow mal-distribution in two identical parallel channels. However, we make an interesting observation that with different channel properties, we regain controllability from the pump and observability from a single channel flow rate measurement.

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