Multiple-evaporator vapor compression cycles may be used for distributed cooling of high heat-flux systems, such as arrays of high-power electronics. Under transient heating conditions, these systems must be carefully controlled to avoid critical heat flux (CHF) due to evaporator dryout. An active control strategy is presented that regulates two-phase flow quality in multiple evaporators in order to avoid critical quality under transient heating conditions. A two-loop control system is used, in which an outer loop uses model-based feedforward combined with evaporator wall temperature feedback to determine the necessary coolant flow rate to avoid CHF, while an inner loop uses system actuators (variable speed compressor, electronic expansion valves) to track to the desired flow rate. An advantage of this approach is that the inner-loop control handles the system complexity arising from pressure coupling and actuator nonlinearity. Additionally, the outer-loop quality control may be applied to other two-phase cooling schemes, for instance pumped systems, by providing coolant flow rate setpoints. Simulations and corresponding experimental controller validation were conducted using a three-evaporator vapor compression testbed with transient imposed heat-flux.
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