Model-based control of vapor compression cycles for transient heat-flux removal

Systems-level design and control techniques have great potential for increasing efficiency, performance, and reliability of refrigeration cycles for electronics cooling. In contrast to traditional VCC applications, evaporator heat-flux can be large and time-varying in electronic systems. We perform in-depth analysis of direct-expansion, multiple-evaporator vapor compression cycles (VCC) with imposed heat-flux. Static analysis provides nominal operating conditions and inputs for open-loop control, which are tested experimentally. Observation of critical heat-flux (CHF) during transients demonstrates need for closed-loop control. A low-order lumped dynamic model based on one-dimensional mass and energy balance for the heat exchangers is applied and validated with experimental data. The model is then linearized and used to design CHF avoidance controllers. A decoupled approach using proportional-integral (PI) control of compressor speed and accumulator heating is taken for single-evaporator VCC, and multivariable H2 control is applied for two-evaporator systems. The control strategies are shown to be effective in experimental tests, avoiding CHF for nominal heat load disturbances while satisfying system constraints.