Experimental Study and Mitigation of Pressure Drop Oscillation using Active Control

Flow boiling in microchannel heat exchanger is widely recognized as an efficient mode for high heat flux removal due to its compact structure, lower coolant usage, high heat transfer coefficient, and relatively uniform temperatures. However, there are some critical issues accompanied with flow boiling in microchannels, including critical heat flux, partial dry-out, and flow instabilities. Flow instabilities, like pressure drop oscillation (PDO) and maldistribution, would lead to non-uniform wall temperature distribution, flow reversal and local dry-out. In this paper, we discuss the experimental results of vapor compression cycle (VCC) system incorporated with two different microchannel designs. For one channel design, the channel pressure drop characteristics show no negative slope under different evaporator heat loads and PDO does not occur, which could be attributed to the small hydraulic diameters and large mass flow rate. For the other microchannel design, PDOs are observed and the effects of several system parameters, including valve setting, accumulator heat load and evaporator heat load, on the occurrence and the amplitude of oscillation are investigated. Based on the understanding gained from this analysis, a dynamic control strategy is developed to stabilize and maintain the system at the optimum operating conditions using a simple feedforward controller, which provides a guideline for future development of more efficient active control methods.