Analysis and Active Control of Pressure Drop Oscillation in Microchannel Vapor Compression Cycle

Using phase-change cooling, microchannel evaporators can dissipate significantly higher heat fluxes with large heat transfer coefficients and faster response time compared to conventional macroscale evaporators. However, it is well known that microchannel heat exchangers are prone to flow instabilities. The analysis of flow instabilities has been limited to simple open cycle systems consisting of the microchannel evaporator with fixed boundary conditions, which has limited application in closed cycled systems with temporally varying operating conditions. This paper analyzes the pressure drop oscillation phenomenon occurring in a vapor compression cycle (VCC) consisting of a microchannel evaporator, compressor, condenser, electronic expansion valve, and accumulator. Using a combination of lumped dynamic and static models for the system components, we predict the occurrence of pressure drop oscillation in the VCC under certain operating conditions with linearized analysis and nonlinear simulations. Furthermore, we demonstrate how a VCC with flow oscillations can be stabilized by changing the expansion valve openness. Once the flow oscillations in the system is removed, active control of the compressor speed and accumulator heat input based on evaporator wall temperature feedback can effectively regulate the evaporator temperature, even with a fluctuating heat input to the evaporator. As a result, this allows the use of a microchannel-integrated VCC system for dissipating high heat fluxes as well as addressing time-varying cooling demands.