Analysis and active control of flow instabilities in microchannel cooling systems

Abstract

Systems involving multiphase flow and phase-change phenomenon are used extensively in industry, including power generation, heating, air-conditioning, and heat management in electronics. Controlling the phase-change characteristics can improve the performance of many industrial applications. Specifically, phase-change occurring in confined geometries, like microchannel evaporators, has gained significant interest due to its potential to dissipate large heat fluxes, which cannot be done using conventional evaporators. However, systems incorporating microchannel evaporators are prone to challenges. Instabilities like pressure drop oscillation and flow maldistribution in the channels of the evaporator can cause an irreversible failure of the system. This thesis analyzes these instability mechanisms in vapor compression and pumped liquid cycles, which represent a large portion of two-phase systems used in the industry.

The thesis characterizes the multiphase flow and heat transfer via experiments and computational modeling and conducts a system-level investigation of various factors affecting the overall stability. In both cycles, the pressure drop oscillations occur only under certain combinations of system parameters, which could be predicted using computational modeling. The ability to predict system behavior led to the development of active control strategies that successfully avoid flow instabilities, even in the presence of significant variations in the heat loads. This approach allows maintaining conditions that maximize the efficiency of the two-phase systems. The control methodology was also extended to handle multiple evaporators experiencing asynchronous and unanticipated heat loads. This thesis also analyzes the challenge posed by flow maldistribution accompanying pressure drop oscillation in parallel channels and multi-evaporator systems. The computational model and experiments indicate that the extent of thermal and flow coupling between the parallel channels or evaporators can affect flow maldistribution. The study demonstrates better synchronization in performance when the coupling between the channels or evaporators is improved. Several strategies advanced in this thesis can be readily implemented in several industrial applications, making them more robust and efficient in handling a wide range of operating conditions.