The Steady-state Modeling and Static System Design of a Refrigeration System for High Heat Flux Removal

This paper investigates the steady-state modeling and static system design of a refrigeration system for high heat flux removal of high power electronics system. The refrigeration cycle considered consists of multiple evaporators, liquid accumulator, compressor, condenser and expansion valves. In contrast with conventional refrigeration systems with liquid-to-liquid heat exchangers for temperature control where the critical heat flux (CHF) is not a major concern, refrigeration systems for high heat flux removal have to ensure that the incoming heat flux is lower than the CHF to prevent device burnout. Since the superheated region in the evaporator has much lower heat transfer coefficient than the two-phase region, the evaporator exit should be two-phase for ensure sufficiently high CHF. The two-phase evaporator exit necessitates the inclusion of a heated liquid accumulator for the safe operation of the compressor to ensure only saturated vapor enters the compressor. The evaporators and condenser of the cycle are modeled by the mass balance, momentum balance, and energy balance equations. Due to the future utilization of microchannels to enhance heat transfer in heat exchangers, the momentum equation, rarely seen in previous modeling efforts, is included here to capture potentially significant pressure drops. The expansion valve and compressor are modeled as static components. The accumulator is modeled to regulate the active refrigeration charge of the system and to provide just enough heat to the outflow of the evaporator such that the inflow of the compressor is always saturated vapor. Based on the steady-state model, the static system design issues include determining the total refrigerant charge of the system to accommodate the varying operation conditions, sizing of the compressor and accumulator, and finding the optimal operation condition for given incoming heat flux to optimize the Coefficient of Performance (COP) while satisfying the CHF and other constraints. The steady-state model will be validated on a testbed currently in construction. The testbed consists of a reciprocating compressor with variable frequency drive, a plate condenser, a heated accumulator (tank with electric heater), three evaporators with immersed electrically controlled heaters, and one electronic expansion valve for each evaporator.