This paper presents an experimental study and theoretical interpretation of two-phase flow in a closed loop. The objective of this work is to find the optimum flow rate with respect to the thermal design power (amount of heat to be rejected). We assume that forced-convection boiling characteristics are explained based on mass and energy conservation, and claim that our proposed coefficient (CQL / Q : a ratio of amount of evaporated liquid to the flow rate) indicates the optimum flow rate for wide variation of evaporator-shapes and working fluids. In order to verify our model, we have measured the thermal resistance of evaporator with respect to heater input power for various flow rates. Hydrofluoro ether (HFE) and Fluorinert™ refrigerants were used as the working fluid in the experiment. Here flow rate of 40∼120ml/min and thermal design power of 50∼200W were controlled by the pump and by the heater, respectively. We observed that the coefficient resulted in the optimum flow rate is almost the same regardless of working fluids and evaporator shapes. The data which indicates the optimum flow rate were quite well reproduced by our proposed theory when the value of this coefficient is C ≈ 0.7∼0.95. For the demonstration, we designed the assembled-type two-phase cooling module with the optimum flow rate based on our model, and we observed that the evaporator had a relatively small thermal resistance of 0.1K/W.

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