Performance analysis and simplified modelling of a capillary jet loop heat pipe

Abstract

In this study, a prototype Capillary Jet Loop Heat Pipe has been tested to investigate its performances, and a lumped parameters simplified model has been developed to study the key parameters of the device. The tested Capillary Jet Loop is composed of an evaporator, a fluid loop with an ejector and a condenser. The working fluid used is R1233zd(e); an electric resistor of 40x40 mm2, dimensionally similar to a CPU, provides the thermal load, and a water cooling circuit removes it. An experimental investigation on the thermal behaviour has been conducted, varying the orientation, the thermal load from 40 W to 160 W, and the condenser cooling water temperature from 15 °C to 30 °C. The temperatures measured in different locations of the device have been used to estimate the equivalent thermal resistances. The results show a strong relation between performances and orientation and low dependence on thermal load and condensing temperature. The lowest equivalent thermal resistance value is obtained in the horizontal configuration with a value of 0.104 ± 0.005 K/W (CI 64%) at 80 W (5 W/cm2) while it reached the maximum value of 0.202 ± 0.002 K/W at 160 W (10 W/cm2) in vertical bottom heated orientation. The simulation includes a lumped parameter model focused on the two-phase evaporation condensation and capillary phenomena coupled with the fluid dynamics in the loop and in the ejector. Steady-state conditions are simulated. Under some simplified hypotheses, a parametric analysis was performed to investigate the influence on the thermal performances of the different construction and operative variables of the Capillary Jet Loop: loop length, tube diameter, ejector diameter, pore diameter, filling ratio, the fluid temperature. The maximum stable thermal load is calculated, and a stability map is found as a function of the fluid temperature. The matching point between the Loop and the ejector curve identifies the operating system point as a function of the studied parameters.