Experimental studies on the transient characteristics and start-up behaviour of a miniature loop heat pipe

To expand the knowledge of miniature loop heat pipes under different conditions, i.e., vacuum and ambient, it is essential to study the start-up and transient characteristics of the device. Transient experiments were performed on a cylindrical evaporator miniature loop heat pipe at different heat loads from 5 W to 148 W and sink temperatures from −10 °C to 35 °C. The device was tested under ambient and vacuum conditions to better understand the transients under different loading conditions. Temperature oscillations were observed at the condenser outlet for all heat loads at lower sink temperatures and for a limited range of heat loads at higher sink temperatures. Experiments were also performed to study the hysteresis during heating and cooling cycles under different sink temperatures. Thermal hysteresis was observed in the device under lower sink temperature (0 °C). Start-up studies were conducted for different heat loads and sink temperatures, in which temperature oscillations were observed for specific ranges of heat loads. Overshoot, oscillating, and smooth startups were observed under different operating conditions. With increase in the heat load, the startup time was found to decrease initially and then increase for higher heat loads. The condenser outlet temperature showed large oscillations of about 2.5 °C amplitude and 80 s period for a 40 W heat load. The device can start at a low heat load of 4.8 W for the sink temperature of 10 °C. The trends observed in this work are similar to those reported in the literature.