Local heat transfer approach to the start-up analysis of an ultra-thin loop heat pipe

Loop heat pipes are two-phase, passive heat transfer devices that exhibit attractive features for thermal management applications, including micro-electronics and battery packs cooling. To enhance the modelling and optimization of such heat transfer devices, a better understanding of their working behaviour is needed, especially in terms of device response to start-up transients. To this aim, a novel local heat transfer approach is proposed and applied to the experimental investigation of a copper loop heat pipe partially filled with ethanol, whose ultra-thin layout has been specifically designed for embedment in electronic devices. The evaporator section is heated by means of an electrical resistance, while the condenser is cooled by free convection. The outer wall temperature along the whole condenser is monitored during the start-up phase of the device at varying heat loads through a medium-wave infrared camera. The temperature signals, referred to six wall sections, are postprocessed by means of the Inverse Heat Conduction Problem resolution approach, resulting in the assessment of the heat fluxes exchanged between the working fluid and the device wall over time in both the vapor and liquid lines. The inverse method is successfully validated by means of synthetic data, whereas the experimental procedure is calibrated and verified by preliminary experimental tests. Start-up results show comparable trends in the wall-to-fluid heat flux profiles with the heat input, exhibiting peak values of about 2300 W/m2. Through the present non-intrusive technique, fluid velocity in the vapour line is also estimated in the range 0.008 - 0.012 m/s. To the authors' knowledge, this represents one of the first attempts of characterizing both local heat transfer quantities and inner fluid dynamics in loop heat pipes via experimental approaches.