An original look into Pulsating Heat Pipes: Inverse heat conduction approach for assessing the thermal behavior

A promising solution in the field of passive two-phase heat transfer devices is represented by Pulsating Heat Pipes (PHPs). These relatively new devices, which achieves resounding interest in terms of high heat transfer capability, efficient thermal control, flexibility and low cost, have been extensively studied in the last years by many researchers. Several studies have been carried out to deeply characterize the thermal behavior of Pulsating Heat Pipes. Although many authors have investigated the heat flux values for these devices, almost all of them have presented the results only in terms of the mean values at the evaporator and the condenser area. High-speed and high-resolution infrared imaging is performed on a single loop PHP designed with sapphire inserts. Sapphire, being almost transparent in the IR-spectrum, allows to measure the temperature of the fluid inside the pipe by means of IR analysis. The sapphire tube is partially coated with a highly emissive paint, allowing in this way to determine at the same time the external wall temperature and the fluid temperature. In this work a novel approach to investigate the local heat flux in PHPs is presented and tested: the temperature distributions on the external wall of the PHP were used as input data for the inverse heat conduction problem in the wall under a solution approach based on the Tikhonov regularization method. Heat is released from the fluid to the sapphire wall when a two-phase flow is pushed with a high temperature from the evaporator; increasing the wall tube temperature. On the contrary, when a cold flow is pushed back from the condenser, the tube releases the heat previously accumulated; thereby decreasing its temperature. This approach allows to analyze the thermal behavior of the device by investigating the direct interconnection between the thermo-fluid dynamic phenomena within the PHP and the local heat flux measurements. The results proposed in this work could be a breakthrough for developing and validating advanced lumped parameter models and emerging CFD simulations of PHPs.