Infrared analysis of the two phase flow in a single closed loop pulsating heat pipe

A Single Loop Pulsating Heat Pipe (SLPHP) filled at 60% vol. with pure ethanol, having an inner diameter of 2 mm, is designed with two sapphire tubes mounted between the evaporator and the condenser. Being the sapphire almost transparent in the infrared spectrum, such inserts allow simultaneous high-speed visualizations and infrared analysis of the fluid regimes. Two highly accurate pressure transducers measure the pressure at the two ends of one of the sapphire inserts. Three heating elements are controlled independently, in such a way to vary the heating distribution at the evaporator. It is found that particular heating distributions promote the slug/plug flow motion in a preferential direction, allowing to establish a self-sustained circulatory motion. It is demonstrated that a direct infrared visualization of the two-phase flow passing through the sapphire inserts is a valuable technique to measure the liquid slug bulk temperature after a proper calibration of the camera, with uncertainty of 1.5 °C (99.7% confidence level). Additionally, the fluid infrared visualization allows to appreciate the liquid film dynamics (wetting and dewetting phenomena) during the device operation, and to map the temperature gradients of liquid slugs thanks to the high-sensivity of the infrared measurements (0.05 K).