Experimental investigation of a flat-plate closed-loop pulsating heat pipe

The thermal performance and operating modi of a flat-plate closed-loop pulsating heat pipe (PHP) are experimentally observed. The PHP is manufactured through computer numerical controlled milling and vacuum brazing of stainless steel 316 L. Next to a plain closed-loop PHP, also one that promotes fluid circulation through passive Tesla-type valves was developed. Each channel has a 2 * 2 mm² square cross section, and in total, 12 parallel channels fit within the 50 * 200 mm² effective area. During the experimental investigation, the power input was increased from 20 W to 100 W, while cooling was performed using a thermo-electric cooler (TEC) and thermostat bath. Three working fluids were assessed: water, methanol, and ammonia. The PHP was charged with a 40% filling ratio. Thermal resistances were obtained for different inclination angles. It was observed that the PHP operates well in vertical evaporator-down orientation but not horizontally. Moreover, experiments show that the minimum operating orientation is between 15 and 30 deg. Two operating modi are observed, namely, the thermosyphon modus, without excessive fluctuations, and the pulsating modus, in which both the temperature and pressure responses oscillate frequently and violently. Overall thermal resistances were determined as low as 0.15 K/W (ammonia) up to 0.28 and 0.48 K/W (water and methanol, respectively) at a power input of 100 W in the vertical evaporator-down orientation. Infrared thermography was used to visualize the working fluid behavior within the PHPs. Infrared observations correlated well with temperature and pressure measurements. The experimental results demonstrated that the developed flat-plate PHP design, suitable for high-volume production, is a promising candidate for electronics cooling applications.