Experimental investigation on thermal characteristics and performance enhancement of pulsating heat pipe with ultra-maximum hydraulic diameter

Abstract

The lower dry-out limit of the conventional small-diameter pulsating heat pipe (PHP) hinders its large-scale application in solar collectors. In this paper, a new type of ultra-maximum hydraulic diameter PHP (UHDPHP) with a diameter of 6 mm was designed, exceeding the theoretical maximum critical diameter by 92.93 % when ethanol served as the working fluid. The start-up and thermal characteristics of UHDPHP filled with water and ethanol were comparatively investigated. Three strategies were tried to further optimize UHDPHP performance. The results show that the UHDPHP can still be activated and display excellent performance. Compared to the ethanol-based UHDPHP, water-filled UHDPHP exhibits a 45.04 % reduction in thermal resistance and a 4.10 ℃ decrease in average temperature deviation, and also shows lower operating temperature fluctuations. Ultrasonic degassing of water enhances the thermal performance of UHDPHP by nearly 20 %. Among various mixtures, the 1:1 ethanol–water UHDPHP demonstrates the highest performance improvement, with an enhancement ratio of approximately 66 % compared to ethanol. The sensitivity of UHDPHP performance to the inclination angle is reduced by integrating a buffer tank, and this strategy can enhance performance by up to 35.08 %. Even at an inclination angle of 45°, the thermal performance of the enhanced UHDPHP can still reach 16.3 times that of the copper pipe. These results may be useful for designing efficient solar collectors based on large-diameter PHPs.