A novel triple-diameter pulsating heat pipe: Flow regimes and heat transfer performance

Abstract

A single-turn triple-diameter pulsating heat pipe (TD-PHP) with a total length of 250 mm (the evaporation, adiabatic, and condensation lengths were 25 mm, 125 mm, and 100 mm, respectively) made of a transparent Pyrex glass was designed and fabricated for the first time. The thermal performance and flow characteristics of the TD-PHP were compared with two other fabricated PHPs, the single-diameter (SD) and dual-diameter (DD) PHPs, to determine the effect of non-uniform channel diameters. The PHPs were mounted on a tilting frame, and their thermal performances were tested with various heat inputs, inclination angles, and filling ratios. The temperatures of the evaporation, condensation, and adiabatic sections and flow behaviors were monitored. Results indicated that circulating flow was enhanced by using TD-PHP, and thermal resistance was reduced considerably. The visualization results showed that the bubbles' generation, growth, and breakage leading to enlarged vapor plugs were enhanced because of the unbalanced gravitational forces owning to the channel with non-uniform diameter. Hence, the TD-PHP was functional even at inclination angles near the horizontal angle.