Analysis of thermally driven flow behaviors for two-turn closed-loop pulsating heat pipe in ambient conditions: An experimental approach

A two-turn closed-loop pulsating heat pipe (CLPHP) with a transparent glass tube (borosilicate glass) having an inner diameter of 2 mm is manufactured and designed. The manufactured CLPHP is tested under ambient environmental conditions, mounted in the bottom heat vertical mode position and charged with various filling ratios (FRs), using ethanol as a working fluid. The overall length of the device is 210 mm, the length of the evaporator section is 50 mm, and the remainder of the device consists of the condenser section, which is fully exposed to the ambient room temperature. A series of experiments are performed to investigate the thermal performance and observe the corresponding flow patterns. The thermal resistance (R-th) results indicate that the optimal FR is 50%, and the R-th significantly decreases with an increase in the heat input power (Q(in)) for an FR of 50% ethanol. The initial flow motions of the liquid slugs (LSs) and vapor plugs (VPs) are observed at a Q(i)(n) of 10 W for all FRs. Notably, oscillations of small VPs and LSs appear, along with the contributions of dispersed bubbles and large VPs at lower heat loads, while the oscillations of annular VPs, semi-annular VPs, and bulk circulations are larger at higher Q(i)(n), values. Liquid thin films are observed in the evaporator and condenser at high Q(i)(n) values. The present experimental approach facilitates the design of a glass-tube CLPHP that operates under ambient conditions. The ambient temperature provides the necessary cooling source for recondensing the fluid to operate the device successfully. (C) 2019 Elsevier Ltd. All rights reserved.