Review of Pulsating Heat Pipe Working Fluid Selection

The pulsating heat pipe (PHP) is a novel, simply formed, wickless heat pipe that relies on the phase change induced motion of a contained working fluid to transport heat between the evaporator (the hot end) and condenser (the cold end). The improved heat transfer capability, simplicity, and reduced mass of PHPs have lead to great interest in the PHP. This paper reviews one crucial, yet often overlooked, aspect of PHP operation: the working fluid. Analytical analysis is used to show that R-134a and its replacement, HFO-1234yf, are particularly well suited as PHP working fluids. The following five conclusions are reached: 1) surface tension and density play an important role in sizing PHPs for operation in varying gravitational environments; 2) at low vapor pressures, noncondensable gas considerably increases the PHP system pressure, thus subcooling the system; 3) dynamic contact angle and surface tension significantly affect the capillary resistance force acting to damp PHP operation; 4) large viscosities act to damp PHP operation; and 5) latent heat of vaporization, surface tension, and density play a significant part in PHP startup.