Cryogenic pulsating heat pipe – dependence on geometrical parameters like adiabatic length and number of turns

A two-dimensional CFD model has been developed to study the effect of adiabatic section length and number of turns on the thermo-hydraulic behaviour and the thermal performance of a pulsating heat pipe (PHP) using nitrogen as the working fluid. The adiabatic sections of the single-loop PHP are assigned as 40 mm, 80 mm, and 200 mm, while the number of turns has been varied from a single loop to two turns and three turns. The PHP with a 50 % fill ratio is operated with the evaporator heat load ranging from 0.5 W to 4.0 W. In a single loop PHP, the effective thermal conductivity increases with the adiabatic length, attaining a maximum value of 6.3 kW/m-K in the 200 mm unit. Nonetheless, the average evaporator temperature quickly escalates with the heat load in PHPs with longer adiabatic sections, sometimes leading to operational instability beyond a certain thermal load. On the contrary, with the increase in the number of turns, the effective thermal conductivity diminishes causing improvement in heat load handling capability (due to reduced average evaporator temperature). The other thermal performance parameter thermal resistance, exhibits an opposite trend with the rise in adiabatic length in the single-loop PHP, while it reduces with the increasing number of turns, showcasing a minimum thermal resistance of 3.12 K/W at 4.0 W in the three-turn PHP. Dimensionless Karman number evaluated for different heat loads illustrates a combined effect of driving force and frictional losses across various adiabatic lengths. The dominating frequency obtained from the Fast Fourier Transform (FFT) depends mainly on both the geometrical variables as well as thermal input. The typical frequency is below one Hertz.