Simulation and experimental validation of pulsating heat pipes

The one-dimensional transient numerical code of JJ Cooling Innovation has been significantly improved to simulate the thermal/hydraulic performance of pulsating heat pipes (PHPs). Using a mechanistic approach, the liquid slug and vapor plug flows and their interaction with the surrounding wall are modeled. The heat transfer is predicted using the Three-Zone Model of flow boiling in microchannels, which includes liquid film evaporation and liquid film condensation of vapor plugs and convection of liquid slugs. Discretizing the entire PHP serpentine into one continuous grid, the code solves all variables based on local conditions. The code allows the user to define the working fluid, the channel's shape, material and wall roughness, the serpentine's pitch, the geometrical parameters of evaporator, adiabatic region and condenser zones, and the operating parameters. The boundary conditions in the evaporator zone can either be constant heat flux or constant temperature, while on the condenser side, the coolant temperature is constant as well as the heat transfer coefficient. The code works without any fitting factor and predicts all local variables in the 1D grid as a function of time, including the liquid film thickness and the onset of boiling for the formation of new vapor plugs within the evaporating and adiabatic zones, if local nucleation conditions are reached. Growth, collapse and coalescence of generated bubbles are taken into account. The film thermal resistance is corrected to a radial conduction model. The implementation of the initial liquid film thickness laid behind a liquid slug is updated to include the effect of the bubble velocity and acceleration as well as the length of the leading slug. The nucleation of new vapor plug has also been updated. The onset of nucleation is only dependent on the local thermodynamic state. If the threshold is reached, a new method is used to simulate micro-bubbles formation and coalescence to a channel-size vapor plug. These modifications greatly improve the code accuracy and stability allowing simulations with high pressure fluid rather than ethanol only. Furthermore, in the on-going EU project Pulsating Heat Pipes for Hybrid Propulsion Systems (PHP2), a new PHP test bench has been designed and built at Provides Metalmeccanica. A copper tube PHP with a 19-turn of 2.0 mm internal diameter and 12 mm tube pitch was tested over a wide range of imposed heat loads. A microchannel water-cooling plate was used at the condenser side with an inlet temperature of 20 degrees C. Then, tests were done for vertical and horizontal orientations with R1233zd(E) as the working fluid. The code, which was previously validated for ethanol PHP test data from KAIST, is now successfully validated for the present experimental data points. Accurate results were obtained from low heat loads up to 250 W for a set of 49 data points, predicting 97% within an error bandwidth of 30%.