Dynamic liquid-film model for numerical simulation of flow and heat transfer in pulsating heat pipes

Abstract

In the present study, a one-dimensional numerical model for pulsating heat pipes (PHPs) was presented. The balance equations of mass, momentum, and energy were solved for liquid slugs, vapor plugs and also for liquid films, along with the heat conduction equation for the tube wall. In the present study, the spatial and temporal variations in the liquid-film thickness were directly simulated, and the film was allowed to dry out when the local thickness decreased to the roughness height of the tube wall. Using previous experimental data, the present liquid-film model was examined along with the other liquid-film models. For a self-sustained oscillation of a unit-cell, a previous model neglecting the film dynamics could not predict the oscillating motion. The predictions of the other models considering the film dynamics depended largely on the fitting parameter on the film thickness. In the present model, a constitutive model on the initial film thickness was introduced, and the interfacial thermal resistance at the film-vapor interface was safely removed through a sensitivity test. The present numerical model predicted most of the experimental data not only for vertical PHPs but also for horizontal and inclined PHPs having various different parameters, within 20% error. Similarly to the case of the unit-cell, the previous liquid-film models could not predict the oscillating motion of the fluid inside a horizontal PHP, or the predicted thermal performance was strongly dependent on the fitting parameters. Also, the thermodynamic state of a vapor plug was always in saturation condition as long as the vapor plug is closely surrounded by liquid film, while the vapor plug can be locally superheated where the film is dried and the superheated wall is exposed. For a vertical PHP, on the other hand, circulating motion was predicted regardless of the film models, and the role of the film dynamics on the prediction of thermal performance was not significant if the number of turns were comparably small. In this case, the thermodynamic state of a vapor plug was almost always in saturation condition due to the continuous liquid supply. If the number of turns is larger than the critical number, the thermal performance was orientation-independent. The fluid motion and the thermodynamics state of vapor plugs were similar to the case of a horizontal PHP, regardless of the orientation. The present model is considered the first to predict the thermal performance of a horizontal PHP without introducing any fitting parameters.