Numerical modelling of dry-out in pulsating heat pipe using VOF model

Operational limits are critical in the continuous functioning of a pulsating heat pipe (PHP). A computational fluid dynamics study was conducted to analyze the influence of working-fluid properties on the dry-out limit of closed-loop water-based PHP with a 50 % fill ratio. Working fluid (water) was boiled decisively at low temperature (35 °C) under reduced operating pressures to ease and analyze dry-out. Although, vapor pressure of water at 35 °C is 5.6 kPa; different evacuated pressures ranging from 2 to 10 kPa were attempted to validate the numerical model using different boiling characteristics and their influence on the dry-out phenomena. 2-D transient pressure based Navier-Stokes solver (ANSYS Fluent) was used to predict the field variables using the SIMPLE algorithm. The standard k-ε model with the enhanced wall treatment approach was used to capture the turbulent physics. VOF model was used to shape the two-phase flow characteristics. The adiabatic section was noticed to act as a functional barrier (for the pressure drop to occur) between evaporator and condenser sections. New concepts such as inverse flow dynamics, zero-gradient and equivalent points were established from time variation plots of working fluid properties.