High pressure pool boiling: Mechanisms for heat transfer enhancement and comparison to existing models

Pressure has a huge impact on the performance of pool boiling heat transfer due to the change in thermofluid properties of water. The specific volume of water and the latent heat of vaporization decrease with increasing system pressure. High pressure pool boiling experiments were conducted for water on a horizontal smoothed copper surface at pressures ranging from 0 psig (0 kPa) to 60 psig (413.7 kPa). The hydrodynamic behavior of bubbles was studied at high pressure using the flow visualization technique. The primary bubble size, detachment frequencies, and nucleation site densities were studied at each pressure. The performance of the pool boiling curve was critically analyzed with different predictive models. To compare the pressure data and validate with the existing model, Stephan and Abdelsalam's model was used. At low heat flux, no changes in the heat transfer coefficient were noticed. At high flux, a 50%, 75%, 125%, and 175% enhancement in the heat transfer coefficient was found against the atmospheric pressure (0 kPa) for 15 psig (103.4 kPa), 30 psig (206.8 kPa), 45 psig (310.2 kPa), and 60 psig (413.7 kPa) respectively. The critical heat flux value was also measured and compared to the predicted theories. A new mechanistic lift model, similar to the Zuber's model, was used to explain the dry out mechanisms at high pressure in which pressure delays the surface dry out behavior. The results show that change in thermodynamic properties of water due to pressure not only change the hydrodynamic behavior, but also impacts the formation of the thermal boundary layer. The present study shows that there is a need for a theoretical model to predict the critical heat flux and the heat transfer coefficient at high pressure. At high pressure the contact angle shows a dynamic behavior due to change in the surface tension of liquid. The current research work aims to be a fundamental ground to develop for manufactures/designers of high pressurized boilers. (C) 2019 Elsevier Ltd. All rights reserved.