An analytical model for predicting growth rate and departure diameter of a bubble in subcooled flow boiling

Abstract

Bubble formation in flow boiling undergoes an ebullition cycle in which nucleation is followed by bubble growth, departure and a waiting period. Bubble growth rate plays an important role in bubble dynamics and also affects the whole ebullition cycle. It is difficult to make analytical calculation of bubble growth rate and departure diameter due to involvement of various forces and heat transfer processes on a nucleating bubble. In this paper, an analytical technique is proposed to calculate the growth rate and departure diameter of a nucleating bubble in subcooled flow boiling. Both force and energy balance approaches have been applied to calculate the growth and departure diameter of the bubble. Heat transfer contributions from evaporative microlayer beneath the bubble, superheated liquid surrounding the bubble and condensation from the bubble cap have been considered to calculate the bubble growth rate. In place of using a constant coefficient to represent the fraction of bubble in subcooled bulk liquid by existing analytical models, the present model correlates the coefficient with the operating parameters of flow boiling. An effort has also been made for accurate calculation of temperature distribution in heated surface and flowing liquid. Bubble growth rate and departure diameter have been calculated for different values of coolant mass flux, system pressure, applied heat flux, surface orientation angle and liquid subcooling. The heat transfer contribution from individual layer towards the growth rate of the bubble has been quantified and analyzed. The present analytical model has been validated with a wide range of experimental data and it produces better agreement with the experimental data compared to existing analytical models.