Fusion Science and Technology / Volume 61 / Number 1T / January 2012 / Pages 186-192
Fission / Proceedings of the Fifteenth International Conference on Emerging Nuclear Energy Systems
In this paper an attempt has been made to quantitatively determine the heat transfer contribution of the different mechanisms during the bubble collapse process in subcooled flow boiling condition.
For achieving this objective, the bubble behavior was studied during the subcooled flow boiling on a vertical heating rod with upward coolant water using the results of high-speed photography obtained by Nematollahi in 1999 at Tohoku University. Subsequently, some parameters including superheated layer thickness and active nucleation sites density as well as bubble parameter such as maximum diameter, life time, generation period and so forth were measured for calculation purpose of this study.
In the present investigation, four procedures of superheated layer mixing, turbulence induced by bubble collapse phenomenon, latent heat transport and transferring the energy by stable micro bubbles (SMB) (remained after original bubble collapse) were considered as the bubble mechanisms during the collapse process in subcooled flow boiling. Consequently, the heat transfer contributions of the bubble mechanisms were calculated using the applied models in the literature as well as the results obtained from the bubble behavior analysis for different experimental conditions of inlet subcooled temperature, linear power density (W/cm), flow velocity as well as two heights of a heated rod.
According to the calculated results, the most effective mechanism of heat transfer during the bubble collapse process was nominated to be superheated layer mixing at the moment of bubble departure with contribution ranging from 2.62% to 34.11% and average value of 9.8% to total heat flux. Also it was concluded that an average value of about 18% from total heat flux is transferred during the condensation of bubble, as it starts to shrink and finally collapses in the subcooled liquid at the end of its life in the subcooled flow boiling.