Experimental and theoretical investigation of bubble break-up and coalescence in bubbly flows

Abstract

The population balance equation considers the change of the bubble size and the bubble number density due to coale scence and break-up in bubbly flows. Although a number of theoretical models exist for coalescence and break-up rates, nearly no experimental data for the validation of these models exist for turbulent flow with high void fraction. To over come this lack, vertical air-water pipe flows were examined. The validation concept consists of two steps: The observation of single coalescence and break-up events with a high-speed video system and the measurement of time averaged influence parameter ssuch as number density and liquid turbulence. For measuring the number density a combination of single and double fibre optical probes was used that could be applied in high void fraction flows. X-Hot film probes were used to measure the liquid flow turbulence. p]The observation of single events gave information about the efficiency of collisions leading to coalescence. Based on these observations, the coalescence model of Prince was modified using a critical Weber number to calculate the coalescence efficiency. The break-up model of Luo was corrected because comparison of calculated with measured turbulent spectra showed that this model over estimates the turbulent kinetic energy for large eddies. The number density was obtained from the solution of the one-dimensional conservation equation and was compared with measured data. Good agreement between measured and calculated values within the error range could be obtained. That proves the applicability of the adapted models for coalescence and break-up rates.