(18c) Contact Time for Mechanistic Modeling of Heat Transfer Coefficient in Slurry Bubble Columns

The dependence of heat transfer coefficient on bubble dynamics in a 0.44 m diameter bubble column has been experimentally and analytically examined using combined measurement techniques and mechanistic approach. Filtered dry air is used as the gas phase while, filtered soft water is used as the liquid phase. The superficial gas velocity is varied up to 45 cm/s covering bubbly, transition and the churn turbulent flow regimes. The effects of local gas holdup and bubble dynamics on the heat transfer coefficient are experimentally studied by using combined measurement techniques of a fast-response heat transfer probe and four-point optical probe. A consecutive film and unsteady state surface renewal model has been used to analyze the heat-transfer rate and coefficient based on the combined measurements that have been performed simultaneously.  Quantification of the contact time between the thin liquid film on the heating surface and the bulk liquid is needed for the used mechanistic approach. A correlation based on the local bubble properties has been proposed and successfully used to estimate the contact time. The present study indicates that the heat transfer coefficient depends upon the combined effects of bubble parameters such as; bubble frequency, local gas hold-up, bubble size, bubble velocity (both downward and upward) and their distributions over the heating surface. The mechanistic analysis shows that the contact time is a function of the local gas holdup and bubble passage frequency. Consequently the variation of the local heat transfer coefficient with the contact time is via the bubble passage frequency and the local gas phase hold-ups. Hence the heat transfer coefficient at the center of the bubble column is noted to be significantly higher than at the column wall. Detailed results and findings will be presented and discussed in the presentation.