Heat transfer in a liquid film flowing over a rotating disk and a diverging open channel with and without boiling and surfactant addition

A series of experiments and analyses were conducted to investigate heat transfer into a boiling liquid film dispersed on a horizontal rotating disk using a controlled impinging jet and dispersed in an open diverging channel. Two sets of experiments were conducted on the horizontal disk to determine Nusselt numbers with evaporation and nucleate boiling of the liquid film. Rotation rates up to ω = 100 revolutions per minute and water flow rates up to 12 liters per minute were tested at ambient conditions, with inlet water temperatures from 40°C to 97°C. Results showed that the water film Nusselt number increased with increasing rotation rate, flow rate and temperature. Rapid boiling that was found in the subcritical flow region also had a large effect on the Nusselt number. A third experimental study was conducted to determine the influence that a surfactant would have on the liquid film heat transfer characteristics. These experiments were conducted on a flat plate with a diverging open-channel that could be angled downward to accelerate the flow. Heat transfer experiments were conducted with similar water inlet conditions to the rotating disk experiments, and with the plate inclines from horizontal to 45 degrees downward. Results show that a boiling surfactant solution created a thick foam layer with high heat transfer rates and Nusselt numbers that tended to be invariant with inlet Reynolds number.^ A semi-analytical model based on a Karman-Pohlhausen type integral analysis was formulated to describe the liquid film evaporation on the rotating disk. Results suggested that evaporation contributed less to the increase in Nusselt number at high temperatures than expected. Functional dependencies from this model and from the experimental data were used to develop semi-empirical correlations for the Nusselt number of non-boiling thin film liquid flows. A similar model was devised of a single sliding bubble of the type found during the experiments in order to determine the temperature profile of the micro layer of water present underneath the bubble. The analysis suggested that the temperature gradient within the micro layer was steep and resulted in nearly linear radial growth of the sliding bubbles.