Experimental study on the onset of local dryout in an evaporating falling film on horizontal plain tubes

In this paper, an investigation on the onset of local dryout of a saturated R134a falling film with nucleate boiling on a vertical array of horizontal plain tubes is presented. Experiments were performed at saturation temperatures of 5, 10 and 20 °C, a tube pitch of 22.25 mm, heat fluxes from 15 to 70 kW/m2 and film Reynolds numbers up to 3000. Commercial plain tubes with external diameters of 19 mm were used in the experiments. Previous experimental results from Roques obtained for R134a at 5 °C and tube pitches of 22.25, 23.9 and 25.5 mm with the same test facility were also included in the analysis. The criterion adopted to characterize the onset of dryout was a sharp falloff in the heat transfer coefficient with decreasing film flow rate at a fixed heat flux, since prior to this condition the effects of film flow rate on heat transfer were almost negligible. The onset of dryout was observed to depend mainly on the heat flux and film flow rate. A general method to predict the heat transfer deterioration due to dry patch formation was proposed. The proposed method to predict heat transfer coefficient at dryout conditions was also found to work reasonably well at non-dryout conditions without major modifications.