RR-4 Scoping Experiments on Boiling Inception in Falling Films

J. C. Chen and K. Tuzla

Phase II-A, a short-term effort added to the end of the Phase II project, was implemented to obtain scoping measurements of bubble nucleation and boiling inception in falling film evaporation.

Using the existing facility and some of the test fluids from Phase II experiments, the conditions for boiling inception were measured during 28 experimental runs. Data were obtained for two different concentrations of glycerol-water mixtures at system pressures varying from 0.482 to 8.37 psia (3.32 to 57.7 kPa) and film flow rates of 580 to 3140 lb/ft hr (0.24 to 1.3 kg/m s). The local wall superheat at the point of initial bubble nucleation (boiling inception) varied from 22 to 54 °F (12 to 30 °C), with corresponding heat fluxes varying from 4376 to 8879 Btu/hr ft2 (13800 to 28000 W/m2).

These scoping experiments provided preliminary indications of the conditions that trigger boiling inception but did not generate sufficient data to truly characterize the phenomenon. Nevertheless, the Phase II-A project revealed that the wall superheat for boiling inception decreased with either increasing system pressure or film flow rate and showed that boiling inception at low vacuum pressures led to film disruption.