Liquid-film thickness during flow boiling of pure hydrocarbons and their mixtures

Abstract

The present study concerns an experimental investigation of the liquid-film thickness and void fraction during flow boiling of hydrocarbons and their zeotropic mixtures. High-speed flow images were also gathered. Experiments were performed for R600a, R290, R1270, R600a/R290 (70/30 M fraction) and R600a/R1270 (75/25 M fraction) in a horizontal stainless-steel tube with an internal diameter of 9.43 mm. Liquid-film thickness and flow footage were gathered in an adiabatic section, 50 mm downstream the heating section. The saturation temperature was kept at 5 °C, mass velocities ranged from 50 to 250 kg/m2s, vapor qualities from 0.07 to 0.81, and heat fluxes from 5 to 80 kW/m2. The experimental data comprehends annular flows, including results for the intermittent dryout-rewetting phenomenon. The rewetting front was found to be composed of a two-stage wave. Initially it approached the dry wall, increasing the film thickness sharply until reaching a peak. Downstream the wave-peak the liquid-film thickness and its interfacial roughness decreased smoothly. Ripples were observed on top of the rewetting wave and their velocity and amplitude decreased at the wake of the wave. Additionally, footage showed that the entrained droplets flow within intermittent clusters and not homogeneously distributed along the vapor core. The void faction was estimated based on the liquid-film thickness measurements and compared with prediction methods available in the literature. In general, reasonable agreement between experimental data and their predictions was found.