Experimental and Numerical Investigation of Shear-Driven Film Flow and Film Evaporation

Abstract

Shear-driven liquid film flows can occur in several locations of fuel preparation systems, e.g. inside air-driven atomizers or in Lean Pre-mixing Pre-vaporizing (LPP) combustion chambers of modern gas turbines. In LPP chambers the liquid fuel is primary atomized by a pressure nozzle and sprayed onto a pre-filmer. Fine fuel droplets accumulate at the pre-filmer surface and form a thin liquid film driven by hot compressed air to the inlet section of the combustion chamber. While the thin liquid film is accelerating along the wall, it evaporates and mixes with the hot air. The turbulent air flow induces strong shear forces at the air-liquid interface leading to a destabilization of the liquid film and the development of waves. The hydrodynamics of the wavy film flow govern the heat and mass transport and, hence, the entire fuel preparation process. Hydrodynamics and heat and mass transport strongly depend on the microstructure of the pre-filmer wall surface. In this work, the fundamentals of gravity-driven as well as air-driven film flow and evaporation on unstructured and microstructured wall surfaces have been investigated experimentally and numerically. It has been shown that longitudinal microgrooves have a stabilizing effect on the film flow. Flow regimes leading to a strong increase of evaporation efficiency have been identified. Local film thickness distributions have been measured using high-speed shadowgraphy. Wall temperature distributions have been measured using embedded thermocouples. The measurements have been performed for film Reynolds numbers varying from 225 to 650, for gas Reynolds numbers varying from 104 to 7·104, and for wall heat fluxes up to 40 W/cm2. High-speed infrared images have been recorded to visualize local film break-up and rewetting. Corresponding numerical studies of the gas–liquid flow and heat transfer along a heated wall have been conducted using Computational Fluid Dynamics (CFD). In order to track the moving gas–liquid interface, the volume of fluid (VOF) method has been adopted. Parametric numerical studies have been performed and compared with experimental data.