Experimental study of turbulent flame propagation under wall film conditions

Abstract

The presence of a liquid wall film caused by spray impingement is inevitable in modern direct injection engines and can have a substantial effect on flame dynamics and pollutant emissions. The present study investigated the effects of a wall film on turbulent flame propagation characteristics under different initial conditions. In these trials, methane-air premixed turbulent jet flames were established in a constant volume vessel with a small pre-chamber connecting to the main chamber through a narrow orifice. Various fuel films were prepared on the surface of a side plate installed in the main chamber. The flame morphologies were captured using a high-speed schlieren photographic system and the intensity of pool fire was examined by an image intensifier with UV lens. Meanwhile, temporal pressures in both chambers were recorded by two pressure sensors respectively. Changes in turbulent combustion phases in conjunction with single and multi-component wall films were analyzed based on flame propagation features, pressure data and film pyrolysis characteristics over ranges of equivalence ratios (0.8–1.2) and initial pressures (0.2–0.4 MPa). Gas phase emissions were also evaluated to further explore the effects of wall films on turbulent combustion. The high reactivity of the fuel films was found to increase the turbulent flame speed and provide higher peak pressures, although the heat absorbed by evaporation delayed the pressure rise time. The volatility of the film material affected the propagation speed together with the jet flame area growth and these effects were more obvious under lean combustion conditions. Film pyrolysis was triggered as multiple turbulent flows converged near the film region and produced a pool fire with soot deposition. The peaks of pressure and pressure rise rate observed when using a diesel film were intermediate between those obtained with n-dodecane and n-tetradecane films. A lubricating oil film pool fire also appeared after increasing the initial pressure but its intensity was weaker than those of the fires generated by the fuel films.