Burning velocity of turbulent premixed flames in a high-pressure environment

To explore the effects of ambient pressures on the turbulent burning velocity in a high-pressure environment, turbulent premixed flames of lean methane-air mixtures stabilized with a nozzle-type burner in a high-pressure chamber were investigated experimentally. Continuous combustion was investigated up to pressures of 3.0 MPa. Measurements of turbulent burning velocity were made using a mean-angle method based on a technique involving laser tomography and image processing. Results show that the effects of elevated pressure on turbulent burning velocity are significant and that the ratio of turbulent to laminar burning velocities S_T/S_L increases with both turbulence intensity u′ and pressure, reaching a value of 30 at 3.0 MPa under the present experimental conditions. The increases in S_T/S_L with increasing u′/S_L, are rapid at high pressure, particularly for small u′, that is, in the region of weak turbulence. An interesting similarity, of S_T/S_L variations was observed between the effect of pressure found in this experiment and the effect of a density jump as analyzed by Cambray and Joulin. Flame front instability theory based on Sivashinsky's formulation was applied to flames in high-pressure environments: it was found that the region of wave numbers where the flame front is unstable extends to larger wave numbers with increasing pressure because the diffusive-thermal effect, which stabilizes the hydrodynamic instability, weakens. This suggests that hydrodynamic instability which enlarges the total flame area, plays an important role in the rapid increase of S_T/S_L with pressure in high-pressure environments.