On interaction of centrally-ignited, outwardly-propagating premixed flames with fully-developed isotropic turbulence at elevated pressure

Abstract

A new apparatus for the study of turbulent premixed flames at atmospheric and elevated pressures is proposed. The apparatus includes a high-pressure fan-stirred cruciform burner, the inner chamber, which is resided in a relatively large pressure-absorbing safety chamber (the outer chamber). Both chambers are optically accessible, allowing direct visualization and measurement of flame and turbulence interactions. The inner burner applies the same turbulence generating mechanism as that previously reported by Shy et al. [10], capable of generating intense near-isotropic turbulence. The additional modification lies in its vertical vessel which has four sensitive pressure-releasing valves installed symmetrically, so that the pressure difference between the inner and outer chambers during explosion can be eliminated. Flame speed measurements for centrally-ignited, outwardly-propagating lean CH4–air flames at the equivalence ratio ϕ=0.8 under both quiescent and turbulent conditions are conducted over an initial pressure range of p=0.1–1MPa. It is found that, contrary to the popular scenario for laminar flames, the coupling influence of elevated pressure and turbulence significantly enhances turbulent flame speeds. Our experimental data show that the unstretched laminar burning velocities (SL) decrease with p−0.52, while turbulent burning velocities (ST) increase with p0.14 when a constant turbulent fluctuating velocity u′≈1.4m/s is applied. In terms of the power law relation proposed by Kobayashi and his co-workers, we found that ST/SL≈1.07[(u′/SL)(p/p0)]0.44 where p0 is the atmospheric pressure, showing a similar increasing trend but with much lower values of ST/SL to what they found in a Bunsen-type burner. It is suggested that ST∼p0.14 is attributed to further flame surface area increment induced by the enhancement of hydrodynamic instability due to the decrease of kinematic viscosity at elevated pressure.