Flame structure, turbulent burning velocity and its unified scaling for lean syngas/air turbulent expanding flames

A systematic experimental study of lean premixed syngas/air turbulent expanding flames has been conducted under a wide range of turbulence intensities (0–3.54 m/s), initial pressures (0.5–5 bar), and hydrogen volumetric fractions up to 80% (20%, 50% and 80%). Flame structure and turbulent flame propagation dynamics were investigated. Results show that the flame becomes more refined and wrinkled with the increasing of both turbulence intensity and initial pressure, leading to a larger flame area and the associated turbulent burning velocity (S_T). With hydrogen fraction increased, S_T is also enhanced significantly, which is mainly due to the promotion of laminar burning velocity (S_L) and diffusional-thermal instability. S_T/S_L is nearly kept constant with hydrogen fraction, which is a trade-off between strengthened diffusional-thermal instability and weakened turbulence stretch. A unified scaling of S_T is obtained, indicating that turbulent Reynolds number (Re_T) is a practical method to correlate S_T when Lewis number is close to unity. Furthermore, at least in the interpretation domain, S_T of spherical flames continually increases as the flame expands, which has been referred as flame acceleration phenomenon. It appears that only effective turbulence intensity itself is not able to reflect acceleration phenomenon completely. Turbulent expanding flames follow a self-similar propagation law and the quantitative S_T dependence with flame expanding is S_T∼R^0.5 approximately.