A Note on Premixed Flame-Turbulence Interactions 2005-01-0241
This note focuses on the three fundamental mechanisms behind premixed flame-turbulence interactions that result in progressive acceleration of a spark-ignited flame in a turbulent environment such as that inside a spark-ignition engine cylinder. In addition, as a small step in further advancing our understanding on flame-turbulence interactions, experiments were conducted to quantify the changing turbulence parameters associated with a near-isotropic turbulent free-stream as it approaches a solid sphere in a wind tunnel.
It has been observed in some previous studies that when a premixed combustible mixture is ignited in a turbulent environment, the turbulent flame speed / turbulence intensity ratio increases as the flame grows. Depending on the chemical and physical parameters involved, this accelerating turbulent flame may develop into a detonation wave. Over the years, three fundamental mechanisms have been postulated by different experts to explain this progressive turbulence enhancement in flame speed. 1) During the initial stages after ignition, only eddies that are smaller than the flame ball influence the flame front significantly. As the flame grows, the flame / eddy size ratio increases and consequently, increasingly larger portion of the eddying motion associated with the turbulent energy cascade becomes effective in affecting the flame front. 2) Analogous to the exponential increase of a material surface in homogeneous isotropic turbulence, proposed by Batchelor in 1952, the ongoing flame surface-turbulence interactions can lead to continuous increase in reacting flame front surface. 3) In 1995, Ashurst postulated that for a wrinkled flame, volume expansion of a portion of the reacting surface pushes the adjacent flame surface away from it. This expansion-pushing effect enhances flame front corrugation, and hence, accelerates the flame.
The characterization of near-isotropic turbulence as it approaches a sphere was conducted in a 75 cm by 75 cm wind tunnel. The turbulent free-stream was generated by a 43% solid, perforated plate having orifice holes of diameter D of 3.75 cm. The stream-wise root mean square velocity fluctuation, turbulence intensity, integral and Kolmogorov scales were deduced from instantaneous velocity measurements via a normal hot-wire anemometer. These measurements were made at 6.4 cm (2.5″) and 8.9 cm (3.5″) upstream of a 10 cm diameter (d) sphere fixed at 20D downstream of the perforated plate, at a Reynolds number based on the mean flow velocity and the diameter of the sphere of 5 x 104.