Wrinkling and curvature of laminar and turbulent premixed flames

Heat Release Imaging in Turbulent Premixed Ethylene-Air Flames Near Blow-off

“…A length of 1 mm was chosen as the integrating length for HR along the flame normal. The two-dimensional flame front curvature was calculated using the method described by Haq et al [12]. With 's' denoting the length of the flame front measured from a fixed origin on the filtered contour, the first and second derivatives in the x and y directions along the flame front with respect to s were evaluated and filtered using the same zero-phase digital filter.…”

The structure of turbulent flames in fractal- and regular-grid-generated turbulence

“…Flame contour images are usually obtained by binarising laser induced fluorescence images [66][67][68], Mie scattering images [69], Rayleigh scattering images [70] or CPIV images [35,37,71]. During the process of binarisation, continuous flame contours get pixelated and this causes originally smooth contours to become less smooth [72].…”

“…Different smoothing procedures are applied before the curvature calculation in order to filter the pixilation noise and obtain an approximately continuous contour again. Filters often used are the Savitzky-Golay filter [68], spatial filters [70,73] or polynomial curve fits [74]. In each case the filtering 27 parameters such as the kernel size of the Savitzky-Golay filter or the order and length of the polynomial have to be adjusted appropriately as these affect the accuracy and range of curvature values obtained [72].…”

“…The best filter settings are then defined as the settings which give the least deviation from the analytical curvature values over the entire range of curvature values assessed. Three test cases are widely used for optimizing the filter settings: a circle [68], a sine wave [70] and a rosette [72]. The rosette test curve [72] can be described as a circle with an oscillating single sinusoidal pattern and is arguably the most accurate test curve to date as it accounts for the undulating shape of the flame contour and provides a means of pixelating different curvature values differently [72].…”

Variation of Turbulent Burning Rate of Methane, Methanol, and Iso-Octane Air Mixtures With Equivalence Ratio at Elevated Pressure

“…have shown that areas of flame front which become concave to the reactants can locally quench, whilst those areas convex to the reactants demonstrate elevated levels of OH radicals (Bradley et al, 2000). It might be argued that the elevated OH is a result of change in the local equivalence ratio associated with differential molecular diffusion; these flame structural effects have been shown to survive into turbulent flames at atmospheric pressure (Haq et al, 2002). The applicability of flamelet concepts to such highly unstable flames, exemplified at these four conditions, might be questionable; as the flame structure appears to be strongly modified by the local flame front geometry, ultimately resulting in areas of quench.…”