Burning Characteristics of Premixed Flames in Laminar and Turbulent Environments

• Main Author: Mannaa, Ossama
• Other Authors: Roberts, William L.
• Language: en
• Published: 2018
• Subjects: Laminar burning velocity; Turbulent burning velocity; High temperature; Jet premixed flame; Expanding spherical flame; High pressure
• Online Access: Mannaa, O. (2018). Burning Characteristics of Premixed Flames in Laminar and Turbulent Environments. KAUST Research Repository. https://doi.org/10.25781/KAUST-G3VB9; http://hdl.handle.net/10754/630077

Considering the importance of combustion characteristics in combustion applications including spark ignition engines and gas turbines, both laminar and turbulent burning velocities were measured for gasoline related fuels. The first part of the present work focused on the measurements of laminar burning velocities of Fuels for Advanced Combustion Engines (FACE) gasolines and their surrogates using a spherical constant volume combustion chamber (CVCC) that can provide high-pressure high-temperature (HPHT) combustion mode up to 0.6 MPa, 395 K, and the equivalence ratios ranging 0.7-1.6. The data reduction was based on the linear and nonlinear extrapolation models considering flame stretch effect. The effect of flame instability was investigated based on critical Peclet and Karlovitz, and Markstein numbers. The sensitivity of the laminar burning velocity of the aforementioned fuels to various fuel additives being knows as octane boosters and gasoline extenders including alcohols, olfins, and SuperButol was investigated. This part of the study was further extended by examining exhaust gas re-circulation effect. Tertiary mixtures of toluene primary reference fuel (TPRF) were shown to successfully emulate the laminar burning characteristics of FACE gasolines associated with different RONs under various experimental conditions. A noticeable enhancement of laminar burning velocities was observed for blends with high ethanol content (vol ≥ 45 %). However, such enhancement effect diminished as the pressure increased. The reduction of laminar burning velocity cause by real EGR showed insensitivity to the variation of the equivalence ratio. The second part focused on turbulent burning velocities of FACE-C gasoline and its surrogates subjected to a wide range of turbulence intensities measured in a fan-stirred CVCC dedicated to turbulent combustion up to initial pressure of 1.0 MP. A Mie scattering imaging technique was applied revealing the mutual flame-turbulence interaction. Furthermore, considerable efforts were made towards designing and commissioning a new optically-accessible fan-stirred HPHT combustion vessel. A time-resolved stereoscopic particle image velocimetry (TR-PIV) technique was applied for the characterization of turbulent flow revealing homogeneous-isotropic turbulence in the central region to be utilized successfully for turbulent burning velocity measurement. Turbulent burning velocities were measured for FACE-C and TPRF surrogate fuels along with the effect of ethanol addition for a wide range of initial pressure and turbulent intensity. FACE-C gasoline was found to be more sensitive to both primarily the primary contribution of turbulence intensification and secondarily from pressure in enhancing its turbulent burning velocity. Several correlations were validated revealing a satisfactory scaling with turbulence and thermodynamic parameters. The final part focused on the turbulent burning characteristics of piloted lean methane-air jet flames subjected to a wide range of turbulence intensity by adopting TR-SPIV and OH-planar laser-induced florescence (OH-PLIF) techniques. Both of the flame front thickness and volume increased reasonably linearly as normalized turbulence intensity, u^'/ S_L^0, increased. As u^'/ S_L^0 increased, the flame front exhibited more fractalized structure and occasionally localized extinction (intermittency). Probability density functions of flame curvature exhibited a Gaussian like distribution at all u^'/ S_L^0. Two-dimensional flame surface density (2D-FSD) decreased for low and moderate u^'/ S_L^0, while it increased for high u^'/ S_L^0Turbulent burning velocity was estimated using flame area and fractal dimension methods showing a satisfactory agreement with the flamelet models by Peters and Zimont. Mean stretch factor was estimated and found to increase linearly as u^'/ S_L^0increased. Conditioned velocity statistics were obtained revealing the mutual flame-turbulence interaction.