Experiment and Simulation of Autoignition in Jet Flames and its Relevance to Flame Stabilization and Structure
AuthorsAl-Noman, Saeed M.
KAUST DepartmentPhysical Sciences and Engineering (PSE) Division
Permanent link to this recordhttp://hdl.handle.net/10754/615946
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AbstractAutoignition characteristics of pre-vaporized iso-octane, primary reference fuels, gasolines, and dimethyl ether (DME) have been investigated experimentally in a coflow with elevated temperature of air. With the coflow air at relatively low initial temperatures below autoignition temperature Tauto, an external ignition source was required to stabilize the flame. Non-autoignited lifted flames had tribrachial edge structures and their liftoff heights correlated well with the jet velocity scaled by the stoichiometric laminar burning velocity, indicating the importance of the edge propagation speed on flame stabilization balanced with local flow velocity. At high initial temperatures over Tauto, the autoignited flames were stabilized without requiring an external ignition source. The autoignited lifted flames exhibited either tribrachial edge structures or Mild combustion behaviors depending on the level of fuel dilution. For the iso-octane and n-heptane fuels, two distinct transition behaviors were observed in the autoignition regime from a nozzle-attached flame to a lifted tribrachial-edge flame and then a sudden transition to lifted Mild combustion as the jet velocity increased at a certain fuel dilution level. The liftoff data of the autoignited flames with tribrachial edges were analyzed based on calculated ignition delay times for the pre-vaporized fuels. Analysis of the experimental data suggested that ignition delay time may be much less sensitive to initial temperature under atmospheric pressure conditions as compared with predictions. For the gasoline fuels for advanced combustion engines (FACEs), and primary reference fuels (PRFs), autoignited liftoff data were correlated with Research Octane Number and Cetane Number. For the DME fuel, planar laser-induced fluorescence (PLIF) of formaldehyde (CH2O) and CH* chemiluminescence were visualized qualitatively. In the autoignition regime for both tribrachial structure and mild combustion, formaldehyde were found mainly between the fuel nozzle and the lifted flame edge. On the other hand, they were formed just prior to the flame edge for the non-autoignited lifted flames. The effect of fuel pyrolysis and partial oxidation were found to be important in explaining autoignited liftoff heights, especially in the Mild combustion regime. Flame structures of autoignited flames were investigated numerically for syngas (CO/H2) and methane fuels. The simulations of syngas fuel accounting for the differential diffusion have been performed by adopting several kinetic mechanisms to test the models ability in predicting the flame behaviors observed previously. The results agreed well with the observed nozzle-attached flame characteristics in case of non-autoignited flames. For autoignited lifted flames in high temperature regime, a unique autoignition behavior can be predicted having HO2 and H2O2 radicals in a broad region between the nozzle and stabilized lifted flame edge. Autoignition characteristics of laminar nonpremixed methane jet flames in high- temperature coflow air were studied numerically. Several flame configurations were investigated by varying the initial temperature and fuel mole fraction. Characteristics of chemical kinetics structures for autoignited lifted flames were discussed based on the kinetic structures of homogeneous autoignition and flame propagation of premixed mixtures. Results showed that for autoignited lifted flame with tribrachial structure, a transition from autoignition to flame propagation modes occurs for reasonably stoichiometric mixtures. Characteristics of Mild combustion can be treated as an autoignited lean premixed lifted flame. Transition behavior from Mild combustion to a nozzle-attached flame was also investigated by increasing the fuel mole fraction.
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Role of the outer-edge flame on flame extinction in nitrogen-diluted non-premixed counterflow flames with finite burner diametersChung, Yong Ho; Park, Daegeun; Park, Jeong; Kwon, Oh Boong; Yun, Jin Han; Keel, Sang In (Fuel, Elsevier BV, 2013-03) [Article]This study of nitrogen-diluted non-premixed counterflow flames with finite burner diameters investigates the important role of the outer-edge flame on flame extinction through experimental and numerical analyses. It explores flame stability diagrams mapping the flame extinction response of nitrogen-diluted non-premixed counterflow flames to varying global strain rates in terms of burner diameter, burner gap, and velocity ratio. A critical nitrogen mole fraction exists beyond which the flame cannot be sustained; the critical nitrogen mole fraction versus global strain rate curves have C-shapes for various burner diameters, burner gaps, and velocity ratios. At sufficiently high strain-rate flames, these curves collapse into one curve; therefore, the flames follow the one-dimensional flame response of a typical diffusion flame. Low strain-rate flames are significantly affected by radial conductive heat loss, and therefore flame length. Three flame extinction modes are identified: flame extinction through shrinkage of the outer-edge flame with or without oscillations at the outer-edge flame prior to the extinction, and flame extinction through a flame hole at the flame center. The extinction modes are significantly affected by the behavior of the outer-edge flame. Detailed explanations are provided based on the measured flame-surface temperature and numerical evaluation of the fractional contribution of each term in the energy equation. Radial conductive heat loss at the flame edge to ambience is the main mechanism of extinction through shrinkage of the outer-edge flame in low strain-rate flames. Reduction of the burner diameter can extend the flame extinction mode by shrinking the outer-edge flame in higher strain-rate flames. © 2012 Elsevier Ltd. All rights reserved.
On the statistics of flame stretch in turbulent premixed jet flames in the thin reaction zone regime at varying Reynolds numberLuca, Stefano; Attili, Antonio; Lo Schiavo, Ermanno; Creta, Francesco; Bisetti, Fabrizio (Proceedings of the Combustion Institute, Elsevier BV, 2018-07-21) [Article]Direct Numerical Simulations (DNS) are conducted to study the statistics of flame surface stretch in turbulent jet premixed flames. Emphasis is placed on the rates of surface production and destruction and their scaling with the Reynolds number. Four lean methane/air turbulent slot jet flames are simulated at increasing Reynolds number and up to Re ≈ 22 × 103, based on the bulk velocity, slot width, and the reactants’ properties. The Karlovitz number is held approximately constant and the flames fall in the thin reaction zone regime. The simulations feature finite rate chemistry and mixture-average transport. Our data indicate that the area of the flame surface increases up to the streamwise position corresponding to 80% of the average flame length and decreases afterwards as surface destruction overtakes production. It is observed that the tangential rate of strain is responsible for the production of flame surface in the mean and surface destruction is due to the curvature term. In addition, it is found that these two terms are both significantly larger than their difference, i.e., the net surface stretch.The statistics of the tangential strain rate are in good agreement with those for infinitesimal material surfaces in homogeneous isotropic turbulence. Once scaled by the Kolmogorov time scale, the means of both contributions to stretch are largely independent of location and equal across flames with different values of the Reynolds number. Surface destruction is due mostly to propagation into the reactants where the surface is folded into a cylindrical shape with the center of curvature on the side of the reactants. The joint statistics of the displacement speed and curvature of the reactive surface are nuanced, with the most probable occurrence being that of a negative displacement speed of a flat surface, while the surface averaged displacement speed is positive as expected.
Generalized flame surface density transport conditional on flow topologies for turbulent H2-air premixed flames in different regimes of combustionChakraborty, N.; Papapostolou, V.; Wacks, D. H.; Klein, M.; Im, Hong G. (Numerical Heat Transfer, Part A: Applications, Informa UK Limited, 2019-01-24) [Article]The generalized flame surface density (FSD) transport conditional on local flow topologies in premixed turbulent flames has been analyzed based on a detailed chemistry direct numerical simulation database of statistically planar turbulent hydrogen-air premixed flames with an equivalence ratio of 0.7 representing the corrugated flamelets, thin reaction zones and broken reaction zones regimes of combustion. The local flow topologies have been categorized by the values of the three invariants of the velocity gradient tensor and the statistical behaviors of the generalized FSD and different terms of its transport equation conditional on these flow topologies have been analyzed in detail for different choices of the reaction progress variable. The qualitative behavior of the different terms of the generalized FSD transport equation has been found to be similar for different choices of reaction progress variable but the statistical behaviors of the tangential strain rate term and its components have been found to be affected by the regime of combustion. The topologies, which exist for all values of dilatation rate, contribute significantly to the generalized FSD transport in premixed turbulent flames for all regimes of combustion. An unstable nodal flow topology, which is representative of a counter-flow configuration, has been found to be a dominant contributor to the FSD transport for all regimes of combustion irrespective of the choice of reaction progress variable. Moreover, a focal topology which is obtained only for positive values of dilatation rate, has been found to contribute significantly, especially to the curvature and propagation terms of the FSD transport equation for all regimes of combustion including the broken reaction zones regime. However, the contributions of the flow topologies to the turbulent transport and tangential strain rate term, which are obtained only for positive dilatation rates, have been found to weaken from the corrugated flamelets to the broken reaction zones regime.