The nonlinear problem of the propagation of curved stationary flames in tubes of different widths is studied by means of direct numerical simulation of the complete system of hydrodynamic equations ...including thermal conduction, viscosity, fuel diffusion and chemical kinetics. While only a planar flame can propagate in a narrow tube of width smaller than half of the cut-off wavelength determined by the linear theory of the hydrodynamic instability of a flame front, in wider tubes stationary curved flames propagate with velocities considerably larger than the corresponding velocity of a planar flame. It is shown that only simple 'single-hump' slanted stationary flames are possible in wide tubes, and 'multi-hump' flames are possible in wide tubes only as a nonstationary mode of flame propagation. The stability limits of curved stationary flames in wider tubes and the secondary Landau-Darrieus instability are investigated. The dependence of the velocity of the stationary flame on the tube width is studied. The analytical theory describes the flame reasonably well when the tube width does not exceed some critical value. The dynamics of the flame in wider tubes is shown to be governed by a large-scale stability mechanism resulting in a highly slanted flame front. In wide tubes, the skirt of the slanted flame remains smooth with the length of the skirt and the flame velocity increasing progressively with the increase of the tube width above the second critical value. Results of the analytical theory and numerical simulations are discussed and compared with the experimental data for laminar flames in wide tubes.
Gaseous potassium (K) species released from biomass during thermochemical conversion pose challenges to reactors and human health. Photofragmentation tunable diode laser absorption spectroscopy ...(PF-TDLAS) was used for simultaneous, high-dynamic range measurements of atomic K, potassium hydroxide (KOH) and potassium chloride (KCl) in flat flames seeded with KCl salt. An expression for the PF-TDLAS signal is presented and experimentally verified. Axial K species concentration profiles recorded at fuel-air equivalence ratios of 0.8 and 1.3 are compared to 2D axisymmetric reacting flow simulations. An overall good agreement is found, but KOH is over-predicted in simulations of fuel-rich flames at the expense of atomic K. Quantification of K species close to softwood and wheat straw particles converted in the flames is demonstrated.
Absorption spectroscopy is attractive for combustion diagnostics because it allows in-situ and calibration-free quantification of reactants/products and thermometry. However, spectra measured at ...atmospheric pressure in the near-infrared telecom range, where laser sources and optical components are readily available, suffer from strong water interference. Cavity-enhanced optical frequency comb spectroscopy (CE-OFCS) is well suited for detection of other species, as it provides broad bandwidth with high signal-to-noise ratio and resolution, and allows de-convolving the spectra hidden among water transitions. Here we report detection of OH in the presence of H2O in an atmospheric premixed methane/air flat flame by CE-OFCS at 1.57 μm. We demonstrate a new water line list that is more accurate than HITEMP 1 and we isolate the OH lines by dividing spectra taken at different heights above the burner (HABs) to retrieve OH concentration and flame temperature.
We explain the physical mechanism of ultrafast flame acceleration in obstructed channels used in modern experiments on detonation triggering. It is demonstrated that delayed burning between the ...obstacles creates a powerful jetflow, driving the acceleration. This mechanism is much stronger than the classical Shelkin scenario of flame acceleration due to nonslip at the channel walls. The mechanism under study is independent of the Reynolds number, with turbulence playing only a supplementary role. The flame front accelerates exponentially; the analytical formula for the growth rate is obtained. The theory is validated by extensive direct numerical simulations and comparison to previous experiments.
It was demonstrated recently in Bychkov et al. Bychkov et al., Phys. Rev. Lett. 101 (2008) 164501, that the physical mechanism of flame acceleration in channels with obstacles is qualitatively ...different from the classical Shelkin mechanism. The new mechanism is much stronger, and is independent of the Reynolds number. The present study provides details of the theory and numerical modeling of the flame acceleration. It is shown theoretically and computationally that flame acceleration progresses noticeably faster in the axisymmetric cylindrical geometry as compared to the planar one, and that the acceleration rate reduces with increasing Mach number and thereby the gas compressibility. Furthermore, the velocity of the accelerating flame saturates to a constant value that is supersonic with respect to the wall. The saturation state can be correlated to the Chapman–Jouguet deflagration as well as the fast flames observed in experiments. The possibility of transition from deflagration-to-detonation in the obstructed channels is demonstrated.
We measure broadband H 2 O and OH spectra in a flame using near-infrared cavity-enhanced Fourier transform optical frequency comb spectroscopy, we retrieve temperature and OH concentration, and ...compare water spectra to an improved line list.
New high order implicit-explicit Runge-Kutta methods have been developed and implemented into a finite volume code to solve the Navier-Stokes equations for reacting gas mixtures. If only the stiff ...chemistry is treated implicitly, the linear systems in each Newton iteration are simple and solved directly. Numerical simulations of deflagration-to-detonation transition (DDT) show the potential of the new time integration for computational combustion.
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