Symposium (International) on Combustion最新文献

The effects of combustion on the change of the flowfield in a model SCRamjet combustor with a backward step is investigated experimentally and numerically. The main airflow has a Mach number of 2.0. The total temperature is 1000 K for cold flow and 1800 K for hot flow. Hydrogen fuel is injected parallel to the main airflow through a slit on the backward face of the step. The combustion mode is categorized in two modes. One is a weak combustion (WC) mode that is not accompanied by a shock wave, where the flowfield is similar to that in cold flow. Another is an intensive combustion (IC) mode that is accompanied by a shock wave, where the flowfield is much different from that in cold flow. In IC mode, a large separation region is generated behind the step by the shock wave, and the vortex generated at the region rolls the fuel up. The main reacting region is in the shear layer just behind the shock wave, where the main airflow bumps the rolled-up fuel, and the temperature is relatively high due to the shock wave. The flowfield is then controlled by the rate of mixing, leading to fast heat release, which raises the pressure level in the combustor and supports the shock wave. This passive feedback works, and both the mixing efficiency and the combustion efficiency become high. On the other hand, in WC mode, the reacting region spreads over the shear layer downstream of the step, and its heat release rate is lower than that in IC mode. The flowfield is then controlled by the rate of chemical reaction, and the combustion efficiency remains low.

The self-induced baroclinic instability of flames, the Landau-Darrieus instability, is studied numerically in the nonlinear range. A level set (G equation) based approach accounting for heat-release effects is used to follow the flame response to initial perturbation and shape evolution. It is shown that the instability leads to the development of product bubbles moving into the unburned mixture and cold mixture spikes penetrating into the burned gases similar to the bubble-spike configuration produced by the nonlinear Rayleigh-Taylor instability of interfaces in a gravitational field. Independently of the initial perturbation, the flame bubble approaches asymptotically a shape close to a paraboloid. The substantial growth of the flame-surface area due to the instability increases the flame propagation speed to the asymptotic value;$+0.29\sqrt{\alpha -1)}{S}_l$where α is the density ratio and S_l the laminar burning velocity. The asymptotic amplitude of the flame is approximately$A=0.37d\sqrt{\alpha -1}$, where d is the flame width. The burning velocity has a minor effect on the asymptotic shape of the flames. When the turbulence scale is much smaller than the size of the combustion apparatus, the results can be directly applied to turbulent flames by replacing S₁ by the turbulent burning velocity.

This paper is a continuation of our work on edge-flames in premixed combustion. An edge-flame is a two-dimensional structure constructed from a one-dimensional configuration that has two stable solutions (bistable equilibrium). Edge-flames can display wavelike behavior, advancing as ignition fronts or retreating as failure waves. Here we consider two one-dimensional configurations: twin deflagrations in a straining flow generated by the counterflow of fresh streams of mixture; and a single deflagration subject to radiation losses. The edge-flames constructed from the first configuration have positive or negative speeds, according to the value of the strain rate. But our numerical solutions strongly suggest that only positive speeds (corresponding to ignition fronts) can exist for the second configuration. We show that this phenomenon can also occur in diffusion flames when the Lewis numbers are small. And we discuss the asymptotics of the one-dimensional twin deflagration configuration, an overlooked problem from the 70s.

Among many other methods for simplifying chemical kinetics for laminar and turbulent flame calculations, the method of intrinsic low-dimensional manifolds (ILDM) has shown to be an efficient tool for the development of reduced kinetic schemes. Based on a numerical analysis, it identifies and decouples the fast relaxing timescales of the chemical system. The results, for example, the thermokinetic state of the system or the reaction rates, are then stored in terms of a small number of parameters (mixture fraction, reaction progress variables) for subsequent use in reacting flow calculations. Furthermore, together with the reduced mechanism, information about the coupling of the chemical kinetics with the physical processes (molecular transport, turbulent mixing) is obtained.

In this paper, we present a method that allows an efficient implementation of the ILDM method in flame calculations and overcomes several problems that had been discussed in previous work. It is based on three ingredients: A robust numerical method to calculate the ILDM, a storage scheme that allows an easy implementation in CFD codes, and a model for the coupling of the chemical kinetics with transport processes. In this way, not only the chemistry can be calculated beforehand but also a reduced set of diffusion coefficients. The method is verified by simulations of laminar syngas-air flames with an emphasis on the non-premixed case.

An experimental investigation of the flame response to strain rate in the case of unsteady premixed low-turbulent flames is presented. In order to point out the fundamental aspects of the mutual interaction between combustion and turbulence, measurements of local flame properties (curvature, displacement speed) and tangential strain rate were performed under varying conditions of Lewis number and turbulence.

Three different mixtures (methane/air, propane/air, and hydrogen/air) were successively spark ignited in a vertical wind tunnel. The expanding flame freely propagated in a grid-generated decaying turbulent flow. An advanced field imaging technique coupling high-speed laser tomography and cross-correlation particle image velocimetry (PIV) was used to measure the temporal evolution of local flame stretch exerted by the turbulent cold flow.

Local flame curvature and local displacement speed were calculated from flame-front contours. Curvature probability density functions (PDFs) were negatively skewed, especially for nonunity Lewis numbers, and displacement speed distributions underlined the influence of local stretch and thermodiffusive effects on flame-speed variations. Tangential strain rate was determined by using the velocity field in the neighborhood of the flame front and appears to be independent of the Lewis numbers. A strong correlation between local flame curvature and tangential strain rate was demonstrated, underlining the cold flow effects on the local flame structure. The influences of turbulence and Lewis number were evaluated and compared with numerical simulations. Then, local flame stretch distributions were determined versus time, indicating that a significant proportion of the flame was under compression.

Instantaneous measurements of temperature, major species, OH, CO, and NO are performed in turbulent premixed flames using simultaneous Rayleigh scattering, Raman scattering, and laser-induced fluorescence (LIF). Temperature is determined from Rayleigh scattering, and concentrations of CH₄, O₂, N₂, H₂O, CO₂, and H₂ are obtained from Raman scattering. Linear LIF is used to measure OH and NO, while two-photon LIF is used to determine CO concentrations. The two-photon CO-LIF system provides significant improvements over CO-Raman measurements. This combination of diagnostic techniques is used to investigate the detailed compositional structure of turbulent lean premixed methane-air flames. Three turbulent flames having different equivalence ratios are considered. The ratio of rms velocity to laminar flame speed ranges from approximately 6 to 17. The correlation of major species, OH, and CO concentrations with temperature are similar to those predicted by one-dimensional laminar flame calculations. However, NO concentrations in the leaner flames are higher than the predicted values.

Two-dimensional laser-induced fluorescence (2D-LIF) measurements are applied to the chemical vapor deposition (CVD) of diamond by an oxyacetylene flame to visualize the distributions of atomic hydrogen, C₂, and CN in the gas phase during diamond growth. Experiments are carried out in laminar flames and reveal that atomic hydrogen is ubiquitous at and beyond the flame front. Its presence extends to well outside the diamond deposition region, whereas the C₂ distribution is limited to the flame front and the acetylene feather. CN is found to be present mostly at the outer edge of the flame, where ambient air interacts with flame gases. The diamond layers obtained are characterized by optical as well as scanning electron microscopy (SEM) and cathodoluminescence topography (CL). Clear relations are observed between the local variations in growth rate of the diamond layer and the distribution of H, C₂, and CN in the boundary layer just above the substrate. Further relations between CN and the morphology and the nitrogen incorporation as identified by CL of the deposited diamond layer are found as well. These relations agree with theoretical models describing the importance of the mentioned species in (flame) deposition processes of diamond. Three separate regions can be discerned in the flame and the diamond layer, where the gas phase and diamond growth are predominantly governed by the flame source gases, the ambient atmosphere, and the interaction of both, respectively.

The reaction of toluene with molecular oxygen was studied behind reflected shock waves. Mixtures of 0.5–1 mol% toluene and 5–10% oxygen in argon were investigated in the temperature range between 1050 and 1400 K at total pressures between 2 and 4 bar. We followed the rate of formation of the benzyl radicals by time-resolved UV absorption at 257 nm. The measured concentration-time profiles of the benzyl radicals were numerically reproduced using a simple reaction mechanism. For the initial reaction$C_6{H}_{5}C{H}_3+O_2\to C_6{H}_{5}C{H}_2+H{O}_2(R1)$ a rate coefficientk₁ of $k_1=3\times {10}^{14}exp\left[\frac{-180kj/mol}{RT}\right]\frac{{cm}^3}{mols}(1050K<T<1400K)$was determined with an accuracy of 30%. The rate constant k₂of the subsequent reaction $C_6{H}_{5}C{H}_3+H{O}_2\to C_6{H}_{5}C{H}_2+H_2{O}_2(R2)$was determined to be.$k_2=3\times {10}^{14}exp\left[\frac{-92kj/mol}{RT}\right]\frac{{cm}^3}{mols}(1150K<T<1250K)$The reaction of p-xylene with molecular oxygen was investigated using the same technique. Mixtures of 0.25–0.5 mol % p-xylene and 2.5–10 mol% oxygen in argon were shock-heated to temperatures between 1130 and 1380 K. We followed the formation of p-methyl-benzyl radicals by time-resolved UV a