Role In Flame Propagation Research Articles

Quantifying the role of Darrieus–Landau instability in turbulent premixed flame speed determination at various burner sizes

To probe the impact of Darrieus–Landau (DL) instability on turbulent premixed flame propagation at various burner sizes, methane–air premixed flames from five Bunsen-type burners with different nozzle diameters (4 mm, 6 mm, 8 mm, 10 mm, and 12 mm) were investigated at Reynolds numbers ranging from 1000 to 8500. The flame curvatures used to identify DL instability were determined using Mie scatter images captured by a particle image velocimetry system. The flame speed was further derived by applying an asymmetric hypothesis to the images. The energy-frequency spectrum of the inflow disturbance was determined using a hot-wire anemometry system, and specific wavelet transform analysis was performed to investigate the dependence of DL instability on the proportion of effective disturbances (Ped) and quantify the role of DL instability in determining the turbulent flame speed. The results showed that the burner diameter had an obvious effect on the presence of DL instability and its role in flame propagation. The ability of DL instability to enhance the flame curvature skewness and the turbulent flame speed was closely related to Ped. Ped increased when the burner diameter increased from 6 mm to 12 mm, thus enhancing the DL instability. Changing the burner diameter also affected the interplay between DL instability and turbulence. The above interactions and their effects on the flame speed during the change of inflow disturbances could be formulated by Ped. Finally, a Ped-based correlation was proposed to describe the dependence of the turbulent flame speed on the burner size.

The Critical Pressure at the Onset of Flame Instability of Syngas/Air/Diluent Outwardly Expanding Flame at Different Initial Temperatures and Pressures

Syngas has gained attention recently due to its high energy density and environmentally friendly characteristics. Flame stability plays an important role in flame propagation in energy conversion devices. Experimental studies were performed in a cylindrical chamber to investigate flame instability of syngas/air/diluent mixture. A Z-shape Schlieren system coupled with a high-speed complementary metal–oxide–semiconductor camera was used to record flame pictures up to 40,000 frames per second. In this research, syngas is a mixture of hydrogen and carbon monoxide and diluent is a blend of 14% CO2 and 86% N2 with the same specific heat as the burned gases. Three main flame instabilities namely Rayleigh–Taylor (body force) instability, hydrodynamic instability, and thermal-diffusive instability have been studied. For the onset of flame instability, a power law correlation for the ratio of critical pressure to initial pressure of syngas/air/diluent flames over a wide range of initial temperatures (298–450 K), initial pressures (1.0–2.0 atm), equivalence ratios (0.6–3.0), diluent concentrations (0–10%), and hydrogen percentages (5–25%) in the fuel has been developed.

Effect of side venting areas on the methane/air explosion characteristics in a pipeline

To reduce the great losses caused by gas explosions in the gas transportation process and to improve the explosion venting efficiency in pipelines, a small pipeline with both a side vent and an end vent was developed, and premixed methane/air explosion characteristics were studied in the pipeline with different side venting areas. The experimental results show that in the same side venting position, with an increasing venting area, the propagation time for flames through the pipeline increases on a large scale, the peak overpressure of the methane explosion drops significantly, and the venting effect is greatly enhanced. Meanwhile, when the flame propagates upstream a side vent, the propagation can be slightly promoted by a side vent with a small area in the corresponding period of time, whereas a side vent with a large area can hinder the flame propagation all the time. After the flame passes the side vent, side venting plays a strong role in flame propagation, and the flame propagation velocity drops rapidly under a large side venting area. It is also found that overpressure in the vicinity of a side vent is less affected by the side venting area, while the effect of the venting area on overpressure is relatively large at a far distance from the side vent position.

Analysis of the influence of nanometric aluminium particle vaporisation on flame propagation in bulk powder media

The combustion of nanometric aluminium (Al) powder with an oxidiser such as molybdenum trioxide (MoO3) is studied analytically. The analysis was performed to correlate individual Al particle gasification rates to macroscopic flame propagation rates observed in flame tube experiments. Examination of various characteristic times relevant to propagation of a deflagration reveals that particles below about 1.7 nm in diameter evaporate before appreciable chemical reactions occur. Experimental studies used Al particles greater than 1.7 nm in diameter such that a diffusion flame model was developed to better understand the combustion dynamics of multiphase Al particles greater than 1.7 nm diameter relative to experimentally measured macroscopic flame propagation rates. The diffusion flame model predicted orders of magnitude slower propagation rates than experimentally observed. These results imply that (1) another reaction mechanism is responsible for promoting reaction propagation and/or (2) modes other than diffusion play a more dominant role in flame propagation.

Combustion Wave Speeds of Sol−Gel-Synthesized Tungsten Trioxide and Nano-Aluminum: The Effect of Impurities on Flame Propagation

Nanoparticles of tungsten trioxide (WO3) were synthesized using sol−gel chemistry and combined with nanoscale aluminum (Al) particles to form a thermite. The as-made sol−gel-derived WO3 aerogel contains impurities such as hydroxyl groups that are inherent to the synthesis process. These impurities can be removed via heat-treating the powder, and both as-made and heat-treated mixtures were examined for ignition and flame propagation as a function of the mixture bulk density. Results showed that the hydroxyl impurity impedes flame propagation in high density (compressed pellets) by acting as a heat sink and absorbing energy during flame propagation. In loose powders (low density mixtures), convection plays a more dominant role in flame propagation, such that the reduced particle size of the as-made WO3 produces higher flame propagation speeds than the enhanced thermal transport properties associated with the heat-treated WO3.

A numerical study of confined triple flames using a flamelet-generated manifold

Laminar triple flames are investigated numerically using detailed and reduced chemical reaction mechanisms. Triple flames are believed to play an essential role in flame propagation in partially premixed systems. In order to reduce the computational cost of the simulations, the flamelet-generated manifold (FGM) method is used to simplify the chemistry model. The FGM method is based on a library of premixed laminar flamelets. Mixture fraction variations in partially premixed flames are taken into account by using the mixture fraction as an extra degree of freedom. A comparison of the results computed with FGM and with detailed chemistry shows that FGM predicts the structure of triple flames very accurately. The gradient of the mixture fraction in the unburnt mixture is varied, and its influence on the structure and propagation of triple flames is studied. For decreasing gradients, the curvature of the premixed flame branch decreases and the propagation velocity increases. Due to the diffusive nature of the flow and the use of symmetry boundary conditions in the lateral direction, high mixture fraction gradients could not be realized at the position of the flame. As a result, the trailing diffusion flame branch is only weakly present. The heat release in the premixed flame branches is two orders of magnitude higher than in the diffusion flame branch. The premixed and diffusion flame branches are studied using a flamelet analysis. Flame stretch and curvature appear to be significant in the premixed flame branch of a triple flame. These effects cause a decrease in the local mass burning rate of almost 15%. The structure of the diffusion flame branch appears to be similar to the structure of a counterflow diffusion flame with the same scalar dissipation rate.

The evolution equation for the flame surface density in turbulent premixed combustion

One basic effect of turbulence in turbulent premixed combustion is for the fluctuating velocity field to wrinkle the flame and greatly increase its surface area. In the flamelet theory, this effect is described by the flame surface density. An exact evolution equation for the flame surface density, called the Σ-equation, may be written, where basic physical mechanisms like production by hydrodynamic straining and destruction by propagation effects are described explicitly. Direct numerical simulation (DNS) is used in this paper to estimate the different terms appearing in the Σ-equation. The numerical configuration corresponds to three-dimensional premixed flames in isotropic turbulent flow. The simulations are performed for various mixture Lewis numbers in order to modify the strength and nature of the flame-flow coupling. The DNS-based analysis provides much information relevant to flamelet models. In particular, the flame surface density, and the source and sink terms for the flame surface density, are resolved spatially across the turbulent flame brush. The geometry as well as the dynamics of the flame differ quite significantly from one end of the reaction zone to the other. For instance, contrary to the intuitive idea that flame propagation effects merely counteract the wrinkling due to the turbulence, the role of flame propagation is not constant across the turbulent brush and switches from flame surface production at the front to flame surface dissipation at the back. Direct comparisons with flamelet models are also performed. The Bray-Moss-Libby assumption that the flame surface density is proportional to the flamelet crossing frequency, a quantity that can be measured in experiments, is found to be valid. Major uncertainties remain, however, over an appropriate description of the flamelet crossing frequency. In comparison, the coherent flame model of Marble & Broadwell achieves closure at the level of the Σ-equation and provides a more promising physically based description of the flame surface dynamics. Some areas where the model needs improvement are identified.

A model for flame kernel growth at aircraft relight conditions

A one-dimensional (1D), comprehensive numerical model for the prediction of flame kernel development at high altitude relight conditions in aircraft gas turbines is proposed and tested. The main purpose of this study is to develop a new methodology to numerically solve the very complicated governing equations involved in the flame kernel initiation and propagation in turbulent flow fields. Generally speaking, the surface area of a wrinkled flame cannot be directly described by one-dimensional approaches. However, this problem is overcome by incorporating a fractal model into the one-dimensional governing equations. Meanwhile, the concept of high-activation-energy asymptotic analysis is also applied to provide the relationships between the average turbulent transport coefficients and reaction rate and their laminar counterparts. An artificial thermal conductivity submodel is used to reflect the fact that rapid mixing always existsinside spark kernels in the early stages of kernel development. The effect of flame stretch, which plays an important role in flame propagation, is also included in the model. Numerically, the governing equations are first discretized by the power-law scheme and then solved by the PISO algorithm. In general, the numerical predictions are in good agreement with the experimental results. With minor modification, this numerical approach may be applied to spark ignition problems in SI engines as well.