The Effect of Jet-Induced Disturbances on the Flame Characteristics of Hydrogen–Air Mixtures

In this work the interaction between turbulence and premixed combustion is studied by means of direct numerical simulations using a level set equation based on the scalar G that describes the motion of the flame front represented by an iso-surface G = G0. The G-equation contains terms accounting for flame propagation, flame curvature and gas expansion effects. The flame front is transported and wrinkled by the turbulent flow field and propagates in a direction normal to itself with its laminar burning velocity. The turbulent flow field is based on the constant density Navier-Stokes equations. Results are presented for cases with and without heat release at the flame front. In the former case heat release is modeled by volume sources located on the flame front. For the case without heat release the flame surface area ratio and the source terms appearing in its balance equation accounting for production, kinematic restoration and dissipation are evaluated from the DNS results. The flame surface area ratio shows a linear increase with increasing ratios of υ/sL in the corrugated flamelets regime but a bending in the thin reaction zones regime. This is consistent with the evaluation of the source terms where it is found that kinematic restoration is the main sink term in the corrugated flamelets regime, while scalar dissipation becomes the dominant sink term in the thin reaction zones regime. By evaluating the pdf of G-fluctuations the link between the flame surface area ratio and the flame surface density is established. Agreement with existing models for the latter is found. When gas expansion effects are added, the induced velocity at the flame front can be evaluated. This leads to a purely kinematic explanation for counter gradient diffusion. Effects of gas expansion on the flame surface area ratio are found to be significant only if the laminar burning velocity is of the same order of magnitude as the turbulence intensity or larger.

This paper is devoted to the general correlation of turbulent burning velocities in terms of straining rates for premixed flames propagating in intense turbulence. This problem was investigated by the Leeds group led by Professor Bradley and many other researchers. We present here a new methodology based upon a downward propagating premixed CH4–air flame through a nearly isotropic turbulent flow field with a pair of specially designed ion probes for quantitative measurements of turbulent burning velocities. The improvements are that the flame propagation is not influenced by the ignition source and the unwanted turbulence from walls, effects of buoyancy and pressure rise due to burning are minimized, and a greater parameter range than hitherto is covered. The results show that both the turbulent burning velocity bending and the vitality of turbulent premixed flames are certain and surprising. Logarithmic plots of turbulent burning velocities S T/ S L – 1 against the turbulent intensities u '/ S L reveal a transition, where S L is the laminar burning velocity. Across the transition, the slope n changes from positive to negative when values of u '/ S L and/or Karlovitz number are greater than some critical values. This transition seems to correspond to the Klimov–Williams criterion that separates corrugated flamelets from distributed reaction zones. Interestingly, no global quenching of premixed turbulent flames is observed, even at u ' S L ≊ 40, a value significantly higher than in most previous measurements. At a fixed u '/ S L, values of the S T/ S L data vary with the equivalence ratio ϕ. This indicates that the common expression of the form S T/ S L = 1 + C ( u '/ S L) n cannot be applicable generally, because values of the constant C are different for different mixture compositions. It is found that all of the present data with different values of ϕ can be approximated by a simple expression, ( S T – S L)/ u ' ≊ 0.06 Da 0.59, where Da is the Damkohler number. Hence a better correlation of turbulent burning velocities in terms of straining rates for premixed turbulent (methane–air) combustion is proposed.

Investigation of flame-generated turbulence in premixed flames at low and high burning velocities

Pressure influence on the flame front curvature of turbulent premixed flames: comparison between experiment and theory

Numerical simulation of turbulent Bunsen flames with a level set flamelet model

Laminar flamelet concepts in turbulent combustion

For accurate prediction of the laminar flame front propagation the influence of the stretch effect on the burning velocity has to be considered. Thus, only burning velocity and Markstein number together give complete information about the laminar flame front behavior. The Markstein number quantifies the influence of the stretch effect on the burning velocity and accordingly, indicates the flame front stability. Due to the analogy between the laminar and the turbulent flames these two parameters, laminar burning velocity and Markstein number must be also considered as essential for describing the turbulent flame front stability [1]. Nevertheless, the experimental data of commercial liquid fuels regarding these parameters are scarce, especially at elevated pressure. Combustion characteristics (laminar burning velocity and Markstein number) of Kerosene Jet A-1 are investigated experimentally in an explosion bomb vessel. For this purpose an optical laser method is employed based on the Mie-scattering of the laser light by smoke particles. Unlike analogous experiments conducted with gaseous fuels [1], the major challenge connected with the present experiments arises from the liquid state of the investigated fuel at ambient condition. Thus, a main difficulty in the present experiments is pre-evaporation of the fuel and achieving of homogeneous gaseous fuel/air mixture prior to ignition. This is solved by mounting a heating system into the walls of the bomb vessel that provides a homogeneous temperature distribution in the vessel and therewith of the mixture itself. The experimental investigation is practically done through the following steps: heating the vessel up to the requested temperature; filling the vessel with an appropriate mixture by the partial pressure method (providing a fuel in gaseous state through the liquid fuel injection and its instantaneous evaporation due to the elevated temperature); attaining an uniform mixture by means of fans; ignition and acquisition of the data; post-processing and data analyses. Within the experimental study influence on the burning velocity and Markstein number of three crucial parameters — initial temperature, initial pressure and mixture composition — are investigated. Observed results for the burning velocity and Markstein number follow the theoretically expected tendencies resulting from the variation of the initial parameters in almost all cases. Where that was not the case the reasons for discrepancies are discussed. Impact of the results on emissions influenced by different operating modes of jet turbines is considered. Due to the common substitution of the kerosene with n-decane in numerical simulations their burning velocities are compared.

Turbulent premixed flame front dynamics and implications for limits of flamelet hypothesis

Combustion phenomena are of high scientific and technological interest, in particular for energy generation and transportation systems. Direct Numerical Simulations (DNS) have become an essential and well established research tool to investigate the structure of turbulent flames, since they do not rely on any approximate turbulence models. In this work two complementary DNS codes are employed to investigate different types of fuels and flame configurations. The code π$^3$ is a 3-dimensional DNS code using a low-Mach number approximation. Chemistry is described through a tabulation, using two coordinates to enter a database constructed for example with 29 species and 141 reactions for methane combustion. It is used here to investigate the growth of a turbulent premixed flame in a methane-air mixture (Case 1). The second code, Sider is an explicit three-dimensional DNS code solving the fully compressible reactive Navier-Stokes equations, where the chemical processes are computed using a complete reaction scheme, taking into account accurate diffusion properties. It is used here to compute a hydrogen/air turbulent diffusion flame (Case 2), considering 9 chemical species and 38 chemical reactions. For Case 1, a perfectly spherical laminar flame kernel is initialized at the center of a cubic domain at zero velocity. A field of synthetic homogeneous isotropic turbulence is then superposed and the turbulent flow and the flame can begin to interact. Various species can be used as an indicator for the flame front in a combustion process. Among them, the isosurface of species CO$_2$ at a mass fraction of 0.03 is retained here, since this value corresponds to the steepest temperature gradient in the associated one-dimensional laminar premixed flame. The results obtained have been post processed in order to study the interesting aspects of the coupling between flame kernel evolution and turbulence, such as straining and curvature impact on the flame surface area and local thickness. For Case 2, the instantaneous structure of a non-premixed hydrogen/air flame evolving in a turbulent flow and starting from an initially planar structure is investigated. Here again, the properties of the resulting turbulent flame are of high interest and will be visualized, defining the flame front in a classical manner for non-premixed combustion using a mixture fraction isosurface. Considering the context of this publication, the emphasis is clearly set on the post-processing and visualization of the DNS data, not on the fundamental issues associated with turbulent combustion.

The development of wrinkled turbulent premixed flames

The influence of the Lewis number on turbulent flame front geometry is investigated in a premixed turbulent stagnation point flame. A laser tomography technique is used to obtain the flame shape, a fractal analysis of the multiscale flame edges is performed and the distribution of local flame front curvature is determined. Lean H2/Air and C3H8/Air mixtures with similar laminar burning rates were investigated with Lewis numbers of 0·33 and 1·85 respectively. At the conditions studied the laminar H2/Air mixture is unstable and a cellular structure is observed. Turbulence in the reactant stream is generated by a perforated plate and the turbulent length scale (3 mm) and intensity (7%) at the nozzle exit are fixed. The equivalence ratio is set so that the laminar burning velocity is the same for all the cases. The results show clearly that the turbulent flame surface area is dependent on the Lewis number. For a Lewis number less than unity surface area production is observed. The shape of the flame f...

Study on premixed flame dynamics of CH4/O2/CO2 mixtures

Experimental investigation of three-dimensional flame-front structure in premixed turbulent combustion—I: hydrocarbon/air bunsen flames

Experimental data obtained using a new multiple-camera digital particle image velocimetry (PIV) technique are presented for the interaction between a propagating flame and the turbulent recirculating velocity field generated during flame–solid obstacle interaction. The interaction between the gas movement and the obstacle creates turbulence by vortex shedding and local wake recirculations. The presence of turbulence in a flammable gas mixture can wrinkle a flame front, increasing the flame surface area and enhancing the burning rate. To investigate propagating flame/turbulence interaction, a novel multiple-camera digital PIV technique was used to provide high spatial and temporal characterization of the phenomenon for the turbulent flow field in the wake of three sequential obstacles. The technique allowed the quantification of the local flame speed and local flow velocity. Due to the accelerating nature of the explosion flow field, the wake flows develop ‘transient’ turbulent fields. Multiple-camera PIV provides data to define the spatial and temporal variation of both the velocity field ahead of the propagating flame and the flame front to aid the understanding of flame–vortex interaction. Experimentally obtained values for flame displacement speed and flame stretch are presented for increasing vortex complexity.

Biomass-derived syngas is prone to leakage during transportation. To safely use biomass-derived syngas, we need to study the combustion characteristics of material syngas the purpose of this paper is: at T = 303 K, P = 0.1 MPa, under the condition of the spherical expansion flame method, calculate the laminar burning velocity, and used the Chemkin module of ANSYS to simulate four mechanisms (GRI-3.0?FFCM-1?Li-2015?SanDiego +NOx-2018) to compare, select more appropriate reaction mechanism through experimental data for related research. It was found that the chemical reaction mechanism of GRI-3.0 is more in line with the experimental results. It is found that the experimental results are in good agreement with the linear extrapolation method. When the H2 concentration in-creases from 22-42%, the peak laminar burning velocity moves in the direction of the lean fuel side. When the H2 concentration increases to 42%, the laminar burning velocity is the fastest, reaching 0.78 m/s. The effect of H2 on thermal diffusivity is high. When H2 concentration reaches 42%, its thermal diffusivity is much higher than other gas components. The adiabatic flame temperature of F1 (22% H2, 45% CO, 9.6% CH4, 23.4% CO2)-air mixtures is the highest, approaching 2196 K. The peak adiabatic flame temperature of F5 (42% H2, 25% CO, 9.6% CH4, 23.4% CO2)-air mixtures is 2082 K, which is comparatively low. Nonetheless, the H2 concentration in F5-air mixtures is higher than that in F1-air mixtures, indicating that H2 has less influence on adiabatic flame temperature than CO. The positive reactions to accelerate laminar burning velocity mainly include R99, R38, and R46. The R52 and R35 can inhibit laminar burning velocity. There are many factors affecting laminar burning velocity, among which high reactive free radicals are the main factors, and the competition between chain branching reaction and chain termination reaction for high reactive free radicals also affects laminar burning velocity. With the increase of concentration of H2, participate in the reaction of the molar mass fraction of highly reactive free radicals and the laminar burning velocity.