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This paper presents an experimental study and similarity analysis of the flame behavior from an opening of a compartment under horizontal ambient back wind, passing over the roof (back roof-wind). This is a basic scenario for a room fire occurring at the uppermost storey in urban environment, but it has not been quantified yet. Four basic flame morphologic characteristic parameters are considered: flame height, flame width, flame downwind horizontal extension distance and overall flame length. Experiments are conducted with a reduced-scale compartment employing a wind tunnel. The mentioned flame morphologic characteristic parameters are quantified comprehensively involving various opening dimensions, heat release rates and wind speeds. Results show that the flame morphologic characteristics all vary little at relatively lower wind speeds, i.e., ≤ 0.5 m/s. With further increasing wind speed, the flame height decreases, while the flame width and the flame downwind horizontal extension distance increase monotonically, resulting in a complex evolution of overall flame length. The flame height changes more remarkably with heat release rate (HRR) for relatively lower wind speeds, while the flame width and the flame downwind horizontal extension distance change more remarkably with HRR for relatively higher wind speeds. Non-dimensional similarity analyses are performed based on the combined physical mechanisms of tilting of the flame, the change in air entrainment/mixing caused by ambient back roof-wind for the flame body above the roof and partly bound by the facade for that below the roof, as well as the competition of the horizontal momentum of the outflow at the opening and the buoyancy induced upward flow by the flame itself. Basic formulae are proposed, based on the derived non-dimensional quantities, to describe these flame morphologic characteristics systematically. The experimental results obtained and the physical model proposed provide an essential basis to quantify the flame behavior from an opening of a fire compartment under ambient back roof-wind conditions.

The focus of the present work is on the prediction of soot in the turbulent Delft III/Adelaide natural gas flame at a Reynolds number of 9700. A parabolic flow solver with the SSG (Speziale, Sarkar and Gatski) Reynolds stress transport model for turbulence is coupled to a joint-scalar transported PDF (probability density function) approach allowing exact treatment of the interactions of turbulence with the solid and gas phase chemistries. Scalar mixing is treated via the modified Curl's coalescence/dispersion model and two different closures for the scalar dissipation rate are explored. The gas phase chemistry is represented by a systematically reduced mechanism featuring 144 reactions, 15 solved and 14 steady-state species. The dynamics of soot particles, including coagulation and aggregation in the coalescent and fractal aggregate limits, is treated either via a simplified two-equation model or via the MOMIC (method of moments with interpolative closure). The inclusion of soot surface reactions based on a second ring PAH (polycyclic aromatic hydrocarbon) analogy is also investigated. Soot oxidation via O, OH and O(Formula presented.) is taken into account and the sensitivity to the applied rates is investigated. An updated acetylene-based soot nucleation rate is formulated based on consistency with detailed chemistry up to pyrene and combined with a sectional model to compute soot particle size distributions in the NIST well-stirred/plug flow reactor configuration. The derived rate is subsequently used in turbulent flame calculations and subjected to a sensitivity analysis. Radiative emission from soot and gas phase species is accounted for using the RADCAL method and by the inclusion of enthalpy as a solved scalar. Computed soot levels reproduce experimental data comparatively well and approximately match absolute values. The axial location of peak soot in the Delft III/Adelaide flame is consistent with previous large eddy simulations and possible causes for the discrepancy with experimental data are analysed.

In the present work, detailed experimental investigations have been carried out to understand the effect of coflow air on the stabilization of lifted spray flames with kerosene fuel. Two full cone type pressure swirl nozzles N1 and N2 () are considered for experiments with a moderate fuel injection pressure range (4-12 bar). The flame liftoff height is observed to increase with fuel injection pressure for lower coflow velocity range ( 0.2m/s). At higher coflow velocities (0.2-0.4m/s), the flame liftoff height decreases with an increase in fuel injection pressure. At lower injection pressures, the flame liftoff height has been observed to increase significantly for higher coflow velocities. Detailed droplet velocity measurements using PIV and droplet size measurement using the shadowgraphy technique in cold flow helped understand the stabilization of lifted spray flames in the domain. At higher coflow velocities and lower injection pressure, smaller droplet number density allows additional air entrainment and flame stabilizes at the downstream location, where SMD and droplet flow velocities are small. At higher injection pressure, high droplet number density reduces the additional air entrainment, which stabilizes lifted spray flame at a relatively downstream location. At lower coflow velocity and lower injection pressure, flame stabilizes upstream, where mixture burning velocity is nearly equal to flow velocity irrespective of higher SMD and droplet flow velocity. At higher injection pressure, flame stabilizes only after the end of high velocity zones, as observed from PIV measurements.

In the present work, detailed experimental investigations have been carried out to understand the role of various spray parameters, such as fuel injection pressure, droplet size distribution, and velocity field on the stabilization of lifted spray flames with kerosene fuel. Six full cone pressure swirl nozzles N1-N6 (fuel flow rate, m(f) = 1.7-7.08 kg/h at P-inj = 9 bar) are considered for experiments with fuel injection pressure varying from 4-9 bar. The Sauter mean diameter (SMD) has been observed to decrease (58-34 mu m) with an increase in fuel injection pressure (4-9 bar). The flame liftoff height of the spray flame increases with an increase in the fuel injection pressure, even though the spray SMD decreases. To understand the role of the various important spray parameters on stabilization of lifted spray flames, particle image velocimetry measurements have been carried out near the flame stabilization zone of the spray. It has been observed that a zone of high velocity is formed near the nozzle exit. The size of this zone varies with injection pressure and this has been attributed to increased fluctuations in a lifted spray flame along with the increased momentum of the droplets emanating from the spray at higher injection pressures.

Recently, an extinction model was successfully integrated with a conditional moment closure (CMC) approach to predict lift-off height accurately for the lifted jet diffusion flame discharging to hot surroundings Roy et al. (2014). This method, referred as CMCE, is a low-cost model based on the RANS and first-order CMC approach. The proposed methodology may not be superior compared to involved LES and second-order CMC combination, but enables to simulate industrial problems like gas turbine processes in a reasonable CPU time. Though the credibility of this novel approach was well established by validating its predictions against the experimental data, the applicability of this method and constants involved in this model, for other test cases was questionable. In this brief communication, CMCE model is further validated against lift-off studies of two different fuels issuing in cold surroundings, without changing the model constants used in the earlier work. The results are encouraging and a good match is found for lift-off heights for both the test cases and for varying jet velocities. Large variations are found between the predictions of lift-off heights by CMC and CMCE models. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

The structure and characteristics of a turbulent inverse and normal oxy-fuel diffusion flame are investigated. Previous investigations reported in the literature looked at flame characteristics of laminar inverse diffusion flames and their differences to normal diffusion flames. Only few investigations are reported for turbulent inverse diffusion flames and they did not compare the results to the corresponding normal configuration. The present study uses a combined experimental and numerical approach to compare and analyze a turbulent non-premixed inverse oxy-fuel and a corresponding normal flame, both are non-piloted. Measurements were conducted using simultaneously recorded planar Rayleigh scattering and OH-LIF signals. Due to the significant variation of the effective Rayleigh cross section in mixture fraction space and the unknown OH quenching contributions, a comparison of derived quantities such as temperature and OH mole fraction is not possible. Therefore, the Rayleigh and OH-LIF signals were incorporated in the LES flamelet/progress variable approach used here. This allows for a direct comparison of experimentally recorded and numerically predicted Rayleigh and OH-PLIF signals for the flame structure analysis, which includes the joint PDF of both quantities. The two flames are compared in terms of the local flame structure. In addition, differences in the mixing field and especially in the location of turbulent/non-turbulent interface are investigated.

Investigations of structure and reaction zones of unconfined methane-hydrogen laminar jet diffusion flames are presented. In a lab-scale burner, experiments have been conducted using a mixture of pure methane and pure hydrogen in different volumetric proportions. Digital photographs of the flames have been captured and the radial temperature profiles at different axial locations outside the flame zone have been measured. Numerical simulations are carried out with a C2 chemical kinetics mechanism having 25 species and 121 reaction steps and an optically thin radiation sub-model. The numerical results are validated against the experimental data. Parametric studies have been carried out for a range of methane-hydrogen mixtures with volumetric proportion of hydrogen in the mixture varying from 0% to 80%. Variation of flame height, contours of temperature, mass fractions of product species and the net reaction rates of methane and hydrogen for various cases are presented and discussed in detail. Further, analysis of net reaction rates of important reactions involving methane and hydrogen with radicals such as O, H and OH are analyzed. The maximum temperature in the domain is seen to decrease for the fuel mixtures with higher hydrogen content. The overall flame length also decreases. For a fuel mixture having 40% hydrogen by volume, the net molar consumption rates of methane and hydrogen are found to be almost equal. Examination of individual reactions of fuel species with radicals shows that hydrogen is mainly consumed by its reaction with OH, whereas methane consumption is mainly through its reactions involving H as well as OH radicals. � 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Numerical and experimental investigations highlighting the heat and mass transfer phenomena in a laminar co-flowing jet diffusion flame have been carried out. The fuel under consideration is ethylene, with ambient air as the co-flowing oxidizer. The diffusion flame is modeled using a 17-step reduced reaction mechanism with finite rate chemistry and the effects of soot on the radiative heat transfer of the flame have been demonstrated. Soot growth and oxidation processes are studied using a two-equation transport model, while the radiative heat transfer is modeled using the P1 approximation. An in-house finite volume code has been developed to solve the axi-symmetric Navier-Stokes equations in cylindrical coordinates, along with the soot mass fraction, soot number density, energy and species conservation equations. Comparison of predictions with experimental results shows reasonable agreement with regard to the flame height and temperature distribution. A parametric study is also presented, which illustrates the effects of the fuel jet Reynolds number and the flow rate of co-flow air. � 2008 Elsevier Ltd. All rights reserved.

International audience; ortex interactions with flames play a key role in many practical combustion applications, Such interactions drive a large class of combustion instabilities, they control to a great extent the structure of turbulent flames and the corresponding rates of reaction, they occur under transient operations or when flames travel in ducts containing obstacles. Vortices of various types are often used to enhance mixing, organize the flame region, and improve the flame stabilization process, The analysis of flame/vortex interactions has value in the development of our understanding of basic mechanisms in turbulent combustion and combustion instability, The problem has been extensively investigated in recent years. Progress accomplished in theoretical, numerical and experimental investigations on flame/vortex interactions is reviewed in this article.

Beside appropriate turbulence and combustion modeling, the problem of accurate predictions of turbulent diffusion flames, especially in industrial applications, usually requires accurate radiative heat transfer predictions as well. However, it seems at the moment that a full account of accomplished radiation modeling is too much of a burden for efficient overall modeling of diffusion flames, and simplified radiation models are usually retained in the industrial applications. In most of the early works concerned with the modeling of the relatively simple jet diffusion flames, like these simulated in this work, the radiation heat transfer has been neglected, presuming the radiant heat transfer fraction having negligible impact onto overall energy balance. When used, then usually the optically thin radiation model has been applied, mostly due to its simplicity and ease of implementation. On the other hand, in recent publications the importance of radiation inclusion into simulations of simple jet flames has been recognised, where even subtle radiation details, like spectral effects, showed to play an important role. Two different jet diffusion flame configurations are simulated in this work by using a computational fluid dynamics code FIRE – a hydrogen jet flame and a methane piloted jet diffusion flame. Predictions are compared with the experimental measurements. Based on the stationary laminar flamelet methodology, chemistry related calculations were obtained in pre-processing step by using the CSC solver together with detailed chemical mechanisms used in both cases. Statistical behaviour of the reactive scalars is presumed according to the beta probability density function of the mixture fraction moments. Turbulence modeling is based on a hybrid closure and comparisons against the classical k-eps model are performed. The aim of such a hybrid turbulence modeling was to partly overcome the limitations of the classical k-eps model by using the resolved Reynolds stresses to calculate the turbulent kinetic energy only, but still retaining the eddy-viscosity based closure in the momentum equations. This way a more accurate, but still a numerically robust turbulence closure is obtained. A conservative form of the discrete transfer radiation method (DTRM) is applied in the radiative heat transfer calculations, with the radiative properties being modeled as a weighted sum of grey gases. Conservative DTRM considerably contributed to improved temperature profiles of the jet flames simulated in this work, proving to be a sufficiently accurate method as well. Number of rays used in DTRM calculations has been varied and its impact on the predictions has been investigated.

A conservative form of the discrete transfer radiation method (DTRM) has been applied in a computational fluid dynamics (CFD) simulation of the radiative heat transfer and other phenomena in an experimental furnace. Although the furnace is nearly square in cross section, the modelling of relatively small-sized cooling tubes, aligned beside the walls in the first third of the furnace length, was quite challenging, leading to computationally demanding calculations. The furnace was operated under non-premixed conditions, burning preheated heavy fuel-oil. For combustion simulation a semi-empiric oil combustion model has been applied, while for the evaluation of radiative properties the weighted sum of grey gases model (WSGGM) has been adopted. This work focuses on the assessment of the conservative DTRM behaviour in a CFD simulation of a furnace with moderate complex geometry. The heat fluxes on the domain boundaries and the overall heat balance were of particular interest in this respect. An unstructured computational mesh, consisting of 836951 control volumes, has been used. A simple sensitivity analysis for two different DTRM set-ups- with 4 (1x4) and 8 (2x4) rays per boundary face, respectively - has been performed. More rays, due to a limited computational power, could not be used. The simulation results were compared with available experimental data, showing acceptable level of accuracy, with DTRM able to catch direction dependent shadowing effects on the net-flux rates at the walls, caused by cooling-tubes positioned in front of them. A comparison of the original DTRM formulation vs. the conservative DTRM formulation has been performed as well, showing the superiority of the conservative formulation over the original one.

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