Measurement of the instantaneous flame front structure of syngas turbulent premixed flames at high pressure

Instantaneous flame front structure of syngas turbulent premixed flames including the local radius of curvature, the characteristic radius of curvature, the fractal inner cutoff scale and the local flame angle were derived from the experimental OH-PLIF images. The CO/H₂/CO₂/air flames as a model of syngas/air combustion were investigated at pressure of 0.5 MPa and compared to that of CH₄/air flames. The convex and concave structures of the flame front were detected and statistical analysis including the PDF and ADF of the local radius of curvature and local flame angle were conducted. Results show that the flame front of turbulent premixed flames at high pressure is a wrinkled flame front with small scale convex and concave structures superimposed with large scale flame branches. The convex structures are much more frequent than the concave ones on flame front which reflects a general characteristic of the turbulent premixed flames at high pressure. The syngas flames possess much wrinkled flame front with much smaller fine cusps structure compared to that of CH₄/air flames and the main difference is on the convex structure. The effect of turbulence on the general wrinkled scale of flame front is much weaker than that of the smallest wrinkled scale. The general wrinkled scale is mainly dominated by the turbulence vortex scale, while, the smallest wrinkled scale is strongly affected by the flame intrinsic instability. The effect of flame intrinsic instability on flame front of turbulent premixed flame is mainly on the formation of a large number of convex structure propagating to the unburned reactants and enlarge the effective contact surface between flame front and unburned reactants.     

An a priori model for the effective species Lewis numbers in premixed turbulent flames

An a priori model for the effective species Lewis numbers in premixed turbulent flames is presented. This a priori   analysis is performed using data from a series of direct numerical simulations (DNS) of lean (?=0.4$?=0.4$) premixed turbulent hydrogen flames, with Karlovitz number ranging from 10 to 1562 (Aspden et al., 2011). The conditional mean profiles of various species mass fraction versus temperature are evaluated from the DNS and compared to unstretched laminar premixed flame profiles. The turbulent flame structure is found to be different from the laminar flame structure. However, the turbulent flame can still be mapped onto a laminar flame with an appropriate change in the Lewis numbers of the different species. A transition from “laminar” Lewis numbers to unity Lewis numbers as the Karlovitz number increases is clearly captured. A model for those effective Lewis numbers with respect to the turbulent Reynolds number is developed. This model is derived from a Reynolds-averaged Navier–Stokes (RANS) formulation of the reactive scalar and temperature balance equations. The dependency of the effective Lewis numbers on the Karlovitz number instead of the Reynolds number is discussed in this paper. Unfortunately, given that the ratio of the integral length to the laminar flame thickness is fixed throughout this series of DNS, a change in the Karlovitz number is equivalent to a change in the Reynolds number. Incorporating these effective Lewis numbers in simulations of turbulent flames would have several impacts. First, the associated laminar flame speed and laminar flame thickness vary by a factor of two through the range of obtained effective Lewis numbers. Second, the turbulent premixed combustion regime diagram changes because a unique pair of laminar flame speed and laminar flame thickness cannot be used, and a dependency on the effective Lewis numbers has to be introduced. Finally, a turbulent flame speed model that takes into account these effective Lewis numbers is proposed.     

Burning rates and surface characteristics of hydrogen-enriched turbulent lean premixed methane–air flames

The burning rates and surface characteristics of hydrogen-enriched turbulent lean premixed methane–air flames were experimentally studied by laser tomography visualization method using a V-shaped flame configuration. Turbulent burning velocity was measured and the variation of flame surface characteristics due to hydrogen addition was analyzed. The results show that hydrogen addition causes an increase in turbulent burning velocity for lean premixed CH₄–air mixtures when turbulent level in unburned mixture is not changed. Moreover, the increase of turbulent burning velocity is faster than that of the corresponding laminar burning velocity at constant equivalence ratio, suggesting that the kinetics effect is not the sole factor that results in the increase in turbulent burning velocity when hydrogen is added. The further analysis of flame surface characteristics and brush thickness indicates that hydrogen addition slightly decreases local flame surface density, but increases total flame surface area because of the increased flame brush thickness. The increase in flame brush thickness that results in the increase in total surface area may contribute to the faster increase in turbulent burning velocity, when hydrogen is added. Besides, the stretched local laminar burning velocity may be enhanced with the addition of hydrogen, which may also contribute to the faster increase rate of turbulent burning velocity. Both the variation in flame brush thickness and the enhancement in stretched local laminar burning velocity are due to the decreased fuel Lewis number when hydrogen is added. Therefore, the effects of fuel Lewis number and stretch should be taken into account in correlating burning velocity of turbulent premixed flames.     

Analysis of flame surface density measurements in turbulent premixed combustion

In premixed turbulent combustion, reaction rates can be estimated from the flame surface density. This parameter, which measures the mean flame surface area available per unit volume, may be obtained from algebraic expressions or by solving a transport equation. In this study, detailed measurements were performed on a Bunsen-type burner fed with methane/air mixtures in order to determine the local flame surface density experimentally. This burner, located in a high-pressure combustion chamber, allows investigation of turbulent premixed flames under various flow, mixture, and pressure conditions. In the present work, equivalence ratio was varied from 0.6 to 0.8 and pressure from 0.1 to 0.9 MPa. Flame front visualizations by Mie scattering laser tomography are used to obtain experimental data on the instantaneous flame front dynamics. The exact equation given by Pope is used to obtain flame surface density maps for different flame conditions. Some assumptions are made in order to access three-dimensional information from our two-dimensional experiments. Two different methodologies are proposed and tested in term of global mass balance (what enters compared to what is burned). The detailed experimental flame surface data provided for the first time in this work should progressively allow improvement of turbulent premixed flame modeling approaches.     

Large eddy simulation of a lifted turbulent jet flame

The flame index concept for large eddy simulation developed by Domingo et al. [P. Domingo, L. Vervisch, K. Bray, Combust. Theory Modell. 6 (2002) 529–551] is used to capture the partially premixed structure at the leading point and the dual combustion regimes further downstream on a turbulent lifted flame, which is composed of premixed and nonpremixed flame elements each separately described under a flamelet assumption. Predictions for the lifted methane/air jet flame experimentally tested by Mansour [M.S. Mansour, Combust. Flame 133 (2003) 263–274] are made. The simulation covers a wide domain from the jet exit to the far flow field. Good agreement with the data for the lift-off height and the mean mixture fraction has been achieved. The model has also captured the double flames, showing a configuration similar to that of the experiment which involves a rich premixed branch at the jet center and a diffusion branch in the outer region which meet at the so-called triple point at the flame base. This basic structure is contorted by eddies coming from the jet exit but remains stable at the lift-off height. No lean premixed branches are observed in the simulation or and experiment. Further analysis on the stabilization mechanism was conducted. A distinction between the leading point (the most upstream point of the flame) and the stabilization point was made. The later was identified as the position with the maximum premixed heat release. This is in line with the stabilization mechanism proposed by Upatnieks et al. [A. Upatnieks, J. Driscoll, C. Rasmussen, S. Ceccio, Combust. Flame 138 (2004) 259–272].     

Measurements and analysis of turbulent consumption speeds of H2/CO mixtures

This paper describes measurements of global turbulent consumption speeds, S_T,GC, of hydrogen/carbon monoxide (H₂/CO) mixtures. The turbulent flame properties of such mixtures are of fundamental interest because of their strong stretch sensitivity, and of practical interest since they are the primary constituents of syngas fuels. Data are reported at mean flow velocities and turbulence intensities of 4 < U₀ < 50 m/s and , respectively, for H₂/CO blends ranging from 30% to 90% H₂ by volume. Two sets of experiments are reported. In the first, fuel blends ranging from 30% to 90% H₂ and mixture equivalence ratio, ?, were adjusted at each fuel composition to have nominally the same un-stretched laminar flame speed, S_L,0. In the second set, equivalence ratios were varied at constant H₂ levels. The data clearly corroborate results from other studies that show significant sensitivity of S_T,GC to fuel composition. In particular, at a fixed and S_L,0, values of S_T,GC increase by a factor of almost 2 when H₂ levels are increased from 30% (at ? = 0.61) to 90% (at ? = 0.48). Moreover, S_T,GC in the 90% H₂ case is three times larger than the ? = 0.9 CH₄/air mixture with the same S_L,0 value. An important conclusion from this work is these fuel effects are not simply a low turbulence intensity phenomenon – they clearly persist over the entire range of turbulence intensities used in the measurements. We also describe physics-based correlations of these data, using leading points concepts and detailed kinetic calculations of the stretch sensitivity of these mixtures. These results are used to develop an inequality for negative Markstein length flames that bounds the turbulent flame speed data and show that the data can be collapsed using the maximum stretched laminar flame speed, S_L,max, rather than S_L,0.     

High pressure hydrogen fires

Within the scope of the French national project DRIVE and European project HyPER, high pressure jet flames of hydrogen were produced and instrumented.The experimental technique and measurement strategy are presented. Many aspects are original developments like the direct measurement of the mass flow rate by weighing continuously the hydrogen container, the image processing to extract the flame geometry, the heat flux measurement device, the thermocouples arrangement…Flames were observed from 900 bar down to 1 bar with orifices ranging from 1 to 3 mm. An original set of data is now available about the main flame characteristics and about some thermodynamic aspects of hydrogen releases under high pressure.A brief comparison of some available models is presented.     

An analytic model for the effects of nitrogen dilution and premixing characteristics on NOx formation in turbulent premixed hydrogen flames

Advanced hydrogen gas turbine is a promising technology to achieve near-zero emission of carbon dioxide and higher cycle efficiency. With the increased firing temperature and pressure ratio, nitrogen reinjection combined with dry premixed combustion is promising to achieve the challenging low NOx emission. In this study, the effects of nitrogen dilution and fuel/air premixing characteristics on the flame characteristics and NOx emission are investigated first through simulating one-dimensional premixed flames with a 13-species and 39-reaction mechanism at the elevated engine operation conditions. The variation of flame thicknesses and laminar flame speeds with nitrogen dilution is investigated. The NOx formation is characterized by the flame-front NOx and the constant NOx formation rates in the post-flame region. It is shown that the flame-front NOx is an order of 1 ppm and does not change significantly (within 20%) with nitrogen dilution. In contrast, the NOx formation rates in the post-flame region decrease monotonically with nitrogen dilution due to the decrease of oxygen concentration. A detailed analysis of NOx formation reveals that the N₂O pathway is significant and it can account for at least 20% of the NOx formation in the post-flame region. Then an analytic model considering both the extended Zeldovich mechanism and the N₂O pathway is constructed by assuming the involved radicals being in chemical equilibrium. The model can be employed to efficiently estimate the NOx formation in fully premixed hydrogen gas turbines. Next, the effects of fuel/air premixing characteristics on the mean NOx formation rate in the post-flame region are quantified by reconstructing the PDF of mixture fraction. It is shown that without the nitrogen dilution, the NOx formation rate increases dramatically with fuel/air unmixedness due to the existence of local hot spots. Nitrogen dilution can dramatically reduce the NOx formation rate at the same level of unmixedness through reducing the local hot spots. Moreover, nitrogen dilution reduces the sensitivity of the NOx formation rate to fuel/air unmixedness, which greatly alleviates the mixing requirement for the premixing nozzles in gas turbines. Finally, a model for the estimation of NOx emission is constructed, which builds the connection between NOx emission, nitrogen dilution, unmixedness and flow residence time in combustors.     

Radiation intensity imaging measurements of methane and dimethyl ether turbulent nonpremixed and partially premixed jet flames

Quantitative time-dependent images of the infrared radiation intensity from methane and dimethyl ether (DME) turbulent nonpremixed and partially premixed jet flames are measured and discussed in this work. The fuel compositions (CH₄/H₂/N₂, C₂H₆O/H₂/N₂, CH₄/air, and C₂H₆O/air) and Reynolds numbers (15,200–46,250) for the flames were selected following the guidelines of the International Workshop on Measurement and Computation of Turbulent Nonpremixed Flames (TNF Workshop). The images of the radiation intensity are acquired using a calibrated high speed infrared camera and three band-pass filters. The band-pass filters enable measurements of radiation from water vapor and carbon dioxide over the entire flame length and beyond. The images reveal localized regions of high and low intensity characteristic of turbulent flames. The peak mean radiation intensity is approximately 15% larger for the DME nonpremixed flames and 30% larger for the DME partially premixed flames in comparison to the corresponding methane flames. The trends are explained by a combination of higher temperatures and longer stoichiometric flame lengths for the DME flames. The longer flame lengths are attributed to the higher density of the DME fuel mixtures based on existing flame length scaling relationships. The longer flame lengths result in larger volumes of high temperature gas and correspondingly higher path-integrated radiation intensities near and downstream of the stoichiometric flame length. The radiation intensity measurements acquired with the infrared camera agree with existing spectroscopy measurements demonstrating the quantitative nature of the present imaging technique. The images provide new benchmark data of turbulent nonpremixed and partially premixed jet flames. The images can be compared with results of large eddy simulations rendered in the form of quantitative images of the infrared radiation intensity. Such comparisons are expected to support the evaluation of models used in turbulent combustion and radiation simulations.     

Investigation of dilution effects on partially premixed swirling syngas flames using a LES-LEM approach

The dilution effects of CO₂ and H₂O on partially premixed swirling syngas flames are investigated with the large eddy simulation (LES) method. The linear-eddy model (LEM) is employed to directly resolve the unclosed molecular diffusion, scalar mixing and chemical reaction processes occurring at subgrid scale level using their specific length and time scales instead of modelling, which makes the LES-LEM approach quite attractive for hydrogen fuel combustion as the obviously different diffusion and reaction characteristics of H₂ and H compared to other species in the syngas mixture. Firstly, adding CO₂ into the fuel stream can significantly decrease the flame temperature during the partially premixed combustion. The concentration of H and OH radicals decreases upon CO₂ dilution and thus the chemical reaction processes are modified. Compared with CO₂, H₂O is less effective in changing the temperature field because of the chemical effects of H₂O. The simultaneous addition of H₂O and CO₂ as dilution gases with volume ratio 1:1 into the fuel stream is also conducted to identify the effects of H₂O and CO₂ on partially premixed combustion dynamics by comparing with single H₂O and CO₂ cases. The obtained results are expected to provide helpful information for the design and operation of gas turbine combustion systems with syngas fuels.