Assaad R. Masri

The structure of turbulent nonpremixed flames revealed by Raman-Rayleigh-LIF measurements

Abstract This paper reviews recent advances in our understanding of the structure of turbulent nonpremixed flames due to extensive data acquired from single-point and planar imaging experiments using the Raman, Rayleigh, and LIF diagnostic methods. These techniques, used either separately or jointly, have become standard tools in combustion research. Flames with simple streaming flows as well as complex flows with recirculating zones are discussed for a variety of fuel mixtures and a range of turbulent mixing rates. The chemistry—turbulence interaction and other related issues like local flame extinction and the bimodality of the approach toward blowoff are discussed. Additional single-point data are also presented illustrating the effects of partially premixing the fuel with air, diluting it with nitrogen or adding methane to a mixture of nonhydrocarbon fuels. The bimodality of the conditional pdfs of various reactive scalars as the flames approach blowoff, and the start of occurrence of localized extinction, are correlated with two simple parameters: (a) the stoichiometric mixture fraction, ξ s , and (b) the reaction zone width, Δξ R . The latter parameter may be easily determined from standard laminar flame calculations for a given fuel mixture.

Instantaneous and Mean Compositional Structure of Bluff-Body Stabilized Nonpremixed Flames

Abstract Turbulent nonpremixed flames stabilized on an axisymmetric bluff-body burner are studied with fuels ranging from simple H 2 /CO to complex H 2 /CH 4 and gaseous methanol. The fuel-jet velocity is varied to investigate the Damkohler number effects on gas emissions, localized extinction (LE) in the neck zone, and the structure of the recirculation zone dependency on the flow field. Simultaneous, single-point measurements of temperature, major species, OH, and NO are made using the Raman/Rayleigh/Laser induced fluorescence (LIF) technique. The data are collected at different axial and radial locations along the full length of most flames and are presented in the form of ensemble means, root-mean-square (rms) fluctuations, scatter plots, and probability density functions (PDF). It is found that up to three mixing layers may exist in the recirculation zone, one on the air side of the outer vortex, one between the inner and the outer vortices, and one between the fuel jet and the inner vortex. With increasing jet momentum flux, the average mixture in the outer vortex loses its strength and the stoichiometric contour shifts closer to the fuel jet. The decay rate of the mixture fraction on the centerline exhibits similar trends to the ordinary jet flame downstream of the recirculation zone whereas different trends are found inside the recirculation zone. The laminar flame computations with constant mass diffusivities and Lewis number (Le) = 1 are found to be better guides for the measured temperature and stable species mass fraction in the turbulent flames. The measured peak mass fractions of CO and H 2 are similar to those reported earlier for pilot-stabilized flames of similar fuels. Compared with laminar flame compositions with equal diffusivities and Le = 1.0, measured CO may be in superflamelet concentration. Hydroxyl radical and H 2 are found not to be in superflamelet levels contrary to earlier findings in piloted flames. The start of LE and the bimodality of the conditional PDF are consistent with those reported earlier for piloted flames of similar fuels.

Flow and mixing fields of turbulent bluff-body jets and flames

The mean structure of turbulent bluff-body jets and flames is presented. Measurements of the flow and mixing fields are compared with predictions made using standard turbulence models. It is found that two vortices exist in the recirculation zone; an outer vortex close to the air coflow and an inner vortex between the outer vortex and the jet. The inner vortex is found to shift downstream with increasing jet momentum flux relative to the coflow momentum flux and gradually loses its circulation pattern. The momentum flux ratio of the jet to the coflow in isothermal flows is found to be the only scaling parameter for the flow field structure. Three mixing layers are identified in the recirculation zone. Numerical simulations using the standard k-ϵ and Reynolds stress turbulence models underpredict the length of the recirculation zone. A simple modification to the C1 constant in the dissipation transport equation fixes this deficiency and gives better predictions of the flow and mixing fields. The mixed-is-b...

The spontaneous raman scattering technique applied to nonpremixed flames of methane

Abstract Simultaneous space- and time-resolved measurements of the concentrations of CH 4 , O 2 , N 2 , H 2 O, H 2 , CO, and CO 2 have been made using spontaneous Raman scattering, in the blue regions of CH 4 turbulent nonpremixed flames. The temperature is measured from the Rayleigh scattered signal. A “fluorescence” interference, which is broadband and contaminates in varying degrees the Rayleigh and all the Raman lines, is believed to be due to a number of molecules or flame radicals, including C 2 and CN or even incandescence of small particle nuclei. The “fluorescence” has been monitored at a bandhead of C 2 (516.5 nm) and its effect reduced by placing a Polaroid filter at the entrance slit of the spectrometer. The remaining “fluorescence” has been corrected for, using correction curves generated from measurements made in a laminar counterflow CH 4 diffusion flame and a diluted CH 4 N 2 = 1 2 (by vol.) laminar diffusion flame. Measurements of CO and CO 2 are not reliable in the rich regions of the flame where the “fluorescence” is intense. With minor modifications to the optical system, CO and CO 2 could also be measured with acceptable accuracy in regions of intense “fluorescence” and the “fluorescence” correction further refined. This work is considered to be an important extension of the applications of spontaneous Raman scattering as a measurement technique in flames.

Pdf calculations of turbulent lifted flames of H2/N2 fuel issuing into a vitiated co-flow

This paper presents detailed calculations of the flow, mixing and composition fields of a simple jet of hydrogen–nitrogen mixture issuing into a vitiated co-flowing stream. The co-flow contains oxygen as well as combustion products and is sufficiently hot to provide an ignition source for a flame that stabilizes at some ten diameters downstream of the jet exit plane. This configuration forms a good model problem for studying lifted flames as well as issues of auto-ignition. The calculations employ a composition probability density function (PDF) approach coupled to the commercial CFD package FLUENT. The in situ adaptive tabulation method is adopted to account for detailed chemical kinetics. A simple k–ε model is used for turbulence along with a low Reynolds number model for the walls. Calculations are optimized to obtain a numerically accurate solution and are repeated for two different H2 mechanisms, each consisting of ten species. The flame is found to be largely controlled by chemical rather than mixing processes. The mechanisms used yield different lift-off heights and compositions that straddle the data. Ignition delays are found to be extremely sensitive to the chemical kinetic rates of some reactions in the mechanisms.

A numerical study of auto-ignition in turbulent lifted flames issuing into a vitiated co-flow

This paper presents a numerical study of auto-ignition in simple jets of a hydrogen–nitrogen mixture issuing into a vitiated co-flowing stream. The stabilization region of these flames is complex and, depending on the flow conditions, may undergo a transition from auto-ignition to premixed flame propagation. The objective of this paper is to develop numerical indicators for identifying such behavior, first in well-known simple test cases and then in the lifted turbulent flames. The calculations employ a composition probability density function (PDF) approach coupled to the commercial CFD code, FLUENT. The in-situ-adaptive tabulation (ISAT) method is used to implement detailed chemical kinetics. A simple k–ϵ turbulence model is used for turbulence along with a low Reynolds number model close to the solid walls of the fuel pipe. The first indicator is based on an analysis of the species transport with respect to the budget of convection, diffusion and chemical reaction terms. This is a powerful tool for investigating aspects of turbulent combustion that would otherwise be prohibitive or impossible to examine experimentally. Reaction balanced by convection with minimal axial diffusion is taken as an indicator of auto-ignition while a diffusive–reactive balance, preceded by a convective–diffusive balanced pre-heat zone, is representative of a premixed flame. The second indicator is the relative location of the onset of creation of certain radical species such as HO2 ahead of the flame zone. The buildup of HO2 prior to the creation of H, O and OH is taken as another indicator of autoignition. The paper first confirms the relevance of these indicators with respect to two simple test cases representing clear auto-ignition and premixed flame propagation. Three turbulent lifted flames are then investigated and the presence of auto-ignition is identified. These numerical tools are essential in providing valuable insights into the stabilization behaviour of these flames, and the demarcation between processes of auto-ignition and premixed flame propagation.

Turbulent nonpremixed flames of methane near extinction: Probability density functions☆

Abstract Space- and time-resolved measurements of major species concentrations and temperature have been made using the Raman-Rayleigh scattering technique in the blue (visibly soot free) regions of turbulent nonpremixed flames of methane close to extinction. The data are presented in this paper in the form of single variate probability density functions (pdfs) for the mixture fraction, temperature, and the mass fractions of CH4, O2, H2O, H2, CO2, and CO. Representative instantaneous scatter plots and joint pdfs are also shown. When the mixing rates are low, the data show mostly fully burnt mixtures indicating that the chemistry is relatively fast. As the flame approaches extinction, most local mixtures become either partially burnt or simply mixed. The joint pdfs shown bimodality for mixture fractions less than ∼.1 and centered distributions for richer mixtures. When close to extinction, fully burnt pockets of fluid are still encountered and these may be responsible for keeping the flame alight. Questions relating to the local, instantaneous flame structure near extinction are discussed in light of existing theoretical models.

The effects of obstructions on overpressure resulting from premixed flame deflagration

Abstract This paper introduces a new experimental set-up for investigating the effects of obstruction geometry, blockage ratio and venting pressure on overpressures resulting from premixed flame deflagration. Obstructions shaped as cylinders, triangles, squares, diamonds and plates or walls are studied here covering blockage ratios ranging from about 10% to more than 75%. It is found that the deflagration overpressure increases with increasing venting pressure. Also, the maximum overpressure increases, generally with increasing blockage ratio but the rate of increase depends on the obstruction geometry. The wall/plate type obstruction leads to the highest overpressures and the cylindrical obstruction yields the lowest overpressure. The time taken to reach the maximum overpressure decreases with increasing blockage ratio and changes with obstruction geometry implying that the flame accelerates faster due to changed local turbulence levels and length scales.

Swirling turbulent non-premixed flames of methane: Flow field and compositional structure

This paper introduces a new swirl burner which has simple, well-defined boundary conditions and which stabilizes complex, turbulent, unconfined flames that are not unlike those found in practical combustors. Two flames with identical swirl numbers but differing bulk jet velocities Uj are selected for further inves- tigations. Flow field measurements reveal that a second recirculation zone may exist on the centerline of the flames further downstream of the primary recirculation zone. This is attributed to vortex breakdown. The measurements also show the presence of highly rotating collar-like flow features present between the primary and secondary recirculation zones. These regions of the flow are characterized by high tangential shear stresses uw� . The compositional structures of these methane flames are measured using the simultaneous Raman- Rayleigh laser-induced fluorescence (LIF) technique. The LIF technique is used to measure concentra- tions of OH, CO, and NO. Results are presented as scatter plots and radial Favre mean profiles of tem- perature, mixing, and composition fields. As the fuel jet velocity increases and the flame approaches blowoff, a higher proportion of fluid samples shifts away from fully burned conditions and closer to a mixing asymptote. An interesting feature of these flames is that these locally extinguished samples originate mostly from regions of the flow near the high-shear, collar-like region, which is not found in similar bluff- body flames.

Recirculation and Flowfield Regimes of Unconfined Non-Reacting Swirling Flows

Abstract This paper focuses on unconfined swirling flows of air surrounding a bluff-body having a central jet of air. Only one co-flowing primary annular air stream is swirled. The flow conditions investigated here cover a range of swirl numbers and streamwise annular velocities. Emphasis is placed on discerning typical flow structures and on discovering the influence of controlling parameters on the downstream regions of the jet and the recirculation within. The paper is part of a larger program aimed at providing an improved understanding of swirling flows. Laser doppler velocimetry is used to map three components of velocity 〈 u 〉, 〈 v 〉 and 〈 w 〉. In the flows investigated, an upstream recirculation zone is established above the burner’s exit plane. This zone is typical of that found behind bluff-bodies placed in strong axial streams. Additionally, it is shown that the formation of a downstream recirculation zone, hence the onset of vortex breakdown, depends not only on the swirl number but on other flow parameters such as the axial velocity of the primary swirling air, or its Reynolds number. Together, these two parameters appear to control the radial spread of the flow. Spatially, the progression towards downstream recirculation with changes in flow parameters is seen to occur around a fixed location in the streamwise direction. Beyond this axial position, the flowfield seems to be less affected by increases in swirl number. Non-recirculating flow structures are resolved in these flows and are found to cause elevated 〈 u ′ w ′ 〉 shear stresses.