Structure Of Turbulent Nonpremixed Flames Research Articles

Thermal and chemical effects of differential diffusion in turbulent non-premixed H2 flames

As a renewable fuel, hydrogen (H2) may play an increasingly important role in the development and control of piston and gas turbine engines to achieve zero carbon emissions. Predictive modeling of H2-fueled combustion processes requires a clear understanding of differential diffusion (DD) due to the high diffusivity of H2. On the assumption that turbulent mixing is a far more dominant process than molecular mixing, DD effects are typically neglected in turbulent combustion simulations to reduce modeling complications. While this assumption is reasonable for hydrocarbon fuels, it is less valid for H2 combustion, where DD is significant. In this work, two three-dimensional direct numerical simulations of temporally evolving turbulent H2 jet flames with and without considering DD are performed and compared with laminar flamelet solutions to assess DD effects under turbulent conditions. The emphasis is placed on assessing the suitability of classical mixture fraction Z and Bilger mixture fraction ZBilger as conditioning variables for non-premixed turbulent combustion modeling through analyzing DD effects on flame structure, chemical reactions, and tangential diffusion (TD). Furthermore, the persistence of DD effects under turbulent conditions and the suitability of a conventional DD parameter are investigated by comparing the turbulent flames to laminar flamelet solutions. It is found that conditioning the thermochemical state on ZBilger helps to capture DD effects and mitigate the relative contribution of TD, which gives ZBilger advantages over Z when employing flamelet modeling. Due to close coupling between DD and local chemical reactions, DD can affect the turbulent/laminar flames in the form of thermal effects due to the change in flame temperature, chemical effects due to the change in chemical reactions, and transport effects due to multiple species with varying diffusivities that could result in the difference between Z and ZBilger. While the transport effects are suppressed, significant chemical and thermal effects of DD still persist under turbulent conditions, which indicates that the DD parameter is probably unsuitable for comprehensively characterizing and assessing DD effects on the structure of turbulent non-premixed flames.

Structure of turbulent nonpremixed syngas flames at high pressure

The effects of both pressure and Reynolds number on the structure of turbulent nonpremixed flames are investigated experimentally. Syngas (CO/H2/N2) flames with constant composition are examined at pressures up to 12 bar and at Reynolds numbers up to 66,800. The baseline atmospheric pressure flame is identical to the “chnA” flame of the turbulent nonpremixed flames (TNF) workshop. Low-speed OH-PLIF is used to reveal the flame structure in a small region 7 diameters downstream of the nozzle. The thickness of the OH layer decreases monotonically with pressure. Increasing pressure inhibits local extinction when comparing conditions at constant Reynolds number; an effect of changing exit strain rate. Neither changes in Reynolds number nor changes in pressure affect the mean flame front location. Corrugation of the flame front is highly sensitive to changes in Reynolds number but relatively insensitive to changes in pressure. Blow-off velocity limits the highest Reynolds number achievable by the TNF jet flames. By increasing the pressure, this ceases to be the limiting parameter, meaning that we can study the effect of Reynolds number on the structure of the Sandia-ETH syngas jet flame over a wider range of turbulence, approaching what is encountered in practical combustors.

Radiation-driven flame weakening effects in sooting turbulent diffusion flames

The objective of the present study is to use detailed numerical modeling to bring basic information on the structure of turbulent non-premixed flames under sooting and radiating conditions. The questions of flame weakening, flame extinction, and soot leakage are studied using direct numerical simulation. Simulations of ethylene–air combustion are performed in both a reference laminar counter-flow flame configuration and a momentum-driven turbulent wall-flame configuration. In the turbulent wall-flame configuration, the flame optical thickness is artificially increased in order to magnify the role played by luminous thermal radiation. The resulting optically thickened flames feature frequent and pronounced low-temperature, low-burning-intensity spots that may be interpreted as quasi-flame extinction events. The data analysis reveals that these flame weakening events are similar to radiation extinction phenomena previously observed in microgravity laminar flames, and are associated with low values of the fuel–air mixing rate and large values of the flame radiant fraction. These events also differ from previously made observations to the extent that they are associated with soot mass leakage across the flame and occur under a broader range of flame stretch conditions, thereby suggesting that turbulent flames are more susceptible to radiation extinction than their laminar counterparts.

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

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.

A Visual Study of the Structure of Turbulent Nonpremixed Flames Near Jet Exit

Abstract An experimental study is made of the structure of turbulent nonpremixed flames in the vicinity of the exit of hydrocarbon jets. The jets are issued upward from a circular tube which is located inside a coflowing low-velocity air stream. Schlieren—direct and TiOz scattering-direct synchronized photography is used to visualize the flame zone and various boundaries. A stable flame zone is seen to exist at the exterior of the boundary of fuel jet. The location of the flame zone is found to be influenced by the diffusion velocity of fuel in addition to the stoichiometric volume ratio and entrainmenl of reactants. The presence of a stable laminar flame region near the burner tip is experimentally confirmed. However, in the core region near the tip, the flow is turbulent from the entrance and eddies grow to large sizes in the downstream direction. A schematic characterization is proposed which illustrates the basic structure of the turbulent nonpremixed flame in the near field.

The structure of turbulent nonpremixed flames of methanol over a range of mixing rates

The blowoff and electrical connectedness characteristics of pilot-stabilized turbulent nonpremixed flames of methanol fuel are presented. Using the joint Raman-Rayleigh-LIF technique, simultaneous space- and time-resolved measurements of temperature and the mass fractions of CH 3OH, CO, CO 2, H 2, H 2O, O 2, and N 2 are also shown for a number of measurement locations. Flames with slow mixing rates as well as ones close to blowoff are investigated. The results are presented in the form of scatter plots as well as probability density functions conditioned with respect to mixture fraction for a number of mixture fraction ranges selected within the reactive zone for methanol. The local extinction characteristics in the flames are also studied by adopting a temperature threshold below which fluid samples are assumed to be extinguished. It is found that the flame becomes electrically disconnected completely while it remains visibly and aurally stable for a range of velocities just below blowoff. Measurements in regions of the flames where the mixing rates are relatively low show little difference between the turbulent flame data and the laminar flame compositions only on the lean side of stoichiometric. For rich mixtures, however, the peak-measured mass fractions of CO and H 2 are about 70% higher than those for laminar flamelets while the mass fractions of CO 2 and H 2O are lower than those of highly stretched laminar flamelets. Localized extinction remains uniformly low until the flame jet velocity is about 80% of the blowoff velocity. With higher flame jet velocities, localized extinction increases sharply until global blowoff is reached. The flame's approach to global blowoff is mainly bimodal for lean and stoichiometric mixtures but less bimodal for rich ones. This bimodality, however, is less distinct than in methane flames.

The local structure of turbulent nonpremixed flames near extinction

Using reported data collected in turbulent jet nonpremixed flames near extinction, various parameters are derived to characterize the structure of the flames as they approach blowoff. The fuels considered are CH 4, H 2, H 2 Ar , and CO/H 2/N 2. The data used consist of velocity and turbulence measurements using the LDA technique and measurements of temperature, species mass fractions, and mixture fraction using the spontaneous Raman-Rayleigh scattering technique. Flames of H 2 Ar and CO/H 2/N 2 fuels exhibit a monomodal behavior in the pdfs of their reactive scalars as they approach extinction while those of CH 4 flames are bimodal. The parameters controlling this behavior are the width of the reaction zone, Δξ R and the scalar scale of turbulence Σ ξ . It is found that this behavior is only slightly affected by Σ ξ and is mainly dependent on Δξ R . Further investigations of other fuels with a range of Δξ R are recommended.

Turbulent non-premixed flames of hydrocarbon fuels near extinction: mean structure from probe measurements

The stability characteristics, electrical conductivity and mean property structure of turbulent non-premixed flames of natural gas and liquified petroleum gas stabilized on a burner by a pilot flame have been studied. The parameters controlling the flame are the mean jet velocity ū j , the mean external air velocity ū e and the pilot burnt gas velocity ū pb . If the jet velocity is increased high enough, the flame blows off apparently from mixing rates becoming excessive in a region extending from x/D j =15 to 35. Measurements of mean velocity, temperature and mass fractions of CO, CO 2 , O 2 , and fuel as well as turbulence intensities are presented for flames over a range of jet velocities from electrically connected and relatively low mixing rate flames to flames that are electrically intermittent and close to extinction. The structure of the flame is interpreted in terms of mean composition and temperature as a function of the local mean of the mixture fraction. Product species and temperature are found to decrease with increase in the jet velocity (and hence mixing rate) while reactant species increase. It is apparent that these effects are due to the finite rate of the chemical kinetics, although it is not possible to prove that it is not due to an increase in unmixedness (or variance of the mixture fraction) from this probe data. Such proof is availble from Raman measurements to be present elsewhere.

Simultaneous measurements of velocity and density in a turbulent nonpremixed flame

Simultaneous velocity and density measurements have been obtained throughout a turbulent nonpremixed flame under high Reynolds number conditions. The objective of the measurements was to obtain a better understanding of the structure of turbulent nonpremixed flames and the interaction between turbulence and combustion chemistry. The measurements were made with an apparatus which utilized both laser velocimetry and laser Rayleigh scattering. From these measurements it was shown previously that while the gradient diffusion submodel for the velocity-density correlations agreed with the radial correlation data, it was seldom consistent with the axial data. In the present paper the analysis of these data is extended to include information on flame intermittency, conditionally sampled results, and joint probability distributions of velocity and density. Comparison made with results in nonreacting and analogous nonpremixed reacting jets show that both mean and fluctuating quantities and conditionally sampled results are dependent on flow conditions and the presence of combustion. 28 references.