Effect of hydrogen on H2/CH4 flame structure of MILD combustion using the LES method

Large eddy simulation (LES) method is employed to investigate the effect of the hydrogen content of fuel on the H₂/CH₄ flame structure under the moderate or intense low-oxygen dilution (MILD) condition. The turbulence–chemistry interaction of the numerically unresolved scales is modelled using the PaSR method, where the full mechanism of GRI-2.11 represents the chemical reactions. The influence of hydrogen concentration on the flame structure is studied using the profiles of temperature, CH₂O and OH mass fractions and the diffusion profiles of un-burnt fuel through the flame front. Furthermore, more details are investigated by contours of OH, HCO and CH₂O radicals in an area near the nozzle exit zone. Results show that increasing the hydrogen content of fuel reinforces the MILD combustion zone and increases the peak value of the flame temperature and OH mass fraction. This increment also increases the flame thickness and reduces the OH oscillations and diffusion of the un-burnt fuel through the flame front.     

Flame-vortex interaction and mixing behaviors of turbulent non-premixed jet flames under acoustic forcing

This study examines the effect of acoustic excitation using forced coaxial air on the flame characteristics of turbulent hydrogen non-premixed flames. A resonance frequency was selected to acoustically excite the coaxial air jet due to its ability to effectively amplify the acoustic amplitude and reduce flame length and NO_x emissions. Acoustic excitation causes the flame length to decrease by 15% and consequently, a 25% reduction in EINO_x is achieved, compared to coaxial air flames without acoustic excitation at the same coaxial air to fuel velocity ratio. Moreover, acoustic excitation induces periodical fluctuation of the coaxial air velocity, thus resulting in slight fluctuation of the fuel velocity. From phase-lock PIV and OH PLIF measurement, the local flow properties at the flame surface were investigated under acoustic forcing. During flame-vortex interaction in the near field region, the entrainment velocity and the flame surface area increased locally near the vortex. This increase in flame surface area and entrainment velocity is believed to be a crucial factor in reducing flame length and NO_x emission in coaxial jet flames with acoustic excitation. Local flame extinction occurred frequently when subjected to an excessive strain rate, indicating that intense mass transfer of fuel and air occurs radially inward at the flame surface.     

Experimental investigation of laminar LPG–H2 jet diffusion flame

This paper presents an experimental investigation of the effect of hydrogen addition on flame length, soot free length fraction (SFLF), and emission level in LPG–H₂ hybrid fuel jet diffusion flame. Result shows that the addition of H₂ upto 20% does not exhibit any change in the flame length. However, when hydrogen level is varied from 20% to 40%, a reduction in flame length is observed which may be attributed to the enhanced diffusivity caused due to the increased gas temperature. It is also observed that the SFLF increases as H₂ is added to fuel stream. This might have caused due to the reduction in the C/H ratio in the flame which is favorable to attenuate PAH formation rate. Moreover, the reduction in radiant fraction with hydrogen addition is pertinent with the reduction in soot concentration level. Besides this, the decrease in _NOx${\mathrm{NO}}_x$ emission level with H₂ addition to the fuel stream is also observed. This can be ascribed to the enhanced flame temperature and diffusivity with hydrogen addition which results in an increase in gas velocity from burner rim to flame tip and hence leads to a decrease in residence time of the gases. The emission level of CO is observed to be enhanced with hydrogen addition. In contrast, the emission index of CO₂ is seen to be reduced. This can be attributed to reduction in the residence time with hydrogen addition which leads to inhibit the conversion of CO to CO₂ and thus results in decreased CO₂ and enhanced CO concentration levels.     

Turbulence and combustion interaction: High resolution local flame front structure visualization using simultaneous single-shot PLIF imaging of CH, OH, and CH2O in a piloted premixed jet flame

High resolution planar laser-induced fluorescence (PLIF) was applied to investigate the local flame front structures of turbulent premixed methane/air jet flames in order to reveal details about turbulence and flame interaction. The targeted turbulent flames were generated on a specially designed coaxial jet burner, in which low speed stoichiometric gas mixture was fed through the outer large tube to provide a laminar pilot flame for stabilization of the high speed jet flame issued through the small inner tube. By varying the inner tube flow speed and keeping the mixture composition as that of the outer tube, different flames were obtained covering both the laminar and turbulent flame regimes with different turbulent intensities. Simultaneous CH/CH₂O, and also OH PLIF images were recorded to characterize the influence of turbulence eddies on the reaction zone structure, with a spatial resolution of about 40 μm and temporal resolution of around 10 ns. Under all experimental conditions, the CH radicals were found to exist only in a thin layer; the CH₂O were found in the inner flame whereas the OH radicals were seen in the outer flame with the thin CH layer separating the OH and CH₂O layers. The outer OH layer is thick and it corresponds to the oxidation zone and post-flame zone; the CH₂O layer is thin in laminar flows; it becomes broad at high speed turbulent flow conditions. This phenomenon was analyzed using chemical kinetic calculations and eddy/flame interaction theory. It appears that under high turbulence intensity conditions, the small eddies in the preheat zone can transport species such as CH₂O from the reaction zones to the preheat zone. The CH₂O species are not consumed in the preheat zone due to the absence of H, O, and OH radicals by which CH₂O is to be oxidized. The CH radicals cannot exist in the preheat zone due to the rapid reactions of this species with O₂ and CO₂ in the inner-layer of the reaction zones. The local PLIF intensities were evaluated using an area integrated PLIF signal. Substantial increase of the CH₂O signal and decrease of CH signal was observed as the jet velocity increases. These observations raise new challenges to the current flamelet type models.     

Turbulent lifted flames of H2/N2 fuel issuing into a vitiated coflow investigated using Lagrangian Intermittent Modelling

This paper reports results of numerical simulations of a turbulent lifted jet flame of hydrogen–nitrogen mixtures including the effects of the autoignition. The impact of burned gases on the flame stabilization is analysed under the conditions of a laboratory jet flame in a vitiated coflow. In this study, mass flow rate, temperature and exact chemical composition of hot products mixed with air sent toward the turbulent flame base are fully determined. The effects of both non-infinitely fast chemistry and partially premixed combustion are taken into account within a Lagrangian intermittent framework. Detailed chemistry effects are incorporated through the use of a tabulation delay. The concept of residence time of the particles and the transport equation for the mean scalar dissipation rate are included. Numerical simulation of the turbulent diluted jet flame of H₂/N₂ studied by Cabra and his co-workers at Berkeley University is performed and satisfactory results are obtained: the flame liftoff height is reasonably captured and the predictions display a reasonable agreement with respect to experimental data.     

Effects of bluff-body shape on LPG–H2 jet diffusion flame

The effects of bluff-body lip thickness on the several physical parameters like flame length, radiant fraction, gas temperature and _NOx${\mathrm{NO}}_x$ emissions in liquefied petroleum gas (LPG)–H₂ jet diffusion flame are investigated experimentally. Results indicate that the flame length reduces with the addition of hydrogen in the bluff-body stabilized flame, which can be attributed to the enhanced reactivity and residence time of the mixture gases. Moreover, with increasing lip thickness of the bluff body, the flame length also gets reduced. The soot free length fraction (SFLF) is observed to be enhanced with H₂ addition to the fuel stream. In contrast, the SFLF gets reduced with increasing lip thickness repetition, which is due to the reduced induction period of soot formation. The emission index of _NOx${\mathrm{NO}}_x$ (_EINOx${\mathrm{EINO}}_x$) is found to be attenuated in coaxial burner with hydrogen addition. In contrast it is observed to be enhanced in bluff-body stabilized flame. The former is due to the reduction in residence time of gas mixture, whereas the latter can be explained on the basis of increased flame temperature. Besides this, _NOx${\mathrm{NO}}_x$ emission level is also found to be enhanced with increasing lip thickness due to enhanced residence time.     

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.     

Monte Carlo modeling of radiative transfer in a turbulent sooty flame

The calculation of radiative transfer within a sooty turbulent ethylene-air diffusion jet flame has been carried out by using a Monte Carlo method and an accurate CK model for the gases. The influence of the turbulence-radiation interaction (TRI) has been studied. In the TRI modeling, the radiative properties of the assumed homogeneous turbulent structures are randomly obtained from a multidimensional probability density function (PDF) of the reaction progress variable, of the mixture ratio and of the soot volume fraction. This joint PDF is obtained from an Eulerian-Lagrangian turbulent combustion model and the sizes of the turbulent structures are directly derived from a k-? model. In the considered flame, the TRI effect is an increase of the radiative heat loss by about 30%. The radiative heat loss becomes almost equal to one-third of the chemical heat release. Soot particles play the most important role in the global radiative heat loss but the influence of gaseous species like CO₂ and H₂O can be important in the local energy balance.     

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.     

Optimization of the flame characteristics of H2–O2 coaxial injection applied to hydrogen-fueled argon cycle engines

The argon power cycle is one of the most promising technologies for high efficiency and low emission hydrogen-fueled internal combustion engines. The application of coaxial injection technology in the hydrogen-fueled argon engine can improve the mixing process and the combustion performance of the H₂/O₂ mixture. In this study, an innovative H₂–O₂ coaxial injection combustion system was designed to investigate the jet flame characteristics of oxygen coaxially wrapped by hydrogen in a controllable argon thermal atmosphere. The findings of this study could provide a new perspective for designing hydrogen-fueled argon engines in the future. The influences of co-flow temperature, jet injection pressure, and excess oxygen coefficient were all determined. Observations of the flame showed a bright blue flame with a reddish glow in the far-burner region. Experimental results show that the flame length, cross-sectional area, and area/perimeter ratio first decrease with increasing jet injection pressure and subsequently increase, reaching maximum values at 0.4–0.6 MPa. When increasing the co-flow temperature from 1023 K to 1223 K, the cross-sectional area of the flame increases significantly by 61.1% at an excess oxygen coefficient of 0.4. Furthermore, the liftoff flame height shrinks when the co-flow temperature and the excess oxygen coefficient increase, while it rises along with an increasing jet injection pressure.     

Flame characteristics in H2/CO synthetic gas diffusion flames diluted with CO2: Effects of radiative heat loss and mixture composition

Numerical study is conducted to understand the impact of fuel composition and flame radiation in flame structure and their oxidation process in H₂/CO synthetic gas diffusion flame with and without CO₂ dilution. The models of Sun et al. and David et al., which have been well known to be best-fitted for H₂/CO synthetic mixture flames, are evaluated for H₂/CO synthetic mixture flames diluted with CO₂. Effects of radiative heat loss to flame characteristics are also examined in terms of syngas mixture composition. Importantly contributing reaction steps to heat release rate are compared for the synthetic gas mixture flames of high contents of H₂ and CO, individually, with and without CO₂ dilution. The modification of the oxidation pathways is also addressed.     

Computed flammability limits of opposed-jet H2/CO syngas diffusion flames

Extensive computations were made to determine the flammability limits of opposed-jet H₂/CO syngas diffusion flames from high stretched blowoff to low stretched quenching. Results from the U-shape extinction boundaries indicate the minimum hydrogen concentrations for H₂/CO syngas to be combustible are larger towards both ends of high strain and low strain rates. The most flammable strain rate is near one s⁻¹ where syngas diffusion flames exist with minimum 0.002% hydrogen content. The critical oxygen percentage (or limiting oxygen index) below which no diffusion flames could exist for any strain rate was found to be 4.7% for the equal-molar syngas fuels (H₂/CO = 1), and the critical oxygen percentage is lower for syngas mixture with higher hydrogen content. The flammability maps were also constructed with strain rates and pressures or dilution gases percentages as the coordinates. By adding dilution gases such as CO₂, H₂O, and N₂ to make the syngas non-flammable, besides the inert effect from the diluents, the chemical effect of H₂O contributes to higher flame temperature, while the radiation effect of H₂O and CO₂ plays an important role in the flame extinction at low strain rates.     

Effects of Lewis number and preferential diffusion on flame characteristics in 80%H2/20%CO syngas counterflow diffusion flames diluted with He and Ar

Numerical study is conducted to grasp flame characteristics in H₂/CO syngas counterflow diffusion flames diluted with He and Ar. An effective fuel Lewis number, applicable to premixed burning regime and even to moderately stretched diffusion flames, is suggested through the comparison among fuel Lewis number, effective Lewis number, and effective fuel Lewis number. Flame characteristics with and without the suppression of the diffusivities of H, H₂, and He are compared in order to clarify the important role of preferential diffusion effects through them. It is found that the scarcity of H and He in reaction zone increases flame temperature whereas that of H₂ deteriorates flame temperature. Impact of preferential diffusion of H, H₂, and He in flame characteristics is also addressed to reaction pathways for the purpose of displaying chemical effects.     

Large eddy simulation of hydrogen piloted CH4/air premixed combustion with CO2 dilution

Large eddy simulation (LES) of constant adiabatic temperature, hydrogen-piloted, turbulent lean premixed methane/air jet flames with varying amounts of CO₂ addition are reported. Constant adiabatic temperature is achieved by increasing the fuel flow rate slightly to account for the higher specific heat of CO₂ compared to N₂. Such flames are relevant to low NOx gas turbines with high hydrogen content fuels and Exhaust Gas Recirculation (EGR). A newly designed burner called Piloted Axisymmetric Reactor Assisted Turbulent (PARAT) flame burner was utilized. The operating conditions in the experiment were selected to highlight the kinetic effects of CO₂ addition by matching the Reynolds numbers, Lewis numbers and adiabatic flame temperatures. The LES simulations utilize a finite rate chemistry solver with DRM19 combustion mechanism with adaptive zoning and a dynamic structure turbulence model. A five-level adaptive mesh refinement (AMR) improves the velocity and temperature gradient resolution. The LES predicts the experimentally observed increase in flame length with CO₂ levels caused by a decrease in the turbulent flame speed. The computational results also capture the experimentally observed departure from the thin flame limit and a collapse of the root mean square (RMS) versus mean temperature profiles for the three levels of CO₂. The flame structure analysis showed super-equilibrium CO concentrations because of non-equilibrium chemistry effects caused by the external addition of CO₂.     

Stability characteristics of non-premixed turbulent jet flames of hydrogen and syngas blends with coaxial air

The stability characteristics of attached hydrogen (H₂) and syngas (H₂/CO) turbulent jet flames with coaxial air were studied experimentally. The flame stability was investigated by varying the fuel and air stream velocities. Effects of the coaxial nozzle diameter, fuel nozzle lip thickness and syngas fuel composition are addressed in detail. The detachment stability limit of the syngas single jet flame was found to decrease with increasing amount of carbon monoxide in the fuel. For jet flames with coaxial air, the critical coaxial air velocity leading to flame detachment first increases with increasing fuel jet velocity and subsequently decreases. This non-monotonic trend appears for all syngas composition herein investigated (50/50 → 100/0% H₂/CO). OH^∗ chemiluminescence imaging was performed to qualitatively identify the mechanisms responsible for the flame detachment. For all fuel compositions, local extinction close to the burner rim is observed at lower fuel velocities (ascending stability limit), while local flame extinction downstream of the burner rim is observed at higher fuel velocities (descending stability limit). Extrema of the non-monotonic trends appear to be identical when the nozzle fuel velocity is normalized by the critical fuel velocity obtained for the single jet cases.     

Chemical effect of diluents on flame structure and NO emission characteristic in methane‐air counterflow diffusion flame

The dilution effect of air stream according to agent type on flame structure and NO emission behaviour is numerically simulated with detailed chemistry in CH₄/air counterflow diffusion flame. The volume percentage of diluents (H₂O, CO₂, and N₂) in air stream is systematically changed from 0 to 10. The radiative heat loss term, based on an optically thin model, is included to clearly describe the flame structure and NO emission behaviour especially at low strain rates. The effect of dilution of air stream on the decrease of maximum flame temperature varies as CO₂>H₂O>N₂, even if heat capacity of H₂O is the highest. It is also found that the addition of CO₂ shows the tendency towards the reduction of flame temperature in both the thermal and chemical sides, while the addition of H₂O enhances the reaction chemically and restrains it thermally due to a super‐equilibrium effect of the chain carrier radicals caused by the breakdown of H₂O in high‐temperature region. The comparison of the nitrogen chemical reaction pathway between the cases of the addition of CO₂ and H₂O clearly displays that the addition of CO₂ is much more effective to reduce NO emission. Copyright © 2002 John Wiley & Sons, Ltd.     

Emission of impinging swirling and non-swirling inverse diffusion flames

The overall pollutants emission from impinging swirling and non-swirling inverse diffusion flames (IDFs) was evaluated quantitatively by the ‘hood’ method. The results of in-flame volumetric concentrations of CO and NO_x and overall pollutants emission of CO and NO_x in terms of emission index were reported. The in-flame volumetric concentrations of CO and NO_x were measured through a small hole drilled on the impingement plate. In comparison with the corresponding open flame, the CO and NO_x concentrations for the impinging swirling IDF are greatly lowered due to the entrainment of much more ambient air which is related to the increased flame surface area. For the swirling and non-swirling IDFs, the EINO_x increases as the nozzle-to-plate distance (H) increases because more space is available for the development of the high-temperature zone in the free jet portion of the impinging flame, which favors the thermal NO formation. The variation of EICO with H is different for the impinging swirling and non-swirling IDFs because they have different flame structures. For both flames, the EICO is high when their main reaction zone or inner reaction cone is impinged and quenched by the copper plate. The parameters of air jet Reynolds number, overall equivalence ratio and nozzle-to-plate distance have significant influence on the overall pollutants emission of the impinging swirling and non-swirling IDFs and the comparison shows that the swirling IDF emits less NO_x and CO under most of the experimental conditions tested. Furthermore, it is found that compared with the open flames, the impinging flames emit lower level of NO_x and higher level of CO.     

Preferential diffusion effects on flame characteristics in H2/CO syngas diffusion flames diluted with CO2

Numerical study is conducted to clarify preferential diffusion effects of H₂ and H on flame characteristics in synthetic diffusion flames of the compositions of 80% H₂/20% CO and 20% H₂/80% CO as representatively H₂-enriched and CO-enriched H₂/CO flames. Impacts of CO₂ addition to the flames are also examined through the variation of added CO₂ mole fraction from 0 to 0.5. A comparison was made by employing a mixture-averaged diffusivity and the suppression of the diffusivities of H and H₂. It is found that preferential diffusion effects on maximum flame temperature cannot be explained by the well-known behavior between maximum flame temperature and scalar dissipation rate but by chemical processes. The concrete evidence is also presented through the examination of the behavior of maximum H mole fraction and the behavior of importantly-contributing reaction steps to overall heat release rate.     

A numerical study on the effects of H2 addition in non-premixed turbulent combustion of C3H8–H2–N2 mixture using a steady flamelet approach

This study has been implemented in two sections. At first, the turbulent jet flame of DLR-B is simulated by combining the k–ε turbulence model and a steady flamelet approach. The DLR-B flame under consideration has been experimentally investigated by Meier et al. who obtained velocity and scalar statistics. The fuel jet composition is 33.2% H₂, 22.1% CH₄ and 44.7% N₂ by volume. The jet exit velocity is 63.2 m/s resulting in a Reynolds number of 22,800. Our focus in the first part is to validate the developed numerical code. Comparison with experiments showed good agreement for temperature and species distribution. At the second part, we exchanged methane with propane in the fuel composition whilst maintaining all other operating conditions unchanged. We investigated the effect of hydrogen concentration on C₃H₈–H₂–N₂ mixtures so that propane mole fraction extent is fixed. The hydrogen volume concentration rose from 33.2% up to 73.2%. The achieved consequences revealed that hydrogen addition produces elongated flame with increased levels of radiative heat flux and CO pollutant emission. The latter behavior might be due to quenching of CO oxidation process in the light of excessive cold air downstream of reaction zone.     

Dissipation length scales in turbulent nonpremixed jet flames

Line imaging of Raman/Rayleigh/CO-LIF is used to investigate the energy and dissipation spectra of turbulent fluctuations in temperature and mixture fraction in several flames, including CH₄/H₂/N₂ jet flames at Reynolds numbers of 15,200 and 22,800 (DLR-A and DLR-B) and piloted CH₄/air jet flames at Reynolds numbers of 13,400, 22,400, and 33,600 (Sandia flames C, D, and E). The high signal-to-noise ratio of the 1D Rayleigh scattering images enables determination of the turbulent cutoff wavenumber from 1D dissipation spectra. The local length scale inferred from this cutoff is analogous to the Batchelor scale in nonreacting flows. The measured thermal dissipation spectra in the turbulent flames are shown to be similar to the model spectrum of Pope for turbulent kinetic energy dissipation. Furthermore, for flames with Lewis number near unity, the 1D dissipation spectra for temperature and mixture fraction are shown to follow nearly the same rolloff in the high-wavenumber range, such that the cutoff length scale for thermal dissipation is equal to or slightly smaller than the cutoff length scale for mixture fraction dissipation. Measurements from the piloted CH₄/air flames are used to demonstrate that a surrogate cutoff scale may be obtained from the dissipation spectrum of the inverse of the Rayleigh signal itself, even when the Rayleigh scattering cross section varies through the flame. This suggests that the cutoff length scale determined from Rayleigh scattering measurements may be used to define the local resolution requirements and optimal data processing procedures for accurate determination of the mean mixture fraction dissipation, based upon Raman scattering measurements or other multiscalar imaging techniques.