The effect of hydrogen containing fuel blends upon flashback in swirl burners

Lean premixed swirl combustion is widely used in gas turbines and many other combustion Processes due to the benefits of good flame stability and blow off limits coupled with low NO_x emissions. Although flashback is not generally a problem with natural gas combustion, there are some reports of flashback damage with existing gas turbines, whilst hydrogen enriched fuel blends, especially those derived from gasification of coal and/or biomass/industrial processes such as steel making, cause concerns in this area. Thus, this paper describes a practical experimental approach to study and reduce the effect of flashback in a compact design of generic swirl burner representative of many systems. A range of different fuel blends are investigated for flashback and blow off limits; these fuel mixes include methane, methane/hydrogen blends, pure hydrogen and coke oven gas. Swirl number effects are investigated by varying the number of inlets or the configuration of the inlets. The well known Lewis and von Elbe critical boundary velocity gradient expression is used to characterise flashback and enable comparison to be made with other available data.Two flashback phenomena are encountered here. The first one at lower swirl numbers involves flashback through the outer wall boundary layer where the crucial parameter is the critical boundary velocity gradient, G_f. Values of G_f are of similar magnitude to those reported by Lewis and von Elbe for laminar flow conditions, and it is recognised that under the turbulent flow conditions pertaining here actual gradients in the thin swirl flow boundary layer are much higher than occur under laminar flow conditions. At higher swirl numbers the central recirculation zone (CRZ) becomes enlarged and extends backwards over the fuel injector to the burner baseplate and causes flashback to occur earlier at higher velocities. This extension of the CRZ is complex, being governed by swirl number, equivalence ratio and Reynolds Number. Under these conditions flashback occurs when the cylindrical flame front surrounding the CRZ rapidly accelerates outwards to the tangential inlets and beyond, especially with hydrogen containing fuel mixes. Conversely at lower swirl numbers with a modified exhaust geometry, hence restricted CRZ, flashback occurs through the outer thin boundary layer at much lower flow rates when the hydrogen content of the fuel mix does not exceed 30%. The work demonstrates that it is possible to run premixed swirl burners with a wide range of hydrogen fuel blends so as to substantially minimise flashback behaviour, thus permitting wider used of the technology to reduce NOx emissions.     

Experimental assessment of the combustion performance of an oven burner operated on pipeline natural gas mixed with hydrogen

With the increasing need to reduce greenhouse gas emission and adopt sustainability in combustion systems, injection of renewable gases into the pipeline natural gas is of great interest. Due to high specific energy density and various potential sources, hydrogen is a competitive energy carrier and a promising gaseous fuel to replace natural gas in the future. To test the end use impact of hydrogen injection into the natural gas pipeline infrastructure, the present study has been carried out to evaluate the fuel interchangeability between hydrogen and natural gas in a residential commercial oven burner. Various combustion performance characteristics were evaluated, including flashback limits, ignition performance, flame characteristics, combustion noise, burner temperature and emissions (NO, NO₂, N₂O, CO, UHC, NH₃). Primary air entrainment process was also investigated. Several correlations for predicting air entrainment were compared and evaluated for accuracy based on the measured fuel/air concentration results in the burner. The results indicate that 25% (by volume) hydrogen can be added to natural gas without significant impacts. Above this amount, flashback in the burner tube is the limiting factor. Hydrogen addition has minimal impact on NO_X emission while expectedly decreasing CO emissions. As the amount of hydrogen increases in the fuel, the ability of the fuel to entrain primary air decreases.     

Hydrogen substitution of natural-gas in premixed burners and implications for blow-off and flashback limits

Two multi-perforated premixed burners, designed for natural gas, are fueled with increasing hydrogen content to assess the limits of H₂ substitution and investigate potential risks associated to it. The burners feature a different design, which affects flame stabilization and heat exchange between the fresh mixture and the hot burner walls. First, results are presented by means of stability maps that were collected at constant power and over a wide range of equivalence ratio, from pure methane-air to pure hydrogen-air mixtures. The impact of hydrogen addition on blow-off and flashback limits is then analyzed. On one side, it is observed that hydrogen addition increases blow off resistance, extending the operating range towards ultra-lean conditions. On the other side, hydrogen raises the thermal load on the burner favoring flashback. It is shown that the competition between the bulk velocity at the burner outlet and the laminar burning velocity is not a reliable parameter to predict flashback occurrence, while the thermal state of the burner represents a determining factor. An analysis of the thermal transient reveals a strict correspondence between the onset of flashback for a given mixture composition and the burner surface temperature. Results highlight the challenges linked to the design of fuel-flexible systems, pointing out practical limits of H₂ substitution in burners designed for operation with natural gas.     

旋流预混燃烧室燃烧特性及排放特性的数值模拟研究

为了综合考察燃气轮机燃烧室在高稳定性、低排放以及燃料适应性等方面的新要求,基于旋流预混燃烧技术,通过三维数值模拟方法开展了甲烷/空气、丙烷/空气预混燃烧特性及排放特性研究。结果表明：在一定的预混气进气质量流量条件下,当量比增大易引发回火,燃烧温度更高,同时NO_x排放指数增大,增加预混气质量流量,可在一定程度上提高回/熄火极限;当量比固定,增加预混气进气质量流量可避免潜在的回火现象,且NO_x排放指数线性降低;旋流器的旋流数增大能形成强旋流,稳定火焰,降低NO_x排放指数,但过大的旋流强度会引发回火现象;相比于甲烷/空气预混燃烧,丙烷/空气预混燃烧温度偏高,NO_x排放指数较大,但回熄火边界更宽,对应更广阔的稳定燃烧区间。     

Experimental investigation of flashback during start-up in practical premixed combustion

An experimental investigation of flashback was conducted during the start-up of a practical premixed kerosene burner. The flame propagations for normal ignition and flashback were visualized with high time-resolution. The timing of the fuel supply and ignition, as well as time sequence variations in pressures and temperatures in the burner, were analyzed to clarify the phenomena. The accumulated data revealed that flashback was caused by reverse flow of the hot combustion products through the flashback arrester and that the probability of flash back increased with increasing ignition delay, due to the higher pressures resulting from ignition and burning of the fuel accumulated in the combustion chamber during the delay. The thermal energy passing through the perforated plate used for flame quenching between the combustion chamber and the evaporating chamber was estimated using the reverse-flow pressures and their periods. The ignition energy estimated for the kerosene vapor mixtures was of the order of 10 mJ, which corresponds to the minimum ignition energy of other hydrocarbon fuels.     

Assessment of the combustion performance of a room furnace operating on pipeline natural gas mixed with simulated biogas or hydrogen

With the inexorable depletion of fossil fuel and the increasing need to reduce greenhouse gas emissions, blending renewable fuels like biogas or renewable hydrogen into natural gas is of great interest. Due to various potential sources and low-carbon or even carbon-free properties, biogas and hydrogen are competitive energy carriers and promising gaseous fuels to replace pipeline natural gas in the future. From the perspective of end users and combustion device manufacturers, one of the major concerns is the influence of the renewable content on the combustion device performance. In addition, the upper limit of renewable gas content percentage in pipeline also interests policy makers and gas utility companies. Therefore, the present study is conducted to investigate the influence of renewable gas content on the operating performance of a residential room furnace. Evaluated combustion performance characteristics include ignition performance, blow-off/flashback limits, burner temperature and emissions (NO, NO₂, N₂O, CO, UHC, NH₃). The results show that 5% carbon dioxide and 15% (by volume) hydrogen can be added to natural gas separately without significant impacts. Above this amount, the risk of blow-off and flashback is the limiting factor. Generally speaking, carbon dioxide addition helps decrease NO_X emission but increases CO emission. However, hydrogen addition up to the amounts studied here in has minimal impact on NO_X and CO emissions.     

Numerical investigations on flashback dynamics of premixed methane-hydrogen-air laminar flames

Injecting hydrogen into the natural gas network to reduce CO₂ emissions in the EU residential sector is considered a critical element of the zero CO₂ emissions target for 2050. Burning natural gas and hydrogen mixtures has potential risks, the main one being the flame flashback phenomenon that could occur in home appliances using premixed laminar burners. In the present study, two-dimensional transient computations of laminar CH₄ + air and CH₄ + H₂ + air flames are performed with the open-source CFD code OpenFOAM. A finite rate chemistry based solver is used to compute reaction rates and the laminar reacting flow. Starting from a flame stabilized at the rim of a cylindrical tube burner, the inlet bulk velocity of the premixture is gradually reduced to observe flashback. The results of the present work concern the effects of wall temperature and hydrogen addition on the flashback propensity of laminar premixed methane-hydrogen-air flames. Complete sequences of flame dynamics with gradual increases of premixture velocity are investigated. At the flame flashback velocities, strong oscillations at the flame leading edge emerge, causing broken flame symmetry and finally flame flashback. The numerical results reveal that flashback tendency increase with increasing wall temperature and hydrogen addition rate.     

Experimental investigation of the stability and emission characteristics of premixed formic acid-methane-air flames in a swirl combustor

Formic acid (FA) is a potential hydrogen energy carrier and low-carbon fuel by reversing the decomposition products, CO₂ and H_2, back to restore FA without additional carbon release. However, FA-air mixtures feature high ignition energy and low flame speed; hence stabilizing FA-air flames in combustion devices is challenging. This study experimentally investigates the flame stability and emission of swirl flames fueled with pre-vaporized formic acid-methane blends over a wide range of formic acid fuel fractions. Results show that by using a swirl combustor, the premixed formic acid-methane-air flames could be stabilized over a wide range of FA fuel fractions, Reynolds numbers, and swirl numbers. The addition of formic acid increases the equivalence ratios at which the flashback and lean blowout occur. When Reynolds number increases, the equivalence ratio at the flashback limit increases, but that decreases at the lean blowout limit. Increasing the swirl number has a non-monotonic effect on stability limits variation because increasing the swirl number changes the axial velocity on the centerline of the burner throat non-monotonically. In addition, emission characteristics were investigated using a gas analyzer. The CO and NO concentrations were below 20 ppm for all tested conditions, which is comparable to that seen with traditional hydrocarbon fuels, which is in favor of future practical applications with formic acid.     

The effect of fuel injection on NOx emissions and undesirable combustion for hydrogen-fuelled piston engines

In order to determine the potential of direct cylinder injection for hydrogen-fuelled engines, an experimental study was performed with an ASTM-CFR engine. Both the standard Otto head and the standard diesel head were used. Measurements were made of power output, thermal efficiency, and oxides of nitrogen emissions. The feasibility was investigated of a scheme in which injection of gaseous hydrogen starts late in the compression stroke, ignition occurs as soon as possible thereafter, and combustion rate is determined by injection rate. This scheme prevents undesirable combustion phenomena such as pre-ignition, high rates of cylinder pressure rise, and high amplitude pressure waves in the cylinder. Furthermore, it obviates flashback into the carburetor. The potential of hydrogen as a low pollution fuel was investigated by operating the Otto head engine on both hydrogen and indolene, and by comparing the resulting NO_x, emissions. Hydrogen yielded very low NO_x emissions provided the fuel-air equivalence ratio was less than 0.5, and provided the hydrogen and air were well mixed. For equivalence ratios greater than 0.5, hydrogen yielded NO_x emissions that were higher than those obtained with indolene. The timing of hydrogen injection was found to have a significant effect on NO_x emissions. With an unthrottled air intake and hydrogen injection at equivalence ratios between 0.3 and 0.8, indicated mean effective pressures ranged from 0.3 to 0.78 MPa. Corresponding indicated thermal efficiencies ranged from 43 to 31%. By decreasing the equivalence ratio to 0.1, the IMEP could be reduced to 0.07 MPa, thus providing an indicated load range of more than a factor of 10.     

Stabilization regimes and pollutant emissions from a dual fuel CH4/H2 and dual swirl low NOx burner

Burning hydrogen in gas turbines is a relevant technological solution to decarbonize power production and propulsion systems. However, ensuring low NOx emission and preventing flashback can be challenging with hydrogen. Stabilization regimes and pollutant emissions from partially premixed CH₄/H₂/air flames above a coaxial Dual Fuel Dual Swirl injector are investigated in a laboratory-scale combustor at atmospheric conditions for increasing hydrogen contents. The injector consists of an external annular swirler providing premixed methane/air and a central channel fed with pure hydrogen. This burner virtually removes the risk of flashback due to the late injection of hydrogen. Flame stabilization regimes, CO and NOx emissions are analyzed for different configurations of the injector and operating points. The effect of swirling the hydrogen stream is investigated together with the influence of the hydrogen injector recess, i.e. its nozzle position with respect to the backplane of the combustion chamber. It is shown that swirling the central hydrogen stream favors aerodynamically stabilized flames resulting in a low thermal stress on the injector and limited NOx emissions. The study also highlights that a small recess of the central hydrogen injector widely extends the operability range of the burner with aerodynamically stabilized flames. With a sufficient inner swirl and a small recess, flames detach from the injector rim when the hydrogen bulk velocity is large enough. In this configuration, it is found that NOx emissions remain low even for operation with pure hydrogen. Moreover, NOx emissions decrease when increasing the thermal power for a fixed equivalence ratio.     

Turbulent combustion evolution of stoichiometric H2/CH4/air mixtures within a spherical space

Turbulent combustion evolutions of stoichiometric H₂/CH₄/air mixtures were experimentally studied within a spherical constant-volume combustion vessel. A series of initial turbulent ambience (with the range of turbulence intensity from 0 to 1.309 m/s) and a series of hydrogen volumetric fraction (with the range from 0.3 to 0.9) were taken as the variables to studied the influences of turbulence intensity and the fuel composition on the turbulent combustion evolutions. The evolutions of explosion overpressure were studied upon the variations of maximal pressure, the influences of turbulence intensity mainly located at heat loss while the influences of fuel composition mainly located at adiabatic explosion. Subsequently, the evolutions of burnt mass were discussed, the competition between pressure rising and temperature rising induced by the heat release during combustion was considered as major influence mechanism. Then, the nexus between burning velocity and the related burnt mass rate were discussed, the variations regulations of maximal burning velocity brought by turbulence intensity and hydrogen volumetric fraction were analysed. Finally, the nexus between maximum burning velocity and heat loss was discussed.     

Boundary layer flashback of non-swirling premixed flames: Mechanisms,fundamental research,and recent advances

Boundary layer flashback in premixed jet flames has been the subject of detailed experimental and numerical investigation since the 1940′s. The traditional approach for characterizing flashback has involved the critical velocity gradient concept, with higher values indicating a higher flashback propensity for a given situation. Recent studies in confined configurations have illustrated that a key assumption underlying the critical velocity gradient concept, namely a lack of interaction between the flame and the approaching flow, is fundamentally incorrect. However, for unconfined configurations, where this interaction is much less important, the critical velocity gradient concept is able to partially capture flashback characteristics. Historically, the critical velocity gradient concept predicts trends of flashback behavior in laminar configurations for a wide range of temperatures, pressures, and fuel compositions more consistently than in turbulent configurations. This is due in part to the fact that many laminar studies establish well behaved velocity conditions in the tube conveying the premixed reactants to the reaction zone. Yet many important practical systems are in the turbulent regime and cannot be approximated by a simplified analysis. Studies to date in either regime, while numerous, generally do not provide a comprehensive methodology for accounting for all parameters. Recent work has attempted to capture the effect of a large number of these parameters in the turbulent regime, with some emphasis on providing design tools that can be used to estimate flashback propensity in more general terms. These approaches have demonstrated reasonable performance for the limited data available at elevated temperature and pressure which are representative of important practical system such as lean premixed combustors for gas turbines. While progress has been made in the last few years relative to predicting flashback for practical systems with high Reynolds numbers, only limited data are available for developing and validating correlations. Open questions remain in terms of using detailed numerical simulations and complex reaction chemistry to predict flashback for unconfined flames. In addition, flame-wall interaction in terms of heat transfer, sensitivity to turbulence levels, the role of general velocity gradients (vs idealized fully developed flow), and the role of high pressure must be further evaluated.     

A compilation of operability and emissions performance of residential water heaters operated on blends of natural gas and hydrogen including consideration for reporting bases

The impact of hydrogen added to natural gas on the performance of commercial domestic water heating devices has been discussed in several recent papers in the literature. Much of the work focuses on performance at specific hydrogen levels (by volume) up to 20–30% as a near term blend target. In the current work, new data on several commercial devices have been obtained to help quantify upper limits based on flashback limits. In addition, results from 39 individual devices are compiled to help generalize observations regarding performance. The emphasis of this work is on emissions performance and especially NOx emissions. It is important to consider the reporting bases of the emissions numbers to avoid any unitended bias. For water heaters, the trends associated with both mass per fuel energy input and concentration-based representation are similar For carbon free fuels, bases such as 12% CO2 should be avoided. In general, the compiled data shows that NOx, NO, UHC, and CO levels decrease with increasing hydrogen percentage. The % decrease in NOx and NO is greater for low NOx devices (meaning certified to NOx <10 ng/J using premixing with excess air) compared to conventional devices (“pancake burners”, partial premixing). Further, low NOx devices appear to be able to accept greater amounts of hydrogen, above 70% hydrogen in some cases, without modification, while conventional water heaters appear limited to 40–50% hydrogen. Reporting emissions on a mass basis per unit fuel energy input is preferred to the typical dry concentration basis as the greater amount of water produced by hydrogen results in a perceived increase in NOx when hydrogen is used. While this effort summarizes emissions performance with added hydrogen, additional work is needed on transient operation, higher levels of hydrogen, system durability/reliability, and heating efficiency.     

Modeling study on the production of hydrogen/syngas via partial oxidation using a homogeneous charge compression ignition engine fueled with natural gas

The production of hydrogen and syngas from natural gas using a homogeneous charge compression ignition reforming engine is investigated numerically. The simulation tool used was CHEMKIN 3.7, using the GRI-3 natural gas combustion mechanism. This simulation was conducted on the changes in hydrogen and syngas concentration according to the variations of equivalence ratio, intake temperature, oxygen enrichment, engine speed, initial pressure, and fuel additives with partial oxidation combustion. The simulation results indicate that the hydrogen/syngas yields are strongly dependent on the equivalence ratio with maxima occurring at an optimal equivalence ratio varying with engine speed. The hydrogen/syngas yields increase with increasing intake temperature and oxygen contents in air. The hydrogen/syngas yields also increase with increasing initial pressure, especially at lower temperatures, yet high temperature can suppress the pressure effect. Furthermore, it was found that the hydrogen/syngas yields increase when using fuel additives, especially hydrogen peroxide. Through the parametric screening studies, optimum operating conditions for natural gas partial oxidation reforming are recommended at 3.0 equivalence ratio, 530 K intake temperature, 0.3 oxygen enrichment, 500 rpm engine speed, 1 atm initial pressure, and 7.5% hydrogen peroxide.     

Direct numerical simulation of laminar flame–wall interaction for a novel H2-selective membrane/injector configuration

Direct numerical simulations are performed to investigate the transient processes of laminar flame–wall interaction and quenching near a porous, permeable wall and compared against a reference case of a non-porous impermeable wall. A boundary condition formulation that models species (hydrogen in this case) transport through a permeable wall, driven by the fuel species partial pressure difference between the feed and the permeate side of a selective membrane, has been implemented in a high-order finite difference direct numerical simulation code for reactive flows (S3D) by Chen et al. (2009) [1]. The present results are obtained for lean, stoichiometric and rich initial mixture conditions on the permeate side of the permeable wall and indicate that the characteristic parameters of the flame–wall interaction (wall heat flux, quenching distance) are affected to a large extent by the presence of the membrane hydrogen flux. Concurrently, the hydrogen flux through the membrane is also strongly affected by the presence of the flame during the transient flame–wall interaction process, finally resulting in a strong feedback mechanism between the membrane hydrogen flux and the flame that greatly increases boundary layer flashback speeds at fuel lean conditions.     

Study of combustion and emission of a SI engine at various CR fuelled with different ratios of biobutanol/hydrogen fuel

The global requirement is shifting to territorial independence of energy sources, and the introduction of alcohols and biofuels are the primary sectors. Recently agriculture products-based ethanol has replaced a larger portion of gasoline. Butanol is another impressive fuel in the same chain, much better than ethanol in many parameters. Butanol has certain limitations, too, such as higher latent heat and low heating value. Therefore, biobutanol/hydrogen is tested experimentally at various compression ratios (CR) in the present study. Brake thermal efficiency was not significantly changed by CR at 90% butanol, while CR is more impressive with increasing hydrogen. The flame development period was reduced by 34%, while the flame propagation phase was reduced by 29% by increasing CR to 15 and hydrogen to 25%. Peak pressure and heat release rate surged by 12.89% and 12.32% and advanced by 6°CA. The coefficient of variations is also reduced by 21% by increasing CR to 15 and hydrogen to 30%. Higher hydrogen faced combustion difficulties due to increasing stratification and heterogeneity during combustion. Unlikely to trend, T_max (peak cylinder temperature) and NO_x were continuously increased with CR and hydrogen due to increased fuel quantity and larger mass burning before TDC. However, CO and HC emissions were reduced by CR due to increased BTE (brake thermal efficiency) and reduced by hydrogen due to less HC supply. A slight increase in HC and CO was noticed for higher hydrogen due to local heterogeneity and disassociation at high temperatures.     

Characteristics of direct injection combustion fuelled by natural gas–hydrogen mixtures using a constant volume vessel

The effects of hydrogen addition and turbulence intensity on the natural gas–air turbulent combustion were studied experimentally using a constant volume vessel. Turbulence was generated by injecting the high-pressure fuel into the vessel. Flame propagation images and combustion characteristics via pressure-derived parameters were analyzed at various hydrogen volumetric fractions (from 0% to 40%) and the overall equivalence ratios of 0.6, 0.8 and 1.0. The results showed that the turbulent combustion rate increased remarkably with the increase of hydrogen fraction in fuel blends when hydrogen fraction is over 11%. Combustion rate was increased remarkably with the introduction of turbulence in the bomb and decreased with the decrease of turbulence intensity. The lean flammability limit of natural gas–air turbulent combustion can be extended with increasing hydrogen fraction addition. Maximum pressure and maximum rate of pressure rise increased while combustion duration decreased monotonically with the increase of hydrogen fraction in fuel blends. The sensitivity of natural gas/hydrogen hybrid fuel to the variation of turbulence intensity was decreased while increasing the hydrogen addition. Maximum pressure and maximum rate of pressure rise increased while combustion duration decreased with the increase of turbulent intensity at stoichiometric and lean-burn conditions. However, slight influence on combustion characteristics was presented with variation of hydrogen fraction at the stoichiometric equivalence ratio with and without the turbulence in the bomb.     

Optimization of fuel cell system operating conditions for fuel cell vehicles

Proton exchange membrane fuel cell (PEMFC) technology for use in fuel cell vehicles and other applications has been intensively developed in recent decades. Besides the fuel cell stack, air and fuel control and thermal and water management are major challenges in the development of the fuel cell for vehicle applications. The air supply system can have a major impact on overall system efficiency. In this paper a fuel cell system model for optimizing system operating conditions was developed which includes the transient dynamics of the air system with varying back pressure. Compared to the conventional fixed back pressure operation, the optimal operation discussed in this paper can achieve higher system efficiency over the full load range. Finally, the model is applied as part of a dynamic forward-looking vehicle model of a load-following direct hydrogen fuel cell vehicle to explore the energy economy optimization potential of fuel cell vehicles.     

Effects of Moisture and Hydrogen Content on the Heating Value of Fuels

Abstract

In this work, effects of moisture and hydrogen contents on lower heating value (LHV) of fuels were investigated. The LHV at constant pressure measures the enthalpy change of combustion with and without water condensed, respectively. Moisture in biomass generally decreases its heating value. Moisture in biomass is stored in spaces within the dead cells and within the cell walls. Higher heating value (HHV) of a fuel decreases with increasing of its moisture content. The LHV of a fuel increases with increasing of its hydrogen content. The LHV of a fuel depends on its oxygen content and the LHV of a fuel decreases with increasing of its oxygen content. The LHV of a fuel increases with increasing the hydrogen content due to cause combustion water. Moisture in a fuel generally decreases its HHV. The LHV of a fuel increases with increasing the sulfur content due to cause SO_x gases absorbed by water.     

Flame structure changes resulting from hydrogen-enrichment and pressurization for low-swirl premixed methane–air flames

With the current focus on alternate and renewable fuels, fuel flexibility has become a driving factor in the design of new turbines. Flame stability is heavily impacted by the presence of hydrogen in the fuel stream (as is common in many alternative fuels). This study examines how the flame dynamics change in response to the systematic addition of hydrogen in a low-swirl lean premixed methane–air burner. Stability maps for these test cases show that adding hydrogen broadens the blow-off limits, with 20% hydrogen resulting in a 7% change while 40% hydrogen results in a 35% larger stable region. The most dramatic manifestation of hydrogen addition is the greatly decreased radius of curvature of the local flame surface, which is visible from the increased wrinkling of the flame front. Increases in both pressure and hydrogen enrichment result in higher means and variances of flame front curvatures. The flame surface density is in agreement with the aforementioned flame front curvature PDFs in that increasing the pressure and hydrogen concentration leads to an increase in the maximum flame surface density.