Turbulent Flame Propagation Research Articles

Anisotropy of Reynolds Stresses and Their Dissipation Rates in Lean H2-Air Premixed Flames in Different Combustion Regimes

The interrelation between Reynolds stresses and their dissipation rate tensors for different Karlovitz number values was analysed using a direct numerical simulation (DNS) database of turbulent statistically planar premixed H2-air flames with an equivalence ratio of 0.7. It was found that a significant enhancement of Reynolds stresses and dissipation rates takes place as a result of turbulence generation due to thermal expansion for small and moderate Karlovitz number values. However, both Reynolds stresses and dissipation rates decrease monotonically within the flame brush for large Karlovitz number values, as the flame-generated turbulence becomes overridden by the strong isotropic turbulence. Although there are similarities between the anisotropies of Reynolds stress and its dissipation rate tensors within the flame brush, the anisotropy tensors of these quantities are found to be non-linearly related. The predictions of three different models for the dissipation rate tensor were compared to the results computed from DNS data. It was found that the model relying upon isotropy and a linear dependence between the Reynolds stress and its dissipation rates does not correctly capture the turbulence characteristics within the flame brush for small and moderate Karlovitz number values. In contrast, the models that incorporate the dependence of the invariants of the anisotropy tensor of Reynolds stresses were found to capture the components of dissipation rate tensor for all Karlovitz number conditions.

Assessment of Machine Learning Techniques for Simulating Reacting Flow: From Plasma-Assisted Ignition to Turbulent Flame Propagation

Combustion involves the study of multiphysics phenomena that includes fluid and chemical kinetics, chemical reactions and complex nonlinear processes across various time and space scales. Accurate simulation of combustion is essential for designing energy conversion systems. Nonetheless, due to its multiscale, multiphysics nature, simulating these systems at full resolution is typically difficult. The massive and complex data generated from experiments and simulations, particularly in turbulent combustion, presents both a challenge and a research opportunity for advancing combustion studies. Machine learning facilitates data-driven techniques to manage the substantial amount of combustion data that is either obtained through experiments or simulations, and thereby can find the hidden patterns underlying these data. Alternatively, machine learning models can be useful to make predictions with comparable accuracy to existing models, while reducing computational costs significantly. In this era of big data, machine learning is rapidly evolving, offering promising opportunities to explore its integration with combustion research. This work provides an in-depth overview of machine learning applications in turbulent combustion modeling and presents the application of machine learning models: Decision Trees (DT) and Random Forests (RF), for the spatio-temporal prediction of plasma-assisted ignition kernels, based on the initial degree of ionization, with model validations against DNS data. The results demonstrate that properly trained machine learning models can accurately predict the spatio-temporal ignition kernel profile based on the initial energy deposition and distribution.

Turbulent flame propagation of C10 hydrocarbons/air expanding flames: Possible unified correlation based on the Markstein number

In this study, we experimentally analyzed the influence of turbulence and thermal-diffusional effect on the morphology and turbulent flame speed of premixed expanding flames. The turbulent flames of the n-decane/air and decalin/air mixtures were measured using a fan-stirred constant volume combustion bomb generating near-isotropic turbulence at elevated pressures (1, 2, and 5 bar), temperature (443 K), and a wide range of equivalence ratios (0.8–1.6). The results found that the wrinkling degree of the flame front was greater for Ma<0 compared to Ma>0. However, the distribution patterns of the curvature radius of the flame contours exhibited similarities across different equivalence ratios. Furthermore, an increase in turbulence intensity would aggravate the randomness of flame contour distribution. The turbulent expanding flames of n-decane/air and decalin/air mixtures were self-similar under different turbulence intensities and pressures, and this self-similar propagation followed a correlation between the normalized turbulent flame speed and the turbulent flame Reynolds number (ReT,f) to the one-half power. The similarity in normalized turbulent flame speeds was extended from normal alkanes (C4-C8) and isomeric alkanes (C8, C16) to longer straight-chain alkane (n-decane) and cycloalkane (decalin). As the increase of equivalence ratio, their normalized turbulent flame speeds increased nonlinearly due to the thermal-diffusional effect. Additionally, it was found that the classical, effective, and experimental Lewis numbers were confirmed as unsuitable parameters for characterizing the role of thermal-diffusional effect on the turbulent flame speed for hydrocarbon fuels with large molecular weight. Finally, two possible unified correlations were proposed based on the Markstein number, (d〈r〉/dt)/(σSL)=0.178e−0.231MaReT,f1/2 and (d〈r〉/dt)/(σSL)=(0.170−0.0447Ma+0.0067Ma2)ReT,f1/2, which could predict the turbulent flame speeds of hydrocarbon fuels with large molecular weight such as n-alkanes, iso-alkanes, and decalin.

Numerical Investigation of the Operating Characteristics of the Passive and Active Prechamber Jet Ignition in a Natural Gas Engine.

Prechamber jet ignition is a promising technology that enables stable ignition and fast combustion by combining thermal effects, chemical kinetics, and turbulent disturbance. The development and application of the prechamber ignition require a comprehensive and in-depth understanding of the operating characteristics of the prechamber ignition in the real engine working cycle. Therefore, numerical simulations are conducted to explore the operating performance of the prechamber ignition applied to a large-bore natural gas engine in this study. The differences between the passive prechamber (PPRE) and active prechamber (APRE) near the lean burn limit are compared. The results show that the jet ignition performance of the PPRE is hampered by the high residual gas coefficient and lean mixture in the prechamber under lean burn conditions. The ignition mode of the PPRE is similar to torch ignition, and the combustion process in the main chamber is mainly turbulent flame propagation. The ignition mechanism of the APRE is flame jet ignition. The main chamber combustion process presents a two-stage heat release characteristic, which can be subdivided into three phases: the initial flame development phase, the rapid combustion phase, and the late combustion phase. The heat release rate during the initial flame development phase depends on the physical and chemical properties of the prechamber jet and the mixture conditions in the main chamber. During the rapid combustion phase, the flame propagation along the radial direction of the jet largely depends on the time scale of the chemical reaction. The heat release rate depends on the coverage area of the jet, the jet residual momentum, and the turbulent flame speed. During the late combustion phase, the flame propagation is mainly affected by the turbulent flame speed. These results provide theoretical guidance for the subsequent application of prechamber ignition systems in various powertrains.

Investigations on conical lean turbulent premixed hydrogenated natural gas flames

Hydrogen's ability to enhance carbon neutrality in combustion processes puts forward the use of hydrogenated fuels, both in the form of fuel and an energy carrier as a potential decarbonization solution. However, because of the nature of hydrogen, blending it with hydrocarbons causes crucial structural changes in the flame structure, including higher flame propagation velocities and higher flame temperatures, decreased instantaneous flame thickness, and increased risks of flame flashback and an increasing potential of NOx emissions due to higher flame temperatures. These attributes encourage a thorough examination of hydrogenated blends of hydrocarbon fuels. Using lean premixed fuels is another technique to achieve efficient and cleaner combustion. However, due to the problem of flame instability in lean premixed combustion, forecasting the design points in terms of flame attributes is critical for better combustor designs.In this study, conical (Bunsen type) lean premixed turbulent flames of hydrogenated natural gas-air mixtures are experimentally studied. Through chemiluminescence measurements of the OH* and CH* radicals and laser-induced Mie scattering, lean natural gas-air premixed flames are examined with subsequently increasing hydrogen addition rates up to 20% by volume and keeping the premixture velocity constant. The obtained data is utilized for exploring the dynamics of the turbulent flame front. The main turbulent premixed flame parameters we identified relate to the instantaneous and average topology of the flame such as the turbulent flame brush thickness and flame height. We also inferred global combustion parameters like the turbulent flame propagation speed from the experimental findings.

Computational insights into flame development and emission formation in an ammonia engine with hydrogen-assisted pre-chamber turbulent jet ignition

Ammonia, as a promising green fuel for achieving carbon neutrality and zero-carbon emissions, encounters obstacles in practical engine applications due to its intrinsic combustion properties like low burning velocity and high autoignition temperature. Hydrogen-assisted pre-chamber turbulent jet ignition (TJI) technology has recently demonstrated great potential in extending the lean limits and enhancing the stability of ammonia combustion. As such, this study conducts a comprehensive analysis on the flame development and emission formation processes under the active pre-chamber TJI mode fueled with ammonia and hydrogen using computational fluid dynamics (CFD) modeling integrated with detailed chemical kinetics, particularly focusing on the effects of hydrogen energy ratio and spark timing. It is found that the active ammonia-hydrogen TJI engine is typically characterized by four primary physical processes involving H2 mixture preparation in the pre-chamber, flame kernel formation and development within the pre-chamber, the formation and ejection of hot jet across the orifices, and the turbulent flame initiation and propagation in the main chamber. Hydrogen injection holds a scavenging effect on the residuals in the pre-chamber while forms fuel stratification near the spark plug, enhancing the ignition stability. Besides, the current combustion mode is commonly characterized by two-stage heat release in the main chamber, but exhibits a three-stage heat release as increasing the premixed H2 ratio. The first stage is dominated by a hot jet flame, the second stage is characterized by turbulent flame propagation or autoignition, and the third stage is induced by a secondary jet ejected from the pre-chamber, which becomes more prominent as the H2 ratio increases. Hydrogen blending could not only facilitate the turbulent flame propagation of NH3-air mixture in the main chamber but also enhance the initiation of flame kernel in the pre-chamber, thereby promoting the overall combustion process and potentially improving the thermal efficiency. The N2O emission is produced in two distinct stages, wherein N2O is generated and undergoes conversion during the main combustion phase, while unburned crevice gases mix with burned gases and convert into N2O during the expansion stroke. Increasing the premixed hydrogen ratio leads to an increased in-cylinder temperature, which in turn facilitates a reduction in N2O emissions and mitigates unburned ammonia. Furthermore, optimization of the spark timing is required after implementing the hydrogen addition into the intake manifold to achieve desirable combustion phasing and further improve the thermal efficiency. Finally, a novel combustion concept, denoted as TJI assisted compression ignition mode, is proposed for achieving high thermal efficiency in ammonia engines via adopting high compression ratio and premixed ammonia-hydrogen charge under the TJI strategy.

Turbulent partially cracked ammonia/air premixed spherical flames

The combustion of ammonia requires, for most energy conversion systems, a combustion promoter such as hydrogen to guarantee the start-up, stability and combustion efficiency. Partially cracked ammonia (PCA) can provide sufficient hydrogen concentrations to enhance the burning velocity in comparison with pure ammonia. However, little work exists on the use of PCA blends operating under relevant turbulent conditions. To that end the outwardly propagating spherical flame configuration was employed to determine the laminar and turbulent flame propagation characteristics of PCA (NH3/(H2+N2)) and corresponding binary (NH3/H2) mixtures across various turbulent combustion regimes. First, PCA and ammonia-hydrogen blends exhibit similar flame propagation rates under various turbulent intensities, even for the laminar case. The highest turbulent burning velocity was observed at leanest conditions, as opposed to laminar flames which exhibited highest flame speed at conditions above stoichiometry. Under rich conditions, no substantial flame enhancement due to turbulence was measured irrespective of the hydrogen content. This lack of flame enhancement under turbulent conditions is attributed to the effect of preferential diffusion with good agreement observed with trends in measured Markstein numbers. The normalized turbulent flame speed is dominated by the enhanced molecular diffusivity afforded by the presence of hydrogen up to 15 % enrichment, prior to decreasing upon further hydrogen addition under lean and stoichiometric conditions. This ‘bending’ phenomenon may be the contribution of several factors including; the transitioning between combustion regimes associated with low Damköhler numbers (Da) and flame thickening; merging of flamelets due to the presence of ammonia enhancing wrinkling; and combined changes in laminar burning velocity and preferential diffusional behavior. Furthermore, good agreement for turbulent flame speed is observed with a correlation that includes the influence of turbulent stretch (Ka) and non-equidiffusion (Le), with the agreement reducing with decreasing chemical to turbulent time scale ratios (Da << 1).

A comparative study on NH3/H2 and NH3/CH3OH combustion and emission in an optical SI engine

Ammonia is an attractive carbon-free fuel but suffers from poor combustion characteristics and heavy emissions. Green hydrogen and methanol are recognized as the most promising combustion promoters for enhancing ammonia reactivity in pursuit of a carbon–neutral solution. Nonetheless, a comparative study on ammonia combustion and emissions, incorporating hydrogen and methanol enrichment, is currently absent under actual engine conditions. In this work, the combustion performance, visualized flame characteristics, and pollution emissions of ammonia were investigated in an optical engine, allowing for low-level hydrogen and methanol enrichment, specifically at energy ratios of 5%, 10%, and 15%. We have identified the crucial role of hydrogen and methanol enrichment in enhancing various aspects of ammonia engine performance, including thermal efficiency, combustion stability, flame initiation and propagation, and nitrogen-based emissions. Specifically, hydrogen enrichment demonstrates a more pronounced effect on reducing cyclic variations, whereas methanol enrichment exhibits a greater influence on increasing thermal efficiency, manifesting improved controllability in combustion phasing. Combustion visualizations show the contribution of hydrogen enrichment to the formation process of the initial flame kernel, which is reflected as a lower cyclic variation from a macroscopic perspective, and meanwhile, methanol enrichment features a stronger acceleration effect during the turbulent flame propagation process. Furthermore, the quantification of flame response to turbulence is solved and observed much higher effects for NH3/CH3OH blends than for NH3/H2 blends, indicating a distinctly different mechanism of facilitation by methanol enrichment. Regarding nitrogen-based emissions, both hydrogen and methanol enrichment lead to a substantial reduction in NH3 slip and N2O emissions, consistent with observed variations in combustion stability. However, methanol enrichment induces noticeable decreases in NOx emissions, while hydrogen enrichment exhibits an opposing trend. The present study demonstrates the beneficial impacts of hydrogen and methanol enrichment on improving ammonia combustion and emissions, with the underlying reason correlated to flame dynamics and thermochemical interactions.

Experimental study on the structure and propagation of the stretched hydrogen-rich turbulent premixed flames in the thin-reaction-zone regime

An experimental study on the structure and propagation of the stretched hydrogen-rich premixed turbulent flames in the thin-reaction-zone regime was conducted. A set of optical diagnostic techniques including the 2D/3D-particle image velocimetry, hot wire anemometer, intensified charge-coupled device (ICCD), and laser tomography visualization (LTV) methods were adopted to quantitatively measure the velocity field, turbulent intensity, and thickness of the stretched flame of the under various conditions. A new method to improve the measurements of the flame preheat zone thickness was developed by using the low-temperature available LTV method to identify the onset of the preheat zone and using the peak location of CH* to identify the reaction zone. The results showed that the flow field was in the inertial turbulent subrange and the turbulent flames obtained in the present counterflow configuration were categorized into the thin reaction zone regime in Borghi/Peters turbulent flame diagram. The experimental results showed that the flame structure of a turbulent H2-rich premixed flame was synergistically influenced by the hydrodynamic stretch, turbulence, and preferential diffusion. New evidence of the phenomenon accumulated that large-scale vortices wrinkled the surface of the flame while small-scale vortices penetrated into the flame preheat zone. Scaling laws for the turbulent flame propagation speed, apparent turbulent thermal conductivity, and were given with the consideration of the synergetic effect of hydrodynamic stretch, turbulence, and preferential diffusion. The scaling of the turbulent flame preheat zone thickness turned out to be sensitive, while those of the turbulent flame propagation speed and apparent turbulent thermal conductivity were insensitive, to the bulk flow strain rate.

A consistent MMC-LES approach for turbulent premixed flames

A multiple mapping conditioning (MMC) approach is coupled with large eddy simulations (LES) for the prediction of the flame propagation, the flame structure and finite rate chemistry effects including NO formation in turbulent premixed flames. The mixing term in MMC relies on a reference field that characterizes the chemical composition within the flame and it is typically taken to be the reaction progress variable. In LES, this reference field needs to be artificially thickened to be resolved on the LES grid. A novel MMC mixing time scale model that had been developed with the aid of direct numerical simulation (DNS), has been adapted to account for the thickening of the reference field. Equally, the particle selection mechanism for particle mixing has been modified to ensure localness of mixing in composition space and to preserve the prediction of a realistic (thin) flame thickness by the stochastic particles. A set of freely propagating turbulent premixed flames with varying equivalence ratios is used to test the performance of the MMC model. Results of MMC-LES are compared with reference DNS that include full chemistry. Additional simulations were carried out where MMC is coupled with quasi-DNS where the flow field is fully resolved but the reaction progress variable field is thickened and its source term is closed with a flamelet approach. This allows to isolate the different modelling assumptions invoked in MMC-LES. Deviations from the exact solution can then be associated with the MMC approach and with thickening of the reference field separately and they are small. The MMC-LES predictions of the turbulent premixed flame propagation speed and flame structure are very good and finite rate chemistry effects can be captured well which is demonstrated by good agreement of predicted NO with DNS data.

Opposite effects of flame-generated potential and solenoidal velocity fluctuations on flame surface area in moderately intense turbulence

To explore the influence of dilatational and rotational motions on iso-scalar surface area within a premixed turbulent flame, three-dimensional compressible direct numerical simulation data obtained by Dave et al. (2018) from a lean, complex-chemistry, hydrogen-air flame propagating in intense small-scale turbulence (a ratio of laminar flame thickness to Kolmogorov length scale is about 20) in a box are analyzed using Helmholtz–Hodge decomposition of velocity field into solenoidal and potential components. The obtained results indicate that the flame surface area is created by potential velocity fluctuations generated due to combustion-induced thermal expansion, whereas the rotational motion acts to smooth wrinkles of iso-scalar surfaces within local preheat and reaction zones and, consequently, to reduce the flame surface area. The latter finding challenges the classical concept of flame-generated turbulence and is attributed to flame-generated solenoidal velocity fluctuations. Specifically, the incoming tangential (to the flame) vorticity is suppressed by baroclinic torque, which also generates vorticity in another tangential direction, with the latter (flame-generated) solenoidal velocity fluctuations working to smooth the flame surface. Thus, even under conditions of moderately intense turbulence, flame surface area can primarily be created by potential velocity perturbations caused by combustion-induced thermal expansion.

Morphology and turbulent burning velocity of n-decane/air expanding flames at constant turbulent Reynolds numbers

In this work we experimentally investigated the propagation and the turbulent burning velocity of n-decane/air turbulent expanding flames at 423 K and 0.5-3.0 atm, with constant turbulent Reynold number, ReT,flow = (urmsLI)/ν, ranging from 174 to 4712, using a medium-scale, fan-stirred combustion chamber, where urms and LI are respectively the turbulence intensity and the integral length scale, and ν is the kinematic viscosity of a fresh mixture. It was found that the n-decane/air turbulent expanding flame is self-similar, and the normalized turbulent flame propagation speed scales with the turbulent flame Reynold number, ReT,flame = (urms/SL)(〈r〉/δL), to the one-half power for mixtures characterized by non-unity Lewis numbers, where the mean flame radius (〈r〉) is the length scale, and the thermal diffusivity (α = SLδL) is the transport property. The turbulent burning velocity (ST,c=0.5) at the mean progress variable (〈c〉) of 0.5 decreases with the pressure at a constant ReT,flow, following the similar pressure dependence of the laminar burning velocity (SL). The ST,c=0.5/SL of the fuel-rich n-decane/air flame (ϕ = 1.4) is twice larger than that of the fuel-lean mixture (ϕ = 0.8), which would be attributed to the promotion of local burning rates by coupling between the differential diffusion and the turbulent flame stretch on the local flame fronts. In addition, five useful modified correlations for the turbulent burning velocity with the experimental Lewis number (LeBM) consideration were obtained based on the present experimental data of n-decane/air turbulent expanding flames. Particularly, the successful scaling of the modified correlation based on Damköhler's second hypothesis, ST,c=0.5/SL = 1 + 0.115(ReT,flowLeBM−2)0.5, suggesting that the ST,c=0.5/SL is inversely proportional to the LeBM. The importance of using experimental determined values of the Lewis number in the scaling of turbulent burning velocity with the differential diffusion consideration should be emphasized.

Turbulent burning velocity of hydrogen/n-heptane/air propagating spherical flames: Effects of hydrogen content

This study investigated the flame propagation and turbulent burning velocity of hydrogen/n-heptane/air mixtures subjected to differential diffusion and flame stretch induced by turbulence at different hydrogen molar fractions (XH2 = 0–100%), equivalence ratios (ϕ = 0.5–1.0) and turbulence intensities (u’ = 0–3.62 m/s). The experiments were carried out on a medium-scale, fan-stirred cylindrical combustion chamber generating near-isotropic turbulence. Results show that as hydrogen content increases, the turbulent flame propagation speed tends to be affected by stronger differential diffusion and weakened turbulence, characterized by decreasing Lewis number (Le) and Karlovitz number (Ka) within a similar range of turbulence intensity. Among them, the effect of differential diffusion on the normalized turbulent propagation speed (d<r>/dtσSL) appears to be well correlated with ReT,flameLe-2, where ReT,flame represents the turbulent flame Reynolds number. However, inconsistencies in the fitting parameters for this scale are observed at different XH2, highlighting the impact of substantial variations in flame stretch induced by turbulence. Therefore, the normalized turbulent flame propagation velocity under varying hydrogen contents is proposed to be accurately predicted by d<r>/dtσSL-1 = 0.07Ka0.32(ReT,flameLe−2)0.5. The synergistic effects of Le and Ka can also account for the peak phenomenon in the normalized turbulent burning velocity (ST/SL) observed at 85% hydrogen content. Furthermore, the quantitative characterization of turbulence and differential diffusion under extensive experimental data (Ka = 0.03–90) is studied. As evidenced by the Damköhler theoretical hypothesis, the two-stage trend between ST/SL and (u’/SL)Le-1 is identified with a transition near unity Ka. The modified correlations for turbulent burning velocity in terms of (u’/SL)Le-1 and Ka0.2 are obtained with consideration of the differential diffusion and flame stretch rate induced by turbulence, which show good predictive performance for data at different turbulence regime diagrams regardless of fuel type and equipment size.

Effect of Turbulence and Diffusional–Thermal Instability on Hydrogen‐Rich Syngas Turbulent Premixed Flame

As a kind of fuel with low‐carbon emissions, hydrogen‐rich syngas has a large resource potential and is a promising alternative energy resource. A series of experiments in a constant volume combustion bomb are carried out to investigate the effects of turbulence and diffusional–thermal (DT) instability on a hydrogen‐rich syngas turbulent premixed flame. The flow field in the constant volume combustion bomb is calibrated, and the turbulent flow field's homogeneity and isotropy were investigated. The effects of different speeds of fan (1366–4273 rpm), hydrogen fractions (55%–95%), pressures (1.0–3.0 bar), and equivalence ratios (0.6–1.0) on the hydrogen‐rich syngas turbulent premixed flame are studied. According to the analyses, the flow field in the experimental device exhibits good homogeneous and isotropic characteristics. With the increase of turbulence intensity, equivalence ratio, hydrogen fraction, or pressure, the turbulent flame propagation speed increases gradually. As the hydrogen fraction increases or the equivalence ratio decreases, the effective Lewis number decreases, and the effect of DT instability on the flame increases. The research in this article is crucial for the clean and efficient utilization of hydrogen‐rich syngas and the design of related burners.

Flame Characteristics and Abnormal Combustion of Methane Port Injection and Isooctane Direct Injection with Injection Timings Considered

Natural gas (NG) is a low-carbon fuel that can achieve low carbon emissions and high efficiencies. However, the low mean molecular mass and low energy density of methane result in low engine performance, and blending methane with gasoline is an effective way of improving the performance of NG engines. In this study, flame characteristics (including abnormal combustion) and engine performance are optically studied under methane port injection and isooctane direct injection conditions. The results indicate that the addition of isooctane can significantly increase the power output of NG engines. Under partial-load conditions, the minimum ignition advance for best IMEP is delayed with the addition of isooctane because of the higher flame speed of isooctane, and isooctane addition primarily improves the turbulent flame propagation rather than the initial flame formation. As for the isooctane injection timing, the indicated mean effective pressure (IMEP) first increases and then decreases with the delay in isooctane injection. The hot spots in the flame images confirm that late injection results in isooctane inhomogeneity, which reduces the engine’s thermal efficiency. At high loads, the low energy density of methane can lower the knocking intensity, even in the presence of auto-ignition at a speed of 120.7 m/s. The knocking pressure can lead to the ejection of oil droplets, which can increase the flame speed in the subsequent cycle.

Spray Flame Characterization of a Dual Injector for Compact Combustion Systems

ABSTRACT The scalability of liquid fuel operated jet stabilized combustion system toward scales and mixing timescales relevant for corresponding micro gas turbine systems is impeded by the lack of suitable injection concepts. The high momentum of the combustion air jet can deteriorate the spray quality of conventional injection systems. In combustors with reduced mixing timescales (e.g. MGT and compact aero-engines) this results in elevated emissions and a prompt primary atomization is essential. For this purpose, a canonical confined jet spray burner is developed and equipped with a novel in-house dual-pressure swirl/airblast injection concept. An extensive parameter variation on the jet bulk velocity (80 , equivalence ratio (0.6 , combustion air preheat temperature (500 (K) 800) and premixing length (0.0 (mm) ≤ 48) is conducted to delineate the operational range. The spray process originating a partially premixed and a direct injection case is characterized by means of phase Doppler interferometry and shadowgraphy. The combustion process is described via OH*-chemiluminescence, flame photographs and exhaust gas measurements. The results show that the developed injection system yields Sauter mean diameters predominantly below 25 μm over the entire parameter range due to robust and prompt primary atomization. Moreover, the primary atomization process of the dual injector is sufficient under all conditions to confine secondary atomization close-to the combustor nozzle exit. The flame stabilization and reaction progress are more sensitive to the air preheat temperature than the bulk jet velocity variation as the mixture homogenization is limited by the vaporization process. Moreover, with decreasing reactivity (i.e. equivalence ratio and preheat temperature) the flame stabilization is increasingly dominated by the recirculated hot exhaust gas, indicating a transition from conventional turbulent flame propagation to auto-ignition influenced reaction progress. The resulting NOx and CO emissions approximate the values measured previously in related methane powered combustion systems. Consequently, the developed injection system is promising to expand the flexibility of compact gas turbines to liquid fuels while maintaining low emissions.

On accelerative propagation of premixed hydrogen/air laminar and turbulent expanding flames

The accelerative propagation of laminar and turbulent premixed hydrogen/air flames are investigated using a constant volume combustion chamber. The relative flame accelerative characteristics are analyzed under different φ, P, and u’. The results show that three distinct stages, namely quasi steady, transition acceleration and saturation acceleration stages are distinguished in laminar flame propagation process, and the onsets of saturation acceleration and transition acceleration nearly satisfy the relationship of Pecr2/Pecr1≈2. The temporal acceleration exponents (αR) of laminar flames increase quickly in transition acceleration stage and nearly remain as a constant (1.1–1.25) in saturation acceleration stage. The acceleration exponents (α) of laminar flames are weakly dependent on flame conditions once the cellular structures are settled. The turbulent flame propagation follows a continuous acceleration law as SF/SL,b∼Pedt and no evident transition point is observed. The temporal acceleration exponents of turbulent flames continuously increase as flame propagates outwardly and are susceptible to turbulence intensity. The acceleration parameters (dt) of turbulent flames continuously increase with u’/SL in different flame regimes and the growth trend becomes slow in corrugated flamelets regime. Besides, the dt of turbulent flames are promoted in the fuel-lean conditions. All of these indicate that the accelerative propagations of turbulent H2/air flames are dominated by turbulent stretch and the diffusive-thermal instability still plays a promotion role.

Transition from Fast-Flame to Detonation

Flame acceleration within a channel is facilitated by repeating obstacles that generate turbulence. As the flame accelerates to supersonic velocities, it generates compression waves that coalesce to form a strong leading shock. This shock-flame complex continues to accelerate until it reaches a maximum velocity equal to the speed of sound in the combustion products; the complex is referred to as a fast-flame. Shock reflection driven onset of detonation may occur, characterized by an instantaneous jump in velocity to the theoretical ‘Chapman-Jouguet’ velocity. A more fundamental problem is the transition from a fast-flame to detonation without the presence of obstacles. &#x0D; In the current study, a 7.6 cm square ‘optical section’ housing a perforated plate was used to study the transition from fast-flame to detonation. A spark- and glow- plug served as ignition sources. The optical section had windows installed on its side and end walls so that Schlieren- and direct- photography could be captured. Soot foils were also utilized to record detonation cell structure. The fast-flame was generated by the passage of a detonation wave through the perforated plate, after which the transition to detonation could be studied. &#x0D; Gaseous fuel-oxygen mixtures of varying degrees of detonation stability over a wide range of pressures were tested. In low-pressure tests, delayed onset of detonation was observed, and detonation initiation sites were located at the channel walls indicating that flame-generated turbulence alone is not sufficient for the transition process. As pressure was increased, onset occurred immediately after the perforated plate at various locations across the channel cross-section, not strictly at the walls. For this abrupt ignition mode, transverse shock collisions played an important role in the transition to detonation. &#x0D;

Direct numerical simulations of methane, ammonia-hydrogen and hydrogen turbulent premixed flames

Three turbulent premixed flames with the same unstretched laminar flame speeds and thicknesses are analyzed and compared using 3D Direct Numerical Simulation (DNS) in a slot burner configuration at atmospheric conditions: a CH4/air flame at ϕ=1, an NH3−H2/air (46% vol. H2) at ϕ=1 and an H2/air flame at ϕ=0.45.While both stoichiometric methane and ammonia-hydrogen flames behave similarly, the lean hydrogen flame brush is twice as short with less flame surface, and exhibits significant alteration of its local flame structure: for the H2 flame, the thickness of flamelets decreases significantly while burning rates increase drastically. This is observed for flame elements convexly curved with respect to fresh reactants (positive curvature) because of preferential diffusion, where thermo-diffusive instabilities generate long-tail structures that continuously head toward the fresh gases, and also for near-flat flame elements because of local strain effects. Opposite behaviors are observed for flame elements concavely curved (negative curvature) where fuel depletion caused by unfocusing of hydrogen and strain effects are unable to increase the consumption, even leading to near-extinction of the flame. The turbulent methane and ammonia-hydrogen flames studied in this work do not exhibit the instabilities seen in the pure hydrogen flame. However, local flame-flame interactions are observed in the cusp regions of all three flames, which significantly increase the displacement speed as well as flame surface destruction.A 1D-3D comparison indicates that strain effects in regions of low curvature and preferential diffusion effects in regions of positive curvature can be modeled using counterflow flamelets to account for stretch effects on the turbulent hydrogen flame propagation in addition to turbulent wrinkling.

Structure and propagation of spherical turbulent iron-methane hybrid flame at elevated pressure

In this communication we demonstrate the role of turbulence intensity in the dual-front structure and self-similar propagation of spherical turbulent iron-methane hybrid flames. We first show that iron-methane hybrid mixture, whose iron concentration is below a critical threshold for the formation of a dust flame front in laminar or weak turbulent environment, can be burned strongly with both separated dual-front and merged single-front structures in intense turbulence. It is suggested that the formation of iron flame front would be attributed to local iron concentration accumulation by preferential sampling with near-unity Stocks number (St), heat transfer enhancement of iron particles to fluid and mixing promotion of iron particles with oxidants by strong turbulence. The propagation of iron front falls behind the methane front in the leading segments which is promoted by flame stretch for sub-unity Lewis number (Le), thus the separated dual-front structure occurs. Furthermore, the strong self-similar propagation of spherical turbulent iron-methane hybrid flame was observed under different turbulence intensities (urms). Mechanistically, such strong self-similar propagation of the hybrid flame is the consequence of the couple effects of flame mode transition at high urms with near-unity St and differential diffusion for sub-unity Le.