Markstein Length Research Articles

Revisiting effective Lewis number of combustible mixtures

Lewis number (Le) is of importance in the field of premixed combustion. In this study, the effective Lewis numbers of combustible mixtures containing a single fuel (CH4/air, C3H8/air, H2/air, C2H4/O2/N2), two fuels (CH4/C2H4/air, H2/NH3/air, H2/CH4/N2O) and two oxidizers (CH4/N2O/O2) over a wide range of equivalence ratios were evaluated via various methodologies. The Markstein length was obtained from previous studies, or extracted experimentally from the outwardly propagating flames. Firstly, the mass diffusivity and Lewis number of a reactant in mixture were evaluated using different models. Secondly, the effective Lewis number (Leeff) was calculated based on various theories, which was compared with the experimental ones. The results indicate that depending on the components, the heat-release, volumetric and diffusion models can be optimal for the depiction of Leeff. For the studied mixtures, the measured Le based on λ(x)=x (Bechtold & Matalon, Combust. Flame, 2001, 127(1–2): 1906–1913) was found to be more universal, where λ is the non-dimensional thermal conductivity. Additionally, the experimental Lewis numbers derived from different models can be very close, and the key parameters were compared and analyzed.

Impact of stretch on the flame dynamics of laminar premixed flames

Flame transfer functions (FTF) of laminar premixed flames acoustically forced are analytically investigated and modelled. The study is based on the linearized G-equation, which is used to kinematically track the flame front. In order to incorporate combustion properties, the laminar consumption speed is considered varying depending on the flame front stretch. Once written in dimensionless form, the G-equation reveals that the FTF depends on 3 dimensionless parameters: a Strouhal number (St2) that accounts for the convective time of the flow perturbation along the flame-front, the flame aspect-ratio (Lf/R) and the dimensionless Markstein length, adimensionalized by the injector radius. It is shown that the latter term is responsible for a flame-flow feedback that acts as damper or amplifier of the flame perturbation, respectively for thermodiffusively stable or unstable flames. A LOM FTF is derived both for Conical and V-flames, highlighting the impact of stretch for each configuration. Ultimately the obtained FTFs are compared to previously proposed analytical FTFs from the literature, underlining the importance of stretch at high frequency.

Analysis of methyl pentanoate/air mixtures spherically expanding flame intrinsic instabilities

Recently, methyl pentanoate (MPe) has been considered an alternative fuel or additive in gasoline and diesel. Therefore, before its adoption in combustion devices, its combustion characteristics have to be thoroughly understood. Intrinsic flame instability and cellularity are combustion characteristics that influence the performance of combustion devices. In this light, the intrinsic instability and cellularity of MPe/air flames were investigated at the initial pressures of 1, 2, and 4 bar, temperatures of 453 K and 483 K, and equivalence ratios of 0.7–1.4 in a constant volume combustion chamber to understand the various combustion dynamics of MPe. The instability and cellularity were examined using physicochemical characteristics such as the stretch rate, Markstein length, Lewis number, thermal expansion ratio, and flame thickness. There were slight differences in the Markstein length at the various pressures, but generally, according to the Markstein length, MPe flames were unstable at higher pressures. The Lewis number revealed that MPe flames were thermal-diffusively stable, whereas the flame thickness showed MPe flames were more unstable at 483 K and 4 bar. Also, the MPe premixed flame growth rate and stability curves were obtained using theoretical analysis. The stability curves showed that at high initial pressures and temperatures, MPe flame instability occurs early at a smaller flame radius. Moreover, the experimental and theoretical critical radii agreed well. The research results provide a concrete foundation for advancing the knowledge of methyl pentanoate combustion.

Experimental and numerical study on laminar premixed NH3/H2/O2/air flames

Adding the product of water electrolysis (i.e. 2:1 volume of H2 and O2) is an effective strategy to enhance the combustion intensity of NH3/air mixtures. In this work, the laminar burning velocity (LBV) of the obtained NH3/H2/O2/air mixtures was measured at 303 K, 0.1 MPa and compared with the values predicted by seven mechanisms. To improve the prediction performance, a new mechanism is developed based on the existing mechanism and adopted for numerical simulation. The results of this study show that the LBV of NH3 is significantly increased by additional H2 and O2. By comparison, it is found that H2 shows a more significant promoting effect on LBV when the volume ratio of additional H2 and O2 is 2. The concentration of key radicals and the flame temperature increase remarkably due to the addition of H2 and O2, which promote the flame propagation. Furthermore, the experimental results also indicated that the additional H2 and O2 make the burned gas Markstein length decrease on the lean side and increase on the rich side.

The reduction in laminar burning velocity and Markstein length extrapolation uncertainty for TRF-air mixtures

The laminar burning velocity and Markstein length of TRF (n-heptane, isooctane, and toluene)-air mixtures were investigated at the initial temperature of 400 K and 453 K, initial pressure of 1 bar, and an equivalence ratio range of 0.8–1.5. The effects of extrapolation radius (Rf) and Markstein length (Lb) were discussed on the linear and nonlinear extrapolation models. The results showed that the linear model leads to higher extrapolation result for all equivalence ratios compared with the nonlinear model due to the differences in the third term of Taylor expansion. Finally, the laminar burning velocity and Markstein length uncertainty caused by the extrapolation model can be minimized by adjusting LbRf1 (Rf1 refers to the flame initial extrapolated radius). When the value of LbRf1 was below 0.012, the laminar burning velocity and Markstein length uncertainty can be controlled at < 0.05 and < 0.5, respectively.

High-Pressure Laminar Flame Speeds and Markstein Lengths of Syngas Flames Diluted in Carbon Dioxide and Helium

Abstract New laminar flame speed and burned-gas Markstein length data for H2–CO–O2–CO2–He mixtures have been measured from spherically expanding flames. Experiments were conducted at 10 atm and room temperature for H2:CO ratios ranging from 2:1 to 1:4 and for overall CO2 mole fractions from 0% to 30%. CO2 dilution had little effect on Markstein length, but CO2 dilutions of 10%, 20%, and 30% caused average reductions in flame speed of 47%, 73%, and 89%, respectively, regardless of H2:CO ratio. The study was designed to isolate the dilution effect of CO2 on flame speed, and a detailed analysis using the FCO2 method was used to show that the chemical-kinetic participation of CO2 was responsible for up to 20% of the reduction in flame speed. Hence, the majority (80% or more) of the reduction in flame speed due to CO2 is from the thermal effect. Accurate flame speed predictions were produced by five different chemical kinetics mechanisms for most conditions, with the slight exception of high-CO, high-CO2 mixtures. A thorough sensitivity analysis highlighted the larger effect of CO2 dilution on the important kinetics reactions than the effect of changing H2:CO. Sensitivity analysis also showed that the chain branching reaction H2O + O ⇌ OH + OH could be modified (albeit beyond its uncertainty) to achieve more accurate flame speed predictions, but also indicated that further improvement of flame speed modeling would require changes to many lesser reactions.

Experimental and numerical study on the laminar flame characteristics for PODE3 and PODE3/iso-octane blends under elevated and sub-ambient initial pressures

• The laminar burning speeds for PODE 3 under different initial pressures are tested. • The laminar burning speeds for PODE 3 / iso -octane blends are investigated. • The accuracy of mechanism for PODE 3 and PRF/PODE n is validated. • The Markstein length and laminar flame instability are analyzed in detail. Polyoxymethylene dimethyl ether (PODE n ) is a promising alternative fuel for diesel due to its high oxygen contents (>40%) and high cetane number, thereby improving the thermal efficiency and reducing the pollutant emissions caused by the internal combustion engine. This study investigated the laminar burning speeds and Markstein length for PODE 3 and PODE 3 / iso -octane blends at wide equivalence ratios (0.7–1.6) and initial temperature of 408 K under elevated and sub-ambient initial pressures (0.5–4 bar) in a cylindrical constant-volume combustion vessel with high-speed Schlieren photography method combined with the nonlinear extrapolation model. The numerical simulation was also carried out to validate the accuracy of the mechanisms of PODE 3 and primary reference fuel (PRF)/PODE n blends and analyze the reaction sensitivities and the mole fraction profiles of key species. In addition, flame instability was discussed experimentally and theoretically. The results show that the laminar burning speed for PODE 3 decreases with the initial pressure, and the laminar burning speed for PODE 3 / iso -octane basically accords with the linear relationship to the PODE 3 volume fractions. The findings indicate that the kinetic effect has a dominant impact on the laminar burning speed for PODE 3 when compared with the thermal effect. The results also reveal that the Markstein length is negative only at an equivalence ratio between 1.5 and 1.6 under P = 2 bar, indicating that PODE 3 has more enhanced flame stability. Furthermore, the flame instability decreases with the increase of PODE 3 volume fractions, increases with the increase of initial pressure, and increases with the increase of the equivalence ratio. The differences between experimental results and theoretical results including the Markstein length, the critical radius and the critical Peclet number might be explained by the stretch rate results.

Influence of pressure and dilution gas on the explosion behavior of methane-oxygen mixtures

This study investigates the influence of initial pressure (P0) and dilution gas (N2, CO2, He, or Ar) on the explosion behavior of CH4-O2 mixtures. Spherical premixed flames in a 20 L spherical vessel are recorded by high-speed schlieren photography. The explosion parameters, explosion duration (τe), maximum explosion pressure (Pmax), maximum explosion pressure change rate ((dP/dt)max), flame speed, burning velocity, and Markstein length, are experimentally investigated at various pressures up to 250 kPa and at room temperature (280 ± 3 K). The experimental results show that Pmax and (dP/dt)max increase linearly with P0. The influence on the explosion intensities, (dP/dt)max, and flame speed, from high to low with the dilution is in the order of CO2, N2, Ar, and He, which indicates that CO2 and N2 more significantly inhibit CH4-O2 explosion. The results suggest that CO2 not only has a strong endothermic ability and reacts with H, but also changes the active free radicals to molecules due to three-element collision. The experimental phenomena are explained and validated by the results from the Chemkin package using two detailed mechanisms, indicating that (dP/dt)max is closely related to the content of H, O, OH, and CH3. The cellular instability is analyzed by the flame thickness and Markstein length, and it becomes more obvious as the flame thickness and Markstein length decrease. The buoyancy instability, Rayleigh-Taylor instability, and a series of phenomena due to the dilution of CO2 are analyzed. The maximum explosion pressure and flame speed decrease significantly with the addition of CO2 into CH4-air mixtures, and CO2 affects the generation of flame. The buoyancy instability of CH4-air mixtures with CO2 addition is examined. The experimental results contribute to a deeper understanding of the explosion behavior of CH4-O2 mixtures and the dilution effect of other gases on it.

Elucidating the challenges in extracting ultra-slow flame speeds in a closed vessel—A CH[formula omitted]F[formula omitted] microgravity case study using optical and pressure-rise data

Refrigerants with a low global warming potential (GWP) possess mild flammability. Hence, a fundamental understanding of their combustion characteristics is required to assess their fire-hazardous potential. The laminar burning velocity is one fundamental property to describe these under safety evaluation aspects. However, measuring laminar burning velocities for slow-burning components, such as low-GWP alternatives, is challenging. This is due to influencing effects such as buoyant flame deformation, stretch, radiation, and confinement, especially at the flammability limits. In the present study, we investigated near-limit flames of the representative refrigerant difluoromethane (CH2F2) with nitrogen-enriched oxidizer mixtures to elucidate the potentials and limitations at ultra-slow flame speeds of two widely used flame speed measurement methods in the closed vessel configuration: the optical flame speed measurement in the early quasi-isobaric regime and the flame speed determination from the pressure-rise during the later phase of the isochoric combustion process. Experiments were performed under terrestrial- and microgravity, conducted in a specially designed setup suitable for drop tower applications, using both methods in parallel. Recommendations for the extraction of flame speed data are derived from the non-buoyant reference investigations under microgravity and transferred to ultra-slow flames at terrestrial gravity. Near-limit flames under microgravity were found to be significantly affected by stretch effects for the optical method so that flame speed extrapolations to unstretched conditions are prone to errors. Stretch also affects the pressure method’s lower evaluation limit, and errors can be estimated by combining flame speed data from the pressure method and Markstein lengths from optical experiments.

Turbulent flame speed and morphology of pure ammonia flames and blends with methane or hydrogen

Ammonia appears a promising hydrogen-energy carrier as well as a carbon-free fuel. However, there remain limited studies for ammonia combustion especially under turbulent conditions. To that end, using the spherically expanding flame configuration, the turbulent flame speeds of stoichiometric ammonia/air, ammonia/methane and ammonia/hydrogen were examined. The composition of blends studied are currently being investigated for gas turbine application and are evaluated at various turbulent intensities, covering different kinds of turbulent combustion regimes. Mie-scattering tomography was employed facilitating flame structure analysis. Results show that the flame propagation speed of ammonia/air increases exponentially with increasing hydrogen amount. It is less pronounced with increasing methane addition, analogous to the behavior displayed in the laminar regime. The turbulent to laminar flame speed ratio increases with turbulence intensity. However, smallest gains were observed at highest hydrogen content, presumably due to differences in the combustion regime, with the mixture located within the corrugated flamelet zone, with all other mixtures positioned within the thin reaction zone. A good correlation of the turbulent velocity based on the Karlovitz and Damköhler numbers is observable with the present dataset, as well as previous experimental measurements available in literature, suggesting that ammonia-based fuels may potentially be described following the usual turbulent combustion models. Flame morphology and stretch sensitivity analysis were conducted, revealing that flame curvature remains relatively similar for pure ammonia and ammonia-based mixtures. The wrinkling ratio is found to increase with both increasing ammonia fraction and turbulent intensity, in good agreement with measured increases in turbulent flame speed. On the other hand, in most cases, the flame stretch effect does not change significantly with increasing turbulence, whilst following a similar trend to that of the laminar Markstein length.

Analysis of the flame dynamics in methane/hydrogen fuel blends at elevated pressures

We investigate the Flame Transfer Function (FTF) of a lean-premixed, laminar slit flame numerically. Based on the reference case at atmospheric pressure, we investigate four different scenarios: (i) varying the hydrogen content in the fuel at constant equivalence ratio (ER) (resulting in an increase of the laminar flame speed); (ii) varying the hydrogen content in the fuel at varying ER (resulting in a constant laminar flame speed); (iii) varying the operating pressures from 1 to 5 bar (resulting in a decrease of the laminar flame speed); and (iv) combining a hydrogen-enriched flame at an elevated pressure of 3 bar (resulting in the same flame speed as the reference case). We identify in this case that the laminar flame speed and the flame thickness impact the FTF independently. We show that the low-pass behavior of the flame is shifted towards higher frequencies when the operating pressure increases, and demonstrate that wrinkles along the flame front preserve in contrast to the atmospheric operating pressure configurations. These results are in line with past studies, that relate the dampening of flame front wrinkling to a decreasing Markstein length. We therefore conclude that a decreasing flame thickness, due to increasing operating pressure, causes a decreasing Markstein length and therefore less pronounced dampening of flame front wrinkles.

Experimental and numerical study on laminar burning velocity and premixed combustion characteristics of NH3/C3H8/air mixtures

Propane (C3H8) is the major component of liquefied petroleum gas, the combustion of C3H8 and ammonia (NH3) should be concerned. However, there is a scarcity of experimental data on NH3/C3H8/air mixtures to validate the kinetic models, especially at elevated pressures. This work aims to provide some new laminar burning velocity data and investigate the premixed combustion characteristics of NH3/C3H8/air mixtures in detail. The measurement was performed at various equivalence ratios (ϕ = 0.6–1.4), ammonia ratios (α = 20 %, 50 %, 80 %), and initial pressures (P0 = 0.1–0.5 MPa). Besides, five kinetic models were compared against the experimental data, the optimal performing model was used for numerical simulation. The results show that when the mole fraction of C3H8 in fuels is 50 %, the peak laminar burning velocity is about 28.5 cm/s. And the peak value decreases to 19.0 cm/s when the initial pressure rises to 0.5 MPa. The burned gas Markstein length decreases with the increase of equivalence ratio mainly due to the remarkable effect of the effective Lewis number, and it also decreases with the increase of initial pressure because of the decreasing flame thickness. C3H8 promotes the combustion intensity of NH3, which is reflected in the enhancement of flame temperature and radical pool, but C3H8 has no obvious effect on the mixture heating value of the NH3/C3H8/air mixtures. NH3 substituting can effectively reduce CO and CO2 emissions, and the reduction effect is more significant under high NH3 mole fraction conditions.

Effects of Water Vapor Dilution on the Laminar Burning Velocity and Markstein Length of Ammonia/Water Vapor/Air Premixed Laminar Flames

Effects of Water Vapor Dilution on the Laminar Burning Velocity and Markstein Length of Ammonia/Water Vapor/Air Premixed Laminar Flames

Characterizing methane and nitric oxide interaction in oxygen-free outwardly propagating spherical flame

Co-firing methane (CH4) and ammonia (NH3) has attracted growing concerns as a feasible greenhouse gas reduction strategy in gas turbine-based power generation, which raises the need to better understand the interaction of methane and nitric oxide (NO) under flame conditions. In this work, laminar flame propagation of CH4/NO mixtures at initial pressure (Pu) of 1 atm, initial temperature (Tu) of 298 K and equivalence ratios of 0.4–1.8 was experimentally investigated using a constant-volume combustion vessel. Laminar burning velocities (LBVs) and Markstein lengths were experimentally determined. A kinetic model of CH4/NO combustion was developed with rate constants of several important reactions updated, presenting reasonable predictions on the measured LBVs of CH4/NO mixtures. The modeling analyses reveal that the reduction of NO can proceed through two mechanisms, i.e. the hydrocarbon NO reduction mechanism and non-hydrocarbon NO reduction mechanism. Among the two mechanisms, the non-hydrocarbon NO reduction mechanism which includes reactions NO+H = N+OH, NO+O = N + O2 and NO+N = N2+O has a higher contribution to NO reduction at the equivalence ratio of 0.6, while the hydrocarbon NO reduction mechanism with hydrocyanic acid (HCN) as the key intermediate plays a more important role at the equivalence ratio of 1.8. NO+H = N+OH and CH3+NOHCN+H2O are found to be the two most sensitive reactions to promote the flame propagation, while the LBVs measured in this work are demonstrated to provide strong constraint for these reactions. Furthermore, previous CH4/O2/NO oxidation data measured in flow reactor and rapid compression machine were also simulated, which provides extended validation of the present model over wider conditions.

Impacts of preferential vaporization on flashback behaviors of multi-component liquid fuels

Liquid transportation fuels are composed of a wide range of molecular structures and weights, therefore exhibiting a relatively large distillation temperature range. When fuel chemical properties change along with the distillation temperature curve, preferential vaporization effects could play a role in near-limit combustion behaviors. The objective of this study is to experimentally evaluate the role of preferential vaporization on flame flashback behaviors. A unique spray burner is developed to control the extent of fuel spray vaporization by adjusting flow rates and/or the spray injection location from the burner exit. Spray characteristics are comprehensively determined using Phase Doppler Particle Analyzer. Two binary component mixtures are formulated (n-octane/iso-cetane and iso-octane/n-hexadecane) to exhibit common combustion behaviors in the fully vaporized condition but have considerably different preferential vaporization characteristics. Identical flashback behaviors of two mixtures are observed for fully pre-vaporized conditions by setting the burner temperature at 700 K, including both propagation- and ignition-driven flashback behaviors. Partially vaporized conditions are investigated at two global equivalence ratios (1.0 and 1.4) by setting the burner temperature at 450 K. The flashback behaviors for both global equivalence ratio conditions are found to be affected by the preferential vaporization characteristics represented by laminar flame speeds of the vaporized fuel mixture composition. The relative significance of local flow perturbation induced by instantaneous fuel droplet evaporation near the flame surface has been also investigated by analyzing planar laser-induced fluorescence images, as well as considering the changes of Markstein length with the extent of fuel vaporization. Finally, the relative contributions of local laminar flame speed representing local fuel vapor deposit, local flow perturbation, and preferential vaporization are evaluated through feature sensitivity analyses.

Effect of ethanol addition on the laminar burning velocities of gasoline surrogates

Laminar burning velocities (LBV) and Markstein lengths of spherical flames of n-heptane (H), iso-octane (O), and ethanol (E) and their binary and ternary mixtures (in equal volume fractions) at elevated pressures and temperatures are experimentally investigated. Measurements are performed in a constant volume combustion vessel using two distinct techniques: flame front imaging and pressure rise rate. For pure components, ethanol shows a higher LBV compared to n-heptane and iso-octane at same conditions. A pressure dependence on LBV of each pure component is observed where ethanol is found to have the strongest pressure dependence. This is also indicated by the higher negative coefficients in three-body termination reactions through a sensitivity analysis. For binary mixtures, the EH mixture was found to have the highest LBVs compared to other blends. The pressure dependence on LBV of ethanol is also present in binary mixtures where at a higher pressures the EO blend is found to have a lower LBV compared to the OH blend due to competing reactions against the H radical pool. Non-linear behaviour of the EH mixture’s LBV was observed and verified through the chemical kinetics suggesting the widely used mixing rules for blends might need additional improvements, especially at higher temperatures. An equal volume EHO ternary mixture shows the LBV to be similar to the OH mixture at 380 K. At 450 K, the LBVs differ by 7% at atmospheric pressure, but due to ethanol’s pressure dependence, this difference decreases to zero as the pressure increases to 4 bar.

Development of laminar burning velocity measurement system in premixed flames with hydrogen-content syngas or strong oxidizer conditions in a slot burner

This study presents the experimental determination of laminar burning velocity for critical premixed flames with multiple fuels or strong oxidizers. Experiments were conducted with a slot burner under methane/air, methane/nitrous oxide, and syngas (CH4/CO/H2)/air premixed flames conditions with varying equivalence ratios from 0.8 to 1.4. With the flame surface area determined from the Schlieren measurement system and stretch effect corrected by Markstein length, unstretched laminar burning velocity can be garnered according to the conservation of mass. First, the experimental results of methane/air premixed flame velocities were validated by comparing with one-dimensional unstretched burning velocities through numerical simulations with GRI 3.0 mechanism. The experimental results in the equivalence ratio ranging from 0.85 to 1.2 demonstrated errors less than 3.8%. Then, the validated burning velocity measuring techniques were implemented under methane/nitrous oxide and syngas/air premixed flame conditions. Finally, appropriate chemical mechanisms, such as USM or UGM, can be validated for the numerical simulation of critical premixed flames via this proposed laminar burning velocity measuring technique.

A comparative study on the laminar C1–C4 n-alkane/NH3 premixed flame

This work aims to comparatively investigate the NH3 blending effects on the combustion of light alkanes, CH4/C2H6/C3H8,/nC4H10. The laminar burning velocity (Su0) and burned gas Marstein length of the stoichiometric alkane/NH3 blends were measured at 298 K, 0.1 MPa using expanding spherical flame and compared with the six mechanisms. Detailed kinetic analyses were conducted using a newly developed mechanism. The results show that the Su0 of each alkane exhibits a distinctive response to NH3 introduction, which amplified the difference of Su0 for the binary fuels. The C2H6/NH3 blend exhibits the maximum Su0 at low ammonia fraction but exceeds by nC4H10/NH3 at intermediate and high ammonia fraction. The Su0 reduction caused by NH3 blending is dominated by the scavenging effect on the OH and H radicals and less affected by the C-N interactions under all studied conditions. Consequently, the Su0 of larger alkanes are less influenced by the NH3 blending because of their diverse production sources of OH and H radicals. Due to the competitive effect of increased mixture Zel’dovich number and decreased Lewis number, burned gas Markstein length of the binary fuels tend to increase firstly and then decrease with increased NH3 fractions. In addition, ammonia blending energy fraction may be a more proper criterion in practical engineering usage compared with mole fraction, since the energy fraction of NH3 governs the relative reduction of Su0 and CO2 emissions independent of the alkane types, and these parameters are the major concerns in NH3 blending combustion.

Experimental and chemical kinetic study on the laminar flame characteristics of the blends of n-propanol and isooctane at elevated temperature and pressure

Laminar flame characteristics of n-propanol-isooctane blends were investigated at the initial temperature of 433 K, two pressures of 0.1 and 0.5 MPa, different equivalence ratios of 0.7–1.6 and various fuel blending ratios in a constant volume bomb. Laminar flame speeds and flame instability parameters were determined, and the effect of initial conditions were evaluated. It was found that laminar flame speeds of n-propanol decrease with the pressure increases, and the addition of n-propanol into isooctane results in the rise of laminar flame speed. To interpret the flame speed variation, the thermal and diffusion properties were discussed by determining the adiabatic flame temperature and Markstein length. However, the chemical kinetics is concluded as the dominant factor since the variation of adiabatic flame temperature and Markstein length exert different behaviors with laminar flame speed does. A high temperature chemical kinetic model was developed, and the model accuracy was validated with present data as well as several sets of data published previously. With this model, detailed sensitivity and reaction pathway analyses were carried out to aid the understanding of the inherent mechanism. It was found that the laminar flame speed is mainly sensitive to the small molecule reactions. The addition of n-propanol into isooctane induces limited effect on the reaction pathways of each fuel. Finally, the flame thickness and density ratio were presented with the hydrodynamic instability evaluated. Combining the schlieren pictures and the flame instability parameters, it was concluded that the thermal-diffusional mechanism dominates the flame instability variation with the addition of n-propanol.

Effect of hydrogen addition on the laminar burning velocity of n-decane/air mixtures: Experimental and numerical study

Hydrogen is a potential aviation alternative fuel. To explore the influence of hydrogen addition on the combustion of aviation kerosene, the spherical expanding flame method was used to measure the laminar burning velocity and Markstein length of n-decane/hydrogen/air mixtures at 1bar, 2bar and 470 K using experimental and numerical analysis. The experimental LBVs agreed well with the simulation at an initial pressure of 1 bar, but lower than the simulation at 2bar. The laminar burning velocity showed a linear relationship with the hydrogen addition ratio (RH) at the different effective fuel-air equivalence ratios (ϕF). The Markstein length decreased with increased RH and initial pressure, hence, increasing the susceptibility of flame front instability. Based on a one-step overall reaction assumption, sensitivity analysis of the premixture mechanism showed that the kinetic effect greatly influenced n-decane/hydrogen/air mixture combustion. Further kinetic analysis indicated that the maximum (H + OH) mole fraction of the chemical reaction is highly responsible for the linear relationship between laminar burning velocity and RH.