140 results found
Tian L, Lindstedt RP, 2023, On the impact of differential diffusion between soot and gas phase species in turbulent flames, Combustion and Flame, Vol: 251, ISSN: 0010-2180
The molecular diffusivities of larger PAHs and soot particles approach zero leading to differential diffusion with gas-phase species. The present work systematically quantifies the impact on soot moments, soot related statistical correlations and Particle Size Distributions (PSDs) using a fully coupled transported joint probability density function (JPDF) method featuring a 78-dimensional joint-scalar space, including enthalpy, gas phase species with the PSD discretised using 62 size classes via a mass and number density preserving sectional method. Differential diffusion of soot (DDS) is treated via a gradual decline of diffusivity among soot sections maintaining realisability and the expected exponential decay of variance. The solution of the flow field features a time-dependent second moment closure and an elliptic solver. The turbulent non-premixed Sandia C2H4 flame from the International Sooting Flame (ISF) data base was selected as a target along with the KAUST (C2H4/N2) variant of the same flame. Results show that reduced soot diffusion leads to a significant increase in the soot volume fraction RMS and that the correlation coefficient between soot volume fraction and temperature is further reduced in a particle-size-dependent manner. Similar observations are made for correlations between the soot volume fraction and the mass fractions of gas-phase species such as CO, OH, H and C2H2. The results suggest that computational methods that presume explicit (e.g. flame structure related) correlations between such scalars and with soot face leading order modeling challenges. It is also shown that the correlation between CO and soot increases due to oxidation of soot and that DDS leads to a modest downstream shift of PSDs towards larger particles.
Tian L, Boyette WR, Lindstedt RP, et al., 2023, Transported JPDF modelling and measurements of soot at elevated pressures, Proceedings of the Combustion Institute, Vol: 39, Pages: 2439-2447, ISSN: 1540-7489
Accurate measurements and modelling of soot formation in turbulent flames at elevated pressures form a crucial step towards design methods that can support the development of practical combustion devices. A mass and number density preserving sectional model is here combined with a transported joint-scalar probability density function (JDPF) method that enables a fully coupled scalar space of soot, gas-phase species and enthalpy. The approach is extended to the KAUST turbulent non-premixed ethylene-nitrogen flames at pressures from 1 to 5 bar via an updated global bimolecular (second order) nucleation step from acetylene to pyrene. The latter accounts for pressure-induced density effects with the rate fitted using comparisons with full detailed chemistry up to 20 bar pressure and with experimental data from a WSR/PFR configuration and laminar premixed flames. Soot surface growth is treated via a PAH analogy and soot oxidation is considered via O, OH and O2 using a Hertz-Knudsen approach. The impact of differential diffusion between soot and gas-phase particles is included by a gradual decline of diffusivity among soot sections. Comparisons with normalised experimental OH-PLIF and PAH-PLIF signals suggest good predictions of the evolution of the flame structure. Good agreement was also found for predicted soot volume statistics at all pressures. The importance of differential diffusion between soot and gas-phase species intensifies with pressure with the impact on PSDs more evident for larger particles which tend to be transported towards the fuel rich centreline leading to reduced soot oxidation.
Greenblatt D, Tian L, Lindstedt RP, 2023, The impact of hydrogen substitution by ammonia on low- and high-temperature combustion, Combustion and Flame, ISSN: 0010-2180
The combustion behaviour of ammonia has attracted intermittent interest with the original patent by Lyon (US3900554A) relating to its use for nitric oxide reduction through selective non-catalytic reduction (SNCR) a focal point. The recent interest in ammonia stems from its use as a hydrogen rich energy carrier with practical use requiring a much wider parameter space. The corresponding challenges (e.g. Kobayashi et al., Proc. Combust. Inst. 37 (2019) 109–133) include different flame dynamics and high emissions of oxides of nitrogen. The current paper explores the complex nature of ammonia oxidation and provides a reduced size reaction mechanism that enables application, without approximation, to the computation of turbulent flames through a joint-scalar transported probability density function (JPDF) method. Comprehensive validation suggests similar accuracy to a reference mechanism (Glarborg et al., Prog. Energy Combust. Sci. 67 (2018) 31–68) and highlights some uncertainties. The selected turbulent flame configuration features auto-ignition stabilised flames supported by a coflow of hot combustion products. The base case features a H2/N2 fuel jet that permits flame stabilisation at 1045 K corresponding to the onset of the SNCR temperature window. The impact of a gradual substitution of hydrogen by ammonia on flame stabilisation, emissions of oxides of nitrogen and the flame structure is quantified. It is shown that ammonia substitution leads to more prevalent local extinction, a more distributed flame structure and requires substantially increased coflow temperatures to achieve a similar flame stabilisation point. A lowering of the coflow temperature to operate within the SNCR regime substantially reduces NOx and leads towards a homogeneous/distributed reaction mode. The reduced fuel reactivity highlights the importance of turbulence-chemistry interactions leading to complexities in the design of practical devices.
Tian L, Lindstedt RP, 2023, The impact of ammonia addition on soot formation in ethylene flames, Combustion and Flame, ISSN: 0010-2180
The present work investigates computationally the impact of blending ammonia with ethylene on soot formation in four laminar diffusion flames and the corresponding turbulent partially premixed flames. The conditions conform to experimental investigations (Bennett et al., Combust. Flame, 2020, and Boyette et al., Combust. Flame, 2021) with both sets of flames featuring identical fuel blends. A mass and number density preserving sectional method was applied to all cases with the soot surface growth and oxidation models updated to include reactions with oxides of nitrogen. The gas phase chemistry was extended to include a comprehensively validated ammonia sub-mechanism. For the laminar flames, the inception of soot particles is based on detailed chemistry with pyrene treated as representative of the PAH pool. The accuracy of fitting a global acetylene-based inception step is evaluated for the different fuel blends and subsequently applied to the turbulent flames using a fully coupled transported joint probability density function method featuring an 84-dimensional joint-scalar space. Results obtained for laminar flames show that detailed chemistry coupled with the sectional soot model provides excellent agreement with the measured suppression of the soot volume fraction with increased use of ammonia. Computed particle size distributions (PSDs) show an increase in smaller soot particles under such conditions, consistent with experimental observations. The experimentally observed reduced impact in turbulent flames is also reproduced computationally. The suppression of soot is principally caused by changes in the radical pool leading to reduced soot surface growth and, to a lesser extent, soot inception. The contribution of oxides of nitrogen to soot oxidation is modest. Computed PSDs in laminar and turbulent flames highlight the importance of differences in flame structures and flowfield timescales.
Shariatmadar H, Lindstedt RP, 2023, Soot particle size distributions in turbulent opposed jet flames with premixed propene–air reactants, Proceedings of the Combustion Institute, Vol: 39, Pages: 1089-1097, ISSN: 1540-7489
Emissions of soot are strongly dependent on turbulence-chemistry interactions due to the relatively slow formation and oxidation processes. Studies of laminar flames have shown that both flow conditions and the chemistry of the parent fuel have a significant impact on measured particle size distributions (PSDs). The current study determines the impact of flow on the development of PSDs in premixed turbulent flames through a variation in the total rate of strain using an opposed jet configuration with fractal grid generated turbulence. The impact of fuel chemistry is investigated under such conditions through the use of propene–air flames with results compared to the corresponding ethene–air flames quantified in earlier studies. Samples were extracted using a quartz probe featuring aerodynamic quenching and dual port dilution at the probe tip and in the transfer-line. Spatially resolved PSD data is obtained along the stagnation point streamline using a scanning mobility particle sizer equipped with nano- and long-DMA columns to show the evolution through the turbulent flame brush. Results confirm that PSDs of soot in premixed turbulent flames are exceptionally sensitive to both the chemistry of the parent fuel and the flow field. The reduced residence times in the current turbulent flows lead to maximum median and mean mobility diameters below 10 nm with higher rates of strain promoting unimodal PSD shapes. It is further shown that the chemistry of the parent fuel has a strong influence on PSDs with propene causing a two order of magnitude increase in smaller particles compared to the corresponding ethene flame.
Shariatmadar S, Aleiferis P, Lindstedt R, 2022, Particle size distributions in turbulent premixed ethylene flames crossing the soot inception limit, Combustion and Flame, Vol: 243, Pages: 1-20, ISSN: 0010-2180
The current study presents novel experimental data on soot particle size distributions (PSDs) with mobility diameters in the size range 4 ≤ Dm [nm] ≤ 230 obtained from four premixed turbulent ethyleneair flames crossing the soot inception limit. The flames are stabilised against hot combustion productsfrom nitrogen diluted hydrogen flames in a back-to-burnt (or fresh-to-burnt) opposed jet configuration.The burner features fractal grid generated turbulence to provide accurate control of the turbulence. Ascanning mobility particle sizer (SMPS) equipped with nano- and long–DMA columns coupled with adual dilution port quartz probe is used and a comprehensive analysis of optimal sampling conditionsto minimise particle losses is presented. Spatially resolved data along the stagnation point streamlineis obtained to show the evolution of PSDs through the turbulent flame brush. It is shown that turbulent transport distributes soot particles across the mixing layer between the two jets with the maximummedian and mean mobility diameters found close to the stagnation point. The impact of the estimatedtotal mean rate of strain (420 ≤ aT [s−1] ≤ 610) and equivalence ratio (1.8 ≤ φUN ≤ 2.2) on PSDs is alsoquantified.
Shariatmadar H, Hampp F, Lindstedt RP, 2022, The evolution of species concentrations in turbulent premixed flames crossing the soot inception limit, Combustion and Flame, Vol: 235, ISSN: 0010-2180
The current study quantifies the impact of equivalence ratio and rate of strain on the spatial distribution of major species and soot precursors in premixed turbulent ethylene flames crossing the soot inception limit. A back-to-burnt (BTB) opposed jet configuration is used to provide accurate control of the turbulent and chemical timescales. The upper nozzle features fractal grid generated turbulence and premixed ethylene/air mixtures with flames stabilised against well-defined hot combustion products emerging from the lower nozzle. The study combines simultaneous PAH/CHOPLIF and elastic light scattering (ELS) with probe sampling. Gas chromatography-mass spectrometry (GCMS) and gas chromatography-thermal conductivity detector (GC–TCD) were used to quantify concentrations of major species and poly-aromatic hydrocarbons (PAHs). The laser based diagnostics show that the rate of strain exerts a dominant impact on the growth of soot particles with low turbulent Reynolds numbers () used to promote soot formation. The probe measurements indicate that acetylene, 1-methylnaphthalene (1–MN), m/z = 154, m/z = 276 and benzo(a)pyrene correlate with soot formation and that the equivalence ratio is the controlling PAH growth parameter. It is further shown that the spatial extent of the PAH containing reaction zone exceeds three integral length scales.
Meyer MP, Lindstedt RP, 2022, Evaluation of Hazard Correlations for Hydrogen-Rich Fuels Using Stretched Transient Flames, Publisher: Springer Singapore
Simatos P, Tian L, Lindstedt RP, 2021, The impact of molecular diffusion on auto-ignition in a turbulent flow, Combustion and Flame, ISSN: 0010-2180
The inclusion of molecular diffusion into the joint-probability density function (JPDF) method requires terms for molecular transport in physical space and mixing in composition space. The former is typically neglected for high Reynolds number flows but becomes important in flows close to surfaces and for flamelet related reaction-diffusion structures associated with high Damköhler numbers. McDermott and Pope (J. Comp. Phys., 2007) proposed an explicit addition of a molecular transport term to the equation for sub-grid molecular diffusion for the filtered density function method. A related approach was used by Fiolitakis et al. (Combust. Flame, 2014) for the JPDF method when combined with moment based closures. A variant of the method is here combined with comprehensive C1−C2 chemistry featuring 44 chemical species and 256 reactions to study the impact on auto-ignition and flame stabilisation in the well-characterised Cabra burner. It is shown that the contribution of the molecular transport term becomes significant at the flame stabilisation point with predictions of carbon monoxide and, in particular, molecular hydrogen showing a marked improvement. Conditional statistics and the PDF of temperature show that the improvement is due to increased molecular diffusion on the fuel rich side of the flame leading to increased chemical activity. The developed approach is viable for boundary layers while the spatial resolution requirements in terms of the mean scalar gradients may prove limiting for complex flows.
du Preez M, Wandel AP, Bontch-Osmolovskaia D, et al., 2021, Parametric sensitivities of the generalized binomial Langevin-multiple mapping conditioning model, PHYSICS OF FLUIDS, Vol: 33, ISSN: 1070-6631
Kraus P, Lindstedt RP, 2021, It's a gas: oxidative dehydrogenation of propane over boron nitride catalysts, Journal of Physical Organic Chemistry, Vol: 125, Pages: 5623-5634, ISSN: 0894-3230
Boron nitride and related boron-containing materials have recently been suggested as very promising catalysts in the oxidative dehydrogenation of propane. The high selectivity toward propylene at comparably high conversion significantly exceeds the performance of established vanadium-based catalysts. In the current work we show that the high selectivity toward propylene and ethylene is fully consistent with a gas-phase conversion mechanism and that it can be modeled reasonably well by the recent detailed microkinetic reaction mechanism of Hashemi and co-workers [ Proc. Combust. Inst. 2019, 37, 461]. Our analysis, using six heterogeneous catalytic reaction pathways, each representing a hypothetical limit case, shows that the boron nitride catalyst is responsible for initiating the gas-phase chemistry. The increased conversion of propane in cases with water cofeed, as well as the trends in the selectivities of minor species upon dilution of the catalytic bed and upon varying the C3H8/O2 inlet ratio, as observed by Venegas and Hermans [ Org. Process Res. Dev. 2018, 22, 1644], are here explained as gas-phase phenomena. Hence, the oxidative dehydrogenation of propane over boron nitride catalysts is an example of a coupled gas and catalytic chemistry system. The current work also highlights the importance of modeling of the complete heated zone, including the rear heat shields and reactor padding if present.
Kraus P, Lindstedt R, 2021, It's a gas: Oxidative dehydrogenation of propane over boron nitride catalysts
Supplemental archive for the above paper. Updated version following first round of reviews at JPCC.
Shariatmadar H, Hampp F, Lindstedt RP, 2021, Quantification of PAH concentrations in premixed turbulent flames crossing the soot inception limit, Proceedings of the Combustion Institute, Vol: 38, Pages: 1163-1172, ISSN: 1540-7489
Soot formation in combustors is a complex process comprising highly intermittent interactions between physical and chemical processes across a wide range of time-scales and the influence of turbulence on the soot inception process remains substantially conjectural. The current study quantifies the impact of flame temperature, equivalence ratio, and strain rate on PAH concentrations in premixed turbulent ethylene flames crossing the soot inception limit using a back–to–burnt opposed jet configuration that provides accurate control of flow parameters. The upper nozzle features fractal grid generated turbulence and provides premixed ethylene/air mixtures with equivalence ratios 1.7 ≤ ϕ ≤ 2.2 with flames stabilised against well-defined lower nozzle hot combustion products (HCP) from N2 diluted H2 flames. The flame structures are initially analysed using PAH/CH2O/PLIF and elastic light scattering with gas chromatography–mass spectrometry (GC–MS) subsequently used to quantify PAH samples from inside the turbulent flame brush. A combination of hot nitrogen dilution and a heated sampling line is used to minimise PAH and particle losses during sample extraction. The work quantifies the growth in PAH concentrations across the soot inception limit and shows that the rate of strain exerts a dominant influence. It is further shown, through centrifuging of samples, that soot particles contain large amounts of PAHs (e.g. benzo[a]pyrene) with EDX used to quantify the growth in carbon deposition.
Tian L, Schiener MA, Lindstedt RP, 2021, Fully coupled sectional modelling of soot particle dynamics in a turbulent diffusion flame, Proceedings of the Combustion Institute, Vol: 38, Pages: 1365-1373, ISSN: 0082-0784
Soot particle dynamics, including particle size distributions (PSDs) and related statistics, are of increasing practical significance due to evolving regulatory demands. The combination of a mass and number density preserving sectional model with a transported joint probability density function (JPDF) method ensures a full coupling of the joint scalar space, e.g. soot and gas phase reactions and radiative heat losses, within a method that can represent ignition/extinction phenomena as well as the slow (low Damköhler number) soot inception and oxidation chemistry in turbulent flames. This approach is here applied to the sooting non-premixed Sandia ethylene jet flame via a 78-dimensional joint-scalar space, including enthalpy, gas phase species and 62 soot sections. Soot nucleation is treated as a global step from acetylene to pyrene with the rate fitted using comparisons with full detailed chemistry. Soot surface growth is treated via a PAH analogy and soot oxidation is considered via O, OH and O2 using a Hertz–Knudsen approach. Comparisons with measured temperature, gas phase species and the mean soot volume fraction show good agreement while the introduction of zero soot diffusivity leads to substantially improved predictions of the RMS of the soot volume fraction. The calculated PSDs at the burner centreline show a transition from one to two-peaks along the axial direction with the mode of the second peak increasing from 14 to 32 nm. Scatter plots, joint statistics of soot parameters and temperature, and the chemical source terms across soot sections suggest that surface growth is dominant when PSDs are unimodal and that the competition of oxidation, coagulation/aggregation and surface growth leads to a PSD shape transition. It is also shown that local extinction events lead to the presence of soot in cool fuel lean mixtures.
Hampp F, Goh KHH, Lindstedt RP, 2020, The reactivity of hydrogen enriched turbulent flames, Process Safety and Environmental Protection, Vol: 143, Pages: 66-75, ISSN: 0957-5820
The use of hydrogen enriched fuel blends, e.g. syngas, offers great potential in the decarbonisation ofgas turbine technologies by substitution and expansion of the lean operating limit. Studies assessingexplosion risks or laminar flame properties of such fuels are common. However, there is a lack of exper-imental data that quantifies the impact of hydrogen addition on turbulent flame parameters includingburning velocities and scalar fluxes. Such properties are here determined for aerodynamically stabilisedflames in a back-to-burnt opposed jet configuration featuring fractal grid generated multi-scale turbu-lence (Ret= 314 ± 19) using binary H2/CH4and H2/CO fuel blends. The binary H2/CH4fuel blend is variedfrom ̨ = XH2/(XH2+ XF) = 0.0, 0.2 and 0.4–1.0, in steps on 0.1, and the binary H2/CO fuel blend from ̨ = 0.3 − 1.0 also in steps of 0.1. The equivalence ratio is adjusted between the mixture specific lowerlimit of local flame extinction and the upper limit of flashback. The flames are characterised using PIVmeasurements combined with a flame front detection algorithm. The study quantifies the impact ofhydrogen enrichment on (i) turbulent burning velocity (ST), (ii) turbulent transport and (iii) the rate ofstrain acting on flame fronts. Scaling relations (iv) that correlate STwith laminar flame properties areevaluated and (v) flow field data that permits validation of computational models is provided. It is shownthat CH4results in a stronger inhibiting effect on the reaction chemistry of H2compared to CO, that tur-bulent transport and burning velocities are strongly correlated with the rate of compressive strain andthat scaling relationships can provide reasonable agreement with experiments.
Hampp F, Lindstedt RP, 2020, Quantification of fuel chemistry effects on burning modes in turbulent premixed flames, Combustion and Flame, Vol: 218, Pages: 134-149, ISSN: 0010-2180
The present work quantifies the impact of fuel chemistry on burning modes using premixed dimethyl ether (DME), ethanol (EtOH) and methane flames in a back-to-burnt opposed jet configuration. The study considers equivalence ratios 0 ≤ Φ ≤ 1, resulting in a Damköhler (Da) number range 0.06 ≤ Da ≤ 5.1. Multi-scale turbulence (Re ≃ 19,550 and Ret ≃ 360) is generated by means of a cross fractal grid and kept constant along with the enthalpy of the hot combustion products (THCP = 1700 K) of the counterflow stream. The mean turbulent rate of strain exceeds the laminar extinction rate for all flames. Simultaneous Mie scattering, OH-PLIF and PIV are used to identify reactants, mixing, weakly reacting, strongly reacting and product fluids. The relative balance between conventional flame propagation and auto-ignition based combustion is highlighted using suitably defined Da numbers and a more rapid transition towards self-sustained (e.g. flamelet type) combustion is observed for DME. The strain rate distribution on the reactant fluid surface for methane remains similar to the (non-reactive) mixing layer (), while DME and EtOH flames gradually detach from the stagnation plane with increasing Φ leading to stabilisation in regions with lower compressive rates of strain. The study further provides information on the conditions leading to burning mode transitions via (i) multi-fluid probabilities, (ii) structural flow field information and turbulence-flame interactions delineated by means of conditional (iii) velocity statistics and (iv) the rate of strain along fluid iso-contours.
Simatos P, Hampp F, Lindstedt RP, 2020, Auto-Ignition of Hydrogen-Rich Syngas-Related Fuels in a Turbulent Shear Layer, Green Energy and Technology, Pages: 333-356
The development of low carbon footprint and clean energy technologies is essential to resolve the concerns of climate change and diminishing fossil fuel resources. Hydrogen-enriched fuel blends offer a route to decarbonise existing technologies. The current work presents an experimental and numerical study of fuel reactivity changes by the gradual enhancement of methane or carbon monoxide/air mixtures with hydrogen on the auto-ignition in a turbulent shear layer formed between a fuel jet and a stream of hot combustion products. Flame stabilisation in such turbulent shear layers is a key consideration for the operation of gas turbines where the injected fuel interacts with (re-circulating) hot combustion products. The study covers a total of 34 fuel-lean premixed binary fuel blends over a wide range of H2/CH4 and H2/CO compositions. The lift-off height of a vitiated jet flame was used to quantify the impact of mixture reactivity on flame stability using chemiluminescence measurements. Selected experimental data were compared with 2D parabolic RANS calculations using a transported PDF approach closed at the joint-scalar level combined with detailed chemistry. The computations show good agreement with the experimental data over a wide range of conditions. The results consistently show a notable difference between blending H2 with CO or CH4. Comparatively small amounts of added CH4 cause a noticeable decline in mixture reactivity while a CO content of up to 50% shows only a modest impact. The data provide a consistent and unique database detailing the reactivity of hydrogen-rich syngas-related fuel blends in a turbulent flow.
Tian L, Lindstedt RP, 2019, Impact of molecular mixing and scalar dissipation rate closures on turbulent bluff-body flames with increasing local extinction, Combustion and Flame, Vol: 206, Pages: 51-67, ISSN: 0010-2180
The Combustion Institute Bluff-body turbulent CH 4 : H 2 (1:1) flames at 50% (HM1), 75% (HM2) and 91% (HM3) of the blow-off velocity (235 m s−1) were studied experimentally by Masri and co-workers and found to exhibit gradually increasing periodic and shear layer instabilities. The latter are coupled with increasing levels of local extinction with subsequent re-ignition further downstream. This study provides a systematic evaluation of the sensitivity of predictions to molecular mixing and scalar dissipation rate closures. The latter include extended forms of the Euclidean Minimum Spanning Tree (EMST) and modified Curl's (MC) models, applicable to premixed turbulent flames via a closure that accounts for local Damköhler number effects (EEMST and EMC), and a conceptually related blended scalar time-scale approach (BEMST and BMC). Computations are performed using a hybrid finite volume (FV) – transported Joint Probability Density Function (JPDF) algorithm featuring stochastic Lagrangian particles, a comprehensive 48-scalar systematically reduced C/H/N/O mechanism, and a second moment method based on the Generalised Langevin Model that provides a partial resolution of the unsteady fluid motion. The sensitivity to solution parameters affecting the temporal resolution is quantified using Fourier transforms of the time histories of velocity and scalar traces. Radial profiles, conditional means and scatter plots are compared to the experimental data along with burning indices based on the conditional mean temperature. Vortex related instabilities ∼ 1 kHz in the outer shear layer appear for all closures with EMC showing periodic local extinction and re-ignition in the neck region for HM3 and flame turbules (i.e., discrete pockets of hot gas) separating periodically at frequencies ∼ 85 Hz. Results are similar to well–resolved JPDF/LES simulations for HM1. It is shown that the EMC and (E)EMST models essentially enclose the experimental data for
Li T, Hampp F, Lindstedt RP, 2019, The impact of hydrogen enrichment on the flow field evolution in turbulent explosions, Combustion and Flame, Vol: 203, Pages: 105-119, ISSN: 0010-2180
The reactivity of fuel-air mixtures has a strong impact on the severity of obstacle-induced turbulent explosions. While the influence on the flame speed and pressure development has been studied for a wide range of mixtures at different physical scales, the experimental quantification of the resulting flow fields, e.g. in the critical recirculation zones behind obstacles, is typically absent. The lack of such data presents a serious impediment to the development of reliable predictive methods. The current study reports velocity measurements obtained from highly reproducible experiments performed in a flame tube with two staggered obstacles using fuel lean H 2 /CH 4 /Air and H 2 /CO/Air mixtures at a stoichiometry of 0.60. The mixture reactivity for both fuels was varied using H 2 substitution levels of 50% and 80% with the pure hydrogen (100%) case used as a reference. The flow field was quantified using high-speed (10 kHz) particle image velocimetry (PIV), time-series PIV and Mie scattering. The time-resolved evolution of the recirculation zone behind the second obstacle was successfully captured with the explosion over-pressure and flame propagation speed also measured. Data is presented for the mean horizontal (u¯) and vertical (v¯) velocity components at 18 spatial locations for each mixture along with the translational velocities of the shear driven recirculating eddies formed behind the second obstacle. It is shown that the temporal evolution of the flows (not velocity magnitudes) can be approximately normalised based on the flame arrival at the second obstacle. The data provides a comprehensive quantitative description of the flow field evolution leading up to explosion events and is expected to facilitate model development.
Hampp F, Shariatmadar S, Lindstedt RP, 2019, Quantification of low Damköhler number turbulent premixed flames, Proceedings of the Combustion Institute, Vol: 37, Pages: 2373-2381, ISSN: 1540-7489
Schiener MA, Lindstedt RP, 2019, Transported probability density function based modelling of soot particle size distributions in non-premixed turbulent jet flames, Proceedings of the Combustion Institute, Vol: 37, Pages: 1049-1056, ISSN: 1540-7489
Wandel AP, Lindstedt RP, 2019, A mixture-fraction-based hybrid binomial Langevin-multiple mapping conditioning model, Proceedings of the Combustion Institute, Vol: 37, Pages: 2151-2158, ISSN: 1540-7489
Tian L, Lindstedt RP, 2019, Evaluation of reaction progress variable - mixture fraction statistics in partially premixed flames, Proceedings of the Combustion Institute, Vol: 37, Pages: 2241-2248, ISSN: 1540-7489
Schiener MA, Lindstedt RP, 2018, Joint-scalar transported PDF modelling of soot in a turbulent non-premixed natural gas flame, Combustion Theory and Modelling, Vol: 22, Pages: 1134-1175, ISSN: 1364-7830
The focus of the present work is on the prediction of soot in the turbulent Delft III/Adelaide natural gas flame at a Reynolds number of 9700. A parabolic flow solver with the SSG (Speziale, Sarkar and Gatski) Reynolds stress transport model for turbulence is coupled to a joint-scalar transported PDF (probability density function) approach allowing exact treatment of the interactions of turbulence with the solid and gas phase chemistries. Scalar mixing is treated via the modified Curl's coalescence/dispersion model and two different closures for the scalar dissipation rate are explored. The gas phase chemistry is represented by a systematically reduced mechanism featuring 144 reactions, 15 solved and 14 steady-state species. The dynamics of soot particles, including coagulation and aggregation in the coalescent and fractal aggregate limits, is treated either via a simplified two-equation model or via the MOMIC (method of moments with interpolative closure). The inclusion of soot surface reactions based on a second ring PAH (polycyclic aromatic hydrocarbon) analogy is also investigated. Soot oxidation via O, OH and O(Formula presented.) is taken into account and the sensitivity to the applied rates is investigated. An updated acetylene-based soot nucleation rate is formulated based on consistency with detailed chemistry up to pyrene and combined with a sectional model to compute soot particle size distributions in the NIST well-stirred/plug flow reactor configuration. The derived rate is subsequently used in turbulent flame calculations and subjected to a sensitivity analysis. Radiative emission from soot and gas phase species is accounted for using the RADCAL method and by the inclusion of enthalpy as a solved scalar. Computed soot levels reproduce experimental data comparatively well and approximately match absolute values. The axial location of peak soot in the Delft III/Adelaide flame is consistent with previous large eddy simulations and possible causes for the
Li T, Hampp F, Lindstedt RP, 2018, Experimental study of turbulent explosions in hydrogen enriched syngas related fuels, Process Safety and Environmental Protection, Vol: 116, Pages: 663-676, ISSN: 1744-3598
The role of hydrogen enriched fuel streams has come to the fore due to the use of syngas and/or biogas related feedstocks in gas engine or gas turbine based power generation applications. The hydrogen addition can enhance the fuel reactivity significantly, leading to improved combustion stability and widened flammable limits, but also raises safety concerns related to accidental explosions. The current work presents a systematic study of turbulent deflagrations generated in an obstructed tube with explosion overpressures and flame speeds measured. The focus is on the use of lean and ultra-lean fuel blends using binary H 2 /CO, H 2 /CH 4 and ternary H 2 /CH 4 /CO mixtures. The H 2 levels were varied between 0% and 100% at stoichiometries of 0.80, 0.60 and 0.40. The results highlight significant differences in explosion behaviour between the two blending components, with CO mixtures providing substantially higher overpressures than the corresponding CH 4 blends. The results suggest that methane has a mitigating effect up to comparatively high hydrogen blending fractions and that synergistic effects between fuel components need to be taken into account. A new scaling parameter (β) is proposed that successfully linearises the peak explosion overpressure between different fuel blends in response to the hydrogen concentration. A scaling based on acoustic theory shows good agreement with experimental data and a simple method for estimating the overpressure change caused by variations in the mixture reactivity in a fixed geometry is also evaluated.
Hampp F, Lindstedt RP, 2018, Quantification of external enthalpy controlled combustion at unity damköhler number, Green Energy and Technology, Pages: 189-215
The use of external enthalpy support (e.g. via heat recirculation) can enable combustion beyond normal flammability limits and lead to significantly reduced emissions and fuel consumption. The present work quantifies the impact of such support on the combustion of lean (Φ = 0.6) turbulent premixed DME/air flames with a Damköhler number around unity. The flames were aerodynamically stabilised against thermally equilibrated hot combustion products (HCP) in a back-to-burnt opposed jet configuration featuring fractal grid generated multi-scale turbulence (Re ≃ 18,400 and Ret > 370). The bulk strain (ab = 750 s−1) was of the order of the extinction strain rate (aq = 600 s−1) of the corresponding laminar opposed twin flame with the mean turbulent strain (aI = 3200 s−1) significantly higher. The HCP temperature (1600 ≤ THCP (K) ≤ 1800) was varied from close to the extinction point (Tq ≃ 1570 K) of the corresponding laminar twin flame to beyond the unstrained adiabatic flame temperature (Tad ≃ 1750 K). The flames were charac-terised using simultaneous Mie scattering, OH-PLIF and PIV measurements and subjected to a multi-fluid analysis (i.e. reactants and combustion products, as well as mixing, weakly reacting and strongly reacting fluids). The study quantifies the (i) evolution of fluid state probabilities and (ii) interface statistics, (iii) unconditional and (iv) conditional velocity statistics, (v) conditional strain along fluid interfaces and (vi) scalar fluxes as a function of the external enthalpy support.
Hampp F, Lindstedt RP, 2017, Quantification of combustion regime transitions in premixed turbulent DME flames, Combustion and Flame, Vol: 182, Pages: 248-268, ISSN: 0010-2180
The current study quantifies the probability of encountering up to five fluid states (reactants, combustion products, mixing fluid, fluids with low and high reactivity) in premixed turbulent DME flames as a function of the Damköhler number. The flames were aerodynamically stabilised in a back-to-burnt opposed jet configuration featuring fractal grid generated multi-scale turbulence (Re≃ 18,400 and Ret > 370). The chemical timescale was varied via the mixture stoichiometry resulting in a wide range of Damköhler numbers (0.08 ≤ Da ≤ 5.6). The mean turbulent strain (≥ 3200 s−1) exceeded the extinction strain rate of the corresponding laminar flames for all mixtures. Simultaneous Mie scattering, OH-PLIF and PIV were used to identify the fluid states and supporting computations show that the thermochemical state (e.g. OH and CH concentrations) at the twin flame extinction point correlates well with flames in the back-to-burnt geometry at the corresponding rate of heat release. For mixtures where the bulk strain (≃ 750 s−1) was similar to (or less than) the extinction strain rate, fluids with low and high reactivity could accordingly be segregated by a threshold based on the OH concentration at the extinction point. A sensitivity analysis of the distribution between the fluid states was performed. The flow conditions were further analysed in terms of Damköhler and Karlovitz numbers. The study provides (i) the evolution of multi-fluid probability statistics as a function of the Damköhler number, including (ii) the flow direction across fluid interfaces and OH gradients, (iii) mean flow field statistics, (iv) conditional velocity statistics and (v) a tentative combustion regime classification.
Tian L, Lindstedt, 2017, The impact of dilatation, scrambling and pressure transport in turbulent premixed flames, Combustion Theory and Modelling, Vol: 21, Pages: 1114-1147, ISSN: 1741-3559
Premixed turbulent flames feature strong interactions between chemical reactions and turbulence that affect scalar and turbulence statistics. The focus of the present work is on clarifying the impact of pressure dilatation/flamelet scrambling effects with a comprehensive second-moment closure used for evaluation purposes. Model extensions that take into account flamelet orientation and molecular diffusion are derived. Isothermal pressure transport is included with an additional variable density contribution derived for the flamelet regime of combustion. Full closure is assessed by comparisons with Direct Numerical Simulations (DNSs) of statistically ‘steady’ fully developed premixed turbulent planar flames at different expansion ratios. Subsequently, the prediction of lean premixed turbulent methane–air flames featuring fractal grid generated turbulence in an opposed jet geometry is considered. The overall agreement shows that ‘dilatation’ effects contribute to counter-gradient transport and can also increase the turbulent kinetic energy significantly. Levels of anisotropy are broadly consistent with the DNS data and key aspects of opposed jet flames are well predicted. However, it is also shown that complications arise due to interactions between the imposed pressure gradient and combustion and that redistribution is affected along with the scalar flux at the leading edge. The latter is strongly affected by the reaction rate closure and, potentially, by pressure transport. Overall, the derived models offer significant improvements and can readily be applied to the modelling of premixed turbulent flames at practical rates of heat release.
Kraus P, Lindstedt RP, 2017, Microkinetic mechanisms for partial oxidation of methane over platinum and rhodium, Journal of Physical Chemistry C, Vol: 121, Pages: 9442-9453, ISSN: 1932-7455
A systematic approach for the development of heterogeneous mechanisms is applied and evaluated for the catalytic partial oxidation of methane over platinum (Pt) and rhodium (Rh). The derived mechanisms are self-consistent and based on a reaction class-based framework comprising variational transition state theory (VTST) and two-dimensional collision theory for the calculation of pre-exponential factors with barrier heights obtained using the unity bond index–quadratic exponential potential (UBI–QEP) method. The surface chemistry is combined with a detailed chemistry for the gas phase, and the accuracy of the approach is evaluated over Pt for a wide range of stoichiometries (0.3 ≤ ϕ ≤ 4.0), pressures (2 ≤ P (bar) ≤ 16), and residence times. It is shown that the derived mechanism can reproduce experimental data with an accuracy comparable to that of the prevalent collision theory approach and without the reliance on experimental data for sticking coefficients. The derived mechanism for Rh shows encouraging agreement for a similar set of conditions, and the robustness of the approach is further evaluated by incorporating partial updates via more accurate DFT-determined barrier heights. Substantial differences are noted for some channels (e.g., where reaction progress is strongly influenced by early transition states) though the impact on the overall agreement with experimental data is moderate for the current systems. Remaining discrepancies are explored using sensitivity analyses to establish key parameters. The study suggests that the overall framework is well-suited for the efficient generation of heterogeneous reaction mechanisms, that it can serve to identify key parameters where high accuracy ab initio methods are required, and that it permits the inclusion of such updates as part of a gradual refinement process.
Tian L, Lindstedt RP, 2017, Transported PDF modelling and analysis of partially premixed flames, 8th European Combustion Meeting, Publisher: The Combustion Institute
A hybrid finite volume – transported joint probability density function (FV/JPDF) method is used to model piloted flames with inhomogeneous inlets. The flames were experimentally investigated using a retractable central tube within the main burner to control the degree of mixing at the exit. A five-gas (C2H2, H2, CO2, N2, air) co–flow pilot located outside the burner was used to match the composition and adiabatic temperature of a stoichiometric methane/air flame. The applied hybrid method features a flow field calculation using a time-dependent finite-volume based method closed at the second-moment level with the scalar field obtained at the joint-scalar (JPDF) level. The current methodology is applicable to both premixed combustion and diffusion-dominated regions without assumption regarding the inclusion of the chemistry. Results show that the current method can accurately capture the stratified premixed flame mode near the burner exit as well as the diffusion-dominated flame far downstream. The transition between the combustion modes occurs around ten tube diameters downstream of the burner exit and it is observed that the flame structure is very sensitive to the prediction of the flow field in this region.
This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.