Imperial College London

Professor of Thermofluids Mechanical Engineering

Central FacultyOffice of the Provost

Assistant Provost (Academic Promotions)
 
 
 
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Contact

 

+44 (0)20 7594 7039p.lindstedt

 
 
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Assistant

 

Ms Serena Dalrymple +44 (0)20 7594 7029

 
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Location

 

613City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

129 results found

du Preez M, Wandel AP, Bontch-Osmolovskaia D, Lindstedt RPet al., 2021, Parametric sensitivities of the generalized binomial Langevin-multiple mapping conditioning model, PHYSICS OF FLUIDS, Vol: 33, ISSN: 1070-6631

Journal article

Kraus P, Lindstedt RP, 2021, It's a Gas: Oxidative Dehydrogenation of Propane over Boron Nitride Catalysts, JOURNAL OF PHYSICAL CHEMISTRY C, Vol: 125, Pages: 5623-5634, ISSN: 1932-7447

Journal article

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.

Journal article

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.

Journal article

Shariatmadar H, Hampp F, Lindstedt RP, 2020, Quantification of PAH concentrations in premixed turbulent flames crossing the soot inception limit, Proceedings of the Combustion Institute, ISSN: 1540-7489

Journal article

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.

Journal article

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.

Book chapter

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

Journal article

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.

Journal article

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

Journal article

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

Journal article

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

Journal article

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

Journal article

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

Journal article

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.

Journal article

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.

Book chapter

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.

Journal article

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.

Journal article

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.

Journal article

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.

Conference paper

Li T, Lindstedt RP, 2017, Thermal radiation induced ignition of multipoint turbulent explosions, PROCESS SAFETY AND ENVIRONMENTAL PROTECTION, Vol: 107, Pages: 108-121, ISSN: 0957-5820

The severity of vapour cloud explosions is typically correlated with the peak over-pressure or, more accurately, with the total impulse caused by the pressure waves. The flame speed arising from strong (e.g. quasi-stable) turbulent deflagrations is frequently used to provide an indication of potential damage. However, conventional flame propagation mechanisms can present difficulties in terms of explaining the resulting damage. For example, the over-pressure in the Buncefield vapour cloud explosion was much higher than that predicted by conventional models. Alternative propagation mechanisms include intermittent localised strong explosions or detonations potentially supported by forward thermal radiation causing multi-point ignition of dust particles ahead of the advancing flame front. Such mechanisms are here explored using particles coated with acetylene black as the radiation target due to their relationship with soot emissions. A continuous wave laser operating in the near infrared was used as the radiation source with experiments performed in a flame tube using fuel lean CH4/H2/Air mixtures. It is shown that ignition kernels caused by irradiated particles can successfully be entrained into the main flow and/or recirculation zones formed around obstacles and cause multipoint explosions. The resulting relationship between fuel consumption ahead of the advancing flame and the evolution of the strength of the explosion is shown to be complex and typically lead to increased explosion durations with reduced peak pressures. It is also shown that chaotic pressure wave interactions can substantially increase both the explosion duration and the peak pressure depending on the timing of the radiation induced ignition.

Journal article

Kraus P, Lindstedt RP, 2016, Variational transition state theory based surface chemistry for the C2H6/H2/O2/Pt system, Energy & Fuels, Vol: 31, Pages: 2217-2227, ISSN: 1520-5029

A reaction class-based framework for the development of heterogeneous mechanisms is applied to study the (partial) oxidation of ethane over platinum. The rate parameters for the surface chemistry were derived using a systematic application of variational transition state theory (VTST) for adsorption, desorption, and Eley–Rideal reactions coupled with two-dimensional (2D) collision theory for reactions occurring on the surface. The approach removes the need for the experimental determination of surface sticking coefficients and the associated major uncertainties. The barrier heights were determined using the unity bond index–quadratic exponential potential (UBI–QEP) method. The combined gas- and surface-phase chemistry was evaluated against independent data sets obtained from three experimental configurations. The associated 18 cases cover a wide range of residence times, stoichiometries (0.1 < ϕ < 10.4), and inlet pressures (1–12 bar). The work highlights the generality of the VTST approach that is shown to outperform the customary sticking coefficient-based methods for key aspects. A sensitivity analysis highlights the importance of the O2 and CO adsorption pathways.

Journal article

Hampp F, Lindstedt RP, 2016, Strain distribution on material surfaces during combustion regime transitions, Proceedings of the Combustion Institute, Vol: 36, Pages: 1911-1918, ISSN: 1540-7489

A multi-fluid state approach is used to analyse the underlying conditions for burning mode transitions from close to the corrugated flamelet regime to distributed reactions. Turbulent (Ret ≃ 380) premixed DME/air flames were aerodynamically stabilised in a back-to-burnt opposed jet configuration with the Damköhler number varied through the mixture stoichiometry. Simultaneous Mie scattering, OH-PLIF and PIV allowed the delineation of five separate fluid states (reactants, combustion products, mixing fluid, mildly and strongly reacting fluids) with associated material surfaces. The analysis shows self-sustained flames in low strain regions with a collocated flow acceleration for higher Damköhler numbers. By contrast, in highly strained regions (e.g. beyond the twin flame extinction point) the burning mode is governed by the counter-flowing hot combustion products resulting in increased levels of vorticity and an absence of a preferential dilatation direction. The current analysis provides novel insights into combustion regime transitions by means of (i) strain rate statistics conditioned upon material surfaces and (ii) the evolution of fluid state interface probabilities as a function of the Damköhler number. The work further shows (iii) that the combustion mode influences scalar transport and that increased levels of turbulence retards the transition to non-gradient transport.

Journal article

Lindstedt RP, Kraus P, 2016, Reaction class-based frameworks for heterogeneous catalytic systems, Proceedings of the Combustion Institute, Vol: 36, Pages: 4329-4338, ISSN: 1873-2704

A systematic and self-consistent approach is applied to study the combustion of hydrogen and syngas over platinum using coupled detailed gas phase and surface reaction mechanisms. The sur-face chemistry is derived using a reaction class-based framework comprising variational transition state theory (VTST), two-dimensional collision theory and the unity bond index { quadratic ex-ponential potential for barrier heights. The latter approach is augmented by the inclusion of more accurate data, such as the heat of adsorption of CO, and VTST is used to systematically removethe need for the surface sticking coefficients associated with adsorption and desorption processes.Transition-state theory estimates for several reaction classes are produced by combining the M06 family of density functionals with the Stuttgart/Dresden e ective core potential for metal atoms.The developed method reproduces experimental data with an accuracy comparable or better than the previously used collision theory approach and without the reliance on experimental parameters.The presented framework is well{suited for the efficient generation of novel heterogeneous reaction mechanisms and also serves to identify key parameters where high accuracy ab initio methods maybe required. The latter is exempli ed via the sensitivity of selected results to the adsorption of carbon monoxide on platinum.

Journal article

Hampp F, Lindstedt RP, 2016, Fractal grid generated turbulence—A bridge to practical combustion applications, CISM International Centre for Mechanical Sciences, Courses and Lectures, Pages: 75-102

Practical applications typically feature high turbulent Reynolds numbers and, increasingly, low Damköhler (Da) numbers leading to distributed combustion. Such conditions are difficult to achieve under laboratory conditions that permit detailed experimental investigations. The aerodynamically stabilised turbulent-opposed jet flame configuration is a case point—an exceptionally flexible canonical geometry traditionally featuring low turbulence levels. It is shown that fractal grids can be used to increase the turbulent Reynolds number, without any negative impact on other parameters, and to remove the classical problem of a relatively low ratio of turbulent to bulk strain. The use of fractal grids to ameliorate such problems is further exemplified for fuel lean combustion with low Da numbers achieved through the stabilisation of premixed flames against hot combustion products. An analysis is presented in the context of a multi-fluid formalism that extends the customary bimodal pdf approach to include combustion regime transitions. The approach is quantified via simultaneous OH-PLIF and PIV permitting the identification of five separate states (reactant, combustion product, mixing, mildly and strongly reacting fluids). The sensitivity of the distribution between the fluid states to threshold values is also evaluated for combustion of methane. The work suggests that a consistent treatment of the delineating thresholds is necessary when comparing different types of simulations (e.g. DNS) and experiments for reacting fluids with multiple states. The use of fractal grids is further exemplified in a flame driven shock tube and used to generate turbulent Re numbers of the order 10 5 for flows with Mach numbers approaching unity. The conditions are of relevance to flame stabilisation in hypersonics and are analysed through OH-PLIF and high speed PIV with optimal fractal grids selected on the basis of maximum flame acceleration.

Book chapter

Hadjipanayis MA, Beyrau F, Lindstedt RP, Atkinson G, Cusco Let al., 2015, Thermal radiation from vapour cloud explosions, PROCESS SAFETY AND ENVIRONMENTAL PROTECTION, Vol: 94, Pages: 517-527, ISSN: 0957-5820

Journal article

Goh KHH, Geipel P, Lindstedt RP, 2015, Turbulent transport in premixed flames approaching extinction, PROCEEDINGS OF THE COMBUSTION INSTITUTE, Vol: 35, Pages: 1469-1476, ISSN: 1540-7489

Journal article

Beyrau F, Hadjipanayis MA, Lindstedt RP, 2015, Time-resolved temperature measurements for inert and reactive particles in explosive atmospheres, PROCEEDINGS OF THE COMBUSTION INSTITUTE, Vol: 35, Pages: 2067-2074, ISSN: 1540-7489

Journal article

Goh KHH, Geipe P, Lindstedt RP, 2014, Lean premixed opposed jet flames in fractal grid generated multiscale turbulence, COMBUSTION AND FLAME, Vol: 161, Pages: 2419-2434, ISSN: 0010-2180

Journal article

Goh KHH, Geipel P, Hampp F, Lindstedt RPet al., 2013, Flames in fractal grid generated turbulence, FLUID DYNAMICS RESEARCH, Vol: 45, ISSN: 0169-5983

Journal article

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