Publications
141 results found
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.
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.
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.
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.
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.
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.
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- Citations: 7
Hadjipanayis MA, Beyrau F, Lindstedt RP, et al., 2015, Thermal radiation from vapour cloud explosions, PROCESS SAFETY AND ENVIRONMENTAL PROTECTION, Vol: 94, Pages: 517-527, ISSN: 0957-5820
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- Citations: 17
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
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- Citations: 15
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
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- Citations: 15
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
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- Citations: 39
Robinson RK, Lindstedt RP, 2013, A comparative ab initio study of hydrogen abstraction from n-propyl benzene, Combustion and Flame, Vol: 160, Pages: 2642-2653, ISSN: 0010-2180
Aromatics form an integral part of typical aviation fuels with n-propyl benzene selected as a representative molecule for inclusion in several EU and US surrogate blends used for design calculations. Despite the practical relevance, kinetic and thermodynamic data obtained using comparatively accurate ab initio methods have to date not been compared with currently used reaction class based estimates. The use of ab initio methods for comparatively complex molecules also necessitates an assessment of the relative benefits of higher levels of theory as it is typically desirable to balance the accuracy of the treatment of individual reactions with the need to consider more complete reaction sequences. The current study examines six hydrogen extractions, via the hydrogen or methyl radicals from the n-propyl side chain. Potential energy surfaces were determined using 10 different approaches, including state-of-the-art DFT (M06, M06-2X and M08-SO) and contemporary composite methods (G4, G4MP2, CBS-QB3 and CBS-4M). Results are presented relative to data obtained using the CCSD(T)/jun-cc-pVTZ//M06-2X/6-311++G(3df,3pd) coupled cluster based method. Rate parameters were determined using transition state theory combined with (i) small curvature tunnelling and energetics at the M06-2X/6-31G(2df,p) level and (ii) Eckart tunnelling corrections and energetics at the CCSD(T)/jun-cc-pVTZ level. Results were found to agree comparatively well with modest differences in rates for several reactions. However, it is also shown that substantial deviations can arise with respect to reaction class based estimation techniques.
Goh KHH, Geipel P, Hampp F, et al., 2013, Flames in fractal grid generated turbulence, Fluid Dynamics Research, Vol: 45, ISSN: 0169-5983
Twin premixed turbulent opposed jet flames were stabilized for lean mixtures of air with methane and propane in fractal grid generated turbulence. A density segregation method was applied alongside particle image velocimetry to obtain velocity and scalar statistics. It is shown that the current fractal grids increase the turbulence levels by around a factor of 2. Proper orthogonal decomposition (POD) was applied to show that the fractal grids produce slightly larger turbulent structures that decay at a slower rate as compared to conventional perforated plates. Conditional POD (CPOD) was also implemented using the density segregation technique and the results show that CPOD is essential to segregate the relative structures and turbulent kinetic energy distributions in each stream. The Kolmogorov length scales were also estimated providing values ∼0.1 and ∼0.5 mm in the reactants and products, respectively. Resolved profiles of flame surface density indicate that a thin flame assumption leading to bimodal statistics is not perfectly valid under the current conditions and it is expected that the data obtained will be of significant value to the development of computational methods that can provide information on the conditional structure of turbulence. It is concluded that the increase in the turbulent Reynolds number is without any negative impact on other parameters and that fractal grids provide a route towards removing the classical problem of a relatively low ratio of turbulent to bulk strain associated with the opposed jet configuration.
Beyrau F, Hadjipanayis MA, Lindstedt RP, 2013, Ignition of fuel/air mixtures by radiatively heated particles, Proceedings of the Combustion Institute, Vol: 34, Pages: 2065-2072, ISSN: 0082-0784
The current work examines the ignition of fuel/air mixtures by particles which have been heated up rapidly by intense electromagnetic radiation from an infrared laser source. Experiments have been conducted at relatively large beam sizes, where ignition times are a function of the irradiance. Particles in the form of fine powders were placed into a chamber filled with ignitable butane/air mixtures. Possible ignition is shown for a range of carbon based materials including different carbon blacks, graphite, the C60 fullerene and diamond powder, as well as for non-reactive powders such as silicon carbide, iron-, copper- and silicon oxides. The irradiance was varied independently and results are shown to become independent of the size of the irradiated area if a sufficiently large area is illuminated. The particle size was found to have a significant impact on the time to ignition. Specifically, finer particles lead to shorter ignition times due to the higher surface area to volume ratio which reduces both particle and gas heating times. Ignition could be achieved across the whole flammability range of butane/air using carbon black and silicon carbide particles, although, near the rich flammability no ignition could be obtained with carbon black.
Wandel AP, Lindstedt RP, 2013, Hybrid multiple mapping conditioning modeling of local extinction, PROCEEDINGS OF THE COMBUSTION INSTITUTE, Vol: 34, Pages: 1365-1372, ISSN: 1540-7489
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- Citations: 17
Lindstedt RP, Waldheim BBO, 2013, Modeling of soot particle size distributions in premixed stagnation flow flames, Proceedings of the Combustion Institute, Vol: 34, Pages: 1861-1868, ISSN: 0082-0784
Soot formation in five laminar premixed ethylene flames was studied computationally in order to investigate the ability of a sectional model to reproduce measured soot particle size distributions (PSDs). The flames were modelled using a burner-stabilised stagnation flow configuration due to the influence of the sampling probe. In the vicinity of the probe, the residence time is relatively long and the temperature reduced. It was found that under such conditions the coagulation efficiency for small (<10 nm) particles becomes important due to the combination of a relatively low nucleation rate with a significant collision frequency. In previous studies, the impact of the probe was only accounted for by shifting the position of the measured PSDs. A model for the collision efficiency was introduced with a particle size dependent interpolation between a lower limit, based on the Lennard–Jones potential, and an upper limit, based on collision stabilisation caused by the surrounding gas. The developed model is consistent with a low collision efficiency for PAH coagulation and explains the measured high number densities of small particles. The size of particles affected by a reduced collision efficiency were found to decrease with increasing flame temperature. The effect is consistent with temperature dependent internal carbonisation processes changing the polarisability of particles leading to a strengthening of attractive forces and enhanced collisional stabilisation. The calculated PSDs were found to be in reasonable agreement with measurements.
Goh KHH, Geipel P, Hampp F, et al., 2013, Regime transition from premixed to flameless oxidation in turbulent JP-10 flames, Proceedings of the Combustion Institute, Vol: 34, Pages: 3311-3318, ISSN: 0082-0784
An opposed jet configuration featuring fractal generated turbulence was used to characterise premixed fuel lean JP-10 flames with equivalence ratios (ϕ) from 0.20 to 0.80 stabilised against combustion products with a temperature of 1720 K. The cold flow turbulent Reynolds (Ret) number was set to 122 and the estimated Damköhler (Da) numbers varied from below 0.3 (ϕ = 0.20) to 6 (ϕ = 0.80) with laminar flame parameters obtained from computations of the corresponding opposed jet configuration. The gradual transition from a turbulent flame to a supported burning mode related to the Homogeneous Charge Diffusion Ignition (HCDI) flameless oxidation regime was detected with features typical of conventional flames gradually disappearing for ϕ < 0.60. A second mode transition to the corrugated flamelet regime at ϕ = 0.80 is shown to coincide with a gradual change from gradient to counter-gradient turbulent transport. A density segregation based technique was applied to Particle Image Velocimetry (PIV) images to detect the instantaneous location of reaction zones and to provide conditional and unconditional statistics to illustrate the combustion regime transitions. Flame surface area characteristics and turbulent burning velocities were also derived to further characterise the burning modes.
Malik NA, Lindstedt RP, 2012, THE RESPONSE OF TRANSIENT INHOMOGENEOUS FLAMES TO PRESSURE FLUCTUATIONS AND STRETCH: PLANAR AND OUTWARDLY PROPAGATING METHANE/AIR FLAMES, COMBUSTION SCIENCE AND TECHNOLOGY, Vol: 184, Pages: 1799-1817, ISSN: 0010-2202
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- Citations: 3
Robinson RK, Lindstedt RP, 2011, On the chemical kinetics of cyclopentadiene oxidation, COMBUSTION AND FLAME, Vol: 158, Pages: 666-686, ISSN: 0010-2180
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- Citations: 67
Vincent RS, Lindstedt RP, Malik NA, et al., 2011, The dynamics of partial oxidation of ethane over platinum, PROCEEDINGS OF THE COMBUSTION INSTITUTE, Vol: 33, Pages: 1809-1817, ISSN: 1540-7489
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- Citations: 7
Lindstedt RP, Milosavljevic VD, Persson M, 2011, Turbulent burning velocity predictions using transported PDF methods, PROCEEDINGS OF THE COMBUSTION INSTITUTE, Vol: 33, Pages: 1277-1284, ISSN: 1540-7489
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- Citations: 12
Geipel P, Goh KHH, Lindstedt RP, 2010, Fractal-generated turbulence in opposed jet flows, Flow, Turbulence and Combustion, Vol: 85, Pages: 397-419, ISSN: 0003-6994
The opposed jet configuration presents a canonical geometry suitable for the evaluation of calculation methods seeking to reproduce the impact of strain and re-distribution on turbulent transport in reacting and non-reacting flows. The geometry has the advantage of good optical access and, in principle, an absence of complex boundary conditions. Disadvantages include low frequency flow motion at high nozzle separations and comparatively low turbulence levels causing bulk strain to exceed the turbulent contribution at small nozzle separations. In the current work, fractal generated turbulence has been used to increase the turbulent strain and velocity measurements for isothermal flows are reported with an emphasis on the axis, stagnation plane and the distribution of mean and instantaneous strain rates. Energy spectra were also determined. The instrumentation comprised hot-wire anemometry and particle image velocimetry with the flows to both nozzles seeded with 1 μm silicon oil droplets providing a relaxation time of ≃ 3 μs. It is shown that fractal grids increase the turbulent Reynolds number range from 48–125 to 109–220 for bulk velocities from 4 to 8 m/s as compared to conventional perforated plate turbulence generators. Low frequency motion of the order 10 Hz could not be completely eliminated and probability density functions were determined for the location of the stagnation plane. Results show that the fluctuation in the position of the stagnation plane is of the order of the integral length scale, which was determined to be 3.1±0.1 mm at the nozzle exits through the use of hot-wire anemometry. Flow statistics close to the fractal plate located upstream of the nozzle exit were also determined using a transparent glass nozzle.
Malik NA, Lindstedt RP, 2010, The Response of Transient Inhomogeneous Flames to Pressure Fluctuations and Stretch: Planar and Outwardly Propagating Hydrogen/Air Flames, COMBUSTION SCIENCE AND TECHNOLOGY, Vol: 182, Pages: 1171-1192, ISSN: 0010-2202
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- Citations: 4
Lindstedt RP, Markaki V, Robinson RK, 2009, Detailed chemical kinetic modelling of aromatic Diesel fuel components, 7th International Symposium Towards Clean Diesel Engines (TCDE 2009), Publisher: CEUR, ISSN: 1613-0073
The ability to predict the inter-conversion of poly-aromatic hydrocarbons (PAHs) of different toxicities and emissions of fine carbon-based particles from Diesel engines are of increasing relevance given their harmful effects. The matter is complicated by the complexity of Diesel fuels and model fuel blends have to be used in numerical simulations of practical engines. The use of aromatic fuel component(-s) in such blends provides a route towards the modulation of the propensity of a fuel to produce such emissions - provided the chemistry is sufficiently well understood. The current work extends past efforts related to the oxidation of 1-methyl naphthalene, which has been identified as a potential key component of surrogate Diesel fuels. Specifically, 1-methyl naphthalene may be used to modulate sooting tendencies and the methyl groups on aromatic rings (e.g. xylenes and tri-methyl benzenes) have also been identified as important in the context of fuel reactivity.
Barlow RS, Ozarovsky HC, Karpetis AN, et al., 2009, Piloted jet flames of CH<sub>4</sub>/H<sub>2</sub>/air: Experiments on localized extinction in the near field at high Reynolds numbers, COMBUSTION AND FLAME, Vol: 156, Pages: 2117-2128, ISSN: 0010-2180
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- Citations: 19
Gkagkas K, Lindstedt RP, Kuan TS, 2009, Transported PDF Modelling of a High Velocity Bluff-Body Stabilised Flame (HM2) Using Detailed Chemistry, FLOW TURBULENCE AND COMBUSTION, Vol: 82, Pages: 493-509, ISSN: 1386-6184
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- Citations: 11
Wandel AP, Lindstedt RP, 2009, Hybrid binomial Langevin-multiple mapping conditioning modeling of a reacting mixing layer, PHYSICS OF FLUIDS, Vol: 21, ISSN: 1070-6631
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- Citations: 16
Gkagkas K, Lindstedt RP, 2009, The impact of reduced chemistry on auto-ignition of H2 in turbulent flows, COMBUSTION THEORY AND MODELLING, Vol: 13, Pages: 607-643, ISSN: 1364-7830
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- Citations: 19
Bhatt JS, Lindstedt RP, 2009, Analysis of the impact of agglomeration and surface chemistry models on soot formation and oxidation, PROCEEDINGS OF THE COMBUSTION INSTITUTE, Vol: 32, Pages: 713-720, ISSN: 1540-7489
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- Citations: 39
Colket M, Edwards T, Williams S, et al., 2008, Identification of target validation data for development of surrogate jet fuels
The engineering and scientific community has been working to identify surrogate fuels that can reasonably represent the performance and emissions behavior of jet fuels (e.g., Jet- A, JP8) in engines. At the last Aerospace Sciences meeting, a review of efforts by a working group on jet fuel surrogates and related chemical kinetic mechanisms was presented. This working group has continued its efforts to refine a set of target parameters at related conditions that should be matched by surrogates. These target parameters are based upon chemical kinetic processes that might limit the design or performance of an engine. Chemical and physical properties and requirements are relatively easy to define and to either model or measure. However, matching most physical/chemical properties with only a few chemical species has proved to be challenging. Determination of combustion properties at relevant conditions that can be used for model validation is more challenging. Target combustion characteristics that have been identified are laminar flame speeds, heat release rates, flame stability (e.g., extinction strain rates), particulate formation, and (pre)ignition behavior. Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc.
Vincent RS, Lindstedt RP, Malik NA, et al., 2008, The chemistry of ethane dehydrogenation over a supported platinum catalyst, JOURNAL OF CATALYSIS, Vol: 260, Pages: 37-64, ISSN: 0021-9517
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- Citations: 45
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