Recent Submissions

  • Laser induced fluorescence investigation of the chemical impact of nanosecond repetitively pulsed glow discharges on a laminar methane-air flame

    Del Cont-Bernard, Davide; Guiberti, Thibault; Lacoste, Deanna (Proceedings of the Combustion Institute, Elsevier BV, 2020-10-08) [Article]
    This paper reports on an experimental investigation of the chemical impact of nanosecond repetitively pulsed (NRP) glow discharges on a laminar methane-air flame. The chosen configuration was a lean wall stabilized flame where NRP discharges were generated across the flame front. After careful selection of the excitation lines, planar laser induced fluorescence of OH and CH was conducted. Comparisons between the OH and CH fluorescence of a base flame (without plasma actuation), and those obtained during the steady state and the transient regimes of plasma actuation, were performed. First it is shown that during the steady state regime, the intensity of OH and CH fluorescence in the flame could be increased by up to 40% and 10%, respectively. In addition, the life time of OH fluorescence in the discharge channel was estimated to be between 3 and 4.5 µs. The transient regime at the beginning of plasma actuation showed that the flame began to be affected by the discharges long before OH fluorescence could be detected in the discharge channel, upstream of the flame. After 40 ms of plasma actuation, OH intensity began to increase simultaneously in both the flame and the discharge area. Based on current knowledge of nanosecond discharge chemistry, explanations for these results are proposed.
  • Multitude of dimple shapes can produce singular jets during the collapse of immiscible drop-impact craters

    Yang, Zi Qiang; Tian, Yuansi; Thoroddsen, Sigurdur T (Journal of Fluid Mechanics, Cambridge University Press (CUP), 2020-10-07) [Article]
    We study singular jets from the collapse of drop-impact craters, when the drop and pool are of different immiscible liquids. The fastest jets emerge from a dimple at the bottom of the rebounding crater, when no bubble is pinched off. The parameter space is considerably more complex than for identical liquids, revealing intricate compound-dimple shapes. In contrast to the universal capillary–inertial drop pinch-off regime, where the neck radius scales as R ∼ t 2/3, for a purely inertial air dimple the collapse has R ∼ t 1/2. The bottom dimple dynamics is not self-similar but possesses memory effects, being sensitive to initial and boundary conditions. Sequence of capillary waves can therefore mould the air dimple into different collapse shapes, such as bamboo-like and telescopic forms. The finest jets are only 12 μm in diameter and the normalized jetting speeds are up to one order of magnitude larger than for jets from bursting bubbles. We study the cross-over between the two power laws approaching the singularity. The singular jets show the earliest cross-over into the inertial regime. The fastest jets can pinch off a toroidal micro-bubble from the cusp at the base of the jet.
  • Optical frequency metrology in the bending modes region

    Lamperti, Marco; Gotti, Riccardo; Gatti, Davide; Shakfa, Mohammad Khaled; Cané, E.; Tamassia, F.; Schunemann, P.; Laporta, P.; Farooq, Aamir; Marangoni, M. (Communications Physics, Springer Science and Business Media LLC, 2020-10-06) [Article]
    Abstract Optical metrology and high-resolution spectroscopy, despite impressive progress across diverse regions of the electromagnetic spectrum from ultraviolet to terahertz frequencies, are still severely limited in the region of vibrational bending modes from 13 to 20 µm. This long-wavelength part of the mid-infrared range remains largely unexplored due to the lack of tunable single-mode lasers. Here, we demonstrate bending modes frequency metrology in this region by employing a continuous-wave nonlinear laser source with tunability from 12.1 to 14.8 µm, optical power up to 110 µW, MHz-level linewidth and comb calibration. We assess several CO2-based frequency benchmarks with uncertainties down to 30 kHz and we provide an extensive study of the v11 band of benzene, a significant testbed for the resolution of the spectrometer. These achievements pave the way for long-wavelength infrared metrology, rotationally-resolved studies and astronomic observations of large molecules such as aromatic hydrocarbons.
  • Electric field measurement in electric-field modified flames

    Butterworth, Tom; Cha, Min Suk (Proceedings of the Combustion Institute, Elsevier BV, 2020-10-05) [Article]
    The application of an electric field to a flame modifies the flame characteristics due to the collective motion of charged particles generated within the flame. The applied field mobilises electrons and ions depending on their polarity, leading to localised regions of space charge with sufficient density to either locally amplify or shield the applied electric field. A fundamental understanding of the local electric field is essential for both electrically and plasma-assisted combustion, because it determines electric body force as well as electron temperature. In this work, we propose a method, which is independent of gas composition and temperature, to measure the local electric field using electric field induced second harmonic generation (E-FISH). We successfully apply this method to a laminar nonpremixed counterflow flame by applying modulated direct current (DC) electric field with a microsecond duration zero volt pulse. Electric potential and space charge distribution are also deduced from the measured electric field. Furthermore, we show the importance of electron attachment to O2 forming O2− by changing the polarity of the applied DC electric field. When the anode is in the oxygen stream, a region of negative charge is obtained near the anode, whereas, when the anode is in the fuel stream, no region of negative charge is found. We also find the qualitative trends of the measured electric fields reasonably agree with previously reported one-dimensional modelling results. The limitations of the methodology are addressed, and we show that the ability to modulate the DC voltage on sub-microsecond timescales is required for accurate quantitative measurements.
  • Coupled in-nozzle flow and spray simulation of Engine Combustion Network Spray-G injector

    Mohan, Balaji; Badra, Jihad; Sim, Jaeheon; Im, Hong G. (International Journal of Engine Research, SAGE Publications, 2020-10-05) [Article]
    A coupled Eulerian-Lagrangian approach was employed to Engine Combustion Network (ECN) Spray-G simulations. The Eulerian in-nozzle flow simulation was conducted with a small plenum attached to the nozzles, and the results were fed to the Lagrangian spray simulation. For Eulerian simulation, the homogeneous relaxation model (HRM) coupled with the volume of fluid (VOF) method was used. HRM proved to be good at predicting the phase change phenomena due to vaporization mechanisms, that is, both cavitation and flash boiling. As a one-way coupling, quantities such as rate of injection (ROI), mass injected through each hole, discharge coefficient, spray plume angle and half cone angle predicted from the Eulerian simulations were used as the initial and boundary conditions for the subsequent Lagrangian spray simulations using the blob injection model. Non-flashing (Spray-G1) and flashing (Spray-G2) spray was simulated, and the results were validated quantitatively against the published data in terms of the liquid and vapor penetration lengths, and good agreements were obtained. Furthermore, the simulation predicted the liquid and gas axial velocity and sauter mean diameter for Spray-G1 condition in agreement with the droplet size and particle image velocimetry (PIV) measurements from literature.
  • Experimental and kinetic modeling study of tetralin: A naphtheno-aromatic fuel for gasoline, jet and diesel surrogates

    Issayev, Gani; Djebbi, Khalil; Kukkadapu, Goutham; Mehl, Marco; Wagnon, Scott W.; Pitz, William J.; Farooq, Aamir (Proceedings of the Combustion Institute, Elsevier BV, 2020-10-03) [Article]
    Distillate fuels contain significant proportions of naphtheno-aromatic components and tetralin is a suitable surrogate component to represent this molecular moiety. The presence of aromatic and naphthyl rings makes kinetic modeling of tetralin very challenging. Primary radicals formed during the oxidation of tetralin can be aryl, benzylic or paraffinic in nature. Using available information on reaction paths and rate constants of naphthenes and alkyl-aromatics, a kinetic model of tetralin has been developed in the current study with emphasis on low-temperature chemistry and high-pressure conditions. Due to the lack of high-level quantum chemical calculations on reaction pathways of tetralin, analogous rates from ab-initio studies on benzylic and paraffinic radicals have been adopted here. Some modifications to the reaction rate rules are incorporated to account for the unique characteristics of tetralin's molecular structure. Important reaction channels have been identified using reaction path and brute force sensitivity analyses. In order to investigate the model performance at low temperatures, new experiments are carried out in a rapid compression machine on blends of tetralin and 3-methylpentane. Blending of low-reactivity tetralin with a high-reactivity alkane allowed the investigation of tetralin ignition at very low temperatures (665 – 856 K). The kinetic model developed in the current study is found to predict the current experiments and literature data adequately. The new model will aid in high-fidelity surrogate predictions at engine-relevant conditions.
  • An experimental and numerical investigation to characterize the low-temperature heat release in stoichiometric and lean combustion

    Waqas, Muhammad; Cheng, Song; Goldsborough, S. Scott; Rockstroh, Toby; Johansson, Bengt; Kolodziej, Christopher P. (Proceedings of the Combustion Institute, Elsevier BV, 2020-10-01) [Article]
    This work reports on an experimental and modeling study on the low-temperature heat release (LTHR) characteristics for three RON 90 binary blends (n-heptane blended with isooctane, toluene and ethanol) in a Cooperative Fuel Research (CFR) engine at lean and stoichiometric conditions that are representative of homogeneous charge compression ignition (HCCI) and spark-ignition (SI) end-gas combustion conditions, respectively. An analysis of the end-gas temperature-pressure (T-P) trajectories was performed to identify the intake conditions leading to similar T-P trajectories between the two lambdas for each fuel blend. A heat release analysis was then conducted for the identified cases, where fuel-to-fuel differences in LTHR were identified and found to be sensitive to the operating condition. Simulations were conducted for these cases using a recently updated chemical kinetic model and a 0-D engine model, where good qualitative and reasonable quantitative agreements in LTHR were obtained. Sensitivity analysis was also performed directly on the rates of LTHR, to understand the controlling chemical reactions of LTHR, providing further insights into the fuel-to-fuel differences. The results demonstrate the significant promoting effect of n-heptane on LTHR rates, while inhibiting effects were seen for ethanol and toluene. Also highlighted was the importance of H-atom abstraction reactions from the chemistry of each fuel component, which could lead to contradictory fuel behavior depending on the locations of the H site of the abstraction reaction due to the different ensuing pathways for the primary fuel radicals.
  • The magnetised Richtmyer–Meshkov instability in two-fluid plasmas

    Bond, D.; Wheatley, V.; Li, Y.; Samtaney, Ravi; Pullin, D. I. (Journal of Fluid Mechanics, Cambridge University Press (CUP), 2020-09-30) [Article]
    We investigate the effects of magnetisation on the two-fluid plasma Richtmyer–Meshkov instability of a single-mode thermal interface using a computational approach. The initial magnetic field is normal to the mean interface location. Results are presented for a magnetic interaction parameter of 0.1 and plasma skin depths ranging from 0.1 to 10 perturbation wavelengths. These are compared to initially unmagnetised and neutral fluid cases. The electron flow is found to be constrained to lie along the magnetic field lines resulting in significant longitudinal flow features that interact strongly with the ion fluid. The presence of an initial magnetic field is shown to suppress the growth of the initial interface perturbation with effectiveness determined by plasma length scale. Suppression of the instability is attributed to the magnetic field’s contribution to the Lorentz force. This acts to rotate the vorticity vector in each fluid about the local magnetic-field vector leading to cyclic inversion and transport of the out-of-plane vorticity that drives perturbation growth. The transport of vorticity along field lines increases with decreasing plasma length scales and the wave packets responsible for vorticity transport begin to coalesce. In general, the two-fluid plasma Richtmyer–Meshkov instability is found to be suppressed through the action of the imposed magnetic field with increasing effectiveness as plasma length scale is decreased. For the conditions investigated, a critical skin depth for instability suppression is estimated.
  • Effects of Differential Diffusion on the Stabilization of Unsteady Lean Premixed Flames Behind a Bluff-Body

    Kim, Yu Jeong; Lee, Bok Jik; Im, Hong G. (Flow, Turbulence and Combustion, Springer Science and Business Media LLC, 2020-09-29) [Article]
    Two-dimensional direct numerical simulations were conducted to investigate the effects of differential diffusion on flame stabilization and blow-off dynamics of lean premixed hydrogen–air and syngas–air flames stabilized on a meso-scale bluff-body in a square channel. The unity Lewis number for all species was imposed to isolate the effects of differential diffusion. Four sets of simulation cases were conducted. Two different inflow temperature with unity Lewis number were applied to examine distinct levels of hydrodynamic instability. Each unity Lewis number case was compared with the non-unity Lewis number case to investigate how differential diffusion affects the overall flame responses, instabilities, and blow-off mechanism. For all cases, the overall flame dynamics were observed in several distinct modes as the inflow velocity approaches blow-off limit. One of the primary effects of unity Lewis number was an increased level of hydrodynamic instability due to the lower flame temperature and thus a lower density ratio. The lower gas temperature also led to a weakening of the re-ignition of the quenched local mixture by the product gas entrainment. The combined effects were manifested as suppression of the re-ignition events, leading to a revised conclusion that the ultimate blow-off behavior at high velocity conditions are mainly controlled by the onset of local extinction.
  • An Experimental and Kinetic Modeling Study of Laminar Flame Speed of Dimethoxymethane and Ammonia Blends

    Elbaz, Ayman M.; Giri, Binod; Issayev, Gani; Shrestha, Krishna Prasad; Mauss, Fabian; Farooq, Aamir; Roberts, William L. (Energy & Fuels, American Chemical Society (ACS), 2020-09-28) [Article]
    Ammonia (NH3) is considered a promising carbon-neutral fuel, with high hydrogen content, that can diversify the global energy system. Blending ammonia with a highly reactive fuel is one possible strategy to enhance it’s combustion characteristics. Here, an investigation of blends of NH3 and dimethoxymethane (DMM), a biofuel with high fuel-born oxygen content and no carbon-carbon bonds, is reported. Unstretched laminar burning velocity (SL) and Markstein length of different NH3/DMM blends were experimentally determined using spherically propagating premixed flames. The DMM mole fraction was varied from 0.2 to 0.6 while measuring SL at 298 K, 0.1 MPa and equivalence ratios () over a range of 0.8 to 1.3. The addition of DMM was found to immensely enhanced the combustion characteristics of ammonia. DMM 20% in NH3/DMM blend increased SL more than a factor of 3 over neat ammonia; such enhancement was found to be comparable with 60% CH4 in NH3 ( = 0.9 -1.1) blends. Increasing  was found to significantly decrease the burned gas Markestein length for lean cases, whereas a negligible effect was observed for rich mixtures. A composite chemical kinetic model of DMM/NH3, aimed at interpreting the high-temperature combustion chemistry, was able to reliably predict SL for neat NH3 and DMM flames. Also, the predictive capability of the kinetic model to describe SL for DMM/NH3 blends is reasonably good. Sensitivity analysis and reaction paths analysis indicated that the NH3/DMM blends could be understood as the dual oxidation processes of the individual fuels which are competing for the same radical pool.
  • Ignition delay time measurements of diesel and gasoline blends

    AlAbbad, Mohammed A.; Li, Yang; Aljohani, Khalid; Kenny, Gavin; Hakimov, Khaiyom; Al-lehaibi, Moaz; Emwas, Abdul-Hamid M.; Meier, Patrick; Badra, Jihad; Curran, Henry; Farooq, Aamir (Combustion and Flame, Elsevier BV, 2020-09-23) [Article]
    Blends of diesel and gasoline can be used to achieve certain desired ignition characteristics in advanced compression ignition engine concepts. In this work, ignition delay times were measured for two blends of diesel and gasoline in two shock tubes and in a rapid compression machine. These blends comprised of 50/50 and 25/75 volumetric% of diesel and gasoline, respectively. To ensure complete vaporization of the blends, the prepared samples were analyzed with nuclear magnetic resonance (NMR) and laser absorption. The analyses revealed full evaporation, and negligible decomposition/oxidation occurred during mixture preparation. Ignition delay measurements covered wide ranges of temperatures (710–1349 K), pressures (10 and 20 bar), and equivalence ratios (0.5, 1.0 and 2.0). The measured ignition delay times of the two dieseline blends are compared with experimental data of low- to mid-octane gasoline and low- to high-cetane fuels. The measured data are also compared with the simulated ignition delay times of primary reference fuel (PRF) and toluene primary reference fuels (TPRF) surrogates. Multi-component surrogates are proposed for the dieseline blends, and the measured ignition delays of the multi-component surrogates and the dieseline blends are in very good agreement.
  • A computational analysis of strained laminar flame propagation in a stratified CH4/H2/air mixture

    Tomidokoro, Takuya; Yokomori, Takeshi; Ueda, Toshihisa; Im, Hong G. (Proceedings of the Combustion Institute, Elsevier BV, 2020-09-22) [Article]
    Propagation of a H2-added strained laminar CH4/air flame in a rich-to-lean stratified mixture is numerically studied. The back-support effect, which is known to enhance the consumption speed of a flame propagating into a leaner mixture compared to that into a homogeneous mixture, is evaluated. A new method is devised to characterize unsteady reactant-to-reactant counterflow flames under transiently decreasing equivalence ratio, in order to elucidate the influence of flow strain on the back-support effect. In contrast to the conventional reactant-to-product configurations, the current configuration is more relevant to unsteady stratified flames back-supported by their own combustion products. Moreover, since H2 distribution downstream of the flame is known to play a crucial role in back-supported CH4/air flames, the influence of H2 addition in the upstream mixture is examined. The results suggest that a larger strain rate leads to a larger equivalence ratio gradient at the reaction zone through increased flow divergence, which amplifies the back-support. Meanwhile, since H2 addition in the upstream mixture does not affect the downstream H2 content, the relative increase in the consumption speed, i.e. the back-support, is suppressed with larger H2 addition. Especially, when the upstream H2 content decreases with the equivalence ratio, the H2 preferentially diffuses toward the unburned gas, which mitigates H2 accumulation in the preheat zone and further weakens the back-support.
  • Experimental and theoretical evidence for the temperature-determined evolution of PAH functional groups

    Liu, Peng; Chen, Bingjie; Li, Zepeng; Bennett, Anthony; Sioud, Salim; Pitsch, Heinz; Sarathy, Mani; Roberts, William L. (Proceedings of the Combustion Institute, Elsevier BV, 2020-09-22) [Article]
    Elucidating the chemical evolution of various functional groups in polycyclic aromatic hydrocarbons (PAH) and soot aids in understanding soot formation chemistry. In this work, the chemical evolution of various functional groups, including aromatic Csingle bondH, aliphatic Csingle bondH, C=O, Csingle bondOH and Csingle bondOsingle bondC bonds, was experimentally investigated online, rather than with offline diagnostics. Oxidation was performed in a jet-stirred reactor (JSR), fueled with benzene/C2H2/air/N2 and benzene/phenol/C2H2/N2 for a temperature range of 600-1400 K. Kinetic modelling, including ab initio quantum chemistry calculations, reaction rate coefficient calculations and JSR simulations, were conducted to interpret the experimental data and the evolutionary chemistry of the various functional groups. Results show that the formation of functional groups on PAH and oxygenated PAH (OPAH) are highly sensitive to temperature. Aliphatic Csingle bondH bonds survive mainly in the form of Csingle bondCH2single bondC, Csingle bondCH2single bondCH2single bondC or Ctriple bondCH functional groups above 1200 K, and exist in the CHdouble bondCH2 functional group below 1000 K. For the OPAH, the Csingle bondOsingle bondC functional group presents stronger thermal stability than Csingle bondOH and C=O functional groups. Simulation results indicate that HACA-like pathway (hydrogen abstraction carbon addition), in which C2H2 attacks the O atom, followed by cyclization and H-atom elimination reactions, qualitatively describe the formation of OPAH with the Csingle bondO-C functional group at different temperatures. The addition reaction involving PAH radical and C2H4 / C2H3 captures the evolution of PAH with the CHdouble bondCH2 functional group, but fails to explain the formation of Csingle bondCH2single bondC and Csingle bondCH2single bondCH2single bondC functional groups.
  • Explosive dynamics of bluff-body-stabilized lean premixed hydrogen flames at blow-off

    Kim, Yu Jeong; Song, Wonsik; Hernandez Perez, Francisco; Im, Hong G. (Proceedings of the Combustion Institute, Elsevier BV, 2020-09-19) [Article]
    Two-dimensional direct numerical simulation (DNS) databases of bluff-body-stabilized lean hydrogen flames representative of complicated reactive–diffusive system are analysed using the combined approach of computational singular perturbation (CSP) and tangential stretching rate (TSR) to investigate chemical characteristics in blow-off dynamics. To assess the diagnostic approaches in flame and blow-off dynamics, Damköhler number and TSR variables are applied and compared. Four cases are considered in this study showing different flame dynamics such as the steadily stable mode, local extinction by asymmetric vortex shedding, convective blow-off and lean blow-out. DNS data points in positive explosive eigenvalue conditions were subdivided into four different combinations in TSR and extended TSR space and categorized in four distinct characteristic regions, such as kinetically explosive or dissipative and transport-enhanced or dissipative dynamics. The TSR analysis clearly captures the local extinction point in the complicated vortex shedding and allows an improved understanding of the distinct chemistry-transport interactions occurring in convective blow-off and lean blow-out events.
  • A method to convert stand-alone OH fluorescence images into OH mole fraction

    Angelilli, Lorenzo; Ciottoli, P.P.; Guiberti, T.F.; Galassi, R. Malpica; Hernández Pérez, F.E.; Boyette, Wesley; Magnotti, Gaetano; Roberts, William L.; Valorani, M.; Im, Hong G. (Proceedings of the Combustion Institute, Elsevier BV, 2020-09-19) [Article]
    Due to the accessibility of the planar laser-induced fluorescence technique, images of OH fluorescence intensity are often used to study the structure of turbulent flames. However, there are differences between the measured OH fluorescence intensity and the actual OH mole fraction. These are often neglected because accurate conversion from fluorescence to mole fraction requires the combined knowledge of all major species mole fractions and temperature, which was rarely achieved in 2-D. Here, a new method to convert images of OH fluorescence intensity into OH mole fraction is proposed. This model relies only on inexpensive 1-D laminar flame calculations and does not require information on major species or temperature. The primary assumption behind the applicability of this model is the local approximation of multi-dimensional flames with 1-D counterflow flames. The method utilizes the fact that both OH mole fraction and OH fluorescence intensity profiles are self-similar with pressure and scalar dissipation rate. Only two empirical constants need to be calibrated using 1-D laminar flame calculations. The model was validated using computed 2-D axisymmetric laminar flames and 3-D turbulent flames computed with LES. The accuracy of the conversion model was estimated to about 8% (for Reynolds number up to Re ), which includes errors due to the 3-D effects that are not included in this method relying on 2-D images. As a proof of concept, the conversion model was finally applied to one single-shot image of OH fluorescence intensity measured with OH-PLIF for syngas at and Re demonstrating potential applications of this new method. The method was tested for hydrogen, syngas and methane fuels but, for brevity, only syngas results are reported in detail.
  • Picosecond Kerr-gated Raman spectroscopy for measurements in sooty and PAH rich hydrocarbon flames

    Yang, Chaobo; Tang, Hao; Magnotti, Gaetano (Proceedings of the Combustion Institute, Elsevier BV, 2020-09-19) [Article]
    The picosecond optical Kerr gate is an effective method for the separation of long-lived laser induced interference from instantaneous Raman scattering. In this paper, an application of the Kerr-gated Raman spectroscopy in combustion environment was demonstrated. The fast gating provided by a Kerr effect based optical shutter extends Raman spectroscopy measurements to sooty flames and flames rich in polycyclic aromatic hydrocarbons. The optical setup of the Kerr gate is described in detail and the gating time, efficiency and spatial uniformity of the ultrafast shutter are reported. To demonstrate the concept, the Kerr-gated Raman spectroscopy was applied in a laminar CH4/air diffusion flame to evaluate the capability to extract a Raman spectrum under different interference levels. In the region deemed inaccessible with µs-gated Raman spectroscopy, the laser induced interference was effectively suppressed by the Kerr gate. A test was also conducted in a C2H4/air premixed flame to explore the limitation of the Kerr-gated Raman technique. The result shows that after combining with polarization separation, Raman spectra can be extracted from locations with 65 ppb soot volume fraction.
  • High temperature branching ratio of acetaldehyde +OH reaction

    Liu, Dapeng; Giri, Binod; Farooq, Aamir (Proceedings of the Combustion Institute, Elsevier BV, 2020-09-18) [Article]
    The reaction of acetaldehyde (CH3CHO) with hydroxyl radicals (OH) plays an important role in atmospheric and combustion chemistry. The low-temperature chemistry of this reaction has been studied widely in the literature. However, the branching of this reaction at high temperatures is not well known. Aiming to deduce the branching ratio of CH3CHO + OH, measurements were carried out in a shock tube by introducing deuterium into the chemical system. Overall rate coefficients for OH reactions with acetaldehyde (CH3CHO), acetaldehyde-2,2,2-d3 (CD3CHO), acetaldehyde-d4 (CD3CDO) were measured over the temperature range of 950-1300 K and 1.5-3.0 bar. In addition, rate coefficients of OH radicals with acetone (CH3C(O)CH3) and acetone-d6 (CD3C(O)CD3) were measured to deduce the kinetic isotopic effect of H-abstraction reaction at the methyl site. The measured rate coefficients can be represented by the following Arrhenius expressions (cm3/molecule/s): k1 (CH3 CHO + OH) = 1.29 × 10-10 exp (-1996.5 K/T) k2 (CH3 CHO + OH) = 1.06 × 10-10 exp (-2151.9 K/T) k3 (CH3 CDO + OH) = 1.18 × 10-10 exp (-2554.1 K/T) k4 (CH3 COCH3 + OH) = 7.15 × 10-11 exp (-2695.7 K/T) k5 (CD3 COCD3 + OH) = 6.02 × 10-11 exp (-3130.5 K/T) In contrast to the low-temperature chemistry, our results indicate that H-abstraction from the methyl site of acetaldehyde is important at high temperatures and the branching fraction of this channel is ~65%. On per H atom basis, however, H-abstraction from the aldehydic group is faster than that of methyl group even at high temperatures.
  • Effect of the plasma location on the deflagration-to-detonation transition of a hydrogen–air flame enhanced by nanosecond repetitively pulsed discharges

    Gray, Joshua A.T.; Lacoste, Deanna (Proceedings of the Combustion Institute, Elsevier BV, 2020-09-18) [Article]
    This work presents a method for using nanosecond repetitively pulsed (NRP) plasma discharges for accelerating a propagating flame such that the deflagration-to-detonation transition occurs. A strategy is developed for bringing the location of the plasma near the tube wall and, thus, reducing the presence of the electrodes in the combustion tube as well as presenting a configuration in which cooling of the electrodes is viable for practical applications. Time-of-flight measurements were used in combination with energy deposition measurements and high-speed OH*-chemiluminescence imagery to investigate the flame acceleration process. For stoichiometric hydrogen–air flames, successful transition to detonation was achieved by applying a burst of 110 pulses at 100 kHz, with energies as low as 10 mJ per pulse. This was also achieved when plasma discharges were applied in the vicinity of the wall. Two enhancement mechanisms for flame acceleration were identified. The essential role of shock–flame interaction was established as being the main mechanism for flame acceleration when the discharges are located near the wall. This work presents an effective alternative that allows for NRP discharges to be applied near the wall while successfully maintaining a promising success rate for detonation transition.
  • Effect of core metal on flame spread and extinction for horizontal electrical wire with applied AC electric fields

    Kang, Min Sung; Park, Sun Ho; Yoo, Chun Sang; Park, Jeong; Chung, Suk Ho (Proceedings of the Combustion Institute, Elsevier BV, 2020-09-17) [Article]
    The effect of metal core on flame spread behavior over polyethylene (PE)-insulated electrical wire was experimentally investigated by varying applied AC voltage (VAC) and frequency (fAC). The present experimental results with Cu-core were compared with those conducted previously with NiCr-core. Flame spread rate (FSR) with Cu-core for the baseline case with no electric field was larger than that with NiCr-core. Both FSR and flame size were appreciably influenced by applied AC electric fields and behaved differently for Cu- and NiCr-cores. For Cu-core, FSR behavior could be classified into four regimes as fAC increases: increasing FSR (regime I), decreasing FSR (II), and again increasing FSR (III) and decreasing FSR (IV). While for NiCr-core, it has been categorized into two regimes I and II. FSR behavior was qualitatively similar to that of flame width in regimes I, II, and IV. While such a relationship was not satisfied in regime III, which can be attributed to the formation of molten PE film in the burnt wire side as well as the formation of globular molten PE in front of spreading flame edge, emphasizing the important role of complex molten PE behavior. Molten PE dripping was observed for NiCr-core, while for Cu-core, such dripping was not observed. Electrospray and di-electrophoresis phenomena occurred with Cu-core along with a formation of molten PE film on the burnt wire side via continuous di-electrophoresis phenomenon from molten PE droplet. When the frequency was excessive, flame extinction occurred via two routes: appreciable reduction of flame size (both for Cu- and NiCr-cores) and detachment of molten PE (for Cu-core). These extinction frequencies were correlated well with the voltage.
  • Structure and stability characteristics of turbulent planar flames with inhomogeneous jet in a concentric flow slot burner

    Mansour, Mohy S.; Elbaz, Ayman M.; Roberts, William L.; Zayed, Mohamed F.; Juddoo, Mrinal; Akoush, Bassem M.; Khedr, Alaa M.; Al-Bulqini, Hazem M.; Masri, Assaad R. (Proceedings of the Combustion Institute, Elsevier BV, 2020-09-17) [Article]
    Turbulent flames with compositionally inhomogeneous mixtures are common in many combustion systems. Turbulent jet flames with a circular nozzle burner were used earlier to study the impact of inhomogeneous mixtures, and these studies showed that the nozzle radius affects the flame stability. Accordingly, planar turbulent flames with inhomogeneous turbulent jet are created in a concentric flow slot burner (CFSB) to avoid this effect in the present study. The stability characteristics, the mixing field structure, and the flame front structure were measured, and the correlations between stability and the mixing field structure were investigated. The mixture fraction field was measured in non-reacting jets at the nozzle exit using highly resolved Rayleigh scattering technique, and the flame front was measured in some selected turbulent flames using high-speed Planar Laser-Induced Fluorescence (PLIF) of OH technique. The data show strong correlations between flame stability and the range of mixture fraction fluctuations. The flames are highly stabilized within a mixing field environment with the range of fluctuation in mixture fraction close to the range of the flammability limits. The mixing field structure is also illustrated and discussed using a mixing regime diagram and showed that the scatter of the data of the different cases is consistent with the classified mixing regimes. Lean flames are stabilized in the current slot burner. The flame front structure topology varies consistently from thin, small curvature at the low level of turbulence and higher equivalence ratio to more wrinkled, larger curvature, but a thicker structure at a higher level of turbulence and lower equivalence ratio.

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