Fuel Equivalence Ratio Research Articles

Effects of Oxygen Enrichment on Scramjet Performance

This investigation quantifies the beneficial effects of oxygen enrichment on the combustion efficiency and specific thrust of a simplified planar scramjet engine fueled with gaseous hydrogen. Numerical modeling was used to examine the influence of fuel equivalence ratio and enrichment percentage. Broad agreement with experimental data is reported, but the present simulations provide considerably more detail, revealing aspects of the flow physics that could not be discerned from experimental data. Multiple Reynolds-averaged Navier–Stokes simulations were performed at Mach 12 equivalent freestream conditions with fuel equivalence ratios of 0.8–1.4 and enrichment percentages up to 20%. Trends in combustion efficiency with enrichment percentage are formulated for use in trajectory studies. The combustion efficiency and specific thrust are found to be significantly increased by enriching fuel with oxygen. The increases are greater than those expected from the additional propellant and complete consumption of premixed oxygen. We found that this is due to oxygen enrichment modifying the mixing layer properties within the scramjet such that the production of turbulence and therefore fuel/air mixing is increased. It is also shown that an optimal combustor length exists beyond which internal losses outweigh contributions from combustion.

Operational sensitivities of an integrated aerodynamic-ramp-injector/gas-portfire flame holder in a supersonic combustor

Performance and operating limits of an ethylene-fueled injection and ignition/flame-holding system consisting of an aerodynamic-ramp injector and a gas-portfire were investigated both experimentally and numerically, using the scramjet direct connect facility at the Beihang University. The experimental operating conditions include (1) the fuel equivalence ratio, (2) the gas-portfire location, and (3) the gas-portfire gas-to-freestream momentum flux ratio (qg). The lean burn limit was at a fuel equivalence ratio of 0.05. The gas-portfire was placed downstream of the injector on the centerline at three different locations in order to find the best suitable gas-portfire/aero-ramp injector geometrical configurations. Results showed that the distance from the gas-portfire to the aero-ramp injector array was a critical parameter which influenced the combustion efficiency obviously. The ignition delay time was sensitive to the qg. When the qg was below 0.55, the ethylene cannot be ignited; when the qg was 0.64, the ignition delay time reached nearly 1s; the increase of the qg from 0.64 to 1.43 shortened the ignition delay time obviously, but had no influence on the combustion efficiency. The detailed combustion flowfields were investigated by the 3-D numerical study and the numerical results agreed well with the static pressure distributions obtained by experiments (average error<5%).

Characterization of Soot Aerosol Produced from Combustion of Propane in a Shock Tube

The knowledge of yields and properties of soot from combustion of hydrocarbon fuels is crucial for accurate evaluation of the impacts of primary aerosols on air quality and climate. This study presents measurements of soot generated from combustion of propane in a shock tube, using independently adjustable fuel equivalence ratio (φ), temperature, and pressure. The characterization of soot yields inside the shock tube by in situ laser extinction is complemented with a set of comprehensive measurements of soot transferred into a fluoropolymer chamber, including particle size distributions, elemental carbon (EC) mass fraction, effective density, mass fractal dimension (Dfm), dynamic shape factor (χ), and optical properties. The properties of soot particles and the soot yield are sensitive to combustion conditions and the duration of the combustion experiment. High-temperature combustion with φ = 2.5 produces small fractal (Dfm = 2) soot particles composed mainly of EC (up to 90%), at a low mass yield. Particles from lower temperature combustion contain a significant fraction of organic material (∼50%). Using rich fuel mixtures (φ = 4.0 and 8.0) significantly increases particle size and soot mass yield. At lower temperatures, compact (Dfm = 3) and nearly spherical (χ = 1.1) aggregates with high organic content are formed, whereas at higher temperatures, the particles are fractal and closely resemble those obtained using φ = 2.5. Single scattering albedo (SSA) varies from 0.15 for fractal particles to 0.75 for compact particles. For soot generated at high equivalence ratios, SSA can be used as a proxy for particle morphology and EC content. Copyright 2012 American Association for Aerosol Research

Formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F) by precursor pathways in oxidation of pesticide alpha-cypermethrin

This contribution assesses the oxidation of the commonly used insecticide and termiticide, alpha-cypermethrin to polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F, dioxins) and their precursors, under non-catalytic vapour-phase oxidation, in a flow reactor. No soot was produced during the reaction ruling out the de novo synthesis of PCDD/F. The results span the effect of temperature (300–650°C) and fuel equivalence ratio (φ of 0.01–7.0), at a residence time of 1s. XAD-2 resin trapped the PCDD/F which were identified and quantitated by high resolution gas chromatography (HRGC)–ion trap mass spectrometry (ITMS). The reactor walls appear to have no noticeable effect on the initiation of the gas-phase oxidation reactions. The decomposition of alpha-cypermethrin produces two types of PCDD/F precursors, chlorinated benzenes (CBz)/chlorinated phenols (CP) and two-ring species, mainly diphenyl ether and chlorinated diphenyl ethers (CDPE). The CBz/CP precursors comprise chlorobenzene, o-, m- and p-dichlorobenzene, 1,2,3-trichlorobenzene, phenol, 2-monochlorophenol (2-CP), 2,4-dichlorophenol (2,4-DCP) and 2,4,6-TriCP, with minor amount of 2,4,5-TriCP. The highest emission factor for PCDD/F was observed at 550°C and a fuel equivalence ratio of 0.03, corresponding to 19ng ∑TEQ–WHO2005/g of alpha-cypermethrin and the total dioxins concentration of 141μg/g of alpha-cypermethrin. These emissions are one of the highest measured from oxidation of presently used pesticides.

Oxidation of dibenzo-p-dioxin: Formation of initial products, 2-methylbenzofuran and 3-hydro-2-methylenebenzofuran

This study combines the results of experimental measurements and theoretical computations to investigate the initial steps in the oxidation of dibenzo-p-dioxin (DD). The aim is to discover new pathways leading to the formation of toxic species in combustion systems, as well as compounds that may act as precursors for polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/F). The experiments were performed in a tubular reactor equipped with sampling systems for condensable and gaseous products. The reaction conditions were characterised by a residence time of 5s and a fuel equivalence ratio of 0.12. We investigated the effect of temperature (400–800°C) on the type and the yield of the oxidative products, trapped in a cartridge, and gases collected in tedlar bags for subsequent FTIR analyses. The analyses of VOC, performed on a high resolution gas chromatograph–triple quadrupole mass spectrometer (HRGC–QQQMS), identified 2-methylbenzofuran and 3-hydro-2-methylenebenzofuran as the initial products. This has been confirmed by injection of authentic standards and the application of collision induced dissociation (CID) that fragmented the isolated parent ions into specific product ions affording the identification of parent species. The oxidative decomposition of DD initiated at around 450°C, with the evolution of VOC peaking at between 650 and 700°C. In excess of 750°C, all VOC were completely oxidised. The potential energy surface (PES), based on the density functional theory (DFT) of B3LYP, mapped the initial steps involved in the oxidation of DD, and yielded a detailed reaction scheme for the onset of oxidation of DD that results in the formation of 2-methylbenzofuran and 3-hydro-2-methylenebenzofuran. Initial oxidation of DD is most likely to commence via addition of the singlet oxygen molecule and the subsequent facile formation of the propagating OH radical.

Laminar flame speed of soy and canola biofuels

In this article, the flame speed values determined experimentally for laminar premixed flames of the vapors of two biofuels in air are presented. The laminar flame speed is a fundamental thermochemical property of fuels, and is essential for analyzing the flame propagation in practical devices, even those employing turbulent flames. The fuels obtained from transesterification of soy and canola oils are tested. Also, the diesel flames are studied to serve as a baseline for comparison. The experiments are performed with a tubular burner; pre-vaporized fuel is mixed with hot air and is ignited. The flame speed is determined at fuel-equivalence ratios of 1; 1,1 and 1,2 by recording the geometry of the flame. The experimental results show that the flame speed of biofuels is lower by about 15% than that of diesel. Also, the maximum value of flame speed is obtained at an equivalence ratio of approximately 1,1.

Experimental and Numerical Studies of Vitiated Air Effects on Hydrogen-fueled Supersonic Combustor Performance

This paper deals with the vitiation effects of test air on the scramjet performance in the ground combustion heated facilities. The primary goal is to evaluate the effects of H2O and CO2, the two major vitiated species generated by combustion heater, on hydrogen-fueled supersonic combustor performance with experimental and numerical approaches. The comparative experiments in the clean air and vitiated air are conducted by using the resistance heated direct-connected facility, with the typical Mach 4 flight conditions simulated. The H2O and CO2 species with accurately controlled contents are added to the high enthalpy clean air from resistance heater, to synthesize the vitiated air of a combustion-type heater. Typically, the contents of H2O species can be varied within the range of 3.5%–30% by mole, and 3.0%–10% for CO2 species. The total temperature, total pressure, Mach number and O2 mole fraction at the combustor entrance are well-matched between the clean air and vitiated air. The combustion experiments are completed at the fuel equivalence ratios of 0.53 and 0.42 respectively. Furthermore, three-dimensional (3D) reacting flow simulations of combustor flowpath are performed to provide insight into flow field structures and combustion chemistry details that cannot resolved by experimental instruments available. Finally, the experimental data, combined with computational results, are employed to analyze the effects of H2O and CO2 vitiated air on supersonic combustion characteristics and performance. It is concluded that H2O and CO2 contaminants can significantly inhibit the combustion induced pressure rise measured from combustor wall, and the pressure profile decreases with the increasing H2O and CO2 contents in nonlinear trend; simulation results agree well with experimental data and the overall vitiation effects are captured; direct extrapolation of the results from vitiated air to predict the performance of actual flight conditions could result in over-fueling the combustor, possible inlet un-start and inappropriate combustion mode transition. The detailed analysis and discussion are presented and the research conclusions are summarized.

Simultaneous Measurements of Temperature and CO Concentration in Stagnation Stabilized Flames

An impinging jet burner was developed to investigate flame-wall interactions (FWI) using laser based diagnostics. CO concentrations were measured with two-photon laser-induced fluorescence (LIF) in combination with coherent anti-Stokes Raman spectroscopy (CARS) gas phase temperature measurements. Besides being the principal factor in chemical kinetics, temperature data is required to correct the CO LIF data for various factors like density variation, quenching and variation in the Boltzmann population. Phosphor thermometry was used to determine surface temperatures of the wall and to estimate the heat flux. In an parameter study Reynolds numbers and fuel equivalence ratio were varied.

Effect of Local Flow Parameters on Skin-Friction Coefficient in Hypersonic Airbreathing Engines

Effects of Rex, Re , Tw=Taw, and Me on the local skin-friction coefficient (Cf ) in a hypersonic airbreathing combustor with various combustion modes were examined. Measurements were performed in the hypersonic airbreathing combustor with airflowMach number and unit Reynolds numbers at the entrance of combustor of 2.5 and 1:6 10 3:5 10 m , respectively. The ranges of total temperature, total pressure of airflow and fuel equivalence ratio were varied, i.e., 1000–2000 K, 1.0–1.8 MPa, and 0–0.4, respectively, to change Rex, Re , Tw=Taw, and Me. Direct skin-friction measurements were conducted. The state of flow was also surveyed in the streamwise locationwhere skin frictionwasmeasured to attain core-flowparameters, velocity profile, andmomentum thickness. When boundary-layer transition occurred due to combustion, evaluation ofCf by the vanDriestmethod using either by Rex or by Re was not effective. When there was separation with a long oblique shock train and a pseudo shock wave system in the precombustion region due to combustion, evaluation of Cf by the method using Rex was not effective because the origin of the turbulent boundary layer was changed in the downstream combustor.

Mechanism and Control of Combustion-Mode Transition in a Scramjet Engine

A sidewall compression scramjet engine operated in two combustion modes under Mach 6 flight condition, weak- and intensive-combustion modes. The weak mode occurred below the overall fuel equivalence ratio (Φ) of around 0.4. Transition from the weak mode to the intensive mode occurred at Φ ~ 0.4, accompanied by a sudden increase in thrust. Mechanisms of the transition were numerically investigated in this study. Our simulations captured the sudden increase in thrust at the mode transition. In the weak mode, combustion occurred in only a region near the topwall where an igniter was installed. The combustion region expanded toward the cowl with boundary-layer separation at the mode transition. Our simulations demonstrated that low ignition capability resulted in the weak mode. We demonstrated that the presence of additional igniters on the sidewalls improved the ignition capability and achieved the intensive mode in the entire Φ range.

Hydrogen production from burning and reforming of ammonia in a microreforming system

Hydrogen (H 2) is produced by burning and reforming ammonia (NH 3) in a microreforming system. A micro-combustor that burns H 2-added NH 3–air mixtures as a heat source is a cylinder with an expanded exhaust outlet that facilitates ignition and an annular-type shield that adopts a heat-recirculation concept. A micro-reformer that converts NH 3 to H 2 using ruthenium as a catalyst surrounds the micro-combustor, which facilitates heat transfer between the micro-combustor and micro-reformer. Performance of the microreforming system is affected by the fuel-equivalence ratio and micro-combustor inlet velocity of NH 3–H 2–air mixtures and the concentration of the substituted H 2 in the fuel gas. The production rate of H 2, the conversion rate of NH 3, the overall efficiency of the microreforming system and the NO x concentration in the exhaust gas from the micro-combustor for optimized operating conditions are 5.4 W (based on lower heating value), 97.0%, 10.4% and 158 ppm, respectively. This supports the potential of using NH 3 as a clean, carbon-free fuel for both burning and reforming in microreforming systems.

An experimental and kinetic study of syngas/air combustion at elevated temperatures and the effect of water addition

Laminar flame speeds of premixed syngas/air mixtures were measured at various fuel equivalence ratios (0.6–3.0), H2 content of the fuel, and preheat temperatures (298–500K) using a spherically expanding flame configuration. The measured laminar flame speeds were compared with simulations using with three existing chemical kinetic models – GRI Mech 3.0, H2/CO Davis Mechanism and San Diego mechanism. Reasonable agreement between computations and measurements was achieved at room temperature that validated the new experimental configuration. However, at higher preheat temperatures discrepancies between computed and measured values were large, especially for fuel rich mixtures. Addition of H2O to two fuels (H2/CO=5/95 and 50/50) up to 40% in the fuel–air mixture was studied to understand the effect of moisture in coal derived syngas. For the H2/CO=5/95 fuel, flame speed was observed to increase with up to 20% H2O addition and then to decrease with any further water addition. However, the higher H2 content fuel (H2/CO=50/50) only showed a decrease in flame speed with an increasing water concentration in the fuel–air mixture. The different trends have been explained as a result of the competing chemical and physical (dilution and thermal) effects of H2O addition on the syngas flames using sensitivity analyses and by analyzing reaction rates and radical concentrations.

Comparing Pyrolysis Gases and Dusts Explosivities: A Clue to Understanding Hybrid Mixtures Explosions?

During the explosion of an organic dust cloud, the following steps are usually encountered: particle heating; its devolatilization/pyrolysis; and, then, a homogeneous combustion of the pyrolysis gases. In order to highlight the influence of the pyrolysis step in such explosions, experiments have been carried out on wheat starch powders and their pyrolysis gases. The maximum rate of pressure rise of the gases reaches 2830 bar s–1, whereas it remains lower than 400 bar s–1 for the starch at the same fuel equivalence ratio. Such a difference can be explained by the predominance of the pyrolysis step, but also, to a lesser extent, by changes in the initial turbulence level of the suspension. A model based on the flash pyrolysis mechanisms of cellulosic compounds and their combustion has been developed to represent the evolution of the explosion pressure of such powders. Applications to the case of gas/dust hybrid mixtures are also discussed.

The Effect of Carbon–Carbon Double Bonds on the Combustion Chemistry of Small Fatty Acid Esters

Abstract Environmentally friendly biodiesel is a mixture of saturated and unsaturated methyl (or ethyl) esters of long-chain fatty acids. To experimentally examine the effect of C=C double bonds on the combustion chemistry of fatty acid esters, low-pressure premixed laminar flames of four small esters have been studied using flame-sampling molecular-beam mass spectrometry. Mole fraction profiles of reactants, products, and well-identified stable and reactive combustion intermediates in flames of the saturated species methyl propanoate (CH3CH2COOCH 3) and its isomer ethyl acetate (CH3COOCH2CH 3) have been compared with results from flames of the unsaturated fuels methyl propenoate (CH2CHCOOCH 3) and vinyl acetate (CH3COOCHCH 2) flames. A total of eight flames have been studied, with two fuel-rich flame conditions investigated (fuel-equivalence ratios φ=1.2 and 1.56) for each fuel. In addition, the underlying oxidation chemistry at these premixed flame conditions has been investigated using a detailed chemical kinetic reaction mechanism, which is largely based on a previously proposed model for saturated esters [B. Yang et al., Phys. Chem. Chem. Phys. 13 (2011) 7205]. The combined results provide a detailed understanding of the similarities and differences between the combustion of saturated vs. unsaturated esters. Meanwhile, the isomeric and stoichiometric effects on their combustion chemistry are also addressed. In this paper, experimental and modeling details are discussed with a special focus on the different reaction pathways.

Cowl and Cavity Effects on Mixing and Combustion in Scramjet Engines

To investigate the supersonic combustion patterns in scramjet engines, a model scramjet engine was tested in the T4 free-piston shock tunnel. The test model had a rectangular intake, which compressed the freestream flow through a series of four shock waves upstream of the combustor entrance. A cavity flame holder was installed in the supersonic combustor to improve ignition. The freestream test condition was fixed at Mach 7.6, at an altitude of 31 km. This experimental study investigated the effects of varying fuel equivalence ratios, the influence of the cavity flame holder, and the effects of cowl shape. As a result, supersonic combustion was observed at equivalence ratios between 0.11 and 0.18. Measurements indicated that the engine thermally choked at a fuel equivalence ratio of 0.40. Furthermore, the cavity flame holder and the W-shaped cowl showed improved pressure distribution due to greater reaction intensity. With the aid of numerical analysis, the cavity and the W-shaped cowl are shown to be effective in fuel–air mixing.

Experimental and Kinetic Modeling Study of the Combustion of n-Decane, Jet-A, and S-8 in Laminar Premixed Flames

Laminar flame speeds and Markstein lengths of n-decane/air, Jet-A/air, and S-8/air flames were measured using spherically expanding premixed flames. The experiment used a spherical combustion chamber housed inside a customized oven, which provides a uniform temperature distribution inside the chamber for fuel evaporation. Linear and nonlinear extrapolation methods to obtain unstretched flame speed were compared. The difference between linear and nonlinear extrapolation is within 0.3–2.0 cm/s over the entire range of fuel equivalence ratio and decreases as the equivalence ratio increases. The measured flame speed data were compared to numerical simulations using several existing kinetic mechanisms and surrogate models. The results show that the JetSurF 0.2 mechanism was able to best represent the present measured flame speed data for n-decane. Surrogate models were simulated to represent the flame speeds of Jet-A and S-8. However, the simulated flame speeds overpredict the measured flame speeds. GC-MS and a negative ionization method were used to determine the composition change of the liquid fuels before and after heating/vaporization. The result shows that when the initial temperature is above 500 K, auto-oxidation occurred during the mixing process for Jet-A and S-8 flames, which likely produced ketones or aldehydes, resulting in falsely lower flame-speed data.

Experimental Study of Test-Medium Vitiation Effects on Dual-Mode Scramjet Performance

An experimental study was performed to characterize the effects of vitiation due to combustion-air preheating on dual-mode scramjet combustion.Major combustion vitiation species (H2O andCO2)were added to the freestreamof an electrical-resistance-heated, direct-connect facility simulating Mach 5 flight enthalpy. With clean, dry air, the combustor operated in the supersonic mode at fuel equivalence ratios below 0.22, and in the subsonic mode for equivalence ratios above 0.26. Hysteresis was observed in the dual-mode transition region between 0.22 and 0.26, as the mode of combustion was dependent on whether the fuel rate was increasing or decreasing. Adding increasing amounts of water vapor and carbon dioxide to the freestream decreased combustor pressures by 10 to 30% for the same fuel equivalence ratio. Vitiation also caused transition between supersonic and subsonic combustion to occur at a higher fuel equivalence ratio thanwith clean air. This work represents the first direct evaluation of the effect of testmedium vitiation on dual-mode scramjet combustion atMach 5 enthalpy simulation in the same facility. The results indicate the importance of accounting for test-medium vitiation when extrapolating from ground-testing to flight, particularly in the dual-mode transition region between subsonic and supersonic combustion regimes.

Transient flame propagation process and flame-speed oscillation phenomenon in a carbon dust cloud

A detailed numerical study was conducted to understand the transient flame propagation process and the flame-speed oscillation phenomenon in a carbon dust cloud. The modeling included the solution of a set of time-dependent conservation equations developed for the gas phase and the particle phase in a spherical coordinate. The gas-phase reactions used detailed chemistry, variable thermodynamic properties, and multicomponent transport properties. The particle-phase equations include the two-phase force interactions in the momentum equation by considering Stoke drag force and thermophoretic force resulting from the gas-phase temperature gradient. Mass and species transfer between the two phases were modeled as a result of both gas-phase and particle surface reactions. Energy transfer between the two phases, including convective, conductive, and radiative heat transfer, were included. Radiation absorption and emission by particles were both especially considered. The results show that because of the different inertia between particles and gas, a velocity slip occurs between the two phases in the region ahead of the flame front. The slip is more significant in the early flame propagation stage than in the later stage. The radiation heat losses of the hot gases and particles to the cold ambient and the radiation gain as a result of the absorption of unburned particles are both important in the present dust flame, because the characteristic time scale of the chemical reactions is longer than that of gaseous flames. Lastly, an analysis of the detailed numerical simulations shows that a slip between the gas and particle velocities is the cause of flame-speed oscillation. The slip leads to a periodic change in local particle number density in the reaction zone, which in turn changes the local fuel equivalence ratio periodically, causing the oscillation.

Carbon Nanofibers Preparation Using Carbon Monoxide from the V-Type Pyrolysis Flame

Synthesis carbon nanofibers from V-type pyrolysis flame use simple carbon monoxide and acetylene /air premixed flame. Catalyst is transported by helium into the burner. The carbon nanofibers from the sampling probe were characterized by scanning electron microscope and transmission electron microscope. The influence of the fuel equivalence ratio and the flow of carbon monoxide were discussed in detail to reveal the formation rule of carbon nanofiber from the pyrolysis flame. The results indicate that carbon monoxide is a good carbon source gas for the preparation of carbon nanofibers. Under appropriate temperature and optimized flow of carbon monoxide, carbon nanofibers with good quality can be prepared from the V-type pyrolysis flame. It can be concluded that this flame method is effective for the preparation of carbon nanofibers.

Experimental Study of Vitiation Effects on Scramjet Mode Transition

M ANY ground test facilities for studying dual-mode scramjet (DMSJ) propulsion rely on stagnation enthalpy simulationvia combustion of hydrogen or hydrocarbon fuel. This heating process results in levels of vitiates in the test flow, such as water and carbon dioxide, that are not present in atmospheric air. The effects of these vitiates on DMSJ performance and operability must therefore be understood if accurate extrapolations to flight can be made. The Supersonic Combustion Facility [1] provides a unique opportunity to study such effects since the facility is electrically heated and supplies a test gas that is free of combustion generated vitiates. However, water vapor, in the form of steam, and carbon dioxide can be added to the flow to examine the effects these vitiates have on a DMSJ. Previous work to study vitiation effects has been performed using up to 7% H2O and 2.5% CO2, both by mole [1]. However, these results were only obtained over a limited range of fuel equivalence ratios and the study did not fully investigate mode-transition with a precombustion shock train due to the lack of a flowpath isolator. The present study was conducted with vitiate levels closer to those encountered in combustion heated facilities and with an isolator installed. Reference [2] provides details of the new configuration. Basically it consists of the DMSJ of [1], a rectangular, direct connect combustor with a single ramp fuel injector, and a new constant area, rectangular isolator that is ten duct heights long. This Note reports on experimental pressure measurements in the scramjet combustor at twovitiated conditions and compares the resultswith that of clean air. Water and carbon dioxide levels are examined such that the effects of water vapor vitiation can be isolated for conditions that are representative of a methane combustion heated facility operating at Mach 5 simulation. Trends in the data are examined, particularly with respect to the effect vitiates have on the mode-transition characteristics of the DMSJ.