Hydrocarbon-air Mixtures Research Articles

A microwave discharge in high-velocity flows initiated by a half-wave antenna

A microwave discharge in high-velocity (150—250 m/s) air flows induced on a half-wave vibrator is studied. A cw magnetron microwave generator with a frequency of 2.45 GHz and an output power of up to 5 kW was used for initiation of the microwave discharge. The high-speed video imaging was used for studying the discharge structure, determining the diameter and length of the plasma channel as a function of flow velocity and pressure. Electron concentration and temperature, along with characteristic gas temperature, were determined based on the optical spectra. The possibility of using this microwave discharge for ignition of hydrocarbon–air mixtures in combustion chambers of ramjet engines is proved experimentally.

Low-current gliding DC discharge in high-speed flows

Abstract A low-current gliding discharge (current range 1–5 A) in high-speed air flows of 100–250 m s−1 was experimentally studied. A high-voltage direct current source with a maximum voltage of 4.5 kV was used to create the discharge. The average electron concentration n e ∼ 1014 cm−3 and the plasma ionization degree were determined by measuring the Stark broadening of the hydrogen H β line (λ Hβ = 486.1 nm). The estimates of the electric field (E ∼ 100 V cm −1 ÷ 600 V cm−1) in the discharge positive column were found using time-synchronized high-speed video recordings and oscillograms. The gas rotational temperature T g = 7000–9500 K and the vibrational temperature T v = 7000–11 000 K were estimated using optical emission spectroscopy. Time-resolved spectroscopy is used to investigate the effective plasma channel spatial regions from which the N, NH, N2 +, O and OH molecules radiate. The difference of the obtained radii indicates the presence of a radial temperature gradient and inhomogeneous plasma composition in the discharge cross section. The possibility of using of gliding discharge to ignite hydrocarbon-air mixtures in the ramjet engines combustors has been experimentally demonstrated.

Experimental Study on the Propagation Characteristics of Rotating Detonation Wave with Liquid Hydrocarbon/High-Enthalpy Air Mixture

Rotating detonation engines (RDEs) are a promising propulsion technology featuring high thermal efficiency and a simple structure. To adapt the practical engineering applications of ramjet RDEs, rotating detonation combustion using a liquid hydrocarbon and pure air mixture will be required. This paper presents an experimental study on the propagation characteristics of rotating detonation waves with a liquid hydrocarbon and high-enthalpy air mixture in a hollow cylindrical chamber. The parameters, such as the equivalence ratio and inlet mass flux, are considered in this experiment. The frequency and the propagation velocity of rotating detonation combustion are analyzed under typical operations. The experimental results show that the peak pressure and propagation velocity of the rotating detonation wave are close to the C-J theoretical values under the inlet mass flux of 400 kg/(m2s). Both the propagation velocity and peak pressure of the rotating detonation wave decrease as the mass flux and equivalence ratio are reduced while the number of detonation wavefronts increases. Detonation wave instability tends to occur when the inlet mass flux decreases. There is a transition progress from thermo-acoustic combustion to rotating detonation combustion in the experiment under the condition of mass flux 350 kg/(m2s) and the equivalent ratio 0.8. The static pressure in the chamber is higher during detonation combustion than during thermo-acoustic combustion. These experimental results provide evidence that rotating detonation waves have the potential to significantly improve propulsion performance. The findings can serve as a valuable reference for the practical engineering application of rotating detonation engines.

Addressing the complex geometric effects on the three-dimensional transition to detonation in hydrocarbon-air mixtures using a parametrized level-set algorithm

A new level-set algorithm is developed that distinguishes the interior from the exterior of complex geometry by parameterizing the correlation between the stereolithography (STL) information and grid points in order to embody three-dimensional boundary interfaces in the Cartesian coordinate system. The method is applied to the flame and shock wave interactions in hydrocarbon fuel blends including the transition to a detonation in various three-dimensional geometric complexities for predicting the maximum chamber pressure. The locations of the strong shock-flame interactions and the onset of the hot spot initiation in the regions of the strongest interaction are investigated. Moreover, the study is intended for providing safety assessments of the pressure confinements that include bends, steps, and narrow channels connecting cavities by utilizing the present algorithm that can identify the triggering effects of the transition to detonation.

К 125-летию со дня рождения лауреата Нобелевской премии академика Николая Николаевича Семенова Матричная конверсия природного газа в синтез-газ и водород как перспективное направление газохимии и энергетики

The basic principles of a new technology of non-catalytic oxidative reforming of hydrocarbon gases into syngas and hydrogen, based on the combustion of rich hydrocarbon-air or hydrocarbon-oxygen mixtures stabilized by the surface of a permeable matrix, are presented. The kinetics of the process, the prospects for its technological implementation and the most interesting applications are discussed.

Evaluation of the Efficiency of a Double Spark Plug to Improve the Performances of Combustion Engines: Pressure Measurement and Plasma Investigations

The ignition sparks provided by the conventional spark plug (CSP) do not always ensure a fast and complete combustion of the hydrocarbon–air mixture. For this reason, we offer a new type of spark plug with two simultaneous discharges generated by a pulsed high voltage–power supply. First, we present results concerning geometrical and electrical characteristics of plasmas produced in air and in engine combustion chamber. A more important volume of plasma is highlighted. The larger surface of interaction of plasma with the air-to-fuel mixture promotes the initiation of a greater number of chemical reactions thus increases the rate of propagation of the combustion. In addition, for long pulse durations the energy delivered by the double spark plug (DSP) to the discharge is 20% higher, as compared to a CSP. The current rise rate is more important for the DSP, which allows better ionization of the mixture. Afterwards, we present a study of two four-strokes engines ignited by this DSP, one powered with gasoline and the second one with propane. The pressure into the cylinder, measured as a function of the crankshaft angle, allowed to compute the IMEP, COVIMEP, and PPP, for both tested engines. In the case of the gasoline engine, when the ignition and flame development conditions are difficult—a high ignition angle was considered—the DSP can offer an almost 3.5 times better operation stability (considering the coefficient of variation of the indicated mean effective pressure as main indicator) and a faster development of the combustion flame (defined by the means of the pressure peak position). A study was also performed as a function of the equivalence ratio for the propane-powered engine, which confirmed the results regarding the DSP performances obtained with the first engine. In addition, this study highlighted that for very lean mixtures, the DSP can provide a better stability in operation (COVIMEP, COVPPP) is assured even for equivalence ratios in the range of 0.65, where the use of CSPs involves misfires and very high engine vibrations.

Explosion characteristics of cyclic hydrocarbon-air mixtures at elevated temperature and pressures

Explosion characteristics of cyclic hydrocarbon (cyclohexane, cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, benzene)-air mixtures were experimentally investigated in a closed vessel at the temperature of 403 K and pressures of 0.1 and 0.5 MPa. These fuels own different degrees of unsaturation. The crucial explosion parameters were determined, including explosion pressure, rate of pressure rise, explosion index as well as explosion delay. Results indicate that the peak explosion pressure and maximum rate of pressure rise all increase with the rise of initial pressure, demonstrating the high explosion risk at elevated pressure. Except benzene-air mixture, the more unsaturated cyclic hydrocarbon-air mixture has higher values of peak explosion pressure, maximum rate of pressure rise as well as explosion index, and lower values of explosion delay and burn period. Therefore, cyclohexadiene-air mixtures propagate the fastest and show the greatest potential explosion hazard. Cyclohexane-air mixture propagates the slowest and shows the smallest explosion potential as well. This is mainly caused by the different heat release of the cyclic fuel flames. The peak explosion pressure and maximum rate of pressure of benzene-air mixture are between those of cyclohexadiene- and cyclohexane-air mixtures at the lean to slightly rich conditions, while present the lowest values at the extremely rich conditions. This is attributed to the high tendency of soot formation in benzene flame and thus enhanced heat radiation to the vessel wall. Such heat loss also contributes the extended explosion delay and burn period of benzene-air mixture. Furthermore, the explosion index could be correlated with the explosion delay and burn period in exponential expression. This exponential relationship was found to be feasible for methanol-air mixture as well.

An integrated model for vent area design of hydrocarbon-air mixture explosion inside cubic enclosures with obstacles

This study aims to develop an integrated model - NFPA-68-BRANN model, which can be used to calculate the vent areas of cubic enclosures with obstacles. Seven experiments regarding vented explosion inside the obstructed enclosure are reviewed and applied to check the accuracy of two existing standards, i.e. the NFPA-68 2018 and the BS EN 14994:2007. Accordingly, the parameters to describe the flame development in the NFPA-68 2018 are amended by adopting the Bauwens model. Bayesian Regularization Artificial Neuron Network (BRANN) model presenting the non-linear relationship between the turbulent flame enhancement factor X and its affecting factors is subsequently developed. Eventually, the NFPA-68-BRANN model is generated by incorporating the BRANN model into the modified NFAP-68 2018. The accuracy of the NFPA-68-BRANN model is validated by using a series of the New Baker Test data.

A New Spark Plug to Improve the Performances of Combustion Engines: Study and Analysis of Unburned Exhaust Gases

The ignition sparks provided by the conventional spark plug do not always ensure a fast and complete combustion of the hydrocarbon-air mixture. For this reason, we offer a new type of plug with a double spark using two simultaneous discharges generated by a pulsed high voltage-power supply. This work presents the comparison of two spark plugs, a classical one and a double spark plug, by analyzing the unburned hydrocarbon gases from the exhaust pipe of the engine. For a first gasoline engine, we measure the oxygen concentration in the exhaust gases with a lambda probe and the unburned hydrocarbon by the use of GC–MS coupled with SPME extraction technique. We can observe a clear decrease of total unburned hydrocarbon (THC). For a second motor test bench, powered with propane, we complete measures in function of air/fuel ratio of the THC, NOx, CO2 and CO. These results confirm that we obtained a better combustion especially for leaner mixtures.

Assessment and Control of Detonation Hazard of Silane-Containing Mixtures

A formula for calculating the induction period of a silane–air gas mixture has been proposed. The dimension of a detonation cell and the energy of direct initiation of gaseous detonation were assessed. Consideration has been given to the issue of control of the parameters of detonation of silane-containing mixtures. The parameters of Chapman–Jouguet detonation, the relative dimension of the cell of a detonation wave, and also the parameters of explosion at constant pressure and volume in a stoichiometric silane–air gas mixture with additions of chemically inert microparticles (Al2O3) have been calculated. A series of experiments was conducted on measurement of the pressure profile resulting from the explosion of the silane–air mixture in a volume of cubic shape with the known average fuel–oxidant ratio. The distribution of the fuel during the silane jet flowing out into this volume was visualized. The distribution of silane in a cloud in its outflow into the cubically shaped volume and in a high-speed jet flowing out into an unbounded space was calculated. The performed investigations can be useful in evaluating the comparative efficiency of explosion of clouds of silane– and hydrocarbon–air mixtures.

Modified combustion efficiency curve for high-velocity model combustors integrated with the inlet

The method of obtaining combustion efficiency curves for fuel mixtures proposed by Annushkin is modified on the basis of data of high-altitude tests of high-velocity combustors integrated with the inlet. The combustion efficiency curve of a hydrocarbon–air mixture obtained in this study can be applied for design of high-velocity combustors.

Methane-coal dust hybrid fuel explosion properties in a large scale cylindrical explosion chamber

The fires and explosions caused by flammable hydrocarbon air mixtures are a major safety concern in the chemical and processing industries. The thermo-physical and chemical properties of the flammable fuels in a hybrid form appear to have a significant impact on the combustion process. This usually occurs due to substantial changes in the flammability concentration regimes. The aim of this study is to investigate the fire and explosive properties of hybrid fuels in the chemical and process industries. In addition, it examines the impact of the ignition energy and vessel geometry on the magnitude of the pressure rise and flame propagation velocity. The experimental work was conducted on a cylindrically shaped explosion chamber constructed as part of this study at The University of Newcastle, Australia. The chamber was made of mild steel and was 30 m in length and 0.5 in diameter. It included a series of high resolution pressure transducers, a pyrometer, as well as a high speed video camera. Methane and coal dust were used as fuels and chemical igniters with a known energy were used to ignite the fuels.The results obtained from this study showed that both the ignition energy and the diluted combustible fuel dust have significant impacts on the Over Pressure Rise (OPR) in an explosion chamber. The significant findings included that the OPR doubled when 30 g m−3 of coal dust was added to a 6% methane/air mixture, and it increased by 60% when 10 kJ was used instead of a 1 kJ ignition source. The initial ignition energy was observed to considerably enhance the speed of both the pressure wave and the flame front, where the pressure wave speed doubled when using a 5 kJ instead of a 1 kJ ignition source. However, the pressure wave speed increased by five times when a 10 kJ was used instead of a 1 kJ ignition source. Additionally, the maximum flame front velocity observed for the ignition source with 5 kJ energy was twice the flame front velocity for the 1 kJ ignition source. Finally, it was observed that the time needed for the initial methane ignition was reduced by about 50% when using a 10 kJ instead of a 1 kJ ignition source.

Prediction of Auto-Ignition Temperatures and Delays for Gas Turbine Applications

Gas turbines burn a large variety of gaseous fuels under elevated pressure and temperature conditions. During transient operations, variable gas/air mixtures are involved in the gas piping system. In order to predict the risk of auto-ignition events and ensure a safe operation of gas turbines, it is of the essence to know the lowest temperature at which spontaneous ignition of fuels may happen. Experimental auto-ignition data of hydrocarbon–air mixtures at elevated pressures are scarce and often not applicable in specific industrial conditions. Auto-ignition temperature (AIT) data correspond to temperature ranges in which fuels display an incipient reactivity, with timescales amounting in seconds or even in minutes instead of milliseconds in flames. In these conditions, the critical reactions are most often different from the ones governing the reactivity in a flame or in high temperature ignition. Some of the critical paths for AIT are similar to those encountered in slow oxidation. Therefore, the main available kinetic models that have been developed for fast combustion are unfortunately unable to represent properly these low temperature processes. A numerical approach addressing the influence of process conditions on the minimum AIT of different fuel/air mixtures has been developed. Several chemical models available in the literature have been tested, in order to identify the most robust ones. Based on previous works of our group, a model has been developed, which offers a fair reconciliation between experimental and calculated AIT data through a wide range of fuel compositions. This model has been validated against experimental auto-ignition delay times corresponding to high temperature in order to ensure its relevance not only for AIT aspects but also for the reactivity of gaseous fuels over the wide range of gas turbine operation conditions. In addition, the AITs of methane, of pure light alkanes, and of various blends representative of several natural gas and process-derived fuels were extensively covered. In particular, among alternative gas turbine fuels, hydrogen-rich gases are called to play an increasing part in the future so that their ignition characteristics have been addressed with particular care. Natural gas enriched with hydrogen, and different syngas fuels have been studied. AIT values have been evaluated in function of the equivalence ratio and pressure. All the results obtained have been fitted by means of a practical mathematical expression. The overall study leads to a simple correlation of AIT versus equivalence ratio/pressure.

Fast gas heating in N2/O2 mixtures under nanosecond surface dielectric barrier discharge: the effects of gas pressure and composition.

The fractional electron power quickly transferred to heat in non-equilibrium plasmas was studied experimentally and theoretically in N(2)/O(2) mixtures subjected to high electric fields. Measurements were performed in and after a nanosecond surface dielectric barrier discharge at various (300-750 Torr) gas pressures and (50-100%) N(2) percentages. Observations showed that the efficiency of fast gas heating is almost independent of pressure and becomes more profound when the fraction of O(2) in N(2)/O(2) mixtures increases. The processes that contribute towards the fast transfer of electron energy to thermal energy were numerically simulated under the conditions considered. Calculations were compared with measurements and the main channels of fast gas heating were analysed at the gas pressures, compositions and electric fields under study. It was shown that efficient fast gas heating in the mixtures with high fraction of O(2) is due to a notable contribution of heat release during quenching of electronically excited N(2) states in collisions with O(2) molecules and to ion-ion recombination. The effect of hydrocarbon addition to air on fast gas heating was numerically estimated. It was concluded that the fractional electron power transferred to heat in air, as a first approximation, could be used to estimate this effect in lean and stoichiometric hydrocarbon-air mixtures.

Combustion kinetics of cyclooctane and its binary mixture with o-xylene over a Pt/γ-alumina catalyst

Combustion kinetics of the cyclooctane and o-xylene, alone and in their binary mixture, have been experimentally investigated over a commercial Pt/γ-alumina catalyst. The studies were carried out in a laboratory set-up, with diluted hydrocarbon-air mixtures, typical for depollution applications. The experiments were conducted at atmospheric pressure and temperatures between 140 and 400°C, hydrocarbons concentration 200–1000ppmv and WHSW (mass flow rate to catalyst weight ratios) in the range 280–590h−1. The results evidenced an important inhibition of cyclooctane combustion by the presence of o-xylene, throughout the working domain. Based on the experimental measurements and postulating surface mechanisms, rate expressions were developed for the combustion of the two hydrocarbons, in pure state and as binary mixtures. The cyclooctane was found to react by an Eley–Rideal type mechanism, whereas the o-xylene combustion was explained by a Langmuir-Hinshelwood scheme.

Numerical study of cellular detonation structures of methane mixtures

A two-step model of the kinetics of detonation combustion of methane in mixtures with an oxygen and air has been developed. The proposed model of the kinetics of detonation combustion of methane appears to be reasonably accurate within the confines of the assumptions and is consistent with the second law of thermodynamics. Constants of the model have a clear physical meaning. The model is useful for multi-dimensional numerical simulations of detonation processes. A numerical simulation of a two-dimensional (2D) structure of the detonation wave (DW) in a stoichiometric methane–air mixture in a wide range of channel height has been performed. The changes in the 2D structure of the self-sustaining DW on variations of the height of the channel have been studied. From the analysis of the flow structure, the dominant size of the detonation cell for stoichiometric methane–air mixture determines to be 34 ± 1 cm, and 0.3÷0.35 cm for methane–oxygen mixture. These values are in good agreement with all known experimental data. The transverse size of the detonation cell is a characteristic size of the DW front structure. Based on this value, it is possible to determine such parameters of detonation as critical conditions of detonation combustion, the critical energy of direct initiation, detonation propagation limits, etc., i.e. to estimate the detonation hazard of gaseous mixture and its use in various schemes of detonation propulsion systems. The 2D simulations were reproduced the DW irregular cellular structure with all its main features observed in the experiment: a chaotic uncoordinated movement of the main transverse waves; the presence of a fine (secondary) cell structure at the transverse waves themselves; the existence of numerous secondary transverse waves at the leading shock front, forming a hierarchy of the decreasing the size DW front perturbations; and a significant number of pockets of the unburned mixture at a considerable distance behind the DW front, etc. Numerical simulation of irregular structure of the multi-front DW has been conducted for a real hydrocarbon–air mixture with an adequate description of all its thermal and chemical properties.

Prediction of flammability limits of fuel-air and fuel-air-inert mixtures from explosivity parameters in closed vessels

Flammability limits of fuel-air and fuel-air-inert gaseous mixtures, especially at non-atmospheric conditions, are essential properties required for establishing safety operating conditions for handling and processing flammable gases. For pure fuels, an important data pool exists, formed by the flammability limits of fuel-air and fuel-air-inert gaseous mixtures at ambient initial conditions measured by standard methods. Such methods can be used for experimental determination of flammability limits for multi-fuels mixed with air, with or without additives, under non-atmospheric conditions. Their use is however a time- and material-consuming process; in addition, the flammability limits obtained by various standard methods may be scattered as a result of different choices in the operating parameters, for each standard method. It appears that a preliminary estimation of the flammability limits for fuel-air and fuel-air-inert gaseous mixtures can minimize the effort of measuring them in specific initial conditions.The present paper describes a new method for estimating the flammability range of fuel-oxidizer gaseous mixtures based on measurements of explosivity properties e.g. the peak explosion pressure and maximum rate of pressure rise recorded during closed vessel laminar explosions of fuel-oxidizer mixtures far from limits. Data obtained for several hydrocarbon-air gaseous mixtures with or without inert gas addition are used to examine the accuracy of estimated flammability limits (LFL – the lower and UFL – the upper flammability limit) as well as of the Limiting Oxygen Concentration (LOC) and the Minimum Inert Concentration (MIC). The predictive ability of the proposed method is examined against the predictive ability of other recently described methods.

129 RCMによる高温・高圧下での炭化水素-空気混合気の燃焼特性に及ぼすCO_2添加の影響

129 RCMによる高温・高圧下での炭化水素-空気混合気の燃焼特性に及ぼすCO_2添加の影響

Scaling of turbulent flame speed for expanding flames with Markstein diffusion considerations

In this paper we clarify the role of Markstein diffusivity, which is the product of the planar laminar flame speed and the Markstein length, on the turbulent flame speed and its scaling, based on experimental measurements on constant-pressure expanding turbulent flames. Turbulent flame propagation data are presented for premixed flames of mixtures of hydrogen, methane, ethylene, n-butane, and dimethyl ether with air, in near-isotropic turbulence in a dual-chamber, fan-stirred vessel. For each individual fuel-air mixture presented in this work and the recently published iso-octane data from Leeds, normalized turbulent flame speed data of individual fuel-air mixtures approximately follow a Re_{T,f}^{0.5} scaling, for which the average radius is the length scale and thermal diffusivity is the transport property of the turbulence Reynolds number. At a given Re_{T,f}^{}, it is experimentally observed that the normalized turbulent flame speed decreases with increasing Markstein number, which could be explained by considering Markstein diffusivity as the leading dissipation mechanism for the large wave number flame surface fluctuations. Consequently, by replacing thermal diffusivity with the Markstein diffusivity in the turbulence Reynolds number definition above, it is found that normalized turbulent flame speeds could be scaled by Re_{T,M}^{0.5} irrespective of the fuel, equivalence ratio, pressure, and turbulence intensity for positive Markstein number flames.

Effect of multi-bend geometry on deflagration to detonation transition of a hydrocarbon-air mixture in tubes

We present a numerical investigation of gaseous deflagration-to-detonation transition (DDT) triggered by a shock in a multi-bend geometry. The ethylene-air mixture filled rigid tube with obstacles is considered for understanding the effects of complex confinement and initial flame size on DDT. Our calculations show generation of hot spots by flame and strong shock interactions, and flame propagation is either restrained or accelerated due to the wall obstacles of both straight and bent tubes. The effect of initial flame size on DDT in complex confinement geometry is analyzed as well as the hot spot formation on promoting shock–flame interaction, leading to a full detonation.