Lean Propane-air Mixture Research Articles

Long gap spark discharge ignition using a boron-doped diamond electrode

A long gap discharge offers an efficient method for achieving stable ignition in lean fuel/air mixtures, because it can reduce the heat loss to the electrodes, achieve efficient electrical energy transfer from the ignition coil to the discharge plasma, and reduce the initial flame curvature by converting the geometry of the ignition from a point to a line. However, the formation of such discharge requires increased discharge voltage to compensate for the reduction in the electric field between the electrodes, and this is not easily achieved in practice. Since the Townsend discharge criteria depend not only on the electron multiplication coefficient (α coefficient) but on the secondary electron emission (γ-process), we focused on the latter to reduce the discharge threshold voltage. In the present study, for the first time, B-doped diamond (BDD), which has a larger secondary electron emission coefficient than the metals used in conventional spark discharge ignition, was utilized as the cathode material, and its discharge and ignition characteristics were investigated. Under the same conditions, use of a BDD cathode resulted in a lower discharge threshold, greater discharge energy, less discharge energy variation, and more rapid flame development, than in the case of a metal cathode. The influence of the cathode material on discharge threshold suggests that the breakdown mechanism of ignition-coil spark discharge is based on Townsend discharge. To elucidate the effect of gap length, the temporal development of flame growth, using the same discharge energy in methane and propane mixtures, was compared between ignition with a short gap discharge with a metal cathode and long gap discharge with a BDD cathode. With a long gap discharge, only the flame growth of lean propane-air mixtures was accelerated, while no difference was observed in the case of lean methane or relatively rich propane mixtures, suggesting the effectiveness of long gap discharge ignition for mixtures with a large Lewis number, due to the small curvature of the initial flame kernel.

A CFD study of ignition of lean propane-air mixtures in a heat recirculating U-bend catalytic microreactor

Heat recirculating (HR) geometries have been studied in the literature for their use in energy generation applications. This work investigates the ignition and cold-start behavior of lean propane-air combustion in a U-bend catalytic microreactor through computational fluid dynamics (CFD) study. Although several studies have focused on stability and combustion behavior, the role of heat recirculation on catalytic and homogeneous ignition from the cold-start conditions has not been analyzed so far. The U-bend microreactor is a typical example of HR geometry, where the hot exhaust gases in the recirculation channel transfer heat to the cold incoming fluid. Significance of heat recirculation across the dividing wall on the ignition temperature, as well as transient evolution of temperature, propane conversion and reaction are presented. Ignition temperature is found to be 20K lower in the U-bend microreactor than straight channel microreactor. However, transient ignition study revealed that at nominal conditions, U-bend microreactor takes about 35s longer than straight channel for ignition and to reach steady state, owing to heat distribution internally within the U-bend microreactor. The effect of flowrate is analyzed, which shows that at higher flowrates, the ignition and steady-state times are also lower for the U-bend microreactor. Finally, the effect of solid wall material is investigated, which shows that lower conductivity material (such as ceramic) showed faster ignition with low thermal inputs.

Experimental investigation of flame stability in the premixed propane-air combustion in two-section porous media burner

The characteristics of combustion and the flame stabilization within a two-layer porous media burner were investigated experimentally in the present study. Lean propane-air mixtures were used with the equivalence ratio values ϕ in the range between 0.1 and 0.5 and the flow rate Q of the mixture in the range between 10 and 45 l/min. It was experimented with three porous ceramics: reticulated foam alumina, honeycomb foam alumina and SiC foam. The axial temperature distribution, reaction zone, maximum temperature, CO and NOx emissions, and combustion wave propagation rate were analyzed. The experimental results showed that, in the three ceramics studied, the combustion wave propagation rate increases with increased inlet velocity and decreased equivalence ratio of premixed gases. Result of the analysis of the information obtained, a porous burner with flame stabilized between two porous ceramics was built that works permanently with the equivalence ratio ϕ = 0.5 and the flow rate of the mixture in the range between 20 and 35 l/min. The honeycomb foam was placed on top of the burner and the reticulated foam on the bottom. The combustion temperatures were in the range between 1160 and 1380 K, where a higher flow rate resulted in higher temperatures. The CO and NOX emissions did not exceed a few ppm showing that combustion was almost complete.

Role of H2/CO Addition to Flame Instabilities and Their Control in a Stepped Microcombustor

ABSTRACT The effect of H2/CO addition on flame dynamics of propane-air mixtures in a stepped microcombustor with heat recirculation was investigated experimentally. Unstable flame propagation regimes and their transition modes were observed for a range of mixture velocities (5–8 m/s) and equivalence ratios (0.6–0.95). Up to 60% H2 addition to lean propane-air mixtures helps improve the flame stability range in the combustor. The suppression of these flame instabilities with H2 addition is attributed to the increase in the overall mixture reactivity and burning velocity. The enhancement in OH intensity with H2 addition indicated a change in the elementary reactions, thereby affecting the flame dynamics. Contrary to H2 addition, CO addition does not show any noticeable effect on the formation of stable flame modes. The change in mixture properties and burning velocity leads to a change in the flame behavior and alters the heat release rate due to H2 addition to propane-air mixtures. A higher percentage of H2 addition led to the reappearance of the unstable (rotating) flames in the microcombustor. Intrinsic flame instabilities appeared at higher H2 percentages due to differential diffusion and increased thermal-wall coupling effects.

A Study of Combustion Improvement of Extremely Lean Propane-air Mixture by using Propagation Flame in a Tube

A Study of Combustion Improvement of Extremely Lean Propane-air Mixture by using Propagation Flame in a Tube

Formation of combustion wave in lean propane-air mixture with a non-uniform chemical reactivity initiated by nanosecond streamer discharges in the HCCI engine

The propagation of combustion wave, which is formed as a result of impact of a filamentary pulsed periodic non-equilibrium discharge on a mixture that is not ignited by only compression in the HCCI engine, has been simulated. The discharge for a short time locally activated the mixture in the engine cylinder at a certain crankshaft rotation angle at the compression stage. The approach to evaluation of the temperature and composition in the region activated by high-frequency corona discharge has been proposed. Multi-pulse and multi-channel nature of discharge was accounted for in our model. The densities of chemically active species produced during streamer propagation were found using two-dimensional axially symmetric fluid model. The obtained composition and temperature in the activated zone were the initial conditions for modeling the propagation of combustion wave along the cylinder radius in 1D approximation. Special terms were included in equations responsible for the compression along the axis of cylinder to mimic the real change in pressure in cylinder. The calculations were performed for a lean C3H8-air mixture, which is characterized by the presence of low-temperature heat release (LTHR) stage, and intermediate and high temperatures heat release (ITHR and HTHR) stages. It was shown that the ignition timing in activated zone depends on the specific energy input into the streamer channel, the radius of activated region coupled with the volume fraction of activated region treated by discharge, and the moment of discharge initiation relative to the top dead center (TDC). The speed of combustion wave, the occurrence of auto-ignition in the end gas and transition to combustion, accompanied by “knocking” or strong pressure fluctuations are largely determined by the moment of discharge initiation and the specific energy input. The significant effect of discharge is explained by the stimulation of oxidation kinetic mechanisms at LTHR and ITHR stages.

Numerical study of combustion-wave propagation initiated by non-equilibrium discharge

In the work, we present the results of a numerical study of the combustion initiation of the lean propane–air mixture under the conditions of an homogeneous-charge-compression-ignition engine. We show that preliminary discharge treatment could accelerate the ignition and study how the size of discharge area, specific input energy and other parameters influence upon the delay time. The role of chemical reactions occurring during the compression cycle (before discharge treatment) is also discussed.

A study of effects of the mixture flow on combustion characteristics for lean propane-air mixtures

A study of effects of the mixture flow on combustion characteristics for lean propane-air mixtures

A study of combustion improvement for lean propane-air mixtures by using a tube flame

A study of combustion improvement for lean propane-air mixtures by using a tube flame

Wrinkling of Large-Scale Flame in Lean Propane–Air Mixture Due to Cellular Instabilities

ABSTRACTIn accidental gas explosions, flame acceleration owing to cellular instabilities such as diffusional–thermal instability and Darrieus–Landau instability can cause considerable damages, for example, the formation of a strong blast wave. In particular, as the flame scale increases, Darrieus–Landau instability, caused by a density jump, progressively dominates in the flame acceleration. In this study, we experimentally investigated the growth and wrinkling owing to Darrieus–Landau instability of a spherically expanding flame in a large-scale experiment, in which a propane-air mixture of the equivalence ratio ϕ = 0.8 was filled and ignited in a plastic tent of 27 m3. Experimental images of large-scale flames of the lean propane–air mixture, for which the flame is diffusional–thermally stable, were analyzed. The edge of flame was detected and rearranged in polar coordinates. The results show that small-scale cells merge and form a bigger cell. The generated bigger cell grows by the instability mechanism and eventually forms a large single cusp. In addition, the peak-to-peak amplitude of the wrinkled flame was evaluated. The value of peak-to-peak amplitude increased as time progressed. Such a cellular flame gives rise to a fractal-like structure and acceleration of its propagation speed. The fractal dimension of the wrinkled flame surface was evaluated by logarithmically plotting the flame speed versus its radius and also by a box-counting method. The results demonstrated that the wrinkled structure of a large-scale flame can be characterized by its fractal dimension and that a transition period into a well-developed self-similar regime exists.

Effects of pulsation frequency and energy deposition on ignition using nanosecond repetitively pulsed discharges

The effects of pulse repetition frequency (PRF) and energy deposition was investigated using nanosecond repetitively pulsed discharges in a constant volume combustion chamber. Quiescent lean propane-air mixtures (equivalence ratio of 0.7) at 0.2MPa were ignited using fixed total energy deposition and number of pulses across the range of PRF from 2 to 90kHz. Individual pulse energies were measured by electrical probes, and the ignition time was quantified through the use of high-frame-rate intensified imaging of the chemiluminescence. For fixed inter-electrode gap and constant total energy, the ignition time was minimized at a certain pulsation frequency, with higher PRF being detrimental and resulting in longer ignition delay times. Decreasing the total energy deposited in the mixture shifted the minimum ignition time to higher PRF. On the contrary, increasing the gap distance at constant total energy allowed for the same minimum ignition time to be achieved at significantly lower PRF. The minimum ignition delay times for fixed inter-electrode gap distance were similar for a given amount of mean energy deposited per unit time. The effect of energy deposition, PRF, and inter-electrode gap was attributed to the competition between characteristic recirculation time from the discharge-induced flowfield and the inter-pulse time.

Nanosecond pulsed discharge in a propane–air mixture: Ignition and energy deposition

The study examined the possibility of using nanosecond pulse discharges as a new ignition and assisted combustion method to solve ignition and combustion stabilization problems encountered in new combustion technologies. To better understand how temperature and the presence of radicals affect nanosecond discharge ignitions, spatiotemporal profiles of rotational and vibrational N2(X) temperatures were measured through spontaneous Raman scattering in a lean propane–air mixture and compared with previous results obtained in air to obtain the space- and time-resolved measurements necessary to validate the kinetic modeling of the discharge in presence of hydrocarbons. The study aims to contribute to a better understanding of the initial ignition processes in stoichiometric mixture (first observed at 1µs) and the rapid displacement of the flame front in propane–air mixtures. In the analyzed propane–air mixture, the gas heated slightly more rapidly than in air. This temperature increase might have occurred in the release of energy resulting from dissociation of propane due to quenching by metastable species. The presence of traces of CO confirmed this assumption. The energy transfer processes were identical in all other respects and occurred over the same time scales in air and in the propane–air mixture. Once the flame in the stoichiometric mixture was ignited, it propagated through a cylindrical channel whose diameter was identical to that of the volume of gas heated to above 900K in the lean propane–air mixture. This early ignition and the spreading of the flame kernel demonstrate the combined effect of radicals and temperature on the nanosecond discharge ignition process. The resulting new database makes it possible to validate simulations of the vibrational kinetics involved in nanosecond discharges of a lean propane–air mixture and provides a first step toward modeling flame initiation.

Ignition of Quiescent Lean Propane–Air Mixtures at High Pressure by Nanosecond Repetitively Pulsed Discharges

We present an experimental study of lean mixture ignition by nanosecond repetitively pulsed (NRP) discharges. The plasma is created in a lean propane/air mixture at pressure up to 10 bar and equivalence ratio 0.7, premixed in a constant volume vessel. We characterize the initial spark radius, the ignition kernel development and the flame propagation as a function of pressure (up to 10 bar) and the pulse energy (1–6 mJ per pulse). Comparisons with a conventional igniter show that better results are obtained with NRP discharges in terms of flame propagation speed, in particular at high pressure, due to the increased wrinkling of the flame front that is induced by NRP discharges.

Study of combustion process with jet-ignition of propane-air mixtures

Abstract The paper presents the study of combustion process of a homogenous lean propane-air mixture in the cylindrical combustion chamber ignited by a hot gas jet from the pre-ignition chamber. A rich propane-air mixture in the pre-chamber is ignited by the spark plug and the exhaust gasses flow from the chamber trough the holes in the wall. The mathematical model of gas exchange and energy balance in chambers with a laminar finite-rate model taking into account the two-step Arrhenius chemical kinetics is presented. The work presents results of thermodynamic parameters of the charge obtained in CFD simulations in Fluent and Kiva3v for three configurations: with one hole in the wall of the ignition chamber, with three holes and without an ignition chamber. Modelling and simulation have shown faster burning of the mixture for jet ignition with three holes of the pre-chamber. The results of simulations were verified by experimental studies in the combustion chamber of the same geometry by the Schlieren method. The work presents flame front propagation, pressure traces and pressure increment speed for two mixtures with a different equivalence fuel-air ratio. Experimental results proved the simulation observation of faster flame propagation in the main chamber with three holes

Experimental assessment of various methods of determination of laminar flame speed in experiments with expanding spherical flames with positive Markstein lengths

Experimental data obtained from lean propane–air expanding spherical laminar flames at pressures of 1 and 0.5atm, with all other things being equal, are analyzed using four different extrapolation equations available in the literature and a unified processing algorithm developed in the present work. For comparison, experimental data obtained recently by Zamashchikov et al. (2014) from a very rich hydrogen–air flame are also processed using the same algorithm. Dependencies of (i) rms differences between measured and calculated flame radii, (ii) laminar flame speeds, and (iii) Markstein lengths on the boundaries of the analyzed range of flame radii are studied in order to (i) investigate the sensitivity of processing results to these boundaries, (ii) determine the proper radius range for each extrapolation equation, and (iii) to find an equation that fits the measured data in the widest radius range. For the lean propane–air mixture under room conditions, which is associated with a relatively weak effect of flame stretching on the flame kernel growth, all four equations yield approximately equal flame speeds, but the associated Markstein lengths are different. Moreover, the flame speeds and Markstein lengths are sensitive to the processed range of flame radii and the same measured curve can be fitted with the same equation using substantially different pairs of Markstein length and flame speed. Due to these two sources of errors, choice of a proper processing method does not seem to be a critical issue when analyzing weakly non-linear dependencies of flame radius on time. To the contrary, for two other flames, which are associated with stronger stretch effects, the two aforementioned sources of errors play a minor role, i.e. flame speeds and Markstein lengths are weakly sensitive to the processed range of flame radii provided that it is properly selected, while the rms differences between measured and calculated flame radii are sufficiently sensitive to variations in Markstein length and flame speed. Accordingly, a proper choice of a processing method is of paramount importance when analyzing substantially non-linear dependencies of flame radius on time. Results of present work indicate that the phenomenological Markstein equation, which is linear with respect to flame curvature, fits the experimental data well in a wider range when compared to three other equations. Because, close results were also obtained using a non-linear equation derived theoretically by Kelley et al. (2012), the latter equation is worth being highlighted from the basic standpoint.

Total oxidation of lean propane over α-Fe2O3 using microwaves as an energy source

A combined process of catalytic oxidation and microwave heating for the treatment of lean propane-air mixture, and “green” hematite synthesis were described in this work. The study demonstrated that removal and oxidation efficiencies of propane over α-Fe2O3 were above 99.9 % after a short microwave radiation time (from 9 min) and they were strongly dependent on the applied microwave power. Comparing to conventional heating, the catalytic performance of hematite was enhanced by lowering the operation temperature under microwave conditions. Nanocrystalline α-Fe2O3 catalyst with mesoporous structure was successfully prepared by a microwave-assisted hydrothermal treatment of iron hydroxide in an aqueous solution without any alkali additive, and characterized by XRD, TEM, TPR-H2, FTIR and N2 adsorption–desorption. The microwave-induced heating over hematite catalyst appears to be a very effective alternative method for air treatment to the destruction and removal of short chain hydrocarbons.

Propagation and Extinction of a Cylindrical Premixed Flame Undergoing Equivalence Ratio Fluctuation Near the Lean Limit

Experimental study was made to investigate the propagation and extinction characteristics of a stretched cylindrical flame undergoing periodic fluctuation of equivalence ratio near the lean limit. With a lean methane-air and a lean propane-air mixture, burning velocity, flame luminosity and flame stretch rate were measured or evaluated for the fluctuation frequencies of 5Hz and 20Hz. The results were summarized as follows: (1) In some part of a period, burning velocity and flame luminosity of the dynamic flame near the lean limit were possible to become lower than those at the lean flammability limit of the static flame. (2) At the high frequency of 20Hz, the burning velocity took a negative value in a certain time range. In spite of this loss of propagation ability, the flame was not extinguished but sustained, indicating the recovery of the flame intensity due to the dynamic effect of fluctuating flame. (3) Flame recovery phenomenon could occur more easily for the methane flame which was strengthened by the Lewis number effect than the propane flame which was weakened by that effect.

21115 微小重力下での希薄炭化水素-空気混合気の燃焼特性に及ぼす雰囲気湿度の影響について(第2報)(燃料と燃焼の新しい展開(3),OS.9 燃料と燃焼の新しい展開,学術講演)

In practical internal combustion engines, where it is very important to achieve the low emissions (CO_2,NO_x,SO_x). It is well known that the lean burn is very useful method to reduce the emissions by control of flame temperature. Therefore, in this study, at low oxygen concentration high humidity mixture was used for flame temperature reduction. Experiments have been carried out with lean propane-air mixtures to examine the influence of oxygen concentration under high-humidity air and dry air conditions on flame shape and flame speed in a combustion tube. The experimental results in this study are as follows. : (1) Under low oxygen concentration, the addition of water vapor is very effective to increase the flame speed under normal and microgravity. (2) Influence of atmosphere humidity on flame speed depends on a fuel property.

Propagation and Extinction of a Cylindrical Premixed Flame Undergoing Equivalence Ratio Fluctuation Near the Lean Limit

Experimental study was made to investigate the propagation and extinction characteristics of a stretched cylindrical flame undergoing periodic fluctuation of equivalence ratio near the lean limit. With a lean methane-air and a lean propane-air mixture, burning velocity, flame luminosity and flame stretch rate were measured or evaluated for the fluctuation frequencies of 5 Hz and 20 Hz. The results were summarized as follows : (1) In some part of a period, burning velocity and flame luminosity of the dynamic flame near the lean limit were possible to become lower than those at the lean flammability limit of the static flame. (2) At the high frequency of 20 Hz, the burning velocity took a negative value in a certain time range. In spite of this loss of propagation ability, the flame was not extinguished but sustained, indicating the recovery of the flame intensity due to the dynamic effect of fluctuating flame. (3) Flame recovery phenomenon could occur more easily for the methane flame which was strengthened by the Lewis number effect than the propane flame which was weakened by that effect.

21811 希薄炭化水素-空気混合気の燃焼特性に及ぼす雰囲気湿度の影響について(第2報)(熱工学(1))

In practical internal combustion engines, where it is very important to achieve the low emissions (CO_2,NO_x,SO_x). It is well known that the lean burn is very useful method to reduce the emissions by control of flame temperature. Therefore, in this study, at low oxygen concentration high-humidity mixture was used for flame temperature reduction. Experiments have been carried out with lean propane-air mixtures to examine the influence of oxygen concentration under high-humidity air and dry air conditions on flame shape and flame speed in a combustion tube. The expaimental results in this study are as follows.: (1) Under low oxygen concentration, the addition of stream is very effective to increase the flame speed. (2) influence of atmosphere humidity on flame speed depends on a fuel property.