Higher Flame Speed Research Articles

Flame Acceleration of Premixed Natural Gas/Air Explosion in Closed Pipe

An experimental study has been done to investigate the flame acceleration in a closed pipe. A horizontal steel pipe, 2m long and 0.1m in diameter (L/D of 20), was used in this work. For tests with 90 degree bends, the bend had a radius of 0.1m and thus, the pipe was lengthened 1m (based on the centreline length of the segment). Ignition was affected at one end of the vessel while the other end was closed. Only stoichiometric concentration (?, = 1.0) of natural gas/air mixtures will be reported in this paper. It was demonstrated that bend pipe configuration gave three times higher in maximum overpressure (5.5 bars) compared to straight pipe (2.0 bars). From the results, the highest flame speed, of 63ms-1, was observed in a gas explosion with bent pipe; greater by a factor of ~3 as compared with straight pipe (23ms-1). This occurs because bending acts similar to an obstacle, in which this mechanism can induce more turbulence, initiating combustion in an unburned pocket at the corner region and causing a high mass burning rate, which increases the flame speed.

Linking molecular level chemistry to macroscopic combustion behavior for nano-energetic materials with halogen containing oxides

Coupling molecular scale reaction kinetics with macroscopic combustion behavior is critical to understanding the influences of intermediate chemistry on energy propagation, yet bridging this multi-scale gap is challenging. This study integrates ab initio quantum chemical calculations and condensed phase density functional theory to elucidate factors contributing to experimentally measured high flame speeds (i.e., >900 m∕s) associated with halogen based energetic composites, such as aluminum (Al) and iodine pentoxide (I2O5). Experiments show a direct correlation between apparent activation energy and flame speed suggesting that flame speed is directly influenced by chemical kinetics. Toward this end, the first principle simulations resolve key exothermic surface and intermediate chemistries contributing toward the kinetics that promote high flame speeds. Linking molecular level exothermicity to macroscopic experimental investigations provides insight into the unique role of the alumina oxide shell passivating aluminum particles. In the case of Al reacting with I2O5, the alumina shell promotes exothermic surface chemistries that reduce activation energy and increase flame speed. This finding is in contrast to Al reaction with metal oxides that show the alumina shell does not participate exothermically in the reaction.

Porous media quenching behaviors of gas deflagration in the presence of obstacles

To understand the mechanism for the porous media quenching of gas deflagration in the presence of obstacles, an experimental study has been conducted, and ten cases in a laboratory scale obstructed chamber varying in terms of the number of obstacles, porous media pore size, thickness, and material have been examined. The results show that if the premixed flame propagating away from an ignition source encounters obstacles before the flame front arrives to the porous media, the interaction between flame and obstacles will result in higher flame speed and overpressure, which may cause a failure in the operation of porous media quenching. The porous media quenching behavior is dependent on the number of the obstacles in the flame path. Moreover, the smaller the pore size or the thicker the porous media plate, the better the flame quenching performance, but the larger the drop of the overpressure for flow-through the porous media. The material of the porous media is also found to have a significant effect on the quenching characteristics of gas deflagration. Compared to Al2O3 and SiC, the Al porous media shows superior flame quenching and overpressure attenuating performances. Mechanisms for the observed phenomena are discussed in detail.

The effect of hydrogen addition on premixed laminar acetylene–hydrogen–air and ethanol–hydrogen–air flames

In the present work, the laminar premixed acetylene–hydrogen–air and ethanol–hydrogen–air flames were investigated numerically. Laminar flame speeds, the adiabatic flame temperatures were obtained utilizing CHEMKIN PREMIX and EQUI codes, respectively. Sensitivity analysis was performed and flame structure was analyzed. The results show that for acetylene–hydrogen–air flames, combustion is promoted by H and O radicals. The highest flame speed (247 cm/s) was obtained in mixture with 95% H2–5% C2H2 at λ = 1.0. The region between 0.95 < XH2 < 1.0 was referred to as the acetylene-accelerating hydrogen combustion since the flame speed increases with increase the acetylene fraction in the mixture. Further increase in the acetylene fraction decreases the H radicals in the flame front. In ethanol–hydrogen–air mixtures, the mixture reactivity is determined by H, OH and O radicals. For XH2 < 0.6, the flame speed in this regime increases linearly with increasing the hydrogen fraction. For XH2 > 0.8, the hydrogen chemistry control the combustion and ethanol addition inhibits the reactivity and reduces linearly the laminar flame speed. For 0.6 < XH2 < 0.8, the laminar flame speed increases exponentially with the increase of hydrogen fraction.

HCNG 엔진용 연료시스템의 적용성 평가

CNG buses has contributed to improve air quality in cities. But it is difficult to meet the next emission regulations such as EURO-VI without the help of additional post-processing device. Hydorgen has higher flame speed and lower combustion temperature that make it thermal efficiency increase with leaner operation. Using hydrogen natural gas blend (HCNG) fuel is promising technology which can reduce <TEX>$NO_x$</TEX> and <TEX>$CO_2$</TEX> emissions for a natural gas vehicle. However, fuel flow rate of HCNG should be increased since hydrogen's energy density per volume is much smaller than natural gas. In the present study, the characteristics of fuel supply system and its applicability were evaluated in a heavy duty natural gas engine. The results showed that the potential of fuel pressure regulator and fuel metering valve had enough capacity with HCNG. Employed mixer did not affect the distribution characteristics of mixture.

Wheat as a Promising Substitute of Corn for Bioethanol Production

Abstract Excessive consumption of fossil fuels, particularly in large urban areas, has resulted in generation of high levels of pollution during the last few decades. All petroleum-based fuels can be replaced by renewable biomass fuels such as bioethanol, bio-diesel, bio-hydrogen, etc. Bioethanol is an attractive alternative fuel because it is a renewable bio-based resource and it is oxygenated thereby provides the potential to reduce particulate emissions in compression ignition engines .It has a higher octane number, and lower cetane number, broader flammability limits, higher flame speeds and higher heats of vaporization than gasoline. The bioethanol can be produced from lignocellulosic biomass, starchy materials such as corn, wheat, cereals etc and sucrose containing feedstocks. Major portion of the production methods uses corn for the same, but since India being the second largest producer of wheat and there is availability of different varieties of wheat, we have selected wheat as a feedstock. After thorough review of extensive literature we finalized two enz ymes viz; glucoamylase and -amylase followed by fermentation process using bake yeast Saccharomyces Cerivisiae. Experiments were carried out to see the day wise decrease in concentration of glucose and increase in concentration of Ethanol by calculating the absorbance using Ultraviolet-visible spectrophotometer. Results obtained were in agreement with the possibility of wheat to be used as a raw material for maximum yield of bioethanol.

Reprint of “Effect of exhaust confinement and fuel type upon the blowoff limits and fuel switching ability of swirl combustors”

The use of swirl burners with premixed hydrogen–methane fuel blends is a promising technology for low-emission power generation. Utilisation of hydrogen containing fuel mixtures can result in low-emission levels, but it is well known that there are many difficulties, primarily because of the very high laminar and turbulent flame speeds of hydrogen. Problems such as blowoff and flashback limits are extremely important where fuel flexibility is required. In this study, a generic swirl combustor at Cardiff University's GTRC is utilised to investigate blowoff and the ability of the premixed combustor to switch fuels whilst still maintain the same thermal load, for a range of alternative hydrogen based fuel mixtures in configurations where the confinement is representative of gas turbine practice. This complements previous work on the same generic combustor, where the focus was entirely on flashback limits.Ideally to achieve fuel switching or dual fuelling for nominally similar combustor geometries, the operating points for pure hydrogen and natural gas should lie in an operational regime between the blowoff and flashback limits of both fuels. Normal concepts of equivalence ratio matching need modification to allow for the varying stoichiometric requirements of different fuel mixtures and the associated differences in their heating values. Here heating input from the various fuels as a function of mass flow is used to compare their ability to operate in the same operational, fuel lean regime of the premixed combustor. In practice this is extremely difficult; however, fuel switching/dual fuelling is possible in the swirl burner with certain fuel blends (where the hydrogen content is limited).The results demonstrate and quantify improvements in blowoff limits for hydrogen-enriched methane flames. Moreover, for all geometrical configurations considerably improved blowoff characteristics were observed for the confined cases in contrast to the unconfined cases. This data offers a significant insight to burner manufacturers aiming to use swirl combustors with hydrogen-containing alternative fuels.

Performance of a Multiple-Injection Dry Low NOx Combustor With Hydrogen-Rich Syngas Fuels

An oxygen-blown integrated coal gasification combined cycle (IGCC) plant with precombustion carbon dioxide capture and storage (CCS) is one of the most promising means of zero-emission generation of power from coal. In an IGCC plant with CCS, hydrogen-rich syngas with a wide variation of hydrogen contents is supplied to a gas turbine. Such hydrogen-rich syngas poses a great challenge to a low NOx combustor based on premixed combustion technology, because its high flame speed, low ignition energy, and broad flammability limits can cause flashback and/or autoignition. On the contrary, a diffusion combustor suffers from the high flame temperature of syngas and the resulting high NOx emission. The authors applied a “multi-injection burner” concept to a preliminary burner for hydrogen-rich syngas simulating that from IGCC with CCS. In a preliminary experiment under atmospheric pressure, the multi-injection burner worked without any flashback or any blowout. A prototype multicluster combustor based on the results of that preliminary study was made to be a dry low NOx combustor for hydrogen-rich syngas of IGCC with CCS. It was tested in experiments, which were carried out under medium pressure (0.6 MPa) using test fuels simulating syngas from IGCC with a 0% carbon capture rate, a 30% carbon capture rate, and a 50% carbon capture rate. The test fuels contained hydrogen, methane, and nitrogen, and had a hydrogen content ranging from 40% to 65%.The following conclusions were drawn from the test results: (1) the tested combustor allows the stable combustion of fuels simulating 0%, 30%, and 50% CCS, (2) a convex perforated plate swirler is effective to suppress combustion oscillation, which allows NOx emissions to be less than 10 ppm through the variation of fuel simulating 0%, 30%, and 50% CCS, (3) the extended stable combustion region and enhanced entrainment and mixing due to the convex perforated plate improves the cooling of the combustor liner metal to be less than the liner metal temperature criterion.

Experiments and zero D modeling studies using specific Wiebe coefficients for producer gas as fuel in spark-ignited engines

The paper addresses experiments and modeling studies on the use of producer gas, a bio-derived low energy content fuel in a spark-ignited engine. Producer gas, generated in situ, has thermo-physical properties different from those of fossil fuel(s). Experiments on naturally aspirated and turbo-charged engine operation and subsequent analysis of the cylinder pressure traces reveal significant differences in the heat release pattern within the cylinder compared with a typical fossil fuel. The heat release patterns for gasoline and producer gas compare well in the initial 50% but beyond this, producer gas combustion tends to be sluggish leading to an overall increase in the combustion duration. This is rather unexpected considering that producer gas with nearly 20% hydrogen has higher flame speeds than gasoline. The influence of hydrogen on the initial flame kernel development period and the combustion duration and hence on the overall heat release pattern is addressed. The significant deviations in the heat release profiles between conventional fuels and producer gas necessitates the estimation of producer gas-specific Wiebe coefficients. The experimental heat release profiles are used for estimating the Wiebe coefficients. Experimental evidence of lower fuel conversion efficiency based on the chemical and thermal analysis of the engine exhaust gas is used to arrive at the Wiebe coefficients. The efficiency factor a is found to be 2.4 while the shape factor m is estimated at 0.7 for 2% to 90% burn duration. The standard Wiebe coefficients for conventional fuels and fuel-specific coefficients for producer gas are used in a zero D model to predict the performance of a 6-cylinder gas engine under naturally aspirated and turbo-charged conditions. While simulation results with standard Wiebe coefficients result in excessive deviations from the experimental results, excellent match is observed when producer gas-specific coefficients are used. Predictions using the same coefficients on a 3-cylinder gas engine having different geometry and compression ratio(s) indicate close match with the experimental traces highlighting the versatility of the coefficients.

Laminar flame speed correlations for pure-hydrogen and high-hydrogen content syngas blends with various diluents

Hydrogen is an attractive fuel for large-scale combustion systems due to its high flame speeds and clean burning characteristics. This paper presents a new set of correlations for the laminar flame speeds of hydrogen-oxygen mixtures with nitrogen (air) and helium as diluents, using a recently updated chemical kinetics mechanism. A wide excursion of equivalence ratios (φ = 0.5–5.0), pressures (1–30 atm) and temperatures (270–620 K) was performed. Flame speed correlations were developed at five pressures, namely 1, 5, 10, 20, and 30 atm for the pure-hydrogen case. The disparities between the kinetic model predictions and the correlation estimates, commonly associated with existing correlations, were significantly reduced, and the correlation estimates are within ±13 cm/s of the model predictions. Also, a correlation for lean and high-hydrogen content (HHC) syngas blends of H2 + CO + H2O was developed from the pure-hydrogen correlations. A wide range of pressures (1–30 atm), initial temperatures (323–550 K), steam contaminant levels (5–15%), and hydrogen content in the fuel blend (15–100%) were simulated. A design of experiments approach was adopted to determine the critical mixtures necessary to develop the correlation. The developed HHC correlation agrees within ±12% of the model predictions.

Development of a gas turbine fuel nozzle for DME and a design method thereof

DME (dimethyl ether, CH3OCH3) is a potentially attractive fuel for gas turbines because of low pollutant emission and easy transportation and storage. However, the high flame speed and a low ignition temperature of DME present a high risk of flash-back, which can be a potentially serious problem. To solve this problem and achieve better combustion performance, the present study contrived a new fuel nozzle for DME that can obtain optimal combustion of DME in the gas turbine combustor, thereby achieving cost reduction of power generation, enhancement in reliability of power plants and diversification of usable fuel. The configuration of a fuel nozzle for DME and a design method using Wobbe Index are both described in detail. The combustion performance of the newly developed DME fuel nozzle was verified through a gas turbine combustion test and the results showed considerable improvement in the performance of NOx and CO emissions and the prevention of flash-back.

Effect of exhaust confinement and fuel type upon the blowoff limits and fuel switching ability of swirl combustors

The use of swirl burners with premixed hydrogen–methane fuel blends is a promising technology for low-emission power generation. Utilisation of hydrogen containing fuel mixtures can result in low-emission levels, but it is well known that there are many difficulties, primarily because of the very high laminar and turbulent flame speeds of hydrogen. Problems such as blowoff and flashback limits are extremely important where fuel flexibility is required. In this study, a generic swirl combustor at Cardiff University's GTRC is utilised to investigate blowoff and the ability of the premixed combustor to switch fuels whilst still maintain the same thermal load, for a range of alternative hydrogen based fuel mixtures in configurations where the confinement is representative of gas turbine practice. This complements previous work on the same generic combustor, where the focus was entirely on flashback limits.Ideally to achieve fuel switching or dual fuelling for nominally similar combustor geometries, the operating points for pure hydrogen and natural gas should lie in an operational regime between the blowoff and flashback limits of both fuels. Normal concepts of equivalence ratio matching need modification to allow for the varying stoichiometric requirements of different fuel mixtures and the associated differences in their heating values. Here heating input from the various fuels as a function of mass flow is used to compare their ability to operate in the same operational, fuel lean regime of the premixed combustor. In practice this is extremely difficult; however, fuel switching/dual fuelling is possible in the swirl burner with certain fuel blends (where the hydrogen content is limited).The results demonstrate and quantify improvements in blowoff limits for hydrogen-enriched methane flames. Moreover, for all geometrical configurations considerably improved blowoff characteristics were observed for the confined cases in contrast to the unconfined cases. This data offers a significant insight to burner manufacturers aiming to use swirl combustors with hydrogen-containing alternative fuels.

Analysis of the effects of reformate (hydrogen/carbon monoxide) as an assistive fuel on the performance and emissions of used canola-oil biodiesel

Evolving technology and a reoccurring energy crisis creates a continued investigation into the search for sustainable and clean-burning renewable fuels. One possibility is hydrogen that has many desirable qualities such as a low flammability limit promoting ultra-lean combustion, high laminar flame speed for increased thermal efficiency and low emissions. However, past research discovered certain limiting factors in its use such as pre-ignition in spark ignition engines and inability to work as a singular fuel in compression ignition engines. To offset these issues, this work documents manifold injection of a hydrogen/carbon monoxide mixture in a dual-fuel methodology with biodiesel. While carbon monoxide does degrade some of the desirable properties of hydrogen, it acts partially like a diluent to restrict pre-ignition. The result of this mixture addition allows the engine to maintain power while reducing biodiesel fuel consumption with a minimal NOx emissions increase.

정전기 방전에너지에 따른 가솔린-공기 혼합물의 화염전파

실린더형 챔버내에서 정전기 방전에너지 변화에 따른 가솔린-공기 혼합물의 화염전파에 관한 영향을 조사하기 위해 실험적 연구를 수행하였다. 3개의 서로 다른 정전기 방전 에너지(1 mJ, 50 mJ 및 98 mJ)를 실험변수로 사용하였으며, 점화원 전극 주변의 미연소가스 유동장을 가시화하기 위해 고속 PIV 시스템을 적용하였다. 정전기 방전 에너지가 증가할 때, 점화원 핵은 찌그러면서 초기화염에 영향을 미치는 것으로 나타났다. 초기화염 동안에 화염속도는 점화에너지가 높을수록 증가하는 것으로 나타났으나, 초기화염 이후에 시간이 증가할수록 화염속도는점화에너지에 관계없이 거의 유사하였으며, 이는 문헌[5]에서 보여진 전산유체 모델링 결과의 경향과 거의 유사하였다. 또한, 점화에너지가 증가할 때 전파하는 화염 전면의 미연소가스 속도장은 증가하는 것으로 나타났다. Experimental studies were carried out to investigate the effects on flame propagation of gasoline-air mixtures by different electrostatic discharge energies in a cylindrical chamber. Three different ignition energies were used: 1 mJ, 50 mJ and 98 mJ. In this work, a high-speed particle image velocimetry technique was applied to visualize the flow-field around ignition electrodes. It was found that as the ignition energy increased, the ignition kernel was different. The different ignition kernel caused different flame initiation. During the flame initiation, the higher ignition energy was applied, the higher flame speed was observed. However, with increasing time, the flame speeds were independent of the ignition energies used. Theses observed flame behaviors were similar to computational simulations shown in the literature. It was also found that as the ignition energies increased, the velocities of unburnt mixtures ahead of propagating flame fronts increased.

Starting a spark-ignited engine with the gasoline–hydrogen mixture

Because of the increased fuel-film effect and dropped combustion temperature, spark-ignited (SI) gasoline engines always expel large amounts of HC and CO emissions during the cold start period. This paper experimentally investigated the effect of hydrogen addition on improving the cold start performance of a gasoline engine. The test was carried out on a 1.6-L, four-cylinder, SI engine equipped with an electronically controlled hydrogen injection system. A hybrid electronic control unit (HECU) was applied to control the opening and closing of hydrogen and gasoline injectors. Under the same environmental condition, the engine was started with the pure gasoline and gasoline–hydrogen mixture, respectively. After the addition of hydrogen, gasoline injection duration was adjusted to ensure the engine to be started successfully. All cold start experiments were performed at the same ambient, coolant and oil temperatures of 17 °C. The test results showed that cylinder and indicated mean effective pressures in the first cycle were effectively improved with the increase of hydrogen addition fraction. Engine speed in the first 20 start cycles increased with hydrogen blending ratio. However, in later cycles, engine speed varied only a little with and without hydrogen addition due to the adoption of close loop control on engine speed. Because of the low ignition energy and high flame speed of hydrogen, both flame development and propagation durations were shortened after hydrogen addition. HC and CO emissions were dropped markedly after hydrogen addition due to the enhanced combustion process. When the hydrogen flow rate increased from 0 to 2.5 and 4.3 L/min, the instantaneous peak HC emissions were sharply reduced from 57083 to 17850 and 15738 ppm, respectively. NOx emissions were increased in the first 5 s and then reduced later after hydrogen addition.

Modeling of a reacting nanofilm on a composite substrate

This article provides a detailed computational analysis of the reaction of dense nanofilms and the heat transfer characteristics on a composite substrate. Although traditional energetic compounds based on organic materials have similar energy per unit weight, non-organic material in nanofilm configuration offers much higher energy density and higher flame speed. The reaction of a multilayer thin film of aluminum and copper oxide has been studied by varying the substrate material and thicknesses. The numerical analysis of the thermal transport of the reacting film deposited on the substrate combined a hybrid approach in which a traditional two-dimensional black box theory was used in conjunction with the sandwich model to estimate the appropriate heat flux on the substrate accounting for the heat loss to the surroundings. A procedure to estimate this heat flux using stoichiometric calculations is provided. This work highlights two important findings. One is that there is very little difference in the temperature profiles between a single substrate of silica and a composite substrate of silicon silica. Secondly, with increase in substrate thickness, the quenching effect is progressively diminished at a given speed. These findings show that the composite substrate is effective and that the average speed and quenching of flames depend on the thickness of the silica substrate, and can be controlled by a careful choice of the substrate configuration. (C) 2011 Elsevier Ltd. All rights reserved.

Bioethanol from Biomass: A Review

As concern about global warming and dependence on fossil fuels grows, the search for renewable energy sources that reduce CO2 emissions becomes a matter of widespread attention. To reduce the net contribution of GHGs to the atmosphere, ethanol has been recognized as a potential alternative to petroleum-derived transportation fuels. Production of ethanol from biomass (bioethanol) is one way to reduce both consumption of crude oil and environmental degradation. It is biodegradable, low in toxicity and causes little environmental pollution if spilt. Bioethanol has higher octane number, broader flammability limit, higher flame speed and higher heat of vaporization than gasoline. These properties allow for a higher compression ratio, shorter burn time and leaner burn engine, which lead to theoretical efficiency advantages over gasoline in an internal combustion engine. Bioethanol currently accounts for more than 94% of global biofuel production. Lignocellulosic biomass is envisaged to provide a significant portion of the feedstocks for bioethanol production in the medium and long form due to their low cost and high availability. Successful utilization of cellulosic materials as renewable resources is dependent on the development of economically feasible process technologies for cellulase production. Cellulase plays a vital role in the production of bioetanol from biomass. Hydrolysis of biomass is usually carried out by the action of cellulolytic enzymes and the sequential process called Simultaneous Saccharification and Fermentation process (SSF). Cellulase production is the most expensive step during ethanol production from cellulosic biomass, and accountes for approximately 40% of the total cost. Significant cost reduction is required in order to enhance the commercial viability of cellulase production technology. Therefore, efforts are to be intensified to produce ethanol efficiently through improved fermentation technologies. Reduction in the cost of cellulases can be achieved by use of cheaper raw materials and economically viable fermentation strategies. Any strategies which can bring down the production cost of cellulases, can significantly reduce the cost of bioethanol. In the present paper, bioethanol production from lignocellulosic wastes and their cost reduction strategies are highlighted.

Combustion and emissions performance of a hybrid hydrogen–gasoline engine at idle and lean conditions

Due to the narrow flammability of gasoline, pure gasoline-fueled spark-ignited (SI) engines always encounter partial burning or even misfire at lean conditions. Gasoline engines tend to suffer poor combustion and expel large emissions at idle conditions because of the high variation in the intake charge and low combustion temperature. Comparatively, hybrid hydrogen engines (HHE) fueled with the mixtures of hydrocarbon fuels and hydrogen seem to achieve lower emissions and gain higher thermal efficiencies than the original hydrocarbon-fueled engines due to the wide flammability and high flame speed of hydrogen. Since a HHE only requires a small amount of hydrogen, it also removes concerns about the high production and storage costs of hydrogen. This paper introduced an experiment conducted on a four-cylinder SI gasoline engine equipped with a hydrogen port-injection system to explore the performance of a hybrid hydrogen–gasoline engine (HHGE) at idle and lean conditions. The injection timings and durations of hydrogen and gasoline were governed by a hybrid electronic control unit (HECU) developed by the authors, which can be adjusted freely according to the commands from a calibration computer. During the test, hydrogen flow rate was varied to ensure that hydrogen volume fraction in the intake was constantly kept at 3%. For the specified hydrogen addition level, gasoline flow rate was reduced to make the engine operate at idle and lean conditions with various excess air ratios. The test results demonstrated that cyclic variations in engine idle speed and indicated mean effective pressure were eased with hydrogen enrichment. The indicated thermal efficiency was obviously higher for the HHGE than that for the original gasoline engine at idle and lean conditions. The indicated thermal efficiency at an excess air ratio of 1.37 was increased from 13.81% for the original gasoline engine to 20.20% for the HHGE with a 3% hydrogen blending level. Flame development and propagation periods were also evidently shortened after hydrogen blending. Moreover, HC, CO and NOx emissions were all improved after hydrogen enrichment at idle and lean conditions. Therefore, the HHE methodology is an effective and promising way for improving engine idle performance at lean conditions.

Simulations of laminar flame propagation in droplet mists

In order to clarify the conditions conducive to propagation of premixed flames in quiescent sprays, a one-dimensional code with detailed chemistry and transport was used. n-Heptane and n-decane, distinguished by their volatility, were studied under atmospheric and low temperature, low pressure conditions. The effects of initial droplet diameter, overall equivalence ratio ϕ 0 and droplet residence time before reaching the flame front were examined. Increasing the residence time had an effect only for n-heptane, with virtually no evaporation occurring before the flame front for n-decane. The trends were only marginally correlated with the local gaseous equivalence ratio ϕ eff at the location of maximum heat release rate. ϕ eff could be as low as 0.4 (beyond the lean flammability limit), but the flame speed could still be 40% of the gaseous stoichiometric flame speed S L,0 . For n-heptane, ϕ eff increased towards ϕ 0 with smaller droplets while high flame speeds occurred when ϕ eff was near 1. This implied that the highest flame speed was achieved with small droplets for ϕ 0 ⩽ 1 and with relatively large droplets for ϕ 0 > 1. In the latter case, the oxidiser was completely consumed in the reaction zone and droplets finished evaporating behind the flame where the fuel was pyrolysed. The resulting small species, mainly C 2H 2, C 2H 4 and H 2, diffused back to the oxidation zone and enhanced the reaction rate there. Ultimately, this could result in flame speeds higher than S L,0 even with ϕ 0 = 4. For n-decane, the same trends were followed but smaller droplets were needed to reach the same ϕ eff due to the slow evaporation rate. Under low pressure and low temperature, the effects of pressure and temperature on ϕ eff and the flame speed were competitive and resulted in values close to the ones at atmospheric conditions.

Modelling of combustion and nitrogen oxide formation in hydrogen-fuelled internal combustion engines within a 3D CFD code

The concerns about global warming and long-term lack of fossil fuels are strong incentives for alternative fuel research and adaptation of the internal combustion engines (ICE) to these fuels. Because it is free of any carbon compounds and can be produced from alternative sources, hydrogen is an interesting candidate for future ICE-based powertrains. However, the peculiar properties of hydrogen, among those its low density and its very high laminar flame speed, impose specific operating strategies and the adaptation of the conventional research tools. In this context, the 3D CFD models dedicated to combustion and pollutant prediction have to be modified. In the present work, the ECFM (Extended Coherent Flame Model) is adapted to hydrogen combustion through the addition of a new laminar flame speed correlation and a new laminar flame thickness expression. Furthermore, the prediction of the NO x emissions is performed with a modified version of the extended Zeldovitch model.