Ignition Of Fuels Research Articles

Operational Flammability Limits of Commercial Fuels – A Review

This manuscript considers the operational flammability limits of commercial fuels used in the automotive and in the aviation industry. Commonly employed fuels are considered in this study. Understanding the flammability limits is an important aspect while choosing right fuel for a given application. An explosion of stored fuel or combustion system can be catastrophic and can create a serious impact to the society. Therefore, researchers while developing fuels and combustion systems must give considerable attention. This manuscript reviews the effect of vapour pressure and temperature on flammability of fuels and reviews the auto ignition and flash point temperature of fuels commonly seen in the automotive and in the aviation industry.

Investigation of the ignition of liquid hydrocarbon fuels with nanoadditives

During our experimental studies we showed a high efficiency of the influence of nanoparticle additives on the stability of the ignition of hydrocarbon fuels and the stabilization of their combustion in a highfrequency high-voltage discharge. We detected the effects of a jet deceleration, an increase in the volume of the combustible mixture, and a reduction in the inflammation delay time. These effects have been estimated quantitatively by digitally processing the video frames of the ignition of a bubbled kerosene jet with 0.5% graphene nanoparticle additives and without these additives. This effect has been explained by the influence of electrodynamic processes.

Unraveling the structure and chemical mechanisms of highly oxygenated intermediates in oxidation of organic compounds

Decades of research on the autooxidation of organic compounds have provided fundamental and practical insights into these processes; however, the structure of many key autooxidation intermediates and the reactions leading to their formation still remain unclear. This work provides additional experimental evidence that highly oxygenated intermediates with one or more hydroperoxy groups are prevalent in the autooxidation of various oxygenated (e.g., alcohol, aldehyde, keto compounds, ether, and ester) and nonoxygenated (e.g., normal alkane, branched alkane, and cycloalkane) organic compounds. These findings improve our understanding of autooxidation reaction mechanisms that are routinely used to predict fuel ignition and oxidative stability of liquid hydrocarbons, while also providing insights relevant to the formation mechanisms of tropospheric aerosol building blocks. The direct observation of highly oxygenated intermediates for the autooxidation of alkanes at 500-600 K builds upon prior observations made in atmospheric conditions for the autooxidation of terpenes and other unsaturated hydrocarbons; it shows that highly oxygenated intermediates are stable at conditions above room temperature. These results further reveal that highly oxygenated intermediates are not only accessible by chemical activation but also by thermal activation. Theoretical calculations on H-atom migration reactions are presented to rationalize the relationship between the organic compound's molecular structure (n-alkane, branched alkane, and cycloalkane) and its propensity to produce highly oxygenated intermediates via extensive autooxidation of hydroperoxyalkylperoxy radicals. Finally, detailed chemical kinetic simulations demonstrate the influence of these additional reaction pathways on the ignition of practical fuels.

The structure and propagation of laminar flames under autoignitive conditions

The laminar flame speed sl is an important reference quantity for characterising and modelling combustion. Experimental measurements of laminar flame speed require the residence time of the fuel/air mixture (τf) to be shorter than the autoignition delay time (τ). This presents a considerable challenge for conditions where autoignition occurs rapidly, such as in compression ignition engines. As a result, experimental measurements in typical compression ignition engine conditions do not exist. Simulations of freely propagating premixed flames, where the burning velocity is found as an eigenvalue of the solution, are also not well posed in such conditions, since the mixture ahead of the flame can autoignite, leading to the so called “cold boundary problem”. Here, a numerical method for estimating a reference flame speed, sR, is proposed that is valid for laminar flame propagation at autoignitive conditions. Two isomer fuels are considered to test this method: ethanol, which in the considered conditions is a single-stage ignition fuel; and dimethyl ether, which has a temperature-dependent single- or two-stage ignition and a negative temperature coefficient regime for τ. Calculations are performed for the flame position in a one-dimensional computational domain with inflow-outflow boundary conditions, as a function of the inlet velocity UI and for stoichiometric fuel–air premixtures. The response of the flame position, LF, to UI shows distinct stabilisation regimes. For single-stage ignition fuels, at low UI the flame speed exceeds UI and the flame becomes attached to the inlet. Above a critical UI value, the flame detaches from the inlet and Lf becomes extremely sensitive to UI until, for sufficiently high UI, the sensitivity decreases and Lf corresponds to the location expected from a purely autoignition stabilised flame. The transition from the attached to the autoignition regimes has a corresponding peak dLf/dUI value which is proposed to be a unique reference flame speed sR for single-stage ignition fuels. For two-stage ignition fuels, there is an additional stable regime where a high-temperature flame propagates into a pool of combustion intermediates generated by the first stage of autoignition. This results in two peaks in dLf/dUI and therefore two reference flame speed values. The lower value corresponds to the definition of sR for single-stage ignition fuels, while the higher value exists only for two-stage ignition fuels and corresponds to a high temperature flame propagating into the first stage of autoignition and is denoted sR′. A transport budget analysis for low- and high-temperature radical species is also performed, which confirms that the flame structures at UI=sR and UI=sR′ do indeed correspond to premixed flames (deflagrations), as opposed to spontaneous ignition fronts which do not have a unique propagation speed.

Ignition of coal-water fuels made of coal processing wastes and different oils

The paper presents results of a comparison of integral characteristics for ignition and combustion of droplets of organic coal-water fuels (OCWF) made using processing wastes (filter cakes) of coals of different grade and typical liquid combustibles: waste engine, turbine oil, and fuel oil. In experiments, sizes (radii) of fuel droplets are from 0.4mm to 1.5mm; a temperature of oxidizer varies from 600K to 1200K; oxidizer flow rate is in a range from 0.5m/s to 5m/s. We consider the two hydrodynamic regimes of a relative motion of the droplets and oxidizer flow: a steady-state regime with droplet position on a junction of a low-inertia thermocouple and a regime of droplet soaring in oxidizer flow. Minimal ignition temperatures (significantly less than 1000K) of filter-cake-based OCWF are reported. Study presents dependences of the integral characteristics of OCWF ignition on properties of liquid fuel components, such as, primarily, the flash point and ignition temperature. Based on experimental results, recommendations for choosing an optimal regime of combustion and a method of slag removal depending on used components of OCWF are given. Experimental data and formulated conclusions can be considered asa significant contribution to a development of OCWF technologies, which can help to solve global problems of humanity, i.e. reduction of an atmospheric pollution, global warming and dimming.

Mechanism of soft x-ray continuum radiation from low-energy pinch discharges of hydrogen and ultra-low field ignition of solid fuels

EUV continuum radiation (10–30 nm) arising only from very low energy pulsed pinch gas discharges comprising some hydrogen was first observed at BlackLight Power, Inc. and reproduced at the Harvard Center for Astrophysics (CfA). The source was determined to be due to the transition of H to the lower-energy hydrogen or hydrino state H(1/4) whose emission matches that observed wherein alternative sources were eliminated. The identity of the catalyst that accepts 3 · 27.2 eV from the H to cause the H to H(1/4) transition was determined to HOH versus 3H. The mechanism was elucidated using different oxide-coated electrodes that were selective in forming HOH versus plasma forming metal atoms as well as from the intensity profile that was a mismatch for the multi-body reaction required during 3H catalysis. The HOH catalyst was further shown to give EUV radiation of the same nature by igniting a solid fuel comprising a source of H and HOH catalyst by passing a low voltage, high current through the fuel to produce explosive plasma. No chemical reaction can release such high-energy light. No high field existed to form highly ionized ions that could give radiation in this EUV region that persisted even without power input. This plasma source serves as strong evidence for the existence of the transition of H to hydrino H(1/4) by HOH as the catalyst and a corresponding new power source wherein initial extraordinarily brilliant light-emitting prototypes are already producing photovoltaic generated electrical power. The hydrino product of a catalyst reaction of atomic hydrogen was analyzed by multiple spectroscopic techniques. Moreover, the mH catalyst was identified to be active in astronomical sources such as the Sun, stars and interstellar medium wherein the characteristics of hydrino match those of the dark matter of the Universe.

Development of combined low-emissions burner devices for low-power boilers

Low-power water boilers are widely used for autonomous heat supply in various industries. Firetube and water-tube boilers of domestic and foreign manufacturers are widely represented on the Russian market. However, even Russian boilers are supplied with licensed foreign burner devices, which reduce their competitiveness and complicate operating conditions. A task of developing efficient domestic low-emissions burner devices for low-power boilers is quite acute. A characteristic property of ignition and fuel combustion in such boilers is their flowing in constrained conditions due to small dimensions of combustion chambers and flame tubes. These processes differ significantly from those in open combustion chambers of high-duty power boilers, and they have not been sufficiently studied yet. The goals of this paper are studying the processes of ignition and combustion of gaseous and liquid fuels, heat and mass transfer and NO x emissions in constrained conditions, and the development of a modern combined low-emissions 2.2 MW burner device that provides efficient fuel combustion. A burner device computer model is developed and numerical studies of its operation on different types of fuel in a working load range from 40 to 100% of the nominal are carried out. The main features of ignition and combustion of gaseous and liquid fuels in constrained conditions of the flame tube at nominal and decreased loads are determined, which differ fundamentally from the similar processes in steam boiler furnaces. The influence of the burner devices design and operating conditions on the fuel underburning and NO x formation is determined. Based on the results of the design studies, a design of the new combined low-emissions burner device is proposed, which has several advantages over the prototype.

On the effects of fuel properties and injection timing in partially premixed compression ignition of low octane fuels

A better understanding on the effects of fuel properties and injection timing is required to improve the performance of advanced engines based on low temperature combustion concepts. In this work, an experimental and computational study was conducted to investigate the effects of physical and chemical kinetic properties of low octane fuels and their surrogates in partially premixed compression ignition (PPCI) engines. The main objective was to identify the relative importance of physical versus chemical kinetic properties in predicting practical fuel combustion behavior across a range of injection timings. Two fuel/surrogate pairs were chosen for comparison: light naphtha (LN) versus the primary reference fuel (PRF) with research octane number of 65 (PRF 65), and FACE (fuels for advanced combustion engines) I gasoline versus PRF 70. Two sets of parametric studies were conducted: the first varied the amount of injected fuel mass at different injection timings to match a fixed combustion phasing, and the second maintained the same injected fuel mass at each injection timing to assess resulting combustion phasing changes. Full-cycle computational fluid dynamic engine simulations were conducted by accounting for differences in the physical properties of the original and surrogate fuels, while employing identical chemical kinetics. The simulations were found to capture trends observed in the experiments, while providing details on spatial mixing and chemical reactivity for different fuels and injection timings. It was found that differences in physical properties become increasingly important as injection timing was progressively delayed from premixed conditions, and this was rationalized by analysis of mixture stratification patterns resulting from injection of fuels with different physical properties. The results suggest that accurate descriptions of both physical and chemical behavior of fuels are critical in predictive simulations of PPCI engines for a wide range of injection timings.

Effect of fatty acid composition on ignition behavior of straight vegetable oils measured in a constant volume combustion chamber apparatus

Straight vegetable oils attracted attention several times as alternative fuel for mobile machinery. Certain efforts have already been undertaken to enable the usage of the high viscous oils in compression ignition engines. While physical properties are well specified for different kind of oils, only few information is available on the ignition behavior of vegetable oils under modern engine like conditions. Therefore, the ignition behavior of nine different vegetable oils and two oil mixtures was investigated using the Advanced Fuel Ignition Delay Analyser (AFIDA). The AFIDA is a modern constant volume combustion chamber apparatus equipped with a high pressure injection system. Analysis was performed at combustion chamber temperatures ranging from 796K to 967K and at air density levels of 4.7kg/m3, 9.3kg/m3 and 17.7kg/m3. Ignition behavior was correlated to the structure indices average number of carbon atoms and average number of double bonds. Both were calculated using the fatty acid composition of the oils. Ignition delay decreased with rising temperature and air density. Coconut oil ignited first at every temperature except for the lowest air density level. Start of combustion was always last for linseed oil, followed by camelina and soybean oil. Ignition delay increased linearly with rising average number of double bonds and decreasing average number of carbon atoms. The effect of the average number of double bonds on ignition delay was more pronounced than that of carbon chain length. Fatty acids with two or more carbon-carbon double bonds led mostly to a stronger increase in ignition delay compared to those with only one double bond.

A Study on Correlation of Secondary Ignition Waveform, Emissions and Fuel Consumption Rate of Decrepit Gasoline Vehicle

The test was done on cars travelling at the speeds of 20km/h, 60km/h and 100km/h using the performance testing mode for chassis dynamometer. In this test, the secondary ignition waveform, exhaust emissions and fuel consumption were measured in case of faulty MAP sensor, faulty oxygen sensor and spark plugs. The following results from the related analysis of secondary waveform, emission and fuel consumption measurements were obtained : 1) The fuel consumption was higher in the order of oxygen sensor trouble, MAP trouble, spark plug trouble, before maintenance and after maintenance. Maximum fuel economy is 9.3km/L, the minimum fuel economy is 3.2km/L, the difference between max. and min. is 65.5%. 2) If you compare the oxygen sensor trouble with after maintenance, the CO has improved an average of 98%, fuel economy average of 60%. And the HC has improved an average of 87%, fuel economy average of 60%. The fuel consumption and exhaust gas was bad in the order of oxygen sensor trouble, MAP trouble and S/P trouble.

Comparison of E10 and E85 spark ignited stratified combustion and soot formation

The combustion process, soot formation and oxidation in spark ignited, spray guided, globally lean, stratified combustion of E10 and E85 was investigated and compared in a single cylinder optical engine with an outward opening piezo actuated injector. Two engine loads and several injection sequences were studied. Cylinder pressure measurements, high speed video recordings, OH∗ chemiluminescence and soot incandescence imaging were used. Stable ignition and combustion of both fuels was achieved with a spark at the end of the last the fuel spray for all injection strategy cases except for the single injection of large fuel quantities. The average rate of heat release and work output increased and the work variability decreased for both fuels with multiple injections of fuel for both engine loads. The changes for each case and variable were approximately equal for the two fuels. Single injections of E85 led to pool fires on the piston surface which were the only source of soot formation. Pool fires persisted after the end of conditions favourable to soot oxidation. For E10 the source of soot formation was a combination of pool fires and gas phase soot formation at 3.6bar while at 5bar pool fires were the only source. Two injections led to a significant reduction of the soot formation for both fuels and both engine loads. Pool fires remained the sole source of soot formation for E85 while the gas phase was the primary source for E10 and largely ceased during the combustion process while soot oxidizing conditions were present. The spatial distribution of combustion was remarkably similar for both fuels when multiple injections were used.

The Application for a Prediction of the Coal Spontaneous Ignition – Predisam

AbstractThe article follows the research of the project number TA01020351 called “The research of possibilities when predicting steam origin and consequent spontaneous ignition of brown coal fuels” which was researched with the support of the Technological Agency in the Czech Republic in 2011-2014 in the connection with a realized technical research. Therefore, it gives a summary information about the evaluation of the risk degree for the origin of spontaneous ignitions of the brown coal. The presented way of evaluation is based on a numeric expression of a value for MHU criteria - the point load of particular indicators is added together with other results gained from this research project. Then, more information is taken from companies running the dumps of brown coal products - both for suppliers (mining companies) and big consumers (power engineering). The complex knowledge about prediction of the origin of the spontaneous ignition enables to make an early response to eliminate a threat of mining fire in open pit mines or on the dumps of coal products. Consequently, it reduces the risk of fire and breakdowns of transportation means DPD, heavy machines and preparation plants. The working injuries are reduced as well - burns by coal in fire or inhalation of gas products from imperfect combustion.

Enhanced ignition of biomass in presence of NOx

Accumulation of combustible biomass residues on hot surfaces of processing machineries can pose fire hazards. In addition, the presence of nitrogen oxides (NOx) from plant equipment alters the local conditions, aggravating the propensity for low temperature ignition risks. This study presents an experimental study on a relative effect of NOx on ignition temperature of morpholine, an important surrogate of biomass, to reveal the sensitising role of NOx in ignition of biomass fuels and to gain mechanistic insights into the chemical aspect of this behaviour in fire. The experiments employed a flow-through tubular reactor, operated at constant pressure and residence time of 1.01bar and 1.0s, respectively, and coupled with a Fourier-transform infrared spectroscope. For a representative fuel-rich condition (Φ=1.25), the concentration of NOx as small as 0.06% lowers the ignition temperature of morpholine by 150°C, i.e., from approximately 500°C to 350°C. The density functional theory (DFT) calculations performed with the CBS-QB3 composite method, that comprises a complete basis set, characterised the dynamics and energies of the elementary nitration reactions. We related the observed reduction in ignition temperature to the formation of unstable nitrite and nitrate adducts, as the result of addition of NOx species to morphyl and peroxyl radicals. Furthermore, the reaction of NOx with low-temperature hydroperoxyl radical leads to the formation of active OH species that also propagate the ignition process. The present findings quantify the ignition behaviour of biomass under NOx–doped atmospheres. The result is of great importance in practical applications, indicating that safe operation of wood-working plants requires avoiding trace concentration of NOx within the vicinity of biomass residues. This can be facilitated by proper (and separate) venting of engine exhausts.

Physics-Based Modeling of Live Wildland Fuel Ignition Experiments in the Forced Ignition and Flame Spread Test Apparatus

ABSTRACTA computational study was performed to improve our understanding of the ignition of live fuel in the forced ignition and flame spread test apparatus, a setup where the impact of the heating mode is investigated by subjecting the fuel to forced convection and radiation. An improvement was first made in the physics-based model WFDS where the fuel is treated as fixed thermally thin elements and then it was utilized in this study. This improvement included bound water in addition to free water in fuel moisture content. The fuel was assumed to undergo evaporation of free and bound water, pyrolysis and char oxidation. Fuels with different moisture contents ranging from 0% to 130% were simulated under an identical heating condition. The simulated and the experimental ignition times compared reasonably well with each other. The time evolutions of simulated and experimental mass loss rates also compared well with each other. For all fuel moisture contents, it was observed that the release of bound moisture starts at temperatures greater than 200°C long after ignition time. This observation was consistent with the release of moisture observed at high temperatures in the experiments of live fuels.

Photo-Induced Ignition of Different Gaseous Fuels Using Carbon Nanotubes Mixed with Metal Nanoparticles as Ignitor Agents

ABSTRACTThis article describes the photo-induced ignition process of multi-walled carbon nano-tubes (MWCNTs)/ferrocene mixtures by pulsed Xe lamps using programmable driving boards with adjustable parameters, such as variable flash rate and pulse’s energy/intensity. Varying the energy of incident light pulse, minimum ignition energy values were found as a function of mixture weight ratio, observing that a higher MWCNT amount with respect to metal nano-particles leads to lower ignition energy. The photo-induced ignition of CNTs mixed with nano-particles was then used in a properly realized experimental setup for triggering the combustion of CNT-enriched fuel mixtures. Different types of gaseous fuels mixed with air (CH4, liquid propane, and H2) were tested. The combustion process triggered by MWCNTs/ferrocene photo-ignition shows better performances, for all used gaseous fuels and for all tested air/fuel ratios, compared with those obtained by using a traditional spark plug. In particular, CNT-based photo-induced combustion evolves more rapidly with shorter ignition delays, higher peak pressure values, and a higher fuel burning rate as observed by reported experimental tests.

A comprehensive iso-octane combustion model with improved thermochemistry and chemical kinetics

Iso-Octane (2,2,4-trimethylpentane) is a primary reference fuel and an important component of gasoline fuels. Moreover, it is a key component used in surrogates to study the ignition and burning characteristics of gasoline fuels. This paper presents an updated chemical kinetic model for iso-octane combustion. Specifically, the thermodynamic data and reaction kinetics of iso-octane have been re-assessed based on new thermodynamic group values and recently evaluated rate coefficients from the literature. The adopted rate coefficients were either experimentally measured or determined by analogy to theoretically calculated values. Furthermore, new alternative isomerization pathways for peroxy-alkyl hydroperoxide (ȮOQOOH) radicals were added to the reaction mechanism. The updated kinetic model was compared against new ignition delay data measured in rapid compression machines (RCM) and a high-pressure shock tube. These experiments were conducted at pressures of 20 and 40 atm, at equivalence ratios of 0.4 and 1.0, and at temperatures in the range of 632–1060K. The updated model was further compared against shock tube ignition delay times, jet-stirred reactor oxidation speciation data, premixed laminar flame speeds, counterflow diffusion flame ignition, and shock tube pyrolysis speciation data available in the literature. Finally, the updated model was used to investigate the importance of alternative isomerization pathways in the low temperature oxidation of highly branched alkanes. When compared to available models in the literature, the present model represents the current state-of-the-art in fundamental thermochemistry and reaction kinetics of iso-octane; and thus provides the best prediction of wide ranging experimental data and fundamental insights into iso-octane combustion chemistry.

An innovative system for piston engine combustion with laser-induced ignition of the hydrocarbon fuel consisting carbon nanotubes

The article proposes the concept of a new piston engine combustion system that is designed to meet future-oriented ecological requirements. The concept is to use ethanol as a fuel, in which a slurry of carbon nanotubes would be formed, which are characterized by the ability to ignite using a pulse of laser light fed into the combustion chamber. Modifying the shape of the light beam that penetrates the combustion chamber would allow to control the position and the size of the area in which the ignition of fuel would occur. The originality of the concept is to combine the latest achievements in the field of nanotechnology in the construction of lasers and the production of biofuels, so as to contribute to improving the environmental performance of engines using the existing synergies.The article discusses the prospects for the use of bioethanol as a fuel with zero carbon balance, a critical review of related research on light pulse initiated ignition of hydrocarbon fuels from carbon nanotubes was presented, and a review of studies of laser ignition for conventional fuels. The results of studies of carbon nanotubes suspensions in a variety of fuels conducted by the authors in order to seek solutions for the stable dispersions formation, that are resistant to nanotube agglomeration and sedimentation. The summary indicates directions for further research highlighting the importance of environmental impact.

An Experimental Study on the Influence of Ethanol and Automotive Gasoline Blends

The objective of this work is to investigate the production possibility of high octane environmental ethanol gasoline blends based on Euro specifications. The environmental gasoline is the key element to keep the environment safe and clean. Moreover, it reduces gas emissions after combustion of gasoline. One of the main methods to produce the environmental gasoline is blending gasoline with oxygenated compounds such as ethanol. Ethanol is chosen among other oxygenated compounds as it has a high influence on physico-chemical characteristics of gasoline rather than other oxygenated compounds. In addition, it has a high octane number as well as it is not polluting the environment and clean additive. In the experimental study, the choice of environmental gasolines are based on Euro- 3 specifications for samples without ethanol blend and Euro-5 specifications for samples with ethanol blend; after upgrading. Various blend stocks have been prepared which have reformate, isomerate, full refinery naphtha (FRN), heavy straight run naphtha (HSRN), hydrocracked naphtha, heavy hydrocracked naphtha, coker naphtha and heavy coker naphtha. In this study, ASTM standard methods are performed for spark ignition fuels to characterize its physical and chemical properties. The results show that one has exhibited the optimum specifications of Euro-3 and thus its physico-chemical characteristics are 755.11 Kg/m3 of density, 55.88 of °API and 95 of RON, 88 of MON, 40% by volume of aromatic content and 0.66% by volume of benzene content. Moreover, ASTM distillation curve shows that the volume percentage at 150°C is 83. At the same time, the final boiling point (FBP) and recovery volume percent are 198°C is 96% respectively. While another sample has the poorest physical as well as chemical properties so that it is blended with ethanol to upgrade its characteristics. Therefore, the target is determining the optimum ethanol volume percent to be blended with poorest sample to yield the highest properties of gasoline. These blends are namely as E0, E5, E10, E15, E20. The results indicate that E5 is the optimum one for Euro-5 specifications after upgrading and thus its physico-chemical characteristics are 745.55 Kg/m3 of density, 58 of oAPI, 101 of RON, 98 of MON, 32.65% by volume of aromatic content and 0.47% by volume of benzene content. Moreover, ASTM distillation curve illustrates that the volume percentage at 150°C is 75. At the same time, the final boiling point (FBP) and recovery volume percent are 190°C and 97% respectively. In addition, its Reid vapor pressure equals 8.1 psi and the heat of combustion equals 35 MJ/L. In the final, Blending gasoline with ethanol is an essential issue concerning the production of environmental gasolines.

Interaction between flaming and smouldering in hot-particle ignition of forest fuels and effects of moisture and wind

Ignition of natural fuels by hot metal particles from powerlines, welding and mechanical processes may initiate wildfires. In this work, a hot steel spherical particle (6–14 mm and 600–1100°C) was dropped onto pine needles with a fuel moisture content (FMC) of 6–32% and wind speed of 0–4 m s–1. Several ignition phenomena including direct flaming, smouldering and smouldering-to-flaming transition were observed. The critical particle temperature for sustained ignition was found to decrease with the particle size (d) and increase with FMC as (°C), and the maximum heating efficiency of particle was found to be . As the particle size increases, the influence of FMC becomes weaker. The flaming ignition delay times for both direct flaming and smouldering-to-flaming transition were measured, and decreased with particle temperature and wind speed, but increased with FMC. The proposed heat-transfer analysis explains the ignition limit and delay time, and suggests that the hot particle acts as both heating and pilot sources like a small flame for direct flaming ignition, but only acts as a heating source for smouldering. This study deepens the fundamental understanding of hot-particle ignition, and may help provide a first step to understanding the mechanism behind firebrand ignition.

Laser ignition and flame kernel characterization of HCNG in a constant volume combustion chamber

Laser ignition of gaseous fuels leads to improved lean-burn combustion, which is a key technology in reducing engine out emissions for meeting stringent future emission legislations. Hydrogen enriched natural gas (HCNG) promotes lean-burn combustion due to higher flammability limits of constituent hydrogen. HCNG is a promising gaseous fuel for spark ignition (SI) engines prior to development and deployment of dedicated hydrogen fueled engines. In this study, laser ignition of HCNG was compared with baseline natural gas for different lambdas (λ) ranging from 1.1, 1.3, to 1.5 using a Q-switched Nd:YAG laser in a constant-volume combustion chamber (CVCC). Experiments were conducted at different ambient pressures, i.e., initial chamber filling pressures of 5bar and 10bar. These pressures simulated the in-cylinder pressures identical to that at the time of spark ignition in an engine cycle. Flame kernel evolution in HCNG blends of different compositions (10%, 20%, and 30% v/v hydrogen) were compared with baseline natural gas for a specified Lambda (λ). A high speed CCD camera was used for triggering the laser and thus the flame kernel evolution in the CVCC, which was recorded using shadowgraphy technique. Images captured were further analyzed to obtain temporal propagation of flames in different directions. Flame kernel propagated faster in ambient pressure of 5bar compared to 10bar. Both, flame speed and peak CVCC pressure increased, and combustion duration shortened with increasing hydrogen fraction in HCNG.