Measuring the Adiabatic Ignition Delay of n-Pentane Mixture using Rapid Compression Machine

Recent combustion research has focused on low temperature combustion to meet engine emission regulations and to advance the development of low temperature homogenous compression ignition engines.Autoignition studies in this temperature regime are primarily performed by Rapid Compression Machines(RCMs)which are sensitive to the heat transfer characteristics of the experimental device.RCMs are widely used to measure autoignition data such as ignition delay and species concentration.Measured ignition delays from RCMs are typically reported at an adiabatic condition;however,this assumption may produce a systematic error in ignition delay measurement as heat transfer is observed to reduce the pressure and temperature during the autoignition process,e.g.,a longer ignition delay has a greater pressure and temperature drops.RCMs are custom built and have unique design characteristics that affect the heat transfer during the autoignition process.In addition,depending on the diluent composition(e.g.,helium versus nitrogen or argon),different heat transfer characteristics are expected.As a result,autoignition results at similar conditions may vary from facility to facility or depending on the used diluent.The dependency of the measured data on the used facility or diluent may produce uncertainty in the data which impact the development of high-fidelity combustion mechanisms.In this work,a new method is developed and utilized to eliminate heat transfer from the ignition delay data.To evaluate the new method,the autoignition of n-pentane mixtures in the low temperature regime were investigated using an RCM.To vary the heat transfer,the compression ratio of the RCM was changed and the ignition delays were measured at similar pressure and temperature conditions.The tests were performed at an equivalence ratio of approximately one and nitrogen and argon as diluents.By applying the new method,the effect of heat transfer on the ignition delays were eliminated successfully and ignition delays at adiabatic condition were determined.A detailed kinetic model of n-pentane was used to simulate the measured adiabatic ignition delay,which agreed well with the experimental data.

Development of the ignition delay prediction model of n-butane/hydrogen mixtures based on artificial neural network

Based on the experimental ignition delay results of n-butane/hydrogen mixtures in a rapid compression machine,a Genetic Algorithm(GA)optimized Back Propagation(BP)neural network model is originally developed for ignition delay prediction.In the BP model,the activation function,learning rate and the neurons number in the hidden layer are optimized,respectively.The prediction ability of the BP model is validated in wide operating ranges,i.e.,compression pressures from 20 to 25 bar,compression temperatures from 722 to 987 K,equivalence ratios from 0.5 to 1.5 and molar ratios of hydrogen(X_(H2))from 0 to 75%.Compared with the BP model,the GA optimized BP model could increase the average correlation coefficient from 0.9745 to 0.9890,in the opposite,the average Mean Square Error(MSE)decreased from 2.21 to 1.06.On the other hand,to assess the BP-GA model prediction ability in the never-seen-before cases,a limited BP-GA model is fostered in the𝑋X_(H2) range from 0 to 50%to predict the ignition delays at the cases of𝑋X_(H2)=75%.It is found that the predicted ignition delays are underestimated due to the training dataset lacking of“acceleration feature”that happened at𝑋X_(H2)=75%.However,three possible options are reported to improve the prediction accuracy in such never-seen-before cases.

Propagation and Coalescence of Blast-Induced Cracks in PMMA Material Containing an Empty Circular Hole Under Delayed Ignition Blasting Load by Using the Dynamic Caustic Method

In this paper,dynamic caustic method is applied to analyze the blast-induced crack propagation and distribution of the dynamic stress field around an empty circular hole in polymethyl methacrylate(PMMA)material under delayed ignition blasting loads.The following experimental results are obtained.(1)In directional-fracture-controlled blasting,the dynamic stress intensity factors(DSIFs)and the propagation paths of the blast-induced cracks are obviously influenced by the delayed ignition.(2) The circular hole situated between the two boreholes poses a strong guiding effect on the coelesence of the cracks,causing them to propagate towards each other when cracks are reaching the circular hole area.(3)Blast-induced cracks are not initiated preferentially because of the superimposed effect from the explosive stress waves on the cracking area.(4) By using the scanning electron microscopy(SEM)method,it is verified that the roughness of crack surfaces changes along the crack propagation paths.

Ignition of nanothermites by a laser diode pulse

Experimental investigation has been carried out for laser ignition and combustion of nanothermites based on aluminum and oxides of copper,bismuth and molybdenum.Ultrasonic mixing of nanosized powders was used to produce compositions.For thermite ignition,initiating laser pulse with a maximum intensity of 770 W/cm2 was generated by a laser diode with a wavelength of 808 nm.The ignition delay times,the minimum initiation energy density,and the average burning rate at various thermite densities and mass fractions of components were determined by recording the emission of radiation of the reaction products using a multichannel pyrometer jointly with a high-speed video camera.The effect of adding carbon black on the threshold parameters of a laser pulse was also studied.Based on the obtained results,certain assumptions were put forward with regard to the mechanism of nanothermites’ignition by laser radiation and their burning.In particular,the assumptions were made on the two-stage process of the reaction initiation and jet burning mechanism of porous nanothermites.

Shock tube study of n-decane ignition at low pressures

Ignition delay times for n-decane/O2/Ar mixtures were measured behind reflected shock waves using endwall pressure and CH＊ emission measurements in a heated shock tube. The initial postshock conditions cover pressures of 0.09-0.26 MPa, temperatures of 1 227-1 536 K, and oxygen mole fractions of 3.9%-20.7% with an equivalence ratio of 1.0. The correlation formula of ignition delay dependence on pressure, temperature, and oxygen mole fraction was obtained. The current data are in good agreement with available low-pressure experimental data, and they are then compared with the prediction of a kinetic mechanism. The current measurements extend the kinetic modeling targets for the n-decane combustion at low pressures.

Influence of Ethanol Addition on the Spray Auto-ignition Properties of Gasoline and Its Relationship with Octane Number

In this study,the spray auto-ignition properties of binary primary reference fuels(PRFs)of 2,2,4-trimethylpentane and n-heptane with different research octane numbers(RONs)were measured according to the industry standard NB/SH/T 6035 to determine their ignition delay times at various initial temperatures.Furthermore,the auto-ignition properties were investigated after blending the PRFs with various amounts of ethanol.The results revealed a very good correlation between the derived cetane number and the RON for the PRFs in both the presence and absence of ethanol.In addition,a concept of ignition delay sensitivity was developed for ethanol-containing fuels that exhibited a close relationship with the octane sensitivity,which is defined as the RON minus the motor octane number(MON).Finally,the developed method was applied to conveniently estimate the RON and MON values of several ethanol-containing fuels by simply measuring their auto-ignition properties.

The vitiation effects of water vapor and carbon dioxide on the autoignition characteristics of kerosene

In ground tests of hypersonic scramjet, the highenthalpy airstream produced by burning hydrocarbon fuels often contains contaminants of water vapor and carbon dioxide. The contaminants may change the ignition characteristics of fuels between ground tests and real flights. In order to properly assess the influence of the contaminants on ignition characteristics of hydrocarbon fuels, the effect of water vapor and carbon dioxide on the ignition delay times of China RP-3 kerosene was studied behind reflected shock waves in a preheated shock tube. Experiments were conducted over a wider temperature range of 800-1 500 K, at a pressure of 0.3 MPa, equivalence ratios of 0.5 and 1, and oxygen concentration of 20%. Ignition delay times were determined from the onset of the excited radical OH emission together with the pressure profile. Ignition delay times were measured for four cases： （1） clean gas, （2） gas vitiated with 10% and 20% water vapor in mole, （3） gas vitiated with 10% carbon dioxide in mole, and （4） gas vitiated with 10% water vapor and 10% carbon dioxide, 20% water vapor and 10% carbon dioxide in mole. The results show that carbon dioxide produces an inhibiting effect at temperatures below 1 300 K when Ф = 0.5, whereas water vapor appears to accelerate the ignition process below a critical temperature of about 1 000 K when Ф = 0.5. When both water vapor and carbon dioxide exist together, a minor inhibiting effect is observed at Ф = 0.5, while no effect is found at Ф = 1.0. The results are also discussed preliminary by considering both the combustion reaction mechanism and the thermophysics properties of the fuel mixtures. The current measurements demonstrate vitiation effects of water vapor and carbon dioxide on the autoignition characteristics of China RP-3 kerosene at air-like O2 concentration. It is important to account for such effects when data are extrapolated from ground testing to real flight conditions.

MECHANISM ON DISTRIBUTION OF PILOT FUEL SPRAY AND COMPRESSING IGNITION IN PREMIXED NATURAL GAS ENGINE IGNITED BY PILOT DIESEL

Numerical simulations of pilot fuel spray and compressing ignition forpre-mixed natural gas ignited by pilot diesel are described. By means of these modeling, the dualfuel and diesel fuel ignition mechanism of some phenomena investigated on an optional engine bytechnology of high-speed CCD is analyzed. It is demonstrated that the longer delay of ignition indual fuel engine is not mainly caused by change of the mixture thermodynamics parameters. Theanalysis results illustrate that the ignition of pre-mixed nataral gas ignited by pilot dieseltaking place in dual fuel engine is a process of homogenous charge compression ignition.

Development of efficient and accurate skeletal mechanisms for hydrocarbon fuels and kerosene surrogate

In this paper,the methodology of the directed relation graph with error propagation and sensitivity analysis（DRGEPSA）,proposed by Niemeyer et al.（Combust Flame 157：1760-1770.2010）.and its differences to the original directed relation graph method are described.Using DRGEPSA,the detailed mechanism of ethylene containing 71 species and 395 reaction steps is reduced to several skeletal mechanisms with different error thresholds.The 25-species and 131-step mechanism and the 24-species and115-step mechanism are found to be accurate for the predictions of ignition delay time and laminar flame speed.Although further reduction leads to a smaller skeletal mechanism with 19 species and 68 steps,it is no longer able to represent the correct reaction processes.With the DRGEPSA method,a detailed mechanism for n-dodecane considering low-temperature chemistry and containing 2115 species and8157 steps is reduced to a much smaller mechanism with249 species and 910 steps while retaining good accuracy.If considering only high-temperature（higher than 1000 K）applications,the detailed mechanism can be simplified to even smaller mechanisms with 65 species and 340 steps or48 species and 220 steps.Furthermore,a detailed mechanism for a kerosene surrogate having 207 species and 1592 steps is reduced with various error thresholds and the results show that the 72-species and 429-step mechanism and the66-species and 392-step mechanism are capable of predicting correct combustion properties compared to those of the detailed mechanism.It is well recognized that kinetic mechanisms can be effectively used in computations only after they are reduced to an acceptable size level for computation capacity and at the same time retaining accuracy.Thus,the skeletal mechanisms generated from the present work are expected to be useful for the application of kinetic mechanisms of hydrocarbons to numerical simulations of turbulent or supersonic combustion.

Effect of the high-power electromagnetic pulses on the reactivity of the coal-water slurry in hot environment

An effect of the high-power electromagnetic pulses onto the droplet of coal-water slurry inside the furnace was investigated.In contrary to the previously investigated laser-induced fuel atomization that occurs at the room temperature,the pre-heated(to 400 K)slurry becomes dry enough to prevent the explosion-like steam formation.Thus,fuel does not atomize and the ignition does not accelerate.Furthermore,the absorption of several laser pulses leads to evident sintering of irradiated surface with following increase of the ignition delay time for up to 24%.Variation of the pulse energy in range 48-118 mJ(corresponding intensity up to 2.4 J·cm^-2)leads to certain variation of the increase of ignition delay.The strong pulsed overheating of the coal water slurry which does not initiate the fine atomization of the fuel generally makes its ignition longer.

A Chemical Kinetics Perspective on the High-Temperature Oxidation of Methane and Propane through Experiments and Kinetic Analysis

In the conversion of methane and propane under high temperature and pressure,the ignition delay time(IDT)is a key parameter to consider for designing an inherently safe process.In this study,the IDT characteristics of methane and propane(700–1000 K,10–20 bar)were studied experimentally and using kinetic modeling tools at stoichiometric fuel-tooxygen ratios.All the experiments were conducted through insentropic compression.The reliable experimental data were obtained by using the adiabatic core hypothesis,which can be used to generate and validate the detailed chemical kinetics model.The IDTs of methane and propane were recorded by a rapid compression machine(RCM)and compared to the predicted values obtained by the NUIGMech 3.0 mechanism.To test the applicability of NUIGMech 3.0 under different reaction conditions,the influence of temperature in the range of 700–1000 K(and the influence of pressure in the range of 10–20 bar)on the IDT was studied.The results showed that NUIGMech 3.0 could reasonably reproduce the experimentally determined IDT under the wide range of conditions studied.The constant volume chemical kinetics model was used to reveal the effect of temperature on the elementary reaction,and the negative temperature coefficient(NTC)behavior of propane was also observed at 20 bar.The experimental data can serve as a reference for the correction and application of kinetic data,as well as provide a theoretical basis for the safe conversion of low-carbon hydrocarbon chemicals.

Effect of plasma on combustion characteristics of boron

As it is very difficult to release boron energy completely, kinetic mechanism of boron is not clear, which leads to the lack of theoretical guidance for studying how to accelerate boron combustion. A new semi-empirical boron combustion model is built on the King combustion model, which contains a chemical reaction path; two new methods of plasma-assisted boron combustion based on kinetic and thermal effects respectively are built on the ZDPLASKIN zero-dimensional plasma model. A plasma-supporting system is constructed based on the planar flame, discharge characteristics and the spectral characteristics of plasma and boron combustion are analyzed. The results show that discharge power does not change the sorts of excited-particles, but which can change the concentration of excited-particles. Under this experimental condition,plasma kinetic effect will become the strongest at the discharge power of 40 W; when the discharge power is less than 40 W,plasma mainly has kinetic effect, otherwise plasma has thermal effect. Numerical simulation result based on plasma kinetic effect is consistent with the experimental result at the discharge power of 40 W, and boron ignition delay time is shortened by 53.8% at the discharge power of 40 W, which indicates that plasma accelerates boron combustion has reaction kinetic paths, while the ability to accelerate boron combustion based on thermal effect is limited.

On the Intelligent Optimization of the Crevice Structure in a Rapid Compression Machine

In this study,the multi-objective intelligent optimization of the crevice structure in a rapid compression machine(RCM)is carried out based on the RCM simulation model modified with the KIVA-3V program.A multi-objective optimization simulation model of the crevice structure based on the large eddy simulation model coupled with the genetic algorithm NSGA-Ⅲis established.Six optimization parameters and seven optimization objectives are selected in the optimization process.The results show that the genetic algorithm can quickly find the values of the optimized parameters.The crevice volume ratio shows a trade-off relationship with the dimensionless temperature ratio T_(max)/T_(aver)and the tumble ratio.A larger crevice volume can reduce the flow of boundary layer cryogenic gas into the combustion chamber,thus improving the temperature uniformity.In addition,the crevice entrance width and the connecting channel length should be smaller,while the volume of the crevice main chamber should be larger,so as to sufficiently introduce the low-temperature gas of the boundary layer into the crevice and reduce their influence on the temperature field of the combustion chamber.When the crevice volume accounts for10%of the total volume,the temperature uniformity of the combustor is significantly enhanced,and when the crevice volume accounts for 30.4%,there is almost no low-temperature vortex in the combustion chamber.

An experimental and modeling study of n-hexadecane autoignition under low-to-intermediate temperatures

N-hexadecane is a potential candidate of diesel surrogate fuels and is also the largest linear alkane(n-alkanes)with known chemical kinetic models.The objective of this study is to investigate the autoignition characteristics of n-hexadecane in the lowto-intermediate temperature region and to validate the existing kinetic models.In this study,the ignition delay times(IDTs)of nhexadecane were measured using a heated rapid compression machine(RCM)at two pressures of 7 and 10 bar,and over equivalence ratios ranging from 0.5 to 1.3.Two-stage ignition characteristic and the negative temperature coefficient(NTC)behavior of total ignition delay time were experimentally captured.This study paid special attention to the influence of pressure,equivalence ratio,and oxygen content on the IDTs of n-hexadecane.It is observed that both the total IDTs and the first-stage IDTs decrease with the rise of those parameters.It is worth noting that the first-stage IDT is found to show a greater dependence on temperature but a weaker dependence on other parameters compared to the total IDT.The observed IDT dependence in the lowtemperature region(LTR)were quantitatively described by ignition delay time correlations.The newly measured IDTs were then validated against two kinetic models(LLNL and CRECK).Simulation results show that both models underpredict the first-stage IDT but generally capture the temperature dependence.The CRECK model well predicts the total IDTs of n-hexadecane while the LLNL model significantly underpredicts the total IDTs at most investigated conditions.To the best of our knowledge,this study is the first investigation on n-hexadecane autoignition under low-to-intermediate temperatures,which deepens the understanding of large n-alkane oxidation and contributes to the improvement of the existing kinetic models.

Statistical characteristics of spray autoignition of transient kerosene jet in cross flow

To study statistical characteristics of the random spray autoignition,aviation kerosene was injected transiently into non-vitiated air crossflow in a flow reactor with optical accesses.The operating conditions were relevant to gas turbine combustor:the air crossflow pressure and temperature were in the range of 1.4-1.7 MPa and 830-947 K,respectively,and the jet-tocrossflow momentum flux ratios were 20,50 and 80.Statistical distributions of random ignition delay times with adequate convergence were estimated based on histograms.The dependences of the distributions on reactor pressure,temperature,and jet-to-crossflow momentum flux ratio were studied.The results show that the resulting distributions appear more concentrated with the increase of air temperature or jet-to-crossflow momentum flux ratio.And then the correlations for the mean and standard deviation of the ignition delay time sample data were developed based on the present results.Compared with the correlations of ignition delay time of homogeneous premixed gas-phase kerosene/air mixture reported in the literature,the results show a greater significance pressure dependence and lower temperature sensitivity of the ignition delay time of nonpremixed kerosene spray.

Implication of Sensitive Reactions to Ignition of Methyl Pentanoate:H-Abstraction Reactions by H and CH_(3) Radicals

Methyl pentanoate(MP)was identified as a potential candidate.To facilitate the application of MP with high efficiency in engines,a comprehensive understanding of combustion chemical kinetics of MP is necessary.In this work,the H-abstraction reactions from MP by H and CH_(3) radicals,critical in controlling the initial fuel consumption,are theoretically investigated at the DLPNO-CCSD(T)/CBS(T-Q)//M06-2X/cc-pVTZ level of theory.The multistructural torsional(MS-T)anharmonicity is characterized using the dual-level MS-T method;the HF/3-21G and M06-2X/cc-pVTZ methods are chosen as the low-and high-level methods,respectively.The conventional transition state theory(TST)is employed to calculate the high-pressure limit rate constants at 298-2000 K with the Eckart tunneling correction.Our calculations indicate that the hydrogen atoms of the methylene functional group are easier to be abstracted by H and CH_(3) radicals.The multistructural torsional anharmonicities of H-abstraction reactions MP+H/CH_(3) are significant within the temperature range investigated.The tunneling effects are more pronounced at low temperatures,and contribute considerably to the rate constants below 500 K.The model from the work of Diévart et al.is updated with our calculations,and the simulations of the updated model are in excellent agreement with the reported ignition delay time of MP/O2/Ar and MP/Air mixtures.The sensitivity analysis indicates that the H-abstraction reactions,MP+H-CH_(3)CH_(2)CHCH_(2)C(-O)OCH_(3)/CH_(3)CHCH_(2)CH_(2)C(-O)OCH_(3)+H2,are critical in controlling the initial fuel consumption and ignition delay time of MP.

Synthesis and Improved Properties of Hypergolic Boronium-Based Ionic Liquids

A series of asymmetric monoimidazolium dihydroboronium-based ionic liquids （ILs） were synthesized from amine-boranes. All the resulting ILs were fully characterized by 1H and 13C NMR, IR spectroscopy, elemental analysis or high resolution mass spectrum. Compared with the symmetric bisimidazolium dihydroboronium-based ILs, these new ILs exhibited improved properties with shorter ignition delay times （IDs）, higher densities, and lower phase transition temperature showing the promising application potential as green propellants. A series of asymmetric monoimidazolium dihydroboronium-based ionic liquids （ILs） were synthesized from amine-boranes. All the resulting ILs were fully characterized by 1H and 13C NMR, IR spectroscopy, elemental anal- ysis or high resolution mass spectrum. Compared with the symmetric bisimidazolium dihydroboronium-based ILs, these new ILs exhibited improved properties with shorter ignition delay times （IDs）, higher densities, and lower phase transition temperature showing the promising application potential as green propellants.

Synthesis,characterization and thermolysis of lanthanide metal nitrate complexes with 1,10-phenanthroline,Part-95

The nitrate complexes of cerium, praseodymium and neodymium with 1,10-phenanthroline （phen） of general formula [Ln（phen）2（NO3）2（H2O）2]·NO3 （where, Ln=Ce, Pr and Nd） were prepared and characterized by X-ray crystallography. Thermolysis of these complexes was investigated by simultaneous thermogravimetry （TG） and differential thermal analysis （DTA）. Isothermal TG was taken to evaluate the kinetic parameters using model fitting as well as model free isoconversional methods. The thermolytic pathways were also suggested, which involves decomposition followed by ignition. All the three complexes had coordination number ten and showed multistep decompositions. In order to evaluate the response of rapid heating, ignition delay （Di） measurements were undertaken. The activation energies for ignition were found to decrease in the order： Nd〉Pr〉Ce.

Flame Morphology and Characteristic of Co-Firing Ammonia with Pulverized Coal on a Flat Flame Burner

Ammonia as a new green carbon free fuel co-combustion with coal can effectively reduce CO_(2)emission,but the research of flame morphology and characteristics of ammonia-coal co-combustion are not enough.In this work,we studied the co-combustion flame of NH_(3)and pulverized coal on flat flame burner under different oxygen mole fraction(X_(i,O_(2)))and NH_(3)co-firing energy ratios(E_(NH_(3))).We initially observed that the introduction of ammonia resulted in stratification within the ammonia-coal co-combustion flame,featuring a transparent flame at the root identified as the ammonia combustion zone.Due to challenges in visually observing the ignition of coal particles in the ammonia-coal co-combustion flame,we utilized Matlab software to analyze flame images across varying E_(NH_(3))and X_(i,O_(2)).The analysis indicates that,compared to pure coal combustion,the addition of ammonia advances the ignition delay time by 4.21 ms to 5.94 ms.As E_(NH_(3))increases,the ignition delay time initially decreases and then increases.Simultaneously,an increase in X_(i,O_(2))results in an earlier ignition delay time.The burn-off time and the flame divergence angle of pulverized coal demonstrated linear decreases and increases,respectively,with the growing ammonia ratio.The addition of ammonia facilitates the release of volatile matter from coal particles.However,in high-ammonia environments,oxygen consumption also impedes the surface reaction of coal particles.Finally,measurements of gas composition in the ammonia-coal flame flow field unveiled that the generated water-rich atmosphere intensified coal particle gasification,resulting in an elevated concentration of CO.Simultaneously,nitrogen-containing substances and coke produced during coal particle gasification underwent reduction reactions with NO_(x),leading to reduced NO_(x)emissions.