Experimental and computational investigation of autoignition of jet fuels and surrogates in nonpremixed flows at elevated pressures

Experimental and computational investigations are carried out to elucidate the fundamental mechanisms of autoignition of surrogates of jet-fuels at elevated pressures up to 6 bar. The jet-fuels tested are JP-8, Jet-A, and JP-5, and the surrogates tested are the Aachen Surrogate made up of 80 % n-decane and 20 % 1,3,5-trimethylbenzene by mass, Surrogate C made up of 60 % n-dodecane, 20 % methylcyclohexane and 20 % o-xylene by volume, and the 2nd generation Princeton Surrogate made up of 40.4 % n-dodecane, 29.5 % 2,2,4-trimethylpentane, 7.3 % 1,3,5-trimethylbenzene and 22.8 % n-propylbenzene by mole. Using the counterflow configuration, an axisymmetric flow of a gaseous oxidizer stream, made up of a mixture of oxygen and nitrogen, is directed over the surface of an evaporating pool of a liquid fuel. The experiments are conducted at a fixed value of mass fraction of oxygen in the oxidizer stream and at a fixed value of the strain rate. The temperature of the oxidizer stream at autoignition, T_ig, is measured as a function of pressure, p. Experimental results show that the critical conditions, of autoignition of the surrogates are close to that of the jet-fuels. Overall the critical conditions of autoignition of Surrogate C agree best with those of the jet-fuels. Computations were performed using skeletal mechanisms constructed from a detailed mechanism. Predictions of the critical conditions of autoignition of the surrogates are found to agree well with measurements. Computations show that low-temperature chemistry plays a significant role in promoting autoignition for all surrogates. The low-temperature chemistry, of the component of the surrogate with the greatest volatility, was found to have the most influence on the critical conditions of autoignition.     

Ignition and extinction of low molecular weight esters in nonpremixed flows

An experimental and kinetic modeling study is carried out to characterize combustion of low molecular weight esters in nonpremixed, nonuniform flows. An improved understanding of the combustion characteristics of low molecular weight esters will provide insights on combustion of high molecular weight esters and biodiesel. The fuels tested are methyl butanoate, methyl crotonate, ethyl propionate, biodiesel, and diesel. Two types of configuration – the condensed fuel configuration and the prevaporized fuel configuration – are employed. The condensed fuel configuration is particularly useful for studies on those liquid fuels that have high boiling points, for example biodiesel and diesel, where prevaporization, without thermal breakdown of the fuel, is difficult to achieve. In the condensed fuel configuration, an oxidizer, made up of a mixture of oxygen and nitrogen, flows over the vaporizing surface of a pool of liquid fuel. A stagnation-point boundary layer flow is established over the surface of the liquid pool. The flame is stabilized in the boundary layer. In the prevaporized fuel configuration, the flame is established in the mixing layer formed between two streams. One stream is a mixture of oxygen and nitrogen and the other is a mixture of prevaporized fuel and nitrogen. Critical conditions of extinction and ignition are measured. The results show that the critical conditions of extinction of diesel and biodiesel are nearly the same. Experimental data show that in general flames burning the esters are more difficult to extinguish in comparison to those for biodiesel. At the same value of a characteristic flow time, the ignition temperature for biodiesel is lower than that for diesel. The ignition temperatures for biodiesel are lower than those for the methyl esters tested here. Critical conditions of extinction and ignition for methyl butanoate were calculated using a detailed chemical kinetic mechanism. The results agreed well with the experimental data. The asymptotic structure of a methyl butanoate flame is found to be similar to that for many hydrocarbon flames. This will facilitate analytical modeling, of structures of ester flames, using rate-ratio asymptotic techniques, developed previously for hydrocarbon flames.     

Ignition of hydrogen in unsteady nonpremixed flows

The effects of unsteady strain on hydrogen (H₂) ignition in nonpremixed flows are investigated with both experimental measurements and numerical computations. A mixing layer is established in a counterflow configuration with a fuel stream containing N₂–diluted H₂ (X_H2=0.08) flowing against heated air. A reproducible ignition process is initiated by introducing atomic oxygen into the mixing layer with a pulsed ArF excimer laser, which photodissociates heated O₂ from the oxidizer stream. The temporal evolution of OH during ignition is measured by planar laser-induced fluorescence. Following the induction phase, the measured OH mole fraction increases rapidly to a super-equilibrium value that is 60% greater than the OH mole fraction in a steady diffusion flame. The peak OH mole fraction occurs at approximately 6 ms after the excimer laser pulse. To study the OH time history under transient strain, the fuel stream is pulsed at a fixed time after the initiation of ignition. The response of the ignition kernel is extremely sensitive to the time delay of the flow transient. The unsteady strain can delay the ignition time or extinguish the kernel. Comparisons between computations and experiments are made for the evolution of OH during autoignition both for steady and unsteady strain. For both steady and unsteady strain, the transient one-dimensional counterflow computations show excellent agreement with the experiment in terms of predicting ignition delays and the rate of OH accumulation during the induction period. The computations also capture the super-equilibrium OH during the transition to the formation of a steady flame, although not to the degree observed experimentally. The computations are further used to understand the influence of unsteady strain on the kernel evolution. It is found that the degree of super-equilibrium OH is sensitive to strain transients applied close to the time of thermal runaway.     

Impact of nitric oxide on n-heptane and n-dodecane autoignition in a new high-pressure and high-temperature chamber

A New One Shot Engine (NOSE) was designed to simulate the thermodynamic conditions at High Pressure-High Temperature like an actual common-rail diesel engine in order to study the compression ignition of spray. The volume of the combustion chamber provided with large optical windows simplified the implementation of various optical diagnostics. The advantage of this kind of set-up in comparison to pre-burn or flue chambers is that the initial gas mixture can be well controlled in terms of species and mole fraction. The purpose of this work was to investigate the impact of nitric oxide (NO) on ignition delay (ID) for two fuels with different cetane numbers: n-heptane, and n-dodecane. In the thermodynamic conditions chosen (60?bar and over 800–900?K), NO had a strong effect on ID, with increases in NO tending to reduce the ignition delay. Results showed that ID and Lift-Off Length (LOL) presented the same trend as a function of temperature and NO concentration. Experimentally, at 900?K the ignition of n-dodecane was promoted by NO up to 100?ppm, whilst higher NO levels did not further promote ignition and a stabilization of the value has been noticed. For n-heptane, stronger promoting effects were observed in the same temperature conditions: the ignition delays were monotonically reduced with up to 200?ppm NO addition. At a lower temperature (800?K) the inhibiting effect was observed for n-dodecane for [NO] greater than 40?ppm, whereas only a promoting effect was observed for n-heptane. The experimental results of LOL showed that NO shortened LOL in almost all cases, and this varied with both the NO concentration and the mixture temperature. Thus, fuels with shorter ignition delays produce shorter lift-off lengths.     

Direct numerical simulations of autoignition in turbulent two-phase flows

Three-dimensional direct numerical simulations (DNS) were carried out to investigate the impact of evaporation of droplets on the autoignition process under decaying turbulence. The droplets were taken as point sources and were tracked in a Lagrangian manner. Three cases with the same initial equivalence ratio but different initial droplet size were simulated and the focus was to examine the influence of the droplet evaporation process on the location of autoignition. It was found that an increase in the initial droplet size results in an increase in the autoignition time, that highest reaction rates always occur at a specific mixture fraction ξ_MR, as in purely gaseous flows, and that changes in the initial droplet size did not affect the value of ξ_MR. The conditional correlation coefficient between scalar dissipation rate and reaction rates was only mildly negative, contrary to the strongly negative values for purely gaseous autoigniting flows, possibly due to the continuous generation of mixture fraction by the droplet evaporation process that randomizes both the mixture fraction and the scalar dissipation fields.     

Experimental investigation of magnetodynamic processes in two-phase flows

Power plants for use in investigation of flows of products of combustion of metallized plasma-forming fuels in a combustion chamber, Laval nozzle, and MHD channel and for determining the physical characteristics of plasma are described. The results of experimental investigations and numerical calculations of the combustion of fuel in a combustion chamber and flows of combustion products through gas-dynamic channels were employed in the design of the power plants.Scientific-Research Institute of Applied Mathematics and Mechanics at Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 92–100, April, 1993.     

An experimental and modeling study on autoignition of 2-phenylethanol and its blends with n-heptane

2-Phenylethanol (2-PE) is an aromatic alcohol with high research octane number, high octane sensitivity, and a potential to be produced using biomass. Considering that 2-PE can be used as a fuel additive for boosting the anti-knocking quality of gasoline in spark-ignition engines and as the low reactivity fuel or fuel component in dual-fuel reactivity controlled compression ignition (RCCI) engines, it is of fundamental and practical interest to understand the autoignition chemistry of 2-PE, especially at low-to-intermediate temperatures (<1000 K). Based upon the experimental ignition delay time (IDT) results of neat 2-PE obtained from our previous rapid compression machine (RCM) investigation and the literature shock tube study, a detailed chemical kinetic model of 2-PE is developed herein, covering low-to-high temperature regimes. Besides, RCM experiments using binary fuel blends of 2-PE and n-heptane (nC7) are conducted in this work to investigate the nC7/2-PE blending effects, as they represent a dual-fuel system for RCCI operations. Furthermore, the newly developed 2-PE model is merged with a well-validated nC7 kinetic model to generate the current nC7/2-PE binary blend model. Overall, the consolidated model reasonably predicts the experimental IDT data of neat 2-PE and nC7/2-PE blends, as well as captures the experimental effects of pressure, equivalence ratio, and blending ratio on autoignition. Finally, model-based chemical kinetic analyses are carried out to understand and identify the controlling chemistry accounting for the observed blending effects in RCM experiments. The analyses reveal that nC7 enhances 2-PE autoignition via providing extra ȮH radicals to the shared radical pool, while the diminished nC7 promoting effect on 2-PE autoignition with increasing temperature is due to the negative temperature coefficient characteristics of nC7.     

Experimental investigation of the auto-ignition of a transient propane Jet-in-Hot-Coflow

Auto-ignition is a complex process which is extremely sensitive to boundary conditions such as local temperature, mixture or strain rate and occurs on very short time-scales. Therefore, measurement techniques with high spatio-temporal resolution have to be applied to test cases with well-defined boundary conditions in order to generate high-quality validation data for numerical simulations. In the current paper, the auto-ignition of a transient propane jet-in-hot coflow was studied with high-speed OH* chemiluminescence imaging and high-speed Rayleigh scattering for the simultaneous determination of mixture fraction, mixture temperature and scalar dissipation rate immediately prior to the onset of auto-ignition. A variation of the coflow temperature showed a pronounced temperature dependence of the auto-ignition location and time, and the temperature sensitivity was higher than for a comparable methane test case from the literature. This is explained by the lower sensitivity of propane ignition delay times to the local strain rate in comparison to methane. The Rayleigh measurements however showed that the formation mechanism of auto-ignition kernels is similar for propane and methane. Ignition kernels were found to form upstream of bulges of the inflowing jet at locations with locally low scalar dissipation rate.     

Experimental and kinetic modeling study of combustion of JP-8, its surrogates and reference components in laminar nonpremixed flows

Experimental and numerical studies are carried out to construct reliable surrogates that can reproduce aspects of combustion of JP-8 and Jet-A. Surrogate fuels are defined as mixtures of few hydrocarbon compounds with combustion characteristics similar to those of commercial fuels. The combustion characteristics considered here are extinction and autoignition in laminar non premixed flows. The “reference” fuels used as components for the surrogates of jet fuels are n-decane, n-dodecane, methylcyclohexane, toluene, and o-xylene. Three surrogates are constructed by mixing these components in proportions to their chemical types found in jet fuels. Experiments are conducted in the counterflow system. The fuels tested are the components of the surrogates, the surrogates, and the jet fuels. A fuel stream made up of a mixture of fuel vapors and nitrogen is injected into a mixing layer from one duct of a counterflow burner. Air is injected from the other duct into the same mixing layer. The strain rate at extinction is measured as a function of the mass fraction of fuel in the fuel stream. The temperature of the air at autoignition is measured as a function of the strain rate at a fixed value of the mass fraction of fuel in the fuel stream. The measured values of the critical conditions of extinction and autoignition for the surrogates show that they are slightly more reactive than the jet fuels. Numerical calculations are carried out using a semi-detailed chemical-kinetic mechanism. The calculated values of the critical conditions of extinction and autoignition for the reference fuels and for the surrogates are found to agree well with experimental data. Sensitivity analysis is used to highlight key elementary reactions that influence the critical conditions of autoignition of an alkane fuel and an aromatic fuel.     

The role of cool-flame chemistry in quasi-steady combustion and extinction of n-heptane droplets

Experiments on the combustion of large n-heptane droplets, performed by the National Aeronautics and Space Administration in the International Space Station, revealed a second stage of continued quasi-steady burning, supported by low-temperature chemistry, that follows radiative extinction of the first stage of burning, which is supported by normal hot-flame chemistry. The second stage of combustion experienced diffusive extinction, after which a large vapour cloud was observed to form around the droplet. In the present work, a 770-step reduced chemical-kinetic mechanism and a new 62-step skeletal chemical-kinetic mechanism, developed as an extension of an earlier 56-step mechanism, are employed to calculate the droplet burning rates, flame structures, and extinction diameters for this cool-flame regime. The calculations are performed for quasi-steady burning with the mixture fraction as the independent variable, which is then related to the physical variables of droplet combustion. The predictions with the new mechanism, which agree well with measured autoignition times, reveal that, in decreasing order of abundance, H₂O, CO, H₂O₂, CH₂O, and C₂H₄ are the principal reaction products during the low-temperature stage and that, during this stage, there is substantial leakage of n-heptane and O₂ through the flame, and very little production of CO₂ with no soot in the mechanism. The fuel leakage has been suggested to be the source of the observed vapour cloud that forms after flame extinction. While the new skeletal chemical-kinetic mechanism facilitates understanding of the chemical kinetics and predicts ignition times well, its predicted droplet diameters at extinction are appreciably larger than observed experimentally, but predictions with the 770-step reduced chemical-kinetic mechanism are in reasonably good agreement with experiment. The computations show how the key ketohydroperoxide compounds control the diffusion-flame structure and its extinction.     

LES based investigation of autoignition in turbulent co-flow configurations

The impact of turbulence on the autoignition of a diluted hydrogen jet in a hot co-flow of air is studied numerically. The LES combustion model used is successfully validated against experimental measurements and 3D DNS. Parametric studies are then carried out by separately varying turbulent intensity and integral length scale in the co-flow, while keeping all other boundary conditions unchanged. It is found that the impact of turbulence on the location of autoignition is non-trivial. For weak to mild turbulence, with a turbulent time scale larger than the minimum ignition delay time, autoignition is facilitated by increased turbulence. This is due to enhanced mixing between fuel and air, creating larger most reactive mixture fraction regions. On the other hand, for turbulent time scales smaller than the ignition delay time, the increased scalar dissipation rate dominates over the effect of increased most reactive mixture fraction regions, which leads to a rise in the autoignition length. Turbulence–chemistry interaction mechanisms are analysed in order to explain these observations.     

An experimental and computational investigation of the reaction between pent-1-en-3-yl radicals and oxygen molecules under autoignition conditions

We have measured the kinetics of the reaction between pent-1-en-3-yl (

) radicals and oxygen molecules using laser-photolysis/photoionization mass spectrometry at temperatures relevant for autoignition (600–710 K). The rate coefficient of the title reaction was found to be relatively large for an allylic radical in the studied temperature range (1.27–1.79×10⁻¹⁵cm³ s⁻¹). With such a large rate coefficient the studied reaction is expected to be an important sink of pent-1-en-3-yl radicals under autoignition conditions. Quantum chemical calculations and master equation simulations were performed to complement the experimental work. Experimental data was used to fix the values of key parameters in the master equation model. The model was then used to investigate the title reaction over a wide range of conditions (200–1500 K and 10⁻⁵ – 10² bar). The simulations predict that the title reaction mainly forms (E/Z)-pent-1,3-diene and hydroperoxyl at elevated temperatures (T > 500 K), but non-negligible amounts of (2R/S)-1,2-epoxypent-3-ene and hydroxyl are also formed. We found experimental evidence for both product channels, but it was not conclusive. Arrhenius representations are given for the product channels to facilitate the use of our results in combustion modelling.     

Experimental and computational investigation of coupled resonator–cavity systems

Launch vehicle noise is broadband in nature and the noise transmitted into the payload fairing is reduced by treating its interior with an acoustic absorption layer. The latest generation payload fairings are made from composite material which offer poor noise attenuation at low frequencies. One possible solution for reducing the low frequency noise is to use Helmholtz resonators tuned to a few of the dominant low frequency components, such as shell ring frequency or the first few cavity modes of the fairing. The paper presents a simplified modelling approach for numerical simulation of a coupled cavity–resonator system which is validated by experiments. The influence of damping and resonator volume fraction on the coupled system performance, to suppress the first axial mode in a cylindrical cavity, is shown and the resonator volume fraction required for significantly (more than 5 dB) suppressing the cavity axial mode is established.     

Experimental investigation of extinction cross-section of single vegetation elements using open resonator

The experimental results of investigation of the total cross — section of scattering and absorption (extinction cross — section) _t of stalks and strip leaves of grass in millimeter waves range are presented. The investigation was carried out using open quasioptical resonator with spherical mirrors. Measirements were carried out as by absolute method as using the relative method. Experimental results obtained by two different methods showed that in spite of some data differences obtained by absolute and relative methods the latter may be used for estimation of the extinction cross — section _t of vegetation elements.     

The structure and extinction of nonpremixed methane/nitrous oxide and ethane/nitrous oxide flames

Knowledge of combustion of hydrocarbon fuels with nitrogen-containing oxidizers is a first step in understanding key aspects of combustion of hypergolic and gun propellants. Here an experimental and kinetic-modeling study is carried out to elucidate aspects of nonpremixed combustion of methane (CH₄) and nitrous oxide (N₂O), and ethane (C₂H₆) and N₂O. Experiments are conducted, at a pressure of 1 atm, on flames stabilized between two opposing streams. One stream is a mixture of oxygen (O₂), nitrogen (N₂), and N₂O, and the other a mixture of CH₄ and N₂ or C₂H₆ and N₂. Critical conditions for extinction are measured. Kinetic-modeling studies are performed with the San Diego Mechanism. Experimental data and results of kinetic-modeling show that N₂O inhibits the flame by promoting extinction. Analysis of the flame structure shows that H radicals are produced in the overall chain-branching step 3H₂ + O₂ ? 2H₂O + 2H, in which molecular hydrogen is consumed. Hydrogen is also consumed in the overall step N₂O + H₂ ? N₂ + H₂O where stable products are formed. Inhibition of the flames by N₂O is attributed to competition between these two overall steps.