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.     

Numerical analysis on auto-ignition of a high pressure hydrogen jet spouting from a tube

In this study, a direct numerical simulation based on compressible flow dynamics has been applied to the autoignition and extinction of a high-pressure hydrogen jet spouting from a tube. The diameter of the tube is 4.8 mm. The length of the tube is 71 mm. At the inlet, pressure is set at 3.6, 5.3 and 21.1 MPa, and temperature is set at 300 K for all cases. To explore the autoignition of hydrogen jet, two-dimensional axisymmetric Navier–Stokes equations with a detailed chemical kinetics and rigorous transport properties have been employed. The hydrogen jet through the tube is choked. The numerical results show that the high-pressure hydrogen jet produces a semi-spherical shock wave in the ambient air at the early time of jetting. The shock wave heats up the air to a high temperature and causes the autoignition of the hydrogen and air mixture in the tube as well as at the tube exit.     

A probabilistic model of thermal explosion in polydisperse fuel spray

This work is concerned with an analysis of polydisperse spray droplets distribution on the thermal explosion processes. In many engineering applications it is usual to relate to the practical polydisperse spray as a monodisperse spray. The Sauter Mean Diameter (SMD) and its variations are frequently used for this purpose [13]. The SMD and its modifications depend only on “integral” characterization of polydisperse sprays and can be the same for very different types of polydisperse spray distributions.The current work presents a new, simplified model of the thermal explosion in a combustible gaseous mixture containing vaporizing fuel droplets of different radii (polydisperse). The polydispersity is modeled using a probability density function (PDF) that corresponds to the initial distribution of fuel droplets size. This approximation of polydisperse spray is more accurate than the traditional ‘parcel’ approximation and permits an analytical treatment of the simplified model. Since the system of the governing equations represents a multi-scale problem, the method of invariant (integral) manifolds is applied.An explicit expression of the critical condition for thermal explosion limit is derived analytically. Numerical simulations demonstrate an essential dependence of these thermal explosion conditions on the PDF type and represent a natural generalization of the thermal explosion conditions of the classical Semenov theory.     

The autoignition of cyclopentane and cyclohexane in a shock tube

Ignition delay times of cyclohexane-oxygen-argon and cyclopentane-oxygen-argon mixtures have been measured in a shock tube, the onset of ignition being detected by OH radical emission. Mixtures contained 0.5 or 1% of hydrocarbon for values of the equivalence ratio ranging from 0.5 to 2. Reflected shock waves allowed temperatures from 1230 to 1840 K and pressures from 7.3 to 9.5 atm to be obtained. These measurements have shown that cyclopentane is much less reactive than cyclohexane, as for a given temperature the observed autoignition delay times were about 10 times higher for the C₅ compound than for the C₆. Detailed mechanisms for the combustion of cyclohexane and cyclopentane have been proposed to reproduce these results. The elementary steps included in the kinetic models of the oxidation of cyclanes are close to those proposed to describe the oxidation of non cyclic alkanes and alkenes. Consequently, it has been possible to obtain these models by using an improved version of the EXGAS software, a computer package for the automatic generation of detailed kinetic models for the gas-phase combustion of alkanes and alkenes. Nevertheless, the modeling of the oxidation of cyclanes requires new types of generic reactions to be considered, and especially to define new correlations for the estimation of the rate constants. Quantum chemical calculations have been used to improve the estimation of some sensitive rate constants in the case of cyclopentane. The main reaction pathways have been derived from flow rate and sensitivity analysis.     

An experimental study of hydrogen autoignition in a turbulent co-flow of heated air

The autoignition behaviour of hydrogen in a turbulent co-flow of heated air at atmospheric pressures was examined experimentally. Turbulent flows of air, with temperatures up to 1015 K and velocities up to 35 m/s, were set up in an optically accessible tube of circular cross-section. The fuel, pure or diluted with nitrogen, was continuously injected along the centreline of the tube, with velocities equal to or larger than those of the air, and temperatures that were lower. The fuel mixing patterns hence obtained were akin to diffusion from a point source or to an axisymmetric jet within a co-flow. For a relatively wide range of temperatures and velocities, a statistically steady condition of randomly occurring autoignition kernels was observed, whose axial location was measured by hydroxyl radical chemiluminescence. The probability density function of autoignition location was sharp enough to allow the accurate determination of a minimum autoignition length and smooth enough to allow the mean and variance to be calculated. It was found that both autoignition lengths increased with the air velocity and decreased with the air temperature, as expected. An estimate of the residence time up to autoignition showed that the autoignition delay times increased with the air velocity for the same temperature, suggesting a delaying effect of the turbulence on autoignition. The connection between these findings and previous experimental and direct numerical simulation studies is discussed.     

Simulation of transient turbulent methane jet ignition and combustion under engine-relevant conditions using conditional source-term estimation with detailed chemistry

The ignition and combustion processes of transient turbulent methane jets under high-pressure and moderate temperature conditions were simulated using a computationally efficient combustion model. Closure for the mean chemical source-terms was obtained with Conditional Source-term Estimation (CSE) using first conditional moment closure in conjunction with a detailed chemical kinetic mechanism, which was reduced to a Trajectory-Generated Low-Dimensional Manifold (TGLDM). The accuracy of the manifold was first validated against the direct integral method by comparing the predicted reactive scalar profiles in three methane–air reaction systems: a laminar premixed flame, a laminar flamelet and a perfectly stirred reactor. Detailed CFD simulations incorporating the CSE-TGLDM model were able to provide reasonably good predictions of the experimental ignition delay and initial ignition kernel locations of the methane jets reported in the literature with relatively low computational cost. Nitrogen oxides formed in the methane jet flame were found to be underpredicted by the model by as much as a factor of 2. The discrepancy may be attributable to the inability of the simulation to account for the effects of the rarefaction wave in the shock-tube experiments.     

Autoignition of gasoline surrogate mixtures at intermediate temperatures and high pressures: Experimental and numerical approaches

Ignition-delay times were measured in shock-heated gases for a surrogate gasoline fuel comprised of ethanol/iso-octane/n-heptane/toluene at a composition of 40%/37.8%/10.2%/12% by liquid volume with a calculated octane number of 98.8. The experiments were carried out in stoichiometric mixtures in air behind reflected shock waves in a heated high-pressure shock tube. Initial reflected shock conditions were as follows: Temperatures of 690-1200 K, and pressures of 10, 30 and 50 bar, respectively. Ignition delay times were determined from CH^∗ chemiluminescence at 431.5 nm measured at a sidewall location. The experimental results are compared to simulated ignition delay times based on detailed chemical kinetic mechanisms. The main mechanism is based on the primary reference fuels (PRF) model, and sub-mechanisms were incorporated to account for the effect of ethanol and/or toluene. The simulations are also compared to experimental ignition-delay data from the literature for ethanol/iso-octane/n-heptane (20%/62%/18% by liquid volume) and iso-octane/n-heptane/toluene (69%/17%/14% by liquid volume) surrogate fuels. The relative behavior of the ignition delay times of the different surrogates was well predicted, but the simulations overestimate the ignition delay, mostly at low temperatures.     

Modeling high pressure ethane oxidation and pyrolysis

A previously developed detailed chemical kinetic model for ethane oxidation and pyrolysis has been extended by considering the impact of much higher pressures on the chemically- and thermally-activated reactions in the mechanism. The resulting mechanism is now compared to data at much higher pressures. These data include both ethane oxidation and pyrolysis shock tube experiments up to pressures of 986 atm as well as autoignition and ethane dissociation data near 10 atm. The temperature and pressure dependencies of the rate coefficients in the model are represented by Chebyshev polynomials. This model, with no adjustments, describes the varied experimental data very well. A first order sensitivity analysis identified the most important reactions in each of the kinetic regimes. These results, coupled with the earlier validation studies at lower pressures, suggest this model is broadly applicable to analysis of ethane pyrolysis and oxidation over very wide temperature and pressure ranges.     

Ignition of a sequential combustor: Evidence of flame propagation in the autoignitable mixture

Ignition of the second stage in a lab-scale sequential combustor is investigated experimentally. A fuel mixing section between jet-in-cross-flow injection and the second stage chamber allows the fuel and vitiated, hot cross-flow to partially mix upstream of the main heat release zone. The focus of the present work is on the transient ignition process leading to a stable flame in the second stage. High-speed OH-PLIF as well as OH chemiluminescence imaging is applied to obtain complementary planar and line-of-sight integrated information on the ignition. We find experimental evidence for the co-existence of two regimes dominating the chamber ignition, i.e. autoignition and flame propagation. As the mass flow of the dilution air injected downstream of the first stage is increased (i.e. mixing temperatures in the fuel mixing section are decreased), we transition from an autoignition to a flame propagation dominated regime. Hysteresis in the ignition behavior is observed indicating that the first stage in a sequential combustor may be operated at leaner conditions than required for ignition of the second stage. The time traces of integral heat release obtained simultaneously with a photomultiplier tube show distinct features depending on the dominating regime, which is important for high-pressure testing with limited optical access.     

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.