A study of ignition and combustion of liquid hydrocarbon droplets in premixed fuel/air mixtures in a rapid compression machine

The combustion of two fuels with disparate reactivity such as natural gas and diesel in internal combustion engines has been demonstrated as a means to increase efficiency, reduce fuel costs and reduce pollutant formation in comparison to traditional diesel or spark-ignited engines. However, dual fuel engines are constrained by the onset of uncontrolled fast combustion (i.e., engine knock) as well as incomplete combustion, which can result in high unburned hydrocarbon emissions. To study the fundamental combustion processes of ignition and flame propagation in dual fuel engines, a new method has been developed to inject single isolated liquid hydrocarbon droplets into premixed methane/air mixtures at elevated temperatures and pressures. An opposed-piston rapid compression machine was used in combination with a newly developed piezoelectric droplet injection system that is capable of injecting single liquid hydrocarbon droplets along the stagnation plane of the combustion chamber. A high-speed Schlieren optical system was used for imaging the combustion process in the chamber. Experiments were conducted by injecting diesel droplet of various diameters (50 µm < d_o < 400 µm), into methane/air mixtures with varying equivalence ratios (0 < ϕ < 1.2) over a range of compressed temperatures (700 K < T_c < 940 K). Multiple autoignition modes was observed in the vicinity of the liquid droplets, which were followed by transition to propagating premixed flames. A computational model was developed with CONVERGE™, which uses a 141 species dual-fuel chemical kinetic mechanism for the gas phase along with a transient, analytical droplet evaporation model to define the boundary conditions at the droplet surface. The simulations capture each of the different ignition modes in the vicinity of the injected spherical diesel droplet, along with bifurcation of the ignition event into a propagating, premixed methane/air flame and a stationary diesel/air diffusion flame.     

Effect of H-atom concentration on soot formation in premixed ethylene/air flames

Soot formation is a major challenge in the development of clean and efficient combustion systems based on hydrocarbon fuels. Fundamental understanding of the reaction mechanism leading to soot formation can be obtained by investigating the role of key reactive species such as atomic hydrogen taking part in soot formation pathways. In this study, two-dimensional laser induced incandescence (LII) measurements using λ?=?1064?nm laser have been used to measure soot volume fraction (f_V) in a series of rich ethylene (C₂H₄)/air flames, stabilized over a McKenna burner fitted with a flame stabilizing metal disc. Moreover, a comparison of UV (λ?=?283?nm), visible (λ?=?532?nm) and IR (λ?=?1064?nm) laser excited LII measurements of soot is discussed. Recently developed, femtosecond two-photon laser-induced fluorescence (fs-TPLIF) technique has been applied for obtaining spatially resolved H-atom concentration ([H]) profiles under the same flame conditions. The structure of the flames has also been determined using hydroxyl radical (OH) planar laser induced fluorescence (PLIF) imaging. The results indicate an inverse dependence of f_V on [H] for a range of C₂H₄/air rich flames up to an equivalence ratio, Φ?=?3.0. Although an absolute relationship between [H] and f_V cannot be easily derived owing to the multiple steps involving H and other intermediate species in soot formation pathways, the present study demonstrates the feasibility to couple [H] and f_V obtained using advanced optical techniques for soot formation studies.     

Emission of carbon oxides during the combustion of lean methane-air premixed mixtures

A one-dimensional problem of propagation of a laminar flame front through a uniform methane-air mixture was solved using the GRI-Mech 3.0 reaction mechanism. An analysis of the composition of the combustion products behind the flame front at a pressure of 10 atm, an initial mixture temperature of 600 K, and two values of the air-to-fuel equivalence ratio (α = 1.8 and 2.5) was performed. It was demonstrated that, at short residence times, the carbon oxide emission increases as the mixture is made leaner, with the opposite tendency being observed at long residence times. Numerical calculations of the characteristics of turbulent flow and combustion in two axisymmetric homogeneous-combustion model chambers with relatively long residence times were performed within the framework of a bulk (quasi-laminar) combustion model. In calculations, the methane-air mixture composition and the wall temperature of one of the chambers were varied. The case of cooling air inflow through the chamber wall was considered. It was demonstrated that, over a wide range of parameters in the combustion chamber and on its wall, the CO emission monotonically decreases as the degree of mixture leaning grows, but it increases when the chamber wall is cooled and when cooling air is blown through the wall.     

Transient simulation of the combustion of fuel-lean hydrogen/air mixtures in platinum-coated channels

The start-up of platinum-coated, hydrogen-fuelled planar channels with heights of 1 mm is investigated numerically using 2-D transient simulations with detailed hetero-/homogeneous chemistry, heat conduction in the solid wall and surface radiation heat transfer. Simulations encompass pressures of 1 and 5 bar and fuel-lean H₂/air equivalence ratios of 0.10 to 0.28. Catalytic ignition is inhibited by rising pressure and increasing hydrogen concentration. However, at temperatures above the catalytic ignition temperature T_ign, the dependencies of the heterogeneous reactivity reverse, showing a positive order ～1.5 with respect to hydrogen concentration and an overall positive pressure order of ～0.97. Despite the longer catalytic ignition times for the larger equivalence ratios, the times required to reach steady state are shorter at larger stoichiometries due to their enhanced catalytic reactivity at T > T_ign and the resulting higher exothermicity. Following catalytic ignition, the wall temperatures eventually attain superadiabatic values due to the diffusional imbalance of hydrogen. Homogeneous chemistry considerably moderates the superadiabatic surface temperatures at 5 bar, as the gaseous combustion zone extends parallel to the channel wall and thus shields the catalyst surface from the hydrogen-rich channel core. Furthermore, gas-phase chemistry reduces the steady-state times and substantially increases the hydrogen conversion.     

DNS Of the ignition process of n-heptane/air premixed combustion with low-temperature chemistry in turbulent boundary layer

In the present work, three-dimensional direct numerical simulation (DNS) of $n$-heptane/air premixed combustion in turbulent boundary layer was performed to explore the near-wall ignition process with low-temperature chemistry. A reduced chemical mechanism with 58 species and 387 elementary reactions for $n$-heptane combustion was used in the DNS. The general characteristics of the ignition process near the wall were examined. It was found that low-temperature ignition (LTI) dominates the upstream region, and high-temperature ignition (HTI) appears in the downstream region. The ignition process and the low-temperature chemistry pathways of the DNS are compared with those of a corresponding laminar case. It was found that the ignition process was affected by turbulence, which results in thickened reaction zones. However, the carbon flow analysis of low-temperature chemistry showed that turbulence rarely affects the low-temperature chemistry pathway. The combustion modes of various regions were scrutinized based on the budget terms of species transport equations and the chemical explosion mode analysis (CEMA). It was shown that the reaction term of ${\text{RO}}_2$ is significant during the LTI process of the upstream region, and the reaction terms of ${\text{CH}}_2\mathrm{O}$ and ${\text{CO}}_2$ are evident in the downstream region, indicating the occurrence of HTI. It was also shown that auto-ignition is dominant in the upstream region. With increasing streamwise distance, the contribution of flame propagation increases, which takes over that of auto-ignition in the near-wall region.     

LES/PDF for premixed combustion in the DNS limit

We investigated the behaviour of the composition probability density function (PDF) model equations used in a large-eddy simulation (LES) of turbulent combustion in the direct numerical simulation (DNS) limit; that is, in the limit of the LES resolution length scale Δ (and the numerical mesh spacing h) being small compared to the smallest flow length scale, so that the resolution is sufficient to perform a DNS. The correct behaviour of a PDF model in the DNS limit is that the resolved composition fields satisfy the DNS equations, and there are no residual fluctuations (i.e. the PDF is everywhere a delta function). In the DNS limit, the treatment of molecular diffusion in the PDF equations is crucial, and both the ‘random-walk’ and ‘mean-drift’ models for molecular diffusion are investigated. Two test cases are considered, both of premixed laminar flames (of thickness δ_L). We examine the solutions of the model PDF equations for these test cases as functions of Δ/δ_L and h/δ_L. Each of the two PDF models has advantages and disadvantages. The mean-drift model behaves correctly in the DNS limit, but it is more difficult to implement and computationally more expensive. The random-walk model does not have the correct behaviour in the DNS limit in that it produces non-zero residual fluctuations. However, if the specified mixing rate Ω normalised by the reaction timescale τ_c is sufficiently large (Ωτ_c ? 1), then the residual fluctuations are less than 10% and the observed flame speed and thickness are close to their laminar values. Away from the DNS limit (i.e. h/δ_L ? 1), the observed flame thickness scales with the mesh spacing h, and the flame speed scales with Ωh. For this case it is possible to construct a non-general specification of the mixing rate Ω such that the flame speed matches the laminar flame speed.     

Determination of spatial temperature distributions in a laminar premixed ethylene/air flame by laser-induced atomic fluorescence

A quick and simple detection system for spatially resolved temperature measurements in flames based on laser-induced thermally assisted atomic line fluorescence of seeded rubidium atoms is described. The fluorescence light from two atomic states is dispersed and simultaneously recorded by a CCD camera. The fluorescence ratio distributions lead directly to absolute temperature distributions. The practical use, the spatial and temperature resolution and error limits of the method are discussed and compared with other procedures for temperature measurements.     

Experimental and numerical investigation of the hetero-/homogeneous combustion of lean propane/air mixtures over platinum

The pure heterogeneous and the coupled hetero-/homogeneous combustion of fuel-lean propane/air mixtures over platinum have been investigated at pressures 1 bar  p  7 bar, fuel-to-air equivalence ratios 0.23  φ  0.43, and catalytic wall temperatures 723 K  T_w  1286 K. Experiments were performed in an optically accessible catalytic channel-flow reactor and involved 1-D Raman measurements of major gas-phase species concentrations across the reactor boundary layer for the assessment of catalytic fuel conversion and planar laser induced fluorescence (LIF) of the OH radical for the determination of homogeneous ignition. Numerical predictions were carried out with a 2-D elliptic CFD code that included a one-step catalytic reaction for the total oxidation of propane on Pt, an elementary C₃ gas-phase chemical reaction mechanism, and detailed transport. A global catalytic reaction step valid over the entire pressure–temperature-equivalence ratio parameter range has been established, which revealed a p^0.75 dependence of the catalytic reactivity on pressure. The aforementioned global catalytic step was further coupled to a detailed gas-phase reaction mechanism in order to simulate homogeneous ignition characteristics in the channel-flow reactor. The predictions reproduced within 10% the measured homogeneous ignition distances at pressures p  5 bar, while at p = 7 bar the simulations overpredicted the measurements by 19%. The overall model performance suggests that the employed hetero-/homogeneous chemical reaction schemes are suitable for the design of propane-fueled catalytic microreactors.     

A combustion chemistry study of tetramethylethylene in a laminar premixed low-pressure hydrogen flame

The combustion chemistry of tetramethylethylene (TME) was studied in a premixed laminar low-pressure hydrogen flame by combined photoionization molecular-beam mass spectrometry (PI-MBMS) and photoelectron photoion coincidence (PEPICO) spectroscopy at the Swiss Light Source (SLS) of the Paul Scherrer Institute in Villigen, Switzerland. This hexene isomer with the chemical formula C₆H₁₂ has a special structure with only allylic CH bonds. Several combustion intermediate species were identified by their photoionization and threshold photoelectron spectra, respectively. The experimental mole fraction profiles were compared to modeling results from a recently published kinetic reaction mechanism that includes a TME sub-mechanism to describe the TME/H₂ flame structure. The first stable intermediate species formed early in the flame front during the combustion of TME are 2-methyl-2-butene (C₅H₁₀) at a mass-to-charge ratio (m/z) of 70, 2,3-dimethylbutane (C₆H₁₄) at m/z 86, and 3-methyl-1,2-butadiene (C₅H₈) at m/z 68. Isobutene (C₄H₈) is also a dominant intermediate in the combustion of TME and results from consumption of 2-methyl-2-butene. In addition to these hydrocarbons, some oxygenated species are formed due to low-temperature combustion chemistry in the consumption pathway of TME under the investigated flame conditions.     

A shock tube study of the auto-ignition of toluene/air mixtures at high pressures

The auto-ignition of toluene/air mixtures was studied in a shock tube at temperatures of 1021-1400 K, pressures of 10-61 atm, and equivalence ratios of Φ = 1.0, 0.5, and 0.25. Ignition times were measured using endwall OH∗ emission and sidewall piezoelectric pressure measurements. The measured pressure time-histories do not show significant pre-ignition energy release, in agreement with the rapid compression machine study of Mittal and Sung [G. Mittal, C.-J. Sung, Combust. Flame 150 (2007) 355-368] and disagreement with the shock tube study of Davidson et al. [D.F. Davidson, B.M. Gauthier, R.K. Hanson, Proc. Combust. Inst. 30 (2005) 1175-1182]. Kinetic modeling predictions from three detailed mechanisms are compared. Sensitivity analysis indicates that the reaction of toluene (C₆H₅CH₃) and the benzyl radical (C₆H₅CH₂) with molecular oxygen are important and examination of the rate coefficients for these reactions suggests that improved rate parameters for the multi-channel C₆H₅CH₂ + O₂ reaction may improve model predictions.     

Stability and emissions control using air injection and H2 addition in premixed combustion

We experimentally study lean premixed combustion stabilized behind a backward-facing step. For a propane–air mixture, the lean blowout limit is associated with strong pressure fluctuation arising simultaneously with strong flame–vortex interactions, which have been shown to constitute the mechanism of heat release dynamics in this flow. A high-speed air jet, issuing from a small slot and injected perpendicular to the main flow near the step, is used to disrupt this mechanism. For momentum ratio of jet to main flow below unity, the jet dilutes the mixture, further destabilizing the flame or leading to complete blowout. Above unity, the flame becomes more stable, and the pressure oscillations are suppressed. Flow visualization and OH*/CH* chemiluminescence measurements show that a strong jet produces a more compact flame that is less driven by the wake vortex, anchored closer to the step, and deflected upwards away from the lower wall of the channel. This renders the flame less vulnerable to heat loss and strong strains, which improves its stability and extends the flammability limit. Adding hydrogen to the main fuel improves the flame stability over the entire range of the air jet mass flow, with better results for momentum ratio larger than 1; H₂ pulls the flame further upstream, away from the shear zone and the unsteady vortex. NO_x emission benefits from the air jet, while, with H₂ addition, NO_x concentration is higher in the products as the overall burning temperature rises. However, hydrogen addition enables extending the flammability limit further by increasing air supply in the primary stream, hence achieving lower NO_x. The study suggests a simpler, almost passive, multi-objective combustion control technique and indicates that hydrogen addition can be a successful in situ approach for NO_x reduction.     

An experimental and numerical investigation of the hetero-/homogeneous combustion of fuel-rich hydrogen/air mixtures over platinum

The hetero-/homogeneous combustion of hydrogen/air mixtures over platinum was investigated experimentally and numerically in a channel-flow configuration at fuel-rich equivalence ratios ranging from 2 to 7, pressures up to 5 bar and wall temperatures 760–1200 K. Experiments involved in situ one-dimensional Raman measurements of major gas-phase species concentrations over the catalyst boundary layer and planar laser induced fluorescence (LIF) of the OH radical, while simulations included an elliptic 2-D model with detailed heterogeneous and homogeneous reaction mechanisms. The employed reaction schemes reproduced the measured catalytic reactant consumption, the onset of homogeneous ignition, and the post-ignition flame shapes at all examined conditions. Although below a critical pressure, which depended on temperature, the intrinsic gas-phase kinetics of hydrogen dictated lower reactivity for the fuel-rich stoichiometries when compared to fuel-lean ones, homogeneous ignition was still more favorable for the rich stoichiometries due to the lower molecular transport of the deficient oxygen reactant that resulted in modest catalytic reactant consumption over the gaseous induction zone. Above the critical pressure, the intrinsic gaseous hydrogen kinetics yielded higher reactivity for the rich stoichiometries, which resulted in vigorous gaseous combustion at pressures up to 5 bar, in contrast to lean stoichiometry studies whereby homogeneous combustion was altogether suppressed above 3 bar. Computations at fuel-rich stoichiometries in practical channel geometries indicated that homogeneous combustion was not of concern for reactor thermal management, since the larger than unity Lewis number of the deficient oxygen reactant confined the flames to the core of the channel, away from the solid walls.     

Experimental and numerical study of premixed, lean ethylene flames

Ethylene is a key intermediate in the combustion mechanisms of most practical fuels. It plays also an important role in the formation of aromatic hydrocarbons and soot particules. The latter has motivated many experimental and numerical studies carried out on rich ethylene-air mixtures. Less studies have been devoted to lean mixtures, and the development of strategies based on lean, premixed flames to reduce soot and NO_x production requires additional experimental data in lean conditions. In this work, the chemical structure of lean premixed ethylene-oxygen-nitrogen flames stabilized on a flat-flame burner at atmospheric pressure was determined experimentally. The species mole fraction profiles were also computed by the Premix code (Chemkin II version) and four detailed reaction mechanisms. A very good agreement was observed for the main flame properties: reactants consumption, final products (CO₂, H₂O) and the main intermediates: CO and H₂. Marked differences occurred in the prediction of active intermediate species present in small concentrations. Pathways analyses were performed to identify the origins of these discrepancies. It was shown that the same reactions were involved in the four mechanisms to describe the consumption of ethylene, but with marked differences in their relative importance. C₂H₃ and CH₂HCO are the main radicals formed in this first step and their consumption increases the differences between the mechanisms either by the use of different kinetic data for common reactions or by differences in the nature of the consumption reactions.     

Experimental investigation of ethylene/air combustion instability in a model scramjet combustor using image-based methods

The combustion instabilities of supersonic combustion were investigated experimentally in a laboratory-scale scramjet combustor with a cavity flame holder. Ethylene was injected transversely from an orifice to the supersonic flow of Mach 2 with a stagnation temperature of 1900 K and a total pressure of 0.37 MPa. The dynamic pressure, CH* chemiluminescence and shadowgraph images were measured with a pressure sensor and a high-speed video camera. Dynamic pressure was analyzed by fast Fourier transform, and time-resolved CH* chemiluminescence images were modally decomposed by the sparsity-promoting dynamic mode decomposition (SP-DMD). The results indicated that two combustion instabilities were observed for cavity shear-layer stabilized combustion and the oscillation between jet-wake stabilized and cavity shear-layer ram combustions for the power spectral density (PSD) of pressure. In the case of the combustion instability of cavity shear-layer stabilized combustion, a dominant peak of approximately 128 Hz was observed for the PSD of pressure. This instability corresponded to an entire flame oscillation of the cavity shear-layer stabilized combustion, which was validated by the SP-DMD and a low rank reproduction with 10 modes. This was driven by a fuel injection oscillation in the injection orifice. In the case of oscillation between the jet-wake stabilized and the cavity shear-layer ram combustions, peaks around 1600 Hz were observed for the PSD of pressure. This mechanism was also explained by the SP-DMD modes and a low rank reproduction using within 10 modes. The DMD and shadowgraph images indicated that the vortex formed by a separation of the boundary layer induced a strong jet-wake flame, resulting in the temporal thermal choke followed by cavity shear-layer stabilized ram combustion. The data-driven approach with SP-DMD clarified the combustion instability mechanisms of the supersonic combustion in detail.