Numerical study of the ignition behavior of a post-discharge kernel in a turbulent stratified crossflow

Ensuring robust ignition is critical for the operability of aeronautical gas-turbine combustors. For ignition to be successful, an important aspect is the ability of the hot gas generated by the spark discharge to initiate combustion reactions, leading to the formation of a self-sustained ignition kernel. This study focuses on this phenomena by performing simulations of kernel ignition in a crossflow configuration that was characterized experimentally. First, inert simulations are performed to identify numerical parameters correctly reproducing the kernel ejection from the ignition cavity, which is here modeled as a pulsed jet. In particular, the kernel diameter and the transit time of the kernel to the reacting mixture are matched with measurements. Considering stochastic perturbations of the ejection velocity of the ignition kernel, the variability of the kernel transit time is also reproduced by the simulations. Subsequently, simulations of a series of ignition sequences are performed with varying equivalence ratio of the fuel-air mixture in the crossflow. The numerical results are shown to reproduce the ignition failure that occurs for the leanest equivalence ratio ($?=0.6$). For higher equivalence ratios, the simulations are shown to capture the sensitivity of the ignition to the equivalence ratio, and the kernel successfully transitions into a propagating flame. Significant stochastic dispersion of the ignition strength is observed, which relates to the variability of the transit time of the kernel to the reactive mixture. An analysis of the structure of the ignition kernel also highlights the transition towards a self-propagating flame for successful ignition conditions.     

Diethoxymethane as tailor-made fuel for gasoline controlled autoignition

Within the cluster of excellence “Tailor-Made Fuels from Biomass”  diethoxymethane (DEM) was identified as a promising fuel candidate from a production perspective. Synthesized by combining a bio-based feedstock and $C{O}_2$ as carbon source together with “green hydrogen” from water electrolysis DEM is defined as “bio-hybrid fuel” . To determine the molecules general applicability to a combustion system and to develop up combustion models a rapid screening of the ignition characteristics is performed in a rapid compression machine and a shock tube. Those suggest DEM being a potential fuel for gasoline controlled autoignition (GCAI) because of a relatively wide range of temperature independent ignition delay, a good autoignition behavior compared to conventional gasoline fuel and a multi-stage ignition behavior. To test the suitability of those molecules as a fuel and determine possible improvements to the production side, DEM was used in a single cylinder research engine operated in GCAI combustion mode. Compared to GCAI combustion with conventional RON95 E10 fuel, DME shows a significantly decreased ignition delay. As a consequence, the internal residual gas fraction, whose enthalpy is used to initiate autoignition, can be reduced and combustion stability is increased. Starting from similar combustion phasing using external exhaust gas recirculation to align the ignition behavior of DEM and RON95 E10, a variation of the intake temperature reveals that DEM has the potential to reduce the sensitivity of the combustion system.     

Towards the development of an efficient low-NOx ammonia combustor for a micro gas turbine

Recent studies have demonstrated stable generation of power from pure ammonia combustion in a micro gas turbine (MGT) with a high combustion efficiency, thus overcoming some of the challenges that discouraged such applications of ammonia in the past. However, achievement of low NOx emission from ammonia combustors remains an important challenge. In this study, combustion techniques and combustor design for efficient combustion and low NOx emission from an ammonia MGT swirl combustor are proposed. The effects of fuel injection angle, combustor inlet temperature, equivalence ratio, and ambient pressure on flame stabilization and emissions were investigated in a laboratory high pressure combustion chamber. An FTIR gas analyser was employed in analysing the exhaust gases. Numerical modeling using OpenFOAM was done to better understand the dependence of NO emissions on the equivalence ratio. The result show that inclined fuel injection as opposed to vertical injection along the combustor central axis resulted to improved flame stability, and lower NH₃ and NOx emissions. Numerical and experimental results showed that a control of the equivalence ratio upstream of the combustor is critical for low NOx emission in a rich-lean ammonia combustor. NO emission had a minimum value at an upstream equivalence ratio of 1.10 in the experiments. Furthermore, NO emission was found to decrease with ambient pressure, especially for premixed combustion. For the rich-lean combustion strategy employed in this study, lower NOx emission was recorded in premixed combustion than in non-premixed combustion indicating the importance of mixture uniformity for low NOx emission from ammonia combustion. A prototype liner developed to enhance the control and uniformity of the equivalence ratio upstream of the combustor further improved ammonia combustion. With the proposed liner design, NOx emission of 42?ppmv and ammonia combustion efficiency of 99.5% were achieved at 0.3?MPa for fuel input power of 31.44?kW.     

Effect of injection dynamics on detonation wave propagation in a linear detonation combustor

The effect of reactant injection and mixing on detonation wave propagation is studied in a self-excited, optically-accessible linear detonation combustor operated with natural gas and oxygen. Fuel injection and mixing processes are captured with 100 kHz planar laser induced fluorescence (PLIF) measurements of acetone tracer injected into the fuel stream. Measurements are acquired at multiple orthogonal planes downstream of the reactant injection site to investigate the three-dimensional mixing field in the chamber. The fuel distribution field is correlated with simultaneously acquired OH${}^{*}$ chemiluminescence measurements that provide a qualitative indication of heat release in the combustor. These measurements are used to provide quantitative information of the fuel injector recovery process and its impact on detonation wave structure across a range of equivalence ratios. While significant differences in the detonation wavefront are observed with change in equivalence ratio, the characterization of the fuel refill process into the chamber after the passage of the detonation wave highlights some key generalizable features. The time available for fuel recovery is consistently between 12 – 19% of the detonation wave period across an equivalence ratio range of 0.83 – 1.48. A linear correlation between injector recovery times and the ratio of the average detonation wave pressure amplitude relative to the pressure drop across the fuel injector is observed. Instantaneous and phase-averaged measurements of acetone-PLIF with the time-coincident OH${}^{*}$ chemiluminescence images provide qualitative information of wave structure and injection dynamics along with quantification of fuel injector recovery, a key metric that drives combustor operation and performance. These measurements significantly enhance the ability to obtain detailed information on the intra- and inter-cycle spatiotemporal evolution of the reactant refill process and its coupled effects on the detonation wave structure and propagation.     

Flame kernel characterization of laser ignition of natural gas–air mixture in a constant volume combustion chamber

In this paper, laser-induced ignition was investigated for compressed natural gas–air mixtures. Experiments were performed in a constant volume combustion chamber, which simulate end of the compression stroke conditions of a SI engine. This chamber simulates the engine combustion chamber conditions except turbulence of air–fuel mixture. It has four optical windows at diametrically opposite locations, which are used for laser ignition and optical diagnostics simultaneously. All experiments were conducted at 10 bar chamber pressure and 373 K chamber temperature. Initial stage of combustion phenomena was visualized by employing Shadowgraphy technique using a high speed CMOS camera. Flame kernel development of the combustible fuel–air mixture was investigated under different relative air–fuel ratios (λ=1.2?1.7) and the images were interrogated for temporal propagation of flame front. Pressure-time history inside the combustion chamber was recorded and analyzed. This data is useful in characterizing the laser ignition of natural gas–air mixture and can be used in developing an appropriate laser ignition system for commercial use in SI engines.     

An experimental study of coal particle group combustion in conventional and oxy-fuel atmospheres using multi-parameter optical diagnostics

The present work reports an experimental study of particle group combustion of pulverized bituminous coal in laminar flow conditions using advanced multi-parameter optical diagnostics. Simultaneously conducted high-speed scanning OH-LIF, diffuse backlight-illumination (DBI), and Mie scattering measurements enable analyses of three-dimensional volatile flame structures and soot formation in conventional (i.e., N${}_2$/O${}_2$) and oxy-fuel (i.e., CO${}_2$/O${}_2$) atmospheres with increasing O${}_2$ enrichment. Particle-flame interaction is assessed by calculating instantaneous particle number density (PND), whose uncertainties are estimated by generating synthetic particles in DBI image simulations. Time-resolved particle sequences allow the evaluation of the particle velocity, which indicates a PND dependency and interactions between particles and volatile flames. 3D flame structure reconstruction and soot formation detection are first demonstrated in single-shot visualizations and then extended to analyze effects of O${}_2$ concentration, PND, and inert gas composition statistically. The increasing O${}_2$ concentration significantly reduces local flame extinction and suppresses soot formation in N${}_2$ and CO${}_2$ atmospheres. Volatile flames reveal higher intensities and lower lift-off heights as O${}_2$ concentration increases. In contrast to that, an increased PND leads to earlier flame extinction and stronger soot formation due to the local gas temperature reduction and oxygen depletion. The lift-off height reduces with increasing PND, which is explained by the complex interaction between particle dynamics, heat transfer, and volatile reactions. Slightly stronger soot formation and delayed ignition are observed in CO${}_2$ atmospheres, whereas CO${}_2$ replacement reveals insignificant influences on the flame extinction behavior. Finally, non-flammability is quantified for particle group combustion at varying PNDs in different atmospheres.     

Ignition by moving hot spheres in H2-O2-N2 environments

A combined experimental and numerical study was carried out to investigate thermal ignition by millimeter size ($d=6$ mm) moving hot spheres in H₂-O₂-N₂ environments over a range of equivalence ratios. The mixtures investigated were diluted with N₂ to keep their laminar flame speed constant and comparable to the sphere fall velocity (2.4 m/s) at time of contact with the reactive mixture. The ignition thresholds (and confidence intervals) were found by applying a logistic regression to the data and were observed to increase from lean ($\Phi =0.39$; T_sphere = 963 K) to rich ($\Phi =1.35$; T_sphere = 1007 K) conditions. Experimental temperature fields of the gas surrounding the hot sphere during an ignition event were, for the first time, extracted using interferometry and compared against simulated fields. Numerical predictions of the ignition thresholds were within 2% of the experimental values and captured the experimentally observed increasing trend between lean and rich conditions. The effect of stoichiometry and dilution on the observed variation in ignition threshold was explained using 0-D constant pressure delay time computations.     

Structure and propagation of two-dimensional,partially premixed,laminar flames in diesel engine conditions

We investigate the influence of inflow velocity (V_in) and scalar dissipation rate (χ) on the flame structure and stabilisation mechanism of steady, laminar partially premixed n-dodecane edge flames stabilised on a convective mixing layer. Numerical simulations were performed for three different χ profiles and several V_in (V_in = 0.2 to 2.5m/s). The ambient thermochemical conditions were the same as the Engine Combustion Network’s (ECN) Spray A flame, which in turn represents conditions in a typical heavy duty diesel engine. The results of a combustion mode analysis of the simulations indicate that the flame structure and stabilisation mechanism depend on V_in and χ. For low V_in the flame is attached. Increasing V_in causes the high-temperature chemistry (HTC) flame to lift-off, while the low-temperature chemistry (LTC) flame is still attached. A unique speed S_R associated with this transition is defined as the velocity at which the lifted height has the maximum sensitivity to changes in V_in. This transition velocity is negatively correlated with χ. Near $V_{in}=S_R$ a tetrabrachial flame structure is observed consisting of a triple flame, stabilised by flame propagation into the products of an upstream LTC branch. Further increasing the inlet velocity changes the flame structure to a pentabrachial one, where an additional HTC ignition branch is observed upstream of the triple flame and ignition begins to contribute to the flame stabilisation. At large V_in, the LTC is eventually lifted, and the speed at which this transition occurs is insensitive to χ. Further increasing V_in increases the contribution of ignition to flame stabilisation until the flame is completely ignition stabilised. Flow divergence caused by the LTC branch reduces the χ at the HTC branches making the HTC more resilient to χ. The results are discussed in the context of identification of possible stabilisation modes in turbulent flames.     

Reference natural gas flames at nominally autoignitive engine-relevant conditions

Laminar natural gas flames are investigated at engine-relevant thermochemical conditions where the ignition delay time τ is short due to very high ambient temperatures and pressures. At these conditions, it is not possible to measure or calculate well-defined values for the laminar flame speed s_l, laminar flame thickness δ_l, and laminar flame time scale ${\tau }_l={\delta }_l/s_l$ due to the explosive thermochemical state. Here, the corresponding reference values, s_R, δ_R, and ${\tau }_R={\delta }_R/s_R,$ that account for the effects of autoignition, are numerically estimated to investigate the enhancement of flame propagation, and the competition with autoignition that arises under nominally autoignitive conditions (characterised here by the number τ/τ_R). Large values of τ/τ_R indicate that autoignition is unimportant, values near or below unity indicate that flame propagation is not possible, and intermediate values indicate that a combination of both flame propagation and autoignition may be important, depending upon factors such as device geometry, turbulence, stratification, et cetera. The reference quantities are presented for a wide range of temperatures, equivalence ratios, pressures, and hydrogen concentrations, which includes conditions relevant to stationary gas turbine reheat burners and boosted spark ignition engines. It is demonstrated that the transition from flame propagation to autoignition is only dependent on residence time, when the results are non-dimensionalised by the reference values. The temporal evolution of the reference values are also reported for a modelled boosted SI engine. It is shown that the nominally autoignitive conditions enhance flame propagation, which may be an ameliorating factor for the onset of engine knock. The calculations are performed using a recently-developed, detailed 177 species mechanism for C0–C3 chemistry that is derived from theoretical chemistry and is suitable for a wide range of thermochemical conditions as it is not tuned or optimised for a particular operating condition.     

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.     

An extended flamelet/progress variable model for coal/biomass co-firing flame

Coal/biomass co-firing (CBCF) is regarded as one of the sustainable alternatives to reduce emissions from the utilization of fossil fuels. It features complex reacting stages and fuel streams caused by the asynchronous reaction behaviors of coal and biomass particles, which cannot be represented well by the traditional two-mixture-fractions (2Z) coal flamelet/progress variable (FPV) model. To address this issue, we developed an extended FPV model for the CBCF flame in the present study. Firstly, a three-dimensional (3D) point-particle direct numerical simulation (PP-DNS) was conducted to explore the combustion characteristics of the co-firing flame and served as a reference for the model development. Secondly, an extended FPV model was developed by introducing an extra parameter to distinguish the volatiles sources, and the model performance was evaluated by the $a$ $priori$ study as well as comparison with those of the traditional coal-/bio- 2Z-FPV models. The results showed that there were three reacting stages with four fuel streams in the CBCF flame, and their corresponding flame behaviors were obviously different from each other as demonstrated in both the one-dimensional flamelets and 3D PP-DNS solutions. The $a$ $priori$ results showed that the coal-/bio- 2Z-FPV models would give large deviations in the predictions of gas temperature and major species due to the lack of distinguishing the volatiles sources. In contrast, the extended FPV model could well reproduce the flame behaviors (both temperature and species profiles) for different reacting stages with complex fuel streams in the CBCF flame. This validated the extended FPV model and demonstrated its superiority against the traditional 2Z-FPV models.     

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.     

Application of a multiple mapping conditioning mixing model to ECN Spray A

The engine combustion network (ECN) Spray A is modelled using the Reynolds-averaged Navier–Stokes-transported probability density function (RANS-TPDF) approach to validate the application of a new multiple mapping conditioning (MMC) mixing model to multiphase reactive flows. The composition TPDF equations are solved using a Lagrangian stochastic approach and the spray is modelled with a discrete particle approach. The model is first validated under non-reacting conditions (at 900 K) using experimental mixture-fraction data. Reactive simulations are then performed for three different ambient temperatures (800, 900, 1100 K) and oxygen concentrations (13, 15, 21%) at an ambient density of 22.8 kg/m³. The MMC mixing model is compared with the interaction by exchange with the mean (IEM) mixing model. The ignition delay predictions are not sensitive to the mixing model and are predicted well by both the mixing models under all the tested ambient conditions. The IEM model overpredicts the flame lift-off length (FLOL) at high temperature and high oxygen conditions with a mixing constant $C_{?}=2$. The MMC model with $C_{?}=2$ and a target correlation coefficient $r_t=0.935$ between the mixture fraction and a reference variable used to condition mixing predicts good FLOL under all the conditions except 800 K. It is demonstrated that the lift-off length is controllable by changing the target correlation coefficient, while C_? and therefore the mixing fields are held fixed. In comparison to the MMC model, the IEM model predicts a higher variance of temperature conditioned on mixture fraction near the flame base owing to its lacking the property of localness. The mixing distance between the notional TPDF particles in the composition space is also higher with the IEM model and it is demonstrated that by changing r_t, different levels of mixing locality can be achieved.     

Modeling the non-Arrhenius ignition behavior of 2‐butanol: A detailed theoretical and experimental study

2-butanol is a promising alternative to fossil fuels for which production pathways are already established and which was proven to be suitable for usage within modern internal combustion engines. It is a potential octane booster for gasoline fuels, can be used in diesel engines to reduce soot production, and is applicable as a stand-alone fuel. In the present study, the previously not discussed non-Arrhenius ignition behavior of 2-butanol was revealed by recalculating and adjusting the thermodynamic properties of the fuel and its most important radicals. All relevant fuel consumption reaction rate coefficients were replaced by a consistent set of analogies without any further modifications of the rate parameters. The existing spectrum of validation targets for 2-butanol was extended by new high-pressure Rapid Compression Machine (RCM) experiments for end-of-compression pressures of $p_{eoc}$ =  20, 30, 40, and 50 bar with high resolution of temperature to highlight the non-Arrhenius ignition behavior and to study the model performance in more detail. The kinetic model proposed in the present study can reproduce the observed non-Arrhenius behavior within the RCM regime. Kinetic analysis demonstrated that simulated Ignition Delay Times (IDTs) are very sensitive to the equilibria of the $\stackrel{.}{\text{R}}$ + ${\text{O}}_2$ reactions for all 2-hydroxybutyl radicals. With the newly determined thermodynamic properties, the equilibria of the $\stackrel{.}{\text{R}}$ + ${\text{O}}_2$ reactions are moved towards lower temperatures, enabling low-temperature chain branching. The shift of the equilibrium from the R${\stackrel{.}{\text{O}}}_2$ side to $\stackrel{.}{\text{R}}$ + ${\text{O}}_2$ within the temperature regime covered by the RCM experiments could be identified as the main reason for the observed non-Arrhenius behavior.     

Experimental study on downward/opposed flame spread and extinction over electric wires in partial gravity environments

Downward/opposed flame spread over laboratory wire samples under varied gravity conditions were investigated in the range from 0 G to 1 G. Reduced gravity experiments are conducted by parabolic flights of an airplane. Limiting oxygen concentrations (LOCs) and flame spread rates ($V_{\mathrm{f}}$) are obtained as a function of gravity level, with oxygen concentration, forced flow velocity, and wire characteristics such as insulation thickness and core material as experimental variables. The samples used in this study consist of low-density polyethylene (LDPE) insulation over metallic cores. Copper (Cu) and nickel-chrome (NiCr) were selected as core materials. It is found that the effect of gravity on the insulation flammability varies with the thermal conductivity of the wire core; the LOCs of the Cu sample are less affected by gravity, while those of the NiCr sample decrease with decreasing gravity level. On the other hand, $V_{\mathrm{f}}$ increase monotonically with increasing gravity level in the Cu sample, while $V_{\mathrm{f}}$ of the NiCr sample show a peak value under the low gravity conditions. It is suggested that these differences in the response of LOCs and $V_{\mathrm{f}}$ to the gravity level due to the difference in core materials are controlled by the fuel concentration in the reaction zone, which is a function of $V_{\mathrm{f}}$. It is also found that the molten LDPE produced during the flame spread process shows unique behaviors depending on the gravity levels and wire characteristics. Some characteristics of the dynamic motion of the molten LDPE during the flame spread process, such as deformation and dripping, are also summarized in this paper. The experimental data obtained in this study provide useful information on the flammability of materials in a partial gravity environment and will serve as a database for fire safety design in future space exploration.