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This work shows experiments and simulations of the fired operation of a spark ignition engine with port-fuelled injection. The test rig considered is an optically accessible single cylinder engine specifically designed at TU Darmstadt for the detailed investigation of in-cylinder processes and model validation. The engine was operated under lean conditions using iso-octane as a substitute for gasoline. Experiments have been conducted to provide a sound database of the combustion process. A planar flame imaging technique has been applied within the swirl- and tumble-planes to provide statistical information on the combustion process to complement a pressure-based comparison between simulation and experiments. This data is then analysed and used to assess the large eddy simulation performed within this work. For the simulation, the engine code KIVA has been extended by the dynamically thickened flame model combined with chemistry reduction by means of pressure dependent tabulation. Sixty cycles have been simulated to perform a statistical evaluation. Based on a detailed comparison with the experimental data, a systematic study has been conducted to obtain insight into the most crucial modelling uncertainties. [ABSTRACT FROM PUBLISHER]

Laser induced fluorescence of sulfur dioxide (SO-PLIF) has been demonstrated as a useful tool for flame imaging. Advantage was taken from the strong temperature dependence of the SO fluorescence signal. SO fluorescence intensity increases by more than one order of magnitude if the temperature changes from ambient conditions to adiabatic flame temperatures of stoichiometric methane-air flames. This results in a steep gradient of SO-PLIF intensities at the reaction zone and therefore can be used as a reliable flame marker. SO can be excited electronically using the fourth-harmonic of an Nd:YAG laser at 266 nm. This is an attractive alternative to OH-LIF, a well-recognized flame front marker, because no frequency-doubled dye lasers are needed. This simplifies the experimental setup and is advantageous for measurements at high repetition rates where dye bleaching can become an issue. To prove the performance of this approach, SO-PLIF measurements were performed simultaneously with OH-PLIF on laminar premixed methane-air Bunsen flames for equivalence ratios between 0.9 and 1.25. These measurements were compared to 1D laminar flamelet simulations. The SO fluorescence signal was found to follow the temperature rise of the flame and is located closer to the steep temperature gradient than OH. Finally, the combined SO- and OH-PLIF setup was applied to a spark ignition IC-engine to visualize the development of the early flame kernel. [ABSTRACT FROM AUTHOR]

A more detailed understanding of coal combustion as required for predictive engineering is still lacking. To gain insights into physically relevant sub-processes the community follows a stepwise approach. Experiments have been conducted starting from lab-scale gas-assisted coal flames up to self-sustaining industrial-scale coal combustors in the MW-range. In the intermediate range, however, experiments are sparse. To close this gap in this contribution a new generic test rig is presented that is suitable for operating gas-assisted coal flames. In close similarity to a burner designed for oxycoal combustion the nozzle and quarl assembly exhibit most important characteristics of a state-of-the-art combustor. Quarl and combustion chamber provide excellent optical access such that advanced laser diagnostic methods can be applied for detailed studies of flow and scalar fields. Geometrical requirements for future numerical simulations such as easy meshing of the nozzle have been considered during the re-design of the burner. Although prepared for gas-assisted oxycoal firing, in a first step various gas flames operated in air and oxyfuel atmospheres are investigated here. The focus of the present study is on flame stabilization. The leading edge of the flame is located inside the quarl such that the optical access to this most important region is a mandatory requirement. For non-reacting and reacting conditions flow fields inside the quarl and the combustor are studied using stereoscopic particle image velocimetry (SPIV). To visualize reaction zones and flame stabilization regions planar laser induced fluorescence of the OH molecule (OH-PLIF) and broadband chemiluminescence (CL) measurements are performed for reacting conditions. It is observed that close to the nozzle the leading edge of the flame stabilizes as a diffusion flame in the slow moving shear layer located between the central recirculation zone and the fuel inlet. Further downstream the flame stabilizes in the high-momentum annular jet flow generated by the primary and secondary flow. Most probably it has switched to the premixed regime. [ABSTRACT FROM AUTHOR]

Homogeneous ignition and the evolution of near-spherical volatile flames from coal and biomass particles are investigated under laminar flow conditions experimentally with a collaborative endeavor of numerical simulations. Specifically, high-volatile bituminous (hvb) coal and walnut shell biomass particles are combusted in N 2 / O 2 and CO 2 / O 2 atmospheres with an ambient temperature of approximately 1800 K. Simultaneous laser-induced fluorescence of OH radicals (OH-LIF), diffuse back-illumination, and Mie scattering measurements are employed to study early-stage volatile combustion of two fuels. Particle size, morphology, dynamics, homogeneous ignition delay times, and volatile flame structures are evaluated by implementing advanced data processing approaches. Detailed numerical simulations are performed to encompass hard-to-measure information such as the bulk rates of particle heating and volatile release rate. Experimental results indicate that hvb coal and walnut shell particles of the same size exhibit slight difference in ignition. This is explained by the fact of early release of a relative large amount of moisture from biomass observed in detailed simulations. Moreover, ignition is delayed as particle size increases, however, only marginal difference was observed when replacing N 2 with CO 2. Increasing particle diameter also weakens the flame, which is visualized by the OH-LIF for the first time. Although the flame structure remains similar for two fuels (up to 8 ms after ignition), its intensity is further suppressed by the higher heat capacity (ρ c p ) of CO 2. The dimensionless flame stand-off distance increases with particle diameter. After the ignition, a continuously rising flame distance is observed for small particles, however, it shows a decrease-and-increase behavior of large particles. This implies a dynamic process: volatile out-gassing accelerates at increasing temperature after ignition, pushing the gas flame away from the particle. The volatile release rate plays a critical role in shaping flame structure and its evolution, in contrast to the relatively minor influence of inert gas composition and the chemical composition of fuels released. • Ignition and volatile flame structures of single walnut shell and bituminous coal particles are focused on. • Advanced multi-parameter diagnostics are supported by detailed numerical simulations. • Air and oxy-fuel conditions are compared with the same ambient gas temperature of about 1800 K. • Effects of oxygen enrichment and particle diameter on ignition delay time and volatile flame development. • Temporal evaluation of volatile flame structures and flame stand-off distances are evaluated. [ABSTRACT FROM AUTHOR]

• Local laminar flame speed measured in laminar lifted iron dust flames. • Preheat zone position identified from particle image velocimetry analysis. • Thorough assessment of statistical and systematic errors conducted. In this study, a method adapted from gas-phase combustion is presented to determine the local laminar burning velocities of iron dust flames. This method is also applicable for non-stationary flames, which presents a notable advantage for metal dust flames, as temporally stationary flames remain a challenge for practical reasons. This approach relies on the simultaneous measurement of flame contours, preheat zones, and gas velocities. These parameters were determined in two dimensions by processing multiplane particle image velocimetry measurements. For the investigations, lifted iron dust flames were selected in which the oxygen volume fraction of 30 and 40 vol% and the particle size distribution with a fixed median diameter were varied. Statistical and systematic errors were estimated. Notably, this analysis also underscores the necessity of simultaneous gas and particle velocity measurements, especially for larger particle fractions, which poses a challenge for future experimental research. [ABSTRACT FROM AUTHOR]

Effects of increased pressure on the thermochemistry of turbulent flame-wall interaction (FWI) were investigated within this study. Quantitative, pointwise measurements of three state parameters, i.e., gas-phase temperature, CO 2 and CO mole fraction, were performed by means of dual-pump coherent anti-Stokes Raman spectroscopy (CARS) and two-photon laser-induced fluorescence (LIF) of CO. The flame front was simultaneously tracked by planar OH-LIF imaging. The measurements were carried out in a fully premixed methane-air flame in an enclosed side-wall quenching burner test rig under atmospheric and pressurized conditions (3 bar absolute pressure). An evaluation of near wall flame front topologies showed that the interaction of the flame with the wall is predominantly characterized by one single, extended reaction zone, similar to a so-called head-on quenching configuration. The dominance of this quenching scenario occurred most likely due to the confined burner configuration preventing an unhindered expansion of the hot exhaust gases. Thermochemical states were studied using pairwise correlations of temperature, CO 2 and CO mole fraction. Regarding the measurements at atmospheric pressure, an overall good agreement with other studies on atmospheric FWI in a similar unconfined burner was observed considering differences concerning the flow field and the burner configuration. For the thermochemistry measurements at elevated pressure, similar trends as for the atmospheric measurements were identified. A comparison of both operating cases suggested that FWI processes were more strongly affected by heat losses at elevated pressure, but differences are moderately pronounced. To the authors' best knowledge, these experiments are the first attempt to explore the near-wall thermochemistry of FWI processes at elevated pressure using multi-parameter measurements. As the near-wall measurements were further complicated by effects arising with increasing pressure, e.g., intensified beam steering and reduced length scales, implications on the application of the laser diagnostics are reported. [ABSTRACT FROM AUTHOR]

Abstract Understanding the detailed flame transport in IC engines is important to accurately predict ignition and combustion phasing, rate of heat release and assess engine performance. This is particularly important for RANS and LES engine simulations, which often struggle to accurately predict flame propagation and heat release without first adjusting model parameters. Detailed measurements of flame transport in technical systems are required to guide model development and validation. This work introduces an experimental dataset designed to study the detailed flame transport and flame/flow dynamics for spark-ignition engines. Simultaneous dual-plane OH-LIF and stereoscopic PIV are used to acquire 3D measurements of unburnt gas velocity (U ⇀ g a s), flame displacement speed (S D) and overall flame front velocity (U ⇀ F l a m e) during the early flame development. Experiments are performed in an optical engine operating at 800 and 1500 RPM with premixed, stoichiometric isooctane-air mixtures. Analysis reveals several distinctive flame/flow configurations that yield a positive or negative flame displacement for which the flame progresses towards the reactants or products, respectively. For the operating conditions utilized, S D exhibits and inverse relationship with flame curvature and a strong correlation between negative S D and convex flame contours is observed. Trends are consistent with thermo-diffusive flames, but have not been quantified in context of IC engines. Flame wrinkling is more severe at the higher RPM, which results in a broader S D distribution towards higher positive and negative velocities. Spatially-resolved distributions of U ⇀ g a s and S D are presented to describe in-cylinder locations where either convection or thermal diffusion is the dominating mechanism contributing to flame transport. Findings are discussed in relation to common engine flow features, including flame transport near solid surfaces. Findings provide a first insight into the detailed flame transport within a technically relevant environment and are designed to support the development and validation of engine simulations. [ABSTRACT FROM AUTHOR]

• Interdisciplinary methodical approach to assess the mode of action of flame retardants in polypropylene. • Four different flame retardant additives with varying retardation effects are experimentally investigated. • Thermogravimetric analysis coupled to Fourier-transform infrared spectroscopy provides knowledge on the thermal decomposition products. • Laser-induced fluorescence of the OH radical visualizes the combustion of micrometer-sized particles and stick-shaped specimens in custom-built test environments. • The methodical combination provides a comprehensive knowledge on polymer chemistry and combustion dynamics for the selected flame retardants in polypropylene. The assessment of the efficacy and mode of action of flame retardants in polymers requires a thorough knowledge of the coupling of the presence of chemical inhibitors with the underlying heat and mass transport processes. In this experimental work, we combine a thermal decomposition analysis with optical combustion diagnostics in different test environments to gain a comprehensive understanding of the relevant sub-processes and demonstrate how this approach provides future opportunities for the research regarding the mode of action of flame retardants. Four different composites of polypropylene (PP) and 10 wt% of selected flame retardants as well as neat PP are prepared. Flame retardants include pentaerythritol spirobis(methylphosphonate) (PSMP), zinc diethylphosphinate (DEPZn), aluminum hydroxide (i.e. aluminum trihydrate, ATH) and ammonium polyphosphate (APP). The released substances in the gas phase are investigated by using thermogravimetric analysis coupled to Fourier-transform infrared spectroscopy (TGA-FTIR), and the burning behavior regarding carbon monoxide concentration and heat release rate (HRR) is studied in cone calorimeter experiments. The ignition and combustion of flame retarded micrometer-sized particles of PP are experimentally investigated in a laminar flat flame burner by means of simultaneous scanning planar laser-induced fluorescence of OH radicals (OH-PLIF) and diffuse back-illumination (DBI) at a repetition rate of 10 kHz. Particles with gas-phase activity show a decreased normalized OH signal intensity during combustion. A dependency of the dimensionless flame radius on the particle size is found which can be linked to the faster heating and pyrolysis of smaller particles compared to large particles and agglomerations. Eventually, the interaction with an external premixed flame and the subsequent extinction of self-sustaining flames of polymer sticks is investigated using OH-PLIF in an adapted horizontal burning test suited for the application of optical diagnostics. This approach allows to link the findings of investigations encompassing multiple scales, resulting in a thorough understanding of both polymer chemistry and combustion dynamics in flame retarded polymers. [ABSTRACT FROM AUTHOR]

This paper reports on simultaneous measurement of temperature and CO concentration in atmospheric methane/air jet flames impinging vertically against a water-cooled stainless-steel wall. Flame–wall interactions are investigated for statistically stationary flames and propagating flames, representing the recognized case of head-on quenching. Instantaneous temperatures are determined using nanosecond coherent anti-Stokes Raman spectroscopy of nitrogen (CARS); CO concentrations are measured using two-photon laser-induced fluorescence (LIF). Statistically stationary flames are investigated in a parametric study for equivalence ratios (0.83 < ϕ < 1.2) and two turbulence intensities. Surface temperatures were measured using phosphor thermometry (TP). Extrapolation of the gas phase to the wall temperature allows estimation of the error in determining the wall position. For transient flame–wall interactions flames are initiated by a laser-spark 27 mm below the wall and propagate against the wall. Head-on flame quenching is studied in these cases for 0.83 < ϕ < 1.0. Quenching distances and maximum wall heat fluxes are derived from the quantitatively measured gas phase temperatures. Conditional statistics are deduced from 200 individual quenching events and are analyzed for distance from the wall. Enthalpy losses of the flame to the wall severely impact the thermo-chemical state, causing significant deviation from stationary conditions. Spatial and temporal profiles of the transient flames are also investigated. The quenching layer is found to be in the range of 0.17–0.32 mm with corresponding dimensionless quenching distances between 0.38 and 0.68. During transient flame quenching the wall heat flux is enhanced by a factor of two and reaches values ranging from 0.24 to 0.48 MW/m2. The normalized quenching heat flux is found to be 0.29 for lean and 0.52 for stoichiometric methane/air flames. These values are in agreement with experimental studies that used very different measurement techniques and with results from direct numerical simulations (DNS) reported in the literature. [ABSTRACT FROM AUTHOR]

The homogeneous ignition and volatile combustion of pulverized solid fuel in single-particle and particle group configurations were studied numerically in a laminar flat flame burner. Simulations with increasing particle streams were performed to investigate the influence of the interactions in particle groups on homogeneous ignition and combustion. An extensive set of simulations are conducted considering models with different levels of detail for both the gas-phase and solid fuel chemistry. The reference simulations employ the chemical percolation devolatilization model coupled with a detailed chemistry model for gas-phase reactions. The particle-fluid interactions were modeled with a fully coupled Eulerian-Lagrangian framework. Increased ignition delay times for higher particle streams were successfully validated against available experimental measurements. Furthermore, the transition from single-particle ignition to a conically shaped volatile flame with suppressed reactions near the flame base in particle group combustion was observed in both experiments and simulations. The subsequent detailed investigations revealed that the increased heat transfer to particles and, therefore, lower gas temperature for higher particle number densities together with the local oxygen depletion are the primary reasons for this transition. Based on the reference simulation, different simplified model combinations were assessed. The systematic model reduction investigation started with assessing the fixed volatile composition as a required assumption for flamelet models. Finally, the effects of gas-phase chemistry and different simple devolatilization models on ignition and combustion chemistry were studied. Overall, all model combinations provide reasonable predictions of volatile combustion with minor local deficits in the studied conditions. [ABSTRACT FROM AUTHOR]