Search

The work presented in this study aims to understand the spray-wall-flow interaction within a gasoline direct-injection (GDI) engine flow bench under simulated early-injection conditions. The Engine Combustion Network (ECN) Spray G injector is installed in the Darmstadt optically accessible engine flow bench. Under simulated early-injection conditions, the formation of a multi-hole spray and the interaction with characteristic intake flows, such as the intake jet and central tumble flow, are extensively discussed. By reducing the complexity in the number of variables inherent in engine flow and whole-engine simulation, an engine flow bench operating under various mass flow rates is applied in this study. The numerical simulation is carried out using Large Eddy Simulation (LES) under the Eulerian-Lagrangian framework for spray simulation. Experimental data, acquired through particle image velocimetry (PIV) measurements, provides 2-D flow fields on both the central tumble and valve planes, facilitating the validation of in-cylinder flow fields. Furthermore, experimental data obtained through Mie scattering is utilized to investigate spray formation and evolution within the GDI engine, providing the liquid penetration length and liquid spray angle. Comparison between the numerical and experimental data demonstrates several agreements. Moreover, the variation of different spray plumes under different mass flow rates is observed in the case of both experimental and numerical data. Increasing the mass flow rate distorts the overall plume shape and shifts it away from the intake port. This phenomenon is examined by extracting the liquid volume fraction and vapor fields of each plume. Spray plumes encounter different convective disturbances and evaporation due to their local characteristic in-cylinder flow. Furthermore, spray-wall-flow interaction and wall film deposits are observed during the injection. Lastly, the influence of the spray-induced turbulence is analyzed under different mass flow rates. [ABSTRACT FROM AUTHOR]

Control of the combustion process in Spark-Ignition (SI) engines operated with extreme dilution from exhaust gas recirculation (EGR) represents one of the major limitations in the industrial design of such technology. Numerical approaches able to describe in detail the formation of the early flame kernel become essential to face such an ambitious task. This work presents a RANS-based multi-dimensional model of the combustion process, including an advanced description of the ignition stage to consider its stochastic re-ignitions within the average cycle prediction. The sparkchannel is described as a column of Lagrangian parcels that represent early flame kernels, whose growth is controlled by the laminar flame speed and energy input from the electrical circuit. The spatial evolution of each parcel is computed according to a scaled value of the average-flow speed, to mimic the smooth but short elongation of the mean-cycle channel produced by stochastic restrikes affecting the single-cycle arcs. To clarify this phenomenon and assess the proposed CFD method, a series of experiments are performed in a single cylinder SI engine with optical access, running at a low-load cruise-speed operating condition. Increasing EGR levels are tested up to the onset of misfire, with measurements of the secondary-circuit features and of the flame evolution through high-speed imaging. Satisfactory results are achieved in terms of numerical-experimental comparison of the cycle-averaged in-cylinder pressure, discharge parameters, and spatial flame distribution, demonstrating the reliability of the proposed numerical approach. [ABSTRACT FROM AUTHOR]

An optically accessible single-cylinder spark-ignition engine operated under homogeneous, part-load conditions is experimentally investigated using optical and spark diagnostics to evaluate the relationship between the spark, flow, and flame with increasing dilution using several levels of exhaust gas recirculation (EGR). Voltage and current measurements of the secondary spark circuit are compared with simultaneous high-speed spark plasma imaging, particle image velocimetry measurements of the flow field, and burned gas images. Specifically, characteristic restrike cycles and normal cycles are examined under the 0 % EGR and 12.9 % EGR conditions to reveal a relationship between the magnitude and direction of the velocity near the spark plug and the spark's behavior coupled with that of the subsequent flame propagation. Through the use of conditional statistics and correlation analysis of data sets of all non-restrike and all restrike cycles, the horizontal velocity across the spark gap was identified as a critical quantity in facilitating more stable and faster combustion under diluted mixture conditions. [ABSTRACT FROM AUTHOR]

Quantified conflict-driven clause learning (QCDCL) is one of the main solving approaches for quantified Boolean formulas (QBF). One of the differences between QCDCL and propositional CDCL is that QCDCL typically follows the prefix order of the QBF for making decisions. We investigate an alternative model for QCDCL solving where decisions can be made in arbitrary order. The resulting system QCDCL A NY is still sound and terminating, but does not necessarily allow to always learn asserting clauses or cubes. To address this potential drawback, we additionally introduce two subsystems that guarantee to always learn asserting clauses ( QCDCL U NI - A NY ) and asserting cubes ( QCDCL E XI - A NY ), respectively. We model all four approaches by formal proof systems and show that QCDCL U NI - A NY is exponentially better than QCDCL on false formulas, whereas QCDCL E XI - A NY is exponentially better than QCDCL on true QBFs. Technically, this involves constructing specific QBF families and showing lower and upper bounds in the respective proof systems. We complement our theoretical study with some initial experiments that confirm our theoretical findings. [ABSTRACT FROM AUTHOR]

QCDCL is one of the main algorithmic paradigms for solving quantified Boolean formulas (QBF). We design a new technique to show lower bounds for the running time in QCDCL algorithms. For this we model QCDCL by concisely defined proof systems and identify a new width measure for formulas, which we call gauge. We show that for a large class of QBFs, large (e.g. linear) gauge implies exponential lower bounds for QCDCL proof size. We illustrate our technique by computing the gauge for a number of sample QBFs, thereby providing new exponential lower bounds for QCDCL. Our technique is the first bespoke lower bound technique for QCDCL. [ABSTRACT FROM AUTHOR]

The turbulent intake flow of an optically accessible internal combustion engine is modeled using an air flow bench to reduce the complexity in the number of variables inherent within engine flows. By removing the piston and introducing a new optically accessible housing and outlet channel, the flow bench design simulates engine flows in the region just downstream of the intake valves and offers the possibility to measure and calculate quantities that would be difficult or impossible to obtain in the unsteady environment of a dynamic engine. Velocity data obtained via high-speed particle image velocimetry of the flow bench in the symmetry and valve planes are compared with data from a base operating condition of the motored engine at an intake pressure of 0.95 bar and a speed of 800 rpm at - 270 °CA ( 270 crank-angle-degrees before compression top dead center), beginning with stationary valves at the corresponding valve lift of - 270 °CA, then with moving valves. Analysis of the intake jet turbulence for increasing mass flow rates reveals a coherent flapping of the jet at a frequency of 752.5 Hz for only the 100 % mass flow rate case. The vortex shedding frequency of the valve stem is estimated to being in the range of 634 – 799 Hz, indicating a possible link between the coherent jet flapping and the vortex shedding surviving the acceleration through the valve gap. Through comprehensive analysis, this study provides valuable validation data and insight into the intake flows of internal combustion engines. [ABSTRACT FROM AUTHOR]

QBF solvers implementing the QCDCL paradigm are powerful algorithms that successfully tackle many computationally complex applications. However, our theoretical understanding of the strength and limitations of these QCDCL solvers is very limited. In this paper we suggest to formally model QCDCL solvers as proof systems. We define different policies that can be used for decision heuristics and unit propagation and give rise to a number of sound and complete QBF proof systems (and hence new QCDCL algorithms). With respect to the standard policies used in practical QCDCL solving, we show that the corresponding QCDCL proof system is incomparable (via exponential separations) to Q-resolution, the classical QBF resolution system used in the literature. This is in stark contrast to the propositional setting where CDCL and resolution are known to be p-equivalent. This raises the question what formulas are hard for standard QCDCL, since Q-resolution lower bounds do not necessarily apply to QCDCL as we show here. In answer to this question we prove several lower bounds for QCDCL, including exponential lower bounds for a large class of random QBFs. We also introduce a strengthening of the decision heuristic used in classical QCDCL, which does not necessarily decide variables in order of the prefix, but still allows to learn asserting clauses. We show that with this decision policy, QCDCL can be exponentially faster on some formulas. We further exhibit a QCDCL proof system that is p-equivalent to Q-resolution. In comparison to classical QCDCL, this new QCDCL version adapts both decision and unit propagation policies. [ABSTRACT FROM AUTHOR]

A single-cylinder full-metal engine with a real combustion chamber geometry was used to investigate particulate number emissions resulting from transient engine operation. The formation of particulate number emissions depends on mixture formation influenced by the in-cylinder flow and injection, and the formation of fuel films on the in-cylinder walls. For the investigation of this multi-parameter process, simultaneous endoscopic PIV and combustion visualization were applied. Hence, the measurement techniques allowed the investigation of in-cylinder flow, flame propagation, and soot formation. The test rig was modified to apply a generic load step and a realistic tip-in with Miller cycle. The reproducibility of the engine parameters during the transient allowed statistical analysis and the comparison between steady-state operating points. Cause-and-effect chains concerning the formation of soot are concluded by correlation analysis of parameters extracted from the flow field, the flame propagation and the soot luminosity. [ABSTRACT FROM AUTHOR]

A well-characterized multi-hole gasoline injector, the Engine Combustion Network's (ECN) Spray G injector, was investigated in an optically accessible research engine under four motored operating conditions with early injection. The experiments were conducted at intake pressures of 0. 4 bar and 0. 95 bar , nearly matching the ECN's standard early injection operating conditions, Spray G2 (flash boiling) and Spray G3 (early injection), respectively. This was combined with two engine speeds at 800 rpm and 1500 rpm. Using particle image velocimetry and volumetric Mie scatter imaging, the in-cylinder flows were evaluated and the effects on the spray morphology were characterized. The in-cylinder flow was evaluated to understand the spray-flow interaction, including the turbulent kinetic energy. Little effect on turbulent energy was observed in the region examined near the exit of the fuel injector nozzle shortly after injection. Mie scatter imaging was used to characterize the spray morphology and wall wetting was clearly visible on the spark plug. Cyclic variability of the sprays was found to be insignificant; and major differences in spray morphology are attributed to the in-cylinder velocity and intake pressure at the time of injection. Decreasing the bulk gas density by decreasing the intake pressure had a number of effects on the evolution of the spray including faster evaporation, increased axial liquid penetration, and decreased spray angle. Increasing the in-cylinder flow magnitudes by increasing the engine speed had a similar effect on spray morphology by also increasing the evaporation rate, increasing the axial penetration, and decreasing the spray opening angle. Comparison of the motored spray cases with a no-flow case (when the fuel is sprayed into the engine without the piston present) further illustrated the extent to which the intake flow influenced the spray shape. [ABSTRACT FROM AUTHOR]

A combined numerical and experimental investigation is carried out to analyze the cycle-to-cycle variations (CCV) in an optically accessible spark-ignition engine with port fuel injection. A stable and an unstable operating condition is considered. Well-established turbulence, combustion, and ignition models are employed in the large-eddy simulations (LES). High-speed measurements of the velocity field via particle image velocimetry and flame imaging in the tumble plane are conducted in the experiments. A detailed comparison between LES and experiments is carried out, including the in-cylinder pressure, the flow fields, the spatial flame distribution, and the fields conditioned on fast and slow cycles. Good agreement is achieved for the variables considering all cycles; yet, some discrepancies are observed for the conditionally averaged quantities. A systematic quantitative correlation analysis between the selected influencing variables and the CCV is presented, in which the influencing variables are extracted from different length scales (r = 3 mm, 12 mm, and 43 mm) and the CCV are distinguished between the early flame kernel development and later flame propagation. Even though the most relevant influencing parameters are different for the two operating conditions, the location of the coherent vortex structure is found to be important for the CCV of both cases. [ABSTRACT FROM AUTHOR]

Cyclic variability is investigated in an optically accessible single-cylinder spark-ignition research engine by introducing artificial exhaust gas in controlled amounts to the homogenous air–fuel mixture before ignition. A skip-fire scheme ensures the absence of internal exhaust gas recirculation (EGR) and allows the engine to be fired continuously for acquisition of large statistics. Four operating conditions ranging from a stable 0% EGR case up to a highly unstable extreme EGR case are analyzed to examine the increasing effects of homogeneous EGR on the cycle performance. To that end, high-speed measurements of the velocity field via particle image velocimetry and flame imaging in the tumble plane allow the determination of phenomena leading to various flame positions and sizes as well as faster and slower combustion cycles. Through extensive conditional statistical and multivariate correlation techniques, flames are found to be heavily influenced by large-scale velocity motion, especially with the presence of greater EGR which leads to lower flame speeds. The greater sensitivity of slower flames to variations in the velocity field manifests itself in an exponential increase in cyclic variability of the maximum in-cylinder pressure and causes misfire cycles where the flame is blown off or quenched at the cylinder roof. In the most extreme cycles at the highest EGR level, the state of the large-scale velocity structures at the time of ignition determines whether the flame propagates towards the center of the cylinder (and is blown off or quenched) or if the flame sustains growth by propagating within the lingering tumble vortex. [ABSTRACT FROM AUTHOR]

In this work, the Engine Combustion Network Spray G injector was mounted in the Darmstadt optical-accessible engine to study phenomena typical of multi-hole, early direct-injection events in spark-ignition engines characterized by tumble flow charge motion. Dedicated experimental measurements of both in-cylinder spray morphology and flow velocities before and after the injection process were carried out to assess the adopted numerical setup under real engine conditions. A dynamic secondary breakup model from the literature was coupled with an atomization multi-motion regime model. The model was validated against state-of-the-art ECN Spray G experiments for a constant-volume chamber under low evaporating condition. Then, the simulation of the spray injection in the engine was carried out and the achieved results were compared against the experimental data. Overall, good agreement between experiments and simulations was observed for the spray morphology and velocity fields in both cases. With reference to engine calculations the intake flow was seen to induce spray asymmetry. A partial vortex generated during the intake phase on the tumble plane interacts with the spray, developing into a full vortex which induces an upward flow that stabilizes the spray. The upward flows below the intake valve increase the dilution of the plume outside the tumble plane, which therefore exhibits reduced penetration. Moreover, the intake valves protect from the energetic intake flow the recirculation vortex generated at the tip of the plumes that lie outside the tumble plane. The intake flow helps fuse the vapor fuel clouds of the individual plumes near the injector tip, obtaining a vapor fuel with a shape like that generated by a horseshoe multi-hole injector. Finally, a phenomenological model of the interaction between the multi-hole injector jets and the engine intake flow was introduced to describe the spray evolution in a typical DISI engine. [ABSTRACT FROM AUTHOR]

Transient engine operation of direct-injection spark ignition engines can result in high particulate number emissions. To investigate the causes of soot formation, an engine test rig was developed to perform detailed measurements of real transient operation. For this purpose, a single-cylinder full-metal engine with a real combustion chamber geometry was equipped with minimally invasive optical accesses. Simultaneous high-speed endoscopic PIV, spray visualization, and combustion imaging were applied to investigate the in-cylinder processes in detail. Endoscopic PIV was first compared in the central symmetry plane with classical PIV performed at the equivalent optical engine at steady-state operation for verification. Then the engine parameters of a tip-in performed by the corresponding four-cylinder engine, which led to high particle number emissions, were applied to the single-cylinder engine. The engine parameters were in a good agreement and particle number emissions due to the maneuver were within the same range of several 106 #/cm3. In total, 19 repetitions of the tip-in maneuver were analyzed with respect to the in-cylinder processes and repeatability of engine parameters. Furthermore, the in-cylinder flow field during the late compression stroke, flame propagation, and soot luminosity of single cycles during the tip-in indicated cause-and-effect chains for the formation of pool fire and soot at the injector tip. The direction of the flow below the spark plug influenced the direction of flame propagation. An early arrival of the flame enhanced the formation of soot from fuel films formed on the piston surface or at the injector. In engine applications, counter measures can be applied to reduce the particle number emissions when accounting for these indicated cause-and-effect chains. [ABSTRACT FROM AUTHOR]

The application of a compact pulsed fiber laser for high-speed time-resolved particle image velocimetry (TR-PIV) measurements in the potential core of a turbulent round jet is presented at repetition rates of up to 500 kHz . The master oscillator power amplifier laser architecture consists of a pulsable seed diode whose emission is amplified in a Yb-doped fiber. The pulsed laser is operated continuously at repetition rates ranging from 10 kHz to 1 MHz with adjustable pulse widths at an output power of 50 W. The maximum rated pulse energy of 486 μ J is reached at 100 kHz , making the laser suitable for measurements of turbulent flows and highly transient phenomena. To demonstrate the feasibility of the laser for flow velocimetry, TR-PIV is conducted in a turbulent round jet. Two different high-speed camera systems are employed: a high-speed CMOS camera running at 200 kHz and 400 kHz and an in situ storage CCD camera for burst-mode PIV measurements at 500 kHz . For the CMOS system, the capability of measuring several characteristic quantities of turbulent flows is discussed with regard to the effects of uncertainty and spatial resolution. The presented system extends the range of suitable laser systems for high-speed PIV measurements offering continuous pulsing at repetition rates of up to 1 MHz at a compact footprint. The amount of consecutive images is solely limited by the onboard storage of the camera, which enables unprecedented temporal dynamic ranges. [ABSTRACT FROM AUTHOR]

While existing engineering tools enable us to predict how homogeneous surface roughness alters drag and heat transfer of near-wall turbulent flows to a certain extent, these tools cannot be reliably applied for heterogeneous rough surfaces. Nevertheless, heterogeneous roughness is a key feature of many applications. In the present work we focus on spanwise heterogeneous roughness, which is known to introduce large-scale secondary motions that can strongly alter the near-wall turbulent flow. While these secondary motions are mostly investigated in canonical turbulent shear flows, we show that ridge-type roughness—one of the two widely investigated types of spanwise heterogeneous roughness—also induces secondary motions in the turbulent flow inside a combustion engine. This indicates that large scale secondary motions can also be found in technical flows, which neither represent classical turbulent equilibrium boundary layers nor are in a statistically steady state. In addition, as the first step towards improved drag predictions for heterogeneous rough surfaces, the Reynolds number dependency of the friction factor for ridge-type roughness is presented. [ABSTRACT FROM AUTHOR]

Machine learning (ML) models based on a large data set of in-cylinder flow fields of an IC engine obtained by high-speed particle image velocimetry allow the identification of relevant flow structures underlying cycle-to-cycle variations of engine performance. To this end, deep feature learning is employed to train ML models that predict cycles of high and low in-cylinder maximum pressure. Deep convolutional autoencoders are self-supervised-trained to encode flow field features in low dimensional latent space. Without the limitations ascribable to manual feature engineering, ML models based on these learned features are able to classify high energy cycles already from the flow field during late intake and the compression stroke as early as 290 crank angle degrees before top dead center (− 290 ° CA) with a mean accuracy above chance level. The prediction accuracy from − 290 ° CA to − 10 ° CA is comparable to baseline ML approaches utilizing an extensive set of engineered features. Relevant flow structures in the compression stroke are revealed by feature analysis of ML models and are interpreted using conditional averaged flow quantities. This analysis unveils the importance of the horizontal velocity component of in-cylinder flows in predicting engine performance. Combining deep learning and conventional flow analysis techniques promises to be a powerful tool for ultimately revealing high-level flow features relevant to the prediction of cycle-to-cycle variations and further engine optimization. [ABSTRACT FROM AUTHOR]

Ignition and combustion of pulverized solid fuel is investigated in a laminar burner. The two-dimensional OH radical field is measured in the experiments, providing information on the first onset of ignition and a detailed characterization of the flame structure for the single particle. In addition, particle velocity and diameter are tracked in time in the experiments. Simulations are carried out with a Lagrangian point-particle approach fully coupled with an Eulerian solver for the gas-phase, which includes detailed chemistry and transport. The numerical simulation results are compared with the experimental measurements in order to investigate the ignition characteristics. The effect of the slip velocity, i.e. the initial velocity difference between the gas-phase and the particle, is investigated numerically. For increasing slip velocity, the ignition delay time decreases. For large slip velocities, the decrease in ignition delay time is found to saturate to a value which is about 40% smaller than the ignition delay time at zero slip velocity. Performing a simulation neglecting the dependency of the Nusselt number on the slip velocity, it is found that this dependency does not play a role. On the contrary, it is found that the decrease of ignition delay time induced by the slip velocity is due to modifications of the temperature field around the particle. In particular, the low-temperature fluid related to the energy sink due to particle heating is transported away from the particle position when the slip velocity is non-zero; therefore, the particle is exposed to larger temperatures. Finally, the effect of particle swell is investigated using a model for the particle swelling based on the CPD framework. With this model, we observed negligible differences in ignition delay time compared to the case in which swelling is not included. This is related to the negligible swelling predicted by this model before ignition. However, this is inconsistent with the experimental measurements of particle diameter, showing a significant increase of diameter even before ignition. In further simulations, the measured swelling was directly prescribed, using an analytical fit at the given conditions. With this approach, it is found that the inclusion of swelling reduces the ignition delay time by about 20% for small particles while it is negligible for large particles. [ABSTRACT FROM AUTHOR]

Near-wall processes in internal combustion engines strongly affect heat transfer and pollutant emissions. With continuously improving capabilities to model near-wall processes, the demand for corresponding measurements increases. To obtain an in-depth understanding of the near-wall processes within spark-ignition engines, flame distributions and flow fields were measured simultaneously near the piston surface of an optically accessible engine operating with homogeneous, stoichiometric isooctane–air mixtures. The engine was operated at two engine speeds (800 rpm and 1500 rpm) and two different intake pressures (0.95 bar and 0.4 bar). Flame distributions were obtained at high spatial resolution using high-speed planar laser induced fluorescence of sulfur dioxide ( SO 2 ). Particle tracking velocimetry was utilized to measure the flow field above the piston at high spatial resolution, which enabled the determination of hydrodynamic boundary layer profiles. Flame contours were extracted and statistical distributions of the burnt gas area determined. The burnt gas distributions were compared with the simultaneously recorded high-speed flow field measurements in the unburnt gas. A direct comparison with motored engine operation showed comparable boundary layer profiles until the flame approaches the wall. Flow acceleration due to flame expansion rapidly increases velocity gradients and the boundary layer development becomes highly transient. The interaction of flame and flow depends on the operating conditions, which results in a different evolution of burnt gas positions within the field-of-view. This has additional implications on the development of the velocity boundary layer. Depending on the operating conditions, the flame strongly affects the velocity boundary layer profiles resulting in boundary layer thicknesses (defined by 50% maximum velocity) in the order of 80 - 180 μ m . [ABSTRACT FROM AUTHOR]

Cycle-to-cycle variations in an optically accessible four-stroke direct injection spark-ignition gasoline engine are investigated using high-speed scanning particle image velocimetry and in-cylinder pressure measurements. Particle image velocimetry allows to measure in-cylinder flow fields at high spatial and temporal resolution. Binary classifiers are used to predict combustion cycles of high indicated mean effective pressure based on in-cylinder flow features and engineered tumble features obtained during the intake and the compression stroke. Basic in-cylinder flow features of the mid-cylinder plane are sufficient to predict combustion cycles of high indicated mean effective pressure as early as 180 degree crank angle before the top dead center at 0 degree crank angle. Engineered characteristic tumble features derived from the flow field are not superior to the basic flow features. The results are independent of the tested machine learning method (multilayer perceptron and boosted decision trees) and robust to hyper-parameter selection. [ABSTRACT FROM AUTHOR]

The bivariate two dimensional empirical mode decomposition (Bivariate 2D-EMD) is extended to estimate the turbulent fluctuations and to identify cycle-to-cycle variations (CCV) of in-cylinder flow. The Bivariate 2D-EMD is an adaptive approach that is not restricted by statistical convergence criterion, hence it can be used for analyzing the nonlinear and non-stationary phenomena. The methodology is applied to a high-speed PIV dataset that measures the velocity field within the tumble symmetry plane of an optically accessible engine. The instantaneous velocity field is decomposed into a finite number of 2D spatial modes. Based on energy considerations, the in-cylinder flow large-scale organized motion is separated from turbulent fluctuations. This study is focused on the second half of the compression stroke. For most of the cycles, the maximum of turbulent fluctuations is located between 50 and 30 crank angle degrees before top dead center (TDC). In regards to the phase-averaged velocity field, the contribution of CCV to the fluctuating kinetic energy is approximately 55% near TDC. [ABSTRACT FROM AUTHOR]

The development of high-speed volumetric laser-induced fluorescence measurements of formaldehyde ( CH 2 O -LIF) using a pulse-burst laser operated at a repetition rate of 100 kHz is presented. A novel laser scanning system employing an acousto-optic deflector (AOD) enables quasi-4D CH 2 O -LIF imaging at a scan frequency of 10 kHz . The diagnostic capability of time-resolved volumetric imaging is demonstrated in a partially premixed DME/air lifted turbulent jet flame near the flame base. Simultaneous imaging of laser beam profiles is performed to account for the laser pulse energy fluctuation and laser sheet inhomogeneity. With the accurate registration of laser sheet positions, the volumetric reconstruction of CH 2 O -LIF signals is performed within a detection volume of 17.3 × 11.9 × 2.3 mm 3 with an average out-of-plane spatial resolution of 250 μ m . A surface detection algorithm with adaptive thresholding is used to determine the global maximum intensity gradient by calculating gradient percentiles. The flame topology characteristics are investigated by evaluating the 3D curvatures of CH 2 O surfaces. Curvatures calculated using 2D data systematically underestimate the full 3D curvature due to the lack of out-of-plane information. The inner surfaces near the turbulent fuel jet exhibit higher probabilities of large mean curvature than the outer surfaces. The saddle and cylindrical structures are dominant on both the inner and outer surfaces and the elliptic structures occur with lower probability. The results suggest that the damping of turbulent fluctuations by the temperature increase through the CH 2 O region reduces the curvature, but the local structure topology remains self-similar. [ABSTRACT FROM AUTHOR]

The combustion of micrometer-sized polypropylene (PP) particles is analyzed in situ using a combination of high-speed planar laser-induced fluorescence of the OH radical (OH-PLIF) and a thermal decomposition analysis. The gas phase is investigated by multiple analytical techniques to gain comprehensive knowledge on the decomposition products of flame retardants and their effect on the combustion process. Neat PP is compared with a formulation consisting of 10 wt% of a phosphorus-containing flame retardant (pentaerythritol spirobis(methylphosphonate), PSMP) which is known to provide gas phase activity. The decomposition of the neat flame retardant, PP and the flame retardant formulation is investigated using a simultaneous analysis (STA) consisting of a thermal gravimetric analysis and a differential thermal analysis device which is coupled to Fourier-transform infrared spectroscopy and mass spectrometry devices. By this, the release of decomposition products of the flame retardant additive can be determined. The excitation of OH radicals is used to temporally track the diffusion flame surrounding the particles during combustion in a laminar flow reactor. The radial distance to the peak reactivity zone of flame retardant containing particles increased by about 70 µm compared with neat PP particles. Tracking the peak OH signal in the diffusion flame, during ignition and the early phase of combustion, a decrease in the peak intensity is observed for flame retardant polymer particles. Additionally, cone calorimeter tests are used to evaluate the combustion behavior as a standard test in flame retardancy. [ABSTRACT FROM AUTHOR]

Mid-level ethanol/gasoline blends can provide knock resistance benefits for stoichiometric spark-ignition engine operation, but previous studies have identified challenges associated with spray impingement and wall wetting, leading to excessive particulate matter emissions. At the same time, stratified-charge spark-ignition operation can provide increased thermal efficiency, but care has to be exercised to avoid excessive in-cylinder soot formation. In support of the use of mid-level ethanol/gasoline blends in advanced spark-ignition engines, this study presents spray and fuel-film measurements in a direct-injection spark-ignition engine operated with a 30 vol.%/70 vol.% ethanol/gasoline blend (E30). Crank-angle resolved fuel-film measurements at the piston surface are conducted using two different implementations of the refractive index matching technique. A small-angle refractive index matching implementation allows quantification of the wetted area, while a large-angle refractive index matching implementation enables semi-quantitative measurements of fuel-film thickness and volume, in addition to fuel-film area. The fuel-film measurements show that both the amount of fuel deposited on the piston and the shape of the fuel-film patterns are strongly influenced by the injection timing, duration, intake pressure, and coolant temperature. For combinations of high in-cylinder gas density and long injection duration, merging of the individual spray plumes, commonly referred to as spray collapse, can cause a dramatic change to the shape and thickness of the wall fuel films. Overall, the study provides guidance to engine designers aiming at minimizing wall wetting through tailored combinations of injection timings and durations. [ABSTRACT FROM AUTHOR]

An investigation of the interaction between the in-cylinder flow and the spray topology in two spray-guided direct injection optical engines is reported. The bulk flow field in the combustion chamber is characterized using particle image velocimetry. Geometrical parameters such as the axial penetration and the spray angle of the liquid spray are measured using Mie scatter imaging and/or diffuse back-illumination. The measured parameters are compared with data from a constant volume chamber available in the literature. For a late injection strategy, the so-called ECN Spray G standard condition, the mean values of the spray penetration do not seem to be significantly perturbed by the in-cylinder flow motion until the plumes approach the piston surface. However, spray probability maps reveal that cycle-to-cycle fluctuations of the spatial distribution of the liquid spray are affected by the magnitude of the in-cylinder flow. Particle image velocimetry during injection shows that the flow field in the vicinity of the spray plumes is heavily influenced by air entrainment, and that an upward flow in-between spray plumes develops. Consistent with previous research that demonstrated the importance of the latter flow structure for the prevention of spray collapse, it is found that increased in-cylinder flow magnitudes due to increased intake valve lifts or engine speeds enhance the spray-shape stability. Compared with cases without injection, the influence of the spray on the in-cylinder flow field is still noticeable approximately 2.5 ms after the start of injection. [ABSTRACT FROM AUTHOR]

Understanding the causes and mechanisms of cyclic variation would enable engineers to optimize engine designs, to devise new control strategies and to potentially implement new combustion strategies. For conventional spark-ignited engines, strategies exist to avoid such extremes: Weak knock can be detected by knock sensors and mitigated by a later spark timing, but this reduces cycle efficiency. B Preface b This Special Topic Issue presents a collection of articles from current research on cyclic variations in engine combustion. [Extracted from the article]

For an improved understanding of the in-cylinder processes in internal combustion (IC) engines, advanced diagnostics are needed to measure the thermodynamic state of the in-cylinder fluid. This is essential to validate numerical simulations which aim at predicting the in-cylinder processes during motored engine operation. Here, knowledge of the in-cylinder temperature is essential, as it strongly influences in-cylinder gas viscosity as well as the in-cylinder mass and thus the peak pressure. This work presents temperature measurements at 2.3 ∘CA resolution of a motored, optically accessible engine by applying tunable diode laser absorption spectroscopy (TDLAS) to in-cylinder water vapor. The temperature was investigated in different heights along the cylinder axis for variations of intake temperature and engine speed. Temperature uncertainties of ± 2.3 K were achieved at various cycle phases, and the absolute temperatures are verified using thermodynamic calculations of simultaneously measured water vapor concentration. The measured temperature dependency is compared to isentropic and polytropic changes of state, the incylinder gas mass is calculated and an estimation of the heat transfer to the walls is given. Based on the in-cylinder mass, different numerical simulations of the engine found in literature are compared. [ABSTRACT FROM AUTHOR]

The presence of a turbulent premixed flame strongly influences the properties of the adjacent velocity boundary layer. This influence is studied here using a generic configuration where at atmospheric pressure turbulent premixed methane/air flames interact with a temperature stabilized wall. The experiment is optimized for well-defined boundary conditions and optical accessibility in the zone where the flame impinges at the wall. Laser based diagnostic methods are used to measure two components of the velocity field by particle image velocimetry simultaneously with the flame front position using laser induced fluorescence of the OH molecule. Two measurement planes are selected that are aligned perpendicularly to the surface of the wall. Based on this data, the flow field near the wall is analyzed by different methodologies using laboratory-fixed and flame-conditioned statistics, a quadrant splitting analysis of the Reynolds stresses and an evaluation of the production term of the turbulent kinetic energy. The results of chemically reactive cases are compared to their corresponding non-reactive flows for otherwise identical inflow conditions. In the zone of flame-wall interactions the boundary layer structure and its turbulence are dominated by the turbulent flame. Important features are that the flame compresses the boundary layer already upstream the location where the flame is finally quenched and that ejection and sweeps are no longer the dominant mechanisms as in non-reactive boundary layers. This experimental data may serve additionally as a database for model development for near wall reactive flows. [ABSTRACT FROM AUTHOR]

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]

Structured laser illumination planar imaging (SLIPI) is combined with gas phase thermometry measurements using thermographic phosphor (TGP) particles. The technique is applied to a heated jet surrounded by a coflow which is operated at ambient temperature. The respective air flows are seeded with a powder of BaMgAlO:Eu (BAM) which is used as temperature-sensitive gas phase tracer. Upon pulsed excitation in the ultraviolet spectral range, the temperature is extracted based on the two-color ratio method combined with SLIPI. The main advantage of applying the SLIPI approach to phosphor thermometry is the reduction of particle-to-particle multiple light scattering and diffuse wall reflections, yielding a more robust calibration procedure as well as improving the measurement accuracy, precision, and sensitivity. For demonstration, this paper focuses on sample-averaged measurements of temperature fields in a jet-in-coflow configuration. Using the conventional approach, which in contrast to SLIPI is based on imaging with an unmodulated laser light sheet, we show that for the present setup typically ~40% of the recorded signal is affected by the contribution of multiply scattered photons. At locations close to walls even up to 75% of the apparent signal is due to diffuse reflection and wall luminescence of BAM sticking at the surface. Those contributions lead to erroneous temperature fields. Using SLIPI, an unbiased two-color ratio field is recovered allowing for two-dimensional mean temperature reconstructions which exhibit a more realistic physical behavior. This is in contrast to results deduced by the conventional approach. Furthermore, using the SLIPI approach it is shown that the temperature sensitivity is enhanced by a factor of up to ~2 at ~270 °C. Finally, an outlook towards instantaneous SLIPI phosphorescence thermometry is provided. [ABSTRACT FROM AUTHOR]

This paper shows the results of spatially resolved temperature measurements of the piston surface of an optically accessible direct injection spark ignition engine during flame impingement. High-speed thermographic phosphor thermometry (TPT), using GdGaO:Cr,Ce, and planar laser-induced fluorescence of the hydroxyl radical (OH-PLIF) were used to investigate the temperature increase and the time and position of flame impingement at the piston surface. Measurements were conducted at two operating cases and showed heating rates of up to 16,000 K/s. The OH-PLIF measurements were used to localize flame impingement and calculate conditioned statistics of the temperature profiles. The TPT coating was characterized and its influence on the temperature measurements evaluated. [ABSTRACT FROM AUTHOR]

In this paper, a novel scanning system for laser diagnostics was developed and characterized. The system is based on the acousto-optic deflection of a high-speed pulsed laser. Results showed that quasi-volumetric laser illumination with high precision and accuracy can be achieved with a simplified and flexible optical setup. The feasibility of the method for performing high-speed quasi-4D laser diagnostics was demonstrated by the tomographic visualization of a lifted turbulent jet flame using Mie-scattering and multi-plane particle image velocimetry measurements of a turbulent non-reactive mixing case. Three-dimensional flame and flow structures can be detected and tracked with this new scanning system. [ABSTRACT FROM AUTHOR]

This work comprises of experimental and numerical investigations of the volumetric flow in a direct injection spark-ignition engine to analyse the origin of cycle-to-cycle variations during stratified engine operation. High-speed two-dimensional two-component particle image velocimetry measurements are carried out simultaneously in the central tumble and mid-intake valve plane of an optically accessible engine, to capture the three-dimensional characteristic of the in-cylinder flow. Early investigation showed spray formation of stratified operation to be sensitive to cyclic fluctuations of the flow in a specific region below the spark within the central plane prior the first injection. Conditional statistics are used to track the origin of these variations back to the tumble flow in the valve plane during early compression underlining the three-dimensional structure of the flow. Moreover, conditional statistics reveals that the combustion performance is sensitive to the same specific flow region. According to this, computational fluid dynamics simulations are used to describe the in-cylinder flow with a map of four dominant flow structures. A new intake port geometry is derived from computational fluid dynamics simulations, optimized for high tumble generation. High-speed two-dimensional two-component particle image velocimetry in the central tumble plane is utilized to compare the optimized intake port with the original geometry. The new intake ports yield a considerable increased tumble movement with a favourable combustion performance and less cycle-to-cycle variation. [ABSTRACT FROM AUTHOR]

The development of the turbulent flow field inside a spark ignition engine is examined by large-eddy simulation (LES), from the intake flow to the tumble break-down. Ten consecutive cold flow engine cycles on a coarse and twenty cycles on a fine grid are simulated and compared to experiments of the same engine. The turbulent subgrid scales are modeled by the standard Smagorinsky and by the recently developed Sigma model. A comparison of the intake flow is made against Particle Image Velocimetry (PIV) measurements along horizontal and vertical lines and to an LES simulation performed by the Darmstadt group. Furthermore, we show the first LES comparison to Magnetic Resonance Velocimetry (MRV conducted by Freudenhammer et al.) measurements, which provided the 3D flow field inside a full scale dummy of the entire upper cylinder head including the valve seat region, at a time which mimics inflow conditions of the corresponding engine. Our LES is in good qualitative and quantitative agreement with the simulation and the experiments, with the notable exception of the measured in-cylinder pressure, which is discussed in detail and compared to 0D simulations and simulations from other groups. A criterion is proposed for estimating the number of cycles needed in a simulation, if experimental data is available. We put emphasis on the flow in the valve seat region, where turbulence is generated, and discuss the formation of the large scale tumble motion, including a comparison of the radial velocity fields on rolled-up planes around the valve seat. Here, spots of high velocities were found in the under flow region, which cannot been seen by the ensemble averaged MRV measurement. Within the compression stroke, a 2D vortex center identification algorithm is applied on slices inside the combustion chamber, yielding a 3D visualization of the tumble vortex, which is found to have a 'croissant-like' shape. The tumble vortex trajectory is plotted on the symmetry plane and compared to measurements. Finally, we consider a modified definition of the (turbulent) integral length scale that provided further insight to the tumble break-down process. [ABSTRACT FROM AUTHOR]

Time-resolved particle image velocimetry and Mie-scattering of fuel droplets at 16 kHz were used to capture simultaneously the temporal evolution of the in-cylinder flow field and spray formation within a direct-injection spark-ignition engine. The engine was operated in stratified combustion mode, with stratified mixtures created by a triple injection late in the compression stroke. The impact of geometric variation of the intake port on in-cylinder flow and flow–spray interactions was investigated, focusing on the second injection, since it provides ignitable mixtures at the time of ignition and is subject to strong fluctuations, rather than the first injection, which is very reproducible. Flow field statistics conditioned on the spray shape of the second injection revealed regions with macroscopic cycle-to-cycle flow variations, which correlated with the spray for all recorded cycles. The flow–spray interaction was traced back to before the first injection using correlation analysis, which revealed that cycle-to-cycle fluctuations of the large-scale tumble vortex had a big impact on the spray shape of the second injection, while the first injection was unaffected. This indicates that the origin of the spray fluctuations may be during intake. Despite significant flow modifications due to the intake port geometry variation, fluctuation levels of the second injection were the same for both geometries, that is, spray fluctuations were not sensitive to the geometric change. [ABSTRACT FROM AUTHOR]

Planar particle image velocimetry (PIV) and tomographic PIV (TPIV) measurements are utilized to analyze turbulent statistical theory quantities and the instantaneous turbulence within a single-cylinder optical engine. Measurements are performed during the intake and mid-compression stroke at 800 and 1500 RPM. TPIV facilitates the evaluation of spatially resolved Reynolds stress tensor (RST) distributions, anisotropic Reynolds stress invariants, and instantaneous turbulent vortical structures. The RST analysis describes distributions of individual velocity fluctuation components that arise from unsteady turbulent flow behavior as well as cycle-to-cycle variability (CCV). A conditional analysis, for which instantaneous PIV images are sampled by their tumble center location, reveals that CCV and turbulence have similar contributions to RST distributions at the mean tumble center, but turbulence is dominant in regions peripheral to the tumble center. Analysis of the anisotropic Reynolds stress invariants reveals the spatial distribution of axisymmetric expansion, axisymmetric contraction, and 3D isotropy within the cylinder. Findings indicate that the mid-compression flow exhibits a higher tendency toward 3D isotropy than the intake flow. A novel post-processing algorithm is utilized to classify the geometry of instantaneous turbulent vortical structures and evaluate their frequency of occurrence within the cylinder. Findings are coupled with statistical theory quantities to provide a comprehensive understanding of the distribution of turbulent velocity components, the distribution of anisotropic states of turbulence, and compare the turbulent vortical flow distribution that is theoretically expected to what is experimentally observed. The analyses reveal requisites of important turbulent flow quantities and discern their sensitivity to the local flow topography and engine operation. [ABSTRACT FROM AUTHOR]

In the context of toluene laser-induced fluorescence (LIF) thermometry, the two common LIF detection strategies, namely one-color and two-color detection, have been simultaneously applied to compare each strategy's ability to accurately resolve thermal gradients during an engine cycle within an optically accessible internal combustion (IC) engine. Temperature images are obtained from high-speed toluene LIF measurements and are combined with high-speed particle image velocimetry. The combination with flow data and Mie scattering images facilitates the interpretation of differences between the toluene LIF detection strategies. Two-color temperature images are limited in their ability to detect thermal gradients near the end of compression due to larger precision uncertainties. Local regions of cold gases in the two-color images are better identified with the guidance of the one-color images when homogeneous toluene mixtures preside. During expansion, large differences exist between one- and two-color temperature images and likely caused by local mixture fraction heterogeneities that bias the one-color detection strategy. Toluene condensation occurs during the expansion and exhaust stroke and causes local mixture fraction heterogeneities in the combustion chamber. Liquid toluene is in contact with solid surfaces and crevices of the combustion chamber and can evaporate during compression or expansion causing both local temperature and mixture stratification. This work demonstrates the advantage of high-speed imaging and use of multiple image diagnostics to reveal the development of natural temperature and mixture stratification in a motored IC engine. This work also suggests that natural temperature stratification typically regarded from gas-wall heat transfer may also be caused by liquid droplet evaporation on solid surfaces. Such phenomenon, however, is expected to be pertinent for all modern-day engine operating systems. [ABSTRACT FROM AUTHOR]

Magnetic resonance velocimetry (MRV) measurements are performed in a 1:1 scale model of a single-cylinder optical engine to investigate the volumetric flow within the intake and cylinder geometry during flow induction. The model is a steady flow water analogue of the optical IC-engine with a fixed valve lift of $$9.21$$ mm to simulate the induction flow at crank-angle $$270^{\circ }$$ bTDC. This setup resembles a steady flow engine test bench configuration. MRV measurements are validated with phase-averaged particle image velocimetry (PIV) measurements performed within the symmetry plane of the optical engine. Differences in experimental operating parameters between MRV and PIV measurements are well addressed. Comparison of MRV and PIV measurements is demonstrated using normalized mean velocity component profiles and showed excellent agreement in the upper portion of the cylinder chamber (i.e., $$y \ge -20$$ mm). MRV measurements are further used to analyze the ensemble average volumetric flow within the 3D engine domain. Measurements are used to describe the 3D overflow and underflow behavior as the annular flow enters the cylinder chamber. Flow features such as the annular jet-like flows extending into the cylinder, their influence on large-scale in-cylinder flow motion, as well as flow recirculation zones are identified in 3D space. Inlet flow velocities are analyzed around the entire valve curtain perimeter to quantify percent mass flow rate entering the cylinder. Recirculation zones associated with the underflow are shown to reduce local mass flow rates up to 50 %. Recirculation zones are further analyzed in 3D space within the intake manifold and cylinder chamber. It is suggested that such recirculation zones can have large implications on cylinder charge filling and variations of the in-cylinder flow pattern. MRV is revealed to be an important diagnostic tool used to understand the volumetric induction flow within engine geometries and is potentially suited to evaluate flow changes due to intake geometry modifications. [ABSTRACT FROM AUTHOR]

Turbulent opposed jet (TOJ) burners are an interesting test case for fundamental combustion research and a good benchmark for the available modelling approaches. However, these opposed jet flames strongly depend on the turbulence generation inside the nozzle, which is usually achieved through a perforated plate upstream of the nozzle exit. The present work investigates the flow from these perforated plates and the subsequent turbulence generation in great detail. We present results from highly-resolved large eddy simulations (LES) of the in-nozzle flow in turbulent opposed jets alongside state-of-the-art particle image velocimetry (PIV) at standard and high repetition rates taken inside a glass nozzle. The in-nozzle PIV data provides the LES inflow conditions with unprecedented detail, which are used to follow the initial jet development behaviour known from PIV, before jet coalescence, turbulence production and decay further downstream in the nozzles are successfully predicted. In regions where the PIV experiment suffers from inherent limitations like reflections and the velocity bias, the LES data is available to still obtain a detailed picture of the flow. The sensitivity of the simulations to various physical and numerical parameters is discussed in detail. Results from LES and PIV are compared qualitatively and quantitatively in terms of first and second moments of velocity, temporal autocorrelations, and energy density spectra. Significant deviations are found in the frequency (20%) and strength of vortex shedding from the inlet plane only, whereas the qualitative and quantitative agreement between simulation and experiment is otherwise excellent throughout, implying that a successful large eddy simulation of a turbulent opposed jet can be performed in a domain that includes the perforated plates. [ABSTRACT FROM AUTHOR]

Over the past three decades laser combustion diagnostics have guided an improved understanding of turbulent combustion processes. Until recently, this was based on statistically independent sampling using sampling rates much slower than typical integral time-scales of turbulent flames. Recent developments in laser and camera technology enabled an increase in sampling rates by more than three orders of magnitudes. Using these new instruments for particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) at high sampling rates (high-speed diagnostics) allowed the resolution of integral time-scales of turbulent flames. This statistically dependent sampling is increasingly used to temporally track transients in turbulent combustion, such as flame extinction, ignition, flashback and cycle-to-cycle variations in IC engines. The simultaneous application of flow and scalar field measurements makes insights into these transients possible that were not when using statistically independent sampling with low data acquisition rates. Conditioning on distinct flame features with high-speed diagnostics enables the inclusion of time as an additional dimension. This paper reviews the emerging field of multi-parameter, high-speed, planar laser diagnostics in combustion applications. The benefit of high data acquisition rates in turbulent combustion applications is discussed in detail as well as requirements and constraints imposed by the time-scales of the investigated phenomenon are addressed. Recent developments in laser and detector hardware are highlighted, as these are the limiting factors of the sampling rate. Finally, multi-parameter high-speed measurements in combustion are summarized, with a few examples discussed in more detail. [ABSTRACT FROM AUTHOR]

Turbulent opposed jet burners are an excellent test case for combustion research and model development due to the burners’ compactness, relative simplicity, and the good optical access they provide. The flow-field in the flame region depends strongly on the turbulence generation inside the nozzles, so that realistic flow simulations can only be achieved if the flow inside the nozzles is represented correctly, which must be verified by comparison to suitable experimental data. This paper presents detailed particle image velocimetry (PIV) measurements of the flow issuing from the turbulence generating plates (TGP) inside a glass nozzle. The resulting data is analyzed in terms of first and second moments, time-series, frequency spectra and phase averages. The measurements show how individual high velocity jets emerging from the TGP interact and recirculation zones are formed behind the solid parts of the TGP. Vortex shedding is observed in the jet’s shear layer were high levels of turbulent kinetic energy are generated. Time series measurements revealed periodic pulsations of the individual jets and implied a coupling between adjacent jets. The peak frequencies were found to be a function of the Reynolds-number. [ABSTRACT FROM AUTHOR]

In this paper, we compare the capabilities of two open source near-wall-modeled large eddy simulation (NWM-LES) approaches regarding prediction accuracy, computational costs and ease of use to predict complex turbulent flows relevant to internal combustion (IC) engines. The applied open source tools are the commonly used OpenFOAM, based on the finite volume method (FVM), and OpenLB, an implementation of the lattice Boltzmann method (LBM). The near-wall region is modeled by the Musker equation coupled to a van Driest damped Smagorinsky-Lilly sub-grid scale model to decrease the required mesh resolution. The results of both frameworks are compared to a stationary engine flow bench experiment by means of particle image velocimetry (PIV). The validation covers a detailed error analysis using time-averaged and root mean square (RMS) velocity fields. Grid studies are performed to examine the performance of the two solvers. In addition, the differences in the processes of grid generation are highlighted. The performance results show that the OpenLB approach is on average 32 times faster than the OpenFOAM implementation for the tested configurations. This indicates the potential of LBM for the simulation of IC engine-relevant complex turbulent flows using NWM-LES with computationally economic costs. [ABSTRACT FROM AUTHOR]

A new approach for quantitative mixture fraction imaging in the turbulent mixing of a fuel jet and a high temperature oxidizer co-flow was developed by means of planar laser-induced fluorescence of nitric oxide (NO-PLIF). Unlike existing strategies, the new approach is based on seeding NO in the oxidizer stream. The method was first evaluated during the laminar mixing of methane in air and CH 4 / CO 2 blends (synthetic biogas) in air, at room temperature. Mixture fraction measurements were validated against Rayleigh scattering imaging. Then, the measurements were extended to the turbulent mixing of a cold fuel jet issuing into a NO-seeded, hot air co-flow. By seeding the NO in the air stream, high signal-to-noise ratios were achieved at locations around the stoichiometric mixture fraction. Additionally, a stable in situ calibration region is available at every measurement location, which contributes to reduce the uncertainty. Results showed that the approach is not suitable for CH 4 /air mixtures due to the lack of sensitivity of the fluorescence signal to the mixture fraction. For biogas/air mixtures, the addition of CO 2 increased the response of the fluorescence signal to the mixture fraction, making the measurements feasible. The stoichiometric mixture fraction can be fully resolved for each biogas blend. Two-dimensional instantaneous mixture fraction measurements were feasible with an acceptable uncertainty. The high spatial resolution of the measurement was of the same order of the smallest scale in the concentration field (i.e., Batchelor scale). [ABSTRACT FROM AUTHOR]