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A systematic analysis of knocking combustion at the knock limit in a single-cylinder research engine is conducted. Both experimental and numerical methods are used to investigate the physical phenomena involved in knocking combustion. While real gasoline fuel can be used directly in experimental studies, this is not feasible in numerical simulations. Here, surrogate fuels with a reduced number of components defined to match the desired properties, such as knock resistance, are employed. In this work, standard gasoline and three surrogate fuels are considered. Differences in composition complexity are covered by selecting isooctane and two toluene reference fuels (TRF) with ethanol addition, all of which exhibit negative temperature coefficient (NTC) behavior in which auto-ignition delay times increase with increasing temperature. Spark timing sweeps at two engine speeds show that the knock resistance of the fuels correlates with the respective research octane number (RON). Isooctane is found to have higher knock resistance and higher sensitivity to engine speed than standard gasoline. One of the two TRFs studied shows good agreement with gasoline in terms of combustion and knock characteristics. The lower knock resistance of the other TRF indicates a non-linear dependence between mixture composition and knock resistance. A strong relative increase in knock resistance at higher engine speeds suggests a possible influence of NTC behavior at lower engine speeds. In the subsequent model-based analysis, the fuel influence on combustion and auto-ignition is investigated, and the laminar burning velocities are found to correlate well with the observed heat durations. While auto-ignition may be triggered by a cool spot at the lower engine speed and at operating conditions within the NTC regime, auto-ignition at the higher engine speed is assumed to be initiated by hot spots. These different mechanisms for initiating auto-ignition were identified as a potential explanation for the different knock resistances observed.

A novel design of an AdBlue mixing unit to reduce urea deposits at low temperatures in diesel exhaust is described. The main principle of the mixer includes the injection of AdBlue in an axisymmetric swirling flow, which is achieved by splitting the exhaust stream and off‐centred introduction of the sub‐flows. Crucial geometric parameters were analyzed by computational fluid dynamics (CFD) simulations towards pressure loss, flow field, and spray morphology. Deposit formation was experimentally investigated on three upscaling levels implying an optical test bench, a diesel engine test bench, and a hydraulic excavator. In particular, the studies with the hydraulic excavator showed neither deposits nor critical back pressure. Overall, the experiments substantiated the working principle of the AdBlue mixer.

Pulverized iron flames stabilized in a multidimensional hot counterflow burner are simulated using a numerical model, which is extended from the state-of-the-art model developed by Hazenberg and van Oijen (PCI, 2021) considering unsteady effects. The results are compared to available experimental data (McRae et al., PCI, 2019), including particle image velocimetry measurements, a direct flame photo, the flow field velocity and the flame speed for different iron and oxygen concentrations. The comparison shows that the particle dynamics and flame shape can be reasonably well predicted. The flow field velocity and flame speed also show quantitative agreement between the simulation and the experiment. Based on the validated simulation results, the iron combustion characteristics, including the thermal structures and the multidimensional effects, are analyzed for different oxidizer environments. The analysis shows that the iron particles undergo a transition from kinetic-controlled regime (up to ignition) to a diffusion-controlled regime (burning) at the central axis for both environments with the particle temperature being higher than the gas temperature at the flame front, which is indicated by the Damköhler number. For the hot counterflow burner, there exist multidimensional effects, i.e., the temperature and Damköhler number change along the radial direction.

A combined experimental and numerical study is conducted on knocking combustion in turbocharged direct-injection spark-ignition engines. The experimental study is based on parameter variations in the intake-manifold temperature and pressure, as well as the air-fuel equivalence ratio. The transition between knocking and non-knocking operating conditions is studied by conducting a spark timing sweep for each operating parameter. By correlating combustion and global knock quantities, the global knock trends of the mean cycles are identified. Further insight is gained by a detailed analysis based on single cycles. The extensive experimental data is then used as an input to support numerical investigations. Based on 0D knock modeling, the global knock trends are investigated for all operation points. Taking into consideration the influence of nitric oxide on auto-ignition significantly improves the knock model prediction. Additionally, the origin of the observed cyclic variability of knock is investigated. The crank angle at knock onset in 1000 consecutive single cycles is determined using a multi-cycle 0D knock simulation based on detailed single-cycle experimental data. The overall trend is captured well by the simulation, while fluctuations are underpredicted. As one potential reason for the remaining differences of the 0D model predictions local phenomena are investigated. Therefore, 3D CFD simulations of selected operating points are performed to explore local inhomogeneities in the mixture fraction and temperature. The previously developed generalized Knock Integral Method (gKIM), which considers the detailed kinetics and turbulence-chemistry interaction of an ignition progress variable, is improved and applied. The determined influence of spark timing on the mean crank angle at knock onset agrees well with experimental data. In addition, spatially resolved information on the expected position of auto-ignition is analyzed to investigate causes of knocking combustion.

In order to reduce CO2 emissions, hydrogen combustion has become increasingly relevant for technical applications. In this context, lean H2-air flames show promising features but, among other characteristics, they tend to exhibit thermo-diffusive instabilities. The formation of cellular structures associated with these instabilities leads to an increased flame surface area which further promotes the flame propagation speed, an important reference quantity for design, control, and safe operation of technical combustors. While many studies have addressed the physical phenomena of intrinsic flame instabilities in the past, there is also a demand to predict such flame characteristics with reduced-order models to allow computationally efficient simulations. In this work, a H2-air spherical expanding flame, which exhibits thermo-diffusive instabilities, is studied with flamelet-based modeling approaches both in a-priori and a-posteriori manner. A recently proposed Flamelet/Progress Variable (FPV) model, with a manifold based on unstretched planar flames, and a novel FPV approach, which takes into account a large curvature variation in the tabulated manifold, are compared to detailed chemistry (DC) calculations. First, both FPV approaches are assessed in terms of an a-priori test with the DC reference dataset. Thereafter, the a-posteriori assessment contains two parts: a linear stability analysis of perturbed planar flames and the simulation of the spherical expanding flame. Both FPV models are systematically analyzed considering global and local flame properties in comparison to the DC reference data. It is shown that the new FPV model, incorporating large curvature variations in the manifold, leads to improved predictions for the microstructure of the corrugated flame front and the formation of cellular structures, while global flame properties are reasonably well reproduced by both models.

Thermoacoustic instabilities in gas turbine engines are of increasing interest for design of future low-emission technology combustors. Numerical methods used to predict thermo-acoustic instabilities often employ either fully compressible resource-intense Computational Fluid Dynamics (CFD) simulations, or two-step approaches where source terms extracted from CFD simulations are supplied to subsequent Computational Aero Acoustics (CAA) simulations. The present work demonstrates an analysis of self-excited thermoacoustic instabilities using a hybrid method based on runtime coupling of incompressible CFD and CAA simulations. A prior implementation for the computation of combustion noise through one-way coupling of CFD and CAA solvers has been extended to a two-way coupling that allows acoustics to influence the flow field. The resulting hybrid method couples acoustic and convective physical phenomena by using mean flow field quantities and the thermoacoustic energy source term obtained from the solution of the Navier-Stokes equations in the low Mach number limit as an input for the Acoustic Perturbation Equations (APE). The fluctuating acoustic quantities pressure and velocity obtained from the solution of the APE are in turn included in the solution of the low Mach number Navier-Stokes equations, influencing the convective dynamics of the flow field. The suitability of the present implementation to capture thermoacoustic feedback is demonstrated through analysis of the well-known self-excited instability observed in a Rijke Tube. The results are presented and discussed in comparison with a fully compressible reference solution.

Centrifugal pumps conveying gas–liquid flows are often designed based on pure liquid flows due to limited understanding of gas formations within a gas–liquid flow. Computational fluid dynamics (CFD) can provide insights into the flow characteristics, yet standard multiphase models are not able to describe a wide range of spatial scales. In this study, a hybrid multiphase approach for turbulent flows based on an Eulerian-Eulerian solver with volume-of-fluid (VOF) enhancements is applied, which can handle multiple flow regimes (continuous gas/liquid, disperse bubbles) at once. Large eddy simulations (LES) of gas–liquid-flows through a diverging horizontal channel (diffuser) are performed to evaluate the possibilities and limitations of this approach. Three representative cases, each containing different flow characteristics, are computed using a fixed bubble size approach. The results show that the hybrid approach is able to capture all flow characteristics and predict the gas void size and position precisely. However, it can be observed that the performance of the approach significantly depends on the specified bubble size.

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.

A combined experimental and numerical investigation of partially premixed laminar methane-air flames undergoing side-wall quenching (SWQ) is performed. A well-established SWQ burner is adapted to allow the seeding of the main flow with additional gaseous products issued from a (secondary) wall inlet close to the flame's quenching point. First, the characteristics of the partially premixed flame that quenches at the wall are assessed using planar laser-induced fluorescence measurements of the OH radical, and a corresponding numerical simulation with fully-resolved transport and chemistry is conducted. A boundary layer of enriched mixture is formed at the wall, leading to a reaction zone parallel to the wall for high injection rates from the wall inlet. Subsequently, in a numerical study, the wall inflow is mixed with dimethylmethylphosphonat (DMMP), a phosphor-based flame retardant. The DMMP addition allows the assessment of the combined effects of heat loss and flame retardants on the flame structure during flame-wall interaction. With an increasing amount of DMMP in the injected mixture, the flame stabilizes further away from the wall and shows a decrease in the local heat-release rate. Thereby, the maximum wall heat flux is significantly reduced. That results in a lower thermal load on the quenching wall. The flame structure analysis shows an accumulation of the intermediate species HOPO at the wall similar to the CO accumulation during the quenching of premixed flames without flame retardant addition. The study shows how the structure of a partially premixed flame is influenced by a wall that releases either additional fuel or a mixture of fuel and flame retardant. The insights gained from the canonical configuration can lead to a better understanding of the combined effects of flame retardants and heat losses in near-wall flames.

Side-wall quenching (SWQ) is one of the generic configurations for flame-wall interaction and has been widely investigated through simulations using detailed and reduced chemical kinetics. In all p...

A coaxial burner with a hydrogen-supported pulverized coal flame, operated by the Central Research Institute of Electric Power Industry (CRIEPI, Japan), is investigated numerically. The flame is modeled using massively parallel large eddy simulation (LES). A flamelet/progress variable (FPV) approach is used for modeling the complex multiphase flow of the laboratory coal flame. A four-dimensional tabulation method based on non-premixed flamelets is introduced, which uses two mixture fractions for the hydrogen pilot and coal volatiles, respectively, as well as the absolute enthalpy and the reaction progress to parametrize the thermochemical space. Simulations are compared to the experiments in terms of the temperature, gas-phase velocities (with and without consideration of buoyancy), and gas compositions along the centerline and in the radial direction at different heights. The effect of the suction probe on the scalar field measurements is tested by simulating this probing, observing relative changes up to 50% in various quantities and locations. By consideration of these probe effects, the agreement between the experiment and simulation can be improved significantly; at the same time, the simulation also provides the unperturbed scalar fields, without probing effects. The new flamelet model gives a robust and cost-effective prediction of the investigated laboratory flame, provided that the probing effects are considered.

During development of modern lean burn aero engine combustors, prediction and characterization of injector thermoacoustic response plays an important role. The SCARLET (SCaled Acoustic Rig for Low Emission Technologies) test rig fulfills this need by providing acoustically defined upstream and downstream conditions under realistic operating conditions to characterize the acoustic response of full-scale injectors in a single sector setup. This work presents results from the complementary numerical analysis campaign that aims to reproduce the acoustic scattering behaviour as measured in the SCARLET rig. As a first step, non-reacting flow simulations were performed using fully compressible CFD simulations and the CAA (computational aero acoustics) method to compute upstream and downstream acoustic fields. The results obtained are then post-processed based on a multi-microphone method to yield the acoustic scattering matrix, which follows the employed experimental post-processing strategy. Results obtained from both methods show satisfactory agreement with measurements. Geometrical simplifications and their implications on scattering behaviour are investigated to achieve a trade-off between accuracy and geometrical complexity. Both methods are compared in terms of their inherent advantages and disadvantages. Limitations associated with employed models and possible improvements are discussed.

In recent optical flow experiments on a transparent volute-type radial centrifugal pump, an accumulation of air bubbles to adherent gas pockets within the impeller blade channels was observed. A transition of unsteady bubbly flow toward an attached gas pocket at the blade suction side was found for increasing air loading of the liquid water phase. This steadily attached pocket shows a distinctive unsteady wake. A reproduction of the transition from bubbly to pocket flow in a three-dimensional (3D) flow simulation demands the treatment of dispersed bubbly flow, on the one hand, and of coherent air regions, on the other hand. Therefore, a hybrid flow solver is adopted based on an Euler–Euler two-fluid (EE2F) method for dispersed flows and features volume-of-fluid (VOF) properties when air accumulations form. A scale-adaptive simulation (SAS) turbulence model is utilized to account for highly unsteady flow regions. For the time being, a monodisperse bubble size distribution is assumed for the dispersed part of the flow. For an operation range close to the design point and rising air loading, the flow transition from bubbly to pocket flow is well captured by the hybrid simulation method. Even an alternating pocket flow in between bubbly and pocket flow regime is predicted. The simulation method is still limited by an appropriate choice of a monodisperse bubble diameter. Therefore, the disperse model part of the hybrid flow solver will be coupled with population balance and bubble interaction models in future studies.

In the present study, we report the first large eddy simulation (LES) study of the Cambridge CCB2 coal flames, one of the target flames in the Workshop on Measurement and Simulation of Coal and Biomass Conversion, with an extended flamelet progress variable (FPV) model. The extended FPV model is based on two mixture fractions considering the volatiles and char off-gases. The normalized total enthalpy is used for the interphase heat transfer modelling. Turbulence-chemistry interaction is treated with an assumed probability density function approach. The results show that the present LES can generally capture the flow field and particle distribution, while there are considerable deviations in the OH prediction due to the boundary treatment of using a mixture of volatiles and carrier gas to replace the methane-containing mixtures in the primary and pilot flow. It indicates that for such gas-assisted coal flames, the pilot fuel stream needs to be rigorously considered in the flamelet tabulation that could be resolved by extending the two-mixture-fraction model into a three-mixture-fraction model. The instantaneous Lagrangian particles histories show that the increasing of coal load has a negligible effect on devolatilization, but delays the char conversion.