Henning Bockhorn

Euler–Lagrange simulations coupled with the multiphase particle-in-cell (MP-PIC) approach for considering inter-particulate collisions have been performed to simulate a non-reacting fluidized bed at laboratory-scale. The objective of this work is to assess dynamic properties of the fluidized bed in terms of the specific kinetic energy of the bed material kS in J/kg and the bubble frequency fB in Hz, which represent suitable measures for the efficiency of the multiphase momentum exchange and the characteristic timescale of the fluidized bed system. The simulations have reproduced the bubbling fluidization regime observed in the experiments, and the calculated pressure drop Δp in Pa has shown a reasonably good agreement with measured data. While varying the bed inventory mS in kg and the superficial gas velocity uG in m/s, kS increases with uG due to the increased momentum of the gas flow, which leads to a reinforced gas-to-solid momentum transfer. In contrast, fB decreases with mS, which is attributed to the increased bed height hB in m at larger mS. An increased gas temperature TG from 20 to 500 °C has led to an increase in kS by approximately 50%, whereas Δp, hB, and fB are not sensitive to TG. This is due to the increased gas viscosity with TG, which results in an increased drag force exerted by the gas on the solid phase. While up-scaling the reactor to increase the bed inventory, bubble formation is enhanced significantly. This has led to an increased fB, whereas kS, hB, and Δp remain almost unchanged during the scale-up process. The results reveal that the general parameters such as hB and Δp are not sufficient for assessing the hydrodynamic behavior of a fluidized bed while varying the operating temperatures and up-scaling the reactor dimension. In these cases, the dynamic properties kS and fB can be used as more suitable parameters for characterizing the hydrodynamics of fluidized beds.

OpenFOAM is one of the most widely used open-source computational fluid dynamics tools and often employed for chemical engineering applications. However, there is no systematic assessment of OpenFOAM’s numerical accuracy and parallel performance for chemically reacting flows. For the first time, this work provides a direct comparison between OpenFOAM’s built-in flow solvers as well as its reacting flow extension EBIdnsFoam with four other, well established high-fidelity combustion codes. Quantification of OpenFOAM’s numerical accuracy is achieved with a benchmark suite that has recently been established by Abdelsamie et al. (Comput Fluids 223:104935, 2021. https://doi.org/10.1016/j.compfluid.2021.104935) for combustion codes. Fourth-order convergence can be achieved with OpenFOAM’s own cubic interpolation scheme and excellent agreement with other high-fidelity codes is presented for incompressible flows as well as more complex cases including heat conduction and molecular diffusion in multi-component mixtures. In terms of computational performance, the simulation of incompressible non-reacting flows with OpenFOAM is slower than the other codes, but similar performance is achieved for reacting flows with excellent parallel scalability. For the benchmark case of hydrogen flames interacting with a Taylor–Green vortex, differences between low-Mach and compressible solvers are identified which highlight the need for more investigations into reliable benchmarks for reacting flow solvers. The results from this work provide the first contribution of a fully implicit compressible combustion solver to the benchmark suite and are thus valuable to the combustion community. The OpenFOAM cases are publicly available and serve as guide for achieving the highest numerical accuracy as well as a basis for future developments.

This study performs a linear analysis of a turbulent reacting methane-air jet flame, with the goal of predicting the response of the reacting flow to upstream acoustic actuation. Accounting for heat release fluctuations is a vital component when investigating thermoacoustic instabilities and flame noise in a linearized framework. Unlike previous studies this work develops and applies an active flame approach, meaning the heat release oscillations of the flame resulting from the acoustic fluctuations are taken into account. To yield an active flame approach in the linear framework, a combustion model needs to be linearized. It is demonstrated that linearizing Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS) combustion models leads to closure problems, making their application in the linearized framework troublesome. Reynolds-averaged Navier Stokes (RANS) combustion models, however, prove to circumvent this problem, which makes them suitable candidates for this purpose. The RANS combustion models are linearized around the temporal mean flow of the turbulent jet flame, which is obtained by LES. An a priori analysis shows that a linearized RANS-Eddy Break Up (EBU) model is the best suited among all investigated combustion models for the investigated set-up and reproduces with high accuracy the fluctuations in reaction rate obtained in the LES. Furthermore, the linearized governing equations of the flow including the linearized EBU model for the reaction rate are solved for incoming acoustic perturbations. The response modes show that the reaction rate oscillations are caused by Kelvin-Helmholtz vortex rings, which perturb the jet flame. The results are in good agreement with the LES simulations in terms of the mode shapes of both reaction rate and velocity fluctuations.

The aim of this study is to investigate the impact of global operating parameters, e.g., engine speed, brake mean effective pressure, and air–fuel ratio, of a turbocharged 4-cylinder GDI engine on the reactivity of soot particles against oxidation. The knowledge of soot reactivity is crucial for optimizing gasoline particulate filter regeneration strategies and is, consequently, a key parameter for reducing fuel consumption and CO₂ emissions. In this work, time-resolved in-cylinder soot concentrations and exhaust particle size distributions are measured by using two-color pyrometry, engine exhaust particle sizer and smoke meter, respectively. Reactivity against oxidation by molecular oxygen is determined by temperature programmed oxidation analysis. To derive a physicochemical explanation for varying soot reactivity, the morphological and nanostructural primary particle structure of collected samples is analyzed using high-resolution electron microscopy and image analysis algorithms. The results reveal that engine operating parameters affect soot reactivity differently. While engine speed has only a slight effect, a reduction of air/fuel ratio (λ < 1.0) or an increase of BMEP > 10 bar significantly reduces the soot oxidation reactivity. These findings give evidence, that the quality of the fuel/air mixture is a significant parameter influencing soot reactivity. Measured soot concentrations substantiate the hypothesis that low-sooty homogeneous premixed combustion of a homogeneous fuel/air mixture favors formation of high-reactive soot particle fractions. Reactive soot particle aggregates are composed of multiple soot fractions of different reactivity. Reactive primary particles are composed of short graphene-like layers and vice versa, providing a physicochemical explanation for varying soot reactivity depending on engine operating conditions.

Division / Sponsors: Internal Combustion Engine Division (ICED)