TY - CHAP A1 - Movaghar, Amirreza A1 - Linne, Mark A1 - Oevermann, Michael A1 - Meiselbach, Falko T. A1 - Schmidt, Heiko A1 - Kerstein, Alan R. T1 - Numerical study of liquid breakup at the surface of turbulent liquid jets using One-Dimensional Turbulence T2 - 26th Annual Conference on Liquid Atomization and Spray Systems, ILASS, proceedings of the Conference, Bremen, Germany, 08.09.2014 - 10.09.2014 N2 - This paper presents an investigation of primary breakup of planar turbulent liquid jets and breakup properties at the surface of turbulent jets in still air at standard conditions. Numerical simulations are carried out for jet exit Reynolds number 23000 and Weber numbers in the range [102–107]. Due to the limitation of direct numerical simulation (DNS) to moderate Reynolds numbers, a stochastic 1D ansatz based on the one-dimensional turbulence (ODT) model is used to simulate a planar liquid jet with a high lateral resolution. ODT permits an affordable high resolution of interface and single-phase property gradients which are key for understanding the local behavior. ODT is a stochastic model simulating turbulent flow evolution along a notional 1D line of sight by applying instantaneous maps to represent the effect of individual turbulent eddies on property profiles. The most relevant mechanisms that influence the primary breakup of liquid jets are found to be represented accurately based on comparisons to experiments and correlations reported in the literature. Building on this finding, future work will focus on the statistics of droplets generated by primary breakup, both to investigate their relationship to breakup mechanisms and to provide input to models of secondary breakup and subsequent spray evolution. Y1 - 2014 SN - 978-3-00-047070-7 CY - Bremen ER - TY - GEN A1 - Movaghar, Amirreza A1 - Linne, Mark A1 - Oevermann, Michael A1 - Meiselbach, Falko T. A1 - Schmidt, Heiko A1 - Kerstein, Alan R. T1 - Numerical investigation of turbulent-jet primary breakup using One-Dimensional Turbulence T2 - International Journal of Multiphase Flow N2 - Primary breakup to form droplets at liquid surfaces is an important fundamental process to study as it determines the initial properties of the dispersed phase, which affect mixing rates, secondary breakup, droplet collisions, and flow separation within the dispersed flow region. Primary breakup can be regarded as one of the least developed model components for simulating and predicting liquid jet breakup. How- ever, it is of paramount importance in many technical applications, e.g. fuel injection in engines and spray painting. This paper presents a numerical investigation of primary breakup of a turbulent liquid jet in still air at standard conditions using the one-dimensional turbulence (ODT) modeling framework. ODT is a stochastic model that simulates turbulent flow evolution along a notional 1D line of sight by applying instantaneous maps to represent the effect of individual turbulent eddies on property profiles. An important feature of ODT is the resolution of all relevant scales, both temporal and spatial. The restriction to one spatial dimension in ODT permits affordable high resolution of interfacial and single-phase property gradients, which is key to capturing the local behavior of the breakup process and allows simulations at high Reynolds and Weber numbers that are currently not accessible to direct numerical simulations (DNS). This paper summarizes our extensions of the ODT model to simulate geometrically simple jet breakup problems, including representations of Rayleigh wave breakup, turbulent breakup, and shear-driven breakup. Each jet breakup simulation consists of a short temporal channel section to initialize a turbulent velocity profile at the nozzle exit followed by an adjacent jet section. The simulations are carried out for jet exit Reynolds number of 11,500, 23,000, 46,000 and 92,000 while the Weber number is varied within the range 102–107. We present results on breakup statistics including spatial locations of droplet release, droplet sizes and liquid core length. The results on primary breakup are compared to experimental results and models. Y1 - 2017 SN - 1879-3533 VL - 89 SP - 241 EP - 254 ER - TY - GEN A1 - Lignell, David O. A1 - Lansinger, Victoria B. A1 - Medina Méndez, Juan Ali A1 - Klein, Marten A1 - Kerstein, Alan R. A1 - Schmidt, Heiko A1 - Fistler, Marco A1 - Oevermann, Michael T1 - One-dimensional turbulence modeling for cylindrical and spherical flows: model formulation and application T2 - Theoretical and Computational Fluid Dynamics N2 - The one-dimensional turbulence (ODT) model resolves a full range of time and length scales and is computationally efficient. ODT has been applied to a wide range of complex multi-scale flows, such as turbulent combustion. Previous ODT comparisons to experimental data have focused mainly on planar flows. Applications to cylindrical flows, such as round jets, have been based on rough analogies, e.g., by exploiting the fortuitous consistency of the similarity scalings of temporally developing planar jets and spatially developing round jets. To obtain a more systematic treatment, a new formulation of the ODT model in cylindrical and spherical coordinates is presented here. The model is written in terms of a geometric factor so that planar, cylindrical, and spherical configurations are represented in the same way. Temporal and spatial versions of the model are presented. A Lagrangian finite-volume implementation is used with a dynamically adaptive mesh. The adaptive mesh facilitates the implementation of cylindrical and spherical versions of the triplet map, which is used to model turbulent advection (eddy events) in the one-dimensional flow coordinate. In cylindrical and spherical coordinates, geometric stretching of the three triplet map images occurs due to the radial dependence of volume, with the stretching being strongest near the centerline. Two triplet map variants, TMA and TMB, are presented. In TMA, the three map images have the same volume, but different radial segment lengths. In TMB, the three map images have the same radial segment lengths, but different segment volumes. Cylindrical results are presented for temporal pipe flow, a spatial nonreacting jet, and a spatial nonreacting jet flame. These results compare very well to direct numerical simulation for the pipe flow, and to experimental data for the jets. The nonreacting jet treatment overpredicts velocity fluctuations near the centerline, due to the geometric stretching of the triplet maps and its effect on the eddy event rate distribution. TMB performs better than TMA. A hybrid planar-TMB (PTMB) approach is also presented, which further improves the results. TMA, TMB, and PTMB are nearly identical in the pipe flow where the key dynamics occur near the wall away from the centerline. The jet flame illustrates effects of variable density and viscosity, including dilatational effects. KW - Cylindrical ODT Y1 - 2018 U6 - https://doi.org/10.1007/s00162-018-0465-1 SN - 0935-4964 SN - 1432-2250 VL - 32 IS - 4 SP - 495 EP - 520 ER - TY - GEN A1 - Somhorst, Joop A1 - Oevermann, Michael A1 - Bovo, Mirko A1 - Denbratt, Ingemar T1 - Evaluation of thermal barrier coatings and surface roughness in a single-cylinder light-duty diesel engine T2 - International Journal of Engine Research N2 - The effect of two thermal barrier coatings and their surface roughness on heat transfer, combustion, and emissions has been investigated in a single-cylinder light-duty diesel engine. The evaluated thermal barrier coating materials were plasma-sprayed yttria-stabilized zirconia and hard anodized aluminum, which were applied on the piston top surface. The main tool for the investigation was cylinder pressure analysis of the high-pressure cycle, from which the apparent rate of heat release, indicated efficiency, and heat losses were derived. For verification of the calculated wall heat transfer, the heat flow to the piston cooling oil was measured as well. Application of thermal barrier coatings can influence engine operating conditions like charge temperature and ignition delay. Therefore, extra attention was paid to choosing stable and repeatable engine operating points. The experimental data were modeled using multiple linear regression to isolate the effects of the coatings and of the surface roughness. The results from this study show that high surface roughness leads to increased wall heat losses and a delayed combustion. However, these effects are less pronounced at lower engine loads and in the presence of soot deposits. Both thermal barrier coatings show a reduction of cycle-averaged wall heat losses, but no improvement in indicated efficiency. The surface roughness and thermal barrier coatings had a significant impact on the hydrocarbon emissions, especially for low-load engine operation, while their effect on the other exhaust emissions was relatively small. KW - Engine KW - efficiency KW - emissions KW - Thermal barrier coating KW - TBC KW - surface roughness KW - MLR KW - regression KW - data model Y1 - 2019 U6 - https://doi.org/10.1177/1468087419875837 SN - 1468-0874 SN - 2041-3149 VL - 22(2021) SP - 890 EP - 910 ER - TY - GEN A1 - Pollack, Martin A1 - Pütz, Michele A1 - Marchisio, Daniele L A1 - Oevermann, Michael A1 - Hasse, Christian T1 - Zero-flux approximations for multivariate quadrature-based moment methods T2 - Journal of Computational Physics N2 - The evolution of polydisperse systems is governed by population balance equations. A group of efficient solution approaches are the moment methods, which do not solve for the number density function (NDF) directly but rather for a set of its moments. While this is computationally efficient, a specific challenge arises when describing the fluxes across a boundary in phase space for the disappearance of elements, the so-called zero-flux. The main difficulty is the missing NDF-information at the boundary, which most moment methods cannot provide. Relevant physical examples are evaporating droplets, soot oxidation or particle dissolution. In general, this issue can be solved by reconstructing the NDF close to the boundary. However, this was previously only achieved with univariate approaches, i.e. considering only a single internal variable. Many physical problems are insufficiently described by univariate population balance equations, e.g. droplets in sprays often require the temperature or the velocity to be internal coordinates in addition to the size. In this paper, we propose an algorithm, which provides an efficient multivariate approach to calculate the zero-fluxes. The algorithm employs the Extended Quadrature Method of Moments (EQMOM) with Beta and Gamma kernel density functions for the marginal NDF reconstruction and a polynomial or spline for the other conditional dimensions. This combination allows to reconstruct the entire multivariate NDF and based on this, expressions for the disappearance flux are derived. An algorithm is proposed for the whole moment inversion and reconstruction process. It is validated against a suite of test cases with increasing complexity. The influence of the number of kernel density functions and the configuration of the polynomials and splines on the accuracy is discussed. Finally, the associated computational costs are evaluated. KW - Number density function KW - Population balance equation KW - Method of moments KW - EQMOM KW - Multivariate KW - Evaporation Y1 - 2019 U6 - https://doi.org/10.1016/j.jcp.2019.108879 SN - 0021-9991 IS - 398 ER - TY - GEN A1 - Arshad, Salman A1 - Kong, Bo A1 - Kerstein, Alan R. A1 - Oevermann, Michael T1 - A strategy for large-scale scalar advection in large eddy simulations that use the linear eddy sub-grid mixing model T2 - International Journal of Numerical Methods for Heat and Fluid Flow N2 - Purpose: The purpose of this numerical work is to present and test a new approach for large-scale scalar advection (splicing) in large eddy simulations (LES) that use the linear eddy sub-grid mixing model (LEM) called the LES-LEM. Design/methodology/approach: The new splicing strategy is based on an ordered flux of spliced LEM segments. The principle is that low-flux segments have less momentum than high-flux segments and, therefore, are displaced less than high-flux segments. This strategy affects the order of both inflowing and outflowing LEM segments of an LES cell. The new splicing approach is implemented in a pressure-based fluid solver and tested by simulation of passive scalar transport in a co-flowing turbulent rectangular jet, instead of combustion simulation, to perform an isolated investigation of splicing. Comparison of the new splicing with a previous splicing approach is also done. Findings: The simulation results show that the velocity statistics and passive scalar mixing are correctly predicted using the new splicing approach for the LES-LEM. It is argued that modeling of large-scale advection in the LES-LEM via splicing is reasonable, and the new splicing approach potentially captures the physics better than the old approach. The standard LES sub-grid mixing models do not represent turbulent mixing in a proper way because they do not adequately represent molecular diffusion processes and counter gradient effects. Scalar mixing in turbulent flow consists of two different processes, i.e. turbulent mixing that increases the interface between unmixed species and molecular diffusion. It is crucial to model these two processes individually at their respective time scales. The LEM explicitly includes both of these processes and has been used successfully as a sub-grid scalar mixing model (McMurtry et al., 1992; Sone and Menon, 2003). Here, the turbulent mixing capabilities of the LES-LEM with a modified splicing treatment are examined. Originality/value: The splicing strategy proposed for the LES-LEM is original and has not been investigated before. Also, it is the first LES-LEM implementation using unstructured grids. KW - Large eddy simulations KW - LES-LEM KW - Linear eddy model KW - Passive scalar mixing KW - Splicing KW - Turbulent jet flow Y1 - 2018 U6 - https://doi.org/10.1108/HFF-09-2017-0387 SN - 0961-5539 VL - 28 IS - 10 SP - 2463 EP - 2479 ER - TY - GEN A1 - Lackmann, Tim A1 - Nygren, Andreas A1 - Karlsson, Anders A1 - Oevermann, Michael T1 - Investigation of turbulence–chemistry interactions in a heavy-duty diesel engine with a representative interactive linear eddy model T2 - International Journal of Engine Research N2 - Simulations of a heavy-duty diesel engine operated at high-load and low-load conditions were compared to each other, and experimental data in order to evaluate the influence of turbulence–chemistry interactions on heat release, pressure development, flame structure, and temperature development are quantified. A recently developed new combustion model for turbulent diffusion flames called representative interactive linear eddy model which features turbulence–chemistry interaction was compared to a well-stirred reactor model which neglects the influence of turbulent fluctuations on the mean reaction rate. All other aspects regarding the spray combustion simulation like spray break-up, chemical mechanism, and boundary conditions within the combustion chamber were kept the same in both simulations. In this article, representative interactive linear eddy model is extended with a progress variable, which enables the model to account for a flame lift-off and split injection, when it is used for diffusion combustion. In addition, the extended version of representative interactive linear eddy model offers the potential to treat partially premixed and premixed combustion as well. The well-stirred reactor model was tuned to match the experimental results, thus computed pressure and apparent heat release are in close agreement with the experimental data. Representative interactive linear eddy model was not tuned specifically for the case and thus the computed results for pressure and heat release are in reasonable agreement with experimental data. The computational results show that the interaction of the turbulent flow field and the chemistry reduce the peak temperatures and broaden up the turbulent flame structure. Since this is the first study of a real combustion engine (metal engine) with the newly developed model, representative interactive linear eddy model appears as a promising candidate for predictions of spray combustion in engines, especially in combustion regimes where turbulence–chemistry interaction plays an even more important role like, example given, in low-temperature combustion or combustion with local extinction and re-ignition. KW - Representative interactive linear eddy model KW - turbulent spray flame KW - turbulence–chemistry interaction KW - diesel engine Y1 - 2018 U6 - https://doi.org/10.1177/1468087418812319 SN - 1468-0874 VL - 21 IS - 8 SP - 1469 EP - 1479 ER - TY - GEN A1 - Lackmann, Tim A1 - Kerstein, Alan R. A1 - Oevermann, Michael T1 - A representative linear eddy model for simulating spray combustion in engines (RILEM) T2 - Combustion and Flame N2 - The design of new combustion concepts for low emission, high efficiency internal combustion engines often leads to combustion under low temperature conditions. Under those conditions, the assumption of fast chemistry, which has been the cornerstone of many turbulent combustion models, is not strictly valid anymore and the validity and applicability of classical combustion models such as flamelet models might be limited. In this paper we present an updated version of a recently developed regime independent modeling approach for turbulent non-premixed combustion with an emphasis on applications to internal combustion engines. The model utilizes the mode- and regime-independent linear eddy model (LEM) as a combustion and micro-mixing model in a representative way. This is achieved by time advancing only one LEM realization representing the combustion process in the whole engine domain and coupling it to a RANS simulation with a presumed β-function PDF approach for the mixture fraction. The use of LEM rather than flamelet combustion closure has several benefits, an important one being regime independence. Additionally, LEM incorporates a physically based representation of the stochastic variability of turbulent eddy motions, implying an intrinsic representation of scalar dissipation rate fluctuations. In order to capture key features of engine spray-combustion environments, the LEM methodology is extended by introducing a conical LEM domain to approximate spray spatial development, fuel vapor input based on CFD-prescribed spray evaporation, and a representation of large scale turbulent motions distinct from the inertial-range turbulence that develops at smaller scales. The representative character of LEM states is evaluated by comparing mixture fraction statistics and scalar dissipation rates generated by LEM and the CFD. The performance and predictive capability of the model for typical engine applications is evaluated by simulating a standard test case – Spray B of the Engine Combustion Network (ECN) – and comparing the results with experimental data. The results demonstrate the capability of the model to represent the spray combustion process with reasonable accuracy but also reveal some limitations. The limitations and shortcomings of the model are discussed and an outlook for further development of the approach into a regime- and mode-independent combustion model for internal engine applications is given. KW - Turbulent combustion KW - Regime independent KW - Engine simulation KW - Linear eddy model Y1 - 2018 U6 - https://doi.org/10.1016/j.combustflame.2018.02.008 SN - 0010-2180 VL - 193 SP - 1 EP - 15 ER - TY - GEN A1 - Chen, Boxiong A1 - Oevermann, Michael T1 - An Eulerian stochastic field cavitation model coupled to a pressure based solver T2 - Computer and Fluids N2 - Probability density functions (PDF) and the relevant methods have been widely used to describe non-linear phenomena in the realm of turbulence modelling and CFD. In order to solve PDF transport equations, the main trend of previous studies rely on Monte Carlo method with Lagrangian particle tracking. However, as with any Lagrangian based approach, the scalability of the parallelized simulations of such method is less than satisfactory. An Eulerian stochastic field model has been presented recently by Dumond et al. [1] to simulate cavitating flows. Their model uses a fully compressible density based solver. Here we present an adapted version using an iso-thermal cavitation model adopting the homogeneous mixture assumption in a pressure based flow solver which is more relevant to engine simulations. A PDF method is used to represent a distribution of vapour volume fractions, based on which the Eulerian stochastic field (ESF) method is applied to perform a three-dimensional large eddy simulation (LES) of the cavitation phenomena inside an academic fuel injector configuration. The numerical model is based on a volume of fluids approach and coupled with a pressure based solver for the flow field, and is implemented in the framework of the open source C++ toolbox OpenFOAM. The result of the ESF simulation is compared against that from a typical single volume fraction solver for validation. Vortex structures and its correspondence to cavitation are shown, and the behaviour of the PDF at different probe locations at different times are acquired to demonstrate the potential of the ESF model in capturing both transient and stochastically steady cavitation. KW - Multiphase flow KW - Cavitation KW - Volume of fluid KW - Eulerian stochastic field method Y1 - 2018 U6 - https://doi.org/10.1016/j.compfluid.2017.12.002 SN - 0045-7930 VL - 162 SP - 1 EP - 10 ER - TY - GEN A1 - Movaghar, Amirreza A1 - Linne, Mark A1 - Herrmann, Marcus A. A1 - Kerstein, Alan R. A1 - Oevermann, Michael T1 - Modeling and numerical study of primary breakup under diesel conditions T2 - International Journal of Multiphase Flow N2 - A recently introduced stochastic model for reduced numerical simulation of primary jet breakup is evaluated by comparing model predictions to DNS results for primary jet breakup under diesel conditions. The model uses one-dimensional turbulence (ODT) to simulate liquid and gas time advancement along a lateral line of sight. This one-dimensional domain is interpreted as a Lagrangian object that is advected downstream at the jet bulk velocity, thus producing a flow state expressed as a function of streamwise and lateral location. Multiple realizations are run to gather ensemble statistics that are compared to DNS results. The model incorporates several empirical extensions of the original ODT model that represent the phenomenology governing the Weber number dependence of global jet structure. The model as previously formulated, including the assigned values of tunable parameters, is used here without modification in order to test its capability to predict various statistics of droplets generated by primary breakup. This test is enabled by the availability of DNS results that are suitable for model validation. Properties that are examined are the rate of bulk liquid mass conversion into droplets, the droplet size distribution, and the dependence of droplet velocities on droplet diameter. Quantities of greatest importance for engine modeling are found to be predicted with useful accuracy, thereby demonstrating a more detailed predictive capability by a highly reduced numerical model of primary jet breakup than has previously been achieved. KW - Spray KW - Primary breakup KW - Turbulence KW - One-dimensional turbulence KW - Direct numerical simulation (DNS) Y1 - 2018 U6 - https://doi.org/10.1016/j.ijmultiphaseflow.2017.09.002 SN - 0301-9322 VL - 98 SP - 110 EP - 119 ER - TY - GEN A1 - Gerber, Stephan A1 - Oevermann, Michael T1 - A two dimensional Euler-Lagrangian model of wood gasification in a charcoal bed — Particle histories T2 - Powder Technology N2 - An Euler-Lagrangian simulation applied to wood gasification in a fluidized bed is used to investigate individual particle histories and to provide a statistical analysis for temperature, apparent density as well as radial and axial positions of charcoal and dry wood particles. The model and several parameter variations have been discussed in detail in previous articles [1], [2], [3]. Based on simulation results we find much higher particle heating rates than typically used to measure kinetic data for, e.g., pyrolysis models. Although we presented a rather complex interplay of particle heating rate, biomass decomposition, particle shrinkage and particle fluidization the simulation results emphasize the importance of the usage of realistic particle size distributions for the initial bed as well as the fuel inlet. Furthermore, particle shrinkage data in combination with mass decomposition data appear to be crucial for realistic simulations. KW - Wood gasification KW - Pyrolysis KW - Discrete element KW - Fluidized bed KW - Charcoal bed KW - Euler-Lagrange Y1 - 2018 U6 - https://doi.org/10.1016/j.powtec.2017.10.032 SN - 0032-5910 VL - 324 SP - 5 EP - 15 ER - TY - GEN A1 - Gerber, Stephan A1 - Oevermann, Michael T1 - A two dimensional Euler-Lagrangian model of wood gasification in a charcoal bed - Part III: parameter influence and comparison T2 - Powder Technology N2 - The efficient utilization of biomass in fluidized bed reactors depends on numerous operation conditions and parameters which can be investigated systematically with computational models. Based on a previous study [1] we compare a base scenario which mimics the experimental situation with different parameter settings for an Euler-Lagrangian simulation of wood gasification in a fluidized charcoal bed. For the varied parameters superficial velocity, reactor wall temperature, air inlet temperature, wood particle size, and the wood inlet temperature we analyse the simulation results based on data for temporal evolution of reactor outlet temperature, averaged particle temperature, overall wood mass, overall charcoal mass, concentrations of several gaseous species (N2, CO, CO2, H2, H2O, CH4, C2H2, and three virtual tar components) and axial barycenter data of particles bed mass. Furthermore we show time averaged data for gaseous species and gas phase temperature at the reactor outlet. At the end we critically examine our findings under consideration of the models opportunities and limitations. KW - Wood gasification KW - Fluidized bed KW - Euler-Lagrange KW - Discrete element method KW - Charcoal bed KW - Gasification Y1 - 2017 U6 - https://doi.org/10.1016/j.powtec.2016.12.057 SN - 0032-5910 VL - 310 SP - 163 EP - 174 ER - TY - GEN A1 - Wadekar, Sandip A1 - Janas, Peter A1 - Oevermann, Michael T1 - Large-eddy simulation study of combustion cyclic variation in a lean-burn spark ignition engine T2 - Applied Energy N2 - Multi-cycle large-eddy simulation (LES) was performed to investigate combustion cyclic variability (CCV) in a single cylinder spark ignition engine with a homogeneous lean ( = 1.25) isooctane-air mixture. The aim was to obtain physical insights into the early stage of combustion and its influence on CCV. Propagation of the flame was modeled by a transport equation for the filtered flame surface density within the LES framework. The ignition process was represented by the imposed stretch spark ignition model (ISSIM-LES). Ten consecutive cold flow LES cycles followed by two initialization cycles (12 cycles in total) were used to perform the reactive simulations concurrently. The simulation results were compared with experimental data. Although the number of computed cycles was fairly low, the LES was able to reproduce the cyclic variability observed in experiments both quantitatively and qualitatively. Firstly, validation of the simulation was done by comparing measured pressure traces. Secondly, correlations between the timing of the 10% fuel burnt mass fraction with early flame kernel growth and initial-to-turbulent transition period (in which there was an asymmetric flame kernel that persisted through the early development periods) were determined. Thirdly, calculated results of the flame propagation were analyzed at two cross-sections (in swirl and tumble planes) of the combustion chamber, which highlighted differences in instantaneous flame structures and propagation characteristics between the fastest and slowest cycles. Good overall agreement was obtained between the measurements and simulation data. The results revealed that the instantaneous velocity and fluctuation of flows around the spark vicinity affect growth of the early flame kernel and cause combustion cyclic variability. KW - Large-eddy simulation KW - Combustion cyclic variation KW - Flame surface density KW - Ignition modelling KW - Lean combustion Y1 - 2019 U6 - https://doi.org/10.1016/j.apenergy.2019.113812 SN - 0306-2619 VL - 255 ER - TY - GEN A1 - Gerber, Stephan A1 - Oevermann, Michael T1 - A two dimensional Euler-Lagrangian model of wood gasification in a charcoal bed - Part II: parameter influence and comparison T2 - Particuology N2 - A Euler–Lagrangian simulation was employed for a comprehensive parameter study of wood gasification in a fluidized charcoal bed. The parameters that were varied include the initial bed temperature, fuel mass flow rate, inert tar fraction, and kinetic energy losses caused by particle–particle and particle–wall collisions. The results of each parameter variation are compared with a base scenario, previously described in detail in Part I of this study (Gerber & Oevermann, 2014). The results are interpreted by comparing the reactor outlet temperature, averaged particle temperature, overall wood mass, overall charcoal mass, concentrations of several gaseous species, and axial barycenter data for particles obtained with different sets of parameters. The inert tar fraction and fuel mass flow rate are the most sensitive parameter, while the particle–particle and particle–wall contact parameters have only a small impact on the results. Increasing the reactive tar components by 19% almost doubled the amount of reactive tars at the reactor outlet, while decreasing the restitution coefficients of the particle collisions by 0.2 results in higher overall gas production but almost no change in bed height. Herein, our numerical results are discussed in detail while assessing the model restrictions. KW - Wood gasification simulation KW - Gas–solid flow KW - Discrete element method KW - Pyrolysis KW - Gasification KW - Charcoal Y1 - 2017 U6 - https://doi.org/10.1016/j.partic.2017.01.004 SN - 1674-2001 VL - 35 SP - 22 EP - 30 ER - TY - GEN A1 - Lackmann, Tim A1 - Hewson, John A1 - Knaus, Robert A1 - Kerstein, Alan R. A1 - Oevermann, Michael T1 - Stochastic modeling of unsteady extinction in turbulent non-premixed combustion T2 - Proceedings of the Combustion Institute N2 - Turbulent fluctuations of the scalar dissipation rate have a major impact on extinction in non-premixed combustion. Recently, an unsteady extinction criterion has been developed (Hewson, 2013) that predicts extinction dependent on the duration and the magnitude of dissipation rate fluctuations exceeding a critical quenching value; this quantity is referred to as the dissipation impulse. The magnitude of the dissipation impulse corresponding to unsteady extinction is related to the difficulty with which a flamelet is exintguished, based on the steady-state S-curve. In this paper we evaluate this new extinction criterion for more realistic dissipation rates by evolving a stochastic Ornstein–Uhlenbeck process for the dissipation rate. A comparison between unsteady flamelet evolution using this dissipation rate and the extinction criterion exhibit good agreement. The rate of predicted extinction is examined over a range of Damköhler and Reynolds numbers and over a range of the extinction difficulty. The results suggest that the rate of extinction is proportional to the average dissipation rate and the area under the dissipation rate probability density function exceeding the steady-state quenching value. It is also inversely related to the actual probability that this steady-state quenching dissipation rate is observed and the difficulty of extinction associated with the distance between the upper and middle branches of the S-curve. KW - Extinction KW - Unsteady flames KW - Non-premixed flame KW - Scalar dissipation rate KW - Turbulence Y1 - 2016 U6 - https://doi.org/10.1016/j.proci.2016.07.014 SN - 0082-0784 VL - 36 IS - 2 SP - 1677 EP - 1684 ER - TY - GEN A1 - Somhorst, Joop A1 - Uczak De Goes, Wellington A1 - Oevermann, Michael A1 - Bovo, Mirko T1 - Experimental Evaluation of Novel Thermal Barrier Coatings in a Single Cylinder Light Duty Diesel Engine T2 - SAE Technical Papers N2 - The objective of this investigation was to improve the thermal properties of plasma sprayed thermal barrier coatings (TBC) for internal combustion engines. There is a need for further reduction of thermal conductivity and volumetric heat capacity and the negative effects on heat loss and combustion phasing of surface roughness and permeable porosity, typical for plasma sprayed coatings, should be minimized. Four measures for improvement of TBC properties were evaluated: i) modification of the coating's microstructure by using a novel suspension plasma spraying method, ii) application of gadolinium-zirconate, a novel ceramic material with low thermal conductivity, iii) polishing of the coating to achieve low surface roughness, and iv) sealing of the porous coating surface with a polysilazane. Six coating variants with different combinations of the selected measures were applied on the piston crown and evaluated in a single cylinder light duty diesel engine. The experimental data was modeled with multiple linear regression to obtain confidence intervals for the measurement results and to correct the data for variations of surface roughness, combustion phasing and compression ratio for the different pistons. The main tool for evaluation of the coating properties was cylinder pressure analysis, providing the apparent rate of heat release, indicated efficiency, wall heat loss, and exhaust loss. The new TBC microstructure from suspension plasma spraying in combination with the use of gadolinium-zirconate showed promising results with respect to indicated efficiency and heat loss reduction. Y1 - 2019 U6 - https://doi.org/10.4271/2019-24-0062 SN - 2688-3627 SN - 0148-7191 ER - TY - GEN A1 - Wadekar, Sandip A1 - Oevermann, Michael A1 - Lipatnikov, Andrei T1 - Large Eddy Simulation of Stratified Combustion in Spray-guided Direct Injection Spark-ignition Engine T2 - SAE Technical Papers N2 - Stratified combustion in gasoline engines constitutes a promising means of achieving higher thermal efficiency for low to medium engine loads than that achieved with combustion under standard homogeneous conditions. However, creating a charge that leads to a stable efficient low-emission stratified combustion process remains challenging. Combustion through a stratified charge depends strongly on the dynamics of the turbulent fuel-air mixing process and the flame propagation. Predictive simulation tools are required to elucidate this complex mixing and combustion process under stratified conditions. For the simulation of mixing processes, combustion models based on large-eddy turbulence modeling have typically outperformed the standard Reynolds averaged Navier-Stokes methods. Therefore, we investigated spray-guided stratified combustion in a single cylinder engine using large-eddy turbulence modeling with a variant of the flame speed closure (FSC) model for premixed turbulent combustion. This model reveals the influence of the mixture composition on the flame speed. The effect of fluctuations in the composition were accounted for by using a presumed probability density function (PDF) approach for the mixture fraction. The fuel injection process was modeled with a standard Lagrangian spray model. More importantly, the measured in-cylinder pressure traces for three different loading cases with varying injection and ignition timings (leading to different levels of stratification) were accurately reproduced by the simulation. High-speed video images were used to evaluate the ability of the model to accurately simulate flame propagation under stratified conditions. The influence of mixture fluctuations on flame propagation was also investigated. KW - Single cylinder engines KW - Ignition timing KW - Combustion and combustion processes KW - Fuel injection Y1 - 2018 U6 - https://doi.org/10.4271/2018-01-1420 SN - 2688-3627 SN - 0148-7191 ER - TY - GEN A1 - Fistler, Marco A1 - Kerstein, Alan R. A1 - Wunsch, Scott A1 - Oevermann, Michael T1 - Turbulence modulation in particle-laden stationary homogeneous shear turbulence using one-dimensional turbulence T2 - Physical Review Fluids N2 - Turbulence modulation in particle-laden stationary homogeneous shear turbulence (HST) is investigated using one-dimensional turbulence (ODT), a low-dimensional stochastic flow simulation model. For this purpose, an ODT formulation previously used to study turbulence modulation in forced homogeneous isotropic turbulence (HIT) is extended, so that the model emulates the anisotropic character of HST and, potentially, anisotropic turbulence in general. This is done by limiting the kinetic-energy redistribution during an eddy event to an exchange involving two velocity components, where the three possible choices of the omitted component define three eddy types whose relative likelihoods control the anisotropy. Comparisons of ODT and direct-numerical-simulation results with reference to signatures of turbulence modulation are the basis of a broader ODT parameter study that is reported. Owing to the reduced dimensionality of ODT, it is found that the fidelity of the model for single-phase HST does not extend to particle effects on flow anisotropy, but for quantities averaged over components, parametric trends are captured. The consistent approach to case comparisons that was introduced in the HIT study to evaluate sensitivities to particle-phase parameters in a given flow configuration is extended here to a cross-comparison of HST and HIT model results, and its efficacy is again confirmed. The results provide an overall characterization of the potential for ODT to support the incorporation of particle-induced turbulence modulation into subgrid-scale closures of large-eddy simulations. Y1 - 2020 U6 - https://doi.org/10.1103/PhysRevFluids.5.124303 SN - 2469-990X VL - 5 IS - 12 ER - TY - GEN A1 - Wadekar, Sandip A1 - Yamaguchi, Akichika A1 - Oevermann, Michael T1 - Large-Eddy Simulation on the Effects of Fuel Injection Pressure on the Gasoline Spray Characteristics T2 - SAE Technical Papers N2 - Increasing the injection pressure in gasoline direct injection engines has a substantial potential to reduce emissions while maintaining a high efficiency in spark ignition engines. Present gasoline injectors are operating in the range of 20 MPa to 25 MPa. Now there is an interest in higher fuel injection pressures, for instance, around 40 MPa, 60 MPa and even higher pressures, because of its potential for further emission reduction and fuel efficiency improvements. In order to fully utilize the high-pressure fuel injection technology, a fundamental understanding of gasoline spray characteristics is vital to gain insight into spray behavior under such high injection pressures. The understanding achieved may also be beneficial to improve further model development and facilitate the integration of such advanced injection systems into future gasoline engines. In the present study, a gasoline fuel spray has been investigated over a range of fuel injection pressures from 40 to 150 MPa through a numerical simulation study. The numerical calculations have been performed in a constant volume chamber under non-vaporizing conditions to best match the experimental setup. The numerical model utilized a large-eddy simulation (LES) approach for the gas flow and a standard Lagrangian spray model for the liquid phase. The spray atomization has been modeled using the Kelvin Helmholtz - Rayleigh Taylor (KH-RT) atomization model with a droplet size distribution from the injector assumed to follow a Rosin-Rammler distribution function. Simulation results for the spray liquid penetration length are validated with experimental findings under different fuel injection pressures. Afterwards, an arithmetic mean droplet diameter (D10) and a Sauter mean droplet diameter (D32) as a function of pressure are compared against the measured droplet diameters. Simulated drop size distributions are presented and compared with measured droplet sizes. The results indicate that a high fuel injection pressure increases the liquid penetration length and significantly reduces droplet sizes. The results also exhibit that the SMD decreases from 13.4 μm to 7.5 μm, when injection pressure changes from 40 MPa to 150 MPa and that probability of finding the 5-9 μm droplet diameter decreases from 72% to 40% for the injection pressure drops from 150 MPa to 40 MPa. KW - Spark ignition engines KW - Fuel injection KW - Fuel economy KW - Energy conservation Y1 - 2019 U6 - https://doi.org/10.4271/2019-01-0060 SN - 2688-3627 SN - 0148-7191 ER - TY - GEN A1 - Somhorst, Joop A1 - Oevermann, Michael A1 - Bovo, Mirko A1 - Denbratt, Ingemar T1 - A Method to Evaluate the Compression Ratio in IC Engines with Porous Thermal Barrier Coatings T2 - SAE Technical Papers N2 - The compression ratio is an important engine design parameter. It determines to a large extend engine properties like the achievable efficiency, the heat losses from the combustion chamber and the exhaust losses. The same properties are affected by insulation of the combustion chamber. It is therefore especially important to know the compression ratio when doing experiments with thermal barrier coatings (TBC). In case of porous TBCs, the standard methods to measure the compression ratio can give wrong results. When measuring the compression ratio by volume, using a liquid, it is uncertain if the liquid fills the total porous volume of the coating. And for a thermodynamic compression ratio estimation, a model for the heat losses is needed, which is not available when doing experiments with insulation. The subject of this paper is the evaluation of an alternative method to assess the compression ratio. It is based on motored cylinder pressure data like other thermodynamic methods but does not need a model for the heat losses. The validation and application of the method is done with data from experiments involving two types of porous TBCs, performed on a light duty single cylinder diesel engine. The results indicate that the proposed method accurately predicts the compression ratio for porous thermal barrier coatings. KW - Combustion chambers KW - Combustion and combustion processes KW - Coatings, colorants, and finishes KW - Insulation Y1 - 2018 U6 - https://doi.org/10.4271/2018-01-1778 SN - 2688-3627 SN - 0148-7191 ER -