@misc{GerberOevermann, author = {Gerber, Stephan and Oevermann, Michael}, title = {A two dimensional Euler-Lagrangian model of wood gasification in a charcoal bed — Particle histories}, series = {Powder Technology}, volume = {324}, journal = {Powder Technology}, issn = {0032-5910}, doi = {10.1016/j.powtec.2017.10.032}, pages = {5 -- 15}, abstract = {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.}, language = {en} } @misc{GerberOevermann, author = {Gerber, Stephan and Oevermann, Michael}, title = {A two dimensional Euler-Lagrangian model of wood gasification in a charcoal bed - Part III: parameter influence and comparison}, series = {Powder Technology}, volume = {310}, journal = {Powder Technology}, issn = {0032-5910}, doi = {10.1016/j.powtec.2016.12.057}, pages = {163 -- 174}, abstract = {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.}, language = {en} } @misc{WadekarJanasOevermann, author = {Wadekar, Sandip and Janas, Peter and Oevermann, Michael}, title = {Large-eddy simulation study of combustion cyclic variation in a lean-burn spark ignition engine}, series = {Applied Energy}, volume = {255}, journal = {Applied Energy}, issn = {0306-2619}, doi = {10.1016/j.apenergy.2019.113812}, pages = {16}, abstract = {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.}, language = {en} } @misc{GerberOevermann, author = {Gerber, Stephan and Oevermann, Michael}, title = {A two dimensional Euler-Lagrangian model of wood gasification in a charcoal bed - Part II: parameter influence and comparison}, series = {Particuology}, volume = {35}, journal = {Particuology}, issn = {1674-2001}, doi = {10.1016/j.partic.2017.01.004}, pages = {22 -- 30}, abstract = {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.}, language = {en} } @misc{LackmannHewsonKnausetal., author = {Lackmann, Tim and Hewson, John and Knaus, Robert and Kerstein, Alan R. and Oevermann, Michael}, title = {Stochastic modeling of unsteady extinction in turbulent non-premixed combustion}, series = {Proceedings of the Combustion Institute}, volume = {36}, journal = {Proceedings of the Combustion Institute}, number = {2}, issn = {0082-0784}, doi = {10.1016/j.proci.2016.07.014}, pages = {1677 -- 1684}, abstract = {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 Damk{\"o}hler 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.}, language = {en} } @misc{SomhorstUczakDeGoesOevermannetal., author = {Somhorst, Joop and Uczak De Goes, Wellington and Oevermann, Michael and Bovo, Mirko}, title = {Experimental Evaluation of Novel Thermal Barrier Coatings in a Single Cylinder Light Duty Diesel Engine}, series = {SAE Technical Papers}, journal = {SAE Technical Papers}, issn = {2688-3627}, doi = {10.4271/2019-24-0062}, abstract = {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.}, language = {en} } @misc{WadekarOevermannLipatnikov, author = {Wadekar, Sandip and Oevermann, Michael and Lipatnikov, Andrei}, title = {Large Eddy Simulation of Stratified Combustion in Spray-guided Direct Injection Spark-ignition Engine}, series = {SAE Technical Papers}, journal = {SAE Technical Papers}, issn = {2688-3627}, doi = {10.4271/2018-01-1420}, abstract = {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.}, language = {en} } @misc{FistlerKersteinWunschetal., author = {Fistler, Marco and Kerstein, Alan R. and Wunsch, Scott and Oevermann, Michael}, title = {Turbulence modulation in particle-laden stationary homogeneous shear turbulence using one-dimensional turbulence}, series = {Physical Review Fluids}, volume = {5}, journal = {Physical Review Fluids}, number = {12}, issn = {2469-990X}, doi = {10.1103/PhysRevFluids.5.124303}, pages = {28}, abstract = {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.}, language = {en} } @misc{WadekarYamaguchiOevermann, author = {Wadekar, Sandip and Yamaguchi, Akichika and Oevermann, Michael}, title = {Large-Eddy Simulation on the Effects of Fuel Injection Pressure on the Gasoline Spray Characteristics}, series = {SAE Technical Papers}, journal = {SAE Technical Papers}, issn = {2688-3627}, doi = {10.4271/2019-01-0060}, abstract = {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.}, language = {en} } @misc{SomhorstOevermannBovoetal., author = {Somhorst, Joop and Oevermann, Michael and Bovo, Mirko and Denbratt, Ingemar}, title = {A Method to Evaluate the Compression Ratio in IC Engines with Porous Thermal Barrier Coatings}, series = {SAE Technical Papers}, journal = {SAE Technical Papers}, issn = {2688-3627}, doi = {10.4271/2018-01-1778}, abstract = {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.}, language = {en} } @misc{WadekarOevermann, author = {Wadekar, Sandip and Oevermann, Michael}, title = {Large-Eddy Simulation Study of Ultra-High Fuel Injection Pressure on Gasoline Sprays}, series = {Flow, Turbulence and Combustion}, volume = {107 (2021)}, journal = {Flow, Turbulence and Combustion}, number = {1}, issn = {1573-1987}, doi = {10.1007/s10494-020-00231-0}, pages = {149 -- 174}, abstract = {The development of gasoline spray at ultra-high injection pressures was analyzed using Large-Eddy simulation (LES). Two different nozzle hole geometries, divergent and convergent shape, were considered to inject the fuel at injection pressures ranging from 200 to 1500 bar inside a constant volume spray chamber maintained at atmospheric conditions. The discrete droplet phase was treated using a Lagrangian formulation together with the standard spray sub-models. The numerical results were calibrated by reproducing experimentally observed liquid penetration length and efforts were made to understand the influence of ultra-high injection pressures on the spray development. The calibrated model was then used to investigate the impact of ultra-high injection pressures on mean droplet size and droplet size distribution. In addition, the spray-induced large-scale eddies and entrainment rate were evaluated at different ultra-high injection pressures. Overall, simulation results showed a good agreement with available measurement data. At ultra-high injection pressures mean droplet sizes were significantly reduced and comprised very high velocities. Integral length scales of spray-induced turbulence and air entrainment rate into the spray were larger at higher injection pressure compared to lower ones.}, language = {en} } @misc{PuetzMovagharOevermann, author = {P{\"u}tz, Michele and Movaghar, Amirreza and Oevermann, Michael}, title = {Numerical simulation of a gasoline spray using one-dimensional turbulence for primary atomization}, series = {ICLASS 2018 - 14th International Conference on Liquid Atomization and Spray Systems}, journal = {ICLASS 2018 - 14th International Conference on Liquid Atomization and Spray Systems}, abstract = {Predictive and reliable simulations have the potential to constitute a valuable tool for the optimization of spray systems if accurate submodels are developed for the entire range of the governing processes. The primary breakup of the turbulent liquid jet is one the most important mechanisms in sprays, yet the least developed in terms of numerical modeling. The most accurate method to simulate primary breakup is the proper resolution of liquid-gas interfaces and turbulent flow structures. However, a wide range of relevant length and time scales implicate grid requirements that are often prohibitive for real engineering applications. The most widely used method in practice is still the representation of both the continuous liquid core and the dispersed phase by means of discrete Lagrangian particles evolving in and interacting with the Eulerian gas phase. The available models for primary breakup are mainly phenomenological and involve a number of empirical constants. The one-dimensional turbulence (ODT) model is an alternative stochastic approach to model turbulence in flows with a dominant direction of property gradients. The stochastic representation of turbulent eddies on a one-dimensional domain enables high resolution at moderate computational costs. Applications of ODT to atomization revealed a great potential in recent studies. The objective of the present study is to combine ODT as a primary breakup model with a conventional Eulerian-Lagrangian method for the further spray evolution in order to asses ODT as a submodel in full spray models. Our numerical investigations were conducted on the ECN spray G, a gasoline-like, evaporating spray. The results in terms of spray penetration are encouraging, though the applicability of ODT to the transient injection phase and effects on additional spray characteristics require further investigation.}, language = {en} } @misc{FistlerKersteinOevermann, author = {Fistler, Marco and Kerstein, Alan R. and Oevermann, Michael}, title = {A new LES subgrid-scale approach for turbulence modulation by droplets}, series = {ICLASS 14th Triennial International Conference on Liquid Atomization and Spray Systems Chicago, USA, 2018-07-21 - 2018-07-26}, journal = {ICLASS 14th Triennial International Conference on Liquid Atomization and Spray Systems Chicago, USA, 2018-07-21 - 2018-07-26}, pages = {8}, abstract = {We present a new modelling approach for turbulence modulation by droplets on the subgrid-scale (SGS) level of Large-Eddy-Simulations (LES). Many SGS models exist for the effect of gas phase SGS on the droplet phase, but very few for the mechanisms vice versa on the turbulent intensity of the gas phase. The reasons are a lack of physical understanding and limited computational resources for extensive DNS studies. To address both problems a dimension-reduced and consequently less costly model, namely One-Dimensional- Turbulence (ODT), is used to gather information about this specific flow phenomena. ODT is a stochastic tool simulating turbulent flows along a notional 1D line of sights. For modeling the turbulent advection instantaneous maps are applied to the line which represent the effect of individual eddies on property fields and the dispersed phase. After validating the general test case of a droplet-laden shear flow against DNS data, a concept is presented on how to gather turbulence modulation for several parameter ranges in a data base and how to make them accessible on the flight for LES. The three most significant parameters, the unladen flow Reynolds number, the droplet loading and the particle momentum number, are chosen to construct an efficient data base.}, language = {en} } @incollection{MovagharChiodiDesjardinsetal., author = {Movaghar, Amirreza and Chiodi, Robert and Desjardins, Olivier and Oevermann, Michael and Kerstein, Alan R.}, title = {A Subgrid-Scale Model for Large-Eddy Simulation of Liquid/Gas Interfaces Based on One-Dimensional Turbulence}, series = {Turbulent Cascades II}, booktitle = {Turbulent Cascades II}, publisher = {Springer Nature Switzerland AG}, address = {Schweiz}, isbn = {978-3-030-12547-9}, doi = {10.1007/978-3-030-12547-9_10}, pages = {83 -- 91}, abstract = {The interface/turbulence interaction between two fluids in a turbulent environment has an important role in many technical processes, e.g. primary liquid atomization in combustion devices. Primary atomization has a significant role in spray formation and its characteristics. The resulting dynamics typically span 4-6 orders of magnitude in length scales, making detailed numerical simulations exceedingly expensive. This motivates the need for modeling approaches based on spatial filtering such as large-eddy simulation (LES). In this paper, a new approach based on One-Dimensional turbulence (ODT) is presented to describe the subgrid interface dynamics. ODT is a stochastic model simulating turbulent flow evolution along a notional one-dimensional line of sight by applying instantaneous maps that represent the effects of individual turbulent eddies on property fields. It provides affordable high resolution of interface creation and property gradients within each phase, which are key for capturing the local behavior as well as overall trends. ODT has previously been shown to reproduce the main features of an experimentally determined regime diagram for primary jet breakup. Here a new approach called VODT is presented which produces a size-conditioned as well as a total time rate of generation of droplets for given flow conditions at an interface. At the LES level, the total droplet generation from VODT is interpreted as a rate of mass conversion of LES-resolved liquid into unresolved droplets. Preliminary results of applying VODT to a cell with a planar-shear-layer are discussed at the end of the paper.}, language = {en} } @misc{SomhorstOevermann, author = {Somhorst, Joop and Oevermann, Michael}, title = {Effects of thermal barrier coating porosity on combustion and heat losses in a light duty diesel engine}, series = {International Journal of Engine Research}, volume = {25 (2024)}, journal = {International Journal of Engine Research}, number = {5}, issn = {1468-0874}, doi = {10.1177/14680874231215526}, pages = {940 -- 958}, abstract = {Plasma sprayed thermal barrier coatings (TBCs), applied in internal combustion engines, can have a profound impact on the apparent rate of heat release. Upon fuel jet contact with the piston wall, the heat loss seems to increase compared to an uncoated piston and combustion appears delayed. Permeable porosity of plasma sprayed TBCs is identified as one of the possible causes for this unwanted effect. In this article the impact of open porosity and sealing of the TBC surface are investigated with single cylinder engine experiments and with simulations. Based on the results, a mechanism is presented to explain the observations.}, language = {en} } @misc{MedinaMendezFistlerOevermannetal., author = {Medina M{\´e}ndez, Juan Ali and Fistler, Marco and Oevermann, Michael and Schmidt, Heiko and Riebel, Ulrich}, title = {Economical map-based turbulence models: Developments and perspectives for the numerical analysis of electrostatic precipitation}, series = {Book of Abstracts PARTEC International Congress on Particle Technology, September 26-28, 2023, N{\"u}rnberg}, journal = {Book of Abstracts PARTEC International Congress on Particle Technology, September 26-28, 2023, N{\"u}rnberg}, publisher = {VDI Verlag GmbH}, address = {D{\"u}sseldorf}, pages = {634 -- 638}, language = {en} } @misc{MenonKersteinOevermann, author = {Menon, Abhilash and Kerstein, Alan and Oevermann, Michael}, title = {Assessing the multi-regime capability of the Super-Grid Linear Eddy Model (SG-LEM) using the Darmstadt multi-regime burner}, series = {Flow, Turbulence and Combustion}, volume = {2024}, journal = {Flow, Turbulence and Combustion}, publisher = {Springer Science and Business Media LLC}, issn = {1386-6184}, doi = {10.1007/s10494-024-00602-x}, pages = {26}, abstract = {AbstractRecent advances in combustion modelling for Large Eddy Simulation (LES) have increasingly utilised lower-dimensional manifolds, such as Flamelet Generated Manifolds and Flamelet/Progress Variable methods, due to their computational efficiency. These methods typically rely on one-dimensional representations of flame structures, often assuming premixed or non-premixed configurations. However, practical combustion devices frequently operate under partially-premixed conditions and present challenges due to mixture inhomogeneities and complex flow features. The Linear Eddy Model (LEM) offers an alternative by directly simulating turbulence-chemistry interactions without presuming specific flame structures. However, traditional LES-LEM approaches are computationally quite expensive due to the need for resolved LEM domains to be embedded in every LES cell.The authors developed the Super-Grid LEM (SG-LEM) method (Comb. Theor. Model.  28, 2024) to address these computational challenges by coarse-graining the LES mesh and embedding individual LEM domains within clusters of LES cells. This study evaluates SG-LEM in the context of the Multi-Regime Burner (MRB) introduced by Butz et al. (Combust. Flame, 210, 2019), which features both premixed and non-premixed flame characteristics. SG-LEM simulations of the MRB case demonstrate the method's sensitivity to clustering parameters, with flow-aligned clusters significantly improving flame stability. LEM domains on the super-grid were able to represent the MRB flame topology while LES radial profiles including velocity, mixture fraction, temperature, and \$\${\textrm{CO}}\$\$ CO mass fraction, were validated against experimental data and also reference simulations using standard combustion closures. The work also investigates discrepancies in CO profiles using conditional statistics and stand-alone LEM simulations. Finally, the work identifies areas of improvement for the SG-LEM framework, in particular relating to cluster generation, and (advective and diffusive) mass exchange between neighbouring LEM domains, as well as possible solutions for future SG-LEM implementations which could improve the model's predictive capability.}, language = {en} } @misc{KleinNadolskiZenkeretal., author = {Klein, Rupert and Nadolski, Maikel and Zenker, Christian and Oevermann, Michael and Paschereit, Christian Oliver}, title = {Pressure gain combustion for gas turbines : analysis of a fully coupled engine model}, series = {Journal of Engineering for Gas Turbines and Power}, volume = {147}, journal = {Journal of Engineering for Gas Turbines and Power}, number = {2}, publisher = {ASME International}, issn = {0742-4795}, doi = {10.1115/1.4066348}, pages = {26}, abstract = {The "Shockless Explosion Combustion" (SEC) concept for gas turbine combustors, introduced in 2014, approximates constant volume combustion (CVC) by harnessing acoustic confinement of auto-igniting gas packets. The resulting pressure waves simultaneously transmit combustion energy to a turbine plenum and facilitate the combustor's recharging against an average pressure gain. Challenges in actualizing an SEC-driven gas turbine include (i) the creation of charge stratifications for nearly homogeneous auto-ignition, (ii) protecting the turbocomponents from combustion-induced pressure fluctuations, (iii) providing evidence that efficiency gains comparable to those of CVC over deflagrative combustion can be realized, and (iv) designing an effective one-way intake valve. This work addresses challenges (i)-(iii) utilizing computational engine models incorporating a quasi-one-dimensional combustor, zero- and two-dimensional (2D) compressor and turbine plena, and quasi-stationary turbocomponents. Two SEC operational modes are identified which fire at roughly one and two times the combustors' acoustic frequencies. Results for SEC-driven gas turbines with compressor pressure ratios of 6:1 and 20:1 reveal 1.5-fold mean pressure gains across the combustors. Assuming ideally efficient compressors and turbines, efficiency gains over engines with deflagration-based combustors of 30\% and 18\% are realized, respectively. With absolute values of 52\% and 66\%, the obtained efficiencies are close to the theoretical Humphrey cycle efficiencies of 54\% and 65\% for the mentioned precompression ratios. Detailed thermodynamic cycle analyses for individual gas parcels suggest that there is room for further efficiency gains through optimized plenum and combustor designs.}, language = {en} } @misc{MovagharChiodiOevermannetal., author = {Movaghar, Amirreza and Chiodi, Robert and Oevermann, Michael and Desjardins, Olivier and Kerstein, Alan}, title = {Assessment of a multiphase formulation of one-dimensional turbulence using direct numerical simulation of a decaying turbulent interfacial flow}, series = {Physical Review Fluids}, volume = {9}, journal = {Physical Review Fluids}, number = {10}, publisher = {American Physical Society (APS)}, issn = {2469-990X}, doi = {10.1103/PhysRevFluids.9.104003}, abstract = {The interaction between turbulence and surface tension is studied numerically using the one-dimensional-turbulence (ODT) model. ODT is a stochastic model simulating turbulent flow evolution along a notional one-dimensional line of sight by applying instantaneous maps that represent the effects of individual turbulent eddies on property fields. It provides affordable high resolution of interface creation and property gradients within each phase, which are key for capturing the local behavior as well as overall trends, and has been shown to reproduce the main features of an experimentally determined regime diagram for primary jet breakup. Here ODT is used to investigate the interaction of turbulence with an initially planar interface. The notional flat interface is inserted into a periodic box of decaying homogeneous isotropic turbulence, simulated for a variety of turbulent Reynolds and Weber numbers. Unity density and viscosity ratios serve to focus solely on the interaction between fluid inertia and the surface-tension force. Statistical measures of interface surface density and spatial structure along the direction normal to the initial surface are compared to corresponding direct-numerical-simulation (DNS) data. Allowing the origin of the lateral coordinate system to follow the location of the median interface element improves the agreement between ODT and DNS, reflecting the absence of lateral nonvortical displacements in ODT. Beyond the DNS-accessible regime, ODT is shown to obey the predicted parameter dependencies of the Kolmogorov critical scale in both the inertial and dissipative turbulent-cascade subranges. Notably, the probability density function of local fluctuations of the critical scale is found to collapse to a universal curve across both subranges. Published by the American Physical Society 2024}, language = {en} } @misc{MenonOevermannKerstein, author = {Menon, Abhilash M. and Oevermann, Michael and Kerstein, Alan R.}, title = {A super-grid approach for LES combustion closure using the linear eddy model}, series = {Combustion Theory and Modelling}, volume = {28}, journal = {Combustion Theory and Modelling}, number = {1}, publisher = {Informa UK Limited}, issn = {1364-7830}, doi = {10.1080/13647830.2023.2260351}, pages = {99 -- 126}, abstract = {LES-LEM is a simulation approach for turbulent combustion in which the stochastic Linear Eddy Model (LEM) is used for sub-grid mixing and combustion closure in Large-Eddy Simulation (LES). LEM resolves, along a one-dimensional line, all spatial and temporal scales, provides on-the-fly local turbulent flame statistics, captures finite rate chemistry effects and directly incorporates turbulence-chemistry interaction. However, the approach is computationally expensive as it requires advancing an LEM-line in each LES cell. This paper introduces a novel turbulent combustion closure model for LES using LEM to address this issue. It involves coarse-graining the LES mesh to generate a coarse- level 'super-grid' comprised of cell-clusters. Each cell-cluster, instead of each LES cell, then contains a single LEM domain. This domain advances the combined advection-reaction-diffusion solution and also provides suitably conditioned statistics for thermochemical scalars such as species mass fractions. Local LES-filtered thermochemical states are then obtained by probability-density-function (PDF) weighted integration of binned conditionally averaged scalars, akin to standard presumed PDF approaches for reactive LES but with physics-based determination of the full thermochemical state for particular values of the conditioning variables. The proposed method is termed 'super-grid LEM' or 'SG-LEM'. The paper describes LEM reaction-diffusion advancement, the LEM representation of turbulent advection, a novel splicing algorithm (a key feature of LES-LEM) formulated for the super-grid approach, a wall treatment, and a thermochemical LES closure procedure. To validate the proposed model, a pressure-based solver was developed using the OpenFOAM library and tested on a premixed ethylene flame stabilised over a backward facing step, a setup for which some DNS data is available. SG-LEM provides high resolution flame structures, temperature and mass fractions suitable for LES thermochemical closure. Additionally, it provides reaction-rate data at the coarse level, a unique feature compared to other mapping-type closure methods. Quantitative comparisons are made between the proposed model and time-averaged DNS data, focussing on velocity, temperature and species mass fraction. Results show good agreement downstream of the step. Furthermore, comparison with an equivalent Partially-Stirred Reactor (PaSR) simulation demonstrates the superior predictive capability of SG-LEM. Additionally, the paper briefly examines the sensitivity of the model to coarse-graining parameters and finally, explores computational efficiency highlighting the substantial speedup achieved when compared to the standard LES-LEM approach with potentially significant speedup relative to PaSR closure for the intensely turbulent regimes of principal interest.}, language = {en} }