@misc{LackmannKersteinOevermann, author = {Lackmann, Tim and Kerstein, Alan R. and Oevermann, Michael}, title = {A representative linear eddy model for simulating spray combustion in engines (RILEM)}, series = {Combustion and Flame}, volume = {193}, journal = {Combustion and Flame}, issn = {0010-2180}, doi = {10.1016/j.combustflame.2018.02.008}, pages = {1 -- 15}, abstract = {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.}, language = {en} } @misc{ChenOevermann, author = {Chen, Boxiong and Oevermann, Michael}, title = {An Eulerian stochastic field cavitation model coupled to a pressure based solver}, series = {Computer and Fluids}, volume = {162}, journal = {Computer and Fluids}, issn = {0045-7930}, doi = {10.1016/j.compfluid.2017.12.002}, pages = {1 -- 10}, abstract = {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.}, language = {en} } @misc{MovagharLinneHerrmannetal., author = {Movaghar, Amirreza and Linne, Mark and Herrmann, Marcus A. and Kerstein, Alan R. and Oevermann, Michael}, title = {Modeling and numerical study of primary breakup under diesel conditions}, series = {International Journal of Multiphase Flow}, volume = {98}, journal = {International Journal of Multiphase Flow}, issn = {0301-9322}, doi = {10.1016/j.ijmultiphaseflow.2017.09.002}, pages = {110 -- 119}, abstract = {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.}, language = {en} } @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} }