| 1. |
- Arshad, Salman, 1987, et al.
(författare)
-
A strategy for large-scale scalar advection in large eddy simulations that use the linear eddy sub-grid mixing model
- 2018
-
Ingår i: International Journal of Numerical Methods for Heat and Fluid Flow. - 0961-5539. ; 28:10, s. 2463-2479
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
|
|
| 2. |
- Arshad, Salman, 1987, et al.
(författare)
-
Subgrid Reaction-Diffusion Closure for Large Eddy Simulations Using the Linear-Eddy Model
- 2019
-
Ingår i: Flow, Turbulence and Combustion. - : Springer Science and Business Media LLC. - 1573-1987 .- 1386-6184. ; 103:2, s. 389-416
-
Tidskriftsartikel (refereegranskat)abstract
- Turbulent combustion models approximate the interaction between turbulence, molecular transport and chemical reactions. Among the many available turbulent combustion models, the present focus is the linear-eddy model (LEM) used as a subgrid combustion model for large eddy simulations. In particular this paper introduces a new LEM closure with the reaction-rate approach to close the filtered chemical source terms in the governing equations for species mass fractions and enthalpy. The new approach is tested using a non-premixed syngas flame and a bluff-body stabilized premixed flame problem. Simulation results are compared to data from a direct numerical simulation and experiments. This comparison shows that mean and rms quantities compare well with experiments and are in the range of previous simulation studies. These results are obtained with a pressure-based and unstructured computational-fluid-dynamics solver, an approach that is preferred in industry.
|
|
| 3. |
- Chen, Boxiong, 1987, et al.
(författare)
-
An Eulerian stochastic field cavitation model coupled to a pressure based solver
- 2018
-
Ingår i: Computers and Fluids. - : Elsevier BV. - 0045-7930. ; 162, s. 1-10
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 4. |
- Chen, Boxiong, 1987, et al.
(författare)
-
LES Investigation of ECN Spray G2 with an Eulerian Stochastic Field Cavitation Model
- 2018
-
Ingår i: SAE Technical Papers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191 .- 2688-3627. ; 2018-April
-
Tidskriftsartikel (refereegranskat)abstract
- Due to an ongoing trend of high injection pressures in the realm of internal combustion engines, the role of cavitation that typically happens inside the injector nozzle has become increasingly important. In this work, a large Eddy Simulation (LES) with cavitation modeled on the basis of an Eulerian Stochastic Field (ESF) method and a homogeneous mixture model is performed to investigate the role of cavitation on the Engine Combustion Network (ECN) spray G2. The Eulerian stochastic field cavitation model is coupled to a pressure based solver for the flow, which lowers the computational cost, thereby making the methodology highly applicable to realistic injector geometries. Moreover, the nature of the Eulerian stochastic field method makes it more convenient to achieve a high scalability when applied to parallel cases, which gives the method the edge over cavitation models that are based on Lagrangian tracking. The result of the Eulerian stochastic field 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 behavior of the size Probability Density Function (PDF) at different probe locations at different times are acquired to demonstrate the capability of the Eulerian stochastic field model to capture cavitation and potentially providing more statistical information in each cell as compared to the typical single volume fraction solver. Major cavitation zones that are observed in the result indicates the important role of cavitation in ECN spray G2, therefore inferring a need to take cavitation into consideration in future spray studies.
|
|
| 5. |
- Doubiani, Nidal, 1995, et al.
(författare)
-
A pressure-coupled Representative Interactive Linear Eddy Model (RILEM) for engine simulations
- 2024
-
Ingår i: Fuel. - 0016-2361. ; 355
-
Tidskriftsartikel (refereegranskat)abstract
- The Representative Linear Eddy Model (RILEM) was introduced by Lackmann et al. (2018) as an alternative modeling approach to simulate turbulent non-premixed combustion in engines. The model utilizes a RANS approach for turbulence and the Linear Eddy Model (LEM) with a presumed probability density function (PDF) approach for combustion closure. A distinct feature of RILEM is its potential to handle arbitrary combustion regimes and the consideration of complex physical phenomena such as differential diffusion effects. The original version of RILEM implemented a volume-based coupling between LEM and the flow solver. This work presents a new variant of RILEM, i.e., Multiple Representative Interactive Linear Eddy Model (MRILEM) based on a pressure-based coupling, to overcome some deficiencies of the original RILEM, namely statistical fidelity. Due to the introduced pressure coupling, the effects of heat losses (wall heat fluxes, latent heat of evaporation) on combustion are intrinsically included via the pressure trace. Furthermore, we introduce a new step function PDF for the progress variable defined by its mean value only. Issues with an incomplete solution space for mixture fraction and progress variable due to the stochastic nature of LEM are remedied with a PDF scaling technique, aided by a novel parameterization of the progress-variable PDF The new variant of RILEM is evaluated using part- and full-load cases of a heavy-duty metal engine. The impact of utilizing multiple LEM lines on the completeness of the solution space and its influence on the distribution of scalar values in the CFD domain was demonstrated. Results for pressure trace, flame structure, and CO emissions are analyzed and compared with simulations using the Multi-Zone Well-Mixed Model (MZWM) model and experiments. While pressure traces agree well among the different models and experiments, noteworthy differences are observed between the models regarding CO emissions and temperature. Effects of turbulence chemistry interaction were noticed when comparing MRILEM to the results of the MZWM simulation, namely flame brush and species mass fraction distribution.
|
|
| 6. |
- Doubiani, Nidal, 1995, et al.
(författare)
-
Pressure Coupling Of the Spherical Linear Eddy Model to RANS-CFD for Internal-Combustion Engine Simulation
- 2021
-
Ingår i: Proceedings of the 6th World Congress on Momentum, Heat and Mass Transfer (MHMT'21). - : Avestia Publishing.
-
Konferensbidrag (refereegranskat)abstract
- As a result of the increase of the use of combustion engines, more restrictive emission standards are applied. New combustion technologies are being constantly developed to enhance the internal combustion engine, this is motivated by overcoming issues that are relevant for both engine efficiency and the environmental aspects. To do that, turbulent combustion modelling is needed. In this paper, an updated version of the Linear Eddy Model (LEM) will be presented. LEM is capable of simulating premixed, non- premixed and mixed mode combustion, and it is a regime independent model that runs under the assumption of finite-rate chemistry that is sensitive to turbulent chemistry interactions, which makes it suitable for prediction of pollutant formation. A New coupling scheme to the CFD-RANS simulation is proposed, the coupling is based on linking the two models via pressure. The benefits of pressure coupling are that the effects of wall heat losses and latent heat of evaporation that are modelled on the CFD side are an intrinsic part of the pressure term that is linking the two models, in contrast to volume coupling in which these relevant phenomena need supplementary modelling on the LEM side. The pressure coupling results in a radially uniform dilatation of the LEM domain reflecting the combined effects of pressure change, fuel addition, and cylinder volume change during the engine cycle. Consistency of the LEM cone volume and the CFD domain volume is maintained by adopting a split operator strategy involving a volume correction that adjusts the cone angle. A Spherical Stand-Alone Linear Eddy Model (SSALEM) has been created to conduct relatively fast simplified code development and parameter studies. SSALEM input parameters were drawn from a WSR-RANS simulation and a 1D slider-crank model that calculates the combustion chamber volume that corresponds to a given crank-angle. Model results show the capability to physically track the evolution of several scalars that are solved on the LEM line such as temperature, fuel, and intermediate species in the combustion process.
|
|
| 7. |
- Fistler, Marco, 1989, et al.
(författare)
-
A new LES subgrid-scale approach for turbulence modulation by droplets
- 2020
-
Ingår i: ICLASS 2018 - 14th International Conference on Liquid Atomization and Spray Systems. ; 14
-
Konferensbidrag (refereegranskat)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.
|
|
| 8. |
- Fistler, Marco, 1989, et al.
(författare)
-
Numerical studies of turbulent particle-laden jets using spatial approach of one-dimensional turbulence
- 2017
-
Ingår i: 28TH CONFERENCE ON LIQUID ATOMIZATION AND SPRAY SYSTEMS, ILASS-EUROPE 2017. - 9788490485804 ; , s. 83-89
-
Konferensbidrag (refereegranskat)abstract
- To challenge one of the major problems for multiphase flow simulations, namely computational costs, a dimension reduced model is used with the goal to predict these types of flow more efficiently. One-dimensional turbulence (ODT) is a stochastic model simulating turbulent flow evolution along a notional one-dimensional line of sight by applying instantaneous maps that represent the effect of individual turbulent eddies on property fields. As the particle volume fraction is in an intermediate range above 10(-5) for dilute flows and under 10(-2) for dense ones, turbulence modulation is important and can be sufficiently resolved with a two-way coupling approach, which means the particle phase influences the fluid phase and vice versa. For the coupling mechanism the ODT multiphase model is extended to consider momentum transfer and energy in the deterministic evolution and momentum transfer during the particle-eddy interaction. The changes of the streamwise velocity profiles caused by different solid particle loadings are compared with experimental data as a function of radial position. Additionally, streamwise developments of axial RMS and mean gas velocities along the centerline are evaluated as functions of axial position. To achieve comparable results, the spatial approach of ODT in cylindrical coordinates is used here. The investigated jet configuration features a nozzle diameter of 14.22 cm and a Reynolds number of 8400, which leads to a centerline inlet velocity of 11.7 m/s. The particles used are glass beads with a density of 2500 kg/m(3). Two different particle diameters (25 and 70 mu m) were tested for an evaluation of the models capability to capture the impact of a varying Stokes number and also two different particle solid loadings (0.5 and 1.0) were evaluated. It is shown that the model is capable of capturing turbulence modulation of particles in a round jet.
|
|
| 9. |
- Fistler, Marco, 1989, et al.
(författare)
-
Numerical study of stochastic particle dispersion using One-Dimensional-Turbulence
- 2017
-
Ingår i: ILASS-Americas 29th Annual Conference on Liquid Atomization and Spray Systems.
-
Konferensbidrag (refereegranskat)abstract
- A stochastic model to study particle dispersion in a round jet configuration using the one-dimensional-turbulence model (ODT) is evaluated. To address one of the major problems for multiphase flow simulations, namely computational costs, the dimension-reduced model is used with the goal of predicting these flows more efficiently. ODT is a stochastic model simulating turbulent flow evolution along a notional one-dimensional line of sight by applying instantaneous maps which represent the effect of individual turbulent eddies on property fields. As the impact of the particles on the carrier fluid phase is negligible for cases considered, a one-way coupling approach is used, which means that the carrier-phase is affecting the particle dynamics but not vice versa. The radial dispersion and axial velocity are compared with jet experimental data as a function of axial position. For consistent representation of the spatially developing round jet, the spatial formulation of ODT in cylindrical coordinates is used. The investigated jet configuration has a nozzle diameter of 7 mm and Reynolds numbers ranging from 10000 to 30000. The flow statistics of the ODT particle model are compared with experimental measurements for two different particle diameters (60 and 90 μm), thereby testing the Stokes number dependence predicted by ODT.
|
|
| 10. |
- Fistler, Marco, 1989, et al.
(författare)
-
Turbulence modulation in particle-laden stationary homogeneous isotropic turbulence using one-dimensional turbulence
- 2020
-
Ingår i: Physical Review Fluids. - 2469-990X. ; 5:4
-
Tidskriftsartikel (refereegranskat)abstract
- Turbulence modulation in particle-laden stationary homogeneous isotropic turbulence is investigated using one-dimensional turbulence (ODT), a low-dimensional stochastic flow simulation model. For this purpose, ODT is extended in two ways. First, a forcing scheme that maintains statistical stationarity is introduced. Unlike direct numerical simulation (DNS) of forced turbulence, the ODT framework accommodates forcing that is not directly coupled to the momentum equation. For given forcing the ODT energy dissipation rate is therefore the same in particle-laden cases as in the corresponding single-phase reference case. Second, previously implemented one-way-coupled particle phenomenology is extended to two-way coupling using the general ODT methodology for flow modulation through interaction with any specified energy and momentum sources and sinks. As in a DNS comparison case for Re-lambda = 70, turbulence modulation is diagnosed primarily on the basis of the fluid-phase kinetic-energy spectrum. Because ODT involves subprocesses with straightforward physical interpretations, the ODT mechanisms of particle-induced turbulence modulation are clearly identified and they are plausibly relevant to particleladen Navier-Stokes turbulence. ODT results for the ratio of particle-phase and fluid-phase kinetic energies as a function of particle Stokes number and mass loading are reported for the purpose of testing these predictions in the future when these quantities are evaluated experimentally or using DNS.
|
|
| 11. |
- Fistler, Marco, 1989, et al.
(författare)
-
Turbulence modulation in particle-laden stationary homogeneous shear turbulence using one-dimensional turbulence
- 2020
-
Ingår i: Physical Review Fluids. - 2469-990X. ; 5:12
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 12. |
- Gerber, Stephan, et al.
(författare)
-
A two dimensional Euler-Lagrangian model of wood gasification in a charcoal bed - Part III: Parameter influence and comparison
- 2017
-
Ingår i: Powder Technology. - : Elsevier BV. - 1873-328X .- 0032-5910. ; 310, s. 163-174
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 13. |
- Gerber, Stephan, et al.
(författare)
-
A two dimensional Euler-Lagrangian model of wood gasification in a charcoal bed — Particle histories
- 2018
-
Ingår i: Powder Technology. - : Elsevier BV. - 1873-328X .- 0032-5910. ; 324, s. 5-15
-
Tidskriftsartikel (refereegranskat)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 [1–3]. 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.
|
|
| 14. |
- Gerber, Stephan, et al.
(författare)
-
A two dimensional Euler–Lagrangian model of wood gasification in a charcoal bed—Part II: Parameter influence and comparison
- 2017
-
Ingår i: Particuology. - : Elsevier BV. - 2210-4291 .- 1674-2001. ; 35, s. 22-30
-
Tidskriftsartikel (refereegranskat)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.
|
|
| 15. |
- Gonzalez-Juez, Esteban D., et al.
(författare)
-
Effect of the turbulence modeling in large-eddy simulations of nonpremixed flames undergoing extinction and reignition
- 2017
-
Ingår i: AIAA SciTech Forum - 55th AIAA Aerospace Sciences Meeting. - Reston, Virginia : American Institute of Aeronautics and Astronautics.
-
Konferensbidrag (refereegranskat)abstract
- Simulating practical combustion systems requires the approximation of the interaction between turbulence, molecular transport and chemical reactions. Turbulent combustion models are used for this purpose, but their behavior is difficult to anticipate based on their mathematical formulations, making the use of numerical experimentation necessary. Therefore, the present work explores the effect of three turbulent-combustion models, two eddy-viscosity models, and their parameters on a combustion problem which is notoriously difficult to model: flame extinction and reignition. For this purpose, two types of temporal jets are considered, and direct-numerical-simulation results are compared qualitatively with those from large-eddy simulations.
|
|
| 16. |
- Gonzalez-Juez, Esteban D., et al.
(författare)
-
Turbulent-combustion closure for the chemical source terms and molecular mixing using the linear-eddy model
- 2017
-
Ingår i: 53rd AIAA/SAE/ASEE Joint Propulsion Conference, 2017. - 9781624105111
-
Konferensbidrag (refereegranskat)abstract
- Turbulent combustion models approximate the interaction between turbulence, molecular transport and chemical reactions with the purpose of allowing computationally feasible simulations. There are many of these models and the present interest is on the lineareddy model (LEM). Specifically this paper introduces and tests a new method to use LEM to close the filtered chemical source terms in the conservation equations of the thermochemical quantities. For this purpose, this paper discusses the conceptual enhancements of this method over traditional LEM implementations, and its testing in a widely-used bluff-body-stabilized-flame problem.
|
|
| 17. |
- Grosshans, H., et al.
(författare)
-
Sensitivity of VOF simulations of the liquid jet breakup to physical and numerical parameters
- 2016
-
Ingår i: Computers & Fluids. - : Elsevier. - 0045-7930 .- 1879-0747. ; 136, s. 312-323
-
Tidskriftsartikel (refereegranskat)abstract
- In this paper the characteristics of the primary breakup of a liquid jet is analyzed numerically. We applied the Volumes of Fluids (VOF) approach utilizing the Direction Averaged Curvature (DAC) model, to estimate the interface curvature, and the Direction Averaged Normal (DAN) model, to propagate the interface. While being used for the first time to predict liquid atomization, this methodology showed a high accuracy. The influence of varying the fluid properties, namely liquid-gas density and viscosity ratio, and injection conditions is discussed related to the required grid resolution. Resulting droplet sizes are compared to distributions obtained through the One-Dimensional Turbulence (ODT) model.
|
|
| 18. |
- Kerstein, Alan R., et al.
(författare)
-
Parameter dependences of the onset of turbulent liquid-jet breakup
- 2017
-
Ingår i: Journal of Fluid Mechanics. - : Cambridge University Press (CUP). - 0022-1120 .- 1469-7645. ; 811
-
Tidskriftsartikel (refereegranskat)abstract
- Previous studies have predicted We(-2/5) dependence of the streamwise location at which primary breakup of turbulent liquid jets begins and We(-3/5) dependence of the Sauter mean diameter (SMD) of droplets released at that location, where We is the jet Weber number. Measured deviations from these predictions were attributed to measurement uncertainties and to the simplicity of the analysis, which invoked turbulence inertial-range phenomenology. Here, it is proposed that breakup onset is instead controlled by the residual presence of the boundary-layer structure of the nozzle flow in the near field of the jet. Assuming that the size of the breakup inducing eddy is within the scale range of the log-law region. We(-1) dependence of both the onset location and the SMD at onset is predicted. These dependences agree with the available measurements more closely than those previously predicted. lb predict the dependences on the Reynolds number Re, either the friction velocity in conjunction with the Blasius friction law or the hulk velocity can he used, where the former yields Re-3/8 and Re-1/4 dependence of the onset location and the SMD at onset respectively, while the latter implies no Re dependence of either. 'f he latter result is consistent with the available measurements, but the boundary-layer analysis indicates that the velocity scaling should be based on the friction velocity rather than the bulk velocity, so the origin of the measured lack of Re dependence merits further investigation. A plausible hypothesis is that pressure effects associated with the transition from wall-bounded nozzle flow to jet free-slip boundary conditions induce a transient large-scale flow modification that counteracts the Re dependence of the nozzle flow while preserving the logarithmic flow structure near the jet surface. Notwithstanding the absence of direct evidence supporting this hypothesis, the new analysis and comparisons of its predictions with measurements suggest that transient effects such as the residual influence of the nozzle-flow structure are the likely explanations of the observed parameter dependences.
|
|
| 19. |
- Kösters, Anne, 1983, et al.
(författare)
-
RANS predictions of turbulent diffusion flames: comparison of a reactor and a flamelet combustion model to the well stirred approach
- 2015
-
Ingår i: Combustion Theory and Modelling. - : Informa UK Limited. - 1364-7830 .- 1741-3559. ; 19:1, s. 81-106
-
Tidskriftsartikel (refereegranskat)abstract
- The flame stabilisation process in turbulent non-premixed flames is not fully understood and several models have been developed to describe the turbulence-chemistry interaction. This work compares the performance of the multiple Representative Interactive Flamelet (mRIF) model, the Volume Reactor Fraction Model (VRFM), and the Well-Stirred reactor (WS) model in describing such flames. The predicted ignition delay and flame lift-off length of n-heptane sprays are compared to experimental results published within the Engine Combustion Network (ECN). All of the models predict the trend of ignition delay reasonably well. At a low gas pressure (42bar) the ignition delay is overpredicted compared to the experimental data, but the difference between the models is not significant. However, the predicted lift-off lengths differ. At high pressure (87bar) the difference between the models is small. All models slightly underpredict the lift-off length compared to the experimental data. At low gas pressure (42bar) the mRIF model gives the best results. The VRFM and WS models predict excessively short lift-off lengths, but the VRFM model gives better results than the WS model. The flame structures of the models are also compared. The WS model and the VRFM model yield a well defined flame stabilisation point whereas the mRIF model does not. The flame of the mRIF model is more diffuse and the model is not able to predict flame propagation. All models were able to predict the experimental trends in lift-off and ignition delay, but certain differences between them are demonstrated.
|
|
| 20. |
- Lackmann, Tim, 1983, et al.
(författare)
-
A fuel mapping strategy for unsteady injection processes within the representative interactive linear eddy model (RILEM)
- 2015
-
Ingår i: Proceedings of the European Combustion Meeting 2015, 30th March - 2nd April 2015, Budapest, Hungary, 2015. ; , s. 6-
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- This paper presents a fuel mapping strategy for a new regime-independent combustion modeling strategy fornon-premixed combustion in which the linear eddy model (LEM) is used as a representative interactive regimeindependentturbulent combustion model coupled to a 3D CFD solver. Parameters and boundary conditions thatdetermine the evolution of the LEM are supplied by the 3D CFD calculation and updated at each time step. TheLEM is then solved for the corresponding time step, providing the 3D CFD code with an updated compositionstate.This initialization strategy for this new representative interactive linear eddy model (RILEM) is tested bysimulations of an n-heptane spray, demonstrating the ability of the RILEM to describe spray combustionprocesses.
|
|
| 21. |
- Lackmann, Tim, 1983, et al.
(författare)
-
A representative linear eddy model for simulating spray combustion in engines (RILEM)
- 2018
-
Ingår i: Combustion and Flame. - : Elsevier BV. - 1556-2921 .- 0010-2180. ; 193, s. 1-15
-
Tidskriftsartikel (refereegranskat)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 9-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. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
|
|
| 22. |
- Lackmann, Tim, 1983, et al.
(författare)
-
A representative linear eddy model (RILEM) for non-premixed combustion
- 2015
-
Ingår i: SAE Technical Papers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191 .- 2688-3627. ; Volume 2015-April:April
-
Tidskriftsartikel (refereegranskat)abstract
- To further improve the efficiency and emissions profiles of internal combustion engines, many new combustion concepts are currently being investigated. Examples include homogeneous charge compression ignition (HCCI), stratified charge compression ignition (SCCI), lean stratified premixed combustion, and the use of high levels of exhaust gas recirculation (EGR) in diesel engines. The typical combustion temperatures in all of these concepts are lower than those in traditional spark ignition or diesel engines.Most of the combustion models that are currently used in computational fluid dynamics (CFD) simulations were developed to describe either premixed or non-premixed combustion under the assumption of fast chemistry. The refinement of existing combustion concepts for highly efficient clean engines and the development of new ones would be greatly facilitated by the introduction of new computational tools and combustion models that are mode- and regime-independent, i.e. capable of modeling both premixed and non-premixed and also fast and non-fast chemistry. Such tools should enable more accurate simulation of combustion under non-standard conditions such as those established during low temperature combustion.This paper presents a new regime-independent combustion modeling strategy for non-premixed combustion in which the linear eddy model (LEM) is used as a representative interactive regime-independent turbulent combustion model and coupled to a 3D CFD solver. Parameters and boundary conditions that determine the evolution of the LEM are supplied by the 3D CFD calculation and updated at each time step. The LEM is then solved for the corresponding time step, providing the 3D CFD code with an updated composition state.This new representative interactive linear eddy model (RILEM) is used to simulate an n-heptane spray, demonstrating some potential to describe spray combustion processes.
|
|
| 23. |
- Lackmann, Tim, 1983, et al.
(författare)
-
Comparison of a Representative Linear Eddy Model with a Representative Interactive Flamelet Model for Spray Combustion Processes
- 2015
-
Ingår i: SAE Technical Papers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191 .- 2688-3627. ; 2015, s. 7-
-
Konferensbidrag (refereegranskat)abstract
- To further improve engines in terms of both efficiency and emissions many new combustion concepts are currently being investigated.Examples include homogeneous charge compression ignition(HCCI), stratified charge compression ignition (SCCI), lean stratified premixed combustion, and high levels of exhaust gas recirculation (EGR) in diesel engines. All of these combustion concepts have in common that the typical combustion temperatures are lower than intraditional spark ignition or diesel engines.To further improve and develop combustion concepts for clean and highly efficient engines, it is necessary to develop new computational tools that can be used to describe and optimize processes in nonstandardconditions, such as low temperature combustion. Thus, inthe presented study a recently developed model (RILEM:Representative Interactive Linear Eddy Model) for modelingnon-premixed combustion, regime-independently, was used tosimulate a spray combustion process. RILEM consists of a single representative linear eddy model (LEM) coupled to a 3D CFD solver.All fluid dynamics and scalar field equations are solved in the CFD code, while the turbulent combustion is solved simultaneously in a separate, representative one-dimensional LEM. Parameters andboundary conditions that determine the evolution of the LEM are supplied from the 3D calculation at each time step. The LEM code is then solved for the same time step, providing the 3D CFD code with an update of the composition state. In addition to the modelling strategy, a numerical simulation of a n-heptane spray is presented here. The RILEM output is also compared to calculations for the same case by the RIF (representative interactive flamelet) model.
|
|
| 24. |
- Lackmann, Tim, 1983, et al.
(författare)
-
Investigation of turbulence–chemistry interactions in a heavy-duty diesel engine with a representative interactive linear eddy model
- 2020
-
Ingår i: International Journal of Engine Research. - : SAGE Publications. - 1468-0874 .- 2041-3149. ; 21:8, s. 1469-1479
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
|
|
| 25. |
- Lackmann, Tim, 1983, et al.
(författare)
-
Modeling n-dodecane Spray Combustion with a Representative Interactive Linear Eddy Model
- 2017
-
Ingår i: SAE Technical Papers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191 .- 2688-3627. ; 2017-March:March
-
Tidskriftsartikel (refereegranskat)abstract
- Many new combustion concepts are currently being investigated to further improve engines in terms of both efficiency and emissions. Examples include homogeneous charge compression ignition (HCCI), lean stratified premixed combustion, stratified charge compression ignition (SCCI), and high levels of exhaust gas recirculation (EGR) in diesel engines, known as low temperature combustion (LTC). All of these combustion concepts have in common that the temperatures are lower than in traditional spark ignition or diesel engines. To further improve and develop combustion concepts for clean and highly efficient engines, it is necessary to develop new computational tools that can be used to describe and optimize processes in nonstandard conditions, such as low temperature combustion. Thus, in the presented study a recently developed model (RILEM: Representative Interactive Linear Eddy Model [1]) for regime-independent modeling of turbulent non-premixed combustion is used to simulate the so called "Spray B' of the Engine Combustion Network (ECN), which is a heavy-duty optical engine experiment. RILEM directly resolves the interaction of turbulent mixing with the chemistry along a one-dimensional representative line of sight through the combustion chamber via stochastic sequences of statistically independent eddy events. RILEM in its present form consists of a single (one-dimensional) linear eddy model (LEM) instantiation that is coupled to an unsteady Reynolds-averaged Navier-Stokes solver within the OpenFOAM framework. The coupling is similar to unsteady flamelet concepts but features distinct and important differences, e.g. an intrinsic representation of the scalar dissipation rate distribution and its fluctuations. Cylinder pressure, heat release rates and ignition delay time from the computation are compared to experiments under parametric variation of temperature.
|
|
