Browsing by Author "Kahila, Heikki"

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  • DLBFoam: An open-source dynamic load balancing model for fast reacting flow simulations in OpenFOAM
    (2021-10) Tekgül, Bulut; Peltonen, Petteri; Kahila, Heikki; Kaario, Ossi; Vuorinen, Ville
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Computational load imbalance is a well-known performance issue in multiprocessor reacting flow simulations utilizing directly integrated chemical kinetics. We introduce an open-source dynamic load balancing model named DLBFoam to address this issue within OpenFOAM, an open-source C++ library for Computational Fluid Dynamics (CFD). Due to the commonly applied operator splitting practice in reactive flow solvers, chemistry can be treated as an independent stiff ordinary differential equation (ODE) system within each computational cell. As a result of the highly non-linear characteristics of chemical kinetics, a large variation in the convergence rates of the ODE integrator may occur, leading to a high load imbalance across multiprocessor configurations. However, the independent nature of chemistry ODE systems leads to a problem that can be parallelized easily (called an embarrassingly parallel problem in the literature) during the flow solution. The presented model takes advantage of this feature and balances the chemistry load across available resources. Additionally, a reference mapping model is utilized to further speed-up the simulations. When DLBFoam it utilized with both these features enabled, a speed-up by a factor of 10 is reported for reactive flow benchmark cases. To the best of our knowledge, this model is the first open-source implementation of chemistry load balancing in the literature. (C) 2021 The Author(s). Published by Elsevier B.V.
  • The effect of fuel on high velocity evaporating fuel sprays: Large-Eddy simulation of Spray A with various fuels
    (2019-06-19) Kaario, Ossi Tapani; Vuorinen, Ville; Kahila, Heikki; Im, Hong G.; Larmi, Martti
     A2 Katsausartikkeli tieteellisessä aikakauslehdessä
    Lagrangian particle tracking and Large-Eddy simulation were used to assess the effect of different fuels on spray characteristics. In such a two-way coupled modeling scenario, spray momentum accelerates the gaseous phase into an intense, multiphase jet near the nozzle. To assess fuel property effects on liquid spray formation, the non-reacting Engine Combustion Network Spray A baseline condition was chosen as the reference case. The validated Spray A case was modified by replacing n-dodecane with diesel, methanol, dimethyl ether, or propane assuming 150 MPa injection pressure. The model features and performance for various fuels in the under-resolved near-nozzle region are discussed. The main findings of the paper are as follows. (1) We show that, in addition to the well-known liquid penetration (Formula presented.), and vapor penetration (Formula presented.), for all the investigated fuels, the modeled multiphase jets exhibit also a third length scale (Formula presented.), with discussed correspondence to a potential core part common to single phase jets. (2) As a characteristic feature of the present model, (Formula presented.) is noted to correlate linearly with (Formula presented.) and (Formula presented.) for all the fuels. (3) A separate sensitivity test on density variation indicated that the liquid density had a relatively minor role on (Formula presented.). (4) Significant dependency between fuel oxygen content and the equivalence ratio (Formula presented.) distribution was observed. (5) Repeated simulations indicated injection-to-injection variations below 2% for (Formula presented.) and 4% for (Formula presented.). In the absence of experimental and fully resolved numerical near-nozzle velocity data, the exact details of (Formula presented.) remain as an open question. In contrast, fuel property effects on spray development have been consistently explained herein.
  • Engine exhaust plume interactions with a planetary surface
    (2014) Kahila, Heikki
     Perustieteiden korkeakoulu | Master's thesis
  • Immersed boundary method for computational fluid dynamics - A review and verification in OpenFOAM
    (2020-01-20) Väisänen, Ville
     Insinööritieteiden korkeakoulu | Master's thesis
    Fluid-structure interaction (FSI) is a common phenomenon encountered in various computational fluid dynamics (CFD) problems. Conventional methods rely on the generation of a so-called body- or boundary-fitted (BF) grid, which conforms to the structure geometry. Typically, these grids consist of either structured or unstructured cells, of which the former are more efficient. Although, generating a good quality structured BF grid might be straightforward for simple geometries, it may become tedious and take even months to do using the best available software when complex geometries are present. Furthermore, the BF grid needs to be updated at each time step when structure motion is present. In the present work, we concentrate on an immersed boundary (IB) approach implemented into the FOAM-Extend CFD library, which is based on the OpenFOAM software platform. In the IB approach, the grid does not conform to the structure, but instead, the position of the structure is tracked separately and the equations--either continuous or discretized depending on the method in question--are modified to account for the boundary. A literature review of different IB methods is conducted and the implementation is validated by running simulations on various flow cases consisting of stationary (channel flow, pipe flow and flow over a cylinder) and moving (flow over an oscillating cylinder) structures. These results are compared to ones retrieved by using a BF grid in OpenFOAM CFD software and those found in literature. In addition, a user guide for the implemented IB method is presented. Initialization of a flow simulation with the IB method is very quick. However, simulations show that this is acquired with the cost of efficiency and accuracy. Simulation with the BF grids was much faster and more accurate than with the IB grids. Furthermore, resolving values at the surface of a moving boundary was difficult for the IB grid. Even though the IB method does show promising results for the cases with stationary structures, the fast initialization is outweighed by the required high grid resolution and slow simulation times. This is further emphasized by problems with parallel computing, rendering the current IB implementation impractical for computationally demanding CFD simulations.
  • Iteratiiviset menetelmät virtauslaskennassa
    (2015-12-03) Kokkonen, Toni
     Insinööritieteiden korkeakoulu | Bachelor's thesis
  • Kirjallisuuskatsaus polttoainesuihkun dynamiikasta dieselmoottorissa
    (2016-04-21) Mattila, Janne
     Insinööritieteiden korkeakoulu | Bachelor's thesis
  • Komposiittirakenteiden moniskaalamallinnus
    (2013-04-17) Kahila, Heikki
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Large-eddy simulation of dual-fuel ignition: Diesel spray injection into a lean methane-air mixture
    (2019-01-01) Kahila, Heikki; Wehrfritz, Armin; Kaario, Ossi; Vuorinen, Ville
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In the present study, large-eddy simulation (LES) together with a finite-rate chemistry model is utilized for the investigation of a dual-fuel (DF) ignition process where a diesel surrogate (n-dodecane) spray ignites a lean methane-air mixture in engine relevant conditions. The spray setup corresponds to the Engine Combustion Network (ECN) Spray A configuration enabling an extensive validation of the present numerical models in terms of liquid and vapor penetration, mixture distribution, ignition delay time (IDT) and spatial formaldehyde concentration. The suitability of two n-dodecane mechanisms (54 and 96 species) to cover dual-fuel chemical kinetics is investigated by comparing the predicted homogeneous IDTs and laminar flame speeds to reference values in single-fuel methane-air mixtures. LES of an n-dodecane spray in DF conditions is carried out and compared against the baseline ECN Spray A results. The main results of the study are: (1) ambient methane impacts the ignition chemistry throughout the oxidation process. In particular, the activation of the low-temperature chemistry is delayed by a factor of 2.6 with both mechanisms, whereas the high-temperature chemistry is delayed by a factor of 1.6–2.4, depending on the mechanism. (2) The ignition process starts from the spray tip. (3) There exists a characteristic induction time in the order of 0.1 ms between the start of the first high-temperature reactions and the time when maximum methane consumption rate is achieved. (4) The high-temperature ignition process begins near the most reactive mixture fraction conditions. (5) The role of low-temperature reactions is of particular importance for initiation of the production of intermediate species and heat, required in methane oxidation and (6) both applied mechanisms yield qualitatively the same features (1)–(5) in the DF configuration.
  • Large-eddy simulation of dual-fuel spray ignition at different ambient temperatures
    (2020-05-01) Tekgül, Bulut; Kahila, Heikki; Kaario, Ossi; Vuorinen, Ville
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Here, a finite-rate chemistry large-eddy simulation (LES) solver is utilized to investigate dual-fuel (DF) ignition process of n-dodecane spray injection into a methane–air mixture at engine-relevant ambient temperatures. The investigated configurations correspond to single-fuel (SF) ϕCH4= 0 and DF ϕCH4= 0.5 conditions for a range of temperatures. The simulation setup is a continuation of the work by Kahila et al. (2019, Combustion and Flame) with the baseline SF spray setup corresponding to the Engine Combustion Network (ECN) Spray A configuration. First, ignition is investigated at different ambient temperatures in 0D and 1D studies in order to isolate the effect of chemistry and chemical mechanism selection to ignition delay time (IDT). Second, 3D LES of SF and DF sprays at three different ambient temperatures is carried out. Third, a reaction sensitivity analysis is performed to investigate the effect of ambient temperature on the most sensitive reactions. The main findings of the paper are as follows: (1) DF ignition characteristics depend on the choice of chemical mechanism, particularly at lower temperatures. (2) Addition of methane to the ambient mixture delays ignition, and this effect is the strongest at lower temperatures. (3) While the inhibiting effect of methane on low- and high-temperature IDT's is evident, the time difference between these two stages is shown to be only slightly dependent on temperature. (4) Reaction sensitivity analysis indicates that reactions related to methane oxidation are more pronounced at lower temperatures. The provided quantitative results indicate the strong ambient temperature sensitivity of the DF ignition process.
  • Large-eddy simulation of split injection strategies in RCCI conditions
    (2022-04-16) Tekgul, Bulut; Karimkashi, Shervin; Kaario, Ossi; Kahila, Heikki; Lendormy, Eric; Hyvönen, Jari; Vuorinen, Ville
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In this study, we investigate the effect of different split injection strategies on ignition delay time (IDT) and heat release rate (HRR) characteristics in Reactivity Controlled Compression Ignition conditions via large-eddy simulation and finite-rate chemistry. A diesel surrogate (n-dodecane) is injected into a domain with premixed methane and oxidiser in two separate injection pulses. Three different split injection strategies are investigated by fixing the amount of total fuel mass: varying the first injection timing, varying the second injection timing, and changing the fuel mass ratio between the two injections at a fixed injection timing. A compression heating mass source term approach is utilised to take compression heating into account. The main findings of the study are as follows: (1) In general, the IDT shifts towards the top-dead centre when the first injection is advanced or the second injection is retarded. The size and spatial pattern of the ignition kernels are shown to depend on the dwell time between the injections. (2) A precisely timed first injection offered the best control over ignition and HRR characteristics. However, advancing the first injection may lead to over-dilution downstream, preventing volumetric ignition and reducing the peak HRR value. (3) Approximately 21% decrease in the maximum HRR value, as well as a factor of 2.8 increase in combustion duration could be achieved by advancing the first injection timing. (4) As indicated by frozen-flow chemistry analysis, in the investigated configurations, the reactivity stratification is controlled by mixture stratification rather than temperature. The findings indicate that the first injection controls the downstream reactivity stratification, offering ignition and HRR control.
  • Large-eddy simulation of spray assisted dual-fuel ignition under reactivity-controlled dynamic conditions
    (2021-06-01) Tekgül, Bulut; Kahila, Heikki; Karimkashi, Shervin; Kaario, Ossi; Ahmad, Zeeshan; Lendormy, Éric; Hyvönen, Jari; Vuorinen, Ville
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Here, a large-eddy simulation and a finite-rate chemistry solver (see Kahila et al. Combustion and Flame, 2019) is utilized to investigate diesel spray assisted ignition of a lean methane-air mixture. A compression heating model is utilized to emulate the ambient temperature and pressure increase in a compression ignition (CI) system. The key parameter is the start of injection (SOI) relative to a virtual top dead center (TDC), where the peak adiabatic compression pressure/temperature would be achieved. Altogether, five different cases are investigated by advancing the SOI further away from the TDC with constant injection duration. The main findings of the paper are as follows: 1) Advancing the SOI advances the ignition timing of the spray with respect to the TDC from 0.91 to 7.08 CAD. However, beyond a critical point, the ignition time starts retarding towards the TDC to 4.46 CAD due to the excessively diluted diesel spray. 2) Advancing the SOI increases the contribution of leaner mixtures to the heat release rate (HRR). Consequently, the low-temperature combustion HRR mode becomes more pronounced (from 33.9% to 76.7%) while the total HRR is reduced by a factor of 4. 3) Ignition is observed for all investigated SOI's. However, the numerical findings indicate that advancing the SOI decreases the ignition kernel size, resulting in weaker ignition. 4) An ignition index analysis with frozen flow assumption indicates that for the SOI's close to the TDC the HRR mode appears as spray mixing controlled, while for advanced SOI it becomes reactivity controlled, dominated by fuel stratification.
  • Large-Eddy Simulation on the Influence of Injection Pressure in Reacting Spray A
    (2018-05) Kahila, Heikki; Wehrfritz, Armin; Kaario, Ossi; Ghaderi Masouleh, Mahdi; Maes, Noud; Somers, Bart; Vuorinen, Ville
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The Engine Combustion Network (ECN) Spray A target case corresponds to high-pressure liquid fuel injection in conditions relevant to diesel engines. Following the procedure by Wehrfritz et al. 2016, we utilize large-eddy simulation (LES) and flamelet generated manifold (FGM) methods to carry out an injection pressure sensitivity study for Spray A at 50, 100 and 150 MPa. Comparison with experiments is shown for both non-reacting and reacting conditions. Validation results in non-reacting conditions indicate relatively good agreement between the present LES and experimental data, with some deviationin mixture fraction radial profiles. In reacting conditions, the simulated flame lift-off length (FLOL) increases with injection pressure, deviating from the experiments by 4-14%. The respective deviation in ignition delay time (IDT) is noted to be within 10-20%. Analysis of the underlying chemistry manifold implies that the noted discrepancies can be explained partially by the differences between experimental and computational mixing processes.
  • Mukautuvan laskentahilan käyttö virtauslaskennassa
    (2015-12-03) Kidron, Matias
     Insinööritieteiden korkeakoulu | Bachelor's thesis
  • Numerical modeling of spray-assisted dual-fuel ignition
    (2019) Kahila, Heikki
     School of Engineering | Doctoral dissertation (article-based)
    This dissertation belongs to the field of computational physics and chemistry with a research focus on reacting fuel sprays in internal combustion engine context. Computational fluid dynamics (CFD) and chemical kinetics modeling methods are utilized to simulate turbulent reacting fluid flows in engine conditions. The dissertation comprises four journal publications targeting the modeling of dual-fuel pilot ignition system, commonly used in natural gas engines. In such an ignition concept, high-reactivity fuel (e.g. diesel) is shortly injected into a mixture of low-reactivity natural gas and air during the compression stroke. Diesel fuel autoignites and releases enough energy to initiate a premixed natural gas-air flame. Hence, the diesel spray acts similar to a large spark. The use of natural gas as a primary fuel in engines is a contemporary topic of interest from the industrial and academic point of views. Utilization of a lean natural gas-air mixture together with modern low-temperature combustion techniques may enable reductions in emission levels. However, methane, the main component of natural gas, is a harmful greenhouse gas. Incomplete combustion due to e.g. unsuccessful ignition leads to direct methane emissions, degraded thermal efficiency and increase in fuel consumption. To avoid such complications, advanced ignition system designs have been proposed, including the dual-fuel pilot ignition. The focus of the present work is on the analysis of mixture formation and autoignition characteristics in engine conditions. The present study is the first numerical investigation on dual-fuel pilot spray ignition problem by large-eddy simulation (LES) and finite-rate chemistry. A major effort was put on creating a highly efficient finite-rate chemistry solver by utilizing two open source libraries OpenFOAM and pyJack. The developed solver framework enabled the use of high-resolution (62.5 micrometers) grid and complex chemical mechanisms (e.g. 97 species and 997 reactions) in the simulations. The investigated spray setup corresponds to the Engine Combustion Network (ECN) Spray A configuration. ECN provides an open-access data repository and a forum for international experimental and numerical collaboration, enabling an extensive validation of numerical models in terms of a single-fuel diesel spray. The dual-fuel simulations offer the following novel accomplishments: 1) The first published LES study on interactive physics and chemistry of the pilot spray ignition process. 2) The applied zero-, one- and three-dimensional simulations complement one another providing profound understanding on why methane prolongs diesel ignition. 3) Low- and high-temperature combustion characteristics are assessed, indicating the importance of low-temperature reactions. 4) Consistent with experiments, the connection between spray over-leaning and retarded ignition process is explained and quantified.
  • A numerical study on combustion mode characterization for locally stratified dual-fuel mixtures
    (2020-04-01) Karimkashi, Shervin; Kahila, Heikki; Kaario, Ossi; Larmi, Martti; Vuorinen, Ville
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Combustion modes in locally stratified dual-fuel (DF) mixtures are numerically investigated for methanol/n-dodecane blends under engine-relevant pressures. In the studied constant-volume numerical setup, methanol acts as a background low-reactivity fuel (LRF) while n-dodecane serves as high-reactivity fuel (HRF), controlling local ignition delay time. The spatial distribution of n-dodecane is modeled as a sinusoidal function parametrized by stratification amplitude (Y′) and wavelength (0.01 mm<λ<15 mm). In contrast, methanol is assumed to be fully premixed with air at equivalence ratio 0.8. First, one-dimensional setup is investigated by hundreds of chemical kinetics simulations in (Y′, λ) parameter space. Further, the concepts by Sankaran et al. (2005, Proceedings of the Combustion Institute) and Zeldovich (1980, Combustion and Flame) on ignition front propagation speed are applied to develop a theoretical analysis of the time-dependent diffusion–reaction problem. The theoretical analysis predicts two combustion modes, (1) spontaneous ignition and (2) deflagrative propagation, and leads to an analytical expression for the border curve called β-curve herein. One-dimensional chemical kinetics simulations confirm the presence of two combustion modes in (Y′, λ) parameter space while the β-curve explains consistently the position of phase border observed in the simulations. Finally, the role of convective mixing is incorporated to the theoretical expression for the β-curve. The effect of convection on combustion mode is assessed by carrying out two-dimensional fully-resolved simulations with different turbulence levels. Two-dimensional numerical simulation results give evidence on combustion mode switching, which is consistent with predictions of the modified β-curve for turbulent cases. The practical output of the paper is the β-curve which is proposed as a predictive tool to estimate combustion modes for various fuels or fuel combinations.
  • Numerical study on tri-fuel combustion: Ignition properties of hydrogen-enriched methane-diesel and methanol-diesel mixtures
    (2020-02-07) Karimkashi, Shervin; Kahila, Heikki; Kaario, Ossi; Larmi, Martti; Vuorinen, Ville
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Simultaneous and interactive combustion of three fuels with differing reactivities is investigated by numerical simulations. In the present study, conventional dual-fuel (DF) ignition phenomena, relevant to DF compression ignition (CI) engines, are extended and explored in tri-fuel (TF) context. In the present TF setup, a low reactivity fuel (LRF), methane or methanol, is perfectly mixed with hydrogen and air to form the primary fuel blend at the lean equivalence ratio of 0.5. Further, such primary fuel blends are ignited by a high-reactivity fuel (HRF), here n-dodecane under conditions similar to HRF spray assisted ignition. Here, ignition is relevant to the HRF containing parts of the tri-fuel mixtures, while flame propagation is assumed to occur in the premixed LRF/H 2 containing end gas regions. The role of hydrogen as TF mixture reactivity modulator is explored. Mixing is characterized by n-dodecane mixture fraction ξ, and molar ratio x=[Formula presented]. When x < 0.6, minor changes are observed for the first- and second-stage ignition delay time (IDT) of tri-fuel compared to dual-fuel blends (x = 0). For methane, when x > 0.6, first- and second-stage IDT increase by factor 1.4–2. For methanol, a respective decrease by factor 1.2–2 is reported. Such contrasting trends for the two LRFs are explained by reaction sensitivity analysis, indicating the importance of OH radical production/consumption in the ignition process. Observations on LRF/H 2 end gas laminar flame speed (S l) indicate that S l increases with x due to the highly diffusive features of H 2. For methane, S l increase with x is more significant than for methanol.