Browsing by Author "Karimkashi, Shervin"
Now showing 1 - 20 of 23
Results Per Page
Sort Options
Item A comparative study on methanol and n-dodecane spray flames using Large-Eddy Simulation(Elsevier Science Inc., 2024-02) Kaario, Ossi Tapani; Karimkashi, Shervin; Bhattacharya, Atmadeep; Vuorinen, Ville; Larmi, Martti; Bai, Xue Song; Department of Mechanical Engineering; Energy Conversion and Systems; Lund UniversityMethanol (CH3OH) is an attractive alternative fuel that can reduce net carbon release and decrease pollutant emissions. In this study, methanol and n-dodecane spray flames were investigated using Large-Eddy Simulation (LES) and direct coupling with finite-rate chemistry. The selected ambient conditions are relevant to engines and were previously unreported for numerical methanol spray studies, i.e. high pressure (60 bar) and temperature (900 – 1200 K) with high injection pressure (1500 bar). The Engine Combustion Network (ECN) Spray A case was used to validate the n-dodecane spray flame. For methanol, a modified ECN Spray A condition was used with a high initial ambient temperature (1100 K-1200 K) to ensure fast enough ignition relevant to engine time scales. The performed homogeneous reactor (0D) simulations revealed a new phenomenon of a two-stage ignition process for methanol, confirmed by the 3D LES at high pressure, high temperature, and lean conditions. The present numerical results also show that: 1) there is a strong ambient temperature sensitivity for methanol ignition delay time (IDT) with a five-fold decrease in IDT (IDT1100K/IDT1200K=5) and a factor of 2.6 decrease in the flame lift-off length (FLOL1100K/FLOL1200K=2.6) as the ambient temperature is increased from 1100 K to 1200 K, 2) methanol spray ignition takes place at a very lean mixture (ϕMR≈0.2) consistent with the 0D predicted most reactive mixture fraction (ZMR), 3) on average, methanol sprays are significantly leaner than n-dodecane sprays at quasi-steady-state (ϕmeoh,ave≈0.2 vs ϕndod,ave≈0.7), implying very low soot emissions, and 4) the methanol spray flames could have similar temperatures as the n-dodecane sprays depending on the initial conditions, thus a similar level of NOx emissions.Item A Diagnostic Approach to Assess the Effect of Temperature Stratification on the Combustion Modes of Gasoline Surrogates(Taylor & Francis, 2023) Shahanaghi, Ali; Karimkashi, Shervin; Kaario, Ossi; Vuorinen, Ville; Tripathi, Rupali; Sarjovaara, Teemu; Department of Mechanical Engineering; Energy Conversion; Neste CorporationThermal stratification may switch the combustion mode from deflagration to spontaneous (auto-ignition) in spark-ignition engines leading to knock. Despite the available numerical and experimental works, an analytical and systematic method on the role of local thermal stratification on the combustion mode is still missing. Particularly, the effects of heat diffusion before ignition in negative temperature coefficient (NTC) chemistry of gasoline surrogates are typically ignored. In this study, an a-priori diagnostics tool is provided to separate the deflagration and auto-ignition combustion modes by considering the diffusion effects, based on two parameters: stratification wavelength (lambda) and amplitude (delta). The diagnostics tool is an extension of Zeldovich's theory to transient problems by solving the diffusion equation and considering the flame and ignition timescales. It is found that 1) The theory is valid against one-dimensional numerical simulations under different average temperatures and pressures. 2) NTC chemistry promotes spontaneous ignition at both low and high pressures. 3) In the presence of NTC chemistry, the transition region between the two combustion modes is broadened. A third blended mode is observed (spontaneous ignition assisted flame) with front speeds approximate to 6S(1). 4) Finally, estimation of the knock propensity from the generated maps is related to the surrogates' octane sensitivities.Item Effects of ethane addition on diesel-methane dual-fuel combustion in a heavy-duty engine(Elsevier BV, 2021-04-01) Ahmad, Zeeshan; Kaario, Ossi; Karimkashi, Shervin; Qiang, Cheng; Vuorinen, Ville; Larmi, Martti; Department of Mechanical Engineering; Energy ConversionThe present study is a continuation of the previous work by Ahmad et al. (2020), in which ethane (C2H6) enriched diesel-methane (CH4) dual-fuel (DF) combustion was experimentally investigated in a single-cylinder heavy-duty engine. Here, the experiments of ethane enriched DF combustion are carried out with new details together with supporting zero-dimensional (0D) and one-dimensional (1D) chemical kinetics simulations. Three port-fuel injected (PFI) gaseous blends of pure methane with varying ethane concentrations of 0%, 10%, and 20% are used as the main fuels. The PFI gaseous blend provides 97% of the total-fuel energy (TFE), which is ignited by a small 3% (TFE based) pilot diesel. Experiments are performed under lean condition (∅gas = 0.52) for two engine speeds while keeping the TFE and other operating conditions constant. Calculated results from 0D and 1D simulations under engine relevant conditions including theoretical combustion mode analysis (β-curve) are used to deepen the phenomenological understanding of the experimental results. The results reveal that adding ethane into pure methane has minor effects on the pilot-diesel ignition timing. However, ethane addition greatly enhances the ignitability of methane after the start of combustion. Ethane enriched gaseous blends yield higher thermal efficiency and reduce combustion duration compared to pure methane. According to combustion mode analysis, ethane tendency to promote spontaneous autoignition may be one of the reasons for improving overall combustion performance. It is observed that ethane enriched gaseous blends produce lower unburned methane (UB-CH4) and unburned hydrocarbons (THC) accompanied with higher nitrogen oxides (NOx) because of the higher combustion efficiency. Furthermore, ethane addition considerably helps to reduce cycle-to-cycle variations under lean conditions compared to pure methane.Item Embedded direct numerical simulation of ignition kernel evolution and flame initiation in dual-fuel spray assisted combustion(Elsevier Science Inc., 2024-01) Gadalla, Mahmoud; Karimkashi, Shervin; Kabil, Islam; Kaario, Ossi; Lu, Tianfeng; Vuorinen, Ville; Department of Mechanical Engineering; Energy Conversion and Systems; University of ConnecticutThe flame initiation process in dual-fuel spray assisted combustion is presently not fully understood. Here, diesel spray assisted combustion of premixed methane/oxidizer/EGR is explored in the post-ignition phase by scale-resolved simulations. The modified dual-fuel ECN Spray A forms the baseline configuration. An extensive local grid refinement (approaching DNS limit) around one of the first high-temperature ignition kernels is carried out in order to examine the validity of hypothesized flame initiation and deflagration. A high quality LES is used to solve the spray dynamics, while the embedded quasi-DNS (eq-DNS) region offers detailed information on the ignition kernel evolution. The finite-rate chemistry is directly integrated, utilizing 54 species and 269 reactions. Local combustion modes are investigated for the ignition kernel development toward spontaneous ignition and premixed flame propagation using various approaches, including the reaction front displacement speed, energy transport budget, and chemical explosive mode analysis. Furthermore, a new criterion based on reaction flux analysis is introduced, which is compatible with dual-fuel combustion. The spatial and temporal scales associated with the ambient methane consumption and consequent flame initiation are characterized. For the first time in dual-fuel spray assisted simulations, numerical evidence is provided on the initiation of premixed flames, and the corresponding timescale is reported. Particularly, there is a transient mixed-mode combustion phase of approximately 0.2 ms after the spray second stage ignition wherein extinction, ignition fronts, and quasi-deflagrative structures co-exist. After such a transient period, the combustion mode becomes essentially deflagrative. Finally, interactions between turbulence and premixed flame front are characterized mostly in the corrugated regime.Item Experimental and Numerical Study of a Low-Pressure Hydrogen Jet under the Effect of Nozzle Geometry and Pressure Ratio(SAE International, 2023-04-11) Yeganeh, Maryam; Rabensteiner, Samuel; Karimkashi, Shervin; Cheng, Qiang; Kaario, Ossi; Larmi, Martti; Department of Mechanical Engineering; Energy Conversion and Systems; Energy Conversion and SystemsHydrogen (H2), a potential carbon-neutral fuel, has attracted considerable attention in the automotive industry for transition toward zero-emission. Since the H2 jet dynamics play a significant role in the fuel/air mixing process of direct injection spark ignition (DISI) engines, the current study focuses on experimental and numerical investigation of a low-pressure H2 jet to assess its mixing behavior. In the experimental campaign, high-speed z-type schlieren imaging is applied in a constant volume chamber and H2 jet characteristics (penetration and cross-sectional area) are calculated by MATLAB and Python-based image post-processing. In addition, the Unsteady Reynolds-Averaged Navier-Stokes (URANS) approach is used in the commercial software Star-CCM+ for numerical simulations. The H2 jet dynamics is investigated under the effect of nozzle geometry (single-hole, double-hole, and multiple-hole (5-hole)), which constitutes the novelty of the present research, and pressure ratio (PR = injection pressure (Pi) / chamber pressure (Pch)). The results show that the H2 jet from the single-hole nozzle possesses the fastest penetration and smallest cross-sectional area. On the contrary, the H2 jet from the double-hole nozzle possesses the slowest penetration and largest cross-sectional area. The H2 jet from the multiple-hole nozzle shows characteristics between those of the single-hole and double-hole. Overall, since higher pressure ratio and larger jet cross-sectional area lead to higher uniformity of the fuel/air mixture, high-pressure injection with the double-hole nozzle seems more advantageous to attain efficient mixing.Item Experimental investigations of hydrogen pre-ignition phenomenon induced by two different lubricating oils in a rapid compression expansion machine(Elsevier Ltd, 2024-01) Yeganeh, Maryam; Rönn, Kristian; Karimkashi, Shervin; Cheng, Qiang; Hlaing, Ponnya; Hyvönen, Jari; Vuorinen, Ville; Kaario, Ossi; Larmi, Martti; Department of Mechanical Engineering; Energy Conversion and Systems; Department of Mechanical Engineering; Wärtsilä CorporationThe growing interest in utilizing hydrogen (H2) as a zero-carbon fuel has ignited extensive research on its potential application within internal combustion engines (ICEs). However, a major challenge regarding H2 ICEs is the pre-ignition phenomenon. Various factors, including hot spots, oil droplets/deposits, and the ignition system, contribute to pre-ignition. This study focuses on pre-ignition caused by engine lubricating oil droplets/deposits. A Rapid Compression Expansion Machine (RCEM), equipped with optical access is employed to conduct a comparative analysis of the pre-ignition characteristics of two distinct engine lubricating oils called oil A and oil B. Oil A is an API (American Petroleum Institute) Group II lubricating oil with Ca (calcium) detergents, while oil B is of API Group V with a combination of Mg (magnesium) and Ca (calcium) components. The study identifies pre-ignition limits for both oils across various air-to-fuel ratios (λ = 2, 2.5, 3) and compression ratios (ɛ = 11-14.5). Comparative assessments are also performed through a chemical analysis (homogenous constant volume ignition delay time (IDT) simulations) as well as investigating the cylinder pressure and heat release rate (HRR) curves for the tested lubricating oils. This study represents the first exploration of the H2 pre-ignition phenomenon in response to different engine lubricating oils within the context of an RCEM. The findings reveal that oil A is more susceptible to pre-ignition. In contrast, oil B exhibits pre-ignition at higher ɛ while maintaining constant λ. Simultaneously, oil B displays an accelerated flame propagation and more robust combustion due to the incidence of pre-ignition at higher ɛ. Hence, in the context of H2 ICEs, the Group V oil sample (oil B) presents itself as a more advantageous choice compared to the Group II oil sample (oil A) for alleviating undesirable pre-ignition.Item Experimental study of hydrogen jet dynamics : Investigating free momentum and impingement phenomena(Elsevier Ltd, 2024-05-28) Yeganeh, Maryam; Akram, Muhammad Saad; Cheng, Qiang; Karimkashi, Shervin; Kaario, Ossi; Larmi, Martti; Department of Mechanical Engineering; Energy Conversion and Systems; Department of Mechanical EngineeringThere is a growing interest in the utilization of hydrogen (H2), as a zero-carbon fuel, in internal combustion engines (ICEs). Accordingly, the primary focus of this study is to investigate low-pressure H2 jet dynamics, which play a vital role in air-fuel mixing especially in direct injection (DI) engines. High-speed z-type schlieren imaging is employed in a constant volume chamber to study the effect of nozzle geometry (single-hole, double-hole, and multi-hole), pressure ratios (PR = injection pressure (Pi)/chamber pressure (Pch)), injection angle (10°, 15°, and 20°), and injection duration (ID) on the H2 jet characteristics. Image post-processing is executed in MATLAB and Python to extract the H2 jet characteristics, including penetration and cross-sectional area. The novelty stems from the comprehensive investigation of H2 jet dynamics and impingement phenomenon under various engine-like conditions. The results indicate that apart from the fact that higher pressure ratios (PRs) improve the air-fuel mixing, the single-hole nozzle induces the fastest H2 jet penetration and the smallest cross-sectional area. Conversely, the double-hole nozzle leads to the slowest penetration and the most expansive cross-sectional area. The performance of the multi-hole nozzle falls between that of the single-hole and double-hole nozzles. Additionally, changing the injection angle results in jet-piston impingement at the periphery, leading to higher H2 concentration in those areas. This negatively affects the formation of an optimal air-fuel mixture. It is also found that changing the injection duration (ID) has no noticeable impact on the H2 jet's behavior.Item Fast reactive flow simulations using analytical Jacobian and dynamic load balancing in OpenFOAM(American Institute of Physics Publising LLC, 2022-02-01) Morev, Ilya; Tekgül, Bulut; Gadalla, Mahmoud; Shahanaghi, Ali; Kannan, Jeevananthan; Karimkashi, Shervin; Kaario, Ossi; Vuorinen, Ville; Department of Mechanical Engineering; Energy ConversionDetailed chemistry-based computational fluid dynamics (CFD) simulations are computationally expensive due to the solution of the underlying chemical kinetics system of ordinary differential equations (ODEs). Here, we introduce a novel open-source library aiming at speeding up such reactive flow simulations using OpenFOAM, an open-source software for CFD. First, our dynamic load balancing model by Tekgül et al. [“DLBFoam: An open-source dynamic load balancing model for fast reacting flow simulations in OpenFOAM,” Comput. Phys. Commun. 267, 108073 (2021)] is utilized to mitigate the computational imbalance due to chemistry solution in multiprocessor reactive flow simulations. Then, the individual (cell-based) chemistry solutions are optimized by implementing an analytical Jacobian formulation using the open-source library pyJac, and by increasing the efficiency of the ODE solvers by utilizing the standard linear algebra package. We demonstrate the speed-up capabilities of this new library on various combustion problems. These test problems include a two-dimensional (2D) turbulent reacting shear layer and three-dimensional (3D) stratified combustion to highlight the favorable scaling aspects of the library on ignition and flame front initiation setups for dual-fuel combustion. Furthermore, two fundamental 3D demonstrations are provided on non-premixed and partially premixed flames, viz., the Engine Combustion Network Spray A and the Sandia flame D experimental configurations, which were previously considered unfeasible using OpenFOAM. The novel model offers up to two orders of magnitude speed-up for most of the investigated cases. The openly shared code along with the test case setups represent a radically new enabler for reactive flow simulations in the OpenFOAM framework.Item Hyperspectral image reconstruction from colored natural flame luminosity imaging in a tri-fuel optical engine(Nature Publishing Group, 2023-12) Cheng, Qiang; Karimkashi, Shervin; Ahmad, Zeeshan; Kaario, Ossi; Vuorinen, Ville; Larmi, Martti; Department of Mechanical Engineering; Energy Conversion and SystemsThe detection of chemiluminescence from various radicals and molecules in a hydrocarbon flame can provide valuable information on the rate of local heat release, combustion stability, and combustion completeness. In this study, chemiluminescence from the combustion process is detected using a high-speed color camera within the broadband spectrum of visible light. Whereon, a novel hyperspectral reconstruction approach based on the physically plausible spectral reconstruction (PPSR) is employed to reconstruct the spectral chemiluminescence signals from 400 to 700 nm with a resolution of 10 nm to provide 31 different spectral channels. The reconstructed key chemiluminescence signals (e.g., CH*, CH2O*, C2*, and CO2*) from the color images are further analyzed to characterize the chemical kinetics and combustion processes under engine conditions. The spectral chemiluminescence evolution with engine crank angle is identified to comprehend the effect of H2 fraction on flame characteristics and combustion kinetics. Additionally, in this study, a detailed kinetic mechanism is adopted to deepen the theoretical understanding and describe the spectral chemiluminescence from H2/CH4 and H2/CH4/n-dodecane flames at relevant conditions for various species including OH*, CH*, C2*, and CO2*. The results indicate that the PPSR is an adequately reliable approach to reconstructing spectral wavelengths based on chemiluminescence signals from the color images, which can potentially provide qualitative information about the evolution of various species during combustion. Here, the reconstructed chemiluminescence images show less than 1% errors compared to the raw images in red, green, and blue channels. Furthermore, the reconstructed chemiluminescence trends of CH*, CH2O*, C2*, and CO2* show a good agreement with the detailed kinetics 0D simulation.Item Inner Flame Front Structures and Burning Velocities of Premixed Turbulent Planar Ammonia/Air and Methane/Air Flames(SPRINGER, 2022-08) Tamadonfar, Parsa; Karimkashi, Shervin; Kaario, Ossi; Vuorinen, Ville; Department of Mechanical Engineering; Energy ConversionAmmonia (NH3) has attracted interest as a future carbon-free synthetic fuel due to its economic storage and transportation. In this study, quasi direct numerical simulations (quasi-DNS) with detailed-chemistry have been performed in 3-D to examine the flame thickness and assess the validity of Damkohler's first hypothesis for premixed turbulent planar ammonia/air and methane/air flames under different turbulence levels. The Karlovitz number is systematically changed from 4.26 to 12.06 indicating that all the test conditions are located within the thin reaction zones combustion regime. Results indicate that the ensemble average values of the preheat zone thickness deviate slightly from the thin laminar flamelet assumption, while the reaction zone regions remain relatively intact. Following the balance equation of reaction progress variable gradient, normal strain rate and the tangential diffusion component of flame displacement speed variation in the normal direction to the flame surface are found to be responsible for thickening the flame. However, the sum of reaction and normal diffusion components of flame displacement speed variation in the normal direction to the flame surface is in charge of flame thinning for ammonia/air and methane/air flames. In addition, the validity of Damkohler's first hypothesis is confirmed by indicating that the ratio of the turbulent burning velocity to the unstrained premixed laminar burning velocity is relatively equal to the ratio of the wrinkled to the unwrinkled flame surface area. Furthermore, the probability density functions of the density-weighted flame displacement speed show that the bulk of flame elements propagate identical to the unstrained premixed laminar flame.Item Large eddy simulation of diesel spray–assisted dual-fuel ignition: A comparative study on two n-dodecane mechanisms at different ambient temperatures(SAGE Publications Ltd, 2020-08-10) Kannan, Jeevananthan; Gadalla, Mahmoud; Tekgül, Bulut; Karimkashi, Shervin; Kaario, Ossi; Vuorinen, Ville; Department of Mechanical Engineering; Energy ConversionIn dual-fuel compression ignition engines, a high-reactivity fuel, such as diesel, is directly injected to the engine cylinder to ignite a mixture of low-reactivity fuel and air. This study targets improving the general understanding on the dual-fuel ignition phenomenon using zero-dimensional homogeneous reactor studies and three-dimensional large eddy simulation together with finite-rate chemistry. Using the large eddy simulation framework,n-dodecane liquid spray is injected into the lean ambient methane-air mixture at phi=0.5. The injection conditions have a close relevance to the Engine Combustion Network Spray A setup. Here, we assess the effect of two different chemical mechanisms on ignition characteristics: a skeletal mechanism with 54 species and 269 reaction steps (Yao mechanism) and a reduced mechanism with 96 species and 993 reaction steps (Polimi mechanism). Altogether three ambient temperatures are considered: 900, 950, and 1000 K. Longer ignition delay time is observed in three-dimensional large eddy simulation spray cases compared to zero-dimensional homogeneous reactors, due to the time needed for fuel mixing in three-dimensional large eddy simulation sprays. Although ignition is advanced with the higher ambient temperature using both chemical mechanisms, the ignition process is faster with the Polimi mechanism compared to the Yao mechanism. The reasons for differences in ignition timing with the two mechanisms are discussed using the zero-dimensional and three-dimensional large eddy simulation data. Finally, heat release modes are compared in three-dimensional large eddy simulation according to low- and high-temperature chemistry in dual-fuel combustion at different ambient temperatures. It is found that Yao mechanism overpredicts the first-stage ignition compared to Polimi mechanism, which leads to the delayed second-stage ignition in Yao cases compared to Polimi cases. However, the differences in dual-fuel ignition for Polimi and Yao mechanisms are relatively smaller at higher ambient temperatures.Item Large-eddy simulation of diesel pilot spray ignition in lean methane-air and methanol-air mixtures at different ambient temperatures(SAGE PUBLICATIONS, 2023-03) Karimkashi, Shervin; Gadalla, Mahmoud; Kannan, Jeevananthan; Tekgül, Bulut; Kaario, Ossi; Vuorinen, Ville; Department of Mechanical Engineering; Energy ConversionIn dual-fuel compression-ignition engines, replacing common fuels such as methane with renewable and widely available fuels such as methanol is desirable. However, a fine-grained understanding of diesel/methanol ignition compared to diesel/methane is lacking. Here, large-eddy simulation (LES) coupled with finite rate chemistry is utilized to study diesel spray-assisted ignition of methane and methanol. A diesel surrogate fuel (n-dodecane) spray is injected into ambient methane-air or methanol-air mixtures at a fixed lean equivalence ratio phi(LRF) = 0.5 at various ambient temperatures (T-amb = 900, 950, 1000 K). The main objectives are to (I) compare the ignition characteristics of diesel/methanol with diesel/methane at different T-amb, (2) explore the relative importance of low-temperature chemistry (LTC) to high-temperature chemistry (HTC), and (3) identify the key differences between oxidation reactions of n-dodecane with methane or methanol. Results from homogeneous reactor calculations as well as 3 + 3 LES are reported. For both DF configurations, increasing T-amb leads to earlier first- and second-stage ignition. Methanol/n-dodecane mixture is observed to have a longer ignition delay time (IDT) compared to methane/n-dodecane, for example approximate to three times longer IDT at T-amb = 950 K. While the ignition response of methane to T-amb is systematic and robust, the T-amb window for n-dodecane/methanol ignition is very narrow and for the investigated conditions, only at 950 K robust ignition is observed. For methanol at T-amb = 1000 K, the lean ambient mixture autoignites before spray ignition while at T-amb = 900 K full ignition is not observed after 3 ms, although the first-stage ignition is reported. For methanol, LTC is considerably weaker than for methane and in fully igniting cases, heat release map analysis demonstrates the dominant contribution of HTC to total heat release rate for methanol. Reaction sensitivity analysis shows that stronger consumption of OH radicals by methanol compared to methane leads to the further delay in the spray ignition of n-dodecane/methanol. Finally, a simple and novel approach is developed to estimate IDT in reacting LES using zero-dimensional IDT calculations weighted by residence time from non-reacting LES data.Item Large-Eddy Simulation of ECN Spray A(MDPI AG, 2020-07-01) Gadalla, Mahmoud; Kannan, Jeevananthan; Tekgul, Bulut; Karimkashi, Shervin; Kaario, Ossi; Vuorinen, Ville; Department of Mechanical EngineeringIn this study, various mixing and evaporation modeling assumptions typically considered for large-eddy simulation (LES) of the well-established Engine Combustion Network (ECN) Spray A are explored. A coupling between LES and Lagrangian particle tracking (LPT) is employed to simulate liquidn-dodecane spray injection into hot inert gaseous environment, wherein Lagrangian droplets are introduced from a small cylindrical injection volume while larger length scales within the nozzle diameter are resolved. This LES/LPT approach involves various modeling assumptions concerning the unresolved near-nozzle region, droplet breakup, and LES subgrid scales (SGS) in which their impact on common spray metrics is usually left unexplored despite frequent utilization. Here, multi-parametric analysis is performed on the effects of (i) cylindrical injection volume dimensions, (ii) secondary breakup model, particularly Kelvin-Helmholtz Rayleigh-Taylor (KHRT) against a no-breakup model approach, and (iii) LES SGS models, particularly Smagorinsky and one-equation models against implicit LES. The analysis indicates the following findings: (i) global spray characteristics are sensitive to radial dimension of the cylindrical injection volume, (ii) the no-breakup model approach performs equally well, in terms of spray penetration and mixture formation, compared with KHRT, and (iii) the no-breakup model is generally insensitive to the chosen SGS model for the utilized grid resolution.Item Large-eddy simulation of split injection strategies in RCCI conditions(TAYLOR & FRANCIS, 2022-04-16) Tekgul, Bulut; Karimkashi, Shervin; Kaario, Ossi; Kahila, Heikki; Lendormy, Eric; Hyvönen, Jari; Vuorinen, Ville; Department of Mechanical Engineering; Energy Conversion; Wärtsilä Finland OyIn 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.Item Large-eddy simulation of spray assisted dual-fuel ignition under reactivity-controlled dynamic conditions(Elsevier BV, 2021-06-01) Tekgül, Bulut; Kahila, Heikki; Karimkashi, Shervin; Kaario, Ossi; Ahmad, Zeeshan; Lendormy, Éric; Hyvönen, Jari; Vuorinen, Ville; Department of Mechanical Engineering; Energy Conversion; Wärtsilä CorporationHere, 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.Item Large-eddy simulation of tri-fuel combustion: Diesel spray assisted ignition of methanol-hydrogen blends(PERGAMON-ELSEVIER SCIENCE LTD, 2021-06-15) Gadalla, Mahmoud; Kannan, Jeevananthan; Tekgul, Bulut; Karimkashi, Shervin; Kaario, Ossi; Vuorinen, Ville; Department of Mechanical Engineering; Energy ConversionDevelopment of marine engines could largely benefit from the broader usage of methanol and hydrogen which are both potential energy carriers. Here, numerical results are presented on tri-fuel (TF) ignition using large-eddy simulation (LES) and finite-rate chemistry. Zero-dimensional (0D) and three-dimensional (3D) simulations for n-dodecane spray ignition of methanol/hydrogen blends are performed. 0D results reveal the beneficial role of hydrogen addition in facilitating methanol ignition. Based on LES, the following findings are reported: 1) Hydrogen promotes TF ignition, significantly for molar blending ratios β X = [H 2]/([H 2]+[CH 3OH]) ≥0.8. 2) For β X = 0, unfavorable heat generation in ambient methanol is noted. We provide evidence that excessive hydrogen enrichment (β X ≥ 0.94) potentially avoids this behavior, consistent with 0D results. 3) Ignition delay time is advanced by 23–26% with shorter spray vapor penetrations (10–15%) through hydrogen mass blending ratios 0.25/0.5/1.0. 4) Last, adding hydrogen increases shares of lower and higher temperature chemistry modes to total heat release.Item Large-eddy simulation of tri-fuel ignition: diesel spray-assisted ignition of lean hydrogen–methane–air mixtures(TAYLOR & FRANCIS, 2021-04-16) Kannan, Jeevananthan; Gadalla, Mahmoud; Tekgül, Bulut; Karimkashi, Shervin; Kaario, Ossi; Vuorinen, Ville; Department of Mechanical Engineering; Energy ConversionWe present 3D numerical results on tri-fuel (TF) combustion using large-eddy simulation and finite rate chemistry. The TF concept was recently introduced by Karimkashi et al. (Int. J. Hydrogen Energy, 2020) in 0D. Here, the focus is on spray-assisted ignition of methane–hydrogen blends. The spray acts as a high-reactivity fuel (HRF) while the ambient premixed methane-hydrogen blend acts as a low-reactivity fuel (LRF) mixture. Better understanding on such a TF process could enable and motivate more extensive hydrogen usage in e.g. compression ignition marine engines where spray-assisted dual-fuel (DF) combustion is already utilised. The studied spray set-up is based on the modified ECN Spray A case, see Kahila et al. (Combustion and Flame, 2019) for DF combustion. The ambient pressure and temperature are (Formula presented.) 900 K and (Formula presented.) 60 bar. The hydrogen content of the LRF blend is varied systematically by changing the molar fraction (Formula presented.), (Formula presented.). The main added value of the study is that we extend the TF concept to 3D. The particular findings of the study are as follows: 1) Consistent with Karimkashi et al. 2020, hydrogen delays ignition also in 3D and the effect becomes significant for (Formula presented.). 2) The ratio between the first- and second-stage ignition delay times (Formula presented.) and (Formula presented.). Furthermore, the ratio between 3D and 0D ignition delay times is given as (Formula presented.) for all TF cases. 3) Finally, consistent with Karimkashi et al. 2020, also in 3D the high-temperature combustion heat release mode is shown to appear stronger in TF than the low-temperature combustion mode compared to DF methane–diesel combustion.Item Numerical evidence on deflagration fronts in a methane/n-dodecane dual-fuel shear layer under engine relevant conditions(Elsevier BV, 2023-07-15) Kannan, Jeevananthan; Karimkashi, Shervin; Gadalla, Mahmoud; Kaario, Ossi; Vuorinen, Ville; Department of Mechanical Engineering; Energy Conversion and SystemsTo date, high resolution spray-assisted dual-fuel (DF) studies have focused on capturing the ignition process while the subsequent post-ignition events have been largely neglected due to modeling requirements and high computational cost. Here, we use a simplified approach for studying ignition front evolution after ignition. Three-dimensional scale-resolved simulations of igniting shear layers (0≤Re≤1500) are studied to better understand reaction fronts in engine-relevant conditions. We carry out quasi-DNS in a DF combustion setup consisting of premixed n-dodecane/methane/air/EGR at 700K as a fuel stream and premixed methane/air as the oxidizer at a pressure of 60 atmospheres and an ambient temperature of 900 K. The flow solution resolution is δ/10, where δ=laminar flame thickness. The present study primarily focuses on the hypothesized flame formation and its characterization. Under these conditions, the simulations indicate two-stage ignition further leading to reaction front initiation and dual-fuel flame establishment. For Re<1500, a reaction front resembling DF deflagration is demonstrated close to the auto-ignition timescales. At Re=1500, mixing effects promote more rapid dilution and the DF deflagration front formation is slightly delayed although still observed. For the first time, at rather short timescales of 0.2−0.4 IDT (ignition delay time) after the ignition, we provide numerical evidence on DF deflagration front emergence in shear-driven DF combustion processes via 3D numerical simulations for 0Item A numerical study on combustion mode characterization for locally stratified dual-fuel mixtures(ELSEVIER SCIENCE INC, 2020-04-01) Karimkashi, Shervin; Kahila, Heikki; Kaario, Ossi; Larmi, Martti; Vuorinen, Ville; Department of Mechanical Engineering; Energy ConversionCombustion 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.Item A numerical study on premixed laminar ammonia/air flames enriched with hydrogen : An analysis on flame–wall interaction(Elsevier Science Inc., 2024-07) Tamadonfar, Parsa; Karimkashi, Shervin; Zirwes, Thorsten; Vuorinen, Ville; Kaario, Ossi; Department of Mechanical Engineering; Energy Conversion and Systems; University of StuttgartAmmonia (NH3) has been recognized as a potential carbon-free synthetic fuel of the near future. To enhance its low reactivity, one practical option is to blend it with hydrogen (H2). In this study, the transient head-on quenching of premixed laminar ammonia/air flames enriched with hydrogen is explored based on numerical simulations using detailed chemistry and the mixture-averaged transport model. In this respect, nine different test cases are studied for blending ratios (the molar ratio of hydrogen to the ammonia/hydrogen mixture) of 0.0, 0.2, and 0.4, equivalence ratios of 0.8, 1.0, and 1.2, wall temperatures of 300, 500, and 750 K, and pressures of 1, 2, and 5 atm. The results reveal that the quenching distance (maximum absolute wall heat flux) decreases (increases) with increasing the blending ratio, the equivalence ratio, the wall temperature, and the pressure. For all the test cases, the quenching Peclet number changes between 1 and 3.5. In addition, the local heat release rate enhancement and the role of radical recombination reactions are highlighted at the time of quenching in the vicinity of the wall. This effect is augmented as the blending ratio, the equivalence ratio, and the wall temperature increase. Furthermore, the results show that the N2 pathway is the dominant pathway in consumption of NO near the wall at the time of quenching, in which R76 (NH2+NO⇔N2+H2O) poses the rate controlling role. In addition, the leading role of R85 (NH+NO⇔N2O+H) in consuming NO and converting it to N2O is highlighted at the time of quenching near the wall. Moreover, the significant roles of molecular diffusion and reaction source terms over convection are discussed for both NO and N2O species transport in the vicinity of the wall. Novelty and significance statement: The significance of this work is that using ammonia and its blends with hydrogen as promising carbon-free fuels for the future has several technical issues/uncertainties, which needs to be addressed fundamentally. One of the relatively unexplored issues in the combustion devices for ammonia/hydrogen/air flames is the head-on quenching phenomenon, which is investigated in detail in this study. The novelty of this work is that, for the first time, (1) the head-on quenching of various ammonia/hydrogen blends is systematically studied numerically, employing detailed chemistry, (2) effect of wall on the heat release rate chemistry is discussed wherein the role of radical recombination reactions is highlighted, and (3) the formation pathways of pollutant emissions (NO and N2O) for such flames are thoroughly investigated in the freely propagating flame scenario and in the vicinity of the wall at the time of quenching.
