Browsing by Author "Karimkashi, Shervin, Dr., Aalto University, Finland"

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    Computational fluid dynamics studies on spray assisted combustion of alternative energy carriers
    (Aalto University, 2023) Gadalla, Mahmoud; Kaario, Ossi, Assoc. Prof., Aalto University, Finland; Karimkashi, Shervin, Dr., Aalto University, Finland; Konetekniikan laitos; Department of Mechanical Engineering; Energy Conversion Research Group; Insinööritieteiden korkeakoulu; School of Engineering; Vuorinen, Ville, Prof., Aalto University, Department of Mechanical Engineering, Finland
    This dissertation belongs to the research fields of computational combustion physics and chemistry, with a primary focus on the modeling and simulation of diesel spray assisted combustion of alternative energy carriers. Computational fluid dynamics and combustion modeling are utilized within an open-source software framework, to investigate complex fluid flow phenomena involving turbulence, phase change, and chemical reactions under engine-relevant conditions. The dissertation comprises four research articles, investigating the combustion characteristics of various alternative fuels (namely methane, methanol, and hydrogen) when ignited by pilot diesel spray. To cope with the ongoing worldwide decarbonization strategies, utilization of alternative fuels together with researching advanced combustion concepts are sought after. In that regard, spray assisted dual-fuel (DF) combustion is amongst the low temperature combustion (LTC) technologies which relies primarily on low reactivity fuel (LRF), or blend of different fuels, to deliver the main energy in the combustion system. The premixed LRF-air charge is then ignited by a directly injected high reactivity fuel (HRF) such as diesel spray. Due to the adopted lean-burn concept of the premixed charge, LTC is attained with better fuel economy and cleaner emissions. Moreover, early introduction of the LRF into the combustion chamber increases the mixture homogeneity, and thereby mitigates soot formation. Presently, the physicochemical characteristics of DF combustion processes are not well understood for alternative LRFs. Such aspects are further explored in this dissertation. Regarding the numerical framework, large-eddy simulation is utilized for turbulence modeling, Lagrangian-Eulerian coupling for liquid transport and phase change, and the direct integration of finite-rate chemistry for combustion modeling. The Spray A target conditions from the Engine Combustion Network are considered a baseline for simulations, whereas the mixture composition is modified to account for LRF(s) in the spray assisted DF configuration. Throughout the dissertation, the performed simulations (0D, 1D and 3D), the post-processing and analyses are all retained within an open-source software environment. While Publication I revisited the numerical framework and modeling assumptions for evaporating sprays, Publication II investigated the ignition characteristics of methanol, compared with methane, when ignited by a pilot diesel spray. The challenges identified therein, for methanol ignition under DF configuration, were further addressed in Publication III. There, hydrogen enrichment to the premixed charge was proposed as a chemical remedy to facilitate methanol ignition, while extending the operational window. Finally, in Publication IV the focus was shifted to beyond the HRF spray autoignition, wherein local numerical microscopy was conducted to investigate the combustion mode development between autoignition and premixed flame initiation and deflagration.  The main findings of the dissertation are as follows. In Publication I, the fluid dynamical setup is validated and the sensitivity of the modeling assumptions is assessed. In Publication II, for DF spray assisted ignition of methanol, the operational window to achieve smooth ignition is observed to be narrow. In Publication III, hydrogen enrichment as LRF in the previous setup is shown to extend the operational limits, by advancing ignition delay and mitigating ambient reactivity. In Publication IV, we present world's first numerical evidence on emerging deflagration fronts in dual-fuel spray assisted combustion for diesel-methane blends utilizing embedded fully resolved numerical simulations. The present dissertation pioneers high-fidelity numerical investigations on alternative fuel spray-assisted combustion problems.
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    Numerical modeling of spray-assisted dual- and tri- fuel combustion processes
    (Aalto University, 2023) Kannan, Jeevananthan; Kaario, Ossi, Prof., Aalto University, Finland; Karimkashi, Shervin, Dr., Aalto University, Finland; Konetekniikan laitos; Department of Mechanical Engineering; Energy Conversion Research Group; Insinööritieteiden korkeakoulu; School of Engineering; Vuorinen, Ville, Prof., Aalto University, Department of Mechanical Engineering, Finland
    This dissertation is related to the research areas of computational physics and numerical modeling of combustion. The study intends to determine the ignition characteristics of dual-fuel (DF) and tri-fuel (TF) sprays when a high-reactivity fuel spray (n-dodecane) is mixed with a low-reactivity fuel (methane/hydrogen/or its blends) and an oxidizer/EGR in a hot ambient environment. In particular, engine-relevant operating conditions are investigated. A better understanding of the ignition phenomena could lead to improved ignition control, improved thermal efficiency, and lower emissions. By utilizing computational fluid dynamics, numerical combustion modeling and high-performance computing, detailed investigations of the three-dimensional physics and chemistry of such reacting flows can be studied. The present dissertation is based on three journal publications. Large-Eddy Simulation (LES) and quasi-DNS approaches are used in combination with finite rate chemistry and OpenFOAM for CFD simulations. In Publication I, Yao and Polimi reduced mechanisms were studied to determine the effect of temperature on DF spray ignition and methane's inhibition of n-dodecane chemistry. Publication II discusses the use of the tri-fuel (TF) strategy with diesel spray-assisted ignition in mixtures of methane and hydrogen. The ignition characteristics and heat release rate of TF sprays have been investigated in engine-relevant conditions. In Publication III, the DF ignition studies are extended to better understand the combustion progression after ignition. In general Publication I- Publication II are related to understanding spray-assisted ignition phenomena while Publication III focuses on understanding the evolution of ignition fronts to deflagration using a simplified approach based on a three-dimensional reacting shear layer. The main conclusions of this dissertation are as follows: 1) Methane inhibits n-dodecane spray IDT at low temperatures, especially in the simulation of DF sprays at various ambient temperatures relevant to the engine conditions. Moreover, this behavior has been numerically confirmed to be similar with both Yao and Polimi reduced chemical mechanisms. 2) As hydrogen is added to the ambient methane in the same DF setup, it becomes a TF setup, wherein n-dodecane's ignition characteristics are delayed even further than in the DF setup. Moreover, the high-temperature combustion heat release mode in TF appears more pronounced than in the low-temperature combustion mode, in comparison with methane-diesel combustion in DF. 3) The numerical simulation of shear layer-driven dual-fuel combustion processes allows for the numerical evidence of the emergence of deflagration fronts in dual-fuel combustion within a short time interval, 0.2 to 0.4 IDT after the ignition.