| 1. |
- Giramondi, Nicola, 1991-
(författare)
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Diffusive Combustion of Ethanol in a Dual-Fuel Direct Injection Compression Ignition Engine
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The impact of climate change due to global warming necessitates rapid and extensive measures to enhance the sustainability of the energy and transport sectors. In this context, there are large environmental and societal benefits to be gained by replacing diesel with renewable fuels for road freight transport. This solution may facilitate and expedite the transition towards fossil-free, carbon-neutral transport, while the electrification process takes shape. Short-chain alcohol fuels have favorable properties for the enhancement of engine performance and the abatement of pollutant emissions, however, they necessitate ignition aid systems in compression ignition engines. The present research investigates a novel concept of dual-fuel combustion for heavy-duty compression ignition engine applications by means of engine tests and three-dimensional combustion simulations. This concept involves the direct injection of pure ethanol as main fuel through a centrally mounted injector, and minimal quantities of diesel as pilot fuel via a separate injector. The objective is to achieve diffusive combustion of ethanol in a process analogous to conventional diesel combustion throughout the entire engine load range, with a higher thermal efficiency and lower pollutant emissions. Single-cylinder engine tests were carried out to evaluate the influence of combustion characteristics and performance with respect to dual-injection strategy, engine load, ethanol ratio and configuration of the diesel pilot injector. The characteristics and performance of ethanol-diesel direct injection compression ignition (DICI) combustion were compared to two sets of baselines, that are conventional diesel combustion and dual-injections of diesel via the main and pilot injector in the same proportion as in the dual-fuel test points. At low load conditions, increasing the separation between the diesel pilot and ethanol main injection enabled the achievement of diffusive combustion of ethanol, avoiding combustion instability and partial misfire thanks to minimal quantities of diesel injected. At high load conditions, a minimum main-pilot separation was instead required to limit the degree of ethanol premixing at ignition. Using a diesel pilot injector having a lower number of sprays with a wider hole diameter promoted a more robust ignition of ethanol, while also causing a reduction of engine performance. Parallel to the experimental work, three-dimensional combustion simulations were carried out in order to investigate the interaction between diesel and ethanol sprays during ignition at various engine operating conditions, from low to full load. At the operating conditions investigated during engine tests, the ignition of a subset of ethanol sprays was locally triggered by the contact with the products of diesel combustion. Subsequently, ignition propagated towards the neighboring ethanol sprays, until reaching the furthest ones from the diesel pilot injector. The coupling between experimental and numerical results highlighted the noteworthy predictive capability of the adopted combustion model with respect to the ethanol combustion characteristics. In conclusion, the present research work provides a solid starting point for future studies on diffusive combustion of alcohol fuels in compression ignition engines. The structured knowledge built in the course of the doctoral project lays the foundation for the development of a fuel-flexible engine for heavy-duty applications.
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| 2. |
- Bodin, Olle
(författare)
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Simulations of compressible flows associatedwith internal combustion engines
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Vehicles with internal combustion (IC) engines fueled by hydrocarbon compoundshave been used for more than 100 years for ground transportation.During these years and in particular the last decade, the environmental aspectsof IC engines have become a major political and research topic. Followingthis interest, the emissions of pollutants such as NOx, CO2 and unburnedhydrocarbons (UHC) from IC engines have been reduced considerably.Yet, there is still a clear need and possibility to improve engine efficiencywhile further reducing emissions of pollutants. The maximum efficiency ofIC engines used in passenger cars is no more than 40% and considerably lessthan that under part load conditions. One way to improve engine efficiencyis to utilize the energy of the exhaust gases to turbocharge the engine. Whileturbocharging is by no means a new concept, its design and integration intothe gas exchange system has been of low priority in the power train designprocess. One expects that the rapidly increasing interest in efficient passengercar engines would mean that the use of turbo technology will become morewidespread.The flow in the IC-engine intake manifold determines the flow in the cylinderprior and during the combustion. Similarly, the flow in the exhaust manifolddetermines the flow into the turbine, and thereby the efficiency of theturbocharging system.In order to reduce NOx emissions, exhaust gas recirculation (EGR) is used.As this process transport exhaust gases into the cylinder, its efficiency is dependenton the gas exchange system in general. The losses in the gas exchangesystem are also an issue related to engine efficiency. These aspects have beenaddressed up to now rather superficially. One has been interested in globalaspects (e.g. pressure drop, turbine efficiency) under steady state conditions.In this thesis, the flow in the exhaust port and close to the valve as wellas in the exhaust manifold is studied. Since the flow in the port can be transonic,we study first the numerical modeling of such a flow in a more simplegeometry, namely a bump placed in a wind tunnel. Large-Eddy Simulationsof internal transonic flow have been carried out. The results show that transonicflow in general is very sensitive to small disturbances in the boundaryconditions. Flow in the wind tunnel case is always highly unsteady in the transonicflow regime with self excited shock oscillations and associated with that also unsteady boundary-layer separation. The interaction between separationzone and shock dynamics was carried out by one-, and two-point correlationsas well as dynamic mode decomposition (DMD). A clear connection betweenseparation bubble dynamics and shock oscillation was found. To investigatesensitivity to periodic disturbances the outlet pressure in the wind tunnel casewas varied periodically at rather low amplitude. These low amplitude oscillationscaused hysteretic behavior in the mean shock position and appearance ofshocks of widely different patterns.The study of a model exhaust port shows that at realistic pressure ratios,the flow is transonic in the exhaust port. Furthermore, two pairs of vortexstructures are created downstream of the valve plate by the wake behind thevalve stem and by inertial forces and the pressure gradient in the port. Thesestructures dissipate rather quickly. The impact of these structures and thechoking effect caused by the shock on realistic IC engine performance remainsto be studied in the future.The flow in a heavy-duty exhaust manifold was studied under steady andengine-like boundary conditions. At all conditions, significantly unsteady flowis generated in the manifold and at the inlets to the turbine and EGR cooler.The inflow to the turbine is dominated by a combination of the blow-downpulse coming from one cylinder, and the scavenging pulse from another at thefiring frequency.
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| 3. |
- Fjällman, Johan, 1982-
(författare)
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Unsteady simulations of the turbulent flow in the exhaust system of an IC-engine for optimal energy utilization
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This licentiate thesis deals with the ow in pipe bends and radial turbines inan internal combustion engine environment. Looking into the engine bay of apassenger car one cannot avoid noticing all the pipe bends and splits. Duringthe development of internal combustion engines the engine manufacturers arestarting to focus more on simulations than on experiments. This is partly becauseof the reduction in cost but also the reduction in turn around time. This isone of the reasons for the need of more accurate and predictive simulations.By using more complex computational methods the accuracy and predictivecapabilities are increased. The downside is that the computational time isincreasing so the long term goal of the project is to use the results to improvethe predictive capability of the lower order methods used by the industry.By comparing experiments, Reynolds Averaged Navier-Stokes (RANS)simulations, and Large Eddy Simulations (LES), the accuracy of the simulationmethods can be established. The advantages of using LES over RANS for the ows under consideration stems from the unsteadiness of the ow in the enginemanifolds. When such unsteadiness overlaps the natural turbulent spectrum,general RANS models cannot handle the problem specic ow. The thesisconsiders this eect on the chosen numerical model. The LES results have beenshown to be more accurate than the RANS simulations both for global meanvalues and for the uctuating components. The LES calculations have provento predict the mean
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| 4. |
- Jacob, Stefan, Dr. 1980-, et al.
(författare)
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Acoustic scattering in a small centrifugal compressor based on the use of linearized equations in a rotating frame
- 2023
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Ingår i: Journal of Sound and Vibration. - : Elsevier. - 0022-460X .- 1095-8568. ; 544, s. 117315-117315
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Tidskriftsartikel (refereegranskat)abstract
- Numerical solutions of acoustic wave scattering are often used to describe sound propagation through complex geometries. For cases with flow, various forms of the convected equation have been used. A better alternative that includes vortex-sound interaction is instead to use the linearized and harmonic forms of the unsteady fluid flow governing equations. In this paper, a formulation of the linearized equations that include rotational effects, in an acoustic computation using a rotating frame of reference in a stationary geometry, is presented. We demonstrate that rotational effects can be important, e.g., when computing the transmission loss through high-speed compressors. The implementation of the proposed addition to the existing schemes is both simple and numerically inexpensive. The results are expected to have an impact on the research and development related to noise control of high-performance turbo-machinery, e.g., used in automotive or aviation applications at operating conditions that can be represented by steady background flows.
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| 5. |
- Majal, Ghulam Mustafa
(författare)
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Flow dynamics in corrugated pipes: Effect on particle agglomeration
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In recent times, the deleterious impact of particulate emissions from road vehicles on the well-being of humans has garnered a significant amount ofattention. Regulatory bodies have enacted legislation in order to counter act particulate emissions. In order to meet the proposed legislative requirementsthe engineers have to come up with a suitable after treatment treatment technology. It has been understood that typically the emitted particles are distributed such that the particles largest in terms of number are the smallest in size. Recent legislations have increasingly emphasized controlling boththe particle mass and number of the emissions. The present thesis employs a numerical approach in order to study the transport of particles in an exhaust flow. A particle agglomeration concept is considered as a means to shift the particle size distribution. In order to increase the likelihood of interactionbetween the particles the host gas is manipulated in such a way that it successively accelerates and decelerates.As a starting point a 1D model is considered. Extensive parameter studies are performed in order to determine the appropriate flow characteristics,which promote particle grouping. The term grouping refers to particles moving together in a group or a cluster, thereby increasing the likelihood foragglomerating with one another. It is revealed that higher pulse frequency and geometric wavelength promotes particle grouping. A comparison between different pulse shapes highlighted that smoother pulse shapes are marginally better for particle grouping. Finally, it is observed that an idealized sinusoidalpulse form underestimated the extent of particle grouping when compared to an actual engine pulse.In the next step, a 3D computational fluid dynamics (CFD) study is performed on an idealized axi-symmetric sinusoidal pipe-like geometry. The geometrical parameters of the geometry are based on the earlier completed 1D study. Firstly, for a continuous inlet flow scenario the observed flow structures are highlighted. Proper orthogonal decomposition (POD) revealed that for a geometry with a sufficiently large maximum cross sectional diameteran asymmetric oscillatory mode is present. This mode is caused by the dynamics of the recirculation bubble and the shear layer. In a geometry witha much smaller maximum cross sectional diameter only axial modes driven by the shear layer are observed. In the case of pulsatile inflow conditions an additional axial mode, driven by the pulsation frequency, is observed on top of the earlier observed asymmetric oscillatory mode. Particles are later added into this geometry and it is observed that most of the particles have a small residence time within the geometry under both continuous and pulsatile inflow conditions. This is attributed to the narrow recirculation regions within the idealized pipe-like geometry. A second geometry that is utilized in this study is the experimental corrugated pipe-like geometry. Particles are injected again under continuous andpulsatile inflow conditions for this geometry. Earlier experimental work on this experimental pipe prototype revealed that there is no significant difference in the particle distribution obtained using a straight pipe and an experimental pipe prototype. The numerical study conducted revealed that despite the presence of large recirculation zones, within the cavities of the pipe, most of the particles tended to pass through contracted regions of the pipe without entering the recirculation zones.
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| 6. |
- Pietroniro, Asuka Gabriele, et al.
(författare)
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Effects of Boundary Layer and Local Volumetric Cells Refinements on Compressor Direct Noise Computation
- 2022
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Ingår i: SAE Technical Papers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International.
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Konferensbidrag (refereegranskat)abstract
- The use of turbochargers with downsized internal combustion engines improves road vehicles’ energy efficiency but introduces additional sound sources of strong acoustic annoyance on the turbocharger’s compressor side. In the present study, direct noise computations (DNC) are carried out on a passenger vehicle turbocharger compressor. The work focuses on assessing the influence of grid parameters on the acoustic predictions, to further advance the maturity of the acoustic modelling of such machines with complex three-dimensional features. The effect of the boundary layer mesh structure, and of the spatial resolution of the mesh, on the simulated acoustic signatures is investigated on detached eddy simulations (DES). Refinements in the core mesh are applied in areas of major acoustic production, to generate cells with sizes proportional to the local Taylor microscale values. Such an educated guess allows for quality enhancement with a smaller increase in computational costs as compared to more general overall refinements. The reflection-free simulation results are validated against experiments. The experimental data were post-processed with methods from the two-port theory to represent pure acoustic source power density for the acoustic modes, cleaned from test-domain-specific reflections. A detailed comparison between experiments and numerical simulations is carried out. As a result of this study, the most critical parameters for the numerical prediction of turbocharger noise are presented. The results can, furthermore, be used to improve the understanding of grid construction when predicting noise signature for compressor flows.
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