Spray breakup modeling taking into account asymmetric flow conditions

Nagy, Imre Gergely

dc.contributor.author	Nagy, Imre Gergely	en
dc.date.accessioned	2021-11-29T09:37:23Z
dc.date.available	2021-11-29T09:37:23Z
dc.identifier.uri	https://dspace.lib.ntua.gr/xmlui/handle/123456789/54111
dc.identifier.uri	http://dx.doi.org/10.26240/heal.ntua.21809
dc.relation	This research has been funded by the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme FP7/2007-2013/ under Research Executive Agency (REA) grant agreement 607214	el
dc.rights	Default License
dc.subject	Ροή Σπρέι	el
dc.subject	Ναυτικοί Κινητήρες	el
dc.subject	Προσομοίωση Μεγάλων Δινών	el
dc.subject	Διάσπαση	el
dc.subject	Μη Συμμετρική Ροή	el
dc.subject	Sprays	en
dc.subject	Marine Engines	en
dc.subject	LES	en
dc.subject	Breakup	en
dc.subject	Asymmetry	en
dc.title	Spray breakup modeling taking into account asymmetric flow conditions	en
dc.contributor.department	Division of Marine Engineering	el
heal.type	doctoralThesis
heal.generalDescription	In this thesis, a new simulation approach, accounting for the asymmetric and axisymmetric spray formation in large two-stroke marine Diesel engines, was developed, and mainly applied for nonevaporating sprays.	en
heal.classification	Marine Engineering	en
heal.language	en
heal.access	free
heal.recordProvider	ntua	el
heal.publicationDate	2021-07
heal.abstract	Emission regulations and requirements for increased economic operation of marine Diesel engines have become compulsory in modern engine design. Emission and fuel consumption reductions are currently the primary goals in marine Diesel engine development. Recent studies show that the contribution of ships to global emissions of CO2, NOx, and SOx is substantial, corresponding to about 2.5%, 10-15%, and 3-7%, respectively (Eyring et al. (2007) [1], ABS (2018) [2], United Nations (2019) [3]). The international shipping industry is facing an increasingly tighter regulatory frame, with strong pressure from policymakers, especially in terms of limits imposed upon emissions to the atmosphere. Diesel engines are still the main prime movers of ships, while there is at present a broad discussion regarding alternative fuels and ship propulsion technologies (DNV GL (2018) [4]; Wik & Niemi (2016) [5]). In all cases, marine engines and aftertreatment systems should be optimized to meet legislation requirements, while minimizing fuel consumption. To decrease fuel consumption and emission, it is indispensable to optimize the combustion process. The overall performance of marine Diesel engines depends critically on the injection system and the resulting fuel atomization. Several studies have described the symmetric spray formation and influence of fuel injection processes on combustion. However, in marine Diesel engines, it is typical for orifices to be arranged eccentrically with respect to the central bore axis of the injector, thus creating a highly asymmetric spray structure. The widely-used models in the automotive sector give an indication regarding marine Diesel engine processes. Nonetheless, the spray dynamics in a marine compression-ignition engine differs substantially from automotive engines; thus the existing models cannot comprehensively describe the spray processes. The literature on modeling high-pressure non-symmetric Diesel sprays is still extremely limited. The present dissertation is thus an attempt to characterize in detail asymmetric sprays for conditions representative of large marine Diesel engines, using Computational Fluid Dynamics, with a focus on nonevaporating conditions. A new methodology for modeling Diesel sprays in large 2-stroke marine Diesel engines is proposed. The study is supported by experiments. Three representative nozzle layouts are considered: a noneccentric nozzle, a nozzle of medium eccentricity, and a highly eccentric nozzle. First, RANS and LES simulations were performed for different large two-stroke marine Diesel engine atomizer geometries, analyzing the in-nozzle flow. The influence of time discretization and of the initial and boundary conditions on the computed flow field was assessed. A description of the grid requirements and generation was provided, and the importance of proper resolution in LES was discussed. Simulations predicted non-uniform velocity magnitude distribution in the nozzle bores for all nozzle geometries investigated. Consequently, the spray primary breakup zone was analyzed by coupled RANS - LES simulations. Simulation results revealed a highly asymmetric spray behavior for the different nozzle layouts. The resulting spray structures were analyzed, and it was illustrated that the eccentric arrangement of the nozzle results in a deflection normal to the main spray direction. The deflection was found to increase with nozzle eccentricity. The present results also showed that the spray was not just deflected in the spanwise direction, but it also deviated from its symmetry line upwards, in the radial direction. Next, the phenomenon of cavitation was considered in the in-nozzle flow, for different nozzle geometries. Earlier investigations have shown that the strongly asymmetrically and eccentrically arranged nozzle bores of the fuel injectors of large two-stroke marine Diesel engines can lead to undesirable spray deflections that provoke increased levels of component temperature, emissions, and fuel consumption. To investigate the origin of these spray deviations, experiments were performed with diesel fuel in a constant volume spray combustion chamber at Winterthur Gas & Diesel Ltd. Impingement measurements were executed to characterize the nozzle performance and validate CFD simulations. Computational results for the cavitating in-nozzle flow and the evaluated momentum flux were compared against experiments, demonstrating a good qualitative agreement in terms of the cavitation patterns and differences lower than 6% for the momentum flux. A method to investigate the spray structures in primary breakup was introduced. The effect of the role of liquid core on the droplet formation was assessed. A new droplet identification method was introduced, to analyze the droplets that appeared in the vicinity of the core during primary breakup. β-PDF functions were generated for droplet location, velocity, and mass, in properly defined segments, in order to be used as an input in CFD simulations of the spray secondary breakup, in a Lagrangian description of droplets. In the LES simulations of primary breakup, the droplets generated by different atomizer layouts were identified; it was found that the conventional noneccentric nozzle generated the highest number of droplets, while the most eccentric nozzle yielded the smallest number of parent droplets. The droplets resulting from the nozzle of medium eccentricity were characterized by the highest values of Sauter Mean Diameter (SMD); similar SMD values were calculated for the other two nozzles. To further understand and characterize the spray structure and dynamics during the primary breakup, a 3-D Proper Orthogonal Decomposition (POD) analysis was implemented. Characterization, employing the velocity and fuel concentration fields of the asymmetric spray jets emanating from large two-stroke marine Diesel engine injectors calculated by LES, was performed. Hereby, the “method of snapshots” was applied. Simulation results enlightened the role of Kelvin-Helmholtz instabilities in disintegrating the spray liquid core, generating ligaments and large droplets during the primary breakup. The first POD mode qualitatively provided a very good estimate of the flow pattern of the ensemble average. The results demonstrated that the flow dynamics can be represented by a few dominant modes; thus, the flow field can be reconstructed by including the 4-5 most energetic modes of a spatial structure reflecting the asymmetric character of the spray flow. Analysis of time coefficients of the POD modes has shown that they are characterized by dominant frequencies representative of turbulent axisymmetric jets. The β-PDF functions generated by analyzing the LES results of primary breakup for the one noneccentric and the two eccentric nozzles were used as input for the spray secondary breakup calculations. CFD results were compared against experiments in the large spray combustion chamber, as well as with CFD simulations using a conventional (URANS-only) approach. The computational results using the present approach illustrate the asymmetric structure of sprays, even for the case of noneccentric nozzle, which is associated with a nonaxisymmetric flow at the nozzle tip, and is the outcome of in-nozzle flow. In all cases, the results properly accounted for the spray morphology and yielded good predictions of important quantities, such as the spray penetration length, and the spray cone and deflection angles, as verified by comparison with experiments. The results were superior to those of a conventional (URANS-only) approach (which cannot account for nonaxisymmetric spray structure). The present computational study has shown that spray deflection increased with nozzle eccentricity. Furthermore, the spray cone angle also increased with nozzle eccentricity. This resulted in a higher effective spray area, yielding an increased intensity of spray breakup, and thus a decreased penetration length. Overall, the CFD approach of the present study accurately predicted asymmetric sprays of large marine engines, while maintaining the computational cost at an affordable level. An approach for extending the applicability of the new computational framework was introduced and tested at different injection pressure levels. In particular, LES results at a given injection pressure are properly adapted for another pressure, and are used as input for secondary breakup URANS simulations of nonevaporating sprays. The comparison between CFD and experimental results has shown a good agreement regarding the spray tip penetration, for all nozzle layouts investigated. At each pressure level, the new approach outperformed the conventional approach of URANS-only simulations, improving important global parameters as the spray cone angle, quantified both at the horizontal and at the vertical mid-surface. In a final step, the new CFD simulation approach developed in this study was applied for reactive sprays, and first results were compared against new experiments in the spray combustion chamber, in terms of ignition delay. For that, the experimental setup was properly modified, and experiments with injection from a single hole were performed. The present experiments have shown that the reactive sprays exhibited deflections in their structure, similar to those of nonevaporating sprays. The predicted values of ignition delay time exhibited the same trends as those of the experiments. The presentation of work is organized in 12 chapters: Chapter 1 discusses the present status regarding emissions, emission regulations and alternative fuels pertinent to the marine industry, and provides a general introduction to the technology of large 2-stroke marine Diesel engines. A detailed discussion on governing equations and modeling of turbulent flows is given in Chapter 2. In Chapter 3, the physics and simulation techniques of Diesel sprays are discussed. Chapter 4 summarizes the in-nozzle flow and spray primary breakup simulation results. Computational results for cavitating in-nozzle flow are presented in Chapter 5. A detailed characterization of spray structure in the primary breakup regime is presented, on the basis of LES results, in Chapter 6. Results of 3-D Proper Orthogonal Decomposition (POD) analysis of spray flow in the primary breakup zone are presented and discussed in Chapter 7. Chapter 8 presents computational results of spray secondary breakup, utilizing proper input from the LES primary breakup results. Spray characterization uses calculation of proper global parameters, and comparison of their values against experiments. Furthermore, an approach for extending the applicability of the present modeling framework is presented. Experiments and first simulation results of reactive spray flow are discussed in Chapter 9. The novelty of the present Thesis is highlighted in Chapter 10. In Chapter 11, the main conclusions of the present study are summarized. Suggestions for future work are presented in Chapter 12. Finally, the list of references is provided.	en
heal.sponsor	This research has received funding from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/2007-2013/ under REA grant agreement n° 607214.	en
heal.advisorName	Kaiktsis, Lambros	en
heal.committeeMemberName	Kaiktsis, Lambros	en
heal.committeeMemberName	Founti, Maria	en
heal.committeeMemberName	Triantafyllou, George	en
heal.committeeMemberName	Kyrtatos, Nikolaos	en
heal.committeeMemberName	Papadopoulos, Christos	en
heal.committeeMemberName	Bouris, Dimitrios	en
heal.committeeMemberName	Tomboulides, Ananias	en
heal.academicPublisher	Εθνικό Μετσόβιο Πολυτεχνείο. Σχολή Ναυπηγών Μηχανολόγων Μηχανικών	el
heal.academicPublisherID	ntua
heal.numberOfPages	201
heal.fullTextAvailability	false