Vasileios David Sakellarakis


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Sakellarakis

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Vasileios David

ORCID

0000-0002-1511-1462

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2020 - 20213
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Publications 1 - 3 of 3
  • Mixing and Autoignition of Underexpanded Methane Jets at High Pressure Conditions
    Item type: Doctoral Thesis
    Sakellarakis, Vasileios David (2020)
    Dual-fuel internal combustion engines with high-pressure direct injection of natural gas into the cylinder and ignition assist via a diesel pilot offer reduced emissions of carbon dioxide, particulate matter and nitrogen oxides compared to standard diesel engines, while achieving higher thermal efficiency and power output than natural gas engines operating in other modes, while additionally eliminating problems of incomplete combustion at part load, including substantial methane slip. The fundamentals of high pressure gaseous jets, however, are relatively poorly understood. The objective of this thesis to obtain insight into the development, mixture formation and autoignition of underexpanded methane jets at high pressure conditions. To this end, a multi-dimensional computational framework for consistent treatment of single-phase flow with real-gas thermodynamics is established and verified against high fidelity simulation results from extensively validated solvers. The significance of real-gas corrections is also assessed. Subsequently, Reynolds-Averaged Navier-Stokes simulations with the newly established framework are validated against Schlieren and Tracer Laser Induced Fluorescence measurements in an optically accessible constant volume chamber, equipped with a prototype gas injector. For injection pressures up to 300bar and for supercritical pressure ratios up to 10, non-reactive jet development is examined in terms of global jet characteristics, such as mass flow rates, jet tip penetration and jet volume, and compared to scaling laws from literature. The influence of parametric variations in injection pressure, chamber pressure and pressure ratio on both macroscopic and local mixture formation is investigated and interpreted qualitatively with the aid of those laws. In a third step, the autoignition of underexpanded methane jet at injection pressures between 200 and 500bar and at back pressures between 40 to 125bar is investigated numerically. Particular care is taken through a two-stage workflow to simultaneously ensure accurate chemical kinetics, inclusion of real-gas effects, sufficient resolution for the complex shock structures and reliable description of turbulence-chemistry interaction with the Conditional Moment Closure combustion model. The predicted ignition delays are conceptually interpreted and quantified as an interplay of jet reactivity and of jet aerodynamics, whereas ignition locations show much lower variability. A semi-empirical correlation for autoignition delay as a function of chamber temperature, chamber pressure, injection pressure and injection temperature is ultimately constructed, which compares favourably to independent experimental measurements from literature and shows modest improvements over the simpler Arrhenius model.
  • The effect of high-pressure injection variations on the mixing state of underexpanded methane jets
    Item type: Journal Article
    Sakellarakis, Vasileios David; Vera-Tudela, Walter; Doll, Ulrich; et al. (2021)
    International Journal of Engine Research
    This work presents a joint experimental and numerical study of global characteristics and mixing behavior of underexpanded methane jets at high-pressure conditions in a Constant Volume Chamber. Injection pressures of 200, 250, and 300 bar and pressure ratios of 4, 5, 6, 8, and 10 at each of those pressures have been investigated. Tracer LIF with acetone as tracer has been applied to experimentally quantify the mixing of methane and quiescent air. In order to exploit the symmetry of the configuration, accompanying simulations have been carried out in Reynolds-Averaged Navier-Stokes framework with the k –w SST turbulence model and real-gas modelling based on the Soave-Redlich-Kwong Equation of State has been employed to account for high-pressure corrections in thermodynamic and caloric properties. The experiments confirm the hyperbolic decay of axial fuel concentration and the Gaussian shape of traverse concentration profiles in the self-similar region of the jets, while simulation results match well with experimentally determined fuel concentration fields. It is found that scaling laws proposed in literature for steady-state jet propagation can qualitatively interpret the effect of injection variations on jet tip penetration and volume. Increasing pressure ratio at fixed injection pressure leads to the formation of slightly richer jets, with slightly smaller mass percentage in the range of air-to-fuel ratios most favorable to autoignition. By contrast, increasing pressure ratio at fixed chamber pressure leads to virtually identical Probability Distribution Functions of local air-to-fuel ratios and the same is observed when employing a fixed pressure ratio at higher pressure levels.
  • Numerical investigation of the autoignition of underexpanded methane jets
    Item type: Journal Article
    Sakellarakis, Vasileios David; Wright, Yuri; Vera Tudela Fajardo, Walter Martin; et al. (2021)
    Fuel
    This work constitutes a numerical investigation of the autoignition of underexpanded methane jets at high-pressure conditions. Injection pressures from 125 to 500 bar, back pressures from 40 to 125 bar, and pressure ratios between 2.5 and 10 have been targeted. The aim is to identify the effect of the main control variables of gas injection on the autoignition delay and location. To this end, Reynolds-Averaged Navier-Stokes simulations with the model have been carried out for five broad parametric variations. The computational domain represents a Constant Volume Cell with a prototype gas injector. A two-stage workflow enables proper thermodynamic treatment of the conservation equations with real-gas modeling, sufficient resolution for shock structures in the near nozzle area, detailed kinetics described by the San Diego mechanism and treatment of turbulence-chemistry interaction with elliptic Conditional Moment Closure model. The results are interpreted conceptually as an interplay of jet reactivity, effectively described by an exponential dependence on ambient temperature and a power law dependence on ambient pressure, and of jet aerodynamics, empirically described by a quadratic dependence on pressure ratio. Injection temperature is introduced by defining an appropriate characteristic system temperature and a correlation is constructed, whose predictions are juxtaposed against a modified Arrhenius model and measurements from independent experimental studies in literature. The effect of injection variations on ignition location in physical space is also examined.
Publications 1 - 3 of 3