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Assessment of transient effects in diesel injectors affected by fouling and cavitation erosion

Kovolos, K. (2021). Assessment of transient effects in diesel injectors affected by fouling and cavitation erosion. (Unpublished Doctoral thesis, City, University of London)

Abstract

An explicit density-based solver of the compressible Navier-Stokes (NS) and energy conservation equations has been developed and implemented in the open-source CFD code OpenFOAM®®; the flow solver is combined with two thermodynamic closure models for the liquid, vapor and vapor liquid equilibrium (VLE) property variation as function of pressure and temperature. The first is based on tabulated data for a 4-component Diesel fuel surrogate, derived from the Perturbed-Chain, Statistical Associating Fluid Theory (PC-SAFT) Equation of State (EoS), allowing for thermal effects to be quantified. The second thermodynamic closure is based on the widely used barotropic Equation of State (EoS) approximation between density and pressure and neglects viscous heating. The Wall Adapting Local Eddy viscosity (WALE) LES model was used to resolve sub-grid scale turbulence while a cell-based mesh deformation Arbitrary Lagrangian–Eulerian (ALE) formulation is used for modelling the injector’s needle valve movement. Numerical predictions of the fuel heating and cavitation erosion location indicators occurring during the opening and closing periods of the needle valve inside a five-hole common rail Diesel fuel injector are presented. Model predictions are found in close agreement against 0-D estimates of the temporal variation of the fuel temperature difference between the feed and hole exit during the injection period. Two mechanisms affecting the temperature distribution within the fuel injector have been revealed and quantified. The first is ought to wall friction-induced heating, which may result to local liquid temperature increase up to fuel’s boiling point while superheated vapor is formed. At the same time, liquid expansion due to the depressurisation of the injected fuel results to liquid cooling relative to the fuel’s feed temperature; this is occurring at the central part of the injection orifice. The formed spatial and temporal temperature and pressure gradients induce significant variations in the fuel density and viscosity, which in turn, affect the formed coherent vortical flow structures. It is found, in particular, that these affect the locations of cavitation formation and collapse, that may lead to erosion of the surfaces of the needle valve, sac volume and injection holes. Model predictions are compared against corresponding X-ray surface erosion images obtained from injector durability tests, showing good agreement.

Further, investigation of the fuel heating, vapor amount formation and cavitation erosion location patterns occurring during the early opening period of the needle valve (from 2μm to 80μm) inside a five-hole common rail Diesel fuel injector discharging at 180MPa, 350MPa and 450MPa, are presented. These have been obtained using an explicit density-based solver of the compressible Navier-Stokes (NS) and energy conservation equations; the flow solver is combined with tabulated property data for a 4-component Diesel fuel surrogate, derived from the Perturbed-Chain, Statistical Associating Fluid Theory (PC-SAFT) Equation of State (EoS), allowing for the significant variation of the fuel’s physical and transport properties to be quantified. The Wall Adapting Local Eddy viscosity (WALE) LES model was used to resolve sub-grid scale turbulence while a cell-based mesh deformation Arbitrary Lagrangian–Eulerian (ALE) formulation is used for modelling the injector’s needle valve movement. Emphasis is placed on the temperature and vapor volume fraction evolution in needle seat passage. Friction-induced heating has been found to increase significantly with increasing pressure drop, especially at needle valve lifts from 2μm to 40μm. At the same time, liquid cooling is occurring due to fuel expansion at the areas of bulk flow away from walls; up to 25 degrees local fuel temperature drop relative to the fuel’s feed temperature are calculated. As the needle valve reaches 80 μm the fuel vapor volume, the average temperature into this flow passage and at the exit of the orifice converge to the same values for all injection pressures. The extreme injection pressures induce fuel’s jet velocity magnitude of the order of 1100 m/s, which in turn, affect the formation of coherent vortical flow structures into the nozzle’s sac volume. It is found, in particular, that the fuel jet velocity variations with increasing discharge pressure, affect the locations of cavitation formation and collapse, which in turn, lead to different potential locations of erosion of the surface of the needle valve.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TJ Mechanical engineering and machinery
Departments: Doctoral Theses
School of Science & Technology
School of Science & Technology > Engineering > Mechanical Engineering & Aeronautics
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