Modelling of diesel fuel injection processes.

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Study of Multiphase Flow at the Suction of Screw Compressor

Screw compressors are commonly used for industrial and commercial gas processing and refrigeration. These machines are known to be able to admit mixtures of gasses and liquids to a certain concentration. In oil injected compressors, oil is mostly injected in the working domain to seal, cool and lubricate. But would the injection of atomized oil or other liquid in the suction of the compressor be useful for better control of the discharge temperature and reduction in energy consumption, is still to be determined. Similarly, liquid neutral to the process may be injected in an oil free compressor suction to help controlling discharge temperature. It can be erosive and corrosive to the compressor rotors. Therefore mapping a two phase suction flow of a screw compressor may help in understanding the means to improve compressors efficiency and reliability. This paper is the initial phase of PhD program to determine the multiphase flow characteristic at suction of twin screw compressors by means of experimental techniques. Review of most common and up to date measurement techniques in field of multiphase flow was carried out to determine their suitability and feasibility. Also Modelling of single and multiphase flow at the suction domain of a twin screw compressor were performed in order to have a better understanding of flow distribution. The research is performed on an oil free screw compressor with “N” rotor profiles of 128 mm and configuration of 3/5 lobes with L/D of 1.6 and 93 mm centre distance. A simplified CFD model of only suction domain which reduces computational time was compared with the CFD model of the entire compressor and it was found that it predicts most of flow features with same accuracy. The experimental study which will be used to validate the CFD model has been presented

Instantaneous and ensemble average cavitation structures in Diesel micro-channel flow orifices

Cavitation developing upstream and inside the micro-channel orifices of transparent multi-hole fuel injector nozzles has been characterised using a high speed visualisation system. Images have been obtained with short exposure time and sufficiently high spatial and temporal resolution, freezing the formation of cavitation bubbles and their further development during every single injection cycle. Post processing of statistically large number of images collected from many successive injection events and numerous identical nozzles has allowed for the first time estimation of the ensemble average cavitation image and its standard deviation. The instantaneous images reveal the formation of a variety of complex and interacting two-phase flow regimes. Vapour bubbles have been found to exist inside the nozzle prior to start of injection. These bubbles originate from the previous injection cycle as they have not been evacuated from the nozzle and hence, they remain trapped, altering the flow of the subsequent injection event. During the opening and closing stages of the needle valve that controls the fuel flow through the nozzle, cavitation is found to form in the valve’s seat area. Subsequently, vortex or ‘string’ cavitation has been recorded to take place in a rather chaotic manner; its life time and most probable location of appearance have been estimated. The ensemble average images reveal the probability of cavitation appearance at a specific location within the nozzle and the micro-channel flow orifice. The standard deviation from the mean reveals locations with significant cycle-to-cycle variations of the flow. These are linked to significant deviations from the mean of the fuel spray dispersion angle forming downstream of the nozzle exit

Numerical investigation of the aerodynamic breakup of diesel droplets under various gas pressures

[EN] Abstract The present study investigates numerically the aerodynamic breakup of Diesel droplets for a wide range of ambient pressures encountered in engineering applications relevant to oil burners and internal combustion engines. The numerical model solves the Navier-Stokes equations coupled with the Volume of Fluid (VOF) methodology utilized for capturing the interface between the liquid and the surrounding gas. An adaptive local grid refinement technique is used to increase the accuracy of the numerical results around the interface. The Weber (We) numbers examined are in the range of 14 to 279 which correspond to bag, multimode and sheet-thinning breakup regimes. Model results are initially compared against published experimental data and show a good agreement in predicting the drop deformation and the different breakup modes. The predicted breakup initiation times for all cases lie within the theoretical limits given by empirical correlations based on the We number. Following the model validation, the effect of density ratio on the breakup process is examined by varying the gas density (or equivalently the ambient pressure), while the We number is kept almost constant equal to 270; ambient gas pressure varies from 1 up to 146bar and the corresponding density ratios (ε) range from 700 down to 5. Results indicate that the predicted breakup mode of sheet-thinning remains unchanged for changing the density ratio. Useful information about the instantaneous drag coefficient (Cd) and surface area as functions of the selected non-dimensional time is given. It is shown that the density ratio is affecting the drag coefficient, in agreement with previous numerical studies.Financial support from the MSCA-ITN-ETN of the European Union’s H2020 programme, under REA grant agreement n. 675676 is acknowledged.Stefanitsis, D.; Malgarinos, I.; Strotos, G.; Nikolopoulos, N.; Kakaras, E.; Gavaises, M. (2017). Numerical investigation of the aerodynamic breakup of diesel droplets under various gas pressures. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 1052-1059. https://doi.org/10.4995/ILASS2017.2017.4690OCS1052105

Compressible simulations of bubble dynamics with central-upwind schemes

This paper discusses the implementation of an explicit density-based solver, that utilises the central-upwind schemes for the simulation of cavitating bubble dynamic flows. It is highlighted that, in conjunction with the Monotonic Upstream-Centered Scheme for Conservation Laws (MUSCL) scheme they are of second order in spatial accuracy; essentially they are high-order extensions of the Lax–Friedrichs method and are linked to the Harten Lax and van Leer (HLL) solver family. Basic comparison with the predicted wave pattern of the central-upwind schemes is performed with the exact solution of the Riemann problem, for an equation of state used in cavitating flows, showing excellent agreement. Next, the solver is used to predict a fundamental bubble dynamics case, the Rayleigh collapse, in which results are in accordance to theory. Then several different bubble configurations were tested. The methodology is able to handle the large pressure and density ratios appearing in cavitating flows, giving similar predictions in the evolution of the bubble shape, as the reference

Quantitative predictions of cavitation presence and erosion-prone locations in a high-pressure cavitation test rig

Experiments and numerical simulations of cavitating flow inside a single-orifice nozzle are presented. The orifice is part of a closed flow circuit, with diesel fuel as the working fluid, designed to replicate the main flow pattern observed in high-pressure diesel injector nozzles. The focus of the present investigation is on cavitation structures appearing inside the orifice, their interaction with turbulence and the induced material erosion. Experimental investigations include high-speed shadowgraphy visualization, X-ray micro-computed tomography (micro-CT) of time-averaged volumetric cavitation distribution inside the orifice as well as pressure and flow rate measurements. The highly transient flow features that are taking place, such as cavity shedding, collapse and vortex cavitation (also known as ‘string cavitation’), have become evident from high-speed images. Additionally, micro-CT enabled the reconstruction of the orifice surface, which provided locations of cavitation erosion sites developed after sufficient operation time. The measurements are used to validate the presented numerical model, which is based on the numerical solution of the Navier–Stokes equation, taking into account compressibility of both the liquid and liquid–vapour mixture. Phase change is accounted for with a newly developed mass transfer rate model, capable of accurately predicting the collapse of vaporous structures. Turbulence is modelled using detached eddy simulation and unsteady features such as cavitating vortices and cavity shedding are observed and discussed. The numerical results show agreement within validation uncertainty with the obtained measurements

A numerical study on the effect of cavitation erosion in a Diesel injector

The consequences of geometry alterations in a Diesel injector caused by cavitation erosion are inves-tigated with numerical simulations. The differences in the results between the nominal design geometryand the eroded one are analyzed for the internal injector flow and spray formation. The flow in the in-jector is modeled with a 3–phase Eulerian approach using a compressible pressure–based multiphase flowsolver. Cavitation is simulated with a non–equilibrium mass transfer rate model based on the simplifiedform of the Rayleigh–Plesset equation. Slip velocity between the liquid–vapor mixture and the air isincluded in the model by solving two separate momentum conservation equations. The eroded injector isfound to result to a loss in the rate of injection but also lower cavitation volume fraction inside the nozzle.The injected sprays are then simulated with a Lagrangian method considering as initial conditions thepredicted flow characteristics at the exit of the nozzle. The obtained results show wider spray dispersionfor the eroded injector and shorter spray tip penetration