35 research outputs found
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Numerical investigations on bubble-induced jetting and shock wave focusing: application on a needle-free injection
The formation of a liquid jet into air induced by the growth of a laser-generated bubble inside a needle-free device is numerically investigated by employing the compressible Navier–Stokes equations. The three co-existing phases (liquid, vapour and air) are assumed to be in thermal equilibrium. A transport equation for the gas mass fraction is solved in order to simulate the non-condensable gas. The homogeneous equilibrium model is used in order to account for the phase change process between liquid and vapour. Thermodynamic closure for all three phases is achieved by a barotropic Equation of State. Two-dimensional axisymmetric simulations are performed for a needle-free device for which experimental data are available and used for the validation of the developed model. The influence of the initial bubble pressure and the meniscus geometry on the jet velocity is examined by two different sets of studies. Based on the latter, a new meniscus design similar to shaped-charge jets is proposed, which offers a more focused and higher velocity jet compared to the conventional shape of the hemispherical gas–liquid interface. Preliminary calculations show that the developed jet can penetrate the skin and thus, such configurations can contribute towards a new needle-free design
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Numerical simulation of three-phase flow in an external gear pump using immersed boundary approach
This paper presents a three-phase fully compressible model applied along with an immersed boundary model for predicting cavitation occurring in a two dimensional gear pump in the presence of non-condensable gas (NCG). Combination of these models is capable of overcoming numerical challenges such as modelling the contact between the gears and simulating the effect of NCG in cavitation. The model accounting for the effect of NCG also has broader applicability, since gas dissolved in liquids can come out of the solution when exposed to low pressures; this plays a significant role in the pump performance and cavitation erosion. Here the simulation results are presented for the gear pump at different operating conditions including the contact between gear, gear RPM and % of NCG; their effects on performance and cavitation is demonstrated. The results suggest that modelling the contact between the gears play a role in the cavitation prediction inside the gear pump. An increase in cavitation is observed when the contact is modelled even for the small pressure difference considered between the inlet and outlet. An increase in the RPM of the gears also results in increased cavitation within the pump, whereas an increase in the percentage of NCG content by a small amount can reduce the cavitation to a greater extent. This reduction is due to the expansion of the gas at a lower pressure which recovers the pressure and prevents or delays the phase-change process of the working fluid. The fluctuations in the outflow rate is also found to increase when the gears are in contact and also with increasing gas content
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Modelling cavitation during drop impact on solid surfaces
The impact of liquid droplets on solid surfaces at conditions inducing cavitation inside their volume has rarely been addressed in the literature. A review is conducted on relevant studies, aiming to highlight the differences from non-cavitating impact cases. Focus is placed on the numerical models suitable for the simulation of droplet impact at such conditions. Further insight is given from the development of a purpose-built compressible two-phase flow solver that incorporates a phase-change model suitable for cavitation formation and collapse; thermodynamic closure is based on a barotropic Equation of State (EoS) representing the density and speed of sound of the co-existing liquid, gas and vapour phases as well as liquid-vapour mixture. To overcome the known problem of spurious oscillations occurring at the phase boundaries due to the rapid change in the acoustic impedance, a new hybrid numerical flux discretization scheme is proposed, based on approximate Riemann solvers; this is found to offer numerical stability and has allowed for simulations of cavitation formation during drop impact to be presented for the first time. Following a thorough justification of the validity of the model assumptions adopted for the cases of interest, numerical simulations are firstly compared against the Riemann problem, for which the exact solution has been derived for two materials with the same velocity and pressure fields. The model is validated against the single experimental data set available in the literature for a 2-D planar drop impact case. The results are found in good agreement against these data that depict the evolution of both the shock wave generated upon impact and the rarefaction waves, which are also captured reasonably well. Moreover, the location of cavitation formation inside the drop and the areas of possible erosion sites that may develop on the solid surface, are also well captured by the model. Following model validation, numerical experiments have examined the effect of impact conditions on the process, utilizing both planar and 2-D axisymmetric simulations. It is found that the absence of air between the drop and the wall at the initial configuration can generate cavitation regimes closer to the wall surface, which significantly increase the pressures induced on the solid wall surface, even for much lower impact velocities. A summary highlighting the open questions still remaining on the subject is given at the end
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On the effect of realistic multicomponent diesel surrogates on cavitation and in-nozzle flow
Cavitation and cavitation-induced erosion highly depends on the thermodynamic properties of the fluid, which in turn affect the in-nozzle flow. However, many predictive models used today rely on constant properties or very simplified diesel surrogates. In this work, the diesel properties are predicted using a realistic four-component diesel surrogate, named J1D, which is compared with the traditionally used n-dodecane and then additised with n-hexane in amounts of 1% and 10%, named 1C6 and 10C6 respectively. The fuel property variation as function of pressure is modelled using the PC-SAFT EoS. The fluids are then used in simulations for a common rail 5-hole tip injector nozzle. The needle is assumed to be still at a lift of 105µm, which is representative of the lift reached during pilot injection. The injector operating pressure is 180MPa and the collector back pressure is 5MPa. The density of the bulk fluid is assumed to vary according to a barotropic-like scheme, following an isentropic expansion. Regarding the results from the simulations, the value of mass flow rate was proportional to the liquid density of the fluids. From the results, it appears that for substances with similar viscosity and density, such as J1D, 1C6 and 10C6 the vapour pressure is dominant in the cavitation production, as the greater the vapour pressure the greater the cavitation obtained. However, when the vapour pressure is comparable, such as that for J1D and n-dodecane, the difference in density and viscosity of the fluids seems to provide the cause for a greater vaporisation the lighter the fluid is. Despite its exploratory nature, this study offers some insight into the use of complex EoS and surrogate mixtures and their effect on cavitation and preferential vaporisation in diesel
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Simulation of supercritical Diesel jets using the PC-SAFT EoS
A numerical framework has been developed to simulate supercritical Diesel injection using a compressible density-based solver of the Navier-Stokes equations along with the conservative formulation of the energy equation. Multi-component fuel-air mixing is simulated by considering a diffused interface approximation. The thermodynamic properties are predicted using the Perturbed Chain Statistical Associating Fluid Theory (PC-SAFT) real-fluid equation of state (EoS). This molecular-based EoS requires three empirically determined but well-known parameters to model the properties of a specific component, and thus, there is no need for extensive model calibration, as is typically the case when the NIST library is utilised. Moreover, PC-SAFT can handle flexibly the thermodynamic properties of multi-component mixtures, which is an advantage compared to the NIST library, where only limited component combinations are supported. This has allowed for the properties of Diesel fuel to be modelled as surrogates comprising four, five, eight and nine components. The proposed numerical approach improves the overall computational time and overcomes the previously observed spurious pressure oscillations associated with the utilization of conservative schemes. In the absence of experimental data, advection test cases and shock tube problems are included to validate the developed framework. Finally, two-dimensional simulations of planar jets of n-dodecane and a four component Diesel surrogate are included to demonstrate the capability of the developed methodology to predict supercritical Diesel fuel mixing into air
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Initial findings of an investigation on the removal of the cavitation erosion risk in a prototype control orifice inside a diesel injector
A CFD investigation is in progress to study the cavitation characteristics and potential erosion risks of a control orifice in a prototype injector. An early design of the orifice resulted in cavitation erosion after endurance testing. A design modification eliminated the erosion and subsequent prototypes were free from damage. Initial results for the two designs using different simulation methods are discussed, along with the effects of different rates of evaporating and condensing mass transfer. Preliminary findings on possible erosion risk indicators comparing the eroding with the non-eroding design are presented
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Large Eddy Simulation of the internal injector flow during pilot injection
The aim of this work is to simulate the internal flow of a Diesel injector during an entire pilot injection event. In common rail systems a small quantity of fuel can be injected before the main injection is started. This increases the temperature in the combustion chamber and improves the combustion, leading to higher engine efficiency and reduced emissions. The internal nozzle flow during this short event is highly dynamic and vapor cavities may appear at the end of the injection. In order to study the flow characteristics, a numerical methodology based on the Eulerian multi-fluid approach is adopted. The filtered Navier-Stokes equations are discretized with the finite volume method and then solved with an implicit pressure-based solver. The flow field is modelled considering single pressure and velocity fields. The Coherent Structure Model is used to derive the eddy viscosity applied to the Large Eddy Simulation approach. The liquid evaporation rate is evaluated with a cavitation model based on the Rayleigh-Plesset equation for a single bubble. Even though thermodynamic equilibrium is not satisfied a priori, the main parameter is adjusted in order to limit the thermodynamic states to be in a range close to the equilibrium conditions. The liquid compressibility is modelled with a linear correlation between pressure and density variations. The needle longitudinal movement obtained from the experiments is applied to the simulation. The adopted geometry is the Spray A case defined by the Engine Combustion Network. It is an asymmetric single hole Diesel injector that has been extensively studied in the past both experimentally and numerically. The injection pressure is 1,500 [bar] and the ambient pressure is 60 [bar] with a fuel temperature of 363 K inside the injector. Pure n-dodecane is used as fluid in order to have a precise specification of the physical properties. Although both experiments and simulations showed no cavitation for completely open needle at fixed position, recent studies demonstrated that phase-change of the liquid can appear during the needle closing phase. Cavitation erosion prone locations are then evaluated by recording the maximum intensity of pressure on the surface
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Simulating the effect of in-nozzle cavitation on liquid atomisation using a three-phase model
The aim of this article is to present a fully compressible three-phase (liquid, vapor, air, and mixture) cavitation model and its application to the simulation of in-nozzle cavitation effects on liquid atomization. The model employs a combination of barotropic cavitation model with an implicit sharp interface capturing Volume of Fluid (VoF) approximation. The results from the simulation are compared against the experimental results obtained by (1) for injection of water into the air from a stepped nozzle. Large Eddy Simulation (LES) model is utilized for resolving turbulence. Simulations are performed for a condition where developing cavitation is observed. Model validation is achieved by qualitative comparison against the available images for the cavitation, spray pattern. The model predictions suggest that the experimentally observed void inside the nozzle is not purely vapor, but a mixture of both vapor and back-flowing air. The simulation also identified periodic air entrainment that occurs at developing cavitation condition which further improves primary atomization
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Cavitation Induction by Projectile Impacting on a Water Jet
Following the work of Field et al. [4], who experimentally visualised cavity formation and shock propagation in impacted liquids at high velocities, the present study focuses on the simulation of the high velocity impact of a solid projectile on a water jet. The undeformable solid projectile is modelled through a direct forcing Immersed Boundary Method. The simulation is carried out using an explicit density based compressible solver, developed by Kyriazis et al. [6], which employs a two-phase flow model and includes phase change. This study gives a better insight on the phenomena following the impact of solids on liquids, including shock propagation and vapour formation, and demonstrates the capabilities of the presented Immersed Boundary Method to handle compressible cavitating flows
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Heating Effects During Bubble Collapse Using Tabulated Data
An explicit density-based solver for the compressible Navier-Stokes equations able to simulate cavitating flows has been developed and utilised for the simulation of collapsing vapour bubbles. Phase-change is considered by employing the homogeneous equilibrium model (HEM). The wide variation of Mach numbers between the liquid, vapour and mixture regimes is tackled by a Mach consistent numerical flux, suitable for subsonic up to supersonic flow conditions. Time discretisation is performed using a second order low storage Runge-Kutta scheme. Thermodynamic closure is achieved by utilising the Helmholtz energy equation of state (EoS), making feasible simulation of conditions at subcritical and supercritical regions considering the variations of liquid and vapour temperatures during bubble collapse. In order to reduce the computational cost associated with the solution of the Helmholtz EoS at each time step, a tabulated data technique has been followed. The unstructured thermodynamic table, containing the thermodynamic properties derived from the Helmholtz EoS, has been constructed for n-dodecane, which has been the considered as the working fluid. The efficiency of the method is enhanced by a static linked-list algorithm for searching among the elements of the table. In addition, a finite element bilinear interpolation is used for approximating the unknown thermodynamic properties. After validating the numerical method, parametric studies considering 2-D axisymmetric vaporous bubble collapse in the proximity of a wall have been performed at conditions realised in micro-orifice flow passages. The temperature and pressure changes on the wall are estimated as function of the surrounding liquid pressure, the initial bubble radius and the location of the wall from the center of the initial bubble, revealing the expected range of variation as function on the set parameters