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Numerical simulation of cavitation in the presence of non-condensable gas using a three-phase model

Murali-Girija, M. (2020). Numerical simulation of cavitation in the presence of non-condensable gas using a three-phase model. (Unpublished Doctoral thesis, City, University of London)


The interactions between liquid, vapour and gas (three phases) play a vital role in determining the efficiency of many engineering equipments. To study these interactions, a three phase model is developed using barotropic equations of state and implemented into the Navier-Stokes equations; the barotropic fluid is assumed to be in thermal and mechanical equilibrium with the air. The air's motion is simulated using two different approaches, a sharp interface capturing VOF technique and a diffused interface mixture model. Alongside the so-called three-phase model, an immersed boundary method (IBM) is also developed for modelling the complex geometry motions. Both these models are implemented into the Ansys Fluent solver using User Defined Functions (UDF's) and are validated against relevant experimental studies.

Several three-phase flow simulations have been performed, starting from simplified 2D simulations and moving to more complex industrial applications, such as Diesel injectors and gear pumps. Firstly, the three-phase model using the sharp interface VOF approach, in conjunction with the LES model for resolving turbulence, is used for studying the influence of in-nozzle flow on primary atomisation from an asymmetric step-nozzle. The results obtained from the simulation show a good correlation with the experimental observations.

The transient flow phenomena occurring during the opening, closing and dwelt time in fuel injectors are known to significantly contribute to excess exhaust emissions in engines. The three-phase model using the VOF approach with a model for wall-adhesion is used for predicting the flow development inside and outside of the orifices and the wall wetting during the start, end, dwelt time and the subsequent start of the injection cycle. The simulations are performed and validated in a six-hole VCO-type injector imposing realistic conditions of the valve movement, implemented using the immersed boundary method; this allows for simulations to be performed at zero lift during the dwelt time between successive injections.

The three-phase model using the mixture approach in conjunction with the IBM model is further used for simulating the effect of varying gas content in the fluid on cavitation occurring from an external gear pump. These simulations have revealed the importance of modelling contact between the gears in predicting the pump performance accurately. Moreover, it was observed that the main effect of increasing the NCG content in the fluid is a reduction in cavitation occurring inside the pump.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TL Motor vehicles. Aeronautics. Astronautics
Departments: Doctoral Theses
School of Science & Technology > School of Science & Technology Doctoral Theses
School of Science & Technology > Engineering > Mechanical Engineering & Aeronautics
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