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Modelling of nozzle cavitation in newtonian and viscoelastic fluids

Naseri, Homa (2019). Modelling of nozzle cavitation in newtonian and viscoelastic fluids. (Unpublished Doctoral thesis, City, University of London)

Abstract

Cavitation is the abrupt process of vapour formation due to pressure drop in liquid flows, which may occur in various equipment and hydraulic machinery such as propellers, bearings or fuel injectors. In fuel injector nozzles, cavitation formation may be either beneficial or detrimental to the engine performance, depending on the location and type of the vaporous structures. Cavitation can enhance the turbulence levels inside the nozzle and therefore improve the spray atomisation, moreover large vortex cavities known as string cavitation, can increase the spray cone angle. However, cavitation bubbles collapsing near the internal surfaces result in erosion and ultimately failure of the injector parts. Furthermore, excessive vapour formation inside the nozzle significantly reduces the nozzle discharge coefficient and may eventually result in chocked flow conditions. Therefore, understanding the cavitation dynamics and cavitation control is vital for injector design and an active topic in fluid dynamics research.

Newly developed deposit control fuel additives have the potential to reduce the in-nozzle cavitation and enhance the flowrate even in clean injectors due to their non-Newtonian properties. This research aims to provide an understanding about the association between viscoelastic detergent additives and turbulent cavitating nozzle flows using computational fluid dynamics.

An accurate framework for modelling the in-nozzle cavitation is developed. Performance of several RANS and LES models is assessed in predicting incipient and developed cavitation regimes. The Reboud at al. eddy viscosity correction is utilized to compensate the effect of mixture compressibility on turbulent viscosity in k-ɛ RNG and k-ω SST models. WALE LES results are validated against the velocity and rms of turbulent velocity measurements in a step nozzle. A homogeneous equilibrium cavitation model based on Wallis speed of sound formula is utilised to study the effect of vapour-liquid mass transfer rate compared to Zwart Gerber-Belamri (ZGB) and Schnerr-Sauer (SS) cavitation models. Cavitation vapour fraction values are validated against X-ray CT measurements.

The Phan-Thien-Tanner (PTT) fluid model is implemented to simulate the shear-induced viscoelastic behaviour of Quaternary Ammonium Salt (QAS) surfactants in the additised fuel. The model is validated against analytical solution for channel flow and experimental measurements of corner vortex in a square contraction geometry. The effect of liquid viscosity on cavitation development is presented using LES simulations. Finally, the effect of viscoelasticity on
different cavitation regimes, namely cloud cavitation inside a step nozzle and string cavitation in a realistic injector geometry is investigated using LES. The physical flow characteristics are investigated and the effect of the additive on cavitation inside the nozzle is presented. The numerical findings are found to be in agreement with experimental studies of additised fuel.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TJ Mechanical engineering and machinery
Departments: Doctoral Theses
Doctoral Theses > School of Mathematics, Computer Science and Engineering Doctoral Theses
School of Mathematics, Computer Science & Engineering > Engineering > Mechanical Engineering & Aeronautics
URI: https://openaccess.city.ac.uk/id/eprint/23279
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