City Research Online

Abstract

Cavitation, the appearance of vapour in a homogeneous liquid through pressure changes, is common in many fields like naval, automotive and aviation, and often leads to problems such as reduced efficiency, noise and erosion. Consequently, there has been extensive research dedicated to understanding and controlling the effects of cavitation, through experiments and CFD simulations. There is currently no reliable model to predict cavitation erosion. Moreover, simply having cavitation collapse near a solid wall will not always result in erosion. To explore cavitation characteristics and cavitation erosion three different components have been investigated with Computational Fluid Dynamics (CFD) simulations, results of which are presented here. The components investigated included an axisymmetric converging-diverging Venturi nozzle, a thin liquid filled gap between a Shoe and Guide assembly in a high-pressure fuel injection pump, and a control orifice in a prototype diesel injector.

Cavitating flow dynamics are investigated in a Venturi nozzle. Computational Fluid Dynamics (CFD) results are compared with those from previous experiments. Analyses performed on the quantitative results from both data sets reveal a coherent trend and show that the simulations and experiments agree well. The CFD results have confirmed the interpretation of the high speed images of the Venturi flow, which indicated there are two vapor shedding mechanisms that exist under different running conditions: re-entrant jet and condensation shock. Moreover, they provide further detail of the flow mechanisms that cannot be extracted from the experiments. For the first time on this cavitating Venturi nozzle flow, the re-entrant jet shedding mechanism is reliably achieved in CFD simulations. The condensation shock shedding mechanism is also confirmed, and details of the process are presented. These CFD results compare well with the experimental shadowgraphs, space-time plots and time-averaged reconstructed computed tomography (CT) slices of vapor fraction.

For the Shoe and Guide component, real industrial examples were used to evaluate the viability of several cavitation erosion risk indicators (ERIs). Industry standard endurance tests resulted in non critical cavitation erosion of a shoe and shoe-guide assembly in a high-pressure fuel pump. A design modification was made which eliminated the erosion. For the current work, transient CFD simulations of the two designs were run. The distribution and intensity of the resulting ERIs were evaluated against photographic evidence of erosion taken after endurance testing. Details of the component dynamics and the resulting cavitation formation and collapse are presented, along with an analysis of the ERIs for their potential usefulness. Of the 10 ERIs studied, two were found to be particularly good indicators, one of which is newly derived for this research.

Lastly, for the Control Orifice, an early prototype design resulted in cavitation erosion after endurance testing. A design modification eliminated the erosion and subsequent prototypes were free from damage. CFD results for the two designs using different simulation methods are discussed, along with the effects of different rates of evaporating and condensing mass transfer. Findings on the successful ERIs from comparing the eroding with the non-eroding design are presented. The two successful ERIs from the Shoe and Guide component work were also successful for the Control Orifice which emphasises the robustness of these ERIs. It is now anticipated that using these ERIs to guide product design and development will save considerable time and cost

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

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