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A Strongly Coupled Simulation Model of Positive Displacement Machines for Design and Optimization
thesisposted on 15.12.2020, 19:14 by Thomas Ransegnola
Positive displacement machines are used in a wide variety of applications, ranging from fluid power where they act as a transmission of power, to lubrication and fluid transport. As the core of the fluid system responsible for mechanical--hydraulic energy conversion, the efficiencies of these units are a major driver of the total efficiency of the system. Furthermore, the durability of these units is a strong decider in the useful life of the system in which they operate.
The key challenge in designing these units comes from understanding their working principles and designing their lubricating interfaces, which must simultaneously perform a load carrying and sealing function as the unit operates. While most of the physical phenomena relevant to these machines have been studied previously in some capacity, the significance of their mutual interactions has not. For this reason, the importance of these mutual interactions is a fundamental question in these machines that this thesis answers for the first time. In analysis of two different machine types, it is confirmed that mutual interactions of both physical phenomena and neighboring fluid domains of the unit contribute significantly to the overall performance of the machine. Namely, these analyses demonstrate load sharing owing to mutual interactions on average of 20% and as high as 50%, and mutual flow interactions of at least 10%.
In this thesis, the behavior of the thin films of fluid in the lubricating interfaces of the units, the bodies that make up these films, and the volumes which interface with them will be considered. The resulting coupled problem requires a model that can consider the effects of motion of all floating bodies on all films and volumes, and collect the resulting loads applied by the fluid as it responds. This will require a novel 6 degree of freedom dynamics model including the inertia of the bodies and the transient pressure and shear loads of all interfaces of the body and the fluid domain.
During operation, fluid cavitation and aeration can occur in both the displacement chambers of the machine and its lubricating interfaces. To capture this, a novel cavitation algorithm is developed in this thesis, which considers the release of bubbles due to both gas trapped within the fluid and vaporization of the operating fluid in localized low pressure regions of the films. In the absence of cavitation, this model will also be used to find the pressures and flows over the film, communicating this information with the remainder of the fluid domain.
Due to the high pressures that form in these units, the bodies deform. The resulting deformation changes the shape of the films and therefore its pressure distribution. This coupled effect will be captured in one of two ways, the first relying on existing geometric information of the unit, and the other using a novel analytical approach that is developed to avoid this necessity. In either case, the added damping due to the shear of the materials will be considered for the first time. Additionally in regions of low gap height, mixed lubrication occurs and the effects of the surface asperities of the floating bodies cannot be neglected. Accurate modeling of this condition is necessary to predict wear that leads to failure in these units. This work will then develop a novel implementation for mixed lubrication modeling that is directly integrated into the cavitation modeling approach.
Finally, effort is made to maintain a generic tools, such that the model can be applied to any positive displacement machine. This thesis will present the first toolbox of its kind, which accounts for all the mentioned aspects in such a way that they can be captured for any machine. Using both multithreaded and sequential implementations, the tool will be capable of fully utilizing a machine on which it is run for both low latency (design) and high throughput (optimization) applications respectively. In order to make these applications feasible, the various modules of the tool will be strongly coupled using asynchronous time stepping. This approach is made possible with the development of a novel impedance tensor of the mixed universal Reynolds equation, and shows marked improvements in simulation time by requiring at most 50% of the simulation time of existing approaches.
In the present thesis, the developed tool will be validated using experimental data collected from 3 fundamentally different machines. Individual advancements of the tool will also be verified in isolation with comparison to the state of the art and commercial software in the relevant fields. As a demonstration of the use of the tool for design, detailed analysis of the displacing actions and lubricating interfaces of these same units will be performed. These validations demonstrate the ability of the tool to predict machine efficiencies with error averaging around 1% over all operating conditions for multiple machine types, and capture transient behavior of the units. To demonstrate the utility as a virtual optimization tool, design of a complete external gear machine design will be performed. This demonstration will start from only analytical parameters, and will track a route to a complete prototype.