This is a phd thesis

<div><p>Water is inflammable, non-toxic, environmentally friendly--- desirable traits, for a hydraulic fluid. However, its extremely low viscosity diminishes the load-bearing and sealing capacity of lubricating interfaces. Case in point: axial piston machines of swash plate design are compact, highly efficient positive displacement machines at the heart of hydraulic systems in forestry, construction, aerospace, and agricultural equipment, as well as industrial applications (presses, etc.); however, the three main lubricating interfaces decisive to the performance of such units in terms of both component life and efficiency are challenged by the use of water as working fluid. Especially during high-pressure operation, this low-viscosity lubricant can cause the these interfaces to fail in carrying the imposed load, resulting in severe wear, or even pump failure. The piston-cylinder interface is particularly challenging to design for water because it stands under obligation to carry the heavy side load that acts on the pistons of these machines, which increases with operating pressure. Furthermore, the architecture of axial piston machines of swash plate design does not allow this interface to be hydrostatically balanced.</p> <p> </p> <p>Through the development of a methodology that separates the fluid pressure fields of the three main lubricating interfaces of axial piston machines into their hydrostatic and hydrodynamic components, the present work enables a direct comparison of these interfaces in terms of how they support load. A case study of a 75 cc unit running on hydraulic oil conducted via this methodology at three different operating conditions (low pressure/low speed, low pressure/high speed, and high pressure/low speed) demonstrates that in the piston-cylinder interface, the force from hydrostatic pressure reaches such high magnitudes over the high-pressure stroke that less than half of it is needed to counter the load. The excess force from hydrostatic pressure then becomes the load. Consequentially, hydrodynamic pressure must counter a force from hydrostatic pressure that exceeds the original load. In the other two interfaces, by contrast, over half the load is being carried by hydrostatic pressure, thus significantly diminishing the amount of hydrodynamic pressure the interfaces are required to generate in order to achieve full load support. Moreover, nearly all of the moment on the piston is countered by hydrodynamic pressure, while less than half of the moment on the block is countered by hydrodynamic pressure, and the moment on the slipper is negligible by comparison.</p> <p> </p> <p>While this case study only investigates one pump, it shows how critical hydrodynamic pressure can be to load support in the piston-cylinder interface. The use of a low-viscosity fluid, e.g. water, reduces the hydrodynamic pressure that is generated in this interface, which, at challenging operating conditions, can lead to metal-to-metal contact. However, the performance of the interface can be improved via micro surface shaping, i.e. by giving the surface of the piston, or the bore that it moves through, a shape on the order of microns in height. The aim of present work is to pursue design trends leading to surface shapes that will enable this interface to function at higher pressures than currently achievable. </p> <p> </p> <p>This pursuit takes the form of systematic virtual design studies, an optimization procedure, and an algorithm developed specifically for tailoring the bore surfaces through which the pistons travel to piston tilt and deformation. From this emerges not only a set of design trends corresponding to the dimensions of two particularly powerful types of micro surface shaping, but also a profound insight into the behavior of the water-lubricated piston-cylinder interface fluid film, and how that behavior can be manipulated by changing the component surfaces that constitute its borders. Furthermore, in collaboration with Danfoss High Pressure Pumps, a physical prototype of a 444 cc axial piston pump with surface shaping generated via the aforementioned algorithm has been constructed and tested, achieving a total pump efficiency roughly 3% higher than that achievable by the commercial unit that the geometry of the prototype is based on.</p><br></div>

<p dir="ltr">Positive displacement machines are used in various industries spanning the power spectrum, from industrial robotics to heavy construction equipment to aviation. These machines should be highly efficient, compact, and reliable. It is very advantageous for designers to use virtual simulations to design and improve the performance of these units as they significantly reduce cost and downtime. The recent trends of electrification and the goal to increase power density force these units to work at higher pressures and higher rotational speeds while maintaining their efficiencies and reliability. This push means that the simulation models need to advance to account for various aspects during the operation of these machines. </p><p dir="ltr">These machines typically have several bodies in relative motion with each other. Quantifying these motions and solving for their effect on the fluid enclosed are vital as they influence the machine's performance. The push towards higher rotational speeds introduces unwanted cavitation and aeration in these units. To model these effects, keeping the design evaluation time low is key for a designer. The lumped parameter approach offers the benefit of computational speed, but a major drawback that comes along with it is that it typically assumes fluid inertia to be negligible. These effects cannot be ignored, as quantifying and making design considerations to negate these effects can be beneficial. Therefore, this thesis addresses these key challenges of cavitation dynamics, body dynamics, and accounting for fluid inertia effects using a lumped parameter formulation.</p><p dir="ltr">To account for dynamics features related to cavitation, this thesis proposes a novel approach combining the two types of cavitation, i.e., gaseous and vaporous, by considering that both vapor and undissolved gas co-occupy a spherical bubble. The size of the spherical bubble is solved using the Rayleigh-Plesset equation, and the transfer of gas through the bubble interface is solved using Henry's Law and diffusion of the dissolved gas in the liquid. These equations are coupled with a novel pressure derivative equation. To account for body dynamics, this thesis introduces a novel approach for solving the positions of the bodies of a hydraulic machine while introducing new methods to solve contact dynamics and the application of Elasto Hydrodynamic Lubrication (EHL) friction at those contact locations. This thesis also proposes strategies to account for fluid inertia effects in a lumped parameter-based approach, taking as a reference an External Gear Machine. This thesis proposes a method to study the effects of fluid inertia on the pressurization and depressurization of the tooth space volumes of these units. The approach is based on considering the fluid inertia in the pressurization grooves and inside the control volumes with a peculiar sub-division. Further, frequency-dependent friction is also modeled to provide realistic damping of the fluid inside these channels.</p><p dir="ltr">To show the validity of the proposed dynamic cavitation model, the instantaneous pressure of a closed fluid volume undergoing expansion/compression is compared with multiple experimental sources, showing an improvement in accuracy compared to existing models. This modeling is then further applied to a gerotor machine and validated with experiments. Integrating this modeling technique with current displacement chamber simulation can further improve the understanding of cavitation in hydraulic systems. Formulations for body dynamics are tested on a prototype Gerotor and Vane unit. For both gerotor and vane units, comparisons of simulation results to experimental results for various dynamic quantities, such as pressure ripple, volumetric, and hydromechanical efficiency for multiple operating conditions, have been done. Extensive validation is performed for the case of gerotors where shaft torque ripple and the motion of the outer gear is experimentally validated. The thesis also comments on the distribution of the different torque loss contributions. The model for fluid inertia effects has been validated by comparing the lumped parameter model with a full three-dimensional Navier Stokes solver. The quantities compared, such as tooth space volume pressures and outlet volumetric flow rate, show a good match between the two approaches for varying operating speeds. A comparison with the experiments supports the modeling approach as well. The thesis also discusses which operating conditions and geometries play a significant role that governs the necessity to model such fluid inertia effects in the first place.</p>