An Investigation of Cavitation Phenomena in Axial Piston Machines Through Experimental Study and Simulated Scaling Effects
Creators: name: Boland, Hannah Mcclendon
Publisher: name: Purdue University Graduate School
Year: 2023
Subjects: Computational methods in fluid flow, heat and mass transfer (incl. computational fluid dynamics)
Multiphysics flows (incl. multiphase and reacting flows)
Experimental methods in fluid flow, heat and mass transfer

description: Cavitation is one of the most common causes of failures in axial piston machines. Due to the detrimental effects that cavitation has on unit performance, it is of important consideration both in the design of new units and in defining the operational limits of existing market products. The work in this thesis aimed to contribute to the current knowledge in both areas, with a focus on design considerations with respect to cavitation scalability, and on operating conditions by measuring cavitation severity under separate and combined inciting parameters. Though the application of unit scaling is common in industry for the design of pump families, there have been no comprehensive attempts to quantify whether cavitation in fluid power units may be adequately accounted for in published scaling laws. In this thesis, the scalability of cavitation phenomena was examined through a CFD scaling study performed using a modified version of the Full Cavitation Model. Results indicate that linear scaling is consistent in maintaining volumetric efficiency performance within 1% across scaled units up to eight times larger or smaller than the baseline. However, the gas and vapor volume distributions vary significantly between scaled units, due largely to the linear non-scalability of fluid inertia and turbulent factors. Physical exchange between phases within a working fluid was shown to be time-dependent, such that the scaled-down unit exhibits bubble collapse rates up to 30% and 150% greater than the baseline and scaled-up units, respectfully. Considering these effects, the presented work demonstrates a potential for increased cavitation damage area when downscaling a unit and reduced risk in upscaling, despite the scaling law being a reliable indicator for volumetric efficiency. To define a more complete study of cavitation under a variety of operating conditions and inciting parameters, this a new experimental procedure and testing circuit was proposed with focus on repeatability by controlled pressure drops and preliminary quantification of inlet fluid quality. By measuring cavitation conditions under pressure starvation, incomplete filling, and combinations thereof, the direct effect of different inception methods on unit performance was shown to be readily identifiable. Through visualization of the inlet flow, reduction in inlet pressure levels was correlated to fluid cloudiness levels and bubble size, with transparency loss at 0.0 bar<sub>g</sub> and transition from bubbly to plug flow at -0.4 bar<sub>g</sub>. Incomplete filling-induced cavitation was also shown to be detectable by inlet flow conditions, with a distinct change in bubble coalescence rate when operating under shaft speeds greater than or equal to fill speed for a given inlet pressure.