The study of fluid film lubrication in mechanical components is fundamental to the analysis of their dynamic behaviour as well as determining friction losses between moving parts. In several cases, gases present in the system can interact with the oil changing lubrication characteristics - by their dissolution and release from the oil, forming bubbles or even producing foam. Additionally, in low pressure regions the lubricant loses the capacity to flow as a continuous film, and cavitation occurs. Generally cavitation is treated in numerical models via boundary conditions for the pressure equation, which is then used to solve the pr<;>blem only in the full film region instead of the whole solution domain. Several criteria are available, and the results are often sensitive to the chosen conditions. The present work proposes the study of cavitation considering the changes suffered by the lubricant as it flows through the lubricated component, considering the release of gas from the liquid and the existence of a two-phase flow. The numerical model treats the liquid-gas mixture as a homogeneous fluid, whose properties are calculated as weighted averages of the properties from the constituent phases. The model then solves the lubrication equation numerically using the finite volume methodology, considering variable fluid properties and without using any intermediate boundary conditions. The lubricant is considered a mixture of oil and refrigerant, widely studied due to its importance in lubrication and heat transfer problems in refrigeration systems. To advance in the understanding of the two-phase flow model, it was initially applied to simplified geometries, such as that of a partial journal bearing, and gradually the geometrical assumptions were relaxed, applying the model to a full journal bearing, where a . discussion against experimental results was possible. Finally, the problem of the reciprocating motion of the piston in a refrigeration compressor was considered. From the results, it is shown that the model can automatically predict three regions for the lubricated component,the positive pressure region, cavitation and pressure recovery. When the model is compared to solutions considering Reynolds boundary conditions, good agreement is observed for moderate and heavy loads. Studying the main operational parameters and the mixture behaviour, the discussion focuses on the fluid properties, which affect particularly density and as a result the cavitation region - or even the full bearing if bubbles are considered in all the bearing extent. Finally, in the light of the two-phase solution for piston lubrication, existing cavitation criteria for the problem seem inappropriate.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:486397 |
| Date | January 2007 |
| Creators | Grando, Fernando Paulo |
| Publisher | University of Leeds |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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