Power losses in spiral bevel gears

This dissertation describes a numerical modelling strategy for characterising the windage of a spiral bevel gear rotating within a static shroud. The techniques employed include the use of a parametric solid model, and flow field modelling using computational fluid dynamics (CFD) software. A number of hypothetical physical alterations are made to a control system consisting of a gear and a shroud, based on those found in a Rolls-Royce aero engine. Windage is a parasitic power loss that occurs when a gear does work on air and oil within a gas turbine internal gearbox. It leads to degradation of the oil, which wears turbo-machinery and bearing components, shortening their lives. Windage power losses also impact upon the fuel consumption of an engine, reducing its environmental credentials. The requirement of the oil is to cool and lubricate the meshing location of a pair of gears and for it to then be removed from the vicinity of the gear - preventing its re-ingestion and recirculation. The best solution to these issues of reducing windage and managing the oil is to shroud the gear effectively. Additional cooling of oil can mitigate the damage of too much heat generation, but this adds weight and expense to an engine. A parametric model of a shrouded spiral bevel gear is created, which allows for changes to be made to single or multiple dimensions of the gear and shroud. A control volume CFD approach is used, with a single tooth of a gear modelled to reduce computational time. Four gear size variables are tested: inner diameter, outer diameter, cone angle and module (number of teeth). Findings for air-only show that windage can be reduced by: increasing the cone angle; increasing the number of teeth; or decreasing the outer diameter. The effect of changing the inner diameter on windage was found to be complex due to the interaction of gear and shroud upon the system results, but far less significant than the outer diameter, as windage was seen to scale by this variable to the power of 4.3. Changes to the design of the shroud are made, with a comparison of five inlet and five outlet designs. It is found that a sudden restriction at the inlet to the shroud is the most effective way of reducing single phase windage, with additional shroud features that make the flow path more tortuous also helping. The outlet of the shroud is shown to increase windage as it is opened up and permitted to be less restrictive. A series of investigations using a Lagrangian discrete particle model (DPM) to simulate the presence of oil droplets within the system are also presented. It is shown that the destination of oil within the domain is predominantly dictated by the source location of the oil, with little sensitivity to initial velocity or size. Film modelling on the surface of the shroud, using the DPM-based film model, allows the motion of a thin layer of oil to be studied. In conjunction with the five shroud outlet designs, it is shown that a less restrictive, more open shroud outlet design will help prevent re-ingestion of oil that is present within the shrouded area of the gear