Application of CFD to model an aeroengine internal gearbox

This thesis describes research undertaken to improve computational modelling capability for the internal gearbox (IGB) of an aeroengine. Using the commercial computational fluid dynamics (CFD) software ANSYS FLUENT modelling methodologies for regions within the IGB have been developed, applied and refined. The IGB is a bearing chamber that houses the bearings that support the low pressure, intermediate pressure and high pressure shafts and in addition the spiral bevel gear pair that enable power to be taken from the high pressure shaft to power aircraft auxiliary systems. Within civil aeroengines parasitic power loss is a significant issue and as oil is used to lubricate and cool throughout the engine, this power loss largely manifests as increased heat-to-oil (HTO). A significant contributor to HTO is the IGB. The IGB contains complex geometry and a highly rotating two-phase flow consisting of films, droplets, ligaments and mist. Central to the IGB is the spiral bevel gear pair. Previous modelling research has shown that detailed modelling of flow behaviour is too computationally expensive for domains larger than a few teeth. Modelling the meshing gears with full flow fidelity is not yet feasible. In this thesis a significantly less computationally expensive approach is explored. The complex gear-shroud geometry is replaced by a smooth cone with momentum sources used to generate the required fluid motion. In the single phase model these momentum sources were tuned/calibrated against a full tooth model spanning four teeth. The model was capable of generating flow behaviour to within 5% of the full tooth model. Oil was added as a discrete phase with a film model but was less successful as oil motion is strongly affected by geometrical detail. A second approach to whole chamber modelling was proposed where the chamber is split into three zones and coupled via boundary conditions. Single phase investigation showed that the amount of swirl in the front chamber affects computed windage power loss with the maximum occurring at an inlet swirl number of around 0.5. The amount of swirl at gear entry does not however affect the amount of swirl at shroud exit and this shows that decoupling of the front chamber is viable. The investigations into the zonal coupling of the IGB highlighted the importance of the geometry of the rear chamber (between gear and bearing) on the flow through the gear. A study to investigate how best to model the two-phase flow in this rear chamber was conducted. Transient models showed the volume of fluid approach (VOF) to be inadequate whereas a full two-phase Eulerian model converged, yielding viable results consistent with limited qualitative experimental data. The computational model predicts significant accumulation of oil towards the bearing side of the chamber, with this oil stripping periodically through shroud exit slots to the front chamber. In the final part of the research a parametric study on several geometric features in the rear chamber was conducted using the developed two-phase modelling methodology. The chamber size, rear wall geometry, shroud exit slot location and size were investigated. The work in this thesis improves IGB modelling capability through - Establishing the capabilities and limitations of momentum source approach for full two-phase modelling of a shrouded gear - Establishing that to some extent a bearing chamber can be productively modelled as separate but linked zones - Identifying a successful modelling methodology for the high volume fraction two phase flow in the rear chamber In addition the work in this thesis shows that - For single phase flow the amount of swirl at gear inlet affects the windage power loss - The behaviour of oil in the rear chamber, including the amount trapped and the exit condition, are strongly affected by chamber geometry Guidelines for rear chamber design are also suggested