Preliminary CFD Simulations of Lubrication and Heat Transfer in a Gearbox

Abstract

When designing a gearbox it is important to consider the heat rise generated inside the gearbox due to the gear meshing action of gear teeth. Providing efficient lubrication helps keep the gearbox at lower temperatures and reduce friction, which in return leads to a longer lifespan. Given the difficulty in obtaining experimental data within the gearbox, the authors investigate and present the setup and methods using Computational Fluid Dynamics (CFD) modelling of the process. The main purpose of this work is to implement and demonstrate numerical techniques that are needed in order to perform CFD simulations on this subject. There are currently no widely used techniques known to the authors that would allow to carry out parametric CFD study of gearbox lubrication and cooling. There are only limited empirical models that are used to find a best design. When developed, CFD methods may allow to do parametric studies and therefore significantly improve the quality of the gearbox design. In order to capture the fluid behaviour in a continuously changing topology around rotating gears, dynamic mesh technique with remeshing and smoothing is used. Dynamic mesh is a complex and expensive technique on its own; and becomes even more so when have to be implemented along with the two-phase flow and conjugated heat transfer. For that reason the development and implementation of this method requires an incremental approach with very gradual increase of difficulty and separation of the large task into small ones, which essentially what has been done in this work. Furthermore, investigation of how to reduce the cost of the simulation is an important part so that the method can then be used more widely. Two types of lubrication are considered: partial dipping into oil (rotational submersion) and jet spraying. Rotational speeds of up to 8,000rpm are studied. Temperature of the gears and the surrounding fluids are initially defined as uniform. Additional heat sources are created in the solid cells of the gears where the teeth come into contact, also using a UDF. 2.5D dynamic remeshing is used for models with spur gears, whereas full 3D remeshing is used with helical gears. Simulations are performed using the Volume of Fluid method and the standard k-omega turbulence model. Simulations are run with varying degrees of complexity (low- and high-fidelity). Some results of basic preliminary simulations are compared with available results from the literature, demonstrating a good agreement. Validation of the results demonstrate the ability of the presented methods to accurately predict the gear losses and the fluid flow in a gearbox. More complex simulations are run in order to observe and analyse both the fluid flow and the heat distribution in the gearbox. Main attention is given to the temperatures of the housing and the meshing teeth. Since all simulations with meshing gears require a small gap between the gears (i.e. with no direct contact of the gears), three different gap sizes are investigated. For these simulations a comparison of the oil flow is provided. This comparison is used to justify which model can be used most efficiently without significant loss of accuracy when modelling the temperature distribution at the housing. Current work is an essential first step towards the detailed study that is currently of great interest of both research and industry. Future work is necessary to fully justify the methods, however the current work is essential and will hopefully provide an inspiration and encouraging of the topic advancement.