Experimental Investigations of Film Flows Around Obstacles

Within an aero-engine bearing chamber oil is provided to components to lubricate and cool. This oil forms a moving film around the inside of the bearing chamber which provides cooling to the chamber walls. Where the walls have features such as bolt heads or bearing supports protruding from the surface the oil film is forced to flow either over or around these features. If the film becomes too thin, or dries out, the cooling becomes ineffective and local hot spots occur which can precipitate coking. The work reported in this paper is a continuation from two previous papers [9, 10] using experimental methods to characterise the disruption to film flow near obstructions. This fundamental study provides insight into likely flow behaviour near obstructing features and is part of a larger set of projects to understand film flow around bearing chamber walls. In the study two rigs are employed: an inclined plane rig and a stratified liquid-gas tunnel used in the previous work. In both rigs the working fluid is tap water at ambient conditions, chosen for ease of use and because the viscosity and density of water at room temperature (≈20 °C) are representative of those of aero-engine oil at a typical operating temperature (between 100 °C to 200 °C). Two measurement methods are used to map flows around penetrating and sub-film obstacles, a needle contact probe and a Laser Focus Displacement Meter. In addition, visualisation is used to provide more details on flow behaviour. The conclusion from the work indicates that, for a bearing chamber, it is best not to have a series of obstacles directly in a line with each other as there will be a significant variation of film height and velocity caused by the obstacles across the bearing chamber wall. However, the effect is reduced for higher gas flow rates, which more closely match chamber conditions for large turbofans. It is recognized that in an engine there is significant vibration, the surfaces are not hydraulically smooth and that temperature will have a major role on the final effect on film flow and that these effects are not considered in this fundamental work.