On natural convection in a large civil gas turbine

Thesis

As a large civil gas turbine is cooling following operation, natural convective flows develop, which can lead to numerous issues. The ability to predict and understand this transient shutdown cooling cycle is becoming increasingly more important. However, this has proven to be extremely difficult owing to the complex nature of the flow physics and its dependency on a large number of parameters.

Two sophisticated experimental facilities that simulate the natural convective flow field during shutdown cooling of a large civil gas turbine, have been designed and commissioned. This has allowed for the influence of numerous features to be investigated, which has helped to increase understanding of these flow fields. Furthermore, a robust thermocouple probe has been developed that allows for accurate measurements in a natural convective boundary layer, along with a novel method of determining heat flux fluctuations.

An extensive experimental study has shown that the overall heat transfer in a non-isothermal concentric annulus is adequately captured by isothermal correlations when the area-averaged temperature difference is used, as the local heat flux is dominated by the local radial temperature gradient. This finding was validated up to a diameter ratio of 1:5, which is applicable to the majority of the main gas path.

The interaction between the induced draft in the main gas path (due to natural convection) and obstructions in this area has been proven to be critical in defining the severity of thermal rotor bow - the thermal gradient that develops across the rotor drum during shutdown. Two viable solutions to help alleviate rotor bow have been developed: jet impingement cooling at the top of the annulus, and circumferentially non-uniform emissivity distributions.

Authors: Fahy, D

• DOI: 10.5287/ora-mj5jpvq7b
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: gas turbine; heat transfer; natural convection
• Subjects: Engineering; heat; Fluid dynamics