|Physics & Astrophysics colloquium is Nov. 20|
Forrest Ames, mechanical engineering, will present a colloquium titled "Effects of Real World Turbulence and Realistic Roughness on Gas Turbine Vane Heat Transfer" at 4 p.m. Friday, Nov. 20, in 215 Witmer Hall. Coffee and Cookies will be served at 3:30 p.m. in 215 Witmer Hall.
Turbulence is an atmospheric phenomenon we are aware of due to the inconvenience it causes when we are flying. We also sometimes encounter turbulent buffeting when passing a large semi truck on the highway. In engineering we become aware of turbulent flow in pipes and on surfaces in fluid mechanics courses and recognize it is a function of Reynolds number. However, in gas turbine engines turbulence is generated in combustion systems to mix reactants of fuel and air with products of combustion in order to sustain the combustion process. This turbulence generated in combustion systems is then convected downstream into turbine passages. This flow field turbulence can cause substantial increases in heat transfer in regions of laminar flow and it can also lead to early transition to turbulent flow.
Another phenomena which can affect heat transfer rates on turbine surfaces is surface roughness. Generally, the surfaces of turbine components are relatively smooth after the manufacturing process. Sometimes these surfaces are polished to enhance surface smoothness to promote aerodynamic efficiency. Inside gas turbine engines, impurities in air and in fuel can soften or melt during the combustion process and these materials can deposit onto turbine surfaces. Generally, surface roughness in gas turbines can be caused by deposition of particulates, erosion of surfaces due to the impact of particulates and corrosion of turbine surfaces due to impurities in fuel. This roughening is expected to be especially prevalent in gas turbine systems which burn syn gas produced from coal. Sometimes turbine surfaces are coated with a thin layer of thermal barrier materials and these surfaces can roughen over time by similar processes. Thermal barrier coatings can also roughen due to spallation, the process of roughening due to the breaking off of a surface coating.
In engineering we are initially exposed to the influence of surface roughness with surface roughness in pipes. Most engineers are aware of Nikaradse’s famous sand grain roughness experiments where pipes were coated with sand particles and the resulting pressure drop was determined. This information has typically been presented using a Moody diagram. Investigators studying the influence of surface roughness on heat transfer and boundary layer development have often used regular roughness features such as sandgrains, sandpaper, or some type of conical or semispherical roughness. Recently, investigators who have used realistic surface, derived from actual engine surface roughness distributions or from deposition rigs.
In this presentation we will look at the impact of turbulence on vane heat transfer and boundary layer development. We will discuss the response of external turbulence to the surface of a vane and the resulting impact on the mixing across the boundary layer. We will also examine the influence of turbulence on vane aerodynamic losses and the resulting time averaged wake. Next, we will look at the additional influence of realistic roughness on vane surface transition and heat transfer. Finally, we will look at the impact of turbulence on stagnation region heat transfer. We will first look a stagnation heat transfer for a conventional situation to compare this with stagnation heat transfer with our very large cylinder rig. The results show how a larger stagnation region affects the augmentation of heat transfer.
-- Connie Cicha, Administrative Secretary, Physics & Astrophysics, firstname.lastname@example.org, 777-2911