Loren Soma

Date of Award

January 2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Forrest E. Ames

Abstract

The purpose of this study is to experimentally investigate the effects of high turbulence and secondary flows on slot film cooling over an aft loaded vane (turbine blade) with a large leading edge. Turbine nozzle component cooling methods are challenged by very high combustor discharge temperatures, turbulence effects, and high acceleration requiring cooling methods to provide reliable and acceptable component life. Throughout the development of gas turbines, film cooling has become one of the most prevalent techniques to protect high pressure turbine airfoils and endwalls. Film cooling works by supplying a film of cooler than the freestream gas onto the external surface through internal means without significantly increasing turbulence or capturing hot freestream gas. Unfortunately, leading edge and pressure surface film cooling is susceptible to particulate deposition and clogging [1] which slot film cooling is known to significantly mitigate [2]. Understanding the influence of secondary flows in sweeping away film cooling coverage is also important in developing a reliable cooling design. This study investigates a vane designed to be resistant to particulate deposition while providing a highly effective and efficient cooling method. An internal cooling scheme was implemented instead of showerhead cooling arrays in the leading-edge region and shaped holes on the pressure surface, both of which are susceptible to clogging due to particulates in the fuel or air. To accommodate for the internal cooling method, the leading-edge region must be designed with more room; a characteristic that also helps to reduce the leading-edge heat transfer coefficient [3]. Conventionally, the spent internal coolant air is discharged from the vane in a way that can be detrimental to external cooling mechanisms. However, the present approach is to collect the spent internal flow and use it for slot film cooling, a near optimum cooling method which has also been found to be tolerant to particulate deposition. Additionally, film cooling studies have discovered that acceleration inhibits mixing and delays laminar to turbulent transition which enhances the resulting film cooling effectiveness. Conversely, acceleration also inhibits boundary layer growth which contributes to aerodynamic losses. Accordingly, this study uses an aft loaded vane which accelerates the flow over most of the suction surface. Film cooling distributions were acquired in UND's large scale low speed cascade wind tunnel facility. Relevant inlet turbulence conditions were generated using grids or simulated aerocombustors. Two vanes were constructed and tested over a range of Reynolds numbers; one for the pressure side measurements and one for the suction side measurements. Vane film cooling locations are consistent with modern vane cooling application. Cooling air for each slot was fed by individual plenums with controls to measure and regulate film cooling blowing ratios. The film cooling supply and measurement system consisted of an existing blower, heat exchanger, a window air conditioning unit, a thermal inertia system, and an orifice tube for each plenum. Flow from the blower was directed into a duct connected to the evaporator of the air conditioning unit. A similar duct was connected to the exit of the evaporator and directed air into the thermal inertia system. The thermal inertia system consists of stacked aluminum plates spaced to allow air to flow in between the plate surfaces. The purpose of the thermal inertia box is to keep the cooling air supply temperature constant during the experiment. The study has been well documented and analyzed for further development of predictive models. The comprehensive film cooling database generated in this study has improved our understanding of the influence of turbulence and secondary flows on film cooling. This work will be highly useful in grounding high fidelity computational methods and help cooling system designers implement slot cooling throughout the industry.

Download

Share

COinS