The growing need for increased performance from gas turbines has fueled the drive to raise turbine inlet temperatures. This results in high thermal stresses especially along the first stage nozzle guide vane cascade as the hot combustion products exiting modern day gas turbine combustors generally reach temperatures that could endanger the structural stability of these vanes and greatly reduce the vane life. The highest heat transfer coefficients in the vane passage occurs near the endwall, particularly in the leading edge-endwall junction where vortical flows cause the flow of hotter fluid in the mainstream to mix with relatively lower temperature boundary layer fluid. This work documents the computational investigation of air injection at the end wall through a cylindrical hole placed upstream of the nozzle guide vane leading edge-end wall junction. The effect of the secondary jet on the formation of the leading edge horseshoe vortex and the consequent formation of the passage vortex has been studied. For the computations, the Reynolds averaged Navier–Stokes (RANS) equations were solved with the commercial software ANSYS Fluent using the SST k-ω model. Total pressure loss coefficient and kinetic energy loss Coefficient contour plots at the exit of the cascade to estimate the effect of the endwall fluid injection on loss profiles at the vane cascade exit. Swirling strength contours were plotted at several axial chord locations in order to track the path of the passage vortex in and downstream of the vane cascade. Two different hole-positions (located at 1 hole diameter and 2 hole diameters from the leading edge) along a plane parallel to the incident flow were considered in order to study the effect of the hole position with respect to the vane leading edge-endwall junction. Three different streamwise hole inclination angles with respect to the mainstream flow direction were studied to identify the best angle for the injection of fluid through the endwall. This angle was combined with five different compound angles (0°, 30°, 45°, 60° and 90°) in order to study the effect of varying the compound angle on the leading edge vortex and the passage vortex. Each of the above studies were conducted at two different injected fluid-to-mainstream mass flow ratios (0.5% and 1%) in order to study the effect of varying injected flow rate on the formation of the leading edge vortex and the vane passage vortex. From the results it was observed that suitable selection of the secondary injection mass flow rate, injection angle and hole-position caused an absence of the leading edge horseshoe vortex and delayed migration of the passage vortex across the guide vane passage. Heat Transfer studies were also conducted to observe the absence/weakening of the leading edge vortex and the delayed pitch-wise movement of the passage vortex. / Master of Science / Gas turbines are a kind of Internal Combustion engine that convert chemical energy to mechanical energy by way of burning an air-fuel mixture to cause turbine blades to spin and produce power. A typical gas turbine consists of a compressor which compresses the air intake into the combustion chamber, the combustion chamber in which energy is released from fuel by the combustion of the air-fuel mixture, and a turbine coupled to the compressor that is made to spin by the high pressure high temperature exhaust from the combustor. In order to increase the amount of power produced per unit (by weight or volume) of fuel consumed and increase the performance of the engine, the turbine inlet temperature i.e. the temperature of the hot gas products leaving the gas turbine combustor is increased by changing the fuel flow rate into the combustors and the amount of compression of the air entering the combustor. Consequently, the first component of the turbine, the nozzle guide vane faces high thermal loading which could structurally endanger vane life. The existence of complex secondary flows (leading edge vortex, passage vortex, corner vortices) near the junction of vane’s leading edge and the turbine endwall to which the vane is connected to causes increased heat transfer at this point as opposed to other points on the vane surface. The aim of this work is to study through computational simulations how injecting high momentum fluid (air) near the leading edge junction to observe any changes to the secondary flow near the endwall. The angle at which this fluid is injected and the rate of injection of this fluid are, among others, the parameters varied in this study. The flow near the leading edge and through the vane passage is visualized and the pressures at the inlet and outlet of the test domain measured at each step to compute parameters which decide how further studies are designed. The ultimate aim of this project is to identify if injecting fluid through the endwall would prove useful in reducing the vortical flows near the endwall (thereby reducing the thermal load on the endwall).
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/72904 |
| Date | 07 September 2016 |
| Creators | Dhilipkumar, Prethive Dhilip |
| Contributors | Mechanical Engineering, Ekkad, Srinath V., Lowe, K. Todd, Huxtable, Scott T. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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