Simulation and Experimental Validation of the Wheel Space of the Axial Flow Turbine Research Facility Under Low Purge Flow Conditions

Open Access
Author:
Averbach, Michael Alec
Graduate Program:
Aerospace Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 08, 2014
Committee Members:
  • Cengiz Camci, Thesis Advisor
Keywords:
  • Gas Turbines
  • Axial Flow Turbine
  • AFTRF
  • Wheel Space
  • Purge Flow
  • Ingress
Abstract:
Increasing fuel cost and environmental standards create the need for more efficient gas turbine engines. The combustion exit temperature is often raised to increase engine efficiency and reduce specific fuel consumption. Current combustion exit temperatures exceed the melting point of nickel alloys used in the turbine stage of gas turbine engines, which follow the combustor. Ceramic coatings and cooling air bled from the final stages of the compressor allow for the turbine components to maintain temperatures that prevent failure. Between stationary and rotating components of a gas turbine engine, a gap is maintained to prevent a collision. This gap creates an open cavity beneath the hub of the rotating and stationary components. If not properly sealed, air in the hot gas path can enter the wheel space; this process is known as ingress. Conversely, air leaving the wheel space and entering the hot gas path is known as egress. The components beneath the rotor and stator hubs typically do not have ceramic coatings and in the presence of ingress may be heated above acceptable limits. Methods for preventing ingress include geometrical designs, as well as the use of purge flow. Like the air used to cool turbine blades and vanes, purge flow is bled from the final stages of the compressor. The use of purge flow will reduce engine efficiency since that air is no longer used for combustion. It has been experimentally and computationally shown that under low purge flow conditions low frequency structures form in the wheel space, moving slower than the rotor speed and the number of structures is unrelated to the number of vanes or blades. It has also been shown that these structures can cause ingress deep into the wheel space. The Axial Flow Research Turbine Facility (AFTRF) was recently updated with blade, vane, and wheel space geometry representative of the next generation of aircraft turbomachinery. The objective of this study was to indentify low frequency structures that were present in the wheel space of the rig under low purge flow conditions. Experimental and computational studies were completed, in conjunction, to fully understand the flow structure and pressure distribution in the wheel space. An understanding of number of structures and their rotational speed will aid in the validation of future computational models of the AFTRF wheel space which could be used do design a more effective rim seal. It will also be useful in understanding future experimental results from the AFTRF. The experiential study showed 15 structures rotating at 77.5 % of the rotor speed and the computational analysis showed 14 to 15 structures rotating at 81.7 % rotor speed. The agreement was deemed acceptable to validate the computational model. From the computational model, ingress mass flow from the hot gas path into the upper wheel space was shown to be 2.5 times greater than the supplied purge flow and ingress from the upper wheel space into the lower wheel space was 1.75 times greater than the purge flow. Pressure in the wheel space was shown to have the strongest correlation with local ingress and egress patterns and pressure on the rotor hub, upstream of the blade, was also strongly correlated. Pressure on the stator hub downstream of the vanes appeared to have little correlation with local ingress and egress patterns. Further investigation showed locations of maximum ingress were caused when the stator vane wake, rotor blade leading edge, and low pressure in the wheel space coincide at similar circumferential locations.