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The Delta Phi Method of Evaluating Overall Film Cooling Performance

[+] Author Affiliations
James L. Rutledge, Marc D. Polanka

Air Force Institute of Technology, Wright Patterson AFB, OH

David G. Bogard

University of Texas at Austin, Austin, TX

Paper No. GT2015-43288, pp. V05BT13A018; 9 pages
doi:10.1115/GT2015-43288
From:
  • ASME Turbo Expo 2015: Turbine Technical Conference and Exposition
  • Volume 5B: Heat Transfer
  • Montreal, Quebec, Canada, June 15–19, 2015
  • Conference Sponsors: International Gas Turbine Institute
  • ISBN: 978-0-7918-5672-7
  • Copyright © 2015 by ASME

abstract

Film cooling designs are often evaluated experimentally and characterized in terms of their spatial distributions of adiabatic effectiveness, η, which is the nondimensionalized form of the adiabatic wall temperature, Taw. Additionally, film cooling may alter the convective heat transfer coefficient with the possibility of an increase in h that offsets the benefits of reduced Taw. It is therefore necessary to combine these two effects to give some measure of the benefit of film cooling. The most frequently used method is the net heat flux reduction, which gives the fractional reduction in heat flux that accompanies film cooling for the hypothetical case of constant wall temperature. NHFR is imperfect in part due to the fact that this assumption does not account for the primary purpose of film cooling — to reduce the metal temperature to an acceptable level. In the present work we present an alternative method of evaluating film cooling performance that yields the reduction in metal temperature, or in the nondimensional sense, an increase in ϕ that would be predicted with film cooling. This Δϕ approach is then applied using experimentally obtained η and h/h0 values on a simulated turbine blade leading edge region. The delta-phi approach agrees well with the legacy NHFR technique in terms of the binary question of whether the film cooling is beneficial or detrimental, but provides greater insight into the temperature reduction that a film cooling design would provide an actual turbine component. For example, instead of giving an area-averaged NHFR = 0.67 (indicating a 67% reduction in heat flux through film cooling) on the leading edge region with M = 0.5, the Δϕ approach indicates an increase in ϕ of 0.061 (or a 61 K surface temperature decrease with a notional value of T - Tc = 1000 K). Alternatively, the technique may be applied to predict the maximum allowable increase in T against which a film cooling scheme could protect.

Copyright © 2015 by ASME
Topics: Film cooling

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