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The Effect of Unsteadiness on Film Cooling Effectiveness.

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Final rept. 1 Nov 95-9 Jul 96,

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A unique feature of turbine rotor blade film cooling is the main flow unsteadiness caused by the upstream stator vanes. The combined effect of the vane inviscid flow field and the trailing edge wake results in a rapidly changing external pressure at the film cooled blade surface. Because the film flow through the cooling holes is usually unchoked, the varying external pressure results in a modulation of the mass flow through the holes. This study examined the effect of coolant flow modulation on the film effectiveness and the heat transfer downstream of a row of film cooling holes. Coolant oscillation frequencies and amplitudes were selected to match typical modern gas turbine engine conditions when represented in the appropriate non-dimensional forms. Time average blowing rate the ratio of coolant mass flux to free stream mass flux was varied from 0.6 to 1.5. Measurements were made of the flow velocity and temperature fields, the adiabatic film effectiveness, and the film cooling heat transfer using a constant flux heat transfer surface. Both the axial streamwise and lateral cross stream distributions of these quantities were measured from a single row of five circular holes angled at 60 degrees with respect to the surface normal. Frequency spectra taken from measurements of the fluctuating velocity were used to find the extent of the influence of the driving frequency downstream. The observed effect of the coolant flow oscillation was to decrease effectiveness in the streamwise direction, while having little or no influence on effectiveness in the cross stream direction. The rate of decrease of streamwise effectiveness is, however, a strong function of blowing rate, frequency, and amplitude of fluctuations.

Subject Categories:

  • Fluid Mechanics
  • Thermodynamics
  • Jet and Gas Turbine Engines

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