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Chapter 6
Empirical Performance Models Based on Two-Dimensional Cascade Tests

Excerpt

The theoretical methods to analyze the flow in cascades presented in Chapter 5 are useful design and analysis methods, but they are not sufficient for day-to-day axial-flow compressor design activity. The many limitations restricting the accuracy and range of application of those methods have been discussed in Chapter 5. Consequently, empirical models are commonly used as a means of predicting the basic performance of cascades in axial-flow compressor design and analysis. These empirical models are derived from experimental data obtained from two-dimensional cascade testing. Extensive testing of this type has been accomplished, particularly by the NACA. Rather sophisticated empirical methods are available for the standard blade profiles discussed in Chapter 4. In the case of advanced blade types, such as the controlled diffusion airfoils discussed in Chapter 4, alternate empirical models may be required. Typically these will be an adaptation of the methods used for the standard airfoil profiles, specifically addressing the improved performance characteristics achieved by the new profile design.

The basic objective of the empirical modeling process is to predict the fluid turning and total pressure loss for a cascade under fairly general operating conditions. Also, operating conditions where near-optimum performance can be expected need to be identified by the empirical models. These are often referred to as design conditions, since they are appropriate for use under the compressor's design operating condition where optimum performance is usually desired. Indeed, it will be seen in Chapters 10 and 11 that these can be used as a basis for selecting blade geometry to match the desired flow field through the compressor. Special empirical models to address blade tip leakage and seal leakage through stator shroud seals will also be covered in this chapter.

  • 6.1 Cascade Geometry and Performance Parameters
  • 6.2 Design Angle of Attack or Incidence Angle
  • 6.3 Design Deviation Angle
  • 6.4 Design Loss Coefficient and Diffusion Factors
  • 6.5 Positive and Negative Stall Incidence Angles
  • 6.6 Mach Number Effects
  • 6.7 Shock Wave Loss for Supersonic Cascades
  • 6.8 Off-Design Cascade Performance Correlations
  • 6.9 Blade Tip Clearance Loss
  • 6.10 Shroud Seal Leakage Loss
  • 6.11 Implementation, Extensions and Alternate Methods

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