Aircraft Propulsion, 2nd Edition

8.6.4 Aerodynamic Forces on Compressor Blades

The conservation principles that we learned and applied to wings and bodies in external aerodynamics apply to internal aerodynamics as well. There are some complicating factors in a turbomachinery flow that are absent in external aerodynamics. The most dominant feature is the presence of strong swirl in turbomachinery that is either completely absent or weak in external flows. Second, the net turning in the flow is large in turbomachinery and zero in external aerodynamics. We may characterize the flow in a turbomachinery as lacking a unique flow direction, unlike external aerodynamics. As an example, the Kutta–Joukowski theorem on lift predicts a magnitude for an ideal two-dimensional lift (on a body that creates circulation, Γ) as ρ_∞V_∞Γ and a direction for the lift that is normal to V_∞. In turbomachinery blading, there are two distinct flow speeds, one upstream and the other downstream of the blade, that is, W₁ and W₂ and not just a single V_∞. Also, on the question of the direction of this force, is it normal to W₁ or W₂? In turbomachinery, we will define a “mean” flow angle and a “mean” velocity that help us describe the blade lift and its direction. There are other complicating factors in internal flows that deal with flow distortion (e.g., upstream wake transport and interaction with downstream blades) and unsteadiness (i.e., from neighboring blade rows in relative rotation) that are inherent in a turbomachinery stage and have ...