Handbook of Turbomachinery Second Edition Revised - Ventech!

and also increases the critical speeds to those determined by the bearing
support structure. The structural damping (i.e., 5% of critical damping) then
determines the magnitude of the unbalance response as the rotor decelerates
through its critical speeds.
The large amplitude response and resulting rubs render the system
nonlinear. Most blade-out unbalance response analyses are conducted using
piecewise linear equivalent springs and dampers at the rub locations. The
rub can be modeled as a bilinear spring with zero stiffness until the
deflection equals the clearance between the rotating assembly (i.e., the blade
tips) and ‘‘hard’’ metal. The stiffness after hard contact is based on the
lateral bending and ovalization of the casing. The steady-state response at
each spin-speed is determined using an iterative process, continuing until the
assumed response at each rub location matches the predicted response
within a convergence criterion.
Figure 43 illustrates a typical steady-state blade-out response using the
straddle-mounted example with squeeze film dampers presented earlier.
Assuming a 0.10-in. (2.54 mm) cgx of the aft disk after a blade failure and no
rubs, the response at the first critical speed resembles that of the hardmounted,
in-phase unbalance case shown in Fig. 39a except it is over 60
times greater. Due to the large rotor whirl both of the squeeze film dampers
Figure 43 Steady-state blade-out response: straddle-mounted rotor.
Copyright © 2003 Marcel Dekker, Inc.