Geology Reference
In-Depth Information
In mud flows, tip vortices entrain sand; the continuous high-speed rubbing
that results leads to rapid metal loss at rotor tips as shown at the left side of
Figure 8.15. It is known that the greater the pitch angle, the greater the strength
of the vortex. Since blade angles are high for MWD turbines, this erosion is
severe. Reductions in effective radius due to erosion are responsible for large
losses in torque and power. Tip vortices have been studied by aerodynamicists.
Figure 8.16 also shows results for computer simulations and flow visualization.
There are, however, effective ways to deal with rotor tip erosion and power
loss; the remedies used in any particular situation are selected with due attention
to trade-offs and compromises. One simple solution, suggested by the aerospace
examples in Figure 8.17a, is the wrap-around shroud in Figure 8.17b, a
downhole mud turbine prototype for a time-tested aerodynamic concept. The
shroud completely eliminates tip erosion. Also, since loadings no longer vanish
at rotor tips, the shrouded turbine is more effective in generating torque and
power, so that shorter blades are possible. One concern, however, is “sticking”
due to mud gelling when drilling operations are interrupted. The relatively large
surface shroud areas may form strong adhesive bonds that are difficult to break.
Figure 8.17a. Aerospace blade rows with shrouds.
Figure 8.17b. Turbine prototypes with and without shroud.
An alternative to shrouded turbines is twisted blade design. Figure 8.17c
shows aerospace examples commonly used, with three-dimensional velocity and
pressure variations highlighted in color. Sometimes, flexible blades are
employed that deform at high flow rates. In either case, the design objective is
reduced flow efficiency at large angles of attack (induced by flow separation) so
that excessive power is not created. An example of a downhole twisted blade
concept is given in Figure 8.17d.
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