Geology Reference
In-Depth Information
The middle diagram in Figure 8.3 shows how, since pressure loadings
necessarily vanish at the rotor tip, torque creation is least at large radii, an
unfortunate situation. As highly pitched blades are associated with vortex
shedding from the tips and rapid local erosion, power generation worsens, as
suggested at the right of Figure 8.3 (this effect is explained later). From a fluid
mechanics perspective, we have highly separated, three-dimensional, unsteady
flows which cannot be analyzed with rigor. Given the time, cost and labor
constraints associated with typical engineering projects, the outlook for any
turbine design, let alone a good one, at first appeared pessimistic. Figure 8.4
summarizes the major differences between aircraft and MWD turbines.
Aircraft
MWD
Number of stages
Many
Single (small volume size constraint)
Blade separation
Close
Wide (avoid debris jamming)
Tip-to-shroud
clearance
Large (prevent jamming from vibration,
doglegs, gelled mud - means low torque)
Tight
Jamming
No
Yes (vibration, doglegs, debris)
High rotor tip wear (sand recirculation
by vortex flow), constant stator wear
(sand abrasion from rock entrapment)
Erosion
Normal
High cycle fatigue, bit bouncing, strong
transverse loads
Shock and vibration
None
Low (high blade pitch angles and large
tip clearances, flow separation)
Efficiency
High
Figure 8.4.
Qualitative comparison, aircraft versus MWD turbines.
8.2 Why Wind Tunnels Work
Designing an MWD turbine is expensive, time-consuming and labor-
intensive. Once torque and power requirements at a given volume flow rate are
specified, the turbine geometry is to be determined. “Geometry” includes many
parameters: annular inner and outer radii, number of blades, cross-section
contour of a blade, pitch of the blades, rotor tip clearance, stator-rotor
separation, and so on. Usually, because of downhole mechanical packaging
constraints, only one stage can be accommodated, that is, one stator and one
rotor. Still, the number of possible configurations is vast, perhaps hundreds.
Testing of metal models in flowing mud is inconvenient, with theoretical
analysis being equally difficult. Aircraft companies rightly deal with potential
flow analyses such as that presented in Chapter 7. Commercial software
packages are often less rigorous and should be carefully evaluated.
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