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Courtesy of HMA Power Systems,
The Netherlands.
Centrifugal pumps, driven by very large electric motors, are used in
underwater dredges to create shipping lanes. In order to operate effectively
at large depths, the motors must be located underwater close to the pump.
For protection, the motors are filled with oil at a slight overpressure,
with the oil pressure regulated as a function of water depth. The oil
also serves as an effective coolant. One disadvantage of using oil, however,
is a large viscous loss that is a major component of the total power loss.
Engineers at HMA Power Systems used FIDAP to model the oil flow in a
motor in order to quantify the viscous losses and the effectiveness of
the oil as a heat transfer agent. The analysis is especially challenging
because of the complex, three-dimensional geometry inside the motor.
The calculated power loss is in good agreement with experimentally
measured values, adding confidence
to the FIDAP predictions.
Start-up Losses Understood
Experimental measurements showed that during the start-up phase of the
motor, a sudden increase in power loss is observed, possibly because the
oil flow, in the gap between the rotor and stator, transitions to an unstable
flow state. The CFD model confirmed that under certain conditions a Taylor
instability occurs, in which the relatively simple rotor-stator flow changes
to one dominated by a series of ring vortices spaced periodically in the
axial direction. The FIDAP model also identified the effects of operating
temperature, oil viscosity and axial velocity on the Taylor vortex flow.
The calculated power loss is in good agreement with experimentally measured
values, adding confidence to the CFD predictions.
Axial cross-sectional view of a part of the 3.2 MW
suction dredge motor.
Heat Transfer Confirmed
Heat generated by the rotor and frictional losses must be convected by
the oil to the winding end space and then conducted through the outer
wall to the seawater. A 3D analysis of the flow showed that despite the
obstruction caused by the winding end, there is sufficient oil flow in
the radial direction to guarantee good heat exchange. The FIDAP model
also quantified the performance of the outer wall of the motor housing
as a heat exchanger and confirmed the effectiveness of fins placed on
the inside wall of the housing.In June 1998, Fluent and Sun Microsystems,
Inc. entered a joint agreement to explore high-performance network parallel
computing using Fluent's CFD software. Sun provided Fluent with a cluster
of 7 dual-processor 300MHz Sun Ultra 60 workstations, each with 512 MB
memory, enabling Fluent to study network computing on large complex models
typical of the "next-generation" of emerging CFD analyses.
Contours of velocity magnitude show that the oil flow
is mainly in the radial direction, ensuring good heat transfer
to the outer wall.
Reference:
W. A. de Zeeuw, J. C. A. M. Gordens, E. Tuinman, "CFD
Simulation of Fluid Dynamics and Heat Phenomena in a 3.2 MW Hopper Suction
Dredge Motor", Proceedings of the ASME Pressure Vessel and Piping Conference,
San Diego, CA, July 1998.
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