Craft-Tech Works To Improve Efficiency of Turbopumps that Power Space Shuttle, ELVs
At just a little larger than a car engine, sitting in the three 14-foot-long main engines of a 120-ton space
shuttle, turbopumps might not appear to be a crucial part of a space launch.
But for engineers, they are perhaps the most worrisome part of the engines. Since turbopumps feed the liquid
hydrogen and oxygen through the engines at the exact proper flow rate and pressure, they are essential to the
engine's efficiency, which must reach more than a million pounds of thrust for lift-off. A failing pump can
cause catastrophic engine failure.
That's why NASA and industry researchers are constantly working to build better and more efficient
turbopumps for both the space shuttle main engine (SSME) and Expendable Launch Vehicles (EVLs), which
propel rockets into space.
Scientists at Combustion Research and Flow Technology Inc. (CRAFT Tech), a computational fluid dynamics
(CFD) research and development company in Pennsylvania, are using new simulation techniques and
advanced visualization software to study a phenomenon called cavitation, the least-understood problem with
turbopumps.
Powerful Pumps
Though small in size, turbopumps in space shuttles and ELVs have the power to thrust a column of liquid
hydrogen 36 miles into the air.
The pumps are rotational devices used to transfer fluid through spinning blade rows that travel at very high
revolutions per minute. The high-pressure side pushes the fluid through the blades and the low-pressure side
sucks it in. A single blade produces as much horsepower as a NASCAR engine, while all 100 blades working
together can produce 50 percent more power than a jet engine.
"The efficiency of the fluid through the blades is very important to the overall efficiency of the pump," says
Ron Ungewitter, research scientist at CRAFT Tech.
In the past, these pumps have caused technical problems in the engines and had to be removed and inspected
between each mission. Launch schedules often depended on when NASA could get new pumps in place.
Last year, Pratt & Whitney, the company that makes replacement pumps for the SSME, developed a more
efficient pump that doesn't require as much maintenance. The new pump only has to be overhauled every 10
missions, compared to every seven missions for the old one.
Cavitation a Big Issue
While much progress has been made in developing more efficient turbopumps, it is still difficult for engineers
to understand cavitation, which can cause great damage to the pumps.
Cavitation occurs as the blade spins faster and the pressure on the suction side continues to decrease,
Ungewitter says. "A point is reached where the pressure is so low that the liquid vaporizes, forming a gas. The
pump is designed to move through water, so when it encounters gas it offsets the balance and the pump will
vibrate, causing a lot of noise, a lot of wear, and possibly total destruction."
The blades can also get pitted from collapsing bubbles, which also significantly deteriorate the efficiency and
durability of the pump.
Engineers at CRAFT Tech are testing a new method to calculate and visualize cavitation bubbles.
"Most pumps and impellers are designed using computational fluid dynamics, but most codes have difficulty
predicting cavitation," Ungewitter says. "CRAFT Tech has developed a cavitation model and incorporated it
into its unstructured CFD code."
Predicting Bubbles
CRAFT Tech's process starts when the pump passage model is meshed using a hybrid unstructured grid. The
grid consists of tetrahedral, pyramid and hexahedral elements. A typical grid can contain more than a million
elements. Navier-Stokes equations are then solved at each point using the CRAFT-Tech CRUNCH-CFD code,
providing data on pressure, temperature, velocity and volume percentage of gas within the pump.
The researchers then turn to EnSight Gold software from CEI (Apex, N.C.) to visualize pressures on the blade
surface and blade passage that would show if cavitation has occurred. A cylinder cutting tool and unwrapping
feature in EnSight are used to slice the cylinder volume for the model.
"This permits visualization and understanding of the complex 3D fluid phenomena in the blade passages by
projecting the cylindrical surfaces to cascade-like 2D planes," Ungewitter says. "For calculations where
cavitation has occurred, we create an isosurface of the gas volume fraction, which shows the pocket of gas and
its size."
EnSight Gold automatically visualizes flow features such as tip vortex flows, secondary flows, streamline
traces and blade wake interactions.
"Particle traces are sometimes used to see how the liquid travels past the gas bubble, which often blocks the
passage," Ungewitter says. "Multiple analyses are performed to see how the bubble will change at different
speeds. EnSight also helps us look at streamtraces in parts of the flow domain where the flow is very complex
and often counterintuitive including the gap between the blade and housing."
Revealing Images
Using the advanced CFD code and EnSight Gold software, scientists at CRAFT Tech can now see how the
cavitation bubble changes flow in the passage and how that affects the efficiency of the turbopump design.
"We also get a lot of insight into the flow dynamics in critical regions, including why the pressure in certain
regions drops below the vapor pressure," Ungewitter says. "Being able to see the formation and size of the
cavitation bubble provides a unique and powerful design tool. We are also able to better understand complex
flow patterns."
"While the work is still in its early stages, results so far have proven to be very powerful," Ungewitter says.
"We hope to eventually benchmark this tool to more datasets and improve the fidelity of the cavitation onset
model and incorporate it in the design cycle," he says. "We hope to provide a numerical capability to predict
the onset and size of cavitation so that more efficient pumps can be designed to work at higher speeds."
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Anna Turnage is a writer specializing in computer graphics and other technology issues. She works for
Cramblitt & Company in Cary, N.C. She can be reached at (919) 481-4599.