Predicting Bubbles

mechanical engineering power 2002

predicting bubbles

Research to understand cavitation aims to advance the fuel and oxygen pumps in the space shuttle.

This article was prepared by staff writers in collaboration with outside contributors.

At just a little larger than a car engine, sitting in the three 14-foot-long main engines of a 4.5-million-pound space shuttle, the size of a turbo-pump understates its importance to a space launch.

One turbopump feeds hydrogen and another oxygen, at the correct rate and pressure, to the shuttle engine's main combustor, and so they are essential to performance. A failing pump can take an engine with it. That's why NASA and industry researchers are constantly working to build better and more efficient turbopumps.

Pratt & Whitney, which makes replacement pumps for the space shuttle's main engines, developed a more efficient model that doesn't require as much maintenance.

George Hopson, manager of the Shuttle Main Engine Project Office at NASA's Marshall Space Flight Center in Huntsville, Ala., said weight savings made elsewhere in the shuttle's design permitted use of a heavier pump. The P&W pump eliminates welds and has a larger rotor assembly than its predecessor. A ceramic bearing makes it more robust.

Scientists at Combustion Research and Flow Technology Inc., a computational fluid dynamics research and development company in Dublin, Pa., have launched a study of a little-understood problem with turbopumps—cavitation. The work done so far is covered by Phase One funding of a Small Business Innovation Research grant, according to Ashvin Hosangadi, principal scientist at CRAFT Tech.

According to Pratt & Whitney, the pumps drive fluid through stages of spinning blades. The hydrogen pump spins at 36,200 rpm to deliver 162 pounds of hydrogen per second at a discharge pressure of 6,450 psi. The oxidizer turns at 23,700 rpm for 1,180 lbs. of oxygen per second at 7,500 psi. CRAFT Tech modeled a generic pump, tested with water in a laboratory at Marshall.

Cavitation occurs as pressure decreases on the suction side. "A point is reached where the pressure is so low that the liquid vaporizes, forming a gas," said Ron Ungewitter, a research scientist at CRAFT Tech.

Unraveled cylinder cuts of the impeller passage model show pressure contours in the turbopump without cavitation (left) and during cavitation.

A pump designed to move through liquid can vibrate when it encounters gas. Hopson pointed out that, at 36,000 rpm, a vibration can have serious consequences. Collapsing bubbles, moreover, can pit blades and deteriorate the efficiency and durability of the pump.

"Most pumps and impellers are designed using computational fluid dynamics, but most codes have difficulty predicting cavitation," Ungewitter said. "CRAFT Tech has developed a cavitation model and incorporated it into its unstructured CFD code."

CRAFT Tech's Crunch-CFD code solves Navier-Stokes equations at each point to provide data on pressure, temperature, velocity, and volume percentage of gas.

The researchers used EnSight Gold software, developed by CEI of Apex, N.C., to visualize the effects of cavitation. They could investigate, for instance, how a bubble can change flow in the passage and how that can affect the turbopump's efficiency. CRAFT Tech has applied for funding for Phase 2 of the research, which will look at flows using cryogenic fluids.

"We hope to benchmark this tool to more datasets, improve fidelity of the cavitation onset model, and use it in the design. We want a numerical capability to predict the onset and size of cavitation so more efficient pumps can be designed to work at higher speeds," Ungewitter said.