Professor Pitz has developed a new molecular tagging technique that uses lasers to track and analyze airflow and combustion dynamics within hypersonic scramjet engines. 

Scramjets (the nickname for supersonic combustion ramjet) are air-breathing aircraft that can achieve speeds five times the speed of sound or faster. Because they burn air in the atmosphere instead of having to carry oxygen, scramjets present the tantalizing promise of feasible, affordable, super-fast flight.  

Scientists, military leaders and business visionaries have long been intrigued by the prospect of hypersonic flight. Yet despite successes such as NASA’s two recent flight tests of the now-discontinued X43 hypersonic plane, significant technical hurdles remain. 

Keeping the flame from going out in the combustion chamber is one of the major ones.

Engineers are testing a promising solution to this problem—a combustion chamber design that includes a small cavity on the floor of the chamber. This cavity doesn’t look like it would offer much protection, even less than a cupped hand could protect a candle flame from gale-force winds. But the cavity changes the airflow and combustion dynamics just enough to keep the flame going in a hypersonic aircraft’s engine – despite incoming airstreams blasting past at Mach 4 or better. 

So far, the cavity design is working well in the test labs at the Air Force Research Laboratory at Wright-Patterson Air Force Base near Dayton, Ohio.  But for a hypersonic engine to be large enough to propel a vehicle into space, it will need to be scaled up 10-100 times larger than the current test versions. 

“We need to know if our computer models are accurately predicting combustion in these engines,” Professor Pitz said. “To do that we have to find ways to measure and analyze the velocity and other combustion dynamics within scramjet engines.” 

Speed Zone 

To say that you’d need “split-second timing” to make it work is an understatement; proper timing is on the order of milliseconds. That’s one reason why scaling up to a usable size engine is such a challenge. 

“To scale up from our current system, we need to understand all the dynamics involved in initiating and stabilizing a flame,” said Robert A. Mercier, is Deputy for Technology for the Air Force Hypersonic Technology (HyTech) program at the Wright-Patterson Air Force Research Laboratory. “We’re excited about Dr. Pitz’s non-intrusive diagnostic techniques, because the traditional method of using probes in the flow path can’t work at these speeds. No probe can survive the 5,000 degrees Fahrenheit combustion process in a scramjet engine.”

The traditional velocity method of using particles, often made of aluminum oxide or a ceramic material, involves “seeding” them into the air flow. “They’re like bowling balls,” Professor Pitz said, because they are much bigger than molecules naturally occurring in the air. Not only do these large particles disrupt the airflow, but they don’t behave in the same ways as the air molecules. 

Considering this problem, Professor Pitz drew from his earlier award-winning, patented research involving non-intrusive laser diagnostic techniques for combustion. The new technique he developed uses two lasers that first “tag,” and then illuminate, molecules in the air. 

Using test equipment at Wright-Patterson, Professor Pitz first increases the humidity of the air to be studied, then blasts it at about Mach 2 (about 700 meters per second) down a 50-foot wind tunnel.  

Once the air reaches the scramjet test engine, it encounters two of Professor Pitz’s lasers. The first one breaks apart water molecules to form hydrogen atoms and hydroxyl molecules (one hydrogen atom and one oxygen atom). This laser is beamed into the combustion chamber in a grid pattern, so the hydroxyl molecules are formed only within the grid.  

The hydroxyl molecules are then excited by the second laser, which is precisely tuned to excite them to the point of fluorescence. A digital camera records the movement of the lighted grid of “tagged” molecules during a two-microsecond interval. 

“Once a laser line or grid is tagged, the grid moves with the flow. The displacement of tagged grid over a fixed time period yields the velocity,” Professor Pitz says. 

The research has proven helpful in streamlining the computer models used to predict and simulate the flow dynamics, Mercier said. “We are anchoring the codes on a small scale to ensure they are properly predicting the flow path,” he said.  

The Air Force hopes to scale up to vehicle size for flight tests scheduled for 2009. “Ultimately we hope to integrate hypersonic propulsion with high-speed turbines and rockets to make combined engines to power hypersonic cruise missiles and expendable space launch systems,” Mercier said. 

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