Photo by Tom Cogill.

Associate Professor Harsha K. Chelliah

Stand on a busy runway, and you can be overwhelmed by the roar and vibration of powerful jet engines. This sound, startling as it is, represents in part a compromise between power and stability, between energy produced by combustion and the acoustic field within the engine.

“Combustion amplifies resonance acoustic phenomena within an engine that are determined in part by the physical characteristics of the system,” says Associate Professor of Mechanical and Aerospace Engineering Harsha K. Chelliah. “These adverse thermoacoustic phenomena lessen combustion efficiency in engines of all kinds and, if not checked, can even produce a catastrophic failure.” Such acoustic effects are particularly pronounced when a lean fuel mixture designed to reduce pollution and lower energy costs is used.

The acoustic effects produced by single fuels are well studied. Chelliah specializes in modeling and simulation of reaction flows for propulsion and power generation systems. He is particularly interested in mixed fuels such as syngas and different combinations of hydrogen, carbon monoxide and natural gas. In each case, he wants to characterize the thermoacoustic effects under different conditions and evaluate different mitigation strategies, such as adding a secondary fuel source, that allow high efficiency operation.

In the laboratory, Chelliah uses a simplified flow geometry that contains all the important features of an actual gas-turbine engine and which allows for accurate numerical resolution of the flow field. He is currently setting up a micro gas turbine in the former nuclear reactor building to confirm the results of his laboratory tests. He also sees the turbine as a potential prototype for energy cogeneration. “We could use the turbine to generate electricity and pass the waste gases through a heat exchanger to warm the building,” he says.

Chelliah also is tackling the issues of efficiency from another perspective. In a turbine engine, the droplets produced by the injectors must be very fine. In order for turbines to be adapted for biodiesel, engineers must understand its characteristic spray pattern. “We need to create injection conditions that are similar to those produced with diesel fuel,” he notes. “Our challenge is not simply to develop new renewable fuels, but also to modify systems so that we use those fuels efficiently.”