The technology could help military jets fly at high altitudes, passenger planes and unmanned drones cruise for long distances while conserving fuel, and supersonic jets maintain ignition at speeds that would normally suffocate flames with fast-flowing air.

Scientists know that the introduction of plasma to the reaction near or at the location where the flame ignites produces new chemical species that catalyze combustion. However, no one knows precisely what species are involved, what the reactions are, and what their rates are.

To better understand plasma-assisted combustion and to develop future technology, researchers are conducting experiments and creating computer models to determine which chemical processes are involved.

At the meeting of the American Physical Society’s Division of Fluid Dynamics, held 24 – 26 November in Pittsburgh, Igor Adamovich of Ohio State University discussed some of his and his colleagues’ recent experimental results and computer models on low-temperature plasma assisted combustion.

The principal challenges in development of a predictive kinetic model of non-equilibrium plasmas sustained in fuel-air mixtures include (i) lack of “conventional” chemical kinetics mechanisms validated at low temperatures; (ii) lack of data on rates and products of reactions of excited species generated in the plasma, some of which are not well understood, and their coupling with fuel-air plasma chemistry; and (iii) scarcity of data obtained in well-characterized plasma-assisted combustion experiments, which can be used for model validation.

“Conventional” combustion chemistry mechanisms have been developed for relatively high temperature conditions. Their applicability at temperatures below ignition temperature, common in plasma assisted combustion environments, needs to be assessed to determine if they can be used as a basis for a plasma-assisted combustion chemistry mechanism. This requires time-resolved measurements of radical species concentrations during low-temperature fuel oxidation, when an initial pool of primary radicals (O, H, and OH) is generated in the plasma, such as in the late afterglow of an electric discharge.

This allows isolating relatively slow “conventional” low-temperature fuel oxidation reactions triggered by the radicals from the reactions of excited species generated in the discharge, which decay relatively rapidly.

—Adamovich (2013)

The researchers studied reactions and reaction rates at air pressures that represent high-altitude flight and at temperatures between 200 and 400 °Celsius—below ignition temperature and where data and reliable models are particularly lacking. The researchers found that for simpler fuels—such as hydrogen, methane and ethylene—the models agreed fairly well with experimental data, while for propane, the agreement was much worse.

Just over five years ago, relatively little was known about how plasma-assisted combustion works, Adamovich said. But since then, scientists have made significant progress toward identifying the mechanism behind the plasma-assisted combustion chemistry. “We hope in a few years, such a mechanism might emerge,” he said.

Resources

• Igor Adamovich (2013) “Kinetic Modeling of Low-Temperature Plasma Assisted Combustion”