In a bid to simultaneously reduce both fuel consumption and all pollutant emissions–goals that are often at odds–French aerospace research center Onera and engine manufacturer Snecma are working on the next generation of low-NOx combustors

The effort is aimed at meeting Acare 2020 goals, set in 2000 by the European aerospace industry. NOx emissions by 2020 are supposed to be cut by 80 percent from 2000 levels. The challenge is to do so without increasing other pollutant emissions. For example, those conditions that are unfavorable to NOx are favorable to the formation of carbon monoxide (CO).

Ground tests late last year of a row of four injectors, representing a portion of an annular combustion system, demonstrated something critical for the project to proceed: the combustor’s capacity to relight at altitude. The tests were conducted in a low-pressure (0.6 bar), low-temperature (-15 degrees C) environment.

Burning a lean fuel-air mix meets the Acare low fuel burn and low emissions requirements. However, lean combustion is unstable. To address that characteristic, Snecma and Onera engineers employ a trick borrowed from old gas stoves. In each burner, a center “pilot light” burns a rich fuel-air mixture, ensuring stable combustion. The crown-shaped zone around the pilot light indicates the lean area.

The system is sensitive and researchers have to find the right level of interference between the “pilot light” and the “crown.” Too much interference would limit the reduction in emissions; too little would create lighting problems.

Combustors with two distinct zones–one rich, one lean–already exist. However, because the two zones’ dimensions are similar to the combustor’s they create a relatively irregular temperature profile, which accelerates the aging process of the turbine blades downstream. The new architecture combines two or three dozen small dual zones, thus ensuring an even temperature profile.