Technical Challenges and Scientific Approach for a Sustainable Energy Efficient Future - Novel Combustion Concepts for Sustainable Energy Development - page 17

Fig. 8 Colorless distributed combustion, CDC (green combustion turbine)

ef flame (Fig. 8 )

and the reduction in exhaust temperatures have been observed at low pressures.
The challenge is to apply this to higher pressures involved in aircrafts. Though
space is not a limitation in stationary applications (such as in electrical power
plants), it is a challenge in aircraft engines as the add-on components will increase
the weight and volume. Clever designs with minimal weight and volume penalty
should be sought. Computational studies and system optimization consistent with
new fuel characteristics are also challenges.

ciency. The physical color change from yellow fl flame to colorless fl

4.2 Porous Inserts

Studies have been made to evaluate performance of combustors with high-tem-
perature foam material introduced at different locations within the combustion
chamber (Ref. Agrawal). The idea is to stabilize the high-temperature combustion
fl flow and to eliminate hot spots. High-temperature HfC/SiC-coated foam inserts at

appropriate positions (as a ring at the inner periphery of the chamber) has been
found to reduce combustion noise, mitigate combustion instability, and reduce CO
emission (Fig. 9 ).

4.3 Supersonic Combustion

Supersonic combustion will play a major role in the combustion scenario in the near
future due to the advent of hypersonic propulsion. For scramjet combustion, mixing
of fuel and oxidant has been an issue and a number of researchers addressed this.
Radial fuel injection, steps, and countercurrent combustion seem to improve the
mixing and combustion. Recently, fin guided fuel injection (Ref. Yu) has shown to

produce a twofold increase in fuel penetration and 45 % reduction in jet-induced
shock strength (scramjets

— Fig. 10 ).