Originally Posted by
pattern_is_full
Would DAC mean the dual concentric rings of (for want of a better word) nozzles or flameholders shown in this X-Section? Which seems to create an - interesting - pattern of pressures inside the chamber.
Two sets of small torches instead of single larger ones, which may be where the lower emissions come from?
https://www.researchgate.net/figure/...fig6_269050769
Yes - two concentric rings of fuel nozzles. I believe the GE90 was the first engine to use the DAC. They had a lot of trouble making it work right - one particularly troublesome aspect that was - during an accel they'd transition from one ring to both - and there would be a discrete lag before the flame spread to the second ring resulting in a weird discontinuity in the accel characteristics. I left the program before they got it sorted so I don't know how they fixed it (my sneaking suspicion is that they started using both rings at idle in-flight, but I don't really know).
The DAC was an option on the CFM56-7/737NG - it reduced certain emissions but it resulted in a significant increase in fuel burn so very few were actually delivered.