Oxy/fuel Mixture DDT Runup Distances

DYI · Posted: 04/22/2008 19:45 PM Post subject:

I've got something of a handheld in the works, and I don't want what my username abbreviates to happen yet, so here's the question:

How does oxygen enrichment affect the DDT runup distances of gaseous hydrocarbon mixtures?

We all know that it will be decreased, but to what extent? As far as most of us know, DDT runup distance, assuming perfect mixing, no turbulence, and no restrictions, is about 35 feet in air/propane mixes. It's also been tentatively determined that increasing the mix does have an effect on the runup distance, but not a linear one, due to the decreased burn rates caused by higher preignition pressures.

I suspect that oxygen enrichment will have a much more pronounced effect on the runup distance of the mix, for the reasons of increased burn temperature (and therefore increased burn speed) and the fact that more fuel can be burned with a much lower preignition pressure, decreasing the dampening effect that protects traditional hybrids from detonation. Also, the nitrogen in air acts as a kind of buffer, as it undergoes very little change during the burn, and cannot actually support combustion. While the initially present nitrogen will remain in the chamber, it's percentage will steadily decrease as the mix increases, unlike in traditional air/propane mixes where it remains constant as the mix increases.

I believe that, for these reasons, DDT runup distance will be affected by the amount of fuel being burned much more strongly than a traditional hybrid is, because of the decreasing percentage of buffer gas, as well as the reduction of the preignition pressure dampening effect by a factor of roughly 5. I also believe that the runup distance in atmospheric pressure oxy/propane should be less than 8 feet in a 4" pipe. And if the roughly linear and direct relationship between combustion vessel diameter and DDT runup distance remains the same for oxygen enriched mixtures, this would suggest that the runup distance of oxy/propane in a 2" diameter cylinder is less than 4 feet. The vessel in my design is 27" long, and I'm beginning to suspect that even 2x oxy/propane mixes in it would be unneccesarily dangerous.

The only problem is, sealing becomes rather more difficult with the higher pressures required when using air as an oxidiser, and metering becomes less accurate, since the 0-15psig gauge cannot be used for oxidiser metering. Obviously safety trumps ease of use, but I'm wondering whether the two are indeed mutually exclusive in this situation.

Any thoughts?