A gas flowing at greater than 0.6 Mach will not mix with a static volume of gas (the density difference is too great, dynamic pressure at 0.6 Mach is quite high, so density is at least an order of magnitude higher than the same gas at rest). Consequently the high-velocity gas will tend to compress the gas in front of it (much like the piston in a diesel engine). As the downstream gas is compressed, the heat of compression tends to increase the temperature of the static gas rapidly. If the downstream gas reaches a high enough temperature that when the high-velocity gas is dead ended and mixing begins then as soon as the mixture is above the UEL mixture percentage if the mixture is above the autoignition temperature then it will ignite, just like a diesel engine. This all sounds pretty far fetched, but it tends to happen a couple of times a month somewhere in the world, and the Oil & Gas industry suffers about one fatality a month related to this phenomena and has for nearly 100 years.
Wow! That is fascinating. Its also sad to hear that this phenomena regularly kills people.
So I'm assuming this type of thing happens when high velocity gas is piped into a tank initially filled with air.
Would the same type of thing occur if you were to have a flow of gas and a flow of air directed at each other at high velocity. My guess is yes based on what you stated in the previous post.
Thank you David. I try to learn something new every day. Box definitely checked!
It is really hard to get a heat of compression explosion in a tank. The issue there is often improper grounding leads to a static build up (really common in non-metallic tanks) that can discharge a spark hot enough to ignite the mixture. The "dieseling" kind of explosion is limited to pipe. The biggest pipe I've seen evidence of these fires is 6-inch, but I can make the math work for an 8-inch too.
I missed your last statement when I responded to the tank question.
Two high-velocity streams will not mix. I can point a 1.0 Mach methane stream at a 1.0 Mach air stream with an ignition source at the point of contact and the resulting fire will be underwhelming (only the gases slowed by the no-flow boundary will participate). You also will not get any measurable heat of compression. Once everything has slowed to the velocities we consider "incompressible" (depending on the researcher, below somewhere between 0.3 and 0.6 Mach), you will begin to see mixing. I once saw a flow visualization experiment where a high velocity red stream was shot at a high velocity blue stream inside of a clear plastic pipe (no idea how they colored the gases, maybe smoke?) Purple gas started appearing about 20 pipe diameters either side of impact point (between the two purple points, there were streams of red and streams of blue indicated a reluctance to mixing). The purple gas could very well have been between the LEL and UEL of a flammable gas, but the temperature of the purple gas was the same as the red and blue gases so you don't have an obvious ignition source.
