The 3 T's of combustion

[gadget]

I was reading through a bunch of threads last night and noticed lots of info on heat retention and mixing. But....I just don't see much talk on retention time for the burn and I was wonder why this is?

residence time is equally important to mixing and heat. You need all three. Yes you can reduce other by increasing one of the 3 but that works only to an extent. I'm just wondering why no one focuses on keeping the burn hot for longer.

1500F for 2 seconds is a good minimum residence time that I have come up with after studying many white papers. Atleast for solid 2 stage fuel "wood" - (non atomizable) This also seams to be common minimum for thermal oxidizers with 95% and higher efficiencies.


Temperature
Time
Turbulence

[DCish]

I think that the "time" element is generally thought to be addressed by making sure the highly insulated afterburner is long enough to ensure that all flames are contained even at the top of the burn. By ensuring that heat harvest does not begin until after combustion is complete, maximum "time" is assumed to be achieved. Of course, there are competing considerations such as fire viewing or cooking that are often introduced in ways that move away from the perfect ideal, but I think that the ideal is well recognized.

[gadget]

Feb 2, 2019 17:58:35 GMT -8 DCish said:

I think that the "time" element is generally thought to be addressed by making sure the highly insulated afterburner is long enough to ensure that all flames are contained even at the top of the burn. By ensuring that heat harvest does not begin until after combustion is complete, maximum "time" is assumed to be achieved. Of course, there are competing considerations such as fire viewing or cooking that are often introduced in ways that move away from the perfect ideal, but I think that the ideal is well recognized.

Well, looking at a typical RMH I would estimate .5 seconds before the gases hit the barrel. I guess the real question is if that is plenty good and leave well enough alone? Or, is there more improvement that has yet to be explored?

[DCish]

Hmmm. A few thoughts on that. One is that the burn begins before the afterburner, so there is that time to measure somehow. Second, the flame is contained in a highly insulated environment until it burns out, so adding more insulated afterburner length wouldn't necessarily add any additional combustion time. Peter or Matt can correct me if I am wrong, but I think that the last few percent gains were made by getting the air to fuel ratio right, and figuring out how to get the air and fuel mixed optimally. If you check out the flue gas analyzer graphs they have posted I think you will find the efficiency numbers quite high. 

[gadget]

Yes, I have read many post with the flue analyzers. Very fascinating. I wonder with more time under heat some CO spike events would mellow out. What got me thinking about all this is how sensitive some designs are to the mixing. I know temps are decent enough so there must be a lack of time and from what I have read there appears to be to a certain extent.

I'm still trying to figure out how the Testo's figure combustion efficiency without measuring HC's. I know that CO and O2 can look good and that HC can be high under certain conditions, especially if to much excess air. I'm guessing there is an estimation done with the Testo software using excess O2 and CO and just not worry about HC.

Now with temps approaching 2000, residence time may only need to be .5 seconds

[satamax]

Feb 2, 2019 19:14:40 GMT -8 gadget said:

Feb 2, 2019 17:58:35 GMT -8 DCish said:

I think that the "time" element is generally thought to be addressed by making sure the highly insulated afterburner is long enough to ensure that all flames are contained even at the top of the burn. By ensuring that heat harvest does not begin until after combustion is complete, maximum "time" is assumed to be achieved. Of course, there are competing considerations such as fire viewing or cooking that are often introduced in ways that move away from the perfect ideal, but I think that the ideal is well recognized.

Well, looking at a typical RMH I would estimate .5 seconds before the gases hit the barrel. I guess the real question is if that is plenty good and leave well enough alone? Or, is there more improvement that has yet to be explored?

You mean 5 seconds? Or 0.5 seconds? The former i wouldn't believe at all.



I would say, on this video, the gases travel at a good 10 meters per second. Riser is 1.5 meter long.

[gadget]

Feb 2, 2019 21:11:25 GMT -8 satamax said:

Feb 2, 2019 19:14:40 GMT -8 gadget said:

Well, looking at a typical RMH I would estimate .5 seconds before the gases hit the barrel. I guess the real question is if that is plenty good and leave well enough alone? Or, is there more improvement that has yet to be explored?

You mean 5 seconds? Or 0.5 seconds? The former i wouldn't believe at all.



I would say, on this video, the gases travel at a good 10 meters per second. Riser is 1.5 meter long.

I mean .5 or half a second. So... at 10 meters per second, riser would only need to be aprx 20 meters. That would be interesting. Maybe Peter can set one up for testing, assuming its not a windy day.

[Vortex]

At that speed they can't be spending much time mixing in the double vortex.

To me the idea of the rams horn is to keep the gasses moving fast, but at 90 degrees to the flow, so they spend longer in the high temperature zone mixing and burning before leaving.

[satamax]

I have, in the past, made a cyclonic j tube. I'm really keen on a box far bigger than the heat riser. Having turbulent intake and turbulent mixing in that big box. Which stalls the gases. Like a coton shaker cyclone.

[Vortex]

Sounds interesting. What is a "coton shaker cyclone" or "cotton shaker cyclone" ?

The only 2 results for that term on google are yours.

[satamax]

Well, it's a term i have read on Bill Pentz website about cyclones.

It is a cyclone with straight entry, no neutral vanne. No ramp, nothing to avoid the incoming air from crashing into the swirling stream of air inside of the cyclone.

www.pramukhginningmachine.com/cotton_cyclone.html

billpentz.com/woodworking/cyclone/

[Vortex]

Very interesting... So it would be something like this:



Would be a great view from the right-hand end.

[satamax]

Well, i would view it vertical. Bigger diameter. I dont know about the cone. I think this angle is right. In my head, i see a volume of 3 to 5 times the firebox volume. A batch style firebox tucked in. So the port isn't flush with the cylinder's wall. But more inside, of may be a port's width.


What would happen, the gases would fountain in the big cylinder, end up turning. Crash back info the firebox's wall, then back in the incoming gases. May be as a perpendicular jet to the turning gases. Then go up, spin and mix faster and faster. Till it ends up in the Bell. Thé only prob, the energy this process takes.

[graham]

If you're making the gases somehow spend a longer time in the core, then you must also be reducing the air flow into the core as the air flow that enters must also leave. So, if you're not reducing air flow into the core, then maybe what you're doing is making the gas particles take a longer path, which means that they must be travelling faster to avoid a build up of pressure.

[Vortex]

Yeah, the cone isn't necessary for our purposes, it's just there to slow the air down so it drops the dust.

So you want it so it's like a cyclone riser then?:



I don't think it would work the size you're suggesting for the reasons Graham mentions.

This is how I envision it. The inlet and outlet being upto 1 CSA and the rest proportional to that.
Plus a nice window on the right-hand end to watch the show