[Stoves] SX iCan TLUD w/ center disk & Blue flames

Mon Mar 10 15:56:48 CDT 2014

Dear Jock

What a great set of photos. Everything was clearly explained.

>½ inch gap - for photography.  Unit out of its outer safety can.  

Can you please explain what the outer can looks like, meaning how does the air flow through it?

If you were able to place a can around the secondary air inlet so that the air entering was forced to go downwards, not up, it would preheat it, and counter some of the natural draft inside.  This is method in this madness. This is Vesto stuff.

If the flame inside is low in excess air, it is really hot and pulls a lot of air into the secondary holes. You can limit this by putting a cylinder around the outside that creates a negative draft limiting the air flow. As the inside cools, the preheating level lowers, resulting in a reduced negative draft in that portion. The outer sleeve should not be as high as the inner can or the effect is overdone. Using a wall height of even 40mm on a stove that small you will be surprised at the effect is can have.  

The lesson is to create a self-regulating secondary air supply that does not ‘run free’. If the balance is correct (gap, height, material transferring heat) it holds the excess air pretty well even if the primary air is closed to control the power level. 

>…As the flames rises above the center disk, the flame cools and turns yellow.  

Yellow means glowing particles. They may or may not burn out – can’t tell without measuring.

>Reducing the secondary air gap increases the temp in the system, increase ability to withstand drafty conditions, also increase the draft on the pyrolysis zone as well as on the air gap.  Feedstock is standard wood pellets.

The outer ‘inverted’ preheater also gives additional protection to the flame from side winds. This is critical when the power is low or at the very end to prevent the flame going out while there is still gas production.

>Center disk, a 2 inch washer in a 3 inch diameter can, is in place above the fuel but below the top lip of the reactor can.  This also puts it below the secondary air gap as well.  The gap between the reactor and the outer edge of the disk preserves about 55% of the draft capacity.

Very good. The draft will not be limited by such a large hole. No problem to make the gap smaller I think if you wanted.

>According to Tom Miles, this disk, when use in larger furnaces, is called "an aerodynamic flame holder ".

It is also called a ‘bluff body’ and is used in several stoves to retain the flame. The FSP stove (Free State Paraffin Stove) uses a loose bluff body to retain the flame. If the stove is upset, the flame goes out immediately because the fuel spay velocity is higher than the flame velocity.

>Three deflector washers set up inside the bottom of the draft can. The space between the center disk and the deflectors establishes a turbulent combustion zone.

Now seeing this very clearly I wonder if you could put in a larger disc below and cut notches in the periphery, twisting them so as to create turbulence in the smooth ring of secondary air. This would provide stirring and not require the three upper washers. By being a bit clever about the shape of the twists, the total effect might even be better because any metal in the flaming area creates CO.

>You cannot see the blue flames created when the hot pyrolytic gases rise above the edge of the center disk and meet the secondary air.  The flames you see are orange as they have cooled as they rise above the center disk and the three defectors.  At least this is what I think is the reason for them not being blue.

I think there are two reasons. One is excess air cooling it, and the other is the mixing zone could be shorter (with the above modification to the lip of the lower washer). Once the secondary air is introduced, it is best to keep the metalwork away from the flames. It is inevitable that some cooling is taking place in what should be the hottest part of the fire. In the long run, at high or low power, or both, it will affect the quality of the burn.

>As you can see, the deflectors prevent a central spire of flame - the inverted bathtub drain effect.

Clearly shown.

>One effect of this approach is to keep the flames at a relatively constant distance from the target, perhaps a pot, as the the pyrolysis zone works its way down towards the bottom of the reactor and away from the target.

This is ideal. If you get that with minimum excess air, then you have the beginning of a good heat transfer process. 

>Alexis Belonio and Paul Olivier have been able to create clean burning stoves with blue flames in variable speed fan assisted TLUDs.  What I am working on is maximizing the amount of blue flames in a natural draft TLUD.

There are little orange tips on those blue flames as they cool in the air. Generally speaking both of those designs have a lot of excess air available in the flaming zone. As you close and contain it and protect it from winds, the orange tips will shrink, perhaps into nothing. My domestic gas range at home has burners that produce a (what I thought) small number of gas jets – far less than many stoves. It is approved for sale in North America so it has to meet a CO/CO2 ratio of 0.08% instead of the European version of 2%. This is achieved by having a higher flame-to-pot clearance to give the flame a chance to burn completely, and also by not having a large number of very small flamelets. In other words, there is a chance of going too far.

>This is purely an educational device to show the advantages of using a central disk over the reactor can to force the hot gases to the edges to meet the secondary air.  

It is a great example and should be included in demonstrations of TLUD’s along with others so people see the range of possibilities.

>This solves the problem of trying to get secondary air into the middle of the TLUD gas stream.  

It is one approach. When you make they large in diameter they have on drawback which is that you can get a flame on one side and smoke on the other and there is no ‘jumping’ of the flame over to the smoke. When it is physical small, this does not appear often save at very low power.

>This device also shows avoiding the central spire of flame.

That central spire of flame looks great in photos but is just an indicator of poor secondary air mixing. If you check the ZZ Boy Scout Stove on line (a fan assisted stove with preheating etc) it has an enormous fire that is hopelessly too large for the stove. I asked why they showed it like that in the ad and they replied that people think it is a wetter stove if it has a huge flame. That is why technical people and marketing people don’t get along.

Regards

Crispin

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