[Stoves] Inverse Diffusion Flames in TLUDs

Mon Mar 7 20:25:41 CST 2016

Dear Julien

This is a most valuable analysis. At the end I was inspired to add something that could be appended.

The draft of the system can be driven by a chimney. For example the Mongolian TLUD's which are among the cleanest stoves anywhere have to be tested with some length of chimney. The manufacturers make no statements, at least no useful ones, about the chimney height. The height is forgotten by most, save the testers.

The mixing and the flame length and the flame sheet and the length of flamelets are all affected by the draft. China tests all stoves with a four metre tall chimney under the national protocol. The average installed chimney height is five metres, or six, it has been hard to say with clarity. Current testing of 16 stoves for Hebei Province at CAU (BST Lab) is using a six metre chimney because that is what was observed in Hebei.

Whatever result is obtained at a given level of draft, it will ‎change if the draft is changed, especially if the pressure changes by 50%.  In Mongolia the tests are done with a 2.8m chimney because the typical application is a yurt.

The advantage of a fan is that the pressure can be controlled at will, but the added complexity is a problem for many.

The humble candle is a good example of PM formation. Particles exist immediately above the flame, in the clear space, but do not exist 60mm higher up, with no flame in between, meaning it is not necessary to have one to have PM burnout. This can be demonstrated using a saucer and passing it at different heights through the gas stream.

Placing a glass tube above the flame that pulls it into the bottom hole can not only pretty much eliminate the PM, it can also nearly eliminate the visible flame. There are YouTube videos of this phenomenon. The additional draft changes almost everything so it forms part of the discussion. Changing the TLUD power level changes the draft which changes the power level which changes the draft...

Regards
Crispin

Hi all;

Here is an interesting article about inverse diffusion flames IDF similar
to what we see in many TLUDs:

Blevins, L. G., Yang, N. Y., Mulholland, G. W., Davis, R. W., & Steel, E.
B. 2002. Early soot from inverse diffusion flames. Prepr. Pap.-Amer. Chem.
Soc., DiV. Fuel Chem, 47(2), 740-741.

https://web.anl.gov/PCS/acsfuel/preprint%20archive/Files/47_2_Boston_10-02_0236.pdf

I have attached a photo of standard diffusion flame and an IDF of ethylene
flames near their sooting limits. (Kumfer, BM; Skeen, SA; Axelbaum, RL.
2008.  Soot inception limits in laminar diffusion flames with application
to oxy-fuel combustion.  Combustion and Flame 154; 546-556)
[image: Inline image 1]

In her article, Linda Blevins argues that soot forms on the outside edge of
an IDF, then follows a trajectory away from the flame, and escapes
oxidation.  In a standard diffusion flame, soot tends to be oxidized in the
tip of the flame.

One common type of syngas burner for TLUDs has secondary air entering the
gas burner through small holes.  The flamelets arrising from these holes
start out as a type of IDF.  Syngas rises from below, so the underside of
the flamelets should be more fuel rich than the top side (which may be
lean).  Therefore, soot production in a TLUD burner should be greatest on
the underside of the flamelets.  If the flamlets coaless to form a sheet of
flame across the width of the gas burner, then (most?) soot particles will
have to pass through this flame and oxidized to varying degrees.

However, if we turn down the gasification rate in the TLUD, so there is not
a continuous sheet of flame across the width of the burner, is there a
change in the nature of soot particles in the exhause gas from the stove?
Will there be a higher proportion of polycyclic aromatic hydrocarbons vs
black carbon from a turned down TLUD?  Of course, when turned down the mass
of soot emitted may not be very high.  All the same, this is an example of
why we should characterize emissions from stoves over a wide range of power
levels when assessing health risks.

Cheers,
Julien.

--
Julien Winter
Cobourg, ON, CANADA
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