[Stoves] Model R TLUD update

[Crispin Pemberton-Pigott]

Dear Jock

Well done.  I am quoting only the small portion on which I want to comment:

“Primary air is pretty aggressive.  It had to be somewhat increased to adjust for the reduction in draft caused by the 12 cm disk.”

Be very careful about makings assumptions. The draft is the sum of all individual drafts affecting the system at that point (top of the fuel). The disk is definitely not causing ‘drag’ or ‘choking’. It has way too big a gap to do that.

What has happened is that the zone immediately under the disk is much cooler than it was before because the gases are being homogenized under there (which is very good) and then emerging in a slightly cooled state as a ‘ring’. The region above it where the fire is, used to mix with the lower zone which raised the average temperature, which produced more draft on the top of the fuel.

So the disk was the course of the loss of draft, but not because it is a construction of flow. It is a separator between a cool-ish zone and a hot zone.

The draft in the hot zone is of course created by the flames, and it is ‘broken’ by the secondary air apertures, some pulling in air and some pulling on the zone under the disk.

I predict that if you increase the disk diameter yet again, there will not be a corresponding reduction in the draft because the drop has already been achieved. The drop in draft accompanied the breaking into two zones. The reason for wanting to further reduce the clearance around the outside is that as the velocity of the gases increases when it passes through the gap, the more you can do with it, expending a little of the draft energy on getting the gas to split into multiple jets using vee-cuts, not holes. The yare simpler to make, and provide the same useful function of splitting the gas into small, hot flames.

Using the right combination of secondary air and primary gas architecture, you can achieve a short, and therefore very hot fire. Then you tune back the excess air level to about 80% (approx 8-9% O2 in the exhaust where it has not yet mixed with ambient air).

Please keep us informed. Your metalwork is neatly done. Compliments.
Crispin

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