[Stoves] Correcting a misconception that approaches myth status

Crispin Pemberton-Pigott crispinpigott at outlook.com
Mon Dec 15 23:36:54 CST 2014


Dear Kirk

 

In a ND TLDD there will always be a battle between the downward moving air and the hot combustion gasses which will be inclined to rise.  

 

I need to be clear do you mean a Top lit down draft or a bottom lit downdraft? A TLDD is not the same at all – that is like putting fuel on top of an existing fire, except upside down. A BLDD is lit on the bottom and fuel added pushing it down further. The MPF moves up.

 

If you meant BLDD, I will contribute this:

 

I am thinking that gasses cannot go both directions in the same space, so will there be areas in the fuel where the air will win and areas where the hot gasses will win?  

 

In the small spaces, there is hot air buoyancy pushing gas and air up and there is primary air passing down through the fuel. They fight. The passage down is not completely even as it is in a TLUD where every force is upwards. In the BLDD there are little currents of air and gas that rise, cool against the fuel above then descend with the overall primary air flow down. The water vapour and gases are pulled down eventually and have to pass through the permanent char layer at the bottom, cracking the tars and so on to give a high H2 and CO gas that burns below the grate. Ash falls down keeping the grate clear – a significant advantage to low continuous operation.  There are numerous things that accentuate the quality of the gas burn. Adding preheated secondary air is obviously one of them.  There are a couple of recent patents on optimising that burn at low EA.

 

This could lead to the buoyancy caused convection you mention, with the heavier air displacing the hot gasses, pushing them upward.  

 

Correct. This happens between the fuel particles. The vertical distance involved depends on the primary air speed downwards. Generally, when the MPF reaches the top of the fuel, the smoke and gases rise above the fuel and this means a cover of some kind has to be placed over the hopper to prevent them getting into the room. 

 

Here is a picture of such a cover. The primary air hole is in the centre. It has to be large enough not to block the primary air, but small enough to trap the gas and smoke from the pyrolysis of the top layer of fuel inside the stove. The upper hole is a secondary air flow controller (the plate is rotated to regulate it).

 



 

Here is an isometric view of a bottom lit downdraft stove (slightly different from the one above) incorporating the same closer plate on top.



Could this unevenness be caused by local resistance variations to gas flow in the fuel?  

 

I don’t think that is an aspect of the circulation. It is just hot gases rising, cooling against the fuel above and descending again. It also takes place in cross draft stoves with a hopper. In that case the pyrolysation front is approximately stationary and there is no air from above passing down through the fuel in the hopper. The reason is that a cross draft stove would not bring enough air through the fuel to keep the pyrolysation front from rising through the entire hopper. The hopper top is sealed and the layer is starved of air, able only to get air from below rising through the fuel then descending again – perhaps 80mm or so.

 

Or might it be a temperature thing where the air wins in cooler areas and the hot gasses win in hotter areas?  Or am I totally off base here?

 

That is completely on-base. 

 

Here is a photo of a stove taken at night when the MPF has moved halfway up through the fuel.

 



 

The hopper is 2mm thick 3CR12 which can operate at that temperature for sustained periods. All this work was performed at the SeTAR Centre research lab at the University of Johannesburg.

 

The MPF can be made much thinner than above by using chipped fuel. The main reason for chips making it thinner is that the upward movement of hot gases is limited by the compacting of the fuel. They cool more quickly in the small passages.

 

All these stoves have a minimum power level (same as a viable TLUD fire) and it is related to the particle size. Bigger means higher minimum power. If the particle size is larger than 1/6th of the hopper diameter the fire is likely to pyrolyse the fuel then go out. That is experience and observation talking, there is no theory behind it. It just gets too cold.

 

Regards

Crispin

 

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