[Stoves] stove test

kgharris kgharris at sonic.net
Thu Jun 2 12:25:00 CDT 2016


Manqolazi,

Thank you for your interest.

Hi, that's a great design. I'm a complete stove newbie so I hope you don't mind my basic questions. 

I have named the stove the Wonderwerk stove after a cave in South Africa which is an archeological site where signs of controlled fire use by early humans is being found.  I have named the combustor (the section that sits on top of the stove and is removable for fueling) the Strata combustor because in a site like Wonderwerk Cave the layers laid down over time are called strata, and this combustor has a number of layers.   

Is the fan shaped piece of metal for more complete combustion? What effect does it have on the flame at low vs high power? 

The lowest combustor layer is the spider like thing that hangs under the combustor between the legs.  It is not a fan though there is a stationary fan in a higher up layer.  It has two purposes:  to enable turn-down to quite a low power level, and to help clean the high power flame.  

Turn-down:  The full explanation is quite long.  The short version is that it keeps the secondary air from falling into the fuel chamber, diluting the wood gas and putting the flame out.  Also it gives a stable location for the medium and high power flame to sit.

High power:  It divides the wood gas into sheets (18 sheets in this version).  Think of the gas as being extruded through the slits.  Because of the radial arrangement of the sheets, air can move inward between the sheets of gas and feed the sheets from both sides.  Thus no portion of wood gas is more than 3 mm from a source of secondary air.  There is no column of unmixed gas going up the middle as in many TLUD-ND stoves.  This gives good mixing and good combustion.

Do you get a lot of soot on the bottom of the pot? 

There is very very little soot on the pot, mostly if I make a mistake operating the stove.  There needs to be some soot or other black coating on the pot to absorb radiant energy.  A clean metallic pot will reflect radiant energy and so lose some heat.  This can be a problem with this stove if there is not enough soot to coat the pot to absorb radiant energy.

Does all the secondary air come in from the bottom of the reactor module or does the combustor also have secondary inlets? 

Secondary air enters from three locations:  
1.  The pilot holes just below the top of the chamber.  These help give turn-down by injecting a small amount of air before the reduced and lower temperature wood gas is diluted by the not reduced secondary air (at lower  power levels the excess secondary air will further cool the already low temperature wood gas to below combustion temperatures, putting the flame out.  Also it dilutes the wood gas making it to lean to burn.  That is why TLUDs are difficult to turn down).  The pilot flames keep flame presence (I learned this from Dr. Anderson) and heat and ignite the remaining wood gas as it mixes with the secondary air.
2. Secondary air enters over the top edge of the chamber.  Many stoves use an array of holes around the perimeter rather than allowing the secondary air to spill over the top edge.
3. The next layer up in the combustor is an array of tubes with slits in the sides that feeds air into the flame to make sure that every bit of wood gas has access to oxygen to burn.

These secondary air entrances, working together give a thorough mixing of wood gas and air for clean combustion.  

The next layer up is the stationary fan which spins the flame.  This makes the flame take and angled path which is longer than straight up, and so gives the flame more time to finish burning before it gives up its heat to the cooking surface.  Also spinning the flame compacts it, keeping it hotter.

The several layers, if not properly designed, can add considerable resistance to the gas flow through the stove.  This can hinder the achievable high and low power levels.  Properly designed the flow remains strong and both the high and low power levels work well.  The low power flame needs a more open stove with less resistance than the high power flame.

Thanks and cheers!

I hope this answers your questions.

Kirk

----- Original Message ----- 
From: "Mangolazi" <mangolazi at yahoo.com>
To: "Discussion of biomass cooking stoves" <stoves at lists.bioenergylists.org>
Sent: Tuesday, May 31, 2016 1:38 AM
Subject: Re: [Stoves] stove test


> Hi, that's a great design. I'm a complete stove newbie so I hope you don't mind my basic questions. 
> 
> Is the fan shaped piece of metal for more complete combustion? What effect does it have on the flame at low vs high power? 
> 
> Do you get a lot of soot on the bottom of the pot? 
> 
> Does all the secondary air come in from the bottom of the reactor module or does the combustor also have secondary inlets? 
> 
> Thanks and cheers! 
> 
> On May 30, 2016 7:56:15 AM GMT+08:00, kgharris <kgharris at sonic.net> wrote:
>>All,
>>
>> 
>>
>>The TLUD-ND stove which we further developed at Aprovecho Research
>>Center in
>>March was tested at the Lawrence Berkley Lab.  Results are attached. 
>>The
>>photo of the computer screen showing the full power 6+kw graphs was
>>taken by
>>Ashok Gadgil.  They have a good team doing the tests.
>>
>> 
>>
>>The stove is in route to Aprovecho RC where it will be photographed and
>>drawings will be made.  Apro will then update their free online tier 4
>>stove
>>book from the old TLUD-ND design to include this new design.  I am
>>updating
>>the written stove description.
>>
>> 
>>
>>The designs are being put into the public domain.  I hope that the
>>various
>>principles used in the stove, for turn-down and improved efficiency and
>>emissions, will benefit some new stove designs in the future.  
>>
>> 
>>
>>The fuel used was Douglas fir pellets.  The highest power achieved was
>>6+ kw
>>with some cushion above that.  The low power is 1.6 kw with some
>>cushion
>>below that.  The turn-down is then 4 to 1.  Five kw seems to be the
>>highest
>>very clean power level.  The high power used in the tests was around
>>3.25
>>kw.  At these turn-down levels the ambient room temperature makes a big
>>difference in performance.  At 24C room temperature there was no
>>problem
>>keeping the water in the 93C to 97C test simmering range.  At 30C room
>>temperature the simmering water temperature could not be held below
>>97C.
>>The pot skirt had to be raised to release some heat to maintain the
>>test
>>temperature.  This is possibly what lead to the tier 3 rating for low
>>power
>>specific consumption.  The catch 22 is that the more efficiently heat
>>is
>>given to the pot, the lower the stove has to be turned down to maintain
>>a
>>simmer.  
>>
>> 
>>
>>Kirk
>>
>> 
>>
>>Santa Rosa, CA. USA
>>
>>
>>
>>------------------------------------------------------------------------
>>
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> 
> 
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