[Stoves] Rapid mixing

Fri Sep 30 06:12:38 CDT 2016

Kirk,

I'll try and respond to your comments but it may take more than one try.
I lack the numbers and formula to backup my comments, but its fun to waive
my arms around with pretense.

>
> On the question of obstruction, I have to disagree with you.  The Venturi
> mixer I am working on reduces the open area to .5 that of the fuel reactor
> chamber.  Your design reduces the open area from 12 inch dia to 2 inch dia
> which is 113 sq in to 3.14 sq in.  That is .028 the open area of the fuel
> reactor chamber, which is much more restriction than I am using.
>
Fair enough.

>   You must be getting an incredible velocity through the 2 in pipe, which
> would be giving you a substantial drop in pressure via the Venturi effect.
>
I think this Venturi effect would occur at the restriction at the base of
the 2 inch pipe not at the top where it is straight.

>   It would be very interesting to get pressure measurements throughout
> your stove (with the secondary air entrance open/closed, and compared to a
> straight wall stove of the same height for a control).  Your placement of
> the secondary air entrance is perfect to use the Venturi pressure reduction
> for mixing.
>
I don't think so, same as above reason. The low pressure is caused by the
chimney above, there is no solid restriction to cause a venturi effect. The
incoming secondary air could be construed as a restriction. ???

>   I have not found tangential secondary air mixing to work very well, but
> I am guessing that the increased pressure difference makes it work for you.
>
In this case its a marginal difference at best.

>
> I very much like the way you have found to *pre-mix* the wood gas with
> the secondary air.  I had thought it would be much more difficult to
> achieve this.  You have cleverly increased the wood gas velocity so it
> is faster than the flame can burn back, and thus give the gasses time
> to pre-mix before they burn.
>
"Cleverly" implies precognition. There was none.

>   When the mixture ignites in the mixing area, the high gas velocity just
> pushes the flame out.  Or perhaps there is always a small amount of flame
> in the mixing zone,
>
After initial ignition there is no flame in the mixing zone.

> but the gasses are moving so fast that the majority of gas doesn't burn
> until it reaches the burning area.  Or perhaps the gasses mix and begin
> combusting, but because of the gas velocity they don't have time to burn
> completely in the mixer area.  Even well mixed gasses take time to burn.
>
Not much time. I have previously described a paper thin flat horizontal
disc of blue flame. It could also be described as a soap film. This is
where a flame front of premixed gasses has a matching flame speed to the
gas flow. It is a hard flamr to stabilize, but it appears to be only a
molecule thick and could be described as very fast. No unlike a candle
flame on a space shuttle in appearance but not in it function.

> The only way I can see that there would be no flame in the mixing area is
> if the mixture is below ignition temperature.
>
Yes. It needs a spark.

>
> It appears to be a rapid mixing technique, though you have designed for
> narrow and tall whereas I designed for wide and short.  Both will
> find their place.  The height of the cone that accelerates the wood
> gas plus the height of the mixer plus the height of the cone to decelerate
> the mixture and hold the flame adds up to some height.  That is not a
> negative if it works for the application.
>
Your work has more relevance to this list. Mine, so far is mostly a
teaching tool for a class of one.

>
> How do you use the flame for cooking?  A grill over the top?  A sunken pot?
>
I don't. It would have to be in a box with a chimney off to the side.
Likely to complex and fussy and ambitious for the real world.

>
> What is the fire power?
>
I need to put this on a scale sometime. I used to have an intelligent guess
but I forgot what it is.
8 inches deep by 12 inches diameter, wood pellets, 40 lbs/ ft3 @ 8000
btus/lb, apply assumption , 20% weight char residue @ 13000btus/lb,
duration -3 hours.... Anyone:)

> The constriction must be creating a back pressure that is slowing
> pyrolysis, and thus fire power.  This might be countered by the large
> diameter fuel reactor chamber providing a large pyrolysis front and lots of
> wood gas.
>
Controlled mostly by under air primary supply with a top end maximum
limited by the two inch pipe.

> It is natural draft and with how much turn-down?
>
I need a scale to know.

Off to work,
More later.
Still under 60:)
Alex

>
> I agree with you on the concern for material life.  Your design allows the
> materials some cooling by giving off heat to the outside.  My design holds
> the material in the heat.  The only cooling I get is from the secondary air
> entering and picking up some of the heat.  Things often glow red hot.
> Material life will be a serious concern for my design, but this is the
> twenty-first century, and materials science has progressed.  Possible
> solutions include making the mixers from ceramics or other heat resisting
> material, and designing for cheap and easy replacement of the heat stressed
> parts.
>
> Very nice concept!  Thank you for sharing.
>
> Kirk H.
>
> ----- Original Message -----
> *From:* alex english <aenglish444 at gmail.com>
> *To:* Discussion of biomass cooking stoves
> <stoves at lists.bioenergylists.org>
> *Sent:* Thursday, September 29, 2016 4:22 AM
> *Subject:* Re: [Stoves] Rapid mixing
>
> Kirk,
> Its quite wonderful what you have demonstrated. A holy grail of sorts. The
> durability and complexity, and ultimately this is the cost, of the
> structure you have placed in the flame/gas path will determine its future
> usefulness. Interesting solution none the less.
>
> Here is a recent video of my premixed blue tlud-gas flame. Its all
> geometry, no obstruction.
> https://www.youtube.com/watch?v=jnV1e60NTss&feature=youtu.be
>
> Alex
>
> On Wed, Sep 28, 2016 at 12:05 PM, kgharris <kgharris at sonic.net> wrote:
>
>> Crispin and All,
>>
>> Some clarification:  By radial mixing I assume you mean the method in the
>> lower photo where the air travels inward, radially.  By helical mixing I
>> assume you mean swirling gasses by using fan blades like in the top photo.
>> Neither of these represents a conclusion on my part.  Your opening sentence
>> put me on guard immediately because it defines your techniques as my
>> conclusion.  A more accurate starting sentence for your response would have
>> been 'Here are two techniques to look at that have worked for me'.  My
>> thinking is not limited to these two ideas.
>>
>> Your techniques are what I was looking for, ideas for rapid mixing.
>>
>> Kirk H.
>>
>>
>>
>>
>> ----- Original Message -----
>> *From:* Crispin Pemberton-Pigott
>> *To:* 'Discussion of biomass cooking stoves'
>> *Sent:* Tuesday, September 27, 2016 8:09 PM
>> *Subject:* Re: [Stoves] Rapid mixing
>>
>> Dear Kirk
>>
>>
>>
>> I think your conclusion is correct – that you can shorted the height of
>> the combustion zone with radial or helical mixing.
>>
>>
>>
>> I provide two photos with this in mind, though accomplished in completely
>> different ways. The first is (for a wood burner, not pellets) is to put the
>> mixed below the fuel, to continuously create a series of ‘induced vortices’
>> a few of which pass through the fuel and provide mixing by spinning. A
>> vortex tends to perpetuate itself so all that is needed is to get a few
>> going all the time.
>>
>>
>>
>> The material is a low chrome stainless steel. The production is in three
>> stages: blanking, punching the cuts, then forming.
>>
>>
>>
>> The second method of shortening the flame is to use the secondary air
>> entering the fire as a method of driving the flames horizontally. This
>> should be done in a ‘divided’ manner, not a ring of air, so that it breaks
>> up the rising gases into manageable jets. This kind of short clean burn can
>> only be done with preheated secondary air – 300-500˚C works well.
>>
>>
>>
>>
>>
>> The fire above is burning pellet gas (as a TLUD) though it also works for
>> wood if the gas generation is consistent. Consistency requires very good
>> air control. The amount of is required to achieve this is so low that it
>> pretty much rules out having an open bottom with fuel sticking out.
>>
>>
>>
>> Readers may recall Julien in Coburg combined the injection of air and
>> placement of the air feed in the gas stream. A problem commonly reported
>> with that approach is the cooling of the metal by the air can interfere
>> with the combustion.
>>
>>
>>
>> Readers may also recall Dr Nurhuda in Indonesia achieved a downward
>> turning flame consistently in his recent stoves reducing the flame height
>> basically to zero. All of these approaches can assist the provision of
>> lower height stoves – a major challenge for typical Indian rural households
>> where cooks won’t compromise on the cooking height.
>>
>>
>>
>> Everyone should try these approaches and report what works well.
>>
>>
>>
>> Regards
>>
>> Crispin
>>
>>
>>
>>
>>
>>  ……
>>
>> Not to be distracted from the purpose of this list, I have a thought
>> about wood stoves.  A small natural draft wood burning cook stove that
>> mixes the wood gas with secondary air in 2 cm has a considerable advantage
>> over a stove that may take 20 cm or more.  It's a little like transistors,
>> which gave advantage over vacuum tubes in electronics.  Rapid mixing can
>> enable a small wood stove to burn quick and clean at both *high and low*
>> power levels.  Witness the excellent numbers for the Wonderwerk TLUD-ND
>> stove which uses a rapid Venturi mixing technique.  Combined with the user
>> friendly power level controls, the principles used in the Wonderwerk stove
>> hold considerable potential.  Rapid mixing seems to me to be a very good
>> area for research.  I wonder how many rapid mixing techniques will be found
>> over the next few years?  I intend to contribute.
>>
>>
>>
>> List, keep writing about wood stoves, don't be distracted.
>>
>>
>>
>> Kirk H.
>>
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