[Stoves] Types of natural draft Re: Reports from ETHOS meetings: the future of TLUD research
Ronal W. Larson
rongretlarson at comcast.net
Sun Feb 1 17:06:37 CST 2015
Alex: cc list
I like your emphasis on pressure, where you say:
> The measured pressure in the stove is a function of more than just effective chimney height or height of trapped hot gasses. Actual temperatures and flow resistance play a role.
Can you explain your technique/equipment and what sort of pressure differences have you measured? What minimum sensitivity is required?
Ron
On Feb 1, 2015, at 3:23 PM, alex english <aenglish444 at gmail.com> wrote:
> Paul,
> Perhaps I'm drawing a minor distinction, but here goes.
>
> While the pressure drops from outside to inside stoves used to generate gas mixing, or gas flow through packed fuel beds can be similar in stoves using some small electrically powered fans/blowers and stoves with tall hot chimneys, both are prohibitively expensive or inappropriate to certain segments of the market. In other words, there is a technology "ladder" for stoves that could be graded by pressure. As with fuel type/quality or refinement, the pressure/cost/performance trade off can be assessed within the usual socio-economic limits. Most often I hear about this being a 'two rung ladder' world of "natural draft" stoves and "fan"stoves.
>
> In the "natural" gravity world of water wheels there are high head and low head
> (head =pressure) turbines/wheels. I am suggesting that some stoves, perhaps the "natural draft" tallish chimney Mongolian coal stoves may have more in common with fan stoves in some of their design options and even more so in their cost or market than they do with other natural draft, pot topped, TLUDs, Rockets and their kin.
>
> The measured pressure in the stove is a function of more than just effective chimney height or height of trapped hot gasses. Actual temperatures and flow resistance play a role.
>
> You may find that there are distinct pressure based groupings of non electrically assisted stoves that are not best described as only "natural draft" stoves.
>
> So add a rung or two to the pressure ladder and call them something more meaningful.
>
> The 30, 90cm buoyancy comment was based on my past efforts with TLUD fed combustion. The difference was measurable in terms of peak combustion efficiency, and I equated it with pressure drop and mixing. However it was still a much smaller variable than excess air.
>
> My high "head" chimney natural draft gravity fed pellet stove should perhaps be a hybrid. With no electricity assisting ( in blackout mode) with just a chimney it still roars like a powered furnace but it wastes some significant thermal energy out the stack. With a small amount of electrically induced draft ( happy grid connected mode) and more heat exchange surface area, it could have a lower stack temperature/higher net efficiency and likely a higher potential maximum output.
>
> Everything is a compromise, including our terminology.
>
> Alex
>
>
>
> On Sun, Feb 1, 2015 at 12:32 AM, Paul Anderson <psanders at ilstu.edu> wrote:
> Alex,
>
> Please elaborate on
>>
>> I think "natural draft" is still too broad a term. There is a dramatic difference in gas mixing potential of 30, 90 cm of buoyancy.
> Paul
> Doc / Dr TLUD / Prof. Paul S. Anderson, PhD
> Email: psanders at ilstu.edu
> Skype: paultlud Phone: +1-309-452-7072
> Website: www.drtlud.com
> On 1/28/2015 9:45 PM, alex english wrote:
>> Julien,
>> Agreed. Its been a twenty year wait for basic in depth research on natural draft TLUDs.
>> Natural draft mixing was my passion for a while. I think "natural draft" is still too broad a term. There is a dramatic difference in gas mixing potential of 30, 90 cm of buoyancy.
>> It will be interesting to see the results shared, but I am inclined to think the money would be better spent putting portable combustion gas analysers in the hands of folks like you and Harris.
>>
>> My excess air for today,
>>
>> Alex
>>
>> On Wed, Jan 28, 2015 at 12:45 PM, Julien Winter <winter.julien at gmail.com> wrote:
>> I was at ETHOS this year presenting some of my basic investigations of TLUD function, and I will circulate a copy of my presentation shortly. Kirk Harris presented his fascinating burner design.
>>
>> For me, one of the most interesting things at ETHOS was not what was presented, but what is coming. The Colorado State University has received a very large grant from the US Department of Energy to improve our scientific understanding of natural draft TLUDs. This year at ETHOS, they didn't present any results, but they described the laboratory equipment that they have assembled. They will be able to measure the flow rates of primary and secondary air, fuel bed and gas flame temperatures, gas flame structure, the composition of the pyrogas as it exists the fuel bed, and exhaust gas and particulate emissions. They are able to test a variety of gas burners to increase clean-burning --- the most pressing issue today. Knowledge gained on pyrolysis, combustion, and gas flows will be systematized into computer simulation models.
>>
>> This work is much needed. Since the early 1980s, forced draft systems have been extensively studied, however, except for a couple of recent papers, natural draft systems, have not been studied at all. Research on forced draft has provided us with some very important information, but it doesn't extrapolate sufficiently well to natural draft systems. Natural draft systems different from forced draft systems, in three basic ways: (1) they operate at lower gas velocities, near atmospheric pressure, (2) operational temperatures cover a lower range, and (3) and their processes, are interconnected through feed-back mechanisms. We have to understand how these feedbacks work for designing ND-TLUD stoves. For example, how does the size and shape of the pyrogas flame affect draft for primary air --> gasification rate --> size and shape of the pyrogas flame --> ...
>>
>> It will be interesting to see what the workers at Colorado State University come up with. I expect they will be able to tell us what the control or limiting points are in the TLUD reaction, such as resistance to gas flow in the fuel bed, or heat production at the pyrolytic front. They should be able to tell us how the thickness and temperature of the char layer above the pyrolytic front changes the composition of pyrogas rising through it. They should be able to tell us how the composition of pyrogas changes over a turndown curve. Most importantly, I hope they will provide guidelines for designing burners, reasons for why one thing works and not another, and a computer program to simulate design decisions.
>>
>> The results from the Colorado State University lab can't come too soon.
>>
>> Cheers,
>> Julien.
>>
>> ---------------------
>>
>>
>> --
>> Julien Winter
>> Cobourg, ON, CANADA
>>
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