Re: Temperature dilution

[AJH <andrew.heggie@dtn.ntl.com>]

On Thu, 23 May 2002 07:41:36 -0700, "Tom Miles" <tmiles@trmiles.com>
wrote:

>
>----- Original Message -----
>From: "Paul S. Anderson" <psanders@ilstu.edu>
>To: "AJH" <andrew.heggie@dtn.ntl.com>; <stoves@crest.org>
>Sent: Thursday, May 23, 2002 6:50 AM
>Subject: Temperature dilution
>
>
>> Tom Reed was just here in Illinois, so he is on the road and will not get
>> this message for several days.  While here he said that the denser
>(cooler)
>> air was better able to force its way into a body of gases than would
>warmer
>> air, meaning better mixing of air and gas if the air was cooler. 

Makes sense however it seems to me that cool air entering a reaction
can be just as bad at quenching the reaction as a cold surface. If the
flame subsequently reaches sufficient dwell time and temperature it
should make no difference. From my little experiments it looks like
flame holding becomes less significant if the combustion chamber
reaches ~650C. I believe this is the temperature at which most of the
volatiles have dissociated and spontaneous combustion occurs. 

> I said
>to
>> Tom that I believe in the value of pre-heated air and therefore we need to
>> consider how to do better mixing.   (Then he left, and the conversation
>> remains open.)

OK we agree preheated air has a value, it also has a cost: it requires
more power to circulate it for two reasons:
1 The pumping energy is higher
2 intuitively you are also working against gravity
3 it must remove heat from somewhere else in the system, the laws of
thermodynamics limit this to less than 100% efficiency so the loss may
not be worthwhile, I shall continue to look.
>>
>> I would think this is pretty basic information that merits data collection
>> if data are not already available.
>
>

>Paul, Andrew
>
>Mixing gases of similar densities requires energy whether it is for primary
>or secondary air. To conserve energy some designs concentrate small jets of
>combustion air impinging on small areas of fuel. Good secondary air mixing
>is also done with jets as Tom demonstrates in the burner "ring" of his
>turbostove. (See photo at
>http://crest.org/discussiongroups/resources/stoves/ ) We sometimes use
>eductor or venturi designs for mixing.

Tom you are preaching to the converted here, we have covered this
ground in the past, complexity adding to cost is why it has not been
pursued by people in the field.

Vernon Harris suggested to me some while back that a device used in
this mode is an aspirator. Tom Reed has mentioned the benefits of an
electric fan for micro mixing for better combustion. We have also
previously discussed how premixed secondary combustion leads to a
shorter flame. This enables a lower cook stove with implications of
better stability.

> These also require energy to
>penetrate and mix with combustible gases. It seems to me that the challenge
>is to "blow on the fire" without using a bellows (external energy). To do
>that you have to have a pressure differential or a temperature differential.

Again it all comes down to cost, Tom Reed finds the cost of
rechargeable batteries and possibly a solar panel low in his economy.
I see a possible alternative route without this hi tech need, however
even here I do not believe the cost will outweigh the advantage with a
"good" fuel, it might however mitigate the pollution problems when
burning a poor fuel.

I have yet to sit down and consider the energy cost of my aspirator.

AJH

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