[Stoves] FW: Tlud stove testing

Mon Dec 15 12:30:00 CST 2014

Dear Saloop

I had a look at the stove and the calculations.

Please note that the formula for efficiency (which is widely used in that
same format) does not give the correct answer for the numerator ¨C the heat
gained by the pot.

The water cooled during the last 10 minutes and the lost energy was
converted into evaporated water. The correct number of Joules gained by the
pot is:

(I am using grams of water, ¡ãC and Joules)

Final mass of water x ¦¤T1 x 4.186

Plus

Missing mass of water x ¦¤T2 x 4.186

Plus

Missing mass of water x 2257

Where ¦¤T1 = initial and final temperature (not boiling!)

And

Where ¦¤T2 = initial and boiling temperature

This is a common mistake so don¡¯t be unduly alarmed. The formula you are
using only works if the final water temperature is still at the boiling
point. The formula in the paper double-counts the number of Joules lost
cooling from boiling to final temp. This error is covered in the paper KEY
DIFFERENCES OF PERFORMANCE TEST PROTOCOLS FOR HOUSEHOLD BIOMASS COOKSTOVES,
Zhang, Y. et al 2014 at DOI:10.1109/DUE.2014.6827753 

The error is included in several national test methods. Whenever you see
that a test method has as an ¡®ending point¡¯ the water at a temperature
below the boiling point, you should be suspicious that there is a
calculation error in the heat gained by the pot caused by double counting.
Your real efficiency number is lower than you suspect.

Regarding the design: 

I see your note about the secondary air holes being 40mm below the
concentrator ring. This is a good idea. If you bring them down to the top of
the fuel it will burn better for two reasons: there is more heat available
at the fuel surface which promotes faster lighting and a higher flame
temperature throughout the burn. Further, there is more vertical height for
the flame to finish burning before the gases hit the pot. At the moment it
looks about right (40mm + 85mm riser).

The secondary air holes are probably too many and too small. The flames that
shoot in around the secondary air have to reach the centre in order to get a
clean burn before the flames hit the pot. I suggest enlarging every other
one of the lower holes to 10mm and leave the ones in between at 8mm. Then
close all the top holes with a strip of metal round the chamber.

This will leave all the lower holes open (nearest the fuel) and a reduced
total secondary air supply. It may still be too much. It will be difficult
to tune it exactly without an oxygen measurement. You would want to have an
O2 concentration in the exhaust of about 9-11%.

If you can get a combustion analyser and measure the gases as they emerge
from between the pot and the stove, making sure not to allow any dilution by
the air near the sample point, you can reduce the number of holes until your
get the correct O2 level at high power. When it is correct, the heat
transfer efficiency will rise considerably. My guess is that at the moment
your O2 level in the exhaust gases is at least 15-17%.

Once you have that corrected, you want to get the CO as low as possible by
ensuring the air shooting through the secondary holes reaches the centre.
With 10mm you should be OK. 8mm is too small for the air to reach the centre
of a 165mm diameter fuel chamber unless the material around the hole is
shaped like a nozzle.  It is easy to close holes while running by popping an
8mm bolt into a few holes. Spread them around. This is normally done with an
O2 meter running so you can watch the excess air (EA) level.

What you want is to have enough inertia imparted into the secondary air so
it is carries into the centre of the combustion zone. That is accomplished
by converting the force of the draft (calculated from the riser height and
average temperature) into velocity. There is work being done so the energy
to do that is limited by the draft. The energy comes from the available
¡®chimney draft¡¯ which is a form of fan, really. When everything is
correct, the flame will completely cover the fuel bed and the exhaust O2
will be 9-11%. It is that combination which leads to a high efficiency,
clean burn. O2 can drop to 7% but you are running up against limits there to
do with mixing. I would accept 8% at highest power and 10% at other
settings.

You probably have far more primary air than is needed. You could just make
fewer holes. If you make too few, the power will be reduced (lower burn
rate).

When you are making a star pattern of holes like that, rotate each set of
holes 30 degrees to sit midway between the previous set. The reason is to
spread the heat of combustion out more. As you have it now, the ¡®lands¡¯
(the metal between the holes) will deteriorate rapidly from overheating.

The surface temperatures are probably not a major loss from the stove ¨C it
is the excess air that is killing your efficiency. Getting it correct should
give you a ¡Ö1.5x  increase in heat transferred, i.e. into the high 20¡¯s
even if you are making charcoal.

Re the big hole in the square shell (at the top). Cut it 16mm smaller than
the shell that goes into it, and hammer it outwards  to create a vertical
lip. With a bit of care you can get a really good fit between the riser and
the body. That limits air flow through the gap and increases the preheating
of the secondary air. If you can, locate a steep pipe that has the right
inside diameter to suit the hole size you need. Then you can hammer the
material over 90 degrees using the steel pipe as a collar, banging on the
inside. In the end you will have a rigid, round hole in the top of the body.
The lip can go upwards or downwards as you prefer.

Later you can spot-weld the riser onto the lip.

Regards

Crispin

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] 
Sent: Monday, December 15, 2014 12:08 PM
To: 'Discussion of biomass cooking stoves'
Subject: [Stoves] FW: Tlud stove testing

The file with the tests results for this stoves was too large to send
through the list, so I posted it here:

http://stoves.bioenergylists.org/content/metal-chulah-stove

Kind regards,

Erin Rasmussen

Stoves List Administrator

erin at trmiles.com <mailto:erin at trmiles.com>  

From: Saloop T S [mailto:t.s.saloop at gmail.com] 
Sent: Monday, December 15, 2014 3:34 AM
To: Discussion of biomass cooking stoves
Subject: Tlud stove testing

Respected Sir's,
              I am a student from Kerala, India doing project on a tlud
gasifier stove. I have made two 2 models of stove based on Dr. Paul Anderson
and others work. In my second model I have tried to add a riser and
concentrator disc arrangement so that I thought it may increase the
efficiency of the stove. I have done two tests on it 1) with out the riser+
concentrator disc- test I and 2) with riser and concentrator disc to find
WBT efficiency of test one to be 21.7 and test two to be 21%. 

I would like to know whether the addition of riser+ concentrator disc will
increase the efficiency of a stove. Is that anything that wrong with my
experiment?. I would like you to comment on it and help me.

I have attached a detailed report on my experiments and my model herewith.

Thanking you,

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