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Hi Crispin,<br>
<br>
Replies under your questions.<br>
<br>
On 29/11/2011 06:00, Crispin Pemberton-Pigott wrote:
<blockquote cite="mid:00b901ccae08$5f4153f0$1dc3fbd0$@gmail.com"
type="cite">
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<div class="WordSection1">
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Dear
Peter<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Good
to hear from you, I was thinking of you last week testing a
downdraft stove using chunks of wood the same size you use
on your DD BBQ (barbie). The chunks of wood provide quite a
bit of excess air so the way the heat transfer takes place
it affected a lot.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">></span>As
far as I know the difference between kerosene (paraffin) and
higher hydrocarbon fractions, including vegetable oils is the
fact that they are not distillable under atmospheric pressure.<span
style="color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal" style=""><span style="font-size: 11pt;
font-family: "Calibri","sans-serif";
color: rgb(31, 73, 125);">The stove must work then by
boiling something in the veggie oil that boils at a
temperature achievable in the evaporator tube. Perhaps that
is why there is so much gunk left behind. A completely
different approach (and an old one) is dripping oil onto a
flat plate that is completely contained inside the
combustion chamber. People burn old engine oil that way, and
for similar reasons.</span></p>
</div>
</blockquote>
Yes, as the veggie oil decomposes liquids, vapours and solids are
produced, some of the latter may be sticky and decompose further
into char.<br>
Dripping oil on a flat plate is a good idea except that we need
access of air to the resulting char, the reason I thought of a
perforated plate.<br>
<blockquote cite="mid:00b901ccae08$5f4153f0$1dc3fbd0$@gmail.com"
type="cite">
<div class="WordSection1">
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span
style="color:#1F497D">></span>Undistillable hydrocarbons
are fed into the fire as a spray of very fine droplets that
burn completely. <span style="color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal" style=""><span style="font-size: 11pt;
font-family: "Calibri","sans-serif";
color: rgb(31, 73, 125);">With the residence time in the
flame matching the burnabilty of the droplet, right? When a
droplet of diesel burns, has it been evaporated by radiant
heat under that increasing pressure of the pressure wave?</span></p>
</div>
</blockquote>
I don't think so, being a droplet part of it will produce gaseous
decomposition products and the tiny char skeleton should burn given
enough residence time.<br>
<blockquote cite="mid:00b901ccae08$5f4153f0$1dc3fbd0$@gmail.com"
type="cite">
<div class="WordSection1">
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">The
same could be done with vegetable oils at the combustor end.
However, most vegetable oils tend to react slowly with oxygen
to form gunk which eventually blocks the passages it has to
flow through.<o:p></o:p></p>
<p class="MsoNormal" style=""><span style="font-size: 11pt;
font-family: "Calibri","sans-serif";
color: rgb(31, 73, 125);">Is this process dramatically
accelerated by heating the oil? In other words, is the
depositing cause by O2 already in the fuel? That bodes badly
for the future of burning raw oil.</span></p>
</div>
</blockquote>
Yes, the polymerisation would speed up at higher temperatures. At
still higher temperatures the tendency is to break up into smaller
molecules, which certainly happens in a flame.<br>
<blockquote cite="mid:00b901ccae08$5f4153f0$1dc3fbd0$@gmail.com"
type="cite">
<div class="WordSection1">
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">So
for a stove that burns vegetable oil, the piping from the
storage to the burner should be of very simple shape and easy
to clean.<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Daily,
as I understand it.<o:p></o:p></span></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Realising
that producing a spray of fine droplets is out of the question
for domestic stoves, we have to find something that feeds the
oil to the combustion zone where the carbon, resulting from
the decomposition of the oil is burnt as well. <span
style="color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal" style=""><span style="font-size: 11pt;
font-family: "Calibri","sans-serif";
color: rgb(31, 73, 125);">How about using one of those
spinning disks with a spiky periphery that are used in
greenhouses to make as fine a mist as possible? They are
very small (50-75mm) and use only a small amount of power.
It is conceivable they could be driven by electricity, heat
or draft.</span></p>
</div>
</blockquote>
Yes, that should work.<br>
<blockquote cite="mid:00b901ccae08$5f4153f0$1dc3fbd0$@gmail.com"
type="cite">
<div class="WordSection1">
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Possibly
something like a perforated disk where the oil burns in
updraft mode and where the holes occupy a sufficient part of
the disk area that all the char comes in contact with air.<o:p></o:p></p>
<p class="MsoNormal"
style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Good
idea. A variation on the drop-onto-plate idea might do.<o:p></o:p></span></p>
</div>
</blockquote>
Always interested in your comments in the List.<br>
<br>
Best regards,<br>
<br>
Peter
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