REFLECTOR:Re: NPG+ (Leidenfrost effect)
Brian Michalk
reflector@tvbf.org
Mon, 19 Apr 2004 10:59:29 -0500
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Just to interject some more confusion into the discussion by using some
factual information.
What most of us are talking about is the Leidenfrost effect, summarized
nicely by the following paragraph I stole off the internet.
Liquid nitrogen can be handled by an experienced demonstrator without
danger due to the Leidenfrost effect. His or her hands are so hot, compared
to the liquid nitrogen, that the nitrogen first touched is flashed
immediately to vapor, and the rest of the liquid will be insulated from the
hand by that vapor layer. Liquid nitrogen splashed on a smooth surface will
bead up into balls and roll around for a long while like mercury, because it
will float on an insulating vapor film. Water will do the same thing if
dropped on a very hot skillet, while a colder skillet will evaporate the
water away almost immediately.
Interesting to note that some steam locomotives had problems exploding due
to this. I seem to recall that if they stoked up too much fire, the boiler
would get so hot the heat transfer was minimized. I believe the engineer
would let the pressure in the boiler increase, until such time that the
steam layer could not form. Lots of energy going into the water, lots of
pressure in a short time. Things broke.
-----Original Message-----
From: reflector-admin@tvbf.org [mailto:reflector-admin@tvbf.org]On Behalf Of
John Dibble
Sent: Monday, April 19, 2004 9:11 AM
To: reflector@tvbf.org
Subject: Re: REFLECTOR:Re: NPG+
Weeeell, almost. Actually, water cooling systems are usually not
pressurized. To be pressurized the coolant temp would need to be above it's
atmospheric boiling point (212-215F). The thermostat controls the temp
below that. The reason you have pressure when you open the cap is because
once the engine is off, the radiator can not cool, but the engine is still
hot and heats the coolant above the boiling point which results in temporary
pressure buildup.
If the cooling system were pressurized and pressure was lost, there would
be a temporary surge (possibly a steam layer) in the boiling rate and then
normal boiling would resume. The only way to stop nucleate boiling is to
coat the jacket surface with oil. In that case the coolant would superheat
(about 20F above the normal boiling point) and boil spontaneously and the
cooling would be about the same.
Thinking about it further, the coolant temp is probably a good estimate of
the CHT, which means that a hotter coolant temp would be beneficial as Bill
pointed out.
John
Chuck Jensen wrote:
John,I think I've got it straight. You're right (which is probably not
a surprise to you), in that boiling does go on in a pressurized H2O cooling
system. Nucleate boiling forms right at the metal surface and when the
bubble moves away from the metal, they immediately collapse. This is a very
effective heat transfer mechanism. If pressure on the system is lost, then
a steam layer will form on the metal, heat transfer will suffer greviously
and it would be a good idea to have a landing site in mind. So, the NPG+
cooled engine probably runs hotter since it won't support nucleate boiling
for good heat transfer. Is that good?Chuck
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<DIV><!--StartFragment --><SPAN class=3D330305015-19042004>Just to =
interject some=20
more confusion into the discussion by using some factual=20
information.</SPAN></DIV>
<DIV><SPAN class=3D330305015-19042004>What most of us are talking about =
is the=20
Leidenfrost effect, summarized nicely by the following paragraph I stole =
off the=20
internet.</SPAN></DIV>
<DIV> </DIV>
<DIV> Liquid nitrogen can be handled by an experienced demonstrator =
without=20
danger due to the Leidenfrost effect. His or her hands are so hot, =
compared to=20
the liquid nitrogen, that the nitrogen first touched is flashed =
immediately to=20
vapor, and the rest of the liquid will be insulated from the hand by =
that vapor=20
layer. Liquid nitrogen splashed on a smooth surface will bead up into =
balls and=20
roll around for a long while like mercury, because it will float on an=20
insulating vapor film. Water will do the same thing if dropped on a very =
hot=20
skillet, while a colder skillet will evaporate the water away almost=20
immediately. </DIV>
<DIV> </DIV>
<DIV><SPAN class=3D330305015-19042004>Interesting to note that some =
steam=20
locomotives had problems exploding due to this. I seem to recall =
that if=20
they stoked up too much fire, the boiler would get so hot the heat =
transfer was=20
minimized. I believe the engineer would let the =
pressure in=20
the boiler increase, until such time that the steam layer could not =
form. =20
Lots of energy going into the water, lots of pressure in a short =
time. =20
Things broke.</SPAN></DIV>
<DIV><SPAN class=3D330305015-19042004></SPAN><FONT face=3DTahoma><SPAN=20
class=3D330305015-19042004><FONT =
face=3DArial> </FONT></SPAN></FONT></DIV>
<DIV><FONT face=3DTahoma><SPAN =
class=3D330305015-19042004> </SPAN>-----Original=20
Message-----<BR><B>From:</B> reflector-admin@tvbf.org=20
[mailto:reflector-admin@tvbf.org]<B>On Behalf Of </B>John =
Dibble<BR><B>Sent:</B>=20
Monday, April 19, 2004 9:11 AM<BR><B>To:</B>=20
reflector@tvbf.org<BR><B>Subject:</B> Re: REFLECTOR:Re:=20
NPG+<BR><BR></DIV></FONT>
<BLOCKQUOTE dir=3Dltr=20
style=3D"PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: #000000 2px =
solid; MARGIN-RIGHT: 0px">Weeeell,=20
almost. Actually, water cooling systems are usually not=20
pressurized. To be pressurized the coolant temp would need to be =
above=20
it's atmospheric boiling point (212-215F). The thermostat =
controls the=20
temp below that. The reason you have pressure when you open the =
cap is=20
because once the engine is off, the radiator can not cool, but the =
engine is=20
still hot and heats the coolant above the boiling point which results =
in=20
temporary pressure buildup. <BR>If the cooling system were pressurized =
and=20
pressure was lost, there would be a temporary surge (possibly a steam =
layer)=20
in the boiling rate and then normal boiling would resume. The =
only way=20
to stop nucleate boiling is to coat the jacket surface with oil. =
In that=20
case the coolant would superheat (about 20F above the normal boiling =
point)=20
and boil spontaneously and the cooling would be about the same. =
<BR>Thinking=20
about it further, the coolant temp is probably a good estimate of the =
CHT,=20
which means that a hotter coolant temp would be beneficial as Bill =
pointed=20
out.=20
<P>John=20
<P>Chuck Jensen wrote:=20
<BLOCKQUOTE TYPE=3D"CITE"> <SPAN=20
class=3D359083212-19042004>John,</SPAN><SPAN=20
class=3D359083212-19042004></SPAN><SPAN class=3D359083212-19042004>I =
think I've=20
got it straight. You're right (which is probably not a =
surprise to=20
you), in that boiling does go on in a pressurized H2O cooling =
system. =20
Nucleate boiling forms right at the metal surface and when the =
bubble moves=20
away from the metal, they immediately collapse. This is a very =
effective heat transfer mechanism. If pressure on the system =
is lost,=20
then a steam layer will form on the metal, heat transfer will suffer =
greviously and it would be a good idea to have a landing site in =
mind. =20
So, the NPG+ cooled engine probably runs hotter since it won't =
support=20
nucleate boiling for good heat transfer. Is that =
good?</SPAN><SPAN=20
class=3D359083212-19042004></SPAN><SPAN=20
=
class=3D359083212-19042004>Chuck</SPAN></BLOCKQUOTE></BLOCKQUOTE></BODY><=
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