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Auxiliary radiators for pipe-stack air-water heat exchangers

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Posted by David Delaney on December 2, 2004, 4:42 pm
 
Nick

I was thinking about your ceiling pipes and the problem of increasing
the heat transfer power of air to something else heat exchangers.

I had some clear thought about Harry Thomason's rock-water stores
<http://geocities.com/davidmdelaney/shurcliff/shurcliff-thomason-storage-1.html>
only recently, when it occurred to me that most of the enhancement
of the rock-water heat transfer is by radiation.  (I don't know why
Shurcliff never mentioned this.)  Clearly the thermal mass of the
rocks is also important, because it extends the heat transfer time.
But radiation seems to me to be the main mechanism of heat transfer
enhancement.

We can use this insight to enhance air/water-pipe heat
exchange in a heat exchanger consisting of a stack of pipes
of water.

In a stack of pipes, the pipes see each other very well,
and are at a lower temperature than the air that bathes
them, and at nearly the same temperature, so radiation is a
small component of the heat exchange between pipes and air.
We can change this by introducing  *auxiliary radiators*  between the
pipes.

When the auxiliary radiators are positioned transverse to air
flows, they would have to present little resistance to air passing
through them.

The air bathing the auxiliary radiators keeps them close to the
temperature of the air.  They represent an additional heat transfer
mechanismthat that was not present before. If they do not impede the
air flow significantly, they will increase the power of the heat
transfer between the air and the pipes.

Auxiliary radiators would be less effective than fins, but fins and
auxiliary radiators should be complementary to some extent.
Radiators might help build more economical air water heat
exchangers than could be built with fin tubes or with neither
fin tubes nor auxiliary radiators.

David Delaney, Ottawa


Posted by nicksanspam on December 3, 2004, 12:22 am
 


Good idea. I wonder how much this would help.

Nick


Posted by daestrom on December 3, 2004, 12:48 am
 

<http://geocities.com/davidmdelaney/shurcliff/shurcliff-thomason-storage-1.html>

Perhaps because heat transfer by radiation in this sort of situation is much
less than that of the conduction.  Why do you think 'most of the
enhancement... is by radiation'?  Radiant heat transfer to air is very
difficult owing to air being almost completely transparent to infra-red
radiation.

Looking at the sketch in the link you provided, it seems the rocks biggest
contribution (aside from some additional heat capacitance) is the direct
contact they make with the tank.  They act like very inexpensive 'fins' to
provide more surface area for the air to contact for heat transfer.


An interesting idea.  But you might get an order of magnitude more heat
transfer power by just weaving metal straps back and forth between the tubes
in the air stream.  With the straps well bonded to the tubes, these 'fins'
would probably collect heat from the air much more than auxilary radiator
panels.

daestrom



Posted by David Delaney on December 3, 2004, 4:16 am
 On Fri, 03 Dec 2004 00:48:37 GMT, "daestrom"


<http://geocities.com/davidmdelaney/shurcliff/shurcliff-thomason-storage-1.html>

Yes. I meant radiation between the rocks and the water tank.


And very ineffective fins, too. Each rock touches the tank on a very
small area, a very small proportion of its surface area.


If you mean to add enough straps to get an area increase comparable to
a conventiuonal fin tube, I'd agree. Does anyone know the formula for
radiation between concentric cylinders?

Never mind, consider small (and therefore planar) pieces of the pipe
wall.  Assume a parrallel auxiliary radiator at the air temperature,
45C, and  the pipe at 35C.  The power is

5.7e--8*((273+45)^4-(273+35)^4) / (1/0.9 + 1/0.9 -1) = 58 W/m2, or an
equivalent  RSI value of (45-35)/58 = 0.1724 m2.C/W, or approximately
1 ft2.hr.F/Btu, probably very close to the effective resistance to
conduction between the air and the pipe.

This assumes a solid auxiliary radiator, but that might be possible
with adroit placing of curved sheet metal pieces.

David Delaney, Ottawa

Posted by Anthony Matonak on December 3, 2004, 9:18 am
 David Delaney wrote:

Maybe this is a case of over thinking something. Transferring heat from
(or to) a fluid in a pipe seems to be a well developed technology.
Why not simply use ordinary car or truck radiators and fans?

Anthony

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