B: Re: ECI cylinders - Yes they run HOT

[Walter Atkinson]

Guys:

The heat of combustion is about 3800 degrees F, not the EGT temp of  
1600.  The metal (CHT) is 350-400 degrees.  How can that be?

It's the thermal boundary layer which protects the metal. Disrupt  
that (like during detonation) and the metal gets hot really quickly.

Walter


On Oct 30, 2007, at 3:49 PM, Lance Fisher wrote:



Dan Barclay wrote:

> Yup, I'm familiar with the science and the calculations.
>
> My point is that when you say "heat input" the real issue is *net*  
> heat
> exchange.  Completely ignoring any heat generation differences, I  
> would
> still expect the net heat exchange to be significantly different at
> different places around the cylinder.
>
> Sometimes it's useful to consider an extreme (boundary condition)  
> case.  The
> boundary condition case isn't what's happening quantitatively, but  
> can help
> understand what's going on.  To wit, assume there is very hot gas  
> (1600
> degr) in the cylinder full time and perfect cooling with 70F air at  
> the top.
> The top metal would be 70F *at the outside surface* and near 1600F  
> at the
> inside surface.
>
I'm not able to follow your logic here.  You state that the /metal/  
would be 70F on the outside and 1600F on the inside, but that's  
simply not true.  The /gas/ might be 1600 F on the inside (probably  
not in the boundary layer) and the *air* might be 70F on the outside  
(again not likely at the air to metal interface) but the /metal/ will  
be very nearly the same temperature on the inside and outside  
surfaces.  If you suspended a 12" long steel bar so that one end was  
being heated by a hot air gun whose output was air heated to 500F  
with the other end subject to 0 F air blowing on it, the entire bar  
would eventually reach a temperature somewhere around halfway between  
0 F and  500 F.  The heated end of the bar will be hotter than the  
cooled end but probably not by more than 10 F with nearly 250 F  
between the hot air temp and the heated end of the bar and a similar  
difference between the cooler end of the bar and the 0 F air.

>  Assume *no* external cooling at the bottom.  The bottom
> temperature would be near 1600F (though not quite since some of the  
> heat
> transfers toward the top) just before the head started to drip.   
> Remember
> that the heat moving from the bottom of the cylinder, through the  
> walls,
> must travel through several inches of metal in order to be  
> dissipated at the
> top.  There is significant resistance there.
>
The thing is the thermal resistance of the metal is a tiny fraction  
of the resistance between the metal and the air or gas so most of the  
delta temp will be across the metal to gas interface and very little  
difference will be seen from one spot on the outside to another even  
though the distance is much greater along the metal path.

Now just to be clear, I'm *not* certain that a 150 F difference  
between the temp of the upper surface of the cylinder and the temp of  
the lower surface isn't possible, it just seems unlikely to me.

-lance