Re: Natural Convection in a Vertical Column

If you don't _control_ the relation of the cables to each other within the
conduit, then no CFD model could predict what is going to happen in the real
system, because you don't know what the real geometry will be.  Simple example:
 Given, say, seven conductors, six could surround one and completely insulate
it from any air circulation.  Assuming that they will be spaced apart from each
other is not supportable, unless physical spacers enforce separation, and then
the spacers interfere with the airflow.

Wait; it gets worse.  Long parallel conductors will react with each other
physically, because of the interaction of their induced magnetic fields.  I
don't know about a 40 foot column;  I do know that in a 1000 foot navigation
lock, the control cables that share a cable tray will chafe each other's
insulation off, and actually 'whip' from one side of the tray to the other if
they are not physically tied down at close intervals.

-Mike-

>If you are working on a fixed installation, the IEE wiring regulations may
>well limit what you can do, ( U.K. only ).
>
>Even if you are outside the U.K. they are a useful guide to what is safe,
>and are extremely practical ( reading between the lines I smell frying
>electrician.... :-) ).
>
>They also contain a lot of tables giving information on common cable and
>wire types.
>
>--
>Jonathan
>
>Barnes's theorem; for every foolproof device
>there is a fool greater than the proof.
>
>To reply remove AT
>
>"Russ" <[EMAIL PROTECTED]> wrote in message
>news:[EMAIL PROTECTED]
>> I am currently analyzing a problem involving a long vertical column
>> with a relatively small conduit along the length column containing a
>> group of power cables. There is a significant amount of heat
>> generation in the cable due to a large current. I'm primarily
>> interested in the temperature of the conductors and surrounding
>> insulation.
>>
>> It is fairly straightforward to take a control volume and perform a
>> simple conservation of energy calculation to obtain component
>> temperatures. This assumes that the temperature of the surrounding air
>> is determined exclusively by that of the components.
>>
>> In reality, since there is heat generation along the entire cable,
>> natural convection will carry the heat upward and result in a higher
>> air temperature at the top of the column. My problem is to quantify
>> what "higher" is. I can do a parametric study to examine the component
>> temperatures as the surrounding air temperature increases but I need
>> some qualitative means of determining where we're really at along this
>> parametric curve. The simplified analysis yields temperatures that are
>> close to the specified limit for the insulation, even a modest
>> increase in temperature could potentially push the insulation over the
>> limit.
>>
>> I've been considering a CFD analysis of the entire system but I have
>> some concerns about the viability of CFD for natural convection
>> modeling.  Also, CFD modeling, even for this relatively simple
>> geometry may be a massive undertaking. I have done some work in CFD
>> for forced flow without any temperature considerations but I'm afraid
>> that introducing this additional level of complexity will make CFD
>> unreasonable for this application. Any thoughts on that?
>>
>> I'm looking for any suggestions or reference information that you may
>> have. Although CFD is an option I'm considering I'd be interested in
>> any potential analytical approach.
>>
>> Thank you,
>> Russ D.
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>Subject: Re: Natural Convection in a Vertical Column
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>From: "Jonathan Barnes" [EMAIL PROTECTED] 
>Newsgroups: sci.engr.analysis, sci.physics.computational.fluid-dynamics,
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>Date: Thu, 9 Oct 2003 18:26:43 +0100
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