Heat Loss in Pipe-Fluid Not Moving 1

[RJB32482]

Hey,
I am trying to calculate heat loss of a fluid in a pipe where the initial temperature of the fluid in the pipe is greater than ambient temperature. The pipe is open to the atmosphere. The tubing is not insulated. The fluid is not flowing in the pipe, just sitting there.

I am assuming there is no forced convection in this fluid since the Re number would be 0. So I found two correlations to calculate the Nu number for free convection (turbulent and laminar flow). Could I just use the laminar flow correlation (from Churchill and Chu "Correlating Equations for Laminar and Tubular Free Convection from a Horizontal Cylinder) to calculate this heat loss.

Is there another method for this calculation?

Thanks.

 

[Latexman]

RJB32482,

"So I found two correlations to calculate the Nu number for free convection (turbulent and laminar flow)." This is confusing to me. Did you mean natural convection when you said free convection? If so, there should be no flow or velocity components to input to those correlations, because natural convection is for no forced flow situations. The only flow that occurs during natural convection is due to temperature and density differences. Use a relavent natural convection correlation.

Good luck,
Latexman

 

[RJB32482]

Thanks for the reply,

Anyone have any good correlations for this type of problem?

Thanks again.

 

[MintJulep]

"Heat loss" will be a function of time.

Do you really mean rate of heat transfer? This will not be constant, being a function of delta T.

 

[googool]

Hi
You can find your answer in this book

http://web.mit.edu/lienhard/www/

page 416

Good luck

 

[RJB32482]

Thanks for the help,

But wouldn't I use this equation since the flow is neither Laminar or Turbulent:

Q=(Delta T)/[(1/h*A)+(LN(ro/ri)/(2*pi*k*L))]

Now do I use the equations for the Nu number to find h(convective coefficient) or do I just ignore convection for the heat loss estimate? (Can estimate average wind speed). I have already ignored convection through the silicone fluid.

Thanks

 

[RJB32482]

To clear it up, the piping is indoors so wind speed is minimum. Is there a rule of thumb wind speed used for indoor applications so I can get a rough estimate of convective heat loss?

Thanks.

 

[25362]

MintJulep is right. This is an unsteady-state heat transfer situation, since [Δ]T is bound to drop with time.

 

[Latexman]

Speak to someone knowledgeable in the HVAC of the building this pipe goes through to get an average velocity number. If it's essentially nil, you have an unsteady state heat transfer problem with natural circulation on the outside (the air) and natural circulation on the inside (the fluid). The conduction through the metal pipe wall may be significant in your case since the heat transfer coefficient on the inside and outside are going to be relatively low.

Good luck,
Latexman

 

[Latexman]

Excuse me, I meant the resistance to heat transfer through the metal pipe wall may be significant in your case since the heat transfer coefficient on the inside and outside are going to be relatively low.

Good luck,
Latexman

 

[MintJulep]

As a SWAG, air velocity in an industrial type building might be expected to range between 50 and 250 ft/min.

 

[RJB32482]

OK, is there ant sense in finding the convective heat transfer coefficient for the fluid in the pipe (since the flow is 0)and I can just use the resistance from conduction through the pipe and convective heat transfer to the air?

Thanks, now if there was flow of the silicone in the pipe. I would just use the correlations to find the convective heat transfer coefficient for the inside correct?

Thanks for all the responses.