Heat Sink Question 1

[sullivnc]

It's been a few years since I took thermo and heat transfer, need some help with a question:

I am using a 29 J/s heater strip to heat a heat sink (181.4 g of aluminum) to 60 degrees C at room temperature. I am wondering how to tell if this heater strip has enough power to overcome the effects of lowering the air temperature to -25 degrees C. Any help would be appreciated. Thanks.

 

[IRstuff]

mass is less important than surface area; ultimately, it's still area*htc*deltaT

TTFN
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[chicopee]

How do you intend to place the heat strip on the Al heat sink (by winding it around the block or just laying it out on one surface)? is the heat strip insulated on its non contacting side? what is the configuration of the heat sink (another word is it a thick or thin block and how long)? is the heat sink surrounded by air or is it in contact with other material? Is the air still or moving at what speed? a heat transfer problem could be simplified when all parameters are known and a sketch of the set up would be very helpful.

 

[sullivnc]

Thanks for the questions. It is a flexible heater strip, attached to the outer diameter with epoxy. The strip is insulated on its non-contact side, with about .125" of Fiberfrax insulation. The heat sink has an OD of 2.75" and an ID of 50 mm (1.9685"). The OD of the heat sink is covered by the heater strip and insulation. The ID is open to still air. The heat sink is just a pipe with wall thickness ~0.385" and height 1.5".

 

[IRstuff]

So calculate the total area on OD and ID, assume something like 10W/m^2-K htc, and a deltaT of 85°C

TTFN
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[chicopee]

Is the pipe laying flat, inclined or vertical?

 

[MintJulep]

Is your 29 watt heater currently operating continuously?

 

[sullivnc]

The openings are at the top & bottom, and no, I have a thermistor in place to tell me when the heat sink has reached the proper temperature.

 

[MintJulep]

As IRStuff noted.

area*htc*deltaT

Existing DeltaT = 60 - "room temperature" . So about 40

New DeltaT = 60 - (-25) = 85

85/40 = a bit bigger than 2

Over some reasonably long period of time, if your heater is currently on more than half of the time then you don't have enough power.

 

[chicopee]

I have an attachment involving a horizontal tube heated per your description. Trial and error will be needed tempered with good engineering judgment. Note all units are in British thermal units such as Btu/hr-sq.ft.-dF, Btu/hr-ft-dF, Lbm/hr, etc...Two sources of reference were Kreith "heat Transfer" and Kent "Power". There will be some initial engineering estimates such as reasonable surface temperatures and air velocity thru the horizontal tube so that Nusselt and Reynold dimensionless equations can be of use. The bulk temperature will be the combined temperature of the air going in and coming out of the tube. As far as the temperature of the heating element, you'll need to find out the manufacturer's recommendation as a start.