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hammerpe
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| Posted: 02 January 2004 at 3:53pm | IP Logged
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What is the Convection Coefficient (h) that is required for a convection boundry condition? What are it's units? I am using Pro/Mechanica Wildfire. I have a cylinder that has a heat load of 15 lbf/s applied to the inner surface. The outside of the cylinder is in stagnent air. I have my Convection Coefficient set to 100. I want to see how much I can improve cooling with adding some fins to the outside of the cylinder.
Also, I would've expected the max_dyn_temperature to have leveled out by 1000s but it doesn't. Even if I lower my heat load all the way to 1 lbf/s. The slope of the lines seems to stay the same, the max temp just keeps getting lower.
Thanks in advance!
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hammerpe
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| Posted: 02 January 2004 at 6:31pm | IP Logged
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Ok, so I downloaded a heatsink example from PTC's website. I followed all of the instructions and I do not get the same results as they do... They have a nice graph of their max_dyn_temperature leveling out after 1000s with a 100w input load. I must have units of somewhere I guess but I can't figure out where....
I have attached the heatsink file with all of my inputs. Here is the link to PTC...
http://www.ptc.com/cs/cs_24/howto/mth812/mth812.htm
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hammerpe
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| Posted: 05 January 2004 at 8:33am | IP Logged
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Just a FYI for anyone, I found this.....
http://www.tsdengineering.com/pages/HT_natural_conv.html
I haven't had time to apply this yet, will let you know the results. I'd still like to know why mt model doesn't match PTC's model....
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Luis Aguirre
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| Posted: 05 January 2004 at 9:44pm | IP Logged
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Hello Hammerpe,
The values for the convection coefficient also known as the film coefficient are Btu/(hr*ft^2*F) or W/(m^2*K). Adding fins will definetely help to disipate more heat. If you still having problems later on post them I will see If I can fix them.
Luis
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JHardy
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| Posted: 06 January 2004 at 6:40pm | IP Logged
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Hammerpe,
It sounds a little as if you have a units problem. You need to use consistent units throughout. Depending on your chosen units system, you need to make sure all parameters are in appropriate units; eg:
SI Metric:
Heat: Watts (or Joules/second)
Heat Flux: Watts/square metre
Temperature: Degrees Kelvin
Thermal Conductivity: Watts / metre / degree Kelvin
Specific Heat: Joules / kg / degree Kelvin
Convection: Watts / square meter / degree Kelvin
Inch / Pound / Second:
Heat: BTU
Heat Flux: BTU / second /square inch
Temperature: Degrees Fahrenheit
Thermal Conductivity: BTU / second / inch / degree Fahrenheit
Specific Heat: BTU / pound / degree Fahrenheit
Convection: BTU / second / square inch / degree Fahrenheit
Foot / Pound / Second:
Heat: foot . pound force
Heat Flux: foot . pound force / second /square foot
Temperature: Degrees Fahrenheit
Thermal Conductivity: foot . pound force / second / foot / degree Fahrenheit
Specific Heat: foot . pound force / slug / degree Fahrenheit
Convection: foot . pound force / second / square foot / degree Fahrenheit
Note that you may need to adjust the “catalogue data”, depending on your chosen units. Eg if you are using units of inch / pound / seconds, but your published data is in units of BTU / hour / square foot / degree Fahrenheit, you will need to allow an appropriate factor to convert to consistent units of BTU / second / square inch / degree Fahrenheit.
Some typical values of convection coefficient (in metric units) are:
Natural convection in air: 11 W/m2/K
Natural convection in water: 570 W/m2/K
Forced convection in air: 170 W/m2/K
Forced convection in water: 1,100 W/m2/K
Natural convection and boiling in water: 5,700 W/m2/K
Forced convection and boiling in water: 57,000 W/m2/K
Condensation of water at atmospheric pressure 5,700 W/m2/K
When considering convection from a surface, you may also need to consider emission or absorption from free surfaces as well.
Hope this helps.
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hammerpe
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| Posted: 08 January 2004 at 5:47pm | IP Logged
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It was my stupidity that was bringing me down. In mechanica the loads and constraints are automatically highlighted when you rin the simulation. In thermal they aren't, so I neede to highlight the boundry condition and it works perfectly.
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Mechwitch
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| Posted: 15 November 2005 at 9:56am | IP Logged
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Hi all
I know this was all started a long time ago but I have recently been doing the same sort of thing. I've been looking for a realistic convection coefficient for forced convection in air and have come across the info above.
Do you have a reference for the above values so I can quote them in my report or details or link to where I can find them?
Thanks
MW
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Topic: Convection Coefficient?
