Forced Convection
Forced Convection
(OP)
I have a problem similar to a thread I found here:
thread391-227383: Tube Heat Transfer Coefficient (Forced Convection)
I am designing a generator cooling system. The generator is in a sealed steel tube which is air filled and under water. I intend to weld a steel channel to the side of the tube and blow air down it to power circulation. So I need to work out how big a fan I need.
HeatPower = GeneratorPower x (1- Efficiency)
TempGenerator = TempWater + HeatPower x Area x Coefficient
The previous thread recommended a coefficient of 100W/m2K. That's a good start but it doesn't tell me how big a fan I need. I suppose I'll need to use CFD to get a better value than that but is there a rough formula I can use to relate the fans flow rate to the coefficient?
(Relating to the previous thread, under water vehicles use an electrical umbilical which may carry was much as 2MW. These are wrapped on a winch drum which is water cooled.)
thread391-227383: Tube Heat Transfer Coefficient (Forced Convection)
I am designing a generator cooling system. The generator is in a sealed steel tube which is air filled and under water. I intend to weld a steel channel to the side of the tube and blow air down it to power circulation. So I need to work out how big a fan I need.
HeatPower = GeneratorPower x (1- Efficiency)
TempGenerator = TempWater + HeatPower x Area x Coefficient
The previous thread recommended a coefficient of 100W/m2K. That's a good start but it doesn't tell me how big a fan I need. I suppose I'll need to use CFD to get a better value than that but is there a rough formula I can use to relate the fans flow rate to the coefficient?
(Relating to the previous thread, under water vehicles use an electrical umbilical which may carry was much as 2MW. These are wrapped on a winch drum which is water cooled.)





RE: Forced Convection
I would putt he formula up here, but tis kinda ugly and wont look at all nice ont he form style, so it might not be much help to you.
RE: Forced Convection
You need to figure out what you need, and design accordingly.
How deep is this assembly, since you need to account for the pressure at depth for a pump to work?
One also wonders why you don't use the water, since it's already there?
TTFN
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RE: Forced Convection
Thanks IRstuff, The generator is in an environment sealed at atmospheric pressure, so the pressure depends on the temperature.
Pressure = PressureAtmos x Temp / TempAtmos
Water cooling would be a bit risky in case it leaked into the air filled tube. The tube can't be entirely full of water since it needs to float.
RE: Forced Convection
I wasn't suggesting filling the tube with water, just running a pipe through it to the generator. As for leaks, it's only marginally worse that the sealing of the tube itself, no? In fact, you've got way bigger joint seals for the tube than any liquid cooling would actually require. However, liquid cooling can pull way more heat out than the air can.
TTFN
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RE: Forced Convection
To compare liquid cooling to air cooling I still need to answer my original question; how to calculate the convection coefficient.
RE: Forced Convection
http:
There are lots of heat transfer calculators on the web that you can look at. You haven't said how much power you're trying to dissipate and the criticality of the cooling. That will dictate the design approach, not the htc itself.
TTFN
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RE: Forced Convection
E34*(0.3+(0.62*E31*E32)/((1+E33)^0.25)*(1+(B32/282000)^(5/8))^(4/5))
E34 = k/D
E31 = Reynolds number to the 1/2
E32 = Prandtl Number to the 1/3
E33 = (.4/Pr)^2/3
B32 = Reynolds Number
k = Thermal conductivity of the external fluid in W/mK
D = Pipe Diameter in m
So thats what i use to calculate the external forced Convection Coefficient, the formula is different for Natural Convection
Natural Convection (Please note this one is truly messed up as i typically work in metric, but the formula was in Imperial, so all the G units are imperial units, and F is the conversion factor back to metric
=116*((G56^3*G57^2*G58*G59/G60)*(G61/G62))^0.25*F65
G56 = k (BTU/ft/hr/ft^2/°F)
G57 = rho (lb/ft^3)
G58 = cp (Specific Heat of the external Fluid in BTU/lb°F)
G59 = B (Coefficient of Thermal Expansion Length/Length/°F)
G60 = mu (Dynamic Viscosity in cP)
G61 = temp (°F)
G62 = diameter (in)
F65 = 5.678263398
To see all my conversion factors check this website
http://www
I hope this helps and does nto confuse the crap out of you, im still waiting tog et my own copy of Process Heat Transfer, id borrowed a former co-workers copy when i made this spread sheet. (Those silly out of print 1950's books are so hard to get)
RE: Forced Convection
GreenRoger, how are these generators cooled terrestrially?
TTFN
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RE: Forced Convection
Did you try Amazon for "Process Heat Transfer"
h
They have many copies ranging from a new $90 reprint to a like-new original $44.95
RE: Forced Convection
TTFN
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RE: Forced Convection
RE: Forced Convection
I'm not convinced by Jmiles formula; it asks for the conductivity of the external fluid while I am looking at the fluid inside the pipe.
I've downloaded Lienhard but haven't found a useable formula for this in it.
RE: Forced Convection
You can also try the Thermal Wizard website here: http://www.thermal-wizard.com/tmwiz/default.htm
The web pages are coded so they determine laminar/turbulent flow domains before computing.
RE: Forced Convection
For the internal convection coefficient i use
Nu*k/ID
Nu = Nusselt NUmber
k = Thermal conductivity in W/m°K (Internal Fluid)
ID = Internal Diameter in meters
RE: Forced Convection
I'd been trying to calculate the Nusselt number for myself but haven't managed that yet.
RE: Forced Convection
RE: Forced Convection
Your fluid velocity is too small to be in turbulent regime. (0.0001m/s through a 1m dia pipe) You should use the laminar formula.
Look at the theoretical stuff at: http://w
RE: Forced Convection
RE: Forced Convection
RE: Forced Convection