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Name: Kareem
Status: other
Age: 20s
Location: N/A
Country: N/A
Date: 2000-2001


Question:
I am trying to design a system to cool the water in my fish tank. I have the formulas to calculate the rate of heat transfered by convection between the glass the of fish tank and the air around. I have not been able to find a value for the heat transfer coefficient. I realize that this is a subjective question, but even a ball park answer would be helpful. Sincerely, Kareem


Replies:
Hello,

I would use 100 W/m2-K for the convection coefficient at the glass-air interface. You can actually estimate that value, for example, by slightly heating the water in the tank, insulate the top of the tank, and monitor its cooling down temperature.

Use the equation: m Cp dT/dt = A h (Tw-T0) which equates the heat loss from the bulk liquid with heat loss from the walls. m, Cp, and T are the mass, specific heat, and temperature of water. t is time, A is the area of convecting walls, h is the heat transfer coefficient, Tw is the wall temperature, and T0 is the room air temperature.

You can measure Tw and T at, say, 5 minutes intervals, approximate dT/dt as (T(t)-T(t-5))/5, and finally evaluate h in each interval. Then take the average of h. Ideally values of h obtained at different time intervals should be the same.

Please refer to a text on heat transfer for more information.

AK



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