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Heat Transfer
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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