Heat transfer coefficient for a rectangular duct
Heat transfer coefficient for a rectangular duct
(OP)
Hi,
I haven't got luck with my previous post (thread391-197659: Air condiioner specifications). One thing I need to know is the heat transfer coefficient for a rectangular duct fluid (air) inside, three of the walls are isolated from the enviroment and the remaining is an isothermal wall. The Reynold number corresponds to a turbulent flow. I begin my calculations considering a flow parallel to a flat plate and using the Nusselt number as:Nu = h*L/k = 0.037*Re^(4/5)*Pr^(1/3) but I know the actual Nu should be different. Can anybody help me?
I haven't got luck with my previous post (thread391-197659: Air condiioner specifications). One thing I need to know is the heat transfer coefficient for a rectangular duct fluid (air) inside, three of the walls are isolated from the enviroment and the remaining is an isothermal wall. The Reynold number corresponds to a turbulent flow. I begin my calculations considering a flow parallel to a flat plate and using the Nusselt number as:Nu = h*L/k = 0.037*Re^(4/5)*Pr^(1/3) but I know the actual Nu should be different. Can anybody help me?





RE: Heat transfer coefficient for a rectangular duct
...heat transfer coefficient for a rectangular duct fluid (air) inside, three of the walls are isolated from the enviroment and the remaining is an isothermal wall. The Reynold number corresponds to a turbulent flow. I begin my calculations considering a flow parallel to a flat plate and using the Nusselt number as:Nu = h*L/k = 0.037*Re^(4/5)*Pr^(1/3) but I know the actual Nu should be different. Can anybody help me?
desA replies:
What do you think the Nu -> h value should be?
For a first pass, I'd use:
Nu = h*L/k = 0.027*Re^(4/5)*Pr^(1/3)*(v/vs)^0.14
where:
v = mean flow viscosity
vs = viscosity at wall
Works for gases at Re>10,000, for round ducts. For a rectangular duct, you should use hydraulic diameter in your computations.
With this h value, you can then compute the heat extracted from the single isothermal side:
q' = h * A,s * (Tw - Tsur)
where:
A,s = isothermal surface
Tw = wall temperature
T,sur = core (mean) duct temperature.
You may need to take a guess at the Tsur (haven't checked) & iterate until your solution settles (if outlet temp limited).
A good book is "Fundamentals of heat and mass transfer", Incropera F.P. & DeWitt D.P.
Des Aubery...
(adTherm Technology - adthermtech dot com - des@adthermtech dot com)
RE: Heat transfer coefficient for a rectangular duct
Re = p*v*L/u
Where p is air density, v is velocity, u is dynamic viscosity and L is the length (not a perimeter or width) of the plate. Well, this apply for a flat plate with a flow parallel to it. How would you calculate the Reynold's number for the actual problem?
RE: Heat transfer coefficient for a rectangular duct
L -> D,h
Re = dens * Vel * D,h / visc
This should get you to h.
From h, compute q.
Des Aubery...
(adTherm Technology - adthermtech dot com - des@adthermtech dot com)
RE: Heat transfer coefficient for a rectangular duct
RE: Heat transfer coefficient for a rectangular duct
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