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Old   April 30, 2009, 10:47
Unhappy velocity and temperature profile
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Im Panagiotis and i am trying to investigate the flow of air or water in a tube. im interested in temperature and velocity profile of the fluid inside the tube. The tube geometry is quite simple, a 3meters cylindric tube, Diameter 13 cm. the temperature of the cylinder assumed to be constant and higher than the inlet fluid temperature. For example Twall=100oC and Tinlet = 25oC. I would like to work for several flows . for example 5, 10, 20, 50 lit/min. i wanna simulate the flow in fluent. my question is if i need to specify if the flow is laminar or turbulent. in addition i would like to know if i have to specify the length of the thermal and velocity entry zone. i have found that all the cylinder is in the developing region even if i worked with low flows.. Finally i would like to know if i should specify if the heat transfer mechanism is free or forced convection..
So many questions?
Thanks in advance
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Old   April 30, 2009, 11:51
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First, did you make a dimensionless numbers study? cause all ther questions you have can be answered thanks to it:
the flow type is given by the reynolds number
the heat exchange type is given by Nusselt, but anyway, if your fluid has an initial velocity you should be in a case of forced convection

hope it can help you
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Old   May 1, 2009, 07:05
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i calculated the Reynolds Number and for more than 5lit/min is turbulent flow (Re>2100) the problem is that the nusselt number is not a constant in the thermal and velocity entry zone, and using the langhaar equation i found that the entry zone length is extended out of my cylinder( length of cylinder 3 m, entry zone length >7 m)..
i can find a mean Nusselt Number for the system but this represent the Exit temperature of the fluid and not the local temperature as the convection coefficient does not change linear.
another question i have is about the forced or natural convection
in case the fluid i use is water, imagine a vertical cylinder. ever for low floe rate the Reynolds number is high. mathematically the convection seems natural as Grashof Number >>>>1. But. i think thats wrong.
the natural convection says that the hot fluid moves free to regions with lower temperature as the pressure there is lower. but in my case the hot fluid cannot move free upward and even iff this possible, is forced then to come down again and recieve thermal evergy again.
if the free convection is the main heat transfer mechanism then, the mass transfer balance does not stand because the fluid must move upward. there is no exit upward, only downward
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cylinder, flow, heat and mass transfer

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