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Old   June 2, 2012, 19:40
Question Fully Developed Conditions for Microchannel Heat-Sink (MCHS))
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Behzad
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Hi daer friends
I want apply fully developed conditions (hydrodynamics and temperature conditions) at the end of a micro-channel heat sink (MCHS) that nanofluid flowing it.These conditions at outlet include in: 1)pressure equal to ambient pressure. , 2)Velocity gradient across the length of MCHS equal to zero. , 3)Temperature gradient across the length of MCHS equal to zero.
Please tell me how check these conditions for simulation and apply them via CFX?
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Old   June 3, 2012, 07:02
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Don't you mean velocity and temperature gradient is zero in the flow direction?

The simplest way to do these sort of flows is by using a periodic boundary with your defined flow rate and heat addition. Then it will converge on the full developed solution.
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Old   June 3, 2012, 09:48
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no,zero gradient only apply on outlet conditions.Can I give these conditions without apply of periodic conditions?
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Old   June 3, 2012, 19:34
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I see, you want to apply a zero gradient BC, you are not interested in fully developed flow. A zero gradient BC does not mean the flow in front of it is fully developed by the way. To get fully developed flow you need a duct long enough for it to develop. It has nothing to do with the BC - and this is the same for a physical experiment as well.

CFX does not have zero gradient BCs, it has flow variables convected from the domain (with a few exceptions, see the documentation for those).
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Old   June 3, 2012, 20:07
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How I can get enough length for simulation to ensure that flow is fully-developed at outlet.The geometry of my problem is a trapezoidal microchannel haet sink that dimensions of front area include in:400 micron (large base),300 micron (small base),350 micron (height),base angle=54 degree and length of microchannel is 10 millimeter normal to front area.Does these dimensions enough for you to answer me about fully developed conditions?
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Old   June 3, 2012, 21:22
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No, you cannot work out fully developed flow from those details as the fluid properties also contribute. And I do not have time to work it out for you anyway.

Read some basic fluid mechanics textbooks on flow development length. Alternately you can do a sensitivity study by testing various lengths and seeing if you can see it converging with increasing length.
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Old   June 4, 2012, 05:16
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With use of double precision at CFX-Solver Manager all of the equations (mass and momentum,energy,volume fraction) converge but when I use serial option at CFX-Solver Manager, energy equation can't converge.According to this comment, do you think my length is enough for obtaining a fully developed solution?
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Old   June 4, 2012, 07:06
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Whether your simulation achieves your specified residual convergance tolerance or you use double precision numerics has nothing to do with whether fully developed flow has formed.
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