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AlexVin July 10, 2013 07:51

Unexpected Temperature Profile in Rectangular Pipe
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I am using CFX 14.0 to get the temperature profile of water flowing into a rectangular micro pipe. The pipe is 100x100x10000 micrometers and it is placed at the center of a substrate made of aluminum. The two plates at the top and bottom surface of the substrate generate 1 W each and the flow in the pipe is laminar with 1.2 [m s^-1] normal velocity at the inlet. Water and Aluminium are taken from the default materials.

See the attached image and the resulting output file for more details.

The problem is that, despite convergence, the average temperature at the outlet is higher than expected. Indeed, in CFX-Post I get:

areaInt (Heat Flux) @ Outlet = 2 [ W ]

and, if Q = C_p MassFlowRate DeltaT,

(areaAve(Temperature)@Outlet - areaAve(Temperature)@Inlet) * (1.2e-8 [m^3 s^-1] * 997.0 [kg m^-3]) * 4181.7 [ J kg^-1 K^-1 ] = 2.214 [ W ]

How is it possible that the heat that should have been generated to cause the DeltaT from inlet to outlet is always 10% higher than the heat flux at the outlet? Does anybody have an explanation? This result is independent from the accuracy of the solution (target residual and domain imbalance) and also from the amount of heat put as source. Always ~10% higher in both steady state or transient with timestep 5e-3 [ s ].



monkey1 July 10, 2013 08:28

What is the result with areAve(Heat Flux) @ Outlet?
There are significant differences between areaInt and areaAve

AlexVin July 10, 2013 09:29

areaInt (Heat Flux)@Outlet = -1.996e+00 [W]
areaAve (Heat Flux)@Outlet = -1.996e+08 [W m^-2]

btw, the outlet is indeed 100x100 um^2 (1e-8 m^2)

singer1812 July 10, 2013 10:10

Please check massFlowAve(T)@inlet-outlet instead of areaAve.

AlexVin July 10, 2013 10:35

areaAve(T)@Outlet = 343.7 [K]
massFlowAve(T)@Outlet = 338.2 [K]

therefore now

(massFlowAve(T)@Outlet - massFlowAve(T)@Inlet) * (1.2e-8 * 997.0) [kg s^-1] * 4181.7 [J kg^-1 K^-1] = 1.998e+00 [W]

which matches with the 2 W given as total source. Thank You singer1812 !!

AlexVin July 10, 2013 11:03

Hi again :D

do you mind helping me with another last issue? I'd like to profile the quantity massFlowAve(T) over a plane that moves along the direction of the channel.

I created a plane CrossSection in the domain "Channel" and placed on the plane XY at Z=100 micron. Now,

massFlowAve(T)@CrossSection = 300 K

how can I make a plot, or extract a csv file with Z = 0:100:10000 ?

Thanks again


singer1812 July 10, 2013 11:27

Not sure what you mean. You want a plot of average cross section T at 3 different locations (z= 0 , 100, and 1000)?

Cant you just do that by hand?

AlexVin July 10, 2013 11:46

Nope ... every 100. for (i=0 ; i != 10000 ; i += 100)

singer1812 July 10, 2013 14:58

Are you familiar with PERL? You can use that within CFX post. This will allow you to automate the plane movement, data collection, and data write out.

AlexVin July 11, 2013 13:31

Ok. I'll have a look to PERL. I was expecting some automatic way, as chart or export :(

Thanks again

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