Unexpected Temperature Profile in Rectangular Pipe
 

Hi.

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 ].

Thanks

AlexVin

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

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)

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

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 !!

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

AV

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?

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

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.

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

Thanks again