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-   -   Nusselt number in microchannel (https://www.cfd-online.com/Forums/fluent/95197-nusselt-number-microchannel.html)

 micro77 December 8, 2011 23:27

Nusselt number in microchannel

Dear All,

I am doing simulation to analyse flow and heat transfer in nanofluid-cooled rectangular microchannel. It is a rectangular box with rectangular microchannel inside. The main problem is that as I increase the volume fraction, the Nusselt number going down which is contradict to previous researcher. For my simulation:

i. I already did grid independence test with 5 diff. meshes.
ii. My BC are: inlet vel at the inlet with T=293K, pressure outlet=0 gage and To=293K, top heat sink wall with constant heat flux, q"=650kW/m2. I also coupled the solid-liquid interface.
iii. Tref=inlet temperature, L=hyd. diameter.
iv. I calculated Nu in 2 ways:
a) Nu=q"*D/(Tw-Tb)*k where Tw=T at top channel
Tb=T at the centerline of channel
I create a line at top channel & the centerline to get Tw and Tb

while q"=-k(T top heat sink - T top channel)/z

where z=normal distance to the flow

b) Nu=q"*D/(Tw-Tb)*k but q" is taken directly from average value on top channel line.

My question is:
i. How did i get decrease in Nusselt number with increase in nanoparticle volume fraction?
ii. During setting up the BC for solid liquid interface, I set the wall thickness & heat generation =0. As far as I know, the coupled wall shows that the heat from the top can go through the wall to the channel. Am I right?
iii. I also create boundary layer in Gambit but no different in results shown.

I welcome any view/ suggestions.

 shashank312 December 9, 2011 04:11

Are you increasing the volume fraction of the liquid or the vapor?

 micro77 December 9, 2011 09:18

I increase the nano particle volume fraction from 1 to 4%. I did simulate using several different equations used for nanofluid thermal properties. And I also try run with calculated values given in later research but again, the same result, even though the paper got increase in Nu number with increase in vol. fract. I did check my data base, again & again. I do not know what goes wrong here.

 micro77 December 9, 2011 09:20

The nanofluid is alumina with water base. So, I increase the alumina vol. fraction (solid ).

 shashank312 December 9, 2011 16:38

Wait, which MODEL are you using? It cannot be VOF, it has to be some other Eulerian model which considers fluid and solid phases as one continuum.

Let me ask you one more question before I answer your query. Are you doing a nanofluid jet impingement on a heated surface?!

 micro77 December 10, 2011 10:43

I apply single-phase first before going to multi phase model. Forced convection flow.

 akshaygupta March 3, 2014 13:56

same problem, did u get the sollution ?if so,plz reply

[getting same problem if u have got the solloution plz reply
]Dear All,

I am doing simulation to analyse flow and heat transfer in nanofluid-cooled rectangular microchannel. It is a rectangular box with rectangular microchannel inside. The main problem is that as I increase the volume fraction, the Nusselt number going down which is contradict to previous researcher. For my simulation:

i. I already did grid independence test with 5 diff. meshes.
ii. My BC are: inlet vel at the inlet with T=293K, pressure outlet=0 gage and To=293K, top heat sink wall with constant heat flux, q"=650kW/m2. I also coupled the solid-liquid interface.
iii. Tref=inlet temperature, L=hyd. diameter.
iv. I calculated Nu in 2 ways:
a) Nu=q"*D/(Tw-Tb)*k where Tw=T at top channel
Tb=T at the centerline of channel
I create a line at top channel & the centerline to get Tw and Tb

while q"=-k(T top heat sink - T top channel)/z

where z=normal distance to the flow

b) Nu=q"*D/(Tw-Tb)*k but q" is taken directly from average value on top channel line.

My question is:
i. How did i get decrease in Nusselt number with increase in nanoparticle volume fraction?
ii. During setting up the BC for solid liquid interface, I set the wall thickness & heat generation =0. As far as I know, the coupled wall shows that the heat from the top can go through the wall to the channel. Am I right?
iii. I also create boundary layer in Gambit but no different in results shown.

I welcome any view/ suggestions.

 akshaygupta March 3, 2014 14:08

Nusselt number is decreasing when using nanofluid using single phase phase approach

Hi,
i have used single phase approach for calculating the enhancement in HTC in a flow through a channel ,for the same first i have calculated the HTC (both local along the wall surface and Avg) for plane water without any nanofluid thereafter i have inputted the thermophysical properties of water+nanoparticle properties but the HTC is decreasing which is contrary to literature.
the BC are :
inlet v=
temp 293 k
outlet =outflow
wall temp=310 k
in reference values i have changed the length value to be the hydraulic dia
and for temp=the tbulk i have approached two method
1)made a centerline in the channel and calculated the weighted ag along the line
2)(outlet+inlet)/2

but in both these cases the HTC and NU is decreasing
Plz help in telling why there is an anomaly of the current analysis with literature.....