h-x///Nu-x

misagh · April 5, 2012, 06:04

hi.my project is a tube with wall thickness.my goal is to get h(convection heat trans. coef.).Water is entering the tube with definite tempreture and velocity.A constant heat flux is on the tube wall.
how can i draw h-x or Nusselt-x??
thanks alot...

Zigainer · April 5, 2012, 08:20

First of all it's dependent on how you define your temperature difference.
So I would guess its (wall temperature - bulk temperature).
In this case, I would create some planes along the tube and calculate the bulk temperature. Then create a function (4th order or what you need) and put this function as custom-field-function of tube length (hopefully one of your axis is along the tube, but as you wirte h-x I guess it will be the x-axis).
Next step would be a custom-field-function for h dependent on your bulk temperature function. That's it.

misagh · April 5, 2012, 08:42

thank you.yes i can calculate h from the formula h=q/Tb-Tw.since q is constant along tube/x axis/getting T can solve my problem.my question is how to calculate Tbulk?where can i find it in fluent?in REPORT...SURFACE INTEGRAL???there is not BULK tempreture in it.

Zigainer · April 5, 2012, 09:04

Surface Integrals -> Report Type "Mass-Weighted-Average"

misagh · April 5, 2012, 09:16

ok.my equations have not converged yet.i dont know what to do.i have reduced under relaxation... but no effect.
and before convergent i can do nothing.

Zigainer · April 5, 2012, 09:18

Start with first order and less mass flow (like 1/10), then increase massflow to nominal mass flow. All with low under-relaxation factors. Switch to 2nd order. Increase under relaxation factors.

misagh · April 5, 2012, 09:22

so my bc is velocity inlet i increase it gradually?would you please explain the under-relax...??

Zigainer · April 5, 2012, 09:59

If you have velocity inlet, just increase the velocity gradually.
If you have problems with convergence or even divergence it's good to reduce the under-relaxation factors. So your limit the change of your solution from step to step. This can stable your calculation, especially at the beginning. At the end your relaxation factors should be as high as possible to reach convergence quickly.

misagh · April 5, 2012, 10:30

thank you very much for your usefull help.

April 5, 2012, 06:04	h-x///Nu-x	#1
misagh Member misagh Join Date: Apr 2012 Posts: 64 Rep Power: 14	hi.my project is a tube with wall thickness.my goal is to get h(convection heat trans. coef.).Water is entering the tube with definite tempreture and velocity.A constant heat flux is on the tube wall. how can i draw h-x or Nusselt-x?? thanks alot...

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April 5, 2012, 08:20		#2
Zigainer Senior Member Join Date: May 2011 Location: Germany Posts: 130 Rep Power: 14	First of all it's dependent on how you define your temperature difference. So I would guess its (wall temperature - bulk temperature). In this case, I would create some planes along the tube and calculate the bulk temperature. Then create a function (4th order or what you need) and put this function as custom-field-function of tube length (hopefully one of your axis is along the tube, but as you wirte h-x I guess it will be the x-axis). Next step would be a custom-field-function for h dependent on your bulk temperature function. That's it.

April 5, 2012, 08:42		#3
misagh Member misagh Join Date: Apr 2012 Posts: 64 Rep Power: 14	thank you.yes i can calculate h from the formula h=q/Tb-Tw.since q is constant along tube/x axis/getting T can solve my problem.my question is how to calculate Tbulk?where can i find it in fluent?in REPORT...SURFACE INTEGRAL???there is not BULK tempreture in it.

April 5, 2012, 09:04		#4
Zigainer Senior Member Join Date: May 2011 Location: Germany Posts: 130 Rep Power: 14	Surface Integrals -> Report Type "Mass-Weighted-Average"

April 5, 2012, 09:16		#5
misagh Member misagh Join Date: Apr 2012 Posts: 64 Rep Power: 14	ok.my equations have not converged yet.i dont know what to do.i have reduced under relaxation... but no effect. and before convergent i can do nothing.

April 5, 2012, 09:18		#6
Zigainer Senior Member Join Date: May 2011 Location: Germany Posts: 130 Rep Power: 14	Start with first order and less mass flow (like 1/10), then increase massflow to nominal mass flow. All with low under-relaxation factors. Switch to 2nd order. Increase under relaxation factors.

April 5, 2012, 09:22		#7
misagh Member misagh Join Date: Apr 2012 Posts: 64 Rep Power: 14	so my bc is velocity inlet i increase it gradually?would you please explain the under-relax...??

April 5, 2012, 09:59		#8
Zigainer Senior Member Join Date: May 2011 Location: Germany Posts: 130 Rep Power: 14	If you have velocity inlet, just increase the velocity gradually. If you have problems with convergence or even divergence it's good to reduce the under-relaxation factors. So your limit the change of your solution from step to step. This can stable your calculation, especially at the beginning. At the end your relaxation factors should be as high as possible to reach convergence quickly.

April 5, 2012, 10:30		#9
misagh Member misagh Join Date: Apr 2012 Posts: 64 Rep Power: 14	thank you very much for your usefull help.