# calculate friction factor & nusselt number

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 March 5, 2017, 01:14 calculate friction factor & nusselt number #1 New Member   soheil Join Date: Mar 2017 Posts: 7 Rep Power: 7 Hi all I want to simulate tube bank one of my boundary condition in flow direction is periodic wile another is symmetric. (tube temperature is cosntant) I want to calculate friction factor and local and global Nusselt number 1) for calculate friction factor I need to calculate pressure drop across a row how can I calculate this pressure drop across one single tube? 2) can I calculate local Nusselt number by calculate heat flux for each theta and then use the flow formula: Nu=q(theta)/(Tb-Ts) where Ts is tube temperature & Tb is T inlet which is constant please help me thanks

 March 8, 2017, 16:37 #2 New Member   soheil Join Date: Mar 2017 Posts: 7 Rep Power: 7 please someone help me

 March 8, 2017, 18:34 #3 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,112 Rep Power: 60 For streamwise periodic, your input is either a specified pressure drop or a mass-flow rate. If you're using mass-flow rate, then Fluent calculates the pressure drop for you. You have the wrong equation for Nusselt number. Nusselt number is dimensionless. It looks like you have the heat transfer coefficient. If your reference temperature is constant, it is simple. Go to the reference values pane and set the reference temperature to your inlet temperature. Also set the reference length to the appropriate reference length that you need to calculate the Nusselt number (it's probably your tube diameter or radius). Fluent will calculate the local Nusselt number for you. Alternatively, you can let Fluent calculate the local heat transfer coefficient and you can calculate Nusselt number yourself. However, you still need to specify the reference temperature in order to get the right heat transfer coefficient. How do you define global Nusselt number? It should be pretty straightforward to calculate. Usually it is some type of surface average.

March 11, 2017, 10:30
#4
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soheil
Join Date: Mar 2017
Posts: 7
Rep Power: 7
Quote:
 Originally Posted by LuckyTran For streamwise periodic, your input is either a specified pressure drop or a mass-flow rate. If you're using mass-flow rate, then Fluent calculates the pressure drop for you. You have the wrong equation for Nusselt number. Nusselt number is dimensionless. It looks like you have the heat transfer coefficient. If your reference temperature is constant, it is simple. Go to the reference values pane and set the reference temperature to your inlet temperature. Also set the reference length to the appropriate reference length that you need to calculate the Nusselt number (it's probably your tube diameter or radius). Fluent will calculate the local Nusselt number for you. Alternatively, you can let Fluent calculate the local heat transfer coefficient and you can calculate Nusselt number yourself. However, you still need to specify the reference temperature in order to get the right heat transfer coefficient. How do you define global Nusselt number? It should be pretty straightforward to calculate. Usually it is some type of surface average.
Hi Lucky
I calculate global Nusselt number by :
Nu = hD/k ; where h= mdot * cp * (Tout-Tin) / ( A total * LMTD )
I define 2 line in inlet and outlet to calculate Tin & Tout.
I do what you say and I get good result for Nusselt number
but I have 30% error for friction factor
how can I solve it?
Fluent give me pressure drop per meter; which I should multiply it to whole domain length.
Are reference values effect in calculate pressure drop? what should I set length reference value for this case?