Nusselt number and friction factor

inj3ct0r · April 2, 2023, 15:24

Hello,
I have a pipe with fluid inside and heat flux applied from the outside, I want to get the Nusselt and friction factor values for Dittus-Boelter equation validation, is it calculated manually or we get the value from fluent? as I've seen research articles doing validation with the equations and it is in the range, while mine is not.
Thank you

NickFL · April 3, 2023, 04:32

You will have to do a bit of post-processing for the values you need. The pressure drop is going to be the pressure at the inlet minus the pressure at the outlet. You may have set the pressure at the outlet to be 0 [Pa], so in that case the subtraction is simple enough that I can do it in my head. For the heat transfer (Nu) part you could integrate the local heat flux over the entire surface to get the complete heat flux that is typically used when computing the average Nu. Notice the heat flux will be highest near the inlet (when the temperature difference is the greatest) and lower as we move downstream.

Also depending on how long your pipe is and what you have for your inlet velocity boundary condition, that could contribute to a source of error. A more appropriate fully developed inlet profile could be parabolic or a 1/n profile depending on the Reynolds number.

April 2, 2023, 15:24	Nusselt number and friction factor	#1
inj3ct0r New Member inj3ct0r Join Date: Oct 2022 Posts: 1 Rep Power: 0	Hello, I have a pipe with fluid inside and heat flux applied from the outside, I want to get the Nusselt and friction factor values for Dittus-Boelter equation validation, is it calculated manually or we get the value from fluent? as I've seen research articles doing validation with the equations and it is in the range, while mine is not. Thank you

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April 3, 2023, 04:32		#2
NickFL Senior Member Join Date: Jun 2009 Location: Technische Universität Chemnitz Posts: 107 Rep Power: 17	You will have to do a bit of post-processing for the values you need. The pressure drop is going to be the pressure at the inlet minus the pressure at the outlet. You may have set the pressure at the outlet to be 0 [Pa], so in that case the subtraction is simple enough that I can do it in my head. For the heat transfer (Nu) part you could integrate the local heat flux over the entire surface to get the complete heat flux that is typically used when computing the average Nu. Notice the heat flux will be highest near the inlet (when the temperature difference is the greatest) and lower as we move downstream. Also depending on how long your pipe is and what you have for your inlet velocity boundary condition, that could contribute to a source of error. A more appropriate fully developed inlet profile could be parabolic or a 1/n profile depending on the Reynolds number.