double tube heat exchanger

mehyan · December 7, 2015, 07:15

hi everybody,

I have been trying to simulate a double pipe heat exchanger using ansys CFX15. inner tube is used for hot water flow and outer is used for cold water flow.
in defining boundary conditions both hot and cold inlet is defined in terms of mass flow rate (both have the sane mass flow rate). and then interfaces is determined.
both inner and outer tubes have same hydraulic diameter.
when the calculation is finished the following results was obtained:

temp-cold- outlet= the correct result
temp-hot -outlet= the correct result
inner tube heat transfer coefficient =the correct result
inner tube Re number = the correct result
outer tube Re number = the incorrect result
outer tube heat transfer coefficient= the incorrect result

i think outer tube incorrect Re number causes incorrect result in outer tube heat transfer coefficient ,

but the question is that why the outer and inner tube Re number should not be the same whereas the hydraulic diameter, mass flow rate and fluid type in both domain is the same and also temperature changes are not considerable ?

(i have the same problem on fluent)

Kindly help me.

Thanx

ghorrocks · December 7, 2015, 21:16

How are you modelling this? How are you calculating Re? The Re number is not used to calculate anything so how can that affect the result?

mehyan · December 8, 2015, 08:03

Thank u for ur kind repy. i myself consider Re effect on the surface heat transfer coeff, but i dont know is it true or not .
i calculate Re number via velocity that cfx give us after calculation end.
(cfx give me different velocity in each domain, while hydraulic diameter and mass flow rate are same .)
so my main question is that Re in both inner tube and outer tube should be the same in equal hydraulic diameter and mass flow rate (whit a bit changes in temp that cant effects) ?
Subsequently, whit different Re should i have different heat transfer coeff and also different friction factor?

mehyan · December 8, 2015, 13:12

and is there any expression to show nusselt number in a double pipe heat exchanger in cfx?

ghorrocks · December 8, 2015, 16:44

What velocity are you using? Please show where this is coming from.

Nusselt number - this is covered in any heat transfer textbook. If you have problems transferring it to CFX post a new question on it.

December 7, 2015, 07:15	double tube heat exchanger	#1
mehyan New Member mehran Join Date: Dec 2015 Posts: 10 Rep Power: 10	hi everybody, I have been trying to simulate a double pipe heat exchanger using ansys CFX15. inner tube is used for hot water flow and outer is used for cold water flow. in defining boundary conditions both hot and cold inlet is defined in terms of mass flow rate (both have the sane mass flow rate). and then interfaces is determined. both inner and outer tubes have same hydraulic diameter. when the calculation is finished the following results was obtained: temp-cold- outlet= the correct result temp-hot -outlet= the correct result inner tube heat transfer coefficient =the correct result inner tube Re number = the correct result outer tube Re number = the incorrect result outer tube heat transfer coefficient= the incorrect result i think outer tube incorrect Re number causes incorrect result in outer tube heat transfer coefficient , but the question is that why the outer and inner tube Re number should not be the same whereas the hydraulic diameter, mass flow rate and fluid type in both domain is the same and also temperature changes are not considerable ? (i have the same problem on fluent) Kindly help me. Thanx

December 7, 2015, 21:16		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,694 Rep Power: 143	How are you modelling this? How are you calculating Re? The Re number is not used to calculate anything so how can that affect the result? mehyan likes this.

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December 8, 2015, 08:03		#3
mehyan New Member mehran Join Date: Dec 2015 Posts: 10 Rep Power: 10	Thank u for ur kind repy. i myself consider Re effect on the surface heat transfer coeff, but i dont know is it true or not . i calculate Re number via velocity that cfx give us after calculation end. (cfx give me different velocity in each domain, while hydraulic diameter and mass flow rate are same .) so my main question is that Re in both inner tube and outer tube should be the same in equal hydraulic diameter and mass flow rate (whit a bit changes in temp that cant effects) ? Subsequently, whit different Re should i have different heat transfer coeff and also different friction factor?

December 8, 2015, 13:12		#4
mehyan New Member mehran Join Date: Dec 2015 Posts: 10 Rep Power: 10	and is there any expression to show nusselt number in a double pipe heat exchanger in cfx?

December 8, 2015, 16:44		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,694 Rep Power: 143	What velocity are you using? Please show where this is coming from. Nusselt number - this is covered in any heat transfer textbook. If you have problems transferring it to CFX post a new question on it.