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 February 12, 2014, 12:36 hi #1 New Member   Join Date: Oct 2013 Posts: 3 Rep Power: 4 hi iam simulating turbulent flow in concentric double tube heat exchanger. the inner tube is hot water and outer is cold water counter flow. when i have got the heat transfer coefficient for annulus and compare with literature it is wrong ? for nusslet number in literature i use petkhove also ditus-boilter correlation with hydraulic diameter with wetted PERIMETER for Re. and use heat trasfer PERIMETER(i mean just inner tube diameter) for getting heat transfer from nusslet number. but when i use wetted PREMETER for claculating heat transfer coefficient from nusslet number i got true answer. but mainly i should use just inner tube PREMIETER for calculating heat transfer coefficient from nusslet.

 February 12, 2014, 17:41 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,638 Rep Power: 98 FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F But in this case you also have to be careful the results you are comparing to are correct.

 February 13, 2014, 02:23 #3 New Member   AKHILESH S L Join Date: Sep 2013 Location: Trivandrum Posts: 29 Rep Power: 5 hi, can you please show a schematic with dimensions of your problem? which material you are using as heat exchanger wall?

February 14, 2014, 03:17
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for calculating heat transfer coefficient in the annulus the hydraulic diameter of the annulus is (Dh=D0-Di) and i use this diameter for calculating Re number. then i use petukhov correlation for finding nusslet number. it should be noted that for annulus the nusselt number should be based on the equivalent diameter calculated from (De=(4*Ac)/Ph) which here Ph is the perimeter of heat transfer area.(i mean inner tube perimeter) and then there is different between De and Dh which we use Dh for calculating Re and De for calculating heat transfer coefficient. but when i get the heat transfer coefficient with Dh from petukhov it is the same with fluent .but when i use De for getting heat transfer coefficient(HTC) from nusselt number it is different from fluent HTC. mainly i should use De for calculating HTC from nusselt number.

thanks.
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 February 14, 2014, 05:20 #5 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,638 Rep Power: 98 You seem to be comparing against empirical correlations. You should not expect these to be very accurate.

 February 15, 2014, 03:47 #6 New Member   Join Date: Oct 2013 Posts: 3 Rep Power: 4 thanks a lot friends, i found my problem between your guidance. the problem was that fluent solve the momentum and energy equation with Dh and i use another base i mean De for calculating the nusselt number from empirical correlations. bests.

 February 16, 2014, 20:25 #7 Senior Member     Mr CFD Join Date: Jun 2012 Location: Britain Posts: 312 Rep Power: 7 Be careful. Literature often define heat transfer coefficients and Nusselt numbers relative to a freestream or bulk temperature. When you post process the results make sure you do the same. By default CFX uses the neighbouring node as the relative temperature to determine heat transfer coefficients and Nusselt numbers. This can be changed in the expert parameters section.

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