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Problem Nusselt Number

Problem Nusselt Number

Problem Nusselt Number

(OP)
Hello,

I have a question about the calculation of the Nusselt number. In particular, I have to determine this value in a problem inherent a countercurrent heat exchanger and I need to find the value of the heat exchange convection coefficient h. The problem is that I have a concentric annular duct and I don't know what formula I must to use.
The convection is forced and then the Nusselt number should depend essentially on the Reynolds number and the Prandtl number, right? However, now I have many different empirical formulas for the calculation of Nu and I don't know which one I must to use. Also in the book "Handbook of Heat Transfer" of Rohsenow and Hartnett they use a table that provides the values ​​of this number as a function of the problem boundary conditions; however, in this case, the Nusselt number, is independent of Re and Pr. What relationship should I use for the calculation?

Thank you, best regards

Emanuel Bernardi

RE: Problem Nusselt Number

(OP)
Thanks, but I had already read these things as I wrote above. In fact, in these document there are some relationship about Forced convection in turbulent pipe flow (Sieder and Tate, Dittus-Boelter ,etc.. ). My question is: I can use these relationships for the concentric annular duct?

RE: Problem Nusselt Number

WallaWalla,

Im risking sounding "condescending" but im not (really!): This is where book keeping stops and engineering starts! Generally speaking for T&S HX (liquid/liquid) i would use dittus-bolter with the classical parametres.

Best regards

Morten

RE: Problem Nusselt Number

I don't see why not.

See also: http://en.wikipedia.org/wiki/Concentric_tube_heat_...

As ballpark comparative figures for Re>10,000 you can use the following nuumbers which I took, with my old eyes, from a log-log graph in Fluid Flow and Heat Trnasfer by professor Lydersen (Wiley); usually (μ/μw)−0.14 ~ 1.0. Basis: Sieder and Tate equation.

Re --------------------- Nu×Pr-1/3×(μ/μw)−0.14
10,000---------------------- 40
20,000---------------------- 70
30,000---------------------- 95
40,000---------------------- 130
100,000---------------------- 250
200,000---------------------- 450

Don't forget all these formulas are empirical; and errors of +/-15%, or more, are common.

RE: Problem Nusselt Number

For forced convection you can use the Petukhov and Roizen correlation (please check it at this link http://faculty.ksu.edu.sa/Assiry/Documents/Correla... ), where Nu tube is calculated with the Sieder-Tate correlation. What pointed out by 25362 about possible errors stands still.

RE: Problem Nusselt Number

(OP)
Thank to all. In the nest days I will compare the theoretical data with experimental data so as to estimate errors.

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