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
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
Visit:
http://en.wikipedia.org/wiki/Nusselt_number
http://web2.clarkson.edu/projects/subramanian/ch30...
RE: Problem Nusselt Number
RE: Problem Nusselt Number
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
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.
Don't forget all these formulas are empirical; and errors of +/-15%, or more, are common.
RE: Problem Nusselt Number
[b]http://c.ymcdn.com/sites/www.saimeche.org.za/resou...[b]
to get an idea of the possible estimated errors.
RE: Problem Nusselt Number
RE: Problem Nusselt Number