Talk:Natural convection

The Nusselt number

The correlation for calculating the Nusselt number as shown in this Wikipedia contribution is from Churchill and Thelen:

S.W. Churchill and H.-J. Thelen, Eine allgemeine Korrelationsgleichung für den Wärme- und Stoffübergang bei freier Konvektion, Chemie Ingenieur Technik, 47. Jahrg. 1975 / Nr. 10

In this paper the authors propose two different correlations corresponding to eq. (4) and eq. (5). The correlation in the Wikipedia article corresponds to eq. (4). For this equation the authors indicate that the following values for Nu(0) and the following characteristic lengths (for Nu and Ra) have to be used:

Shape, Nu(0), characteristic length: vertical plate, 0.67, L; horiz. cylinder, 0.36, D; sphere, 2, D

In the case of the horizontal cylinder the values shown in the Wikipedia article for Nu(0) and the characteristic length are wrong. The source of the information provided for the other geometries is not known. May be the author of this Wikipedia contribution can add it.

For calculating natural convection from different shapes the following work is recommended:

S. Lee, M.M. Yovanovich and K. Jafarpur, Effects of Geometry and Orientation on Laminar Natural Convection from Isothermal Bodies, Journal of Thermophysics, April-June 1991, Vol. 5, No. 2, pp. 208-216

88.244.91.164 (talk) 13:06, 25 March 2010 (UTC)

Grashof number and Rayleigh number

As far as I understand, Grashof number determines whether convection is laminar or turbulent, but it is Rayleigh number which determines whether convection occurs at all (or, rather, heat is transfered mainly by conduction). Indeed, for earth's mantle Grashof number is extremely small (~ 10^-24) but Rayleigh number is around 1000, and indeed earth's mantle manifests unstable convection.

Am I mistaken? If no objections come up, I will correct accordingly. Dan Gluck (talk) 19:18, 21 April 2010 (UTC)

I think that in the formula it is Grashof number that should be used : it is multiplied by the Prandtl number and the product of Prandtl and Grashof is the Rayleigh number. Also, I have read the formula using Grashof number in some paper : http://www.cheresources.com/convection.shtml#natural and in http://www.cheresources.com/convection.pdf. If no one disagree I will undo your editing. JC44 (talk) 13:53, 16 June 2010 (UTC)

Thermal vs. Mass Convection

I find this article to be biased towards heat transfer, similar to the article on general Convection. For example, the very first sentence states that:

"Natural convection is a mechanism, or type of heat transport, in which the fluid motion is not generated by any external source (like a pump, fan, suction device, etc.) but only by density differences in the fluid occurring due to temperature gradients."

The very first statement is erroneous and limited in a few ways. First, Natural Convection is a mechanism for heat AND mass transport. Second, while it is caused by density differences which MAY occur due to temperature gradients, it can also occur WITHOUT temperature gradients (e.g. in heterogeneous mixtures). I have a strong suspicion that the pages on convection are written from a Mechanical Engineering perspective and should be expanded to include all transport phenomena, not just heat transport. 2001:1948:414:8:0:0:0:64 (talk) 20:18, 14 February 2013 (UTC)

Agreed, convection can be caused by mass density differences from sources other than temperature gradients, eg a denser layer of salt water above a layer of fresher water, can drive convection in the Earth's oceans. I will edit — Preceding unsigned comment added by Rpsear (talk • contribs) 08:25, 20 June 2019 (UTC)

The values indicated are wrong

An IP made this edit in March 2010 to add the disclaimer:

Warning: The values indicated for the Horizontal Cylinder are wrong, see discussion.

See #The Nusselt number above. I'll ask at WP:RD/S whether someone can clean it up. Johnuniq (talk) 09:31, 18 February 2014 (UTC)

Proposed merger with Natural circulation

Both articles seem to describe effectively the same phenomena ..

Since the term "natural convection" contains the information that the circulation process is driven by differences in heat and density (which is only implied in the term "natural circulation") it should probably remain while "natural circulation" would become a redirect.

KaiKemmann (talk) 21:35, 3 February 2020 (UTC)

I agree, redirecting natural circulation to convection (hence dropping existing natural circulation) would be logical. But I am not an earth scientist, so others may have other opinions Rpsear (talk) 17:04, 19 May 2020 (UTC)

  Y Merger complete. Klbrain (talk) 17:39, 9 January 2021 (UTC)