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Natural convection - Transition from laminar to turbulent flow
Posted Sep 1, 2010, 1:58 a.m. EDT 0 Replies
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I am working with natural convection and keep asking myself:
Why do COMSOL define that the transition from laminar to turbulent flow occurs at a Gr value of 10^9 (page 18 of the 3.5a heat transfer module user guide), when one of the books they are refering to on the same page is defining that the transition occurs for an Ra value (Ra = Gr*Pr) of 10^9 (Page 543 of Incropera and DeWitt)?
Support answered that the Pr number in Ra=Gr*Pr corresponded to the turbulent Pr number of value 0.85, and thus assuming that Gr = Ra was not a bad approximation. But why should the turbulent Pr number be used if you are trying to determine if the flow is turbulent or not?
I am working with natural convection in a special oil, which has Pr numbers that are highly dependent on temperature, ranging from 2978 a zero degC to 187 at 60 degC. With such high Pr numbers the two "ways" of determining turbulent flow (Gr>10^9 or Ra>10^9) obiously gives very different results regarding if the flow is to be considered turbulent or not.
Are there any flow experts ot there?
Thanks.
Hello Tore Berge
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