XANSYS Message: 4436 [Go back to message list] [bookmark on del.icio.us]
No rating yet Subject: Re: Thermal analysis in electric motors Author: Han Jiangbo Date: 1999-09-05 21:39:00Is there any rule of thumb, say, based on ratio of surface characteristic length to gap height, to determine when the gap can be treated as mostly in conduction?
----Original Message----- Sent: Friday, September 03, 1999 9:16 PM To: xansys@o... Cc: chrisw@s...
>You can model the air gap as solid70 elements with "air" conductive >properties. I think air is 0.026W/m degC There's a rule for modelling air gaps. In very narrow gaps, heat transfer is mostly conduction, since there's no room for natural convection flow patterns to form. As the gap gets larger the air circulates more, and convection patterns start to form. In the transition area you can assume conduction but with a fudged conductivity to allow for the convective effects. The details in the form of a correlation formula are in Krieth's heat transfer book, among others.
Note that things are different when you have a gap between moving parts because a real honest to God boundary layer forms, and the heat flow is a sort of limited forced convection--limited to the extent that small gaps interfere with the boundary layer development.
Christopher Wright P.E. |"They couldn't hit an elephant from chrisw@s... | this distance" (last words of Gen. ___________________________| John Sedgwick, Spotsylvania 1864) http://www.skypoint.com/~chrisw