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Alain, I think that what happens is that the surface area of the top node is greater than the surface area of the other nodes (because of the tank top). As a result, the top node loses more energy than the node below and thus cools off. The temperature inversion (a colder temperature above and a warmer temperature below) causes a mixing between those two nodes and the result is that there is a net transfer of energy upwards. This continues on down to the bottom node and because there is no node (no source of energy) below that, the bottom node cools off and doesn't get replenished. Essentially, I think that the phenomenon that you are seeing is real (the tank naturally stratifies). In reality, the stratification would not be simply between the bottom and the next to the bottom node but would extend up in the tank. Bear in mind, however, that there are a lot of phenomena that are NOT modeled by Type4 (convection cells that develop within the tank, for example). Kind regards, David On 10/14/2011 05:19, Alain Guiavarch wrote: Dear All, -- *************************** David BRADLEY Principal Thermal Energy Systems Specialists, LLC 22 North Carroll Street - suite 370 Madison, WI 53703 USA P:+1.608.274.2577 F:+1.608.278.1475 d.bradley@tess-inc.com http://www.tess-inc.com http://www.trnsys.com |