# Natural convection problem in CFX 11

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 May 14, 2008, 18:21 Natural convection problem in CFX 11 #1 Willy Guest   Posts: n/a Hello to all, I´m currently working on a natural convection model. In few words, I have a glow plug inserted in a ethanol volume (which will boil after some time as verified on testbench). I would like to check how the ethanol volume is heated and also how the glow plug is cooled. The problem is, we also measured the temperature of the glow plug surface and we know that after 5 seconds it stabilizes on 130°C. The ethanol temperature on a determined point on the model (also measured) is acceptable although the Glow Plug surface temperature keeps increasing more or less until 800 °C!!! I´ve modeled the glow plug components with different meshes and assembled them all together in CFX as interfaces solid-solid, the electrical resistance inside the glow plug was modeled as a heat source subdomain (we´ve 200W power dissipation on it). The working fluids are ethanol pure substance and ethanol gas from the interphase mass transfer library (I tryied to model my own ethanol with measured properties, but also without success). I´m working with Thermal Energy model and am considering both phases as continuous (mixture model). One of the known issues of this model is that I´ve no idea of the order of the Nusselt number in reality (even because it´ll keep changing with time). Please I would appreciate any comments that could help me solving this problem (why the surface temperature of the glow plug does not stabilizes). Another doubt, shouldn´t the liquid temperature freezes at the boiling point temperature I´ve set? It also keeps increasing together with the glow plug surface temperature. Thanks a lot in advance for your help and time.

 May 23, 2008, 00:58 Re: Natural convection problem in CFX 11 #2 Mehul Guest   Posts: n/a Willy, First of all I do not understand why you have used solid- solid interface. After all your glow plug is solid and the fluid which is getting heated ( Ethanol) is fluid. so ideally it should be solid-fluid interface. You can specify the glow plug as heat source.

 May 23, 2008, 23:12 Re: Natural convection problem in CFX 11 #3 Willy Guest   Posts: n/a Hi Mehul, First of all thanks for your reply. I think you did not understand the glow plug. Sure there's a solid-fluid interface between the glow plug and the ethanol domain. The solid-solid interfaces which I refer to are the interfaces of the glow plug domains which are composed of a ceramic composite and the electrical resistance (coil) inside of it. The problem is that the ethanol is not cooling down the glow plug surface as it was suppose to and because of that I'm getting those extremely high temperatures (around 1500K). Hope you can still help me, I tryied everything that I knew already. Thanks again.

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