Problem with natural convection
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I tried to simulate the 3D natural convection flow in pipe with 45° angle to the horizontal. The ambient temperature is 30°C, so I set the Operating Temperature to 303 K. For operating density I used the density of the lowest occurring temperature of the fluid (1020 kg/m³ at 40°C). The fluid is a polypropyleneglycol mixture and so I use a polynomial function for density, cp, lambda and viscosity. I use the komega SST Model with the following boundary conditions: Pressure Inlet: Specification Method: Intensity and Hydraulic DiameterGauge Total Pressure: 0 Pa, Thermal: Total Temperature 313 K Pressure Outlet: Specification Method: Intensity and Hydraulic DiameterGauge Pressure: 0 Pa Thermal: Backflow Total Temperature 363 K On the front side I use a Thermal BC with a Heat Flux of 250 W/m² and on the back side I use a thermal BC (convection) with a Heat Transfer Coefficient of 3 W/m²K and a Free Stream Temperature of 353 K. When I visualize velocity vectors after simulation (after 1500 Iterations) then the velocityvectors have the wrong direction (from outlet to the inlet) and that is impossible. It would be a great help for me if anybody has an idea could cause this phenomena. Thanks! 
The problem is that you should just simulate a simple 2D model at first. After finding all the restrictions of natural convection, turn to 3D model. Natural convection is so sensible of mesh size.

For natural convection simulations, it is quite helpful to initialize the flow field with a simulation of forced convection.
Just set the inlet velocity to a small nonzero value. Run till convergence, then start your natural convection simulation from this initial flow field. "Typical" mistakes in a natural convection simulation could be that the gravity points in the wrong direction and that the heat flux at the surface has the wrong sign (positive > entering the surface, negative> leaving the surface) Sorry if these two possible mistakes are too simple, I just wanted to be sure ;). 
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