# Problem with natural convection

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July 2, 2012, 14:48
Problem with natural convection
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Claudia Hintringer
Join Date: May 2010
Posts: 10
Rep Power: 9
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 polypropylene-glycol mixture and so I use a polynomial function for density, cp, lambda and viscosity.

I use the k-omega SST Model with the following boundary conditions:

Pressure Inlet:
Gauge Total Pressure: 0 Pa,
Specification Method: Intensity and Hydraulic Diameter
Thermal: Total Temperature 313 K

Pressure Outlet:
Gauge Pressure: 0 Pa
Specification Method: Intensity and Hydraulic Diameter
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 velocity-vectors 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!
Attached Images
 geometry.jpg (86.3 KB, 68 views)

 July 3, 2012, 00:06 #2 Member     Join Date: Mar 2011 Posts: 64 Rep Power: 8 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. __________________ Saeed Sadeghi Ansys Fluent CFD Consultant

 July 3, 2012, 03:10 #3 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,663 Rep Power: 26 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 non-zero 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 . thermal energy likes this.

 Tags natural convection, operating density

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