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-   -   Boudaries for bouynat driven flow with additional mass flux at inlet (https://www.cfd-online.com/Forums/cfx/116563-boudaries-bouynat-driven-flow-additional-mass-flux-inlet.html)

 Charon April 22, 2013 09:44

Boudaries for bouynat driven flow with additional mass flux at inlet

Hi,

my modell consists out of two concentric copper cylinders with water beteen them. The cylinders are heated by an constant energy source. the flow inside the fluid domain is driven by the buoyant effect(heat flux from the cylinders) and an external caused mass flow(also caused by buoyant, which is not part of the modell).

I'm using Ansys Workbench 14 with CFX 5

My question is, if my setup of the boundaries is realistic?

The inlet conditions are:
BOUNDARY: Oil_Inlet
Boundary Type = INLET
Location = Part1_Oil_Inlet
BOUNDARY CONDITIONS:
FLOW DIRECTION:
END
FLOW REGIME:
Option = Subsonic
END
HEAT TRANSFER:
Option = Static Temperature
Static Temperature = TempIn
END
MASS AND MOMENTUM:
Option = Static Pressure
Relative Pressure = 0 [Pa]
END
TURBULENCE:
Option = Medium Intensity and Eddy Viscosity Ratio
END
END
BOUNDARY SOURCE:
SOURCES:
EQUATION SOURCE: continuity
Flux = MassflowEintritt
Option = Fluid Mass Flux
VARIABLE: T
Option = Value
Value = TempIn
END
VARIABLE: ke
Option = Value
Value = 0 [m^2 s^-2]
END
VARIABLE: tef
Option = Value
Value = 0 [s^-1]
END
VARIABLE: vel
Option = Cartesian Vector Components
xValue = 0 [m s^-1]
yValue = 0 [m s^-1]
zValue = VEintritt
END

The outlet conditions are:
BOUNDARY: Oil_Outlet
Boundary Type = OUTLET
Location = Part1_Oil_Outlet
BOUNDARY CONDITIONS:
FLOW REGIME:
Option = Subsonic
END
MASS AND MOMENTUM:
Option = Average Static Pressure
Pressure Profile Blend = 0.05
Relative Pressure = 0 [Pa]
END
PRESSURE AVERAGING:
Option = Average Over Whole Outlet
END
END
END

The reason why i have chosen this setup is:

1.Since there is the possibility, that the flow driven by the buoyant effect could be bigger than the one driven by the external caused massflow, I modelled the inlet as if it is an opening(inlet to prevent a backflow).

2.In order to prevent an flow driven by a pressure differnce between inlet and outlet(outlet to prevent backflow), I set the pressure at both boundarys to 0(relative).

3.The external driven massflow i modelled as an boundary source at the inlet.

Greetings
Philipp

 singer1812 April 22, 2013 11:23

I beleive your solution will be driven by the BC's (right or wrong).

Cant you extend the cylindar fluid region out a bit (front and back) to pull the openings away from the heated region of the tube? This might allow a better mix of the heated bouyancy calc, and the BC bouyant enforcement that you are trying to do.

It is hard to tell if you say that your BCs are actual openings (back flow) or if you used actual inlets and outlets (no-backflow)

 Charon April 27, 2013 08:02

I added some volumen below and above the cylinders as you suggested.

Sorry for my unclear formulation of my setup.

I defined my inlet as boundary Type "Inlet" and my outlet as Boundary type "outlet". I set the Pressure of both to 0 [Pa]

I am not definig them as boundary type "opening" to prevent backflow.

Since i read, that i don't have to take the hydrostatic pressure in account when defining the pressure at my inlet and outlet, i set it to 0[Pa].
Since i dont want the flow be driven by an pressure gradient between inlet and outlet.

The mass flux (result of an external natural convection -> not part of the model -> fixed value) entering at the inlet, I set as a boundary source. I did it like this, since i don't know, if the bouyancy inside my cylinders will add up to a higher massflow, then the one defined at the boundary source. If it adds up to a higher massflow, additional mass can be sucked in at the inlet, since it is a zero pressure boundary in the first place.

My question is, if my setup concerning the boundaries is proper or if there are some settings i missed.