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-   -   Fluent-Boundary Conditions (https://www.cfd-online.com/Forums/fluent/50258-fluent-boundary-conditions.html)

 Vijay January 10, 2009 13:26

Fluent-Boundary Conditions

let us assume a flow in simple pipe (air).

Step 1: I used mass flow rate boundary condition at inlet and outflow boundary condition at outlet. I calculated pressure values at inlet and outlet in post processing step from pressure contours.

In step 2 , I used the above obtained pressure values (Total pressure values at inlet and Static Pressure value at outlet)as my pressure inlet and outlet boundary conditions respectively for the same model.

Now question is I am not getting the same mass flow rate at inlet as in step no. 1

 John S. January 10, 2009 14:34

Re: Fluent-Boundary Conditions

Massflow, as you know, is a function of density, velocity and area. In the model, the area of the inlet is set and cannot vary, leaving only density and massflow as variables, with density being a function of pressure and temperature. So when you specify massflow at a boundary it will calculate the area directly from the model and set pressure, temperature and velocity in order to achieve it.

Similarly, when you specify pressure at the boundary it will vary massflow and velocity to set the boundary condition.

Also consider aerodynamic blockage caused by boundary layer buildup. As the boundary layer is resolved, it has the effect of reducing the "effectiveness" of the inlet, causing it to flow as if it were restricted, i.e. the area the flow sees is less than the physical area of the geometry. So in order to truly equate these two analyses you would either have to take the aerodynamic blockage into consideration with the massflow-specified model or apply some kind of loss coefficient with the pressure specified model.

 Vijay January 10, 2009 18:05

Re: Fluent-Boundary Conditions

what parameter should I consider to introduce the loss coefficient. Thank you for your help.

 Vijay January 19, 2009 18:30

Re: Fluent-Boundary Conditions

Can you get back to me to the last question. I appreciate the help that you are doing. It would be very helpful if can extend our discussion.

 furqanrk March 20, 2016 12:12

Quote:
 Originally Posted by John S. ;155902 Massflow, as you know, is a function of density, velocity and area. In the model, the area of the inlet is set and cannot vary, leaving only density and massflow as variables, with density being a function of pressure and temperature. So when you specify massflow at a boundary it will calculate the area directly from the model and set pressure, temperature and velocity in order to achieve it. Similarly, when you specify pressure at the boundary it will vary massflow and velocity to set the boundary condition. Also consider aerodynamic blockage caused by boundary layer buildup. As the boundary layer is resolved, it has the effect of reducing the "effectiveness" of the inlet, causing it to flow as if it were restricted, i.e. the area the flow sees is less than the physical area of the geometry. So in order to truly equate these two analyses you would either have to take the aerodynamic blockage into consideration with the massflow-specified model or apply some kind of loss coefficient with the pressure specified model.
Dear expert.. I have to use Superficial gas velocity 0.0016 m/s. But In fluent we have to put Velocity Magnitude at VELOCITY INLET boundary conditions.( INLET VELOCITY) How can I calculate INLET GAS VELOCITY form superficial gas velocity??? ( Reactor dimensions are 20*5*50 cm W*D*L and inlet dimensions are 2.4*1.2 L*D )

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