Learning Forum

[JonathanSmith]

I would like to build a Fluent case to study the system shown in the diagram below but I need some help to establish an outlet boundary condition that represents the positive displacement nature of the pump. My model/geometry includes a pressurized steam inlet, an inlet vent, a mixing chamber and the outlet.  I would like to study the pressure and flow within the mixing chamber as a function of the volumetric flow rate of the pump.

In this model, the pump should evacuate the mixing chamber at a fixed volumetric rate. While the volumetric flow rate is fixed, the mass-flow should adjust to the density of the fluid in the mixing chamber which would be influenced by upstream vent restrictions and steam input.

This seems like it would be trivial to solve. I know the outlet diameter so I tried (without success) to use a velocity inlet BC at the outlet and reverse the flow direction (choosing a negative velocity [-VolumeRate divided by Area] on the axial component). Regardless of whether I applied the negative to the velocity magnitude or a (-1) to the axial direction, Fluent seems to be forcing an inflow condition at the outlet (i.e. no outflow).

Any tips on an outlet boundary condition? I would really like to figure out how to get this to work. Thanks!

[JonathanSmith]

Ultimately, what I'd like to know is the total mass flow rate out of the system as a function of steam pressure and outlet volumetric flow rate. With outlet mass flow and steam pressure already Fluent functions, this just leaves me with figuring out how to make volumetric outflow an input into the model.

[KR]

Hello:
Using velocity inlet conditions on both the boundaries is not the right way of solving this problem. What happens if you use mass flow inlet and pressure outlet or pressure inlet and mass flow outlet? Have you tried any of these combinations? In your case, do you happen to know if the pump's outlet is connected to the atmosphere?
Karthik

[JonathanSmith]

Karthik, Thank you your for your time in responding.
I'd like to do a better job of explaining why I'm taking this particular approach. There is a physical system that I am modeling in ANSYS Fluent. This physical system includes sensors that I'll be using to validate the Fluent model as I study the flow in the domain. In this particular case, I intend to apply physical measurements as Fluent inputs. My Fluent inputs will be generated from pressure measurements on the inlets and pump flow on the outlet. Let me try to explain this pump a little better. This outlet pump's characteristic feature is that its flow is proportional to its speed. The pump moves a constant volume of fluid for every revolution such that the pump's speed (along with density) determines the total mass flow through the pump. For the physical system, I will be able to vary the steam pressure and the pump speed. In Fluent I want to parameterize steam pressure and volumetric pumping in order to match the physical system. Is there a way to mimic this kind of pump's behavior in Fluent?

Regarding the use of combinations of mass-flow & pressure for inlet and outlet, I'm not able to directly measure mass flow in the physical system so I want Fluent to compute that value.
Regarding the pump outlet pressure, outlet pressure does not change the volume moved through the pump. Each revolution of the pumps moves the same volume of fluid.