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Want Impeller Driven Fluid Flow: What Inlet and Outlet BC to use for Centrifugal Pump

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Old   May 8, 2012, 09:16
Question Want Impeller Driven Fluid Flow: What Inlet and Outlet BC to use for Centrifugal Pump
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Zev Xavier
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Hello

For my analysis I am interested in determining the increase in pressure/mass flow rate a centrifugal pump can provide to a specified flow, based entirely on the rotation of the impeller. (The pump I am simulating is a simple single stage centrifugal pump with one inlet and one outlet and I am using a MRF model in Fluent with water as the fluid for the moment). As I am only interested in the difference between the inlet and outlet flow I assumed that my inlet flow was at gauge pressure = 0 Pa. In order to obtain my results I thought it suitable to then apply an outflow condition to the outlet so as not to over-constrain my simulation by giving it particular mass flow rate or velocity inlet conditions which in turn would drive the fluid. After applying these conditions however, Fluent states that the simulation is incompatible because two pressure conditions have been assigned and that this setting needs to be fixed before proceeding to calculate a solution. (Although it does appear to solve it despite these warnings, however not having a grasp on the significance of this I thought it best to avoid continuing with such warnings given its assumed negative effect on my results).

Following this, I though the next best option would be to use Boundary Conditions of Inlet-Pressure and Pressure- Outlet, both at 0 Pa Gauge pressure. This however in my mind is still over-constraining the simulation and in reality I would like to assign an undefined Pressure- Outlet value to ensure that my results are purely the result of the impeller driving the fluid, however this is not an option (that I have come across anyway).

So my question is does anyone know which boundary conditions to apply to the inlet and outlet pipes of a simple centrifugal pump like mine in order to simulate the fluid flow based entirely on the rotation of the impeller?
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Old   May 9, 2012, 07:40
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When solving a pump (specifically centrifugal) you need to input 3 of the variables (mass flow, head and outlet pressure) to the solver and it will give you the third.
I generally use a mass flow inlet and static pressure outlet which gives me the head of the pump
If you want to then vary the RPM or size of impeller, use the affinity laws to arrive at your new flow rate of head and input one of them to the solver to get the other

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Old   May 9, 2012, 08:52
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Zev Xavier
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Thanks for your reply Luke,

Don't you still think assigning a mass flow inlet would be an over-defining feature? My pumps application is as a ventricular assist device (VAD) which helps pump more fluid around the body. Given that various patients circulatory systems would each have different capabilities of pumping blood around their body, I thought it wise to determine the underlying increase in pressure/mass flow rate a pump could provide regardless of the input conditions (especially given that the input flow would actually by pulsatile – and additionally I do not know how to define in Fluent, and the pumps are not tailored specifically for every individual). Am I naive in thinking my output is significantly independent of the magnitude of defined mass flow rate? and hence incorect in my assumption that an underdefined pump would yeild the most suitable results?

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boundary conditions, centrifugal pump, impeller driven, mrf, undefined bc

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