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Pump starvation on common manifold - use of Fan Interface with specified mass flows |
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October 4, 2016, 09:48 |
Pump starvation on common manifold - use of Fan Interface with specified mass flows
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Hello all
Firstly - I have made a search of CFD-Online and Google using the terms "pump distribution", "pump manifold flow distribution" but have not found anything relevant. It was a choice between the System Analysis and CD-adapco sub-forums for this topic but I have access only to Star and I think the final answer may lie in BC settings specific to the package. The study I am undertaking is of a water suction manifold which draws from a tank open to atmosphere, with the manifold centreline at 3.5m below the tank surface. The manifold feeds seven centrifugal pumps, via two main branches and five further branches off those. Each main branch feeds multiple pumps of different capacity, with each feeding one pump with a relatively high flow rate and two or three with lower flow rates, typically <50% that of the larger pumps, and in the extreme case <13%. Experience has shown that the larger pumps will 'take' the majority of flow through the manifold and branches and that the smaller pumps may struggle to reach their design flow. More specifically, there seems to be a possibility that the larger pumps will create such a low pressure in the manifold that the smaller pumps' pressure rise cannot overcome it to create sufficient flow, or may operate in a cavitating region of the pump curve. The question this study seeks to answer about this system is;
I am attempting the solution as an incompressible, turbulent 3D RANS steady flow model, using a simplified internal domain of the manifold and suction piping (that is, smooth pipes and junctions, without flanges, valves etc.) To represent the pumps I have Fan Interfaces at approximately the location of the pump suction flange +1D of straight pipe (with the pipe centreline as the normal to the interface), and 1D further downstream of this is the outlet boundary associated with each pump. The manifold has one inlet. So it is a one-inlet, seven-outlet domain. The Fan Interfaces use polynomials to describe the pump performance curve which I have created from existing data on the pumps. I have also built a simpler model to understand the behaviour of the Fan Interface in conjunction with other boundary conditions. It consists of a straight pipe of length 10D upstream and 10D downstream of the fan interface, with the inlet one end and outlet the other. The interface has a pump curve applied. From this model I have found that;
Given all of the above, and my initial assumption that specifying a mass flow through the domain (either indirectly by a velocity boundary or directly by a mass flow inlet) would not give a useful result, in that the system would be constrained to see the full design mass flow and pumps could not then be starved without some others taking more than their design flow, I have some questions in mind:
Many thanks |
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