Turbine secondary flow system calculations are usually performed by utilizing the thermal-fluid network approach. The network consists of branches and nodes. Fluid branches usually describe the flow resistance of the correspondent fluid path section, while fluid nodes are used to connect fluid branches between each other. Fluid flow in branches and nodes is described by the set of conservation equations such as mass and momentum equations. For fluid nodes, usually total or static pressure is used as a variable for these equations. However, such an approach for the cases of flow mixing with different total pressures on inlets or flow separation with different static pressures on outlets yields a system of equations that cannot be solved or accurately converged. This paper provides an approach on how to create a model of a fluid lossless junction in order to get the correct system of equations. Such a junction enables the connection of any number of inflow and outflow fluid branches without bringing any flow resistance, which allows fluid branches to handle flow resistance influence by themselves.

The proposed model is based on the idea, that each node should contain not just one pressure (static or total) as a variable, but two variables - both static and total pressures. Then, the momentum equation for each branch should use total pressure from the upstream node and static pressure from the downstream node. The only problem here is to add additional equation for each node because of the additional variables (two pressures instead of one). This equation must describe the connection between static and total pressures at a node. This paper presents a way to derive such an equation. The proposed method can be used to model junctions of different types with flow mixing, as well as separation and sudden cross-section area changes. With this approach it is also possible to model chambers by modifying just one equation. Also, this new method can be applied for both compressible and incompressible calculation types and can handle chocked flows as well. Application of this method is shown for simple examples of flow mixing and separation, which can be checked by hand calculations. The paper will also show more complex cases representative of turbine secondary flow systems that include comparisons of 3D CFD simulation and results from the AxSTREAM NET software package, where this new method is implemented.


An Approach to Model a Lossless Junction for Fluid Network Calculations in Turbomachinery

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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