.Modelica.Fluid.Pipes.BaseClasses.PartialTwoPortFlow

Information

Base class for distributed flow models. The total volume is split into nNodes segments along the flow path. The default value is nNodes=2.

Mass and Energy balances

The mass and energy balances are inherited from Interfaces.PartialDistributedVolume. One total mass and one energy balance is formed across each segment according to the finite volume approach. Substance mass balances are added if the medium contains more than one component.

An extending model needs to define the geometry and the difference in heights between the flow segments (static head). Moreover it needs to define two vectors of source terms for the distributed energy balance:

• Qb_flows[nNodes], the heat flow source terms, e.g., conductive heat flows across segment boundaries, and
• Wb_flows[nNodes], the work source terms.

Momentum balance

The momentum balance is determined by the FlowModel component, which can be replaced with any model extended from BaseClasses.FlowModels.PartialStaggeredFlowModel. The default setting is DetailedPipeFlow.

This considers

• pressure drop due to friction and other dissipative losses, and
• gravity effects for non-horizontal devices.
• variation of flow velocity along the flow path, which occur due to changes in the cross sectional area or the fluid density, provided that flowModel.use_Ib_flows is true.

Model Structure

The momentum balances are formulated across the segment boundaries along the flow path according to the staggered grid approach. The configurable modelStructure determines the formulation of the boundary conditions at port_a and port_b. The options include (default: av_vb):

• av_vb: Symmetric setting with nNodes-1 momentum balances between nNodes flow segments. The ports port_a and port_b expose the first and the last thermodynamic state, respectively. Connecting two or more flow devices therefore may result in high-index DAEs for the pressures of connected flow segments.
• a_v_b: Alternative symmetric setting with nNodes+1 momentum balances across nNodes flow segments. Half momentum balances are placed between port_a and the first flow segment as well as between the last flow segment and port_b. Connecting two or more flow devices therefore results in algebraic pressures at the ports. The specification of good start values for the port pressures is essential for the solution of large nonlinear equation systems.
• av_b: Asymmetric setting with nNodes momentum balances, one between nth volume and port_b, potential pressure state at port_a
• a_vb: Asymmetric setting with nNodes momentum balance, one between first volume and port_a, potential pressure state at port_b

When connecting two components, e.g., two pipes, the momentum balance across the connection point reduces to

pipe1.port_b.p = pipe2.port_a.p

This is only true if the flow velocity remains the same on each side of the connection. Consider using a fitting for any significant change in diameter or fluid density, if the resulting effects, such as change in kinetic energy, cannot be neglected. This also allows for taking into account friction losses with respect to the actual geometry of the connection point.

Contents

Name	Description
FlowModel

Revisions

• 5 Dec 2008 by Michael Wetter:
  Modified mass balance for trace substances. With the new formulation, the trace substances masses mC are stored in the same way as the species mXi.
• Dec 2008 by Rüdiger Franke:
  Derived model from original DistributedPipe models
  • moved mass and energy balances to PartialDistributedVolume
  • introduced replaceable pressure loss models
  • combined all model structures and lumped pressure into one model
  • new ModelStructure av_vb, replacing former avb
• 04 Mar 2006 by Katrin Prölß:
  Model added to the Fluid library