This information is part of the Modelica Standard Library maintained by the Modelica Association.

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.

allowFlowReversal	Value: system.allowFlowReversal Type: Boolean Description: = true to allow flow reversal, false restricts to design direction (port_a -> port_b)
n	Value: nNodes Type: Integer Description: Number of discrete volumes
energyDynamics	Value: system.energyDynamics Type: Dynamics Description: Formulation of energy balances
massDynamics	Value: system.massDynamics Type: Dynamics Description: Formulation of mass balances
substanceDynamics	Value: massDynamics Type: Dynamics Description: Formulation of substance balances
traceDynamics	Value: massDynamics Type: Dynamics Description: Formulation of trace substance balances
p_a_start	Value: system.p_start Type: AbsolutePressure (Pa) Description: Start value of pressure at port a
p_b_start	Value: p_a_start Type: AbsolutePressure (Pa) Description: Start value of pressure at port b
ps_start	Value: if n > 1 then linspace(p_a_start, p_b_start, n) else {(p_a_start + p_b_start) / 2} Type: AbsolutePressure[n] (Pa) Description: Start value of pressure
use_T_start	Value: true Type: Boolean Description: Use T_start if true, otherwise h_start
T_start	Value: if use_T_start then system.T_start else Medium.temperature_phX((p_a_start + p_b_start) / 2, h_start, X_start) Type: Temperature (K) Description: Start value of temperature
h_start	Value: if use_T_start then Medium.specificEnthalpy_pTX((p_a_start + p_b_start) / 2, T_start, X_start) else Medium.h_default Type: SpecificEnthalpy (J/kg) Description: Start value of specific enthalpy
X_start	Value: Medium.X_default Type: MassFraction[Medium.nX] (kg/kg) Description: Start value of mass fractions m_i/m
C_start	Value: Medium.C_default Type: ExtraProperty[Medium.nC] Description: Start value of trace substances
nParallel	Value: 1 Type: Real Description: Number of identical parallel flow devices
lengths	Value: Type: Length[n] (m) Description: Lengths of flow segments
crossAreas	Value: Type: Area[n] (m²) Description: Cross flow areas of flow segments
dimensions	Value: Type: Length[n] (m) Description: Hydraulic diameters of flow segments
roughnesses	Value: Type: Roughness[n] (m) Description: Average heights of surface asperities
dheights	Value: zeros(n) Type: Length[n] (m) Description: Differences in heights of flow segments
momentumDynamics	Value: system.momentumDynamics Type: Dynamics Description: Formulation of momentum balances
m_flow_start	Value: system.m_flow_start Type: MassFlowRate (kg/s) Description: Start value for mass flow rate
nNodes	Value: 2 Type: Integer Description: Number of discrete flow volumes
modelStructure	Value: Types.ModelStructure.av_vb Type: ModelStructure Description: Determines whether flow or volume models are present at the ports
useLumpedPressure	Value: false Type: Boolean Description: = true to lump pressure states together
nFM	Value: if useLumpedPressure then nFMLumped else nFMDistributed Type: Integer Description: Number of flow models in flowModel
nFMDistributed	Value: if modelStructure == Types.ModelStructure.a_v_b then n + 1 else if modelStructure == Types.ModelStructure.a_vb or modelStructure == Types.ModelStructure.av_b then n else n - 1 Type: Integer Description: Number of distributed flow models
nFMLumped	Value: if modelStructure == Types.ModelStructure.a_v_b then 2 else 1 Type: Integer Description: Number of lumped flow models
iLumped	Value: integer(n / 2) + 1 Type: Integer Description: Index of control volume with representative state if useLumpedPressure
useInnerPortProperties	Value: false Type: Boolean Description: = true to take port properties for flow models from internal control volumes

fluidVolumes	Default Value: array(crossAreas[i] * lengths[i] for i in 1:n) * nParallel Type: Volume[n] (m³) Description: Discretized volume, determine in inheriting class