.AixLib.Fluid.FixedResistances.PlugFlowPipe

Information

Pipe with heat loss using the time delay based heat losses and transport of the fluid using a plug flow model, applicable for simulation of long pipes such as in district heating and cooling systems.

This model takes into account transport delay along the pipe length idealized as a plug flow. The model also includes thermal inertia of the pipe wall.

Implementation

The spatialDistribution operator is used for the temperature wave propagation through the length of the pipe. This operator is contained in AixLib.Fluid.FixedResistances.BaseClasses.PlugFlow.

The model AixLib.Fluid.FixedResistances.BaseClasses.PlugFlowHeatLoss implements a heat loss in design direction, but leaves the enthalpy unchanged in opposite flow direction. Therefore it is used in front of and behind the time delay.

The pressure drop is implemented using AixLib.Fluid.FixedResistances.HydraulicDiameter.

The thermal capacity of the pipe wall is implemented as a mixing volume of the fluid in the pipe, of which the thermal capacity is equal to that of the pipe wall material. In addition, this mixing volume allows the hydraulic separation of subsequent pipes.
The mixing volume is either split between the inlet and outlet ports (port_a and port_b) or lumped in at the outlet (port_b) if have_symmetry is set to false. This mixing volume can be removed from this model with the Boolean parameter have_pipCap, in cases where the pipe wall heat capacity is negligible and a state is not needed at the pipe outlet (see the note below about numerical Jacobians).

Note that in order to model a branched network it is recommended to use AixLib.Fluid.FixedResistances.Junction at each junction and to configure that junction model with a state (energyDynamics <> Modelica.Fluid.Types.Dynamics.SteadyState), see for instance AixLib.Fluid.FixedResistances.Validation.PlugFlowPipes.PlugFlowAIT. This will avoid the numerical Jacobian that is otherwise created when the inlet ports of two instances of the plug flow model are connected together.

Assumptions

• Heat losses are for steady-state operation.
• The axial heat diffusion in the fluid, the pipe wall and the ground are neglected.
• The boundary temperature is uniform.
• The thermal inertia of the pipe wall material is lumped on the side of the pipe that is connected to port_b.

References

Full details on the model implementation and experimental validation can be found in:

van der Heijde, B., Fuchs, M., Ribas Tugores, C., Schweiger, G., Sartor, K., Basciotti, D., Müller, D., Nytsch-Geusen, C., Wetter, M. and Helsen, L. (2017).
Dynamic equation-based thermo-hydraulic pipe model for district heating and cooling systems.
Energy Conversion and Management, vol. 151, p. 158-169. doi: 10.1016/j.enconman.2017.08.072.

Revisions

• October 05, 2021, by Baptiste Ravache:
  Made model symmetrical and extends from AixLib.Fluid.Interfaces.PartialTwoPortInterface.
• September 14, 2021, by Michael Wetter:
  Made most instances protected and exposed main variables of interest.
• July 9, 2021, by Baptiste Ravache:
  Replaced the vectorized outlet port ports_b with a single outlet port port_b.
  Expanded the core pipe model that was previously a component. This is for IBPSA, #1494.
  This change is not backward compatible.
  The previous classes definitions were moved to AixLib.Obsolete.Fluid.FixedResistances.PlugFlowPipe. AixLib.Obsolete.Fluid.FixedResistances.BaseClasses.PlugFlowCore.