modelStaticPipe
Extends from AixLib.Fluid.Interfaces.PartialTwoPortInterface (Partial model with two ports and declaration of quantities that are used by many models).
Information
Pipe with heat loss using the time delay based heat losses for the transport delay of the fluid. This model determines the pressure drop either through a static factor or using the sum of zeta values.
This Pipe model is applicable for simulation of long pipes such as in district heating and cooling systems.
Implementation
This model is based on AixLib.Fluid.DistrictHeatingCooling.BaseClassesStatic.StaticCore.
Heat losses are implemented by AixLib.Fluid.FixedResistances.BaseClasses.PlugFlowHeatLoss at each end of the pipe (see AixLib.Fluid.FixedResistances.BaseClasses.PlugFlowCore). Depending on the flow direction, the temperature difference due to heat losses is subtracted at the right fluid port.
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. Thanks to the vectorized implementation of the (design) outlet port, splits and junctions of pipes can be handled in a numerically efficient way.
This mixing volume is not present in the StaticCore model, which can be used in cases where mixing volumes at pipe junctions need to be added manually.
If Boolean use_zeta is set "true" HydraulicResistance is used.
HydraulicResistance takes into account additional pressure drops due to bends/valves/etc. Therefore the sum of zeta values has to be given prior.
If Boolean use_zeta is set "false" the pressureloss is determine through a static factor which has to given prior.
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 ports_b.
Parameters
| Type | Name | Default | Description |
|---|---|---|---|
| Real | ReC | 4000 | Reynolds number where transition to turbulent starts |
| Assumptions | |||
| Boolean | allowFlowReversal (from PartialTwoPort) | true | = false to simplify equations, assuming, but not enforcing, no flow reversal |
| Nominal condition | |||
| Modelica.Units.SI.MassFlowRate | m_flow_nominal (from PartialTwoPortInterface) | Nominal mass flow rate | |
| Modelica.Units.SI.Velocity | v_nominal | 1.5 | Velocity at m_flow_nominal (used to compute default value for hydraulic diameter dh) |
| Advanced | |||
| Modelica.Units.SI.MassFlowRate | m_flow_small (from PartialTwoPortInterface) | 1E-4*abs(m_flow_nominal) | Small mass flow rate for regularization of zero flow |
| Boolean | from_dp | false | = true, use m_flow = f(dp) else dp = f(m_flow) |
| Boolean | homotopyInitialization | true | = true, use homotopy method |
| Boolean | linearized | false | = true, use linear relation between m_flow and dp for any flow rate |
| Advanced › Diagnostics | |||
| Boolean | show_T (from PartialTwoPortInterface) | false | = true, if actual temperature at port is computed |
| Additional pressurelosses | |||
| Boolean | use_zeta | false | = true HydraulicResistance is implemented, zeta value has to be given next |
| Real | fac | 1 | Factor to take into account flow resistance of bends etc., fac=dp_nominal/dpStraightPipe_nominal |
| Real | sum_zetas | 0 | Sum of all zeta values. Takes into account additional pressure drops due to bends/valves/etc. |
| Material | |||
| Modelica.Units.SI.Length | dh | sqrt(4*m_flow_nominal/rho_default/v_nominal/Modelica.Constants.pi) | Hydraulic diameter (assuming a round cross section area) |
| Modelica.Units.SI.Height | roughness | 2.5e-5 | Average height of surface asperities (default: smooth steel pipe) |
| Modelica.Units.SI.Length | length | Pipe length | |
| Modelica.Units.SI.SpecificHeatCapacity | cPip | 2300 | Specific heat of pipe wall material. 2300 for PE, 500 for steel |
| Modelica.Units.SI.Density | rhoPip | 930 | Density of pipe wall material. 930 for PE, 8000 for steel |
| Modelica.Units.SI.Length | thickness | 0.0035 | Pipe wall thickness |
| Thermal resistance | |||
| Modelica.Units.SI.Length | dIns | Thickness of pipe insulation, used to compute R | |
| Modelica.Units.SI.ThermalConductivity | kIns | Heat conductivity of pipe insulation, used to compute R | |
| Real | R | 1/(kIns*2*Modelica.Constants.pi/Modelica.Math.log((dh/2 + dIns)/(dh/2))) | Thermal resistance per unit length from fluid to boundary temperature |
| Initialization | |||
| Modelica.Units.SI.Temperature | T_start_in | Medium.T_default | Initialization temperature at pipe inlet |
| Modelica.Units.SI.Temperature | T_start_out | T_start_in | Initialization temperature at pipe outlet |
| Boolean | initDelay | false | Initialize delay for a constant mass flow rate if true, otherwise start from 0 |
| Modelica.Units.SI.MassFlowRate | m_flow_start | 0 | Initial value of mass flow rate through pipe |
Connectors
| Type | Name | Default | Description |
|---|---|---|---|
| Modelica.Fluid.Interfaces.FluidPort_a | port_a (from PartialTwoPort) | Fluid connector a (positive design flow direction is from port_a to port_b) | |
| Modelica.Fluid.Interfaces.FluidPort_b | port_b (from PartialTwoPort) | Fluid connector b (positive design flow direction is from port_a to port_b) | |
| Modelica.Thermal.HeatTransfer.Interfaces.HeatPort_a | heatPort | Heat transfer to or from surroundings (heat loss from pipe results in a positive heat flow) |
Components
| Type | Name | Default | Description |
|---|---|---|---|
| Modelica.Units.SI.MassFlowRate | m_flow (from PartialTwoPortInterface) | port_a.m_flow | Mass flow rate from port_a to port_b (m_flow > 0 is design flow direction) |
| Modelica.Units.SI.PressureDifference | dp (from PartialTwoPortInterface) | port_a.p - port_b.p | Pressure difference between port_a and port_b |
| Medium.ThermodynamicState | sta_a (from PartialTwoPortInterface) | if allowFlowReversal then Medium.setState_phX(port_a.p, noEvent(actualStream(port_a.h_outflow)), noEvent(actualStream(port_a.Xi_outflow))) else Medium.setState_phX(port_a.p, noEvent(inStream(port_a.h_outflow)), noEvent(inStream(port_a.Xi_outflow))) | Medium properties in port_a |
| Medium.ThermodynamicState | sta_b (from PartialTwoPortInterface) | if allowFlowReversal then Medium.setState_phX(port_b.p, noEvent(actualStream(port_b.h_outflow)), noEvent(actualStream(port_b.Xi_outflow))) else Medium.setState_phX(port_b.p, noEvent(port_b.h_outflow), noEvent(port_b.Xi_outflow)) | Medium properties in port_b |
| Modelica.Units.SI.Velocity | v_med | Velocity of the medium in the pipe | |
| AixLib.Fluid.DistrictHeatingCooling.Pipes.BaseClassesStatic.StaticCore | staticCore | Describing the pipe behavior | |
| Fluid.MixingVolumes.MixingVolume | vol | Control volume connected to ports_b. Represents equivalent pipe wall thermal capacity. | |
| FixedResistances.HydraulicResistance | hydraulicResistance |
Revisions
- September 25, 2019, by Nils Neuland:
Revised variable names and documentation to follow guidelines. Corrected malformed hyperlinks.