Calculation Modes
When running a pipeflow, you can choose between different calculation modes. This page explains the differences between the modes and also describes necessary input parameters.
In summary, there are two modes available to determine the pressure and velocity distribution in the examined network:
One suitable for incompressible and one suitable for compressible media, respectively. A user does not have to define the calculation mode manually. Instead, the fluid used for the calculation contains a parameter specifying if the fluid behaves compressible or incompressible. pandapipes automatically chooses the appropriate calculation mode based on this fluid parameter.
In addition, there is one mode available which calculates the temperature distribution in the network. It is important to note that with the current version of pandapipes, the temperature calculation is only intended to be used in combination with incompressible fluids, although the program will not throw a warning or error message if temperatures are calculated in combination with gases. In this case, a user has to be aware that the temperature calculation is done sequentially with the current pandapipes version.
This means that a calculated temperature does not have an influence on the hydraulic fluid properties, such as the density. This approach is only valid if the fluid properties do not show a strong dependence on the temperature or if temperature variations are very small.
The user has to decide if these assumptions are suitable for his purposes. In future, pandapipes will be extended in order to make sure that calculated temperatures also effect hydraulic fluid properties. To activate temperature calculation, the pipe flow option “mode” has to be set to “sequential”, “bidirectional” or “heat”. If heat is chosen, the user has to provide a solution vector of the hydraulics calculation manually.
Hydraulic calculations for incompressible media
Important parameters of the network main components (junctions and pipes) needed for the calculation are listed in the following table. Note that some parameters which can be specified are not needed for the calculation of incompressible media. The component section of this manual contains an extensive overview of all component parameters.
Parameter |
component |
Explanation |
pn_bar |
junction |
Initial value used for pressure |
tfluid_k |
junction |
No function for hydraulic calculation of incompressible media |
height_m |
junction |
Defines the height level of the junction |
loss_coefficient |
pipe |
A coefficient which introduces additional pressure losses |
length_km |
pipe |
Length of the pipe |
diameter_m |
pipe |
Inner pipe diameter |
k_mm |
pipe |
Pipe roughness |
alpha_w_per_m2K |
pipe |
No function for hydraulic calculation of incompressible media. |
qext_w |
pipe |
No function for hydraulic calculation of incompressible media. |
text_k |
pipe |
No function for hydraulic calculation of incompressible media. |
The main effects of pressure loss accounted for during the calculation of incompressible media are losses due to friction and losses due to bendings or assets. In addition, height differences can influence the pressure and velocity distribution.
Because the fluid is incompressible, the velocity along a pipe is constant and less inputs are needed in comparison with the calculation of compressible fluids.
Hydraulic calculations for compressible media
Important parameters of the network main components (junctions and pipes) needed for the calculation are listed in the following table. Note that some parameters which can be specified are not needed for the calculation of compressible media. The component section of this manual contains an extensive overview of all component parameters.
Parameter |
component |
Explanation |
pn_bar |
junction |
Initial value used for pressure |
tfluid_k |
junction |
Temperature value at the junction. Used for pressure calculation. |
height_m |
junction |
Defines the height level of the junction |
loss_coefficient |
pipe |
A coefficient which introduces additional pressure losses |
length_km |
pipe |
Length of the pipe |
diameter_m |
pipe |
Inner pipe diameter |
k_mm |
pipe |
Pipe roughness |
alpha_w_per_m2K |
pipe |
No function for hydraulic calculation of incompressible media. |
qext_w |
pipe |
No function for hydraulic calculation of incompressible media. |
text_w |
pipe |
No function for hydraulic calculation of incompressible media. |
The law of ideal gases and the comparison with a prescribed reference state are part of the internal calculation, which means that pandapipes also makes use of internal constants, e.g. the normal pressure and normal temperature, to calculate pressure drops.
As mentioned in the introduction of this chapter, temperatures used for hydraulic calculations cannot be calculated for compressible media. Instead, temperature values at junctions are parameters required as input values to calculate pressure losses. The corresponding junction variables “tn_k” will be considered constant throughout the simulation.
Other temperature values than the ones listed in the table are not needed for hydraulic calculations of compressible media. Especially the parameter “text_k”, which can be defined for pipes, does not have an effect in hydraulic calculations.
In gas flows, the velocity is typically not constant along a pipeline. For this reason, result tables for pipes show more entries in comparison with the result tables for incompressible media.
Temperature calculations (pipeflow option: mode = “sequential”, mode = “bidrectional” or mode = “heat”)
Important parameters of the network main components (junctions and pipes) needed for the calculation are listed in the following table. The component section of this manual contains an extensive overview of all component parameters.
Note that temperature values at junctions, the tn_k variables, have a different function than in the hydraulic calculation mode for gases. For temperature calculations, tn_k specifies only the initial temperature value for the calculation. Calculated temperatures will usually differ from the assumed input.
To calculate heat losses along pipelines it is necessary to specifiy the temperature of the surroundings. The temperature needed for loss calculation is stored in the parameter text_k which can be specified for pipe components.
Summary of temperature values
Because a lot of temperature values are needed for the different pandapipes calculations, the following table summarizes available temperatures and their purpose:
Parameter |
component |
Defined in module |
Explanation |
NORMAL_TEMPERATURE |
/ |
constants.py |
The temperature used as a reference value for the calculation of compressible fluids. Should not be changed by a user of pandapipes. |
ambient_temperature |
/ |
pipeflow_setup.py |
The ambient temperature used to calculate air pressure. Needed for hydraulic calculations if junctions have different height levels. Default value of 293 K can be changed by specifying a new value as a pipeflow option. |
tfluid_k |
junction |
create.py |
The fluid temperature at the junction. |
t_k |
external_grid |
create.py |
Fixes the temperature value at a specified junction |
text_k |
pipe |
create.py |
The external temperature used to calculate heat losses. |