EBSILON®Professional Online Documentation
Clicking on "Model settings-->Simulation-->Start/Boundary values" opens a window, that offers the option of specifying start and limit values for pressure, enthalpy and mass flow for the line types water/steam and other lines during the calculation. This makes it possible, for example, to handle pressures below 0.01 bar.
However, changes to these values should only be made with caution, as they also influence the convergence behaviour. It is also not advisable to set values here that lie outside the validity ranges of the substance data.
An error message will be displayed if you enter values outside the validity range of the model options. All values can be reset at once by clicking the "Set to default" button.
As the limits of the material value tables are used as limits for pressures and enthalpies too, the default or standard values of the corresponding model settings are set to values of -10^10, 0 or 10^10.
If required, tighter limits can also be set manually. Some convergence problems can be alleviated by entering significantly smaller upper limits for mass flow, pressure and enthalpy that are realistic for the respective model.
There is also a button to reset all values to the default values.
You can also place a component 147 on a line to change one or more of these values for this individual line.
Zero Mass Flows:
By default, Ebsilon sets the mass flow to at least 10^-6 kg/s on all streams (adjustable under Model Options-->Simulation-->Start/Boundary values). The reason is that with an exact 0 there are problems with the solution of the equation system in many cases. For instance, an energy balance equation of the form
m1*h1+m3*h3-m2*h2 = 0
is no longer solvable for an enthalpy if all mass flows are 0.
In many components precautions have been taken for the treatment of zero mass flows. (e.g. by using for mass flows <10^-6 kg/s, h1+h3-h2 = 0).
Thus if it is necessary to calculate with an exact 0, this will be possible in many cases . The convergence behaviour, however, will usually deteriorate as a result. Normally, controllers with a start value of 0 will then no longer be able to work. KernelScriptings and Kernel expressions must be adjusted in such a way that no problems occur at M=0.
Admissibility of negative mass flows:
In this dialog, it is possible to insert a negative value for the minimum mass flow. This means that negative values are accepted as solutions of the system of equations. This modification was needed in the framework of the validation of mass balances. Another application is the transient calculation (component 119) of a pipe with an input mass flow of 0. If the fluid volume inside the pipe increases or decreases due to changes in the ambient temperature, the resulting flow at the outlet may be positive or negative. This case is modelled in the example file component_119_3b.ebs.
Note that there was no change in the physics of the components due to this modification. It is recommended to check each case whether the solution is physically reasonable. Especially, you should be aware that the behaviour of a mixer with negative input flow is different from the behaviour of a branch.