modelIsenthalpic

Isenthalpic process

Extends from ThermofluidStream.Interfaces.SISOFlow.

Information

Isenthalpic process (h_in = h_out) suitable for modeling a valve.

The following quantities can be specified either as parameters or as time-varying input signals:

  • Pressure loss: dpLoss = p_in - p_out
  • Relative pressure loss: prLoss = dpLoss / p_in = 1 - p_out/p_in
  • Outlet pressure: p_out

Further assumptions for the isenthalpic process:

  • Steady-state conditions: dE_sys/dt = 0, dm_sys/dt = 0
  • No heat transfer: Q_flow = 0
  • No net external forces on the system: The control volume is not subject to acceleration as a rigid body, i.e. \sum F_external = 0.
  • Rigid boundary, no expansion work: w_exp = 0
  • Negligible kinetic and potential energy changes: Differences between inlet and outlet are neglected, i.e. g*z_2 + 1/2*c_2^2 ≈ g*z_1 + 1/2*c_1^2.
  • Constant composition: No change in species mass fractions across the control volume, i.e. X_in = X_out.

Under these assumptions, an increase in pressure in the direction of the flow violates the second law of thermodynamics; for example, the pressure must not increase within a valve. This condition can be enforced with enforcePressureDrop. Then, a red circle highlights if the prescribed pressure loss would cause a pressure rise in flow direction and is clipped. If enforcePressureDrop = false, violation only triggers an assert, which is also indicated on the icon layer. For inverse calculations, enforcePressureDrop = false is recommended, as this simplifies the equation system.

Parameters

TypeNameDefaultDescription
Specification
OutletSpecificationoutletSpecThermofluidStream.Idealized.Types.OutletSpecification.Isenthalpic.PressureLossQuantity used to define the outlet state
ValueSpecificationoutletValueSpecThermofluidStream.Types.ValueSpecification.FixedSpecifies whether the quantity is fixed or prescribed
SI.PressureDifferencedpLoss_fixed0Fixed pressure loss (dpLoss = p_in - p_out)
RealprLoss_fixed0Fixed relative pressure loss (prLoss = dpLoss/p_in)
Medium.AbsolutePressurep_out_fixedMedium.p_defaultFixed outlet pressure
Advanced specification
BooleanenforcePressureDropfalse= true, if pressure drop in flow direction is enforced; use =false for inverse calculations
Warnings
AssertionLevelassertionLevelAssertionLevel.warningAssertion level (pressure drop)
Layout › Display parameters
BooleanshowOutletSpecificationtrue= true to show the fixed outlet specification value (either dpLoss_fixed, prLoss_fixed or p_out_fixed)

Components

TypeNameDefaultDescription
Modelica.Blocks.Interfaces.RealInputoutletSpec_prescribedPrescribed outlet specification [SI-units]
RealisDPLossSetAlignedsign(m_flow*dpLoss_set)= 1.0, if set pressure loss is aligned with flow direction
RealisDPLossAlignedsign(m_flow*dpLoss)= 1.0, if pressure loss is aligned with flow direction
SI.PressureDifferencedpLossp_in - p_outPressure loss
SI.PressureDifferencedpLoss_setSet pressure loss
RealprLossdpLoss/p_inRelative pressure loss
RealprLoss_setSet relative pressure loss
SI.Pressurep_out_setSet outlet pressure

Revisions

  • 2026-08, by Silvan Keim (silvan.keim@dlr.de):
    Improved icon.
  • 2026-06, by Raphael Gebhart (raphael.gebhart@dlr.de):
    The default value of enforcePressureDrop was changed from true to false to improve numerical robustness in inverse calculations. Models that require the previous behavior must now explicitly set enforcePressureDrop=true.
  • 2026, by Raphael Gebhart (raphael.gebhart@dlr.de):
    Initial version.