modelIsobaric

Information

Isobaric process (p_in = p_out) suitable for modeling both heating/cooling (open system, steady-flow, systemSpec==Flow) or periodic heat transfer in a closed cycle system (systemSpec==Cycle), see section Advanced.

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

• Temperature difference: dT = T_out - T_in
• Outlet temperature: T_out
• Specific enthalpy difference: dh = h_out - h_in
• Outlet specific enthalpy: h_out

During simulation, the physical positive direction of the heat flow rate is visualized on the icon layer by a red arrow.

Further assumptions for the isobaric process (open system, steady-flow):

• Steady-state conditions: dE_sys/dt = 0, dm_sys/dt = 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.

Advanced settings

Periodic closed cycle system

The isobaric process (p_in = p_out) can also be used to represent periodic heat transfer in a closed cycle system, which is highlighted on icon layer by p-c (isobaric, closed) compared to p. In this case Inlet amd Outlet refer to the initial and final state of the isobaric process.

Assumptions for the isobaric process (closed system, cyclic operation):

• Cycle-averaged steady state: Over multiple consecutive cycles, the system satisfies dE_sys/dt = 0 and dm_sys/dt = 0.
• The mass flow rate m_flow represents the cycle-averaged mass throughput of the system.
• 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.
• Isobaric boundary work: The process involves a movable boundary with mean expansion work at constant pressure p.
• 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.

Heat flow rate input/output

The heat flow rate can optionally be an EnergyFlowInput or an EnergyFlowOutput; see UsersGuide.EnergyFlow for details.

The model supports four possible operating modes:

1. If the outlet state is set and the heat flow rate is not an input, the model is balanced.
   In this case, the heat flow rate Q_flow is computed from the given specific enthalpy difference dh and mass flow rate m_flow: Q_flow = m_flow*dh.
2. If the outlet state is set and the heat flow rate is an input, the model is locally overdetermined.
   This condition is indicated by a plus symbol on the icon layer. The mass flow rate is then computed from the given heat flow rate Q_flow and specific enthalpy difference dh: m_flow = Q_flow/dh. In this case zero specific enthalpy difference dh = 0 can cause the simulation to fail. To obtain an overall balanced system, a locally underdetermined component, such as Sink_free , must be connected.
3. If the outlet state is not set and the heat flow rate is an input, the model is balanced.
   In this case, the specific enthalpy difference dh is calculated from the given mass flow rate m_flow and heat flow rate Q_flow: dh = Q_flow/m_flow. In this case zero mass flow rate m_flow = 0 can cause the simulation to fail.
4. If the outlet state is not set and the heat flow rate is not an input, the model is not permissible.
   In this case, both the heat flow rate Q_flow and the specific enthalpy difference dh are unknown, and the equation Q_flow = m_flow*dh cannot be balanced. This condition is indicated by a prominent red lightning symbol.

Parameters

Connectors

Components

Revisions

• 2026-08, by Silvan Keim (silvan.keim@dlr.de):
  Improved icon.
• 2026, by Raphael Gebhart (raphael.gebhart@dlr.de):
  Initial version.