Automatically connected streams

You can request that the thermal solver automatically determine thermal stream mass and inlet temperature boundary conditions from other thermal streams to which it is connected to. There are two types of connections for the thermal streams: geometrical connection and thermal streams junction.

For a stream on edges, when the auto-connect option is activated, the solver searches for nodes sharing the same coordinates between inlet and outlet of the wall selection for the specified stream with any other stream inlet or outlet nodes with the same coordinates. When the solver finds a nodes sharing the same coordinates, it creates a geometrical connection between all streams with a shared node coordinates.

For streams on faces, or streams that do not share a geometrical nodes coordinates, you can define a connection between the streams using the thermal stream junction entity. The solver issues a warning if the streams are far apart.

When a stream inlet or outlet has multiple geometrical and user-defined connections, the thermal solver merges all these connections to a single connection.

When all the stream connections are defined, the solver sets two different network of connections: one network to find stream mass flows for the streams with an automatically determined mass flow and reversed mass flow, and the second network to find the temperature for the streams with an automatically determined inlet and outlet temperatures.

At each connection, because it is assumed that the fluids from streams are completely mixed, all streams leaving the connection have the same temperature, which is determined from the enthalpy:

The solver controls the mass flow by the conservation of mass at each connection.

The temperature depends on the temperature and mass flows coming to the connection. For the mass flow, each stream mass can be dependent on other connections. The set of connections, which have dependencies, form mass flow clusters. These clusters represent all the mass connections that needs to be determined simultaneously. Each connection provides one linear equation. The cluster provides a system of linear equations. The solver solves these equations using the singular value decomposition (SVD) method to find mass flows.

There are several cases that the solver cannot resolve because the linear system of equations is either underconstrained or overconstrained. For example, if you define the following two streams:

• The first stream has defined mass flow and the auto-connect option is set for reverse mass flow.
• The second stream, whose inlet shares the same coordinates as the outlet of the first stream, has the auto-connect option set for the mass flow to pass the mass flow value from the first stream, and defined reverse mass flow.

In this case, when the first stream reverses, the solver activates the auto-connect option for the mass flow, which creates one unknown at the connection for the first stream. For the second stream, the mass flow is also unknown because the stream reverse mass flow is activated only if the nominal direction is negative. Therefore, the connection provides one linear equation with two unknowns. The system is under-constrained. The solver issues a fatal error.