The Junction node is used to represent mass transfer in pipe fittings with three or more connected pipes. The node supports reversible flow into each pipe branch by using conditions dependent on the flow direction. When the flow is directed into a connected pipe, an inflow concentration is prescribed. When the flow is out of the pipe, the convective-flux condition of the Mass Outflow node is applied instead.

For a branch b where the flow is from the junction into the pipe, the inlet concentration of species ci is defined by summing over all branches with outflow into in the manner of

Here Ak is the pipe cross section area, uk is the pipe velocity, nk is the pipe outlet normal direction, ci,k is the concentration in pipe k, and N is the number of connected pipes. Since flow is from the junction into the branch b, this contribution is omitted in the summations. The above expression assumes that the junction volume is small enough to not introduce any hold up for the fluid or species.