The Capillary Channel Flow project (CCF) is concerned with flow rate limitations in open capillary channels under low gravity conditions. An open capillary channel is a structure which establishes a liquid flow at low Bond numbers, i.e. when the capillary pressure caused by the surface tension forces dominates in comparison to the hydrostatic pressure induced by gravitational or residual accelerations. The cross section of the flow path is totally or partly confined by free surfaces.

As an example the capillary channel consisting of two parallel plates is shown on the right. At both ends the plates are connected to ducts of closed perimeter. The flow enters the channel via an inlet (bottom), passes the open section (free surface flow) and leaves at the outlet (top). A steady flow is only obtained for a flow rate below a critical value. For a flow rate above the critical value the liquid surfaces collapse at the channel outlet and the flow transits from steady single phase flow to unsteady two phase flow.