Define a path (custom direction) for emitter particles

Question

I use Qt5.3, and I use the QtQuick.Particles2.0 system to display the movement of fluid inside the pipe by the emitter and determine the direction. For a straight pipe line, I have no problem displaying particles inside the pipe (since this is only the target position). But think that I rejected the phone; How to determine the path for particles in the emitter? (see photo)

Thank.