The flow in the runner system has to be divided into sections 1) flow with free surface 2) filling the cavity when the flow is pressurized (see Figures and ). In the first section the gravity affects the air entrainment. A dominate parameter in this case is number. This phenomenon determines how much metal has to be flushed out. It is well known that the liquid interface cannot be a straight line. should we insert the proof to this point, no jump in the pressure in the interface? Above certain velocity (typical to die casting, high number) air leaves streaks of air/gas slabs behind as shown in Figure . These streaks create a low heat transfer zone at the head of the ``jet'' and ``increases'' its velocity. The air entrainment created in this case supposed to be flushed out through the vent system in a proper process design. Unfortunately, at present we know very little about this issue especially as concerned with geometry typical to die casting.
to insert the calculation of the , We numbers to scales of forces
The converging nozzle such as the transition into runner system
(what a good die casting engineer should design) tents to reduce the turbulence and
can even eliminate it.
In that view, the liquid metal enters the runner system
(almost) as a laminar flow (actually close to a plug flow).
For a duct with a typical dimension of 10 [mm] and a mean velocity,
U = [m/sec], (during the second stage), for aluminum die casting,
the Reynolds number is: