3.2.4 Die cavity

All the numerical simulation of die filling are done almost exclusively assuming that the flow is turbulent and continuous (no two phase flow). In the section a discussion about the existence of turbulence and what kind of model is appropriate or not are presented. The liquid metal enters the cavity as a non-continuous flow. Actually, it is preferred that the flow will be atomized (spray). insert the ref about the paper about atomization, the experiments about critical entrance velocity While that there is considerable literature about many geometries non available to typical die casting configurations⁷. The flow can be atomized as either in a laminar or turbulent region. The experiments by the author and by others showed that the flow turns into spray in many cases ( See Figures ). = 2.5in

Figure: Typical flow pattern in die casting, jet entering into empty cavity

In the section it will be shown that the time for atomization is very fast compared with any other process (filling time scale and, of course, the conduction heat transfer or solidification time scales). Atomization requires to have two streams with a significant velocity difference. Numerous experimental studies had shown that castings obtained when the injected velocity is above certain value are superior. This fact alone is enough to convince us that the preferred flow pattern is a spray flow. Yet, very small number of numerical models exist and used for die casting assuming spray flow (for example the paper by Hu at el poro:jia.). Experimental work commonly cited as a ``proof'' of turbulence was conducted in the mid 60's [#!poro:stuhrke!#] utilizing water analogy<sup>8</sup>. The ``white'' spats they observed in their experiments are atomization of the water. Because these experiments were poorly conducted (no similarity to die casting process) the observation/information from these studies is very limited. Yet with this limitation in mind, one can conclude that the spray flow does exist.

Experiments Fondse et al poro:fondse show that atomization is put the full references larger in a laminar flow compared to a turbulent flow in a certain range. This fact further confusing what is the critical velocity needed in die casting. Since the experiments which measure the critical velocity were poorly conducted no reliable information is available on what is the flow pattern and what is the critical velocity⁹.