Reynolds number critical particle diameter

Fig. 11. Effect of density difference at various liquid viscosities on particle Reynolds number evaluation at lower critical particle diameter, (a) Solid-liquid fluidized beds [a = 3.0, Cv = f(s), pi = 1000 kg/m ]. (b) Gas-solid fluidized beds [a = 3.0, Cy = /(e), po = 1 kg/m ]. (c) Unified stability map of particle Reynolds number vs density difference for different values of transition hold-up solid-liquid fluidized beds [a = 3.0, Cy = f(s), p-l = 1 mPas, pi = 1000 kg/m ].

Since the phenomenon only occ irs above a critical Reynolds number (11), it may be most applicable to particles larger than a micron in diameter. 

SOLUTION To analyze the problem, consider particle deposition on a single cylinder placed normal to an aerosol flow. The Reynolds number for the flow, based on the cylinder diameter, is 2320, which is sufficiently large to use the potential flow approximation for the stagnation region. We know that the critical Stokes number for the cylinder is... 

Flows can be classified into streamline, when particles in the fluid follow paths (streamlines) that remain constant with time, and turbulent, when vortices cause unpredictable changes in the flow pattern with time. The changeover occurs at a critical value of the Reynolds number, which is defined as the melt velocity, divided by the viscosity times the channel diameter. The high viscosity of thermoplastic melts causes velocities to be low. Hence, the Reynolds number is very low and the flows are streamline. We will consider steady flows, and ignore the start and end of injection and blow-moulding flows, when the melt accelerates and decelerates, respectively. However, in the RIM process (Section 5.6.5), turbulent flow of the low viscosity constituents in the mixing head achieves intimate mixing. 

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