Gas flow in CVD reactors

A critical aspect, especially in the design of cold-wall reactors is the fluid dynamics of the gas flow. Even though the pressure in the reactor is reduced, the gas density is still sufficient to apply conventional fluid mechanics to its flow. Fluid mechanics tells us that a gas flowing down a smooth tube will move without turbulence as long as the Reynolds number, R , is less than 2000. The Reynolds number in a tube is given by  

The thickness of the stagnant boundary layer can be calculated from fluid mechanics. It is defined as the point at which the viscous force balances inertia under conditions of constant pressure. This may be shown to occur at approximately  

in practice reality is more complex than these simple descriptions, although the concepts remain the same - fluid mechanics is used to control the boundary layer thickness such that the growth rate is constant under conditions of gas-transport limited growth. For real CVD reactors a complete fluid dynamics model is coupled to a film growth model to obtain an optimal susceptor design to fit in the reactor tube (which may be rectangular or round or some other shape as desired). 

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