Fig. 5 Different gas flow patterns possible in chemical vapor deposition reactors. (a) Boundary layer of a gas flowing in a pipe and velocity distribution. (b) Stream lines showing adhered flow and break-away flow. (c) Flow patterns effected by expansion angle of tubes. (d) Effect of Reynolds number on flow properties. Source: Ref 48

The laminar pattern can easily be disturbed by either putting a barrier in the flow path (Fig. 5b) or changing the tube diameter (Fig. 5c). Stream lines are used to show the direction of gas molecules. In Fig. 5(b), the stream lines illustrate gas molecules that both break away from, and adhere to, the wall. In the first barrier in Fig. 5(b), a stream line adheres along the whole of its surface. The second barrier produces a break-away flow. The stream line that touches the edge of the barrier does not touch the plate anywhere else. The flow breaks away from the surface at this point. The wave behind the break-away flow may rotate slowly and form a stable eddy wave. When a susceptor is placed inside the reactor chamber at a large angle with respect to the flow direction, a break-away and return-stream flow is produced. This type of flow should be avoided because it can produce a "memory effect," that is, the composition of the growing layer is contaminated by residual gases used to grow previous layers.

Figure 5(c) illustrates gas flowing in a pipe that has a sudden change in cross-sectional area. When the cross-sectional area either increases or decreases, the gas will experience either a decrease or an increase in pressure, respectively. This kind of sudden momentum change can produce vortices. More importantly, vortices can be created by high flow velocities. As the mean velocity becomes larger, the velocity gradients also increase. The friction between adjacent layers of flow can become so high that local velocity components normal to the flow direction are no longer smoothed out. Instead, they progressively develop into turbulent or chaotic flow. When a pipe expands, the rate of decrease in pressure determines whether or not flow is smooth or turbulent. The rule of thumb for maintaining laminar flow is to avoid linear expansion angles, 9 greater than 7° (Ref 55, 60).

When the flow becomes turbulent, it develops a highly random character with rapid, irregular fluctuations of velocity in both space and time. In this case, an element of gas flow will follow a highly irregular, distorted path (Fig. 5d). Different elements starting at the same place will follow different paths, because the pattern of irregularities changes all the time (Ref 61). The irregular motions do not affect all the molecules at the same instant, so that when the velocity is averaged out, it can still be thought of as a steady flow with a steady average velocity. The criteria that establish whether a gas will flow in a turbulent or laminar regime are characterized by the Reynolds number, Re, which is defined as:

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