Gas Flow Through Pipes and Throttling

The Reynolds number is a dimensionless ratio between inertia and friction in a flowing medium. It is defined as:

In principal, there are two types of flow in a pipe. With Re <2000 the viscous forces dominate in the medium and the flow becomes laminar. This means that different layers of the medium move in relation to each other in the proper order. The velocity distribution across the laminar layers is usually parabolic shaped. With Re≥4000 the inertia forces dominate the behavior of the flowing medium and the flow becomes turbulent, with particles moving randomly across the flow. The velocity distribution across a layer with turbulent flow becomes diffuse. In the critical area, between Re≤2000 and Re≥4000, the flow conditions are undetermined, either laminar, turbulent or a mixture of the both. The conditions are governed by factors such as the surface smoothness of the pipe or the presence of other disturbances. To start a flow in a pipe requires a specific pressure difference to overcome the friction in the pipe and the couplings. The amount of pressure difference depends on the diameter of the pipe, its length and form as well as the surface smoothness and Reynolds number.