In the equation for the Reynolds number for liquid flow, the velocity, v, generally varies in a ten-to-one range; the specific gravity generally ranges from 0.8 to 1.2; and the pipe diameter is constant. However, for some liquids, the viscosity can vary from less than one to thousands of centipises. So, in most liquid flow applications viscosity can be well defined for a given liquid under fixed operating conditions, relatively small changes in temperature can cause order-of-magnitude changes in viscosity. These changes can determine whether the flow is laminar, transitional, or turbulent.
Example 9-4 illustrates a typical Reynolds number calculation to determine flow type.
In the equation for the Reynolds number for gas flow, the velocity,v,generally varies in a ten-to-one rang, the density generally varies over a range of less than two to one, and the pipe diameter is constant. However, the Viscosity is small and virtually constant in most process applications. So, flow and density have the most effect on the Reynolds number in most gas flow applications. Since the flow and density are well established in most applications and the viscosity is low, gas flow is turbulent in properly designed process piping systems.