Types of flow
Gas and liquid flows (fluid flows) can be described as being in one of three states; turbulent, transitional, or laminar.
Turbulent flow is by nature chaotic. The fluid mixes irregularly during turbulent flow. Constant changes in the flow’s behavior (wakes, vortexes, eddies) make flow rates difficult, if not impossible, to accurately measure. Turbulent flow usually occurs at high flow rates and/or in larger diameter pipes. Turbulent flow is usually desirable when solids must remain suspended in the fluid to prevent settling or blockages.
Transitional flow exhibits characteristics of both laminar and turbulent flow. The edges of the fluid flow in a laminar state, while the center of the flow remains turbulent. Like turbulent flows, transitional flows are difficult, if not impossible, to accurately measure.
Laminar or Smooth flow tends to occur at lower flow rates through smaller pipes. In essence, the fluid particles flow in cylinders. The outermost cylinder, touching the pipe wall, does not move due to viscosity. The next cylinder flows against the unmoving fluid cylinder, which exhibits less frictional “pull” than the pipe wall. This cylinder will move the slowest. This continues, with the centermost cylinder having the greatest velocity.
Concepts in Flow
How do we know if a flow is turbulent, transitional or laminar? In the late 1800′s, Osbourne Reynolds discovered that the type of a fluid flow is related to the fluid’s density, mean velocity, diameter and viscosity. This dimensionless (no units) number helps predict changes in flow type. In simple terms, the Reynolds Number can be written as:
density x mean velocity x diameter / viscosity
It is generally accepted that flow is laminar if the Reynolds Number is less than 2000. Transitional flows have a Reynolds Number between 2000 and 4000. Flows are considered turbulent when the Reynolds Number is greater than 4000. Using the Reynolds equation, we can see that reducing the density, mean velocity and/or diameter of a turbulent fluid flow (unchanging viscosity) will make it “more” laminar. This could also be accomplished by increasing the fluid viscosity (keeping density, mean velocity and diameter the same). The inverse is true to make a flow more turbulent.
Pressure drop describes the loss of pressure as a fluid travels through a pipe or channel. If you blew into a mile long pipe, it’s unlikely that anything would come out the other end. This is due to pressure drop. As the fluid flows through the pipe, friction with the pipe walls and between the fluid particles causes a loss of pressure. Pressure drop is approximately proportional to the distance the fluid travels.
Mass Flow vs Volumetric Flow
Mass is a measure of the amount of matter that makes up an object. The mass of an object is considered constant. Volume refers to the amount of space an object takes up. The volume of an object can change depending on pressure, temperature and other factors. In terms of flow, at room temperature and low pressures the volumetric and mass flow rate will be nearly identical, however, these rates can vary drastically with changes in temperature and/or pressure because the temperature and pressure of the gas directly affects the volume. For example, assume a volumetric flow reading was used to fill balloons with 250 mL of helium, but the incoming line ran near a furnace that cycled on and off, intermittently heating the incoming helium. Because the volumetric meter simply measures the volume of gas flow, all of the balloons would initially be the same size. However, if all the balloons are placed in a room and allowed to come to an equilibrium temperature, they would generally all come out to be different sizes. If, on the other hand, a mass flow reading were used to fill the balloons with 250 standard mL of helium, the resulting balloons would initially be different sizes, but when allowed to come to an equilibrium temperature, they would all turn out to be the same size.
Pressure Influence on Volumetric Flow
Alicat V and VC Series Volumetric flow devices are intended for use in low-pressure applications. This is because an accurate measurement of the volumetric flow rate by means of differential pressure requires the flow at the differential pressure sensor to be in a laminar state. The state of the flow is quantified by what is known as the Reynolds Number. If the Reynolds Number gets above a certain quantifiable point the flow will become non-laminar. Most Alicat volumetric flow devices are sized to make valid full-scale measurements with line pressures up to 10 – 15PSIG when using air.
As a general rule, if your line pressures will be above 15PSIG, an Alicat mass flow device will be more appropriate due to the additional sensors required to compensate for the increased densities.