What is laminar flow?

What is laminar flow?

To understand how Alicat differential pressure (DP) mass flow devices work, it is important to first answer the question, “What is laminar flow?” Laminar flow is key to the operating principle of Alicat DP instruments, enabling them to output highly accurate mass flow rates across very wide measurement and control ranges.

What are the types of fluid flow in a channel?

Fluid flows can be described as being in one of three states: turbulent, laminar, or transitional.

chaotic turbulent flow through a pipe, rather than orderly laminar flow

Turbulent flow through a channel

Turbulent flow is the most common kind of flow. This type of flow is chaotic, experiencing rapid pressure and velocity variations throughout the channel it is flowing within. Turbulent flow does not exist in uniform layers, but rather mixes throughout the flow channel.

It is characterized by a pressure drop that is proportional to the square of the velocity of the flow.

 

orderly flow layers, showing what is laminar flow in a pipe

Laminar flow through a channel

Laminar flow, also called streamline flow, is smooth and layered. It is more likely to occur at lower flow rates, in small flow channels, and with high viscosity fluids. Laminar flow exhibits a uniform velocity profile across a channel. The fluid flowing near the center of the channel moves with the highest velocity, and predictably decreases as it approaches the channel walls.

At laminar flow conditions, there is a linear relationship between pressure drop and flow velocity.

 

transitional flow through a pipe, a mixture of laminar flow at the edges and turbulent flow

Transitional flow through a channel

Transitional flow exhibits characteristics of both laminar and turbulent flow. Fluid flow is laminar at the edges of the channel, but turbulent at the center. The exact proportion of turbulent to laminar flow can vary from almost entirely laminar to nearly all turbulent.

It is very difficult to accurately calculate a differential pressure reading using transitional flow, as the pressure drop is proportional to an ill-defined polynomial.

How do you know if a flow is laminar?

In the late 1800s, Osbourne Reynolds came up with something called the Reynolds number (Re). This number can be used to predict the flow type under a certain set of conditions.

Reynolds number (Re)

Re = 2ρVr/η

ρ = Fluid density; V = Average fluid velocity; r = Hydraulic radius of the flow channel; η = Absolute viscosity of the fluid

What does the Reynolds number tell you?

Reynolds numbers generally* fall into ranges that indicate whether flow is laminar, turbulent, or transitional.

  • Laminar flow: Re < 2,000
  • Transitional flow: 2000 < Re < 4,000
  • Turbulent flow: Re > 4,000

*These numbers may be radically affected by surface finishes of channel walls.

To reduce the Reynolds number and obtain laminar flow, you can reduce flow velocity or flow channel dimensions. You can also flow the fluid with a lower density or higher viscosity.