What is Laminar Flow?
Laminar flow is a concept in fluid dynamics which describes the smooth and orderly movement of a fluid (liquid or gas). In laminar flow, fluid particles move in predictable, parallel layers with minimal mixing between layers. Each layer of fluid flows smoothly past the adjacent layers, similar to how playing cards slide past each other in a deck.
A key characteristic of laminar flow is that it occurs at a low Reynolds number (Re). The Reynolds number is a dimensionless parameter. It characterises the flow regime of a fluid within a specific system and relates to the inertial forces in a liquid. The Re is used to predict whether the flow is likely to be laminar or turbulent.
The Reynolds number is calculated using the following formula:
In the above:
- Re is the Reynolds number
- ρ is the density of the fluid
- v is the velocity of the fluid relative to an object or surface
- L is a characteristic length or dimension
- μ is the dynamic viscosity of the fluid
When the Re is relatively small, typically less than 2,000, the flow is considered to be laminar.
Laminar Flow and Turbulent Flow
Laminar and turbulent flow create vastly different fluid movement. They differ mainly in the way the particles move, affecting the characteristics of the flow. Unlike laminar flow, in turbulent air flow, irregular fluctuations and vortices are present. There is significant mixing between adjacent layers, creating a less predictable and more chaotic flow pattern.
Turbulent flow typically occurs at a high Re (above 4,000). Common examples of turbulent flow can be observed in natural phenomena, such as river rapids, ocean waves, and atmospheric turbulence. It is also commonly observed in different engineering applications, ranging from the combustion process in an engine to the mixing of chemicals in reactors.
Laminar Air Flow Uses
Laminar air flow can be used to create an environment which has low levels of contamination. With this, you can conduct sensitive experiments in a confined workspace to reduce the risk of sample contamination. This is important in various areas of research, ranging from thin film electronics to medical biochemistry.
In one application of laminar flow, Levitt et al. (1966) conducted an experiment to understand how substances pass through gut cells in rats. Laminar flow was utilised to accurately predict the absorption rate of carbon monoxide, warfarin, and glucose in varying gut conditions. It was concluded that laminar flow is an accurate model for predicting the stirring effect in a specific area of the rat gut.
Another application of laminar flow is in microfluidics. Laminar flow is commonly used in microfluidic devices, where small volumes of fluids are manipulated for tasks like chemical analysis, DNA sequencing, and drug delivery.
Laminar Flow Hoods
As a key piece of equipment in many laboratory settings, the laminar flow hood (LFH) creates a controlled, contaminant-free workspace using laminar flow. There are two configurations of LFH: vertical and horizontal. Both are equipped with high-efficiency particulate air (HEPA) filters, which remove airborne particles, microbes and contaminants from the incoming air.
The operating mechanism of an LFH is simple. Air is drawn from the room into the hood and moves from the filter, out of the workspace. This directional air flow removes contaminants generated in the hood and prevents contaminants from entering. In the horizonal laminar flow hood, filtered air flows horizontally across the work area. Whereas, in a vertical laminar flow hood, filtered air flows vertically from the top of the cabinet down to the work surface.
It is important to carefully control the air flow in an LFH to maintain laminar flow within the hood.
Laminar Flow Hoods
- Vertical/Horizontal Models
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Levitt, M.D. et al. (1988). ‘Use of laminar flow and unstirred layer models to predict intestinal absorption in the rat’, The Journal of clinical investigation, 81(5). doi:10.1172/JCI113464