CFD Airflow Study of a Bench-Top Laboratory Fume Hood
Steady RANS CFD of a bench-top fume hood at two sash positions, used to understand how it captures fumes and to check a revised liner design.
Project brief
A laboratory equipment manufacturer wanted to understand how air moves through its bench-top fume hood. A fume hood protects the user only if air flows steadily inwards at every opening and carries contaminants to the exhaust without spilling back into the room.
Shirsh Technosolutions ran CFD of the hood in ANSYS Fluent over two design rounds. The first round mapped the flow in the original design. The client then revised the liner profile using those findings, and the second round checked the revised hood under the same conditions so the two designs could be compared directly.
What was at risk
Air enters the hood through several paths at once: the main sash opening, a bypass opening, small openings at each side and the aerofoil slot along the front edge of the work surface. All of it leaves through a single exhaust collar. How the flow divides between these paths, and how the streams meet inside the chamber, decides whether fumes are swept away or left circulating near the user.
In daily use the hood also runs at different sash heights, so the flow had to be understood at both a low and a high sash position.
Solution offered
We extracted the internal air volume from the client's CAD for each sash position and meshed it with a poly-hexcore mesh, with prism layers along the walls to resolve the boundary layer. Mesh quality was checked before solving.
The flow was solved as steady, incompressible and turbulent using the RANS equations with the k-ω SST turbulence model. The exhaust was set as a mass flow outlet at the design extraction rate and every opening was left at atmospheric pressure, so the hood itself decided how much air each path drew, as a real hood does on its exhaust fan.
Results were reviewed as streamlines traced from each opening on its own, velocity contours and vectors on section planes, pressure contours and a mass flow balance across all boundaries. Tracing each inlet separately showed exactly where its air travelled before reaching the exhaust.
Common questions
Why leave the openings at atmospheric pressure instead of fixing inlet velocities? Fixed velocities force the answer. With the exhaust flow set and the openings free, the split between sash, bypass and aerofoil comes out of the geometry, which is the thing being designed.
Can CFD replace a containment test? CFD explains the flow and lets designs be compared quickly and cheaply. Formal containment testing to the applicable standard is still done on the physical hood.
Challenges we overcame
Several inlet paths sharing one exhaust, with the flow split not known in advance
Two sash positions to cover normal use
Recirculation behind the glass and below the exhaust where the bypass and sash streams meet
Mixing at the aerofoil corners close to the work surface
Comparing the original and revised liner on equal terms
How we executed
- 1
Extracted the air volume from the CAD for each sash position
- 2
Meshed with poly-hexcore cells and wall prism layers, and checked mesh quality
- 3
Set the exhaust as a mass flow outlet and all openings at atmospheric pressure
- 4
Solved steady RANS with the k-ω SST turbulence model
- 5
Traced streamlines from each opening and plotted velocity and pressure on section planes
- 6
Checked the mass flow balance and flow split across every boundary
- 7
Shared the findings with the client for the liner revision
- 8
Re-ran the revised liner at both sash positions with the same set-up
Project outcome
At both sash positions most of the incoming air enters through the sash, with the bypass, side and aerofoil openings drawing a small share.
The bypass and sash streams meet behind the glass and below the exhaust, setting up a recirculation that mixes and dilutes the air before it leaves.
Air from the aerofoil slot is swept up by the sash flow at the chamber corners, which keeps air moving across the work surface.
The first round gave the client a flow-based reason for revising the liner, and the second round documented the revised design.
Both designs were analysed with an identical set-up, so the comparison is like for like.
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