ASHRAE Journal - May 2020 - 40

TECHNICAL FEATURE

FIGURE 11 Surface CO2 concentrations for single aisle with overhead supply nozzle
from CFD.
1.90E+03

Leaks Wetter, "Dirtier" Air

1.82E+03
1.75E+03
1.68E+03
1.52E+03

from CFD.
Velocity (ft/s)

Concentration, ppm

1.60E+03

FIGURE 12 Flow pattern for a double aisle cabin with under bin supply nozzle

Downward
Free Jet

1.45E+03
1.38E+03
1.30E+03
1.23E+03
1.15E+03
1.07E+03
1.00E+03
9.25E+02
8.50E+02

Outlet

7.75E+02
7.00E+02
6.25E+02
5.50E+02
4.75E+02
4.00E+02

ceiling flow, Cceiling. The exit rise in concentration at the
floor grille is ΔCexit = 1,100 ppm − 400 ppm = 700 ppm.
The concentration rise on the ceiling is ΔCceiling = 878
ppm − 400 ppm = 478 ppm. Using Equation 5, the effective ventilation air bypass of the occupants is 11.6%
This number applies directly to particulate levels. For
gaseous effluent some of this air is salvaged by the recirculation system, and some is lost to the outflow valve.
The outdoor air effective bypass for systems recirculating 50% of the cabin air should be about half of the particulate bypass value (about 5.8%, a little more than half
that value).

2.90E+00
2.80E+00
2.70E+00
2.60E+00
2.50E+00
2.40E+00
2.30E+00
2.20E+00
2.10E+00
2.00E+00
1.90E+00
1.80E+00
1.70E+00
1.60E+00
1.50E+00
1.40E+00
1.30E+00
1.20E+00
1.10E+00
1.00E+00
9.00E-01
8.00E-01
7.00E-01
6.00E-01
5.00E-01
4.00E-01
3.00E-01
2.00E-01
1.00E-01
0.00E+00

Slot Diffuser
Single Recirculation Vortex

Exit
Symmetrical Half Section of Cabin

FIGURE 13 Surface CO2 concentrations for a double aisle cabin with under bin

nozzle from CFD.

Concentration (ppm)
1.80E+03
1.75E+03
1.70E+03
1.65E+03
1.60E+03
1.55E+03
1.50E+03
1.45E+03
1.40E+03
1.35E+03
1.30E+03
1.25E+03
1.20E+03
1.15E+03
1.10E+03

Case 2 Single Aisle Airplane Overhead Supply Nozzle

1.05E+03

With a slot diffuser located on the ceiling flowing
downward, the slot produces a free jet. The free jet
flows downward at the centerline of the airplane cabin,
producing a single vortex on each side of the cabin
(Figure 10).
Here, stack flow through the upper part of the cabin
will divert less outdoor air from the cabin. The bad news
is that the air is wetter in the ceiling cavity and condensation is increased (Figure 11).
For a 25% bypass flow through the ceiling panels, there
is 25% of the total 60 cfm = 15 cfm (28 L/s = 7.1 L/s) as ceiling flow, Ceiling. The exit concentration rise at the floor
grille is ∆Cexit = 1,100 ppm − 400 ppm = 700 ppm. The
concentration rise on the ceiling and bin is ∆Cceiling =

1.00E+03

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Out

9.50E+02
9.00E+02
8.50E+02
8.00E+02

1,270 ppm − 400 ppm = 870 ppm rise. Using Equation 5
the effective bypass is about −4.9%.

Case 3 A Double Aisle Airplane With Under-Bin Sidewall Supply
With a diffuser located on the sidewall under the
bin flowing inboard, the slot produces a wall jet
attaching to the contour of the overhead stowage bin
and ceiling with a single recirculation vortex on each
side of the cabin (Figure 12).

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ASHRAE Journal - May 2020

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