ASHRAE Journal - May 2020 - 39

TECHNICAL FEATURE

FIGURE 9 Surface CO2 for a single aisle cabin with under bin nozzle from CFD.
Concentration, ppm
Leaks Cleaner, Dryer Air
1.90E+03

FIGURE 8 Flow pattern for a single aisle cabin with under bin nozzle from CFD.

Velocity, fps
1.92E+00
1.85E+00
1.78E+00
1.72E+00
1.65E+00
1.59E+00
1.52E+00
1.46E+00
1.39E+00
1.32E+00
1.26E+00
1.19E+00
1.13E+00
1.06E+00
9.95E-01
9.29E-01
8.63E-01
7.97E-01
7.31E-01
6.65E-01
6.00E-01
5.34E-01
4.68E-01
4.02E-01
3.36E-01
2.70E-01
2.04E-01
1.39E-01
7.27E-02
6.87E-03

1.82E+03
1.75E+03
1.68E+03
1.60E+03
1.52E+03
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
7.75E+02
7.00E+02
6.25E+02
5.50E+02
4.75E+02
4.00E+02

Upper Circulation Vortex

Bin

Lower
Circulation
Vortex

Symmetrical Half Section of Cabin

Leaks Wetter
"Dirtier" Air

FIGURE 10 Flow pattern for a single aisle cabin with overhead supply nozzle from CFD.

Slot Diffuser

ft/s

Effective Bypass % = ( actual bypass ) %
= ( actual bypass ) %

Slot Diffuser

Lower
Concentrations
In Upper
Recirculation
Vortex

( ∆C

exit

dilution CO2 flow
envelope CO 2 flow

)

− ∆Cenvelope Qenvelope

∆Cenvelope Qenvelope
= ( actual bypass ) %

( ∆C

exit

− ∆Cenvelope

∆Cenvelope

(5)

)

In ∆Cexit references the cabin ﬂoor exhaust grilles and
Qenvelope references the bottom of the envelope. If the
effective bypass is greater than zero, it is important to
note the water vapor levels are also lower in the envelope when the occupants are the only source of water
vapor as some of the drier ventilation air will have
entered the envelope above the occupants breathing
space. More importantly, there is no effective bypass loss
if the cabin air is well mixed and the envelope concentration increase at the base of the envelope equals the
cabin concentration increase at the cabin exhaust.

Case 1 Single Aisle Airplane Under-Stowage Bin Supply Nozzle
With a slot diffuser located on the sidewall ﬂowing
inboard, the slot produces a wall jet. The wall jet stays
attached to the underside of the bin, then separates
from the inboard edge, producing an upper circulation
vortex inboard of the bin and a lower circulation vortex
below the bin. Since ceiling panels are typically resting
on panel support (channels), and ﬂow blockers are usually at window height, the following models assume 25%

3.37E+00
3.25E+00
3.14E+00
3.02E+00
2.91E+00
2.79E+00
2.67E+00
2.56E+00
2.44E+00
2.33E+00
2.21E+00
2.09E+00
1.98E+00
1.86E+00
1.75E+00
1.63E+00
1.51E+00
1.40E+00
1.28E+00
1.17E+00
1.05E+00
9.33E-01
8.17E-01
7.01E-01
5.85E-01
4.69E-01
3.53E-01
2.37E-01
1.21E-01
4.75E-03

Bin

Single Circulation Vortex

Exit
Symmetrical Half Section of Cabin

leakage only through the ceiling (Figure 8).
The wall jet and upper circulation vortex isolates the
bin and ceiling from the CO2 and water vapor produced
by the passengers. These concentrations along the surface are lower than the seated area below (Figure 9).
As a result, stack ﬂow through upper part of the cabin
will divert outdoor air from the cabin. The good news is
that the air is drier in the ceiling cavity and condensation is reduced.
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
M AY 2020

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ASHRAE Journal - May 2020

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