ASHRAE Journal - May 2020 - 42

TECHNICAL FEATURE

FIGURE 14 Flow pattern for a double aisle cabin with overhead supply nozzle from CFD.

FIGURE 15 Surface CO2 concentration for a double aisle cabin with overhead

nozzle from CFD.

Velocity (ft/s)
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

Concentration (ppm)
1.80E+03
1.75E+03

Slot Diffuser

1.70E+03
1.65E+03
1.60E+03

1.55E+03
1.50E+03
1.45E+03
1.40E+03

Outboard Recirculation Vortex
Inboard Recirculation Vortex

1.35E+03
1.30E+03
1.25E+03
1.20E+03
1.15E+03

Exit

1.10E+03
1.05E+03

Symmetrical Half Section of Cabin

Out

1.00E+03
9.50E+02
9.00E+02
8.50E+02
8.00E+02

FIGURE 16 Cabin airflows with a crown air sealing liner and the dehumidified air

injection behind the crown insulation seal.

The stack flow through the ceiling panels will divert
outdoor air from the cabin and increase the cabin
contaminant levels. The ceiling panel is exposed to
cleaner, drier air, so condensation would be reduced at
the expense of the (reduced) air quality (Figure 13).
For a 25% bypass flow through the ceiling panels, there
is 25% of the total 90 cfm = 22.5 cfm (42 L/s = 10.6 L/s) as
ceiling flow, Qceiling. The exit concentration rise at the floor
grille is ΔCexit = 1,400 ppm − 400 ppm = 1,000 ppm. The
concentration rise on the ceiling and bins is ΔCceiling = 1,145
ppm − 400 ppm = 745 ppm. Using Equation 5, the effective
bypass is 8.6%.

Case 4 Double Aisle Airplane With Overhead Supply Nozzle
With a slot diffuser located on the ceiling flowing
downward and outboard, the slot produces a free jet.
The free jet produces two vortices on each side of the
cabin (Figure 14).
The stack flow through the ceiling panels will divert
less outdoor air from the cabin and improve the air
quality. Unfortunately, the air is wetter in the ceiling
cavity and condensation is increased (Figure 15).
For a 25% bypass flow through the ceiling panels, there
is 25% of the total 90 cfm = 22.5 cfm (42 L/s = 10.6 L/s)
as ceiling flow, Qceiling. The exit concentration rise at the
floor grille is ΔCexit = 1,400 ppm − 400 ppm = 1,000 ppm.
The concentration rise on the ceiling and bins is ΔCceiling
42

ASHRAE JOURNAL

ashrae.org

M AY 2020

Dry Air Injection
Polyester Film Sheeting
Skin
Liner
Insulation
Envelope

Dry Air Leakage Into Cabin

Flow Blocker
Cabin
Humid Cabin
Air Leakage Into
Envelope
Cargo Space

= 1,575 ppm − 400 ppm = 1,175 ppm. The effective bypass
is −3.7%.
The cases with a negative effective bypass show that
the stack flow can improve cabin air quality but at the
expense of an increase in water condensation.

A Method for Preventing Ventilation Air Loss and Envelope
Moisture Problems
Crown Air Barrier and Dry Air Injection
The first step in eliminating either increased cabin
bioeffluent (e.g., virus) levels or increased envelope
condensation is to seal the crown insulation with a
membrane liner such as polyester film sheeting and
inject the dehumidified air directly into the crown insulation envelope behind this membrane liner as shown
in Figure 16. The amount of dry air injection required to

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ASHRAE Journal - May 2020

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