A330/340 pack exhaust design

[leewan]

I can't understand why A330/340s pack exhaust is designed the way it is. It basically consists of 2 outlets that protrude into the free airstream thereby increasing drag. This goes for the A380 as well except that is installed on the wings. Any aerodynamic reasons for it to be designed in such a way ?

[Touch'n'oops]

Not sure about the A330/A340, but the A320 used to have the same design and now the flap no longer exists. Decreasing the cavity area does reduce drag, but the cost of maintenance of the flap outweighs efficiency gains.

Quick refresh on what the exhausted air is. The aircraft uses air taken from the engines or APU to pressurise and regulate temperature in the cabin. This air of course needs to be cooled as the majority of people can't survive a blast of air at 800c. The cooling of the bleed air is achieved by a mix of heat exchangers, compressors and turbines.
The heat exchangers in this case are air cooled (Like a porsche). On the ground air is draw-in and over the heat exchangers by a turbo fan, thus the FULLY opened flap. In the air the aircrafts forward airspeed achieves a great flow rate over the heat exchangers and the air is cooler, meaning the flap can be almost closed and nearly flush with the aircraft.

Hope that explains it!

[Joetom]

Bleed/duct air temp is often about 200 deg C, can be adjusted to about 150 if air con packs having problems with cooling, 150 option only avail when no NAI or WAI selected on if memory is right.

800 is where the turbine blades live.

[leewan]

Thanks for the info Touch N oops. In essence, ram air is sufficient for the cooling of the bleed air in flight thereby, exhaust pack flaps are open to the bare minimum to reduce drag. It would be much better if the design was like the B744 type with small louvres.