The compressed air for the cabin is bled from an engine core after it passes through the compressor, but before it reaches the combustion chamber, so it tends to be very clean once you’re at altitude. It seems more likely that any gaseous contamination would come from the air conditioning, which actually accounts for a large portion of an airliner’s mechanical power demands.
Why do you need air conditioning when it’s 30 below outside? Thermodynamics. Compressing the air up to cabin pressure necessarily makes it heat up quite a lot. It would be great for cooking in-flight meals with - it’s around 400 deg F.
Air to air heat exchangers
a) Are really, really big
b) Require very high deltaP to work.
A large thing causing a large change in pressure on the atmospheric side means a lot of drag, and I’m sure it would eat more power than a Rankine cycle. The pressure drop on the cabin side would also eat a lot of what we heated the air up for in the first place.
Cool link - looks like they’re working on an airbreathing rocket concept. I did some work on high-performance heat exchangers for this sort of system back in grad school.
It’s a very spiffy and simple solution if you can make it safe. Their heat exchangers work so well because they have abundant really cold stuff in the form of Liquid Hydrogen for rocket propulsion.
Since we don’t have any of that on our airplane, we’ll need an alternate liquid coolant, which was presumably chilled by a heat pump. Guess what we’ve just made? Yup. An air conditioner.
Technically, you could go without liquid coolant. The air bled from the engine is at high temperature but also way higher pressure than needed. Let it lose some of the temperature in a high delta-T (and therefore smaller) heat exchanger, possibly right on the engine using its bypass or suction-side air, then do the rest of cooling by expansion to the desired pressure. For liquid cooling, the fuel itself could be used as a potential coolant (and the hot coolant then dumped into the burners; SR-71 and many other supersonics handle cooling this way; rocket engines are a special case too).
This is getting closer to feasible, but I’m still skeptical. Rankine cycles are pretty darn efficient when the deltaT is high. I think viscous losses in the HEX would outweigh the shaft power requirements of the heat pump pretty quickly.
Bleed hotter air further down the compressor, increases ΔT. Use a low-Δp HEX (e.g. parallel plates one, instead of long meandering thin tubes), lowers viscous losses.
As in precisely vertical? I’ve never seen it defined like this. I got curious if there was a more technical standard for the term but only got as far as “a steep descending flight path” before deciding yeah, it’s purely subjective.
As it turns out, I think you’re getting close to the thermodynamic cycle that they really use. I always assumed it was a Rankine cycle, but on further research, it’s an ACM. Here’s the summary I found - it’s basically a reverse Brayton (i.e. jet engine) cycle.
The system relies on clever use of expansion and compression like any heat pump, but there’s no phase change.
It also turns out that there are ram-air heat exchangers in the system, sometimes as precoolers, and sometimes part of the ACM.
Tangents are fun! Remember when we were talking about boring stuff like flight paths and airspeed?
http://www.daveswarbirds.com/navalwar/divebomb.htm
Let’s throw this in, a text about dive bombers. I think this qualifies as source material for the discussion.
Short version, Allies used 70° dive, Japanese used (uncharacteristically somewhat wimpier) 55° dive.
We all get this, and always have! The airline industry really, really doesn’t want to talk about it. But take a long haul on an older 747 and note your jet lag symptoms, then take a long haul on a 787 or A380 and see the difference.
In the newer aircraft, I get tired, from the clock being out of whack, but not much else. Older aircraft - I actually feel sick for a few days.
Older designs bring cabin air in … tada! via the engine mechanism, newer bring air in … tada! via independent systems that deliberately avoid the engine systems, and any of their i/o flows.
Why, we all ask? Well … the delicate question of the impact of highly volatile organic compounds (jet engine lubricants, anyone?) is a starting point!
If you try to fly from London to NYC, there are certain flights on each carrier’s roster that are the newer aircraft. Very hard to get on those - the business people book 'em up, quick smart.
