Hi Adam,
Intruder is correct; Fuel flow and IAS are fairly well linked at any altitude.
Your initial question is worded in an interesting way:
Basically, what on average is your pph at low levels compared to on top for similar thrust settings?
The key to understanding jet engine performance is to realise that
Fuel flow is proportional to the thrust.
(For a piston engine, Fuel flow is proportional to the
power which is thrust x TAS {or drag x TAS} )
So, to come back to your question, you can see that the fuel flow at sea level will be very similar to the fuel flow at high level
for a similar thrust setting. Actually, due to engine efficiency at higher RPM, fuel flow will be slightly less at higher altitude at the same thrust.
The trick is, of course, that at high level the thrust setting required for a given TAS is much less than at sea level, due to the decrease in drag caused by the lower air density. That's why jets like to fly high! (Pistons, on the other hand, have to produce so much
power at high level to achieve the higher TAS that the fuel flow increases to an uneconomical level {OK, grossly simplified, ignoring supercharging, etc, but you get the point}).
Some examples:
1. CitationJet CJ1+ at sea level. 180 kts IAS. N1 70% FF 720pph. TAS 180 kts
Same machine at FL410. 180 kts IAS. N1 100% FF 650pph. TAS 370 kts
2. B747-400 at sea level. Max EPR 1.72. Thrust 60,000lbs. FF 8,000kg/hr. (each engine)
Same machine at FL410. Max EPR 1.72. Thrust 12,000lbs. FF 2,000kg/hr. (each engine)
All above figures are from memory and after 1/2 a bottle of French red wine, so not to be taken too literally. But I think they illustrate the way that high altitude drastically affects the thrust and therefore the fuel flow.
An old chestnut often crops up along the lines of: "If we have to hold, we'd better stay high because our fuel flow at lower levels will be astronomical."
Actually, the fuel flow at a given holding IAS will be nearly the same at any altitude, as explained above. There will be a slightly better fuel flow at high level, due to the engine RPM being higher and more efficient, but it will only be 5-10% better than at low level.
Now if you plan on diverting a long way after holding, or if there is icing at lower levels, that's a different kettle of fish entirely, and in those cases you would be better off staying high.
Hope that helps. (Now, where's that bottle.....?)