steelpillow said:
1. Yes, internal overpressure causes the intake to stall
And I assume a partial variable would be the amount of pressure gain per stage?
2. Essentially, the sustainable pressure ratio is a function of the compressor stage, i.e. its design and its speed of rotation.
So each blade has a different limit, or the pressure gain per stage as a general rule plays a role in stalling?
Blade tip speed must remain subsonic, which ties the rotation speed to the diameter.
Actually that's not exactly true: Many aircraft engines have supersonic tip-speeds as the RPM nears and reaches either maximum continuous, military, or climb-power.
The inflowing air is subsonic, at first controlled only by compressor RPM, inlet shape, and wind (if applicable); as you go faster it generally remains subsonic. The combined flow with rotational velocity combines to produce a supersonic flow across the whole blade
(presumably this would take hold around 0.30 to 0.70 mach; I don't know if it covers every stage or the first few). If I recall, the compressor blades might have to be properly designed for this to work right, but evidently it increases the maximum pressure-ratio per stage and seems to make the blades more stall resistant
(This does *does* surprise me, as critical alphas are generally lower at transsonic speeds than subsonic speeds).
As I understand it, as tip velocity reaches around 1.4 mach, there may be a tendency for performance to fall off.
I should have said that I was considering conventional jet engines.
I was largely talking about turbojet/turbofan-type engines -- this is the subject of interest for now
Once you go fast enough, the intake geometry and the sheer speed begin to dominate.
In terms of compression, absolutely. Ramjets are more efficient evidently it would appear in terms of power-to-weight at least, if not in terms of the amount of pressure produced by decelerating the airflow
(on a jet you have a turbine to deal with, on a ramjet -- no need).
Faster still and the compression zone outflow becomes supersonic, as in the scramjet.
As I understand it with this kind of propulsion system the argument is that by decelerating the airflow large amounts results in increased temperatures and pressures.
The pressures require a potentially stronger duct, which weighs more, and thus requires a heavier airframe to bear it; exotic materials, active cooling, and any of the following
(though I'm not sure how severe this is) in terms of any of the following: Chemical interaction between O2 and N2; disassociation of O2 to mono-atomic Oxygen; possibly ionization to some degree.
The argument is that it's lighter and more efficient to simply slow the flow from high supersonic/hypersonic to low supersonic speeds as there is still plenty of pressure to be gained this way: It also doesn't require the inlet duct to be convergent-divergent as the flow is never subsonic; the nozzle doesn't require it either as the flow is always supersonic.
The issue usually involves effectively injecting the fuel into the airflow, the traditional spray-bars and flame holders used in ramjets are based on the flow being subsonic. When hit with supersonic flow all sorts of complex shock-wave patterns form. You instead inject off the walls and you're good.
This is largely academic as there have been ways to produce similar or superior efficiencies to ramjets by simply spraying the fuel into the inlet duct substantially upstream of the combustion-chamber: It not only mixes better, possibly vaporizes, but it also absorbs a great deal of heat which is an active cooling effect. Seems simpler to implement.
