Questions about Jet Engines: F-14, F-105 and X-51 Waverider

[blynxGT]

Hello,

I have a question regarding jet engines. Fighter craft such as the F-14, and more obviously (from a visual stand point) the F-105 have some sort of device that controls air flowing to the compressor. Apparently the air must be slowed to subsonic speeds in order to avoid stalling the compressor. Why is this? If the air is entering the compressor above a certain speed, does the fuel air mixture simply not have time to ignite? Is that why for propulsion devices such as the engine for the X-51 waverider the fuel was in vapor form so that it would instantly ignite with the air making its way through the engine at super sonic speeds?

 

[JaredJames]

An aircraft engine ingests air and uses turbines to compress it.

If the air velocity entering the engine is too high, the turbines (which are little wings) are no longer able to continue compressing the air. This gives you an area of high pressure at the rear of the engine and low pressure at the front. The high pressure air can then be expelled from the intake of the engine causing a loss of thrust.

This is why it's important to ensure the air entering the engine is maintained at the correct velocity.

Now the X-51 is a scramjet, which is a different concept. It uses the supersonic air to compress itself and allow combustion. The problem with this is that if you don't have supersonic airflow, it doesn't work. So it's only useful mach 1+. Note this is the opposite of the problem with turbofans / turbojets.

Turbofan / turbojet engines aren't designed to use self compression of the air to allow combustion.

Ideally, you'd employ a turbojet up to supersonic and then deploy a ramjet from there.

 

[minger]

The flow into the compressor is slowed to subsonic to better control the operating conditions. As jared mentioned the airfoils of the compressor have a finite range of efficient operation. If the incoming flow is too slow or even too fast, the flow can have significantly different incident angles relative to the rotating blade.

Inlet guide vanes are often times used to remedy this, however there is always a limit. A good example of controlling inlet conditions that is well documented is the placing the shock in the SR-71. That engine has the memorable moving spike in the inlet which is used to avoid shock reflections. You can wiki it.

I don't have a lot of experience in axial flow compressors, but in centrifugal often times there is a surge baffle, a pocket of space connected to the compressor by small holes that can prevent the onset of surge (the most extreme case of stall) by small amounts.

 

[boneh3ad]

I just want to point out that scramjets don't work near Mach 1 either. It differs from a ramjet in that the combustion chamber is supersonic, so a true scramjet can't be used until Mach 4 or Mach 5.

 

[blynxGT]

Thanks for the reply guys. This is an interesting topic. I always wondered about the problems facing jet propulsion. I had a hunch it had something to do with the operational parameters of the compressor blades but could never quite make the connection. As far as ramjets and scram jets are concerned is there a mechanical (such as the way the inlets are shaped) difference between a ramjet and a scramjet that account for the differences in effective operational speeds?

 

[JaredJames]

Ramjet: http://en.wikipedia.org/wiki/Ramjet

Scramjet: http://en.wikipedia.org/wiki/Scramjet

This shows a turbfan, ramjet and scramjet: http://en.wikipedia.org/wiki/File:Turbo_ram_scramjet_comparative_diagram.svg

As you can see they have different designs. The compression cycle is different in each.

Noted boneh3ad, I wasn't sure of the exact operating parameters of each.

 

[boneh3ad]

Another interesting problem with scramjets is fuel mixing. At high-supersonic and hypersonic speeds, the boundary layer tends to stay laminar for a very long time. If it enters the engine laminar, you pretty much can't mix the fuel for combustion. To fix that they put large trips on the underside of hypersonic cruise vehicles to trip the boundary layer before it enters the engine.

 

[JaredJames]

Another interesting problem with scramjets is fuel mixing. At high-supersonic and hypersonic speeds, the boundary layer tends to stay laminar for a very long time. If it enters the engine laminar, you pretty much can't mix the fuel for combustion. To fix that they put large trips on the underside of hypersonic cruise vehicles to trip the boundary layer before it enters the engine.

Would appear high speed compressor stall is also an issue for these engines too then.

 

[AlephZero]

Would appear high speed compressor stall is also an issue for these engines too then.

The basic issue for any form of compressor is that it has to fool the air into thinking that it can't flow from high pressure to low pressure, like it really really wants to.

OK, that is meant to be a humorous way of making the point, but if the compressor stops performing that confidence trick on the air, the result is usually a loud bang and smoke and flames coming out of the front of the engine as well as the back.. The methods used to prevent this may depend on the engine operating conditions, but all compressors have to overcome the same problem one way or another.

 

[AlephZero]

A good example of controlling inlet conditions that is well documented is the placing the shock in the SR-71. That engine has the memorable moving spike in the inlet which is used to avoid shock reflections. You can wiki it.

Another related issue is relighting an engine if it flames out when flying supersonic. You need some brute force method of slowing down the airflow enough to match the flame speed of the fuel, which is quite slow for jet fuels, otherwise even if you can light the engine the flame won't stay inside the combustor, and the engine can't achieve its correct thermodynamic operating cycle. The variable inlet geometry on Concorde wasn't as pretty as the SR-71 spike, but it did the same basic job.

 

[35 AoA]

Inlet ramps are used to create oblique shockwaves that will slow down the freestream velocity and increase the pressure/density of the air entering the compressor section. Some aircraft use fixed geometry inlets for lower mach numbers, though anytime you get near the Mach 2+ range, you pretty much need variable ramps to do this (ie Tomcat, Eagle, etc). Regardless of design, conventional turbine jet engines require subsonic flow in any case, and this is the most straightforward method of making it happen. Aside from supersonic considerations, there are a number of aircraft out there that have poor inlet designs even in terms of subsonic flight regimes. I remember the T-45 was especially succeptible to compressor stalling at low airspeed, high AoA, and this is not uncommon for tactical jet aircraft. Big yaw rates, high AoA, all generate unusual levels of flow turbulence in the inlet area of the motor, usually by poor intake/duct design, and can cause pop stalls or if really aggravated by poor pilot skill, locked in compressor stalls as well. Either case (super or subsonic) really boils down to flow disruption at the engine inlet in some way, shape, or form.