Pressures in front and behind LP fan on jet engine

[Mark D Larsen]

My friends (all A&P Aircraft mechanics) say I'm nuts. A co-worker and I attended a Honneywell TFE731 jet engine maintenance school. During the class the instructor asked this question, "which side of the front fan on a TFE731 engine has the highest pressure?" His answer was the front. (we are talking a millimeter or less in front or behind each blade) Well my co worker said he was wrong and was an Idiot. (see PS) Well the engine has a large inlet, which is slightly divergent, and the spinner on the fan is quite large causing a large convergence of area. I stated that because of this convergence and fan the incrase of air speed causes a loss of pressure. (bernoulli's pricipal) Every on here at work say's bernoulli's doesn't apply because of the fan is increasing both the speed and the pressure of the air. I thougt if you incrase the speed of air the pressure decreases and if you decreas the speed of air the pressure increases. Am I wrong?
PS I love a good argument!

 

[rcgldr]

If the aircraft's speed exceeds the flow speed of the "intake" fan, it's then acting as a turbine, both reducing speed and pressure, performing "negative" work on the affected air, which violates Bernoulli. If the aircraft is not moving, and assuming there' isn't a reduction of pressure from the intake side of secondary compression stages in the engine aft of the "intake" fan, then the fan increases pressure.

Typical fans and propellers cause an increase in pressure with little change in speed of the air in the immediate vicinity of the fan or prop, called a "pressure jump". This violates Bernoulli because the fan or prop is performing work on the air, increasing it's overall mechanical energy. Once beyond the immediate vicinity of the fan, Bernoulli applies. Upstream of the fan the air accelerates towards the low pressure zone fore of the fan, speed increases as pressure decreases. Downstream of the fan the air accelerates away from the high pressure zone aft of the fan, and again speed increases as pressure decreases. Because of the work done, downstream of the fan there's a point where the affected air's pressure returns to ambient, but it's speed will be non-zero, and this is called "exit velocity".

In order for mass flow within a streamline to remain constant, as the air speed of a streamline increases, the cross section of the streamline decreases. In the case of a jet engine, the flow is enclosed so the engine is designed so that effective area decreases in the sections where air speed increases within the engine, except at the exhaust where the temperature increase causes expansion of the air. In the case of a propeller, the surrounding air is affected due to pressure differential and viscosity, complicating matters:

Link to Nasa article. Note that the effect on the surrounding air is ignored in this simplified explanation. If this were a ducted fan, then the duct shape would be similar to the shape shown in the streamline flow diagram in the article.

http://www.grc.nasa.gov/WWW/K-12/airplane/propanl.html

 

[K^2]

Compressor on the turbojet usually narrows down, though, so you have an increase in flow speed from front to back that's not only due to compressor working on it. You can't look at it as a simple prop. The pressure most certainly increases dramatically after the commpressor flow enters the combustor, but I really can't see any specific argument for why pressure in the flow right after compressor has to be higher or lower than ambient.

 

[Mark D Larsen]

I still thought that Bernoullis principle states that increasing speed decreases pressure. If the pressure behind the fan is lower than in front of the fan then the air should travel to the low pressure? I thought that the only way to increase pressure was to slow air down. Just behind the blades the air is going very fast and in front much slower due to convergence. Whare do I have it wrong?

 

[Topher925]

I still thought that Bernoullis principle states that increasing speed decreases pressure.

It does, for a fluid with constant enthalpy. However, the fan is adding energy to the air by both compression and increase in kinetic energy.

I thought that the only way to increase pressure was to slow air down.

For a fluid with constant enthalpy yes. But this is not the case. How would you explain the compression stage of the engine then? You can think of a fan as a high flow rate low pressure compressor. It takes energy to spin that fan, energy which is transferred to the working fluid.

As for if the professor is an idiot or not, I dunno, I'd have to see a drawing of the problem.

 

[Mark D Larsen]

Here's a drawling of the engine. Figure 7-71-1 http://www.smartcockpit.com/data/pdfs/plane/gulfstream/G150/systems/G150-ENGINES.pdf
The two areas in question are just under the letters T & R in full span stator.
"How would you explain the compression stage of the engine then?" The air in the compressor section is constantly slowing down, if not it would exceed the speed of sound (since the aircraft does Mach .89) and the compressor blades would be ineffective.

 

[sweet springs]

Hi Mark.
I am not sure of general structure of engines but say intake air rho1,p1,v1 → engine → outburst gas rho2,p2,v2. Momentum gain of flying plane engine is rho 2 v2 - rho 1 v1 per unit time which is constant times p2v2 - p1v1. In order plane to fly forward, p2v2 > p1v1. Usually both v2>v1 and p2>p1 because Mol of exhaust gas is larger than intake air by chemical reaction with fuel and hot.
Regards.