I’m a freshman in Aircraft maintenance and my prof in Powerplant 2 (Gas Turbine Engine) explained to us about the venturi duct within the compressor of the Turbine engine. I get that the compressor increases the pressure of the air but what I don’t get is his statement that according to Bernoulli’s principle once the air passes through the inlet duct and towards the compressor the air’s pressure decreases and the velocity increases as it passes through the venturi duct the pressure only increases after it reaches the divergent part of the venturi duct before it gets to the combustion chamber.
So what I want to know is why does the pressure decrease first before it enters the compressor?
In: 1
The way I think about this situation is to know that the mass flow rate through a pipe must be constant. What goes in is what comes out.
Now if we have an upstream area that is bigger than a downstream area, like from the inlet, to the narrowest point of a Venturi, then to achieve the same mass flow rate, the downstream velocity must be higher than the upstream velocity.
From Bernoulli we introduce equivalence between upstream and downstream energies. To keep this equivalence, if the downstream velocity is higher, the downstream pressure must then go lower to compensate.
If you have a smaller section of pipe, then before and after this section (in the bigger pipe) the water is moving slower, but in the smaller section of pipe it has to move faster.
This is because the same amount of flow has to happen all throughout the pipe. All the water going into the large pipe must also go into the small pipe.
So it’s slower in the big pipe.
We know that the energy of movement scales with the square of its speed, so if the water is moving faster in the small pipe, it must have more kinetic energy. That energy has to come from somewhere.
That energy came from the pressure forcing the water through the small pipe. The pressure drops in the smaller pipe, since it resists the pressure by forcing water to flow faster.
At the end of the small pipe, that fast-moving water has to slow down again. The same process happens in reverse, with the fast-moving water ramming into the water in the larger pipe and causing a pressure increase there.
In an ideal constriction, the pressure before and after is the same. In the real world, it is not. And yes, this all applies to air too.
Hiya fellow jet mech! This may help: [https://youtu.be/efB3chu8J3o](https://youtu.be/efB3chu8J3o)
Depending on if you work on Pratt or GE, Pratt calls them Civv’s (compressor inlet variable vanes for the fan) RCVV’s (rear compressor variable vanes for the core) and GE calls them VSV’s throughout (variable stator vanes). They slow and direct the airflow in the fan and core. Depending on the engine you’re working on (220/229/119/404/110/101/108/118 etc) there are various stages of fan and compressor, and not all have variable vanes before each stage of compression.
An easy way to look at it from a convergent/divergent way is to look at an augmentor/afterburner on the engine you’re working on and picture the airflow coming out of the nozzle 🙂
One other thing, the aircraft inlets and ducting also play a part during ground idle and at speed. I’m speaking about fighter aircraft, not heavies 🙂
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