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I have referred to standard author books and online tutorials but I am still quite confused about how the Stall and Throttle characteristics lead to Surge.

1. Why and how does the throttle characteristic affect the compressor performance because as far as i know, the next consecutive element in an engine after a Compressor is a Combustor, and the throttle comes at last,after combustor and turbine, assuming throttle is same as nozzle (if not please correct me).

Assume i am talking about turbojet engines and Axial compressor, just to have a picture in mind.

2. How does the throttle characteristic curves come out to be like that?

And why does the mass flow decrease when we throttle the compressor?

3. I am also unable to grasp the concept which causes the evil fluctuations in the flow during surge, which leads to reverse flow and damage of the components.

enter image description here

Source:- Hill Peterson- Thermodynamics of Propulsion.

Someone please explain the concepts as much as possible.

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  • $\begingroup$ The throttle adjusts the fuel flow, much like the gas pedal does in a car. And: Between the compressor and the combustion chamber is a diffusor which wants to be treated properly to avoid flow separation. $\endgroup$ – Peter Kämpf Mar 8 '15 at 8:52
  • $\begingroup$ Just FYI, the throttle on a car is called that because on a carbureted engine it would restrict (or throttle) the airflow going into the engine, not the fuel directly. $\endgroup$ – Porcupine911 Mar 8 '15 at 20:22
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Since your question is divided in three parts, I do the same with my answer. First, however, we need to explain what the axes of the graph mean. The x-axis plots mass flow through the compressor, which is roughly equivalent to the tip speed of the compressor blades, and the y-axis shows the pressure ratio between entry and exit flow.

Throttle and compressor performance

The throttle regulates fuel flow, and this flow has to match the air flow. The limits are:

  • If fuel flow is too high: The combustion will create too much heat, so the turbine will be damaged.
  • If fuel flow is too low: Too little energy is added to sustain the current operating point. The engine will slow down.

If it is kept within those limits, the throttle will indirectly control the compressor speed and it's mass flow. More fuel flow will translate into higher turbine entry speeds, which will increase the torque produced by the turbine and speed it up. Since it is directly coupled with the compressor, it will see the same speed increase. This leads to higher mass flow and a lower fuel-to-air ratio in the combustor (relative to the state right after the throttle increase), such that a new equilibrium at a higher speed is established.

Throttle lines

For every pressure ratio the compressor will have a matching mass flow, and both move up and down in parallel. This is shown by the dashed lines. Depending on the blockage of flow in the turbine and the engine nozzle, different lines are shown for different grades of blockage. The solid stage characteristic line shows the limits within which the compressor will run, and a higher flow resistance narrows the range of speeds that are possible. The limits are determined by:

  • Lower limit: Here the compressor is run at the lowest possible speed for the given blockage, and the compressor blades run close to their maximum lift coefficient. When the blockage is increased, the flow over the compressor blades will stall and the pressure ratio will collapse.
  • Upper limit: Here the compressor blades run at high speed and a low lift coefficient. Running them any faster will not create enough pressure rise per stage, and again the blocking cannot be overcome.

Surge

Once the pressure rise per stage cannot be maintained (say, due to a stall of the flow over the compressor vanes), the high pressure air in the combustor will find it easier to escape at the front than to flow through the turbine and the nozzle. This leads to reverse flow and a sudden drop in combustor pressure, which in turn allows the compressor to reverse the flow and start pumping air back again. Now the pressure in the combustor rises again, until the whole process repeats.

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  • $\begingroup$ Doubt:- 1. So throttle is used to regulate mass of fuel, fine. But then acc. to u, when we increase fuel mass, it should logically increase air mass flow also, but how can you say that it would lead to a LOWER fuel to air ratio directly, when both of them are increasing. Is their any mathematical, empirical or logical reason to explain that? $\endgroup$ – Manish Mar 8 '15 at 10:36
  • $\begingroup$ 2. Can you please elaborate a bit on the surge part, where on the right side of E we have a stable flow and on the left we have an unstable flow. Because both the sides the pressure ratio decreases, so why is it so that reverse flow effects only while reducing the mass flow beyond E.(even slightly). $\endgroup$ – Manish Mar 8 '15 at 10:39
  • $\begingroup$ @Manish: 1. When you increase the throttle, the fuel-to-air ratio gets richer. Then, as turbine power and mass flow go up, it becomes leaner again. It is lower in relation to the state right after the throttle change. 2. On the left side of E blockage is too high, so no stable point exists. Mass flow equals tip speed, and at the lower speeds the possible pressure ratio is too low to allow the engine to operate. $\endgroup$ – Peter Kämpf Mar 8 '15 at 11:54

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