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Topic: Airflow In The Engine
Username: surajit001
Posted 2012-12-21 03:19:24 and read 3455 times.

When the airflow intake into the engine is disrupted, the engine might surge. Because the high pressured air inside the engine already will have nothing to push it out of the engine and hence it may try to escape from the front or back of the engine creating engine surges. My question is how is the airflow into the engine disrupted sometimes? Because whatever happens in the core engine, I feel the inlet of the engine gets unaffected. Air can enter the engine at any conditions even there is some problem inside the engine. Then how is the airflow intake into the engine gets disrupted? I am a starter into the concept of aerospace and so these questions have been baffling my mind. Please help me.

Topic: RE: Airflow In The Engine
Username: jetlife2
Posted 2012-12-21 08:10:00 and read 3367 times.

The most common cause of inlet disruption is crosswind. All modern engines and inlets are designed and tested to withstand a significant level of crosswind. When the air approaches the inlet from an angle the inlet itself causes distortion into the front to the engine. This provides turbulent and unsteady airflow into the engine. Eventually the level of distortion introduced into the engine is so high that some part of the engine cannot handle it causing aerodynamic stall of one or more stages. Hope that helps.

Topic: RE: Airflow In The Engine
Username: dakota123
Posted 2012-12-21 15:45:25 and read 3248 times.

Another common way for surge to develop is if one or more sets of variable-incidence stator vanes are off-schedule, such that the volume of air fed to its associated compressor stage from the preceding stage is too great and an aerodynamic stall results. When this happens airflow through the compressor stops partially or fully, or in severe cases even reverses. Conditions at the inlet absolutely will be affected, if even only momentarily as the stage unstalls as controls monitoring for such an event ntervene to alleviate the condition (or if the reduction in airflow as a result of the stall itself unstalls the stage, as can happen if the stator vanes are only slightly off-schedule). Worst case, the engine flames out due to the disruption or is tripped off by the control system.

Not quite what you were asking about, I don;t think, but still may help.

[Edited 2012-12-21 15:48:20]

Topic: RE: Airflow In The Engine
Username: Jetlagged
Posted 2012-12-21 17:23:22 and read 3213 times.

Quoting dakota123 (Reply 2):
Another common way for surge to develop is if one or more sets of variable-incidence stator vanes are off-schedule, such that the volume of air fed to its associated compressor stage from the preceding stage is too great and an aerodynamic stall results

It's more the case that variable stator vanes being off schedule will mean the angle of incidence at the next compressor stage is not optimal, which may lead to a stall.

The problem with an axial compressor is that the air is effectively being driven up hill, against the pressure gradient. It doesn't take much disturbance to cause a partial or complete breakdown of flow. Too much airflow at too low rpm will cause surge. Inlet guide vanes and VSVs help to guide the air to more optimal angles. Surge bleeds can be opened to take air out of the compressor, reducing airflow and increasing the surge margin and the fuel control unit will have an acceleration limiting function to keep rpm and airflow relationship away from the surge boundary.

Topic: RE: Airflow In The Engine
Username: TWA772LR
Posted 2012-12-30 23:23:33 and read 2583 times.

Quoting jetlife2 (Reply 1):
causing aerodynamic stall of one or more stages.

Is that how compressor stalls happen?

Topic: RE: Airflow In The Engine
Username: tdscanuck
Posted 2012-12-31 10:48:09 and read 2468 times.

Quoting TWA772LR (Reply 4):
Is that how compressor stalls happen?

One stage (or even one blade on one stage) will go first but the instability tends to cascade through the compressor very quickly. There are stable partial stall configurations (google "Rotating compressor stall") but that's less common.

Tom.

Topic: RE: Airflow In The Engine
Username: Acey559
Posted 2012-12-31 13:09:55 and read 2435 times.

Turbulence can cause engines to stall as well. That's why we turn the ignition on during turbulence, so it reduces the likelihood of that happening.

Topic: RE: Airflow In The Engine
Username: tdscanuck
Posted 2012-12-31 14:28:35 and read 2418 times.

Quoting Acey559 (Reply 6):

Turbulence can cause engines to stall as well. That's why we turn the ignition on during turbulence, so it reduces the likelihood of that happening.

I don't think having the igniters on reduces the chance of stall, it just improves the chance of prompt recovery. Unless the engine changes compressor scheduling with the igniters on...

Tom.


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