Module 212SE – Aircraft maintenance engineering – Research activity 2.

Task name: Surge in an Axial Compressor

Date: 31/10/2013

Student name: Michael Etienne Group: N/A

i. Undertake research to answer the following questions. A good starting place would be the book, “The Jet Engine” produced by Rolls Royce Ltd. However, you will need to expand your research beyond this in order to answer the questions fully. Please feel free to add annotated diagrams to aid explanation. Text within diagrams will not count towards word count.

ii. You should use this template for your answer and insert it into your portfolio of work (CW1).

iii. Ideally your answer should be no more than 1000 to 1500 words.

iv. Additional sheets can be added as required.

Michael Etienne Page 1 4/15/23

Module 212SE – Aircraft maintenance engineering – Research activity 2.

1. Define what constitutes an axial-flow compressor surge and write a technical description of how this phenomenon can occur?

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An axial-flow compressor surge is constituted when the airflow through the compressor breaks down. When this happens the compressor blades stall and it stops the air being forced to flow from front to back. This enables the high-pressure air in the centre of the engine to escape from the front or back in an explosive manner. This phenomenon when looked at in more detail has been found to occur due to the stalling of the blades. The blades are small aerofoils, which behave in the same manner as a wing when airflow passes over them. When the “angle of attack of the blades exceeds the critical angle of attack” (Learnings 2010) or there is a disturbance in the airflow, e.g. FOD, bird strikes and intake distortion, the blades stall causing “a pocket of stagnant air, also known as stall cells, rotate around the circumference of the compressor” (Ward 2010:348). With this pocket of air flowing around the compressor it leads to an increase of structural load on the blades and reduced efficiency of the system. This stalling effect can propagate causing all the blades to stall throughout the whole compressor, also known as an “axisymmetric oscillation”(Ward 2010:348), resulting in the system not being able to maintain a steady airflow against the pressure gradient of the air. The rotator and stator blades create the pressure gradient as the convergent stator blades increase the pressure of the air as it flows from the rotating blades. As a result the velocity of the air decreases and becomes more stable for the compression cycle to be efficient. In this particular situation the compressor begins to act as a turbine, converting the air’s energy into work, forcing it out the front of the engine. This leads to a reverse of the pressure and velocity of the air as it decreases in pressure but increases in the velocity. Some surges may only occur momentarily but a deep surge may occur which is more severe and damaging. This damage can become a deep surge that “involves a large mass flow fluctuation with airflow reversal (expulsion of previously compressed air out of the intake) accompanied by a highly disturbing acoustic effects (large bang)” (Ward 2010:349). With this effect it has known to be accompanied by a flame out which is visible flames seen at both ends of the engine. The stall of the blades of the compressor and the surge are often coupled but can independently occur in flight. When the two phenomenons occur simultaneously this can lead to a surge cycle, this a recurrence of the stall over the blades of the compressor due to the air’s pressure and velocity unchanged from the previous stall.

Module 212SE – Aircraft maintenance engineering – Research activity 2.

2. Describe and evaluate 2 methods utilised by engine manufacturers to alleviate the effects of a compressor surge, including at least 3 examples of actual implementation of these devices to illustrate historical development?

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One of the methods utilised by engine manufacturer is using airflow control devices like the, VIGV (Variable Stagger Intake Guide Valves) and VSV (Variable Stagger Stator Vanes). The VIGV is used to prevent the problem of front-end stall at low or variable rotational speeds, stopping the build up of stagnant air in the forward parts of the compressor. The way in which the vanes prevent this is by directing the airflow at the front of the compressor onto the first row of blades at the correct angle of incidence. During flight, the axial velocity of the air increases and “the vanes must be repositioned to maintain the correct angle of incidence” (Ward 2010:356) otherwise this can lead to the blades stalling and creating a surge. With this type of method of airflow control, the characteristic of the compressor changes and this results in the surge line moving but doesn’t alter the working line. The same method can also be applied to the VSV (figure 1), which are found throughout the compressor as the pressure increases along each stage. The effectiveness of having this type of component within an engine is it a preventative measures so that the stages leading up to a surge do not occur. However one of the problems of having this component integrated within the engine is when it comes to maintenance of the aircraft, which can include inspection or removal of VIGV, this could prove difficult and tricky. The reason for this is that with this integration, if one VIGV or VSV becomes damaged then it requires the entire removal of the axial compressor to fix it. This can be time consuming for engineers during maintenance tasks as well as being costly if one broken part is ingested through the whole working system of the engine and damages other parts. This type of method has been implemented in a wide variety of modern aircraft such as the Boeing 737, most commonly used short-haul aircraft in the world, and military aircraft like the Boeing Globe Master which is the largest transport aircraft the RAF has flown in its 95 year history.

Another method used by engine manufacturers to alleviate the effects of a surge is a blow-off valve. The purpose of this type of component is to not only prevent surge in the compressor but also reduce the damage on the turbocharger and engine. The way which it does this is “relieve the damaging effects of compressor "surge loading" by allowing the compressed air to vent to the atmosphere, making a distinct hissing sound, or recirculate into the intake upstream of the compressor inlet” (Allard 1982). Overall the effectiveness of this type of system is good as it offers an additional safety net for the engine in the scenario where the VIGVS fail at the front of the engine and this system detects the surge (through the ECU controlled by the FADEC). Another effect measure by this system is that it improves the surge margin; this is the “distance between the surge line and the operating point on a vertical line for the constant corrected mass flow value”(GSP 2013). In one scenario the blow-off valve can prove dangerous depending on the position of other components round it. If a MAF, a mass airflow sensor which finds out the mass of the flowing air entering the engine, is located “upstream from the blow-off valve, the engine control unit (ECU) will inject excess fuel because the atmospherically vented air is not subtracted from the intake charge measurements. This leads with a fuel-rich mixture after each actuation which leads to a rich mixing that could possible cause stalling of the engine when the throttle is closed” (Allard 1982). What this is saying is if the detector is behind the blow off valve where it is venting the compressed air for the atmosphere, the computer isn’t then able to calculate the correct fuel to air mixture causing the mixture to become too enriched, as there is less compressed air than it had calculated. This type of scenario could damage other components of the engine it produces “inefficiently combusted fuel, known as soot, which can foul spark plugs and destroy catalyst converters” (Ward 2010). An example of this type of system has been implemented on the C-130J Hercules which is currently used by the RAF to transport of troops/vehicles, air to air refueling or tactical assault sorties. The blow-off valves on the turbo-prop engines on the Hercules are monitored by FADEC (Full Authority Digital Engine Control). This system monitors the engine for its efficiency in temperature and pressure so it will be able to detect a surge and act accordingly.

Figure 1: Variable Stator Vanes in an Axial Compressor (Rolls-Royce 1996:29)Figure 1: Variable Stator Vanes in an Axial Compressor (Rolls-Royce 1996:29)Figure 1: Variable Stator Vanes in an Axial Compressor (Rolls-Royce 1996:29)Figure 1: Variable Stator Vanes in an Axial Compressor (Rolls-Royce 1996:29)Figure 1: Variable Stator Vanes in an Axial Compressor (Rolls-Royce 1996:29)

Module 212SE – Aircraft maintenance engineering – Research activity 2.

3. Describe a typical surge characteristics testing process, including a method for calculating a particular gas turbine engine’s “surge line”?

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With every engine there is an operating map, this aids manufacturers in predicting the performance of it. The map looks in particular at the pressure and temperature rises and efficiency of the engine against the airflow through the engine. The performance characteristics of the engine can be divided into five different parameters:

Correct mass flow Correct rational speed Pressure ratio Component efficiency Reynolds index

(GSP 2013)

The most convenient and easiest way for engineers to read these characteristics is to represent them in component characteristic performance graphs. With a compressor you have a operating map which is plotted with an x-axis of compressor entry flow and the y-axis showing the pressure ratio index. The compressor entry flow is a flow that has been corrected into a non-dimensional flow instead of the actual flow of air. The difference between the two is the corrected flow is the flow of air if the engine was at sea level pressure with the ambient temperature. If this was taken from an ISA table, that would be P=101.325 KPa with the temperature at 288.15K. The standardisation of these units is used as a good estimate for performance to near design conditions as well as allowing engineers to be analytical of the data. The formula used to calculate the corrected entry flow:

corrected entry flow=˙m√C pT 02

A P02

C p=Specific heat constant at pressure constant P02=Static pressure (engine inlet ) T 02=Temperature(engine inlet) A=Area M = Mass

(Cumpsty 2003:127)

With this type of flow, the engine manufacturer is represented by plotting it over a period of time as kg/s or lb/min. The y-axis is plotted with the pressure ratio; this is the difference between the inlet pressure and outlet pressure. On the graph there are two axes, the working line is a line that joins the points of the equilibrium of the engine at different airflows and pressures. Alongside this there is another line known as a surge line, which represents an area above it, the region of unstable airflow, which is what engineers want to avoid, as these are areas where engine surge may occur. This line is plotted after calculating the surge margin; this incorporates both the surge line, and the surge margin. The surge margin is the “distance between the surge line and the operating point on a vertical line for the constant corrected mass flow value”(GSP 2013). The formula to calculate this is:

SurgeMargin=100× PRsurge−PRoperating linePRoperating line

=100×( PRsurgePRoperating line

−1)

(GSP 2013)

Module 212SE – Aircraft maintenance engineering – Research activity 2.

4. Reference and Bibliography: (Must conform to CUHarvard format)

BIBLIOGRAPHY A.Ward, T. (2010). Aerospace Propulsion Systems (1st ed.). Sinapore: John Wiley & Sons.

Allard, A. (1982). Turbocharging and Supercharging. Cambridge, England: Patrick Stevens Limited.

General Electric Aviation (2013) General Electric Aviation J85 [Online] Available at http://Http://www.geaviation.com/engines/v2 military/j85/ [Accessed 2 November 2013]

Cumpsty, N. (2003). Jet Propulsion (2nd Edition ed.). Cambridge: University of Cambridge Press.

Flight Learnings (2010) Turbine Engine Operational Conditions [Online] Available at http://Http://www.flightlearnings.com/2010/03/11/turbine-engine-operational-considerations-part-two-compressor-stalls/ [Accessed 1 November 2013]

Rolls-Royce. (1996). The Jet Engine (5th Edition ed.). Derby, UK: Renault Printing. G.D Team (2013) Surge Margin [Online] Available at

http://Http://www.gspteam.com/GSPsupport/OnlineHelp/index.html?surge_margin.htm [Accessed 3 November 2013]

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