Gas Turbine Observations
Compressor Bleed and Power Extraction
by Charlie Cravens
In a previous article that reviewed Geoffrey Wilde’s book Flow Matching of the Stages of Axial Compressors dealing with variable vane systems in axial compressors, no mention was included of bleed air or power extraction, as these were not addressed in Mr. Wilde’s book. I thought it would be a good idea to discuss these matters briefly to supplement the previously furnished material.
Use of variable vanes as a means of improving stage matching is not the only way of improving compressor stability. Angle of incidence of the compressor airfoils can also be altered by use of interstage bleed. Such bleed is used at low power, when excess flow from the first stages cannot be accepted by the later stages. Most, if not all, modern engines employ these, usually known as "surge bleeds". Normally, they are located between the low pressure (LP) compressor and high pressure (HP) compressor on a typical two-spool engine. They are actuated automatically by the engine’s control system. This air is discharged overboard.
Bleed is also used to power the airplane’s air conditioning and pressurization system and for thermal anti-ice protection. In addition, internal bleeds are used by the engine itself for turbine cooling, bearing compartment sealing and pressurization, and main rotor thrust load control. Bleed and shaft power extraction are discussed further below.
In a modern turbofan, total airflow is divided into core flow plus bypass flow. Core flow is the part which goes through the center of the engine and produces the power to drive the fan and generate the bypass flow, plus core flow itself. The largest internal bleed goes to cool the HP turbine vanes and blades. This generally will amount to around 23% - 27% of core airflow, a very sizable fraction. All this is not lost however, as it reenters the gas path, albeit at lower pressure than before and with some efficiency penalty as a result. Use of labyrinth type seals on the main rotor bearing compartments is universal in these engines, as no other type of seal can live and perform in the environment which prevails. A stationary bushing, usually lined with a soft material is installed in the housing, with pressurizing air fed into the middle of its length. The shaft has several (usually six or so) vee-shaped lands which are a clearance fit in the bore of the bushing. Bleed air flows axially both ways through the assembly, thus forming the "seal". Note that this is not the usual "seal", where leakage is designed to be zero.
Pressure differences across the various main rotor components must be controlled in order to control thrust loads on the bearings. This can be a troublesome problem for the designer, as it is best that the loading remain in one direction only within an upper and lower limit at all operating conditions.
Some engines employ turbine case cooling using an external manifold and bleed air (usually from the fan exit) which impinges on the outside diameter of the case. This is turned on in cruise to reduce clearances and improve fuel burn. This feature is making its way to compressor case applications as well. It must not be used in other flight regimes or blade tip rubs will occur.
The designer will attempt to minimize leakage from the main gas path for performance reasons. However, there will be leakage, and efforts are made to use it constructively as much as possible. Examples would be compartment pressurization, oil sealing and other minor uses. These flows are not actively controlled.
Service bleeds used to drive the air cycle machines (for aircraft cabin environmental control) is, from the engine’s perspective (if engines can have perspectives) an expensive, continuous burden that adversely affects fuel burn and entails compromises in the aerodynamics of the engine as well. However, cabin temperature and pressure must be controlled at all times to provide a habitable environment. Bleed air is extracted from a point near the middle of the HPC and the HPC exit. The LP bleed is used on the ground and at lower altitudes; HP bleed is used during the altitude cruise (especially as fuel is burned off and the engine throttled back). Crossovers may occur depending on demand and other considerations. These bleeds depend on the needs of the airplane and are not normally used for compressor stability augmentation; they may in fact adversely impact stability.
The engine must also furnish shaft power to drive accessories such as generators and hydraulic pumps, also required by the airplane. Power for these purposes is taken from the main gear box, which is (usually) driven by high pressure spool. In the case of airplanes such as the B787, airplane services are driven by electrical power rather than by engine bleed. This entails a large increase in power to drive generators, which in turn requires significant design changes to the engines. There is, of course, no free lunch, but the net result is a decrease in fuel consumption.
All of these items add up to a large burden on the engine and its power management and control systems. Any time power or bleed is extracted, stability margin is affected. In the case of variable or on-demand loads, the transition can trigger instability. In addition to everything else, stability is also reduced by deterioration of the basic engine due to erosion and/or tip wear of the rotor blades, warping of the combustor or turbine vanes, dirty compressor, etc.
Today’s turbine engines are far different things from what they used to be. No more a simple compressor, combustor and (usually single stage) turbine. Today's engines are the result of advances in many technological disciplines Including mechanical design, aerodynamic design, metallurgy, protective coatings, combustor design, materials, manufacturing (including forging and casting and machining of metals that don’t like to be machined). A key item in manufacturing a modern engine is making holes in things; thousands of holes, many in those hard-to-work alloys, which must be sized precisely and aimed exactly ( for cooling turbine parts).
From the description of the bleed and power extraction processes, you might get the idea that the engine control system has a lot to do. You would be more right than you would guess from this article. The control system governs engine start, overspeed and high EGT protection, acceleration rate, ground idle and flight idle settings, responds to changes in air speed and atmospheric conditions, and other functions as well. Oh yes, operates the variable vane system and surge bleeds, and responds to airplane load requirements for bleed and power extraction. We are not dealing with a Stromberg or Holley here.
There is more to be said on many subjects connected with modern turbofans. Low cycle fatigue and disc lives, military versus civil operations, reliability and maintenance costs, maintenance planning and modular engine, derate and flight length effects, etc., etc., etc. On and on it goes. The mind reels. Stay tuned for more.
Thrust and Horsepower
by Charlie Cravens
Discussions of jet engines sometimes include (or attempt to include) comparisons with piston or turboprop/turboshaft engines, whose output is measured in horsepower rather than thrust. The problem is that thrust is a force (measured in pounds) and power is defined as force times distance divided by time. Power is measured on a dynamometer in the form of torque (lb-ft) and speed (RPM). With the appropriate constants included (for the USA, at least) the following equation results:
Horsepower = (Torque (lb-ft) x RPM) / 5252
This is a general equation which applies to any type or size of engine or motor.
On the other hand, the measurement of thrust is not simple or easy to do correctly. Getting a reading is easy; getting one that is useful is not. The first thing to be determined is the type of inlet and exhaust to be used, as well as the proper flow area for the nozzle(s). Note that a turbofan usually requires separate nozzles for the fan and core. If comparisons with the engine specification, or other engines is desired, these nozzles and inlet must be calibrated against another set of known performance, or have been previously so calibrated. This will require the assistance of the engine manufacturer, no doubt accompanied by fees.
Then, the engine is installed in a suitable test cell to provide operator safety and comfort, and supply essential services. The cell and its instrumentation must also be calibrated, with the help of your friendly engine company. You can see where this is going. Data reduction and analysis adds more complication, and at the end of the day you hope you have something useful.
Hope, however, is for old ladies. I can say this because I’m an old man. There is far more to calibration process in general than we can get into here. It is intricate, detailed, and time consuming, and believe me, you don’t want to try this at home. It is possible, of course, to set an engine up on an outdoor stand of some sort (with suitable inlet and exhaust hardware) and simply make a lot of noise and scare your friends, and never mind about the thrust. Before considering this, however, you should remember that all jet engines have a large amount of kinetic energy in their main rotor systems and that failures can be spectacular and extremely dangerous.
If you want to impress your friends, a piston engine is a better idea. I know people have run jet/turbine engines in non-conventional situations, but it remains a risky proposition.
Be safe out there.