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Stall and surge are endemic scourges the gas turbine community about stall and surge incidents they had encountered. A retired pilot I of jet engine and gas turbine operation. know described to me one event, on a flight from Portland bound for Tokyo. He said that during the By Lee S. Langston takeoff run, a single Herring gull was “ingested” into one of the engines. In a blink, the cowling odern jet engines are so reliable that blew right off the front of the engine and de- airline pilots can fly an entire career stroyed the tires of the landing gear. The takeoff without experiencing an engine failure was aborted. Mor having to shut down an engine for The problem wasn’t the bird itself. Jet engines a minor problem. Most passengers never have to are designed to withstand strikes from small birds experience mechanical failure except as the reason like gulls (though the birds are not designed so for an otherwise unexplained grounding of a flight. resiliently). Instead, the bird disturbed the flow of Engine failure—especially one caused by engine air entering the engine and the compressor experi- stall and surge—may be rare, but it is unforgettable. enced a stall. In an instant, superheated gases from Recently, I started discretely asking people in the combustion chamber that normally power MECHANICAL ENGINEERING | APRIL 2017 | P.37

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the turbine in the rear of the engine surged University summarized the progress that has One engine of forward. been made in understanding the phenomenon this C-17A suffers Stall and surge are two words that catch a and highlighted some steps that are being a compressor surge while taxiing gas turbine engineer’s immediate attention. taken to control it. In spite of those eff orts, on an airfield. Those two types of air fl ow disturbances in the stall and surge present a challenge to the gas Photo: Will Mallinson axial compressor of a jet engine or gas tur- turbine research community and a worry to jet bine represent the breakdown of orderly fl ow engine designers. through them. Modern design and fuel control systems try to keep a jet engine or an electrical power gas turbine away from operating condi- Under pressure tions that bring about stall and surge. But they Gas turbines can produce power over don’t always succeed. many orders of magnitude, from kilowatts to I have been thinking about stall and surge hundreds of megawatts. The useful output is, ever since the most recent International Gas however, only a fraction of the power produced Turbine Institute Scholar Lecture, when Ivor by the turbine component of the machine. Day of the Whittle Laboratory at Cambridge Some 50 to 70 percent of the turbine output Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/139/04/36/6360103/me-2017-apr2.pdf by guest on 26 September 2021

Image credit: Robert Mazzawy, Trebor Systems. is diverted to drive the axial flow compressor industrial gas turbines, each stage operates in a upstream. Compressing a gas is not easy. pressure ratio range up to about 1.4:1. Air entering Axial compressors get their name because the a six-stage compressor with a pressure ratio of, path of flowing gas runs more or less in a straight say, 1.2:1 would see an overall compression of 1.2 line parallel to the gas turbine’s axis of rota- raised to the power of 6, or 2.99 times the initial tion. The compressor is assembled from stages, pressure. each comprised of a ring of moving rotor blades The thermal efficiency of a gas turbine is mounted on a rotating disc or drum and a down- directly tied to the overall pressure ratio, which stream ring of case-mounted stationary stator creates an incentive to maximize compression. blades simply known as stators. The rotor blades Some 70 years ago, an axial compressor with 15 act on the gas path air flow, increasing its static stages might have an overall pressure ratio of 4:1. pressure, total pressure, and kinetic energy. The Today, a 231 MW GE 7F.05 gas turbine has a 14 stators remove blade-induced swirl velocity, and stage compressor with an overall pressure ratio of thus decrease the kinetic energy, while serving 18.4:1 and for jet engines the ratio can be as high to increase static pressure and align airflow for as 58:1. As a consequence, gas turbine efficiencies blades in the next stage. have more than doubled. Each additional stage increases both static Together, the compressor blades and stators and total pressure of the gas path. Typically, for operate on the gas path flow to produce what MECHANICAL ENGINEERING | APRIL 2017 | P.39

Over the course of the testing, engineers will determine for each aerodynamists call pressure ratio the air flow to operate properly. Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/139/04/36/6360103/me-2017-apr2.pdf by guest on 26 September 2021 an adverse pressure In industry parlance, gradient in the fl ow that’s called the com- direction—they turn rate below which pressor surge line, and low static pressure that line is labelled as air into high-pressure the compressor fails to such as it runs across air. It’s analogous to the compressor map. pushing water up an But the actual phenom- inclined channel, with operate properly. enon it measures is many small, rapid aerodynamic stall. brush strokes. If the incline is too steep, the water To understand why, consider the streamlines runs backward, down the slope. (By contrast, the of the air fl owing across the compressor blades. turbine operates in a decreasing, favorable static Going from low pressure at the blade’s leading pressure fi eld—as the expanding combustion edge to a higher pressure at its trailing edge, the gases act to turn those blades, their static pressure streamlines closely follow the blade’s suction and decreases.) pressure surfaces. No matter how advanced axial compressors The fl ow around the blade is controlled by its become, they must be carefully controlled in their boundary layer: the very thin, almost immea- operation to avoid the power-robbing eff ects of surable layer of air on the blade surface, where stall and the convulsive eff ects of complete fl ow viscous frictional eff ects are concentrated. Within reversal, brought about by surge. the boundary layer, the air velocity relative to the blade decreases from that of the external stream- lines, to zero at the blade surface. The concept At the boundary of the boundary layer was discovered by the What does this convulsion look like? Another engineer Ludwig Prandtl in 1904, at an appropri- correspondent described a videotape of the fi rst ate time to profoundly infl uence the design and few minutes of an airline fl ight shot through a operation of aircraft—and turbomachinery. cabin window by a passenger. First, the runway In aircraft, when the angle of attack exceeds a streamed by during takeoff and the jet began to certain level, the boundary layer (which is exceed- climb. “Suddenly the inboard right engine gives ingly sensitive to an adverse pressure gradient) out a boom,” he wrote, “and sheets of fl ame start separates from the airfoil and the wing loses lift; pulsating out of the engine as the countryside and the plane stalls. The same eff ect plagues axial fi elds below fall away.” Alarmingly, as the video compressors. Generally, what changes the angle continued, the ground (and farm animals in near- of attack within a compressor is a reduction in by fi elds) started to get closer even as the nose air fl ow due to a blockage downstream in the of the plane continued to point up. Eventually, combustor or turbine or an interference to the the plane recovered enough altitude, though not upstream air inlet. Roughness on the compres- before the tower crew, which saw the plane dip sor blades or too large a gap between the rotating below the tree line, alerted emergency responders blades and the engine casing can also disrupt the of a probable crash. boundary layer. Regardless, when the boundary To avert disaster (and near disasters like the layer separates from the surface such that the one above) any gas turbine company that designs streamlines no longer follow along the suction a new compressor will test it thoroughly, varying side of the blade, the compressor blade is stalled. air fl ow rates and rotor speed to measure pres- When an airplane stalls, the aircraft loses sure ratio and effi ciency, creating what they call altitude. If the pilot cannot restore lift across the a compressor map. Over the course of the testing, wing, the plane will crash. engineers will determine for each pressure ratio A stalled blade in an axial compressor loses its the air fl ow rate below which the compressor fails ability to increase a pressure gradient. What then? With the axial flow so disrupted, flames from the combus- Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/139/04/36/6360103/me-2017-apr2.pdf by guest on 26 September 2021 tor can shoot out the back of the turbine outlet, or even for- ward through the compressor inlet.

As Ivor Day explained in his IGTI Turbo Expo talk, a stall disturbs the compressor fl ow in the tangential direction, while the average axial air- fl ow through the compressor remains steady. But a stalled compressor blade can itself create a block- age which diverts the approaching stage fl ow. This can trigger separation in adjacent blades opposite to the direction of rotation on the same rotor, cre- ating something called a stall cell. the plane took off on a transcontinental fl ight, the In certain circumstances, that stall cell will start engine let out a loud bang—then 57 more in the 70 to move, rotating in the opposite direction of the seconds it took the pilot to shut down the engine. compressor and at half its rotational speed. Need- The crippled plane had to circle and dump fuel for less to say, those rotating stalls can lead to greatly an hour before it was able to make a single-engine shortened blade life, through increased stress and landing. vibration. How can we avert these sorts of engine failures? In extreme cases, however, rotating stall can Stall and surge emerge from basic physics: the cause the compressor fl ow to fail altogether, behavior of the boundary layer on the compres- disturbing the airfl ow in the axial direction. The sor blades and stators. Current technology has no airfl ow rate will pulse on millisecond timescales— means to completely eliminate it. sometimes so violently that the fl ow through the Right now, gas turbine manufacturers resort to compressor is reversed. (This reverse fl ow is often trial and error, plumbing the contour of the surge accompanied by a loud bang, like a backfi re.) With line of a compressor through extensive testing the axial fl ow so disrupted, fl ames from the com- to determine what conditions to avoid. Engine bustor can shoot out the back of the turbine outlet, control systems, such as the Full Authority Digital or even forward through the compressor inlet. Electric Control, or FADEC, are programmed to That degree of compressor fl ow failure is called keep the operating point of the compressor well a surge. It is to be avoided. away from the surge line. Mechanical systems within the engine, such as compressor bleeds, cas- ing treatments, tip clearance controls, and variable Measured in milliseconds pitch stators that control the direction of airfl ow Another surge incident I learned about involved to rotors are all used to avoid stall conditions. an airliner with an engine that had undetected Those mechanical systems work within the erosion in the lining of its compressor casing. As blink of an eye. The time needed to adjust a MECHANICAL ENGINEERING | APRIL 2017 | P.41 Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/139/04/36/6360103/me-2017-apr2.pdf by guest on 26 September 2021

variable stator or a bleed is on the order of 200 in the fan stream. In such cases, however, the low- A Sukhoi T-50 milliseconds. spool rotor increases it rotational speed slowly PAK FA suffers a compressor surge. That is not fast enough. enough for the FADEC to sense the mismatch in Credit: Wikimedia Researchers have been studying stall and surge RPMs between the spools and take measures to for decades, looking for ways to predict and com- counteract it. bat the phenomena. According to Robert Maz- FADEC can also step in during fl ights in heavy zaway of Trebor Systems, who in 1980 was one of rain or hailstorms. In those conditions, extra fuel the fi rst to report on the engine stress created by is needed to process and evaporate the water be- surges, researchers have found that subtle modal ing swallowed by the engine. The electronic con- waves lead to the rotating stall that precipitates trol system can sense the mismatch between the surge. Detect those waves, the thinking goes, and power setting and the fuel fl ow and take action to FADEC could act to prevent the stall. Unfortu- prevent possible engine instability. nately, a rotating stall develops over the course Fortunately for me, my interest in stall and TO LEARN MORE of just a handful of rotor revolutions—which take surge is one of curiosity, spurred by Ivor Day’s I. J. Day. “Stall, Surge only 20 milliseconds for an industrial gas turbine comprehensive IGTI lecture. For others, however, and 75 years of and only 5 millisecond for a jet engine. There’s it is a vivid experience that still raises the hairs on Research.” ASME J. Turbomach. 138 (Oct. not enough time, then, for the mechanical sys- the back of their neck. 2015): 011001, 16 pages. tems to counteract the rotating stall and ensuing “It felt just like we had hit a telephone pole with surge. the right wing,” one acquaintance told me about R.S. Mazzaway. “Surge- Induced Structural Even so, there has been some success in using a surge event on a fl ight he was on. “It was short Loads in Gas Turbines.” FADEC to either prevent a stall and surge or to and abrupt, but scary as hell.” ME ASME J. Engr. Power 102 (Jan. 1980): 162-168. limit the number of repetitions. In twin-spool engines, for instance, stall can be caused by a mis- LEE S. LANGSTON is professor emeritus of mechanical engineer- E.S. Taylor. “Evolution of the Jet Engine.” match between the rotation rates of the low-spool ing at the University of Connecticut in Storrs and a frequent Astronautics & Aero- and high-spool rotors, usually due to a disruption contributor to Mechanical Engineering magazine. nautics 8 (1980): 64-72.