Rotating Stall and Surge

The rotating stall and surge in axial flow compressors are among the most serious problems in turbomachinery aerodynamics. They directly affect the thrust necessary to propel an aircraft at an adequate speed or to drive a generator in AC generation. Both phenomena have to do with the axial flow, and the flow in a multi-stage axial flow compressor is complex in nature because of the proximity of moving blade rows, the build up of endwall boundary layers, and the presence of leakage and secondary flows.

The rotating stall is a flow breakdown at one or more compressor blades. Therefore, it is a stagnated region of air which moves in the circumferential direction of rotor rotation. However, it does so at a fraction of the rotor speed. So, at any given throttle setting, it does not move axially in either direction, but it may cause pressure waves to move upstream (compression waves) or downstream (rarefaction waves). One can distinguish two types of stall; 1) blade stall, which is a two dimensional type of stall, where a significant portion of the blade has large wakes due to substantial thickening or separation of the suction surface boundary layer; 2) wall stall, which is an endwall boundary layer separation.

Surge is the response of the entire engine and it is characterized by a flow stoppage or reversal in the compression system. Upon surge, a compression component will unload by allowing the compressed fluid in downstream stages to expand in the upstream direction, thus forming a more or less planar wave, which at high speeds often leads to flow reversal. The compressor can recover and begin again to pump flow. However, if the surge-inducing cause is not removed, the compressor will surge again.

Bristol Mercury IX

The Bristol Mercury IX was a British radial engine used as the power plant for the Gloster Gladiator Mk I, the Fokker D.XXI, and the Bristol Blenheim. Developed from the Bristol Jupiter, it was conceived and produced to power small and medium size aircraft, such as fighters and small bombers. The Bristol Jupiter had been designed at the end of World War I to power British biplanes of the interwar period. The other widely-used versions developed from this radial engine were the Bristol Mercury VI-S and the VIIIA, which constituted the power plants for the Gloster Gauntlet and the Gladiator Mk II variant respectively.

Technical Characteristics

The Bristol Mercury IX was a 9-cylinder, air-cooled, radial, piston engine, with superchargers. It could deliver between 830 and 845 horsepower. There were two openings in front of each cylinder, working as exhaust ports. These were connected to the collector rings on the cowling. Two spark plugs were fitted atop of each of the nine cylinders. The Mercury had a black crankcase with aluminum cylinders. The nine cylinders drove the airscrew shaft, which turned either a two or a three-blade propeller. It employed 100-octane aviation gasoline.

Specifications

Type: internal-combustion, radial, piston engine.

Weight: 450 kg (980-lb)

Length: 1.2 m (47 inches)

Diameter: 1.3 m (51.5 inches)

Stroke: 16.5 cm (6.5 inches)

Compression Ratio: 6:1

Carburetor: Claudel-Hobson

Supercharger: single-speed, centrifugal type.

Below, a 1930s picture of a Bristol Mercury IX radial engine before it was mounted in the nose nacelle of the Gloster Gladiator biplane fighter.

Union Pacific 4014

The Union Pacific 4014 'Big Boy' is the largest steam locomotive in operation in the world today. It delivers 7,500 horsepower at drawbar, and 8,300 HP at cylinder. It can travel at the top speed of 80 mph (130 km/h) on standard gauge railway track (1.435 m). It runs as a chartered train for the Union Pacific steam program, hauling freight cars.

The 4014 'Big Boy' is a class 4884-1 class locomotive, with four-wheel leading truck, two sets of eight driving wheels, and four-wheel trailing truck (4884 Whyte notation). This wheel configuration has been used only by the Union Pacific 'Big Boy' steam locomotives, which had been built between 1941 and 1944 by the American Locomotive Company (ALCO).

Specifications

Type: steam locomotive

Weight: 386.12 tons

Length: 26.11 m (85 feet, 8 inches)

Width: 3.35 m (11 feet)

Fuel: coal (originally); today fuel oil.

Firebox Volume: 14 m3 (150 square feet)

Boiler Pressure: 2.1 MPa

Cylinders: 4

Valve Type: piston valves

Brake: pneumatic.

Below, photo of UP 4014 steam locomotive in 1948.

The 4014 Big Boy steam locomotive (video)

Volvo Flygmotor RM6C

The Volvo Flygmotor RM6C was a turbojet engine developed from the British Rolls Royce Avon 300 machine. It was built in Sweden in large numbers, under license, to power the Saab J35 Draken combat aircraft. It was a powerful and reliable engine, capable of delivering 17,700 pounds of thrust, enabling the aircraft to fly at Mach 2 (1,350 miles). It was fitted with a Swedish-designed Model 67 afterburner.

Technical Description

The intake of the Volvo Flygmotor RM6C was made of a magnesium annular casing, with variable incidence inlet guide vanes, which were hydraulically-actuated. The engine compressor was a 16-stage axial unit, with rotor blades pinned to the rotor discs, which, in turn, were splined to a shaft. Both the stator and rotor blades were of aluminum alloys. The compressor shaft was both roller and ball-bearing mounted. The front compressor casing was cast magnesium alloy, and it carried the stage 0 to 6 stators. The intermediate casing was cast in aluminum alloy, and carried the stage 7 through 9 stators, while the fabricated steel outlet carried the stage 10 through 15 stators.

The turbine of the Volvo Flygmotor RM6C was a two-stage design, with the turbine shaft being coupled to the compressor shaft. High-pressure and low-pressure blades were both of Nimonic alloys. All turbine blades were shrouded at their tips. The combustion chamber was of the annular type, which was fitted with eight flame tubes and duplex burner. There were interconnecting pipes between each pair of flame tubes. The nozzle casing, of cast chrome steel, had two rows of guide vanes, with one being mounted ahead of each turbine stage.

The accessory drives for the lubrication, fuel pumps, and electrical power equipment consisted of two horizontal power take-off shafts, with one on each side of the engine. They were driven by gearing on the main shaft, just aft of the center bearing assembly. The lubrication system, with its four scavenge pumps, was of the closed circuit type, with lubrication being provided for the three main bearings, the starter reduction gear, and the ancillary drive system.

The Volvo Flygmotor RM6C was started by a Plessy iso-propylnitrate fluid starter, which was mounted in the intake bullet. The starter fluid tank had a capacity for three starts. It was integrated with an automatic high-energy ignition system. A related torch ignition system for the afterburner was also automatically controlled. The afterburner had infinitely variable flap-type nozzle driven by screw-jacks. It had V-type flame holders with fuel injection manifold located at the front of the V.

Below, a photograph of the Volvo Flygmotor RM6C turbojet engine taken out the aircraft fuselage rear section. It was unusual in having an extended exhaust and an angled afterburner.

Cutaway drawing the RM6C

Jet Engine Rotor

The jet engine rotor is a rotating spool which turns airfoils-containing disks around at high speed. It compresses incoming air from the air intake and converts the hot gas flow from the combustion chamber into mechanical shaft power, called torque. Thus, the rotor of a jet engine is essentially composed of two parts: the compressor and the turbine, both sharing the same shaft or spool. Therefore, the rotor is the fundamental part in an axial-flow type engine. In AC power generation, a jet engine rotor spool is attached to a generator rotor shaft, driving it around at high speed in the stator to generate AC electricity.

Function

The main functions of a jet engine rotor structure are airfoil retention, air compression, torque transmission, and provision of inner flow path surface. The rotor structure must be capable of withstanding the centrifugal forces from the rotor mass inertia which results from the high speed rotation. Beside these mechanical functions, the rotor must satisfy aero-mechanical design requirements as well. Geometrically, the rotor parts are axisymmetric structures, which consist of a combination of airfoils-containing disks, shafts, spacers, and rotating seals.

In most jet engine rotors, the performance requirements dictate the number of airfoil stages needed, which in turn dictate the number of corresponding number of disks. Rotors are supported from the static frames by bearings and shafts. Long rotor spools must be supported at both ends due to system dynamics and flight maneuver loads. Rotors, such as the one in the General Electric CF6-80C2 turbofan engine, are cantilever design with bearings at only one end of the rotor.

Below, a picture of a turbojet engine shows the rotor, with its two essential parts: the axial flow compressor and the turbine, sharing the same spool, or shaft. You can see that the compressor is separated from the turbine by the can-type combustion chamber.

Siemens SST-700

The Siemens SST-700 is a dual-casing, geared, high-pressure steam turbine. Designed and produced by Siemens, it is employed for alternating current generation at gas/biomass combined cycle and solar-thermal power plants. It is composed of a geared high pressure (HP) turbine module and a low pressure (LP) turbine module. They both drive an AC generator set up in between

Although each module of the Siemens SST-700 can be combined for optimal configuration, they can also be used independently. This steam turbine also has other applications, such as mechanical drives, heat recovery, and waste incineration plants. This axial-flow machine, together with the generator, can deliver 175 MW.

Technical data (specifications)

Length: 22 m (73 ft)

Width: 15 m (59 ft)

Height: 6 m (20 ft)

Power output: up to 175 MW

Rotational speed: 3,000 – 13,200 rpm

Inlet pressure (with reheat): up to 165 bar/165 psi

Inlet temperature (with reheat): up to 585 degrees Celcius/1085 degrees F.

Exhaust (back) pressure: up to 40 bar (580 psi)

Condensing exhaust pressure: up to 0.6 bar (8.5 psi)

Controlled extraction: up to 40 bar, and up to 415 degrees Celcius/780 degrees F.

Below, the rotor of the SST-700 turbine.

Geared Turbine

A geared turbine is an axial-flow steam engine attached to a set of reduction gears. It has been used to power freight and military ships since the beginning of the 20th century. It was invented by Charles Parsons, who designed the first steam turbine with speed reducing gears in 1896. It was a small 10-HP geared steam turbine that drove a small, 22-foot-long boat on an experimental basis. The gear reduction he had conceived was 14 to 1, with the prop running at 1,400 rmp. Although the speed of the boat was only 9 mph, the performance was good as it encouraged him to keep working on the project.

Charles Parsons would design a double helical gear in 1909, with a speed reduction ratio of 19 to 9, to drive a single prop with a high-pressure (HP) turbine driving one side of the large central gear and a low-pressure (LP) turbine propelling the other side of the gear. This gear was connected to a steam engine on a cargo vessel. The steam that drove the turbine was produced by coal-fired or oil-fired boilers. This type of steam turbine would also be used for power generation as it would be attached to an AC generator.

Charles Parsons had found out that steam turbine could be made smaller but capable of rotating at much higher rpm if a speed reducing gear could successfully be made. He was able to design and produce one. Thus, the geared steam turbine would be used as the propulsion systems of both merchant and warships, especially to power battleships and aircraft carriers in the first half of the 20th century.

Below, a steam turbine, which used a set of reduction gears, on an open museum.