Surge in compressors

Last updated

Compressor surge is a form of aerodynamic instability in axial compressors or centrifugal compressors. The term describes violent air flow oscillating in the axial direction of a compressor, which indicates the axial component of fluid velocity varies periodically and may even become negative. In early literature, the phenomenon of compressor surge was identified by audible thumping and honking at frequencies as low as 1 Hertz, pressure pulsations throughout the machine, and severe mechanical vibration. [1]

Contents

Description

Compressor surge can be classified into deep surge and mild surge. Compressor surge with negative mass flow rates is considered as deep surge while the one without reverse flows is generally termed mild surge. [2] On a performance map, the stable operating range of a compressor is limited by the surge line. Although the line is named after a surge, technically, it is an instability boundary which denotes onsets of discernible flow instabilities, such as compressor surge or rotating stall. [3] When the mass flow rate drops to a critical value at which discernible flow instabilities take place, nominally, the critical value should be determined as a surge mass flow rate on a constant speed line; however, in practice, the surge line on a performance map is affected by specific criteria adopted for determining discernible flow instabilities.

..A typical compressor performance map Compressor map.gif
..A typical compressor performance map

Effects

Compressor surge is catastrophic for the compressor and the whole machine. When compressor surge happens, the operating point of a compressor, which is usually denoted by the pair of the mass flow rate and pressure ratio, orbits along a surge cycle on the compressor performance map. The unstable performance caused by compressor surge is not acceptable to machines on which a compressor is mounted to ventilate or dense air. In addition to affecting performance, compressor surge is also accompanied with loud noises. Frequencies of compressor surge can range from a few to dozens Hertz depending on the configuration of a compression system. [4] Although Helmholtz resonance frequency is often employed to characterize the unsteadiness of mild surge; it was found that Helmholtz oscillation did not trigger compressor surge in some cases. [5] [6] Another effect of compressor surge is on solid structure. Violent flows of compressor surge repeatedly hit blades in the compressor, resulting in blade fatigue or even mechanical failure. While fully developed compressor surge is axisymmetric, its initial phase is not necessarily axisymmetric. Actually, severe damage of compressor surge is often related to very large transverse loads on blades and casing in its initial transient. [7] A chain reaction of compressor surge is the flameout of a jet engine. Due to a lack of air intake in the case of compressor surge, there will be unburnt fuel in the combustion chamber, and that unburnt fuel will burn and cause flameout near the exit of the engine where oxygen is sufficient.

Causes

In most low-speed and low-pressure cases, rotating stall comes prior to compressor surge; [8] [9] however, a general cause-effect relation between rotating stall and compressor surge has not been determined yet. [6] On a constant speed line of a compressor, the mass flow rate decreases as the pressure delivered by the compressor gets higher. Internal flows of the compressor are in a very large adverse pressure gradient which tends to destabilize the flow and cause flow separation. A fully developed compressor surge can be modeled as a one-dimensional global instability of a compression system which typically consists of inlet ducts, compressors, exit ducts, gas reservoir, and throttle valve. [10] [11] A cycle of compressor surge can be divided into several phases. [12] If the throttle valve is turned to be a very small opening, the gas reservoir would have a positive net flux. The pressure in the reservoir keeps increasing and then exceeds the pressure at compressor exit, thus resulting in an adverse pressure gradient in exit ducts. This adverse pressure gradient naturally decelerates flows in the whole system and reduces the mass flow rate. The slope of a constant speed line near surge line is usually zero or even positive, which implies that the compressor cannot provide a much higher pressure as lowering the mass flow rate. Thus, the adverse pressure gradient could not be suppressed by the compressor and the system would rapidly involve an overshoot of adverse pressure gradient which would dramatically reduce the mass flow rate or even cause flows to reverse. On the other hand, the pressure in the reservoir would gradually drop due to less flux delivered by the compressor, thus rebuilding a favorable pressure gradient in exit ducts. And then the mass flow rate would be recovered, and the compressor is back to work on a constant speed line again, which would eventually trigger the next surge cycle. Therefore, compressor surge is a process which keeps breaking the flow path of a compression system down and rebuilding it. [13] Several rules of thumb can be inferred from the interpretation above. Compressor surge in a system with a small gas reservoir is high-frequency and low-amplitude whereas a large gas reservoir leads to low-frequency and high-amplitude compressor surge; another rule of thumb is that compressor surge happens in a compressor with a large external volume and compressor stall tends to show up in a system with a short exit duct. It is also worth noting that the surge line of a compressor can have small variations in different systems, such as a test bench or an engine. [14]

Schematic of compressor control instrumentation Compressor control.jpg
Schematic of compressor control instrumentation

Preventing surge

In the oil and gas industry the operation of gas compressors in surge conditions is prevented by instrumentation around the compressor. [15] The measured flow rate of gas (FT) in the compressor suction line together with the suction pressure (PT), and sometimes the suction temperature (TT) and the pressure (PT) in discharge line is fed into the surge controller. Algorithms in the controller use the data to establish the performance of the machine; the data identifies the operating point in terms of the flow and the developed head. When the compressor’s operation approaches the surge point the controller modulates either a flow control valve (FCV) in the recycle line or adjusts the speed (SC) of the compressor driver. The FCV allows cooled gas from the discharge to spill back to the suction of the compressor, thereby maintaining the forward flow of gas through the machine. The recycle line is ideally located to take cooled gas from downstream of the compressor after-cooler and to discharge it into the feed to the compressor suction drum. [16]

See also

Related Research Articles

<span class="mw-page-title-main">Turbofan</span> Airbreathing jet engine designed to provide thrust by driving a fan

A turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a combination of the preceding generation engine technology of the turbojet, and a reference to the additional fan stage added. It consists of a gas turbine engine which achieves mechanical energy from combustion, and a ducted fan that uses the mechanical energy from the gas turbine to force air rearwards. Thus, whereas all the air taken in by a turbojet passes through the combustion chamber and turbines, in a turbofan some of that air bypasses these components. A turbofan thus can be thought of as a turbojet being used to drive a ducted fan, with both of these contributing to the thrust.

<span class="mw-page-title-main">Turbojet</span> Airbreathing jet engine which is typically used in aircraft

The turbojet is an airbreathing jet engine which is typically used in aircraft. It consists of a gas turbine with a propelling nozzle. The gas turbine has an air inlet which includes inlet guide vanes, a compressor, a combustion chamber, and a turbine. The compressed air from the compressor is heated by burning fuel in the combustion chamber and then allowed to expand through the turbine. The turbine exhaust is then expanded in the propelling nozzle where it is accelerated to high speed to provide thrust. Two engineers, Frank Whittle in the United Kingdom and Hans von Ohain in Germany, developed the concept independently into practical engines during the late 1930s.

<span class="mw-page-title-main">Centrifugal compressor</span> Sub-class of dynamic axisymmetric work-absorbing turbomachinery

Centrifugal compressors, sometimes called impeller compressors or radial compressors, are a sub-class of dynamic axisymmetric work-absorbing turbomachinery.

<span class="mw-page-title-main">Compressor</span> Machine to increase pressure of gas by reducing its volume

A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor.

<span class="mw-page-title-main">Axial compressor</span> Machine for continuous flow gas compression

An axial compressor is a gas compressor that can continuously pressurize gases. It is a rotating, airfoil-based compressor in which the gas or working fluid principally flows parallel to the axis of rotation, or axially. This differs from other rotating compressors such as centrifugal compressor, axi-centrifugal compressors and mixed-flow compressors where the fluid flow will include a "radial component" through the compressor.

<span class="mw-page-title-main">Compressor stall</span> Gas turbine phenomenon

A compressor stall is a local disruption of the airflow in the compressor of a gas turbine or turbocharger. A stall that results in the complete disruption of the airflow through the compressor is referred to as a compressor surge. The severity of the phenomenon ranges from a momentary power drop barely registered by the engine instruments to a complete loss of compression in case of a surge, requiring adjustments in the fuel flow to recover normal operation.

<span class="mw-page-title-main">Turbomachinery</span> Machine for exchanging energy with a fluid

Turbomachinery, in mechanical engineering, describes machines that transfer energy between a rotor and a fluid, including both turbines and compressors. While a turbine transfers energy from a fluid to a rotor, a compressor transfers energy from a rotor to a fluid. It is an important application of fluid mechanics.

<span class="mw-page-title-main">Centrifugal pump</span> Pump used to transport fluids by conversion of rotational kinetic energy

Centrifugal pumps are used to transport fluids by the conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow. The rotational energy typically comes from an engine or electric motor. They are a sub-class of dynamic axisymmetric work-absorbing turbomachinery. The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward into a diffuser or volute chamber (casing), from which it exits.

A compressor map is a chart which shows the performance of a turbomachinery compressor. This type of compressor is used in gas turbine engines, for supercharging reciprocating engines and for industrial processes, where it is known as a dynamic compressor. A map is created from compressor rig test results or predicted by a special computer program. Alternatively the map of a similar compressor can be suitably scaled. This article is an overview of compressor maps and their different applications and also has detailed explanations of maps for a fan and intermediate and high-pressure compressors from a three-shaft aero-engine as specific examples.

A jet engine performs by converting fuel into thrust. How well it performs is an indication of what proportion of its fuel goes to waste. It transfers heat from burning fuel to air passing through the engine. In doing so it produces thrust work when propelling a vehicle but a lot of the fuel is wasted and only appears as heat. Propulsion engineers aim to minimize the degradation of fuel energy into unusable thermal energy. Increased emphasis on performance improvements for commercial airliners came in the 1970s from the rising cost of fuel.

<span class="mw-page-title-main">Vapor-compression refrigeration</span> Refrigeration process

Vapour-compression refrigeration or vapor-compression refrigeration system (VCRS), in which the refrigerant undergoes phase changes, is one of the many refrigeration cycles and is the most widely used method for air conditioning of buildings and automobiles. It is also used in domestic and commercial refrigerators, large-scale warehouses for chilled or frozen storage of foods and meats, refrigerated trucks and railroad cars, and a host of other commercial and industrial services. Oil refineries, petrochemical and chemical processing plants, and natural gas processing plants are among the many types of industrial plants that often utilize large vapor-compression refrigeration systems. Cascade refrigeration systems may also be implemented using two compressors.

<span class="mw-page-title-main">Gas turbine engine compressors</span> Engine component

As the name suggests, gas turbine engine compressors provide the compression part of the gas turbine engine thermodynamic cycle. There are three basic categories of gas turbine engine compressor: axial compressor, centrifugal compressor and mixed flow compressor. A fourth, unusual, type is the free-piston gas generator, which combines the functions of compressor and combustion chamber in one unit.

<span class="mw-page-title-main">Centrifugal fan</span> Mechanical fan that forces fluid to move radially outward

A centrifugal fan is a mechanical device for moving air or other gases in a direction at an angle to the incoming fluid. Centrifugal fans often contain a ducted housing to direct outgoing air in a specific direction or across a heat sink; such a fan is also called a blower, blower fan, or squirrel-cage fan. Tiny ones used in computers are sometimes called biscuit blowers. These fans move air from the rotating inlet of the fan to an outlet. They are typically used in ducted applications to either draw air through ductwork/heat exchanger, or push air through similar impellers. Compared to standard axial fans, they can provide similar air movement from a smaller fan package, and overcome higher resistance in air streams.

In fluid dynamics, flow can be decomposed into primary flow plus secondary flow, a relatively weaker flow pattern superimposed on the stronger primary flow pattern. The primary flow is often chosen to be an exact solution to simplified or approximated governing equations, such as potential flow around a wing or geostrophic current or wind on the rotating Earth. In that case, the secondary flow usefully spotlights the effects of complicated real-world terms neglected in those approximated equations. For instance, the consequences of viscosity are spotlighted by secondary flow in the viscous boundary layer, resolving the tea leaf paradox. As another example, if the primary flow is taken to be a balanced flow approximation with net force equated to zero, then the secondary circulation helps spotlight acceleration due to the mild imbalance of forces. A smallness assumption about secondary flow also facilitates linearization.

<span class="mw-page-title-main">Components of jet engines</span> Brief description of components needed for jet engines

This article briefly describes the components and systems found in jet engines.

Cheng Xu is a Chinese American aerodynamic design engineer and engineering manager. He is a Fellow of the American Society of Mechanical Engineers and a member of the Technical Committee on Energy and Power Systems, IASTED. He also served as a guest editor of International Journal of Rotating Machinery.

Compressor characteristic is a mathematical curve that shows the behaviour of a fluid going through a dynamic compressor. It shows changes in fluid pressure, temperature, entropy, flow rate etc.) with the compressor operating at different speeds.

An axial fan is a type of fan that causes gas to flow through it in an axial direction, parallel to the shaft about which the blades rotate. The flow is axial at entry and exit. The fan is designed to produce a pressure difference, and hence force, to cause a flow through the fan. Factors which determine the performance of the fan include the number and shape of the blades. Fans have many applications including in wind tunnels and cooling towers. Design parameters include power, flow rate, pressure rise and efficiency.

Three-dimension losses and correlation in turbomachinery refers to the measurement of flow-fields in three dimensions, where measuring the loss of smoothness of flow, and resulting inefficiencies, becomes difficult, unlike two-dimensional losses where mathematical complexity is substantially less.

Zoltán S. Spakovszky is an aerospace engineer, academic and researcher. He is best known for his work on fluid system instabilities and internal flow in turbomachinery. He is T. Wilson (1953) Professor in Aeronautics at the Massachusetts Institute of Technology, and the Director of the MIT Gas Turbine Laboratory.

References

  1. H. W. Emmons; C. E. Pearson; H. P. Grant (1955). "Compressor surge and stall propagation". Transactions of the American Society of Civil Engineers. 77: 455–469.
  2. Fink, D. A.; Cumpsty, N. A.; Greitzer, E. M. (1991-06-03). "Surge Dynamics in a Free-Spool Centrifugal Compressor System". Volume 1: Turbomachinery. ASME. doi:10.1115/91-gt-031. ISBN   9780791878989.
  3. Paduano, JD; Greitzer, EM; Epstein, AH (January 2001). "Compression system stability and active control". Annual Review of Fluid Mechanics. 33 (1): 491–517. Bibcode:2001AnRFM..33..491P. doi:10.1146/annurev.fluid.33.1.491. ISSN   0066-4189.
  4. Hafaifa, Ahmed; Rachid, Belhadef; Mouloud, Guemana (2014-10-31). "Modelling of surge phenomena in a centrifugal compressor: experimental analysis for control". Systems Science & Control Engineering. 2 (1): 632–641. doi: 10.1080/21642583.2014.956269 . ISSN   2164-2583.
  5. Day, I. J. (May 1994). "Axial compressor performance during surge". Journal of Propulsion and Power. 10 (3): 329–336. Bibcode:1994JPP....10..329D. doi:10.2514/3.23760. ISSN   0748-4658.
  6. 1 2 Day, I. J. (2015-10-13). "Stall, Surge, and 75 Years of Research". Journal of Turbomachinery. 138 (1): 011001–011001–16. doi:10.1115/1.4031473. ISSN   0889-504X.
  7. A., Cumpsty, N. (2004). Compressor aerodynamics. Krieger Pub. ISBN   978-1575242477. OCLC   824819843.{{cite book}}: CS1 maint: multiple names: authors list (link)
  8. Tan, C.S.; Day, I.; Morris, S.; Wadia, A. (January 2010). "Spike-Type Compressor Stall Inception, Detection, and Control". Annual Review of Fluid Mechanics. 42 (1): 275–300. Bibcode:2010AnRFM..42..275T. doi:10.1146/annurev-fluid-121108-145603. ISSN   0066-4189.
  9. Sundström, Elias; Semlitsch, Bernhard; Mihăescu, Mihai (23 November 2017). "Generation Mechanisms of Rotating Stall and Surge in Centrifugal Compressors". Flow, Turbulence and Combustion. 100 (3): 705–719. doi: 10.1007/s10494-017-9877-z . PMC   6044252 . PMID   30069143.
  10. Greitzer, E. M. (1976). "Surge and Rotating Stall in Axial Flow Compressors—Part I: Theoretical Compression System Model". Journal of Engineering for Gas Turbines and Power. 98 (2): 190–198. doi:10.1115/1.3446138. ISSN   0022-0825.
  11. Greitzer, E. M. (1976). "Surge and Rotating Stall in Axial Flow Compressors—Part II: Experimental Results and Comparison With Theory". Journal of Engineering for Gas Turbines and Power. 98 (2): 199–211. doi:10.1115/1.3446139. ISSN   0022-0825.
  12. Shahin, Ibrahim; Gadala, Mohamed; Alqaradawi, Mohamed; Badr, Osama (2015-06-23). "Large Eddy Simulation for a Deep Surge Cycle in a High-Speed Centrifugal Compressor With Vaned Diffuser". Journal of Turbomachinery. 137 (10): 101007. doi:10.1115/1.4030790. ISSN   0889-504X.
  13. Semlitsch, Bernhard; Mihăescu, Mihai (May 2016). "Flow phenomena leading to surge in a centrifugal compressor". Energy. 103: 572–587. doi:10.1016/j.energy.2016.03.032.
  14. Baines, N. C. (2005). Fundamentals of Turbocharging. Concepts NREC. ISBN   9780933283145.
  15. "Anti Surge Controller Working Principle". instrumentation tool. Retrieved 25 January 2021.
  16. "Controlling Surge in Centrifugal Compressors". Emerson automation. Retrieved 25 January 2021.