THE AXIAL FLOW COMPRESSOR COMPROMISE

THE AXIAL FLOW COMPRESSOR COMPROMISE. 1. Introduction.

For a jet engine to work air comes in through the intake of the engine, it is mixed with fuel then ignited. The hot accelerated air is then expelled through the exhaust of the engine to provide thrust. The power produced is directly proportional to the fuel burned, while the fuel burned is directly proportional to the amount of air present at the time of  combustion. Thus air at normal atmospheric pressure will not provide the power needed for the aircraft to function.Therfore there is a need for a compressor to provide the correct fuel to air ratio for sufficient burning of the fuel thus providing the power needed to fly the aircraft. This led to the search of an appropriate compressor which turned out to be one of the major stumbling blocks of the early jet engines. The axial compressor, although complex, does this compression work easily and efficiently, efficiently, but as it is with machines, it’s not perfect. In this report we will investigate why it is not 100% efficient and the compromise engineers and designers have to make in order for these types of  compressors to function. To To do this, the report is split into two main sections, the description of the compressor, and its design where the compromise arises.

2. Description of axial flow compressors. Axial compressors as seen in fig 1 below, below, are rotating aerofoil based compressors found between the inlet turbine and the exhaust blades where the working fluid, in this case air, flows parallel to the axis of rotation thus the t he name axial flow.

Fig 1 shows the two basic components of an axial flow compressor where the rotor blades are on the left while the stator vanes are in the centre. Both the rotors and stators are assembled to form the compressor on the left. (Sweet haven 2008)

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The axial compressor consists of stationary parts and rotating parts. A shaft shaft that is connected to the turbine drives a central drum retained by bearings. On these bearings are a number of annular rows attached to them. These annular rows alternate between rotating rows known as rotors and stationary rows known as stators or vanes. For the compromise made by engineers and designers to be fully understood, understanding how the compressor functions is essential. Air enters the engine from the intake and is then directed to the compression stage. In some modern aircraft engines, there will be inlet guide vanes which will be discussed later on. The rotor blades spin at a certain velocity tangential to the flow of air. This increases its velocity and decreases its pressure of the air in accordance to Bernoulli’s principle which we will look at in depth. 2.1 Bernoulli’s principle This is a fluid law which states that the pressure of a fluid varies inversely with the speed. An increase in the speed producing a decrease in pressure (such as a drop in hydraulic pressure as the fluid speeds up following a constriction in the pipe) and vice versa. (Farlex 2008).this can be demonstrated in fig 2 below.

Fig 2 shows the Bernoulli’s principle in a venturi where the fluid entering the venture has a high pressure and a low velocity but in the constricted part of the tube the velocity increases while the pressure deacreses.The energy of the fluid however remains a constant all through the tube.(Yeisicanscience 2008)

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The above diagram can be explained by the following equation; Ρ + (k.e / v) = constant. Where p is the pressure of the fluid, k.e is the kinetic energy of the fluid while v is the volume of the fluid passing through the venturi. (Grace, Lovett and Dobson 2002) The increase in velocity of the fluid will in turn increase its kinetic energy, energy, thus for one to maintain the constant the pressure will have to decrease and vice versa. In the compressors case, the air has an increase in velocity after the first stage of  rotor blades therefore a decrease in pressure. Since the rotors are spinning at a high velocity, velocity, normaly between 3500 to 6000 rpm, the air loses its initial angle of attack. The stator vanes, whose main purpose is to diffuse the air from the rotor blades increasing the pressure of the air even more, therefore have to guide the air into the next stage of  rotors. This is done by swirling the airflow into the direction of rotor rotation. Thus the velocity of the air “see-saws” up and down whereas the pressure constantly inceases.This process continues until the last stage of compression where air is mixed with fuel and ignited to produce power. The key to success when using these types of  compressors is in the design of the compressor.

3. Design of an axial flow compressor. An axial compressor is design according to its requirements. Compressors are designed with the objective of minimising the aerodynamic aerodynamic loss of pressure thus achieving an acceptable level of thermodynamic efficiency through its stages, reduced weight of the compressor components and an adequate surge margin to avoid stalling of  the engine.Theoreticaly it sounds simple enough but with the reduction of weight, comes the decrease of thermodynamic efficiency of the compressors while the increase in efficiency will in turn decrease the surge margin which increases the chances of the t he engine to stall. Because of these three factors and their t heir influence on each other, a compromise has to be made among them for optimisation of the engines performance.(Chen,Sun and Wu 2004).

3.1 Aerodynamic design of a compressor. compressor. The building block of the aerodynamic design of these types of compressors is the cascades, a seemingly endless repeating array of aerofoils. The blade which is what works on the air is a key component. We need to distinguish the blade direction which here is denoted as β and the direction which the air flows as, α.The angle between the

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blade and the flowing air is known as the angle of incidence represented by ί. Therefore the equation is:

ί = α1 – β1 Where α1 is the air getting in whereas β1 is the blade angle the air encounters first. The corresponding angle between the flow of air leaving the blades and the blade outlet angle is given by:

δ = α2 – β2 Where α2 is the angle of air leaving the blade while β2 is the angle of the blade at the outlet. The flow does not turn quit as much as the blade thus the deviation which is

δ is

a positive value. A number of factors affect the deviation mainly, angle of incidence, the inlet air speed, the design of the rotor blades, just to mention a few. few. An indication of one of the many problems this type of compressors have is the very narrow range of incidence for which thermodynamic losses is small. When the loss starts to rise rapidly, rapidly, the engine stalls and hence surges. Therefore the angle of  incidence should be closer to zero. This proves to be a stumbling block for designers because when

α1 is 0o, the rotor blades turn the flow to about 20o;the inlet turbine, o

which as we saw is before the first stage of compressor blades, turns the inlet flow of 0 o

o

incidence to about 63 and even 90 on these modern turbine blades. This greatly increases α1 which will lead to stall. (Cumpsty 2001).This problem led to the use of inlet guide vanes (IGVs) as a solution.

3.1.1 Inlet guide vanes. IGVs are basically stationary aerofoil shaped blades that are placed before the first stage of the axial compressors. Their main purpose is to straighten the flow of air  into a suitable angle of incidence for the rotor blades, thus increasing the airs compressibility hence the efficiency of the compressor. Fig 3 on the next page shows a cross section of an axial compressor showing the IGVs, rotor and stator blades. Note the direction in which the aerofoils are pointing. Both the upper camber of the blades of the IGVs and rotors are facing away from the direction of rotation while the stators upper  camber is facing opposite the rotors, towards the direction of rotation. This is to increase the chances of the air being swirled or controlled to enter t he next stage of rotors by providing an opposite aerodynamic effect from the rotors.

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Fig 3 shows the placement of the IGVs, rotors and stator blades. The direction air is flowing and the direction the rotors are moving. (Wikipedia 2008)

Going back to the blade angle, it has already been established that the angle of air  leaving a compressor stage should be less than the t he inlet angle of incidence. Thus

α2 < α1 Taking the velocity of the rotor blades as Vx, the velocity of the air V air V1 at the inlet of the compressor stage will be given by; V1 = Vxcosα1 Where α1 is the angle of attack of  attack of the air. So then the velocity of the air leaving the stage of compressors will be; V2 = Vxcosα2 It is important to note that the velocity of air travelling in a radial movement is small therefore negligable.It also follows that; V1 / V2 = cosα2 / cosα1 And since α2 < α1, it also follows that V2 < V1 thus the flow, although “see-sawing” up and down, is decelerated across each stage of compression. Again Again by Bernoulli’s law, pressure increases. Other additions like increasing the hub diameter have been applied.

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3.1.2. Hub diameter  Another way engineers and designers have improved thermodynamic efficiency, efficiency, is the use of Bernoulli’s principle but instead of varying the velocity, velocity, this time they t hey are increasing the diameter of the hub. Bernoulli’s formula states that; p*A*t = constant Where p is the pressure of the air, A is the cross sectional area of the air passage whereas t is the temperature of the air. As the diameter of the hub increases its cross sectional area does. This in turn reduces the area of the air passage thus to maintain the energy constant, pressure and temperature subsequently increase. (Flack 2005).these improvements increase the efficiency of the compressor but it also becomes more likely to stall and hence surge. 3.2 Compressor surge margin. Compressor surge is normally caused by a complex combination of factors. The most common of these factors is fairly simple. Each blade acts like I miniature plane wing which when subjected to a high angle of attack on oncoming air, air, the blade will stall. Also caused by the pilling of air in the high pressure regions of the compressor which is caused by over compression of air therefore the line that separates super efficiency and zero efficiency and thus stall, is called the surge margin. This problem of air pilling in previous stages can be minimised by reducing the pressure ratio across the compressor stages by giving off airflow. airflow. This can be done efficiently and easily by bleeding air from the middle or towards the end of the compressor stages.BVs (bleed valves) are the solution engineers came up with. As stated above there main function is to release some of the airflow when the ambient pressure of the air exceeds the required pressure preventing choking in the high pressure regions of the compressor thus avoid stalling at the inlet. some BVs are large apertures cut into the casing of the compressor and sealed by a shut off valve of either  the sliding gate type or an outward moving piston normally held against its seating by a spring, while in some engines part or all of the periphery right r ound the casing between the two stages, is perforated by quite large holes which are normally closed by a close fitting ring slid axially towards the front and rear of the engine. (Gunston.B 2008)

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To further explain the above principle, one can use a fairl y simple model illustrated clearly by Hill and Peterson (1992) using pressure coefficients,C coefficients,Cp which is defined as;

Cp= Δp /0.5*pi*ωi2 Where Δp is the change in pressure across a stage, pi is the initial or starting pressure of that stage and ωi is the inlet relative velocity of air in that stage. Thus by dividing the change in pressure of the air by half the initial pressure multiplied by the inlet relative velocity of air one will obtain Cp of the blades in that stage. 4. Conclusion. All these extra additions like valves and extra guide vanes in crease the weight of the compressor an even more the cost of manufacturing the component. But off all the considerations engineers have to make, the thermodynamic efficiency is the most important in that it determines the performance of the engine. compressors are designed for different conditions for example in fighter jets, the compressors are made to be super  efficient for maximum power, power, while in passenger aircrafts there made as light and safe as possible by avoiding the stall margin meaning reducing the efficiency of the compressor. compressor. Major problems of the axial compressor that engineers encountered in thee past are now made easier to solve and understand by the use of computer software and decades of development and improvement in technology. technology. the axial compressor is the most frequently used in jet engines, chances of an easier and more efficient type of  compressor to be developed for future usage are slim but one can never be accurate in predicting technology.

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References. 1. Chen.L, Chen.L, Sun.F Sun.F and Wu.C, Wu.C, (2004).Opti (2004).Optimum mum design design for a sub sonic sonic axial axial compressor stage. Applied Energy. Volume 80. (187-195). 2. Cumsty Cumsty.N. .N. (2001).Jet (2001).Jet Propultion. Propultion.Camb Cambridge ridge.Camb .Cambridge ridge university university press. 3.

Farlex (2008).Online Encyclopaedia.