Axial compressor
An axial compressor is a type of gas compressor that continuously pressurizes a working fluid. It is a rotating, airfoil-based device in which the fluid flows primarily in one direction parallel to the axis of rotation. This distinguishes axial compressors from other types of rotating compressors, such as centrifugal compressors, axi-centrifugal compressors, and mixed-flow compressors, in which the fluid flowing includes a significant radial component directed outward from the axis of rotation.
The rotor blades exert torque on the fluid, increasing its energy as it passes through the compressor. The stationary blades, or stators, slow the fluid, converting the circumferential component of flowing into pressure. Compressors are typically driven by an electric motor, a steam turbine, or a gas turbine.
Axial flow compressors produce a continuous flow of compressed gas. They exhibit high efficiency and large mass flow rate in relation to the size of their cross-section. They are more intricate and expensive relative to other designs, requiring several rows of airfoils to achieve a large pressure rise.
Axial compressors are integral to the design of large gas turbines such as jet engines, high speed ship engines, and small scale power stations. They are also used in industrial applications such as large volume air separation plants, blast furnace air, fluid catalytic cracking air, and propane dehydrogenation. Additionally, due to their performance and operability across the flight envelope, they are widely used in aerospace applications.
| Typical application | Type of flow | Pressure ratio per stage | Efficiency per stage |
| Industrial | Subsonic | 1.05–1.2 | 88–92% |
| Aerospace | Transonic | 1.15–1.6 | 80–85% |
| Research | Supersonic | 1.8–2.2 | 75–85% |
Description
Axial compressors consist of alternating rows of rotating and stationary airfoils. The rotating airfoils, or rotors, are mounted on a rotating drum, which is driven by a central shaft. The stationary airfoils, known as vanes or stators, are mounted to a tubular casing. One row of rotors and one row of stators together form a stage. The rotors impart kinetic energy to the working fluid, increasing its velocity in the axial and circumferential directions. The stators convert the increased kinetic energy into static pressure through diffusion and redirect the flow direction of the fluid to prepare it for the rotor blades of the next stage. The area between the rotor drum and casing decreases along the flow direction to maintain an optimum Mach number as the fluid is compressed.Rotor passages can be designed to achieve diffusive pressure rise, leading to a higher pressure rise per stage. The ratio of pressure rise in the rotor to the total pressure rise of the stage is known as the degree of reaction.
Design
The increase in pressure produced by a single stage is limited by the relative velocity between the rotor and the fluid, and the turning and diffusion capabilities of the airfoils. A typical stage in a commercial compressor will produce a pressure increase of between 15% and 60% at design conditions with a polytropic efficiency in the region of 90–95%. To achieve different pressure ratios, axial compressors are designed with different numbers of stages and rotational speeds. As a rule of thumb, it can be assumed that each stage in a given compressor has the same temperature rise. Therefore, at the entry, temperature to each stage must increase progressively through the compressor and the ratio / entry must decrease, thus implying a progressive reduction in stage pressure ratio through the unit. Hence the rear stage develops a significantly lower pressure ratio than the first stage. Higher stage pressure ratios are also possible if the relative velocity between fluid and rotors is supersonic, but this is achieved at the expense of efficiency and operability. Such compressors, with stage pressure ratios of over 2, are only used where minimizing the compressor size, weight or complexity is critical, such as in military jets.The airfoil profiles are optimized and matched for specific velocities and turning. Although compressors can be run at other conditions with different flows, speeds, or pressure ratios, this can result in an efficiency penalty or even a partial or complete breakdown in flow. Thus, a practical limit on the number of stages, and the overall pressure ratio, comes from the interaction of the different stages when required to work away from the design conditions. These “off-design” conditions can be mitigated to a certain extent by providing some flexibility in the compressor. This is achieved normally through the use of adjustable stators or with valves that can bleed fluid from the main flow between stages. Modern jet engines use a series of compressors, running at different speeds; to supply air at around 40:1 pressure ratio for combustion with sufficient flexibility for all flight conditions.
Kinetics and energy equations
The law of moment of momentum states that the sum of the moments of external forces acting on a fluid which is temporarily occupying the control volume is equal to the net change of angular momentum flux through the control volume.The swirling fluid enters the control volume at radius,, with tangential velocity,, and leaves at radius,, with tangential velocity,.
The rate of change of momentum,, is given by:
Power consumed by an ideal moving blade, P is given by the equation:
Change in enthalpy of fluid in moving blades:
Therefore,
which implies,
Isentropic compression in rotor blade,
Therefore,
which implies
Degree of Reaction,
The pressure difference between the entry and exit of the rotor blade is called reaction pressure. The change in pressure energy is calculated through degree of reaction.
Therefore,
Performance characteristics
Steady-state performance
Axial compressor performance is shown on a compressor map, also known as a characteristic, by plotting pressure ratio and efficiency against corrected mass flow at different values of corrected compressor speed.Axial compressors, particularly near their design point are usually amenable to analytical treatment, and a good estimate of their performance can be made before they are first run on a rig. The compressor map shows the complete running range, i.e. off-design, of the compressor from ground idle to its highest corrected rotor speed, which for a civil engine may occur at top-of-climb, or, for a military combat engine, at take-off on a cold day. Not shown is the sub-idle performance region needed for analysing normal ground and in-flight windmill start behaviour.
The performance of a single compressor stage may be shown by plotting stage loading coefficient as a function of flow coefficient
Stage pressure ratio against flow rate is lower than for a no-loss stage as shown. Losses are due to blade friction, flow separation, unsteady flow and vane-blade spacing.
Off-design operation
The performance of a compressor is defined according to its design. But in actual practice, the operating point of the compressor deviates from the design point; this is known as off-design operation.from equation and
The value of doesn't change for a wide range of operating points till stalling. Also because of minor change in air angle at rotor and stator, where is diffuser blade angle.
Representing design values with
for off-design operations :
for positive values of J, slope of the curve is negative and vice versa.
Instabilities
There are multiple modes of instability an axial compressor could undergo. Edward Greitzer used a Helmholtz resonator type of compression system model to predict the transient response of a compression system after a small perturbation superimposed on a steady operating condition. He found a non-dimensional parameter which predicted which mode of compressor instability, rotating stall or surge, would result. The parameter used the rotor speed, the Helmholtz resonator frequency of the system, and an "effective length" of the compressor duct. These produced a critical value which predicted either rotating stall or surge where the slope of pressure ratio against flow changed from negative to positive.Surge
In the plot of pressure-flow rate, the line separating graph between two regionsunstable and stableis known as the surge line. This line is formed by joining surge points at different rotational speeds. Surging is the unstable flow in axial compressors due to complete breakdown of the steady through flow. This phenomenon affects the performance of compressor and is undesirable.Surge cycle
The following explanation for surging refers to running a compressor at a constant speed on a rig and gradually reducing the exit area by closing a valve. What happens, i.e. crossing the surge line, is caused by the compressor trying to deliver air, still running at the same speed, to a higher exit pressure. When the compressor is operating as part of a complete gas turbine engine, as opposed to on a test rig, a higher delivery pressure at a particular speed can be caused momentarily by burning too-great a step-jump in fuel which causes a momentary blockage until the compressor increases to the speed which goes with the new fuel flow and the surging stops.Suppose the initial operating point D at some rotational speed N. On decreasing the flow-rate at same rotational speed along the characteristic curve by partial closing of the valve, the pressure in the pipe increases which will be taken care by increase in input pressure at the compressor. With further increase in pressure up to point P, compressor pressure will increase. Further moving towards left keeping rotational speed constant, pressure in pipe will increase but compressor pressure will decrease leading to back air flow towards the compressor. Due to this back flow, pressure in pipe will decrease because this unequal pressure condition cannot stay for a long period of time. Though valve position is set for lower flow rate say point G but compressor will work according to normal stable operation point say E, so path E–''F–P''–G–''E will be followed leading to breakdown of flow, hence pressure in the compressor falls further to point H''. This increase and decrease of pressure in pipe will occur repeatedly in pipe and compressor following the cycle E–''F–P''–G–''H–E'' also known as the surge cycle.
This phenomenon will cause vibrations in the whole machine and may lead to mechanical failure. That is why left portion of the curve from the surge point is called unstable region and may cause damage to the machine. So the recommended operation range is on the right side of the surge line.