Jet engine compressors
Encyclopedia
As the name suggests, Jet engine compressors provide the compression part of the jet engine
thermodynamic cycle.
There are three basic categories of jet engine compressor:
Axial compressors are used in medium to large thrust jet engines, because the alternative centrifugal units would be too heavy and too large in cross-sectional area.
At smaller sizes, the axial blading at the rear of the compressor loses its robustness and is difficult to manufacture accurately. Consequently, manufacturers tend replace the rear stages with a centrifugal unit. The compression system is then said to be axial-centrifugal.
At even smaller sizes, particularly in turboprop/turboshaft applications, the compression system is either a single centrifugal or has two centrifugal units connected in series (i.e. double-centrifugal).
In the future, mixed flow compressors may feature at the small end of the market, particularly in turbofans, where the relatively large diameter across the diffuser of a centrifugal compressor is a significant disadvantage.
At high overall pressure ratios, the compression system is usually split into two units; a Low Pressure (LP) Compressor mounted on one shaft followed by an High Pressure (HP) Compressor mounted on the HP shaft and driven by its own (HP) turbine. On civil turbofans, the first stage of the LP Compressor is often a single stage fan. If the HP Compressor pressure ratio exceeds about 4.5:1, then the unit will probably have variable geometry (i.e. variable stators) on the first few stages, to make the surge line on the compressor map
more shallow, to accommodate the shallow working line.
On their large civil turbofans, Rolls-Royce split the compression system into three: a Fan, an Intermediate Pressure (IP) Compressor and an HP Compressor, each unit being driven by its own turbine unit. The RB199 military turbofan also has this arrangement.
Jet engine
A jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...
thermodynamic cycle.
There are three basic categories of jet engine compressor:
- Axial compressorAxial compressorAxial compressors are rotating, airfoil-based compressors in which the working fluid principally flows parallel to the axis of rotation. This is in contrast with other rotating compressors such as centrifugal, axi-centrifugal and mixed-flow compressors where the air may enter axially but will have...
- Centrifugal compressorCentrifugal compressorCentrifugal compressors, sometimes termed radial compressors, are a sub-class of dynamic axisymmetric work-absorbing turbomachinery.The idealized compressive dynamic turbo-machine achieves a pressure rise by adding kinetic energy/velocity to a continuous flow of fluid through the rotor or impeller...
- Mixed flow compressorMixed flow compressorA mixed flow compressor combines the axial and radial components to produce a diagonal flow unit. The exit mean radius is greater than at the inlet, but the flow tends to exit in an axial rather than radial direction...
Axial compressors are used in medium to large thrust jet engines, because the alternative centrifugal units would be too heavy and too large in cross-sectional area.
At smaller sizes, the axial blading at the rear of the compressor loses its robustness and is difficult to manufacture accurately. Consequently, manufacturers tend replace the rear stages with a centrifugal unit. The compression system is then said to be axial-centrifugal.
At even smaller sizes, particularly in turboprop/turboshaft applications, the compression system is either a single centrifugal or has two centrifugal units connected in series (i.e. double-centrifugal).
In the future, mixed flow compressors may feature at the small end of the market, particularly in turbofans, where the relatively large diameter across the diffuser of a centrifugal compressor is a significant disadvantage.
At high overall pressure ratios, the compression system is usually split into two units; a Low Pressure (LP) Compressor mounted on one shaft followed by an High Pressure (HP) Compressor mounted on the HP shaft and driven by its own (HP) turbine. On civil turbofans, the first stage of the LP Compressor is often a single stage fan. If the HP Compressor pressure ratio exceeds about 4.5:1, then the unit will probably have variable geometry (i.e. variable stators) on the first few stages, to make the surge line on the compressor map
Compressor map
Each compressor in a gas turbine engine has an operating map. Complete maps are either based on compressor rig test results or are predicted by a special computer program...
more shallow, to accommodate the shallow working line.
On their large civil turbofans, Rolls-Royce split the compression system into three: a Fan, an Intermediate Pressure (IP) Compressor and an HP Compressor, each unit being driven by its own turbine unit. The RB199 military turbofan also has this arrangement.