Jet Propulsion/Axial compressors

Axial compressors are compressors in which the fluid flows mainly parallel to the rotation axis. They are widely used in gas turbines. Axial flow compressors have large mass flow capacity and high efficiencies, but have a smaller pressure rise per stage than centrifugal compressors. Most commercial jet engines used in transport employ axial compressors. Smaller engines such as those used in helicopters use centrifugal compressors which provide larger pressure ratios.

A typical axial compressor has a rotor which looks like a fan with contoured blades followed by a stationary set of blades, called a stator. Some designs also have inlet guide vanes.

Energy Exchange between rotor and fluid

The relative motion of the blades relative to the fluid adds velocity or pressure or both to the fluid as it passes through the rotor. The fluid velocity is increased through the rotor, and the stator converts kinetic energy to pressure energy. Some diffusion also occurs in the rotor in most practical designs.

The increase in velocity of the fluid is primarily in the tangential direction (swirl) and the stator removes this angular momentum.

The pressure rise results in a stagnation temperature rise. For a given geometry the temperature rise depends on the square of the tangential Mach number of the rotor row. Current turbofan engines have fans that operate at Mach 1.7 or more, and require significant containment and noise suppression structures to reduce blade loss damage and noise.

Velocity Diagrams

The blade rows are designed at the first level using velocity diagrams. The velocity diagram shows the relative velocities of the blade rows and the fluid.

The axial flow through the compressor is kept as close as possible to Mach 1 to maximize the thrust for a given compressor size. The tangential Mach number determines the attainable pressure rise.

The blade rows turn the flow through and angle ß and larger turning allows a higher temperature ratio, but requires higher solidity.

Modern blades rows have lower aspect ratios and higher solidity.

Compressor maps

A compressor map shows the performance of a compressor and allows determination of optimal operating conditions. It shows the mass flow along the horizontal axis, typically as a percentage of the design mass flow rate, or in actual units. The pressure rise is indicated on the vertical axis as a ratio between inlet and exit stagnation pressures.

A surge or stall line identifies the boundary to the left of which the compressor performance rapidly degrades and identifies the maximum pressure ratio that can be achieved for a given mass flow. Contours of efficiency are drawn as well as performance lines for operation at particular rotational speeds

Compressor efficiency

Compressor efficiency defines the performance of the machine. isothermal, adiabatic, and volumetric efficiency are referred for the performance of compressor.

Stage performance parameters

Corrected speed

Corrected mass flow

Multi stage compressors

Multistaging is done to reduce the total work input required for pressurization. This is achieved by facilitating intercoolers between stages of compression, intended to bring done the operating temperatures close to isothermal compression.

Compression stability

Operating efficiency is highest close to the stall line. If the downstream pressure is increased beyond the maximum possible the compressor will stall and become unstable.

Typically the instability will be at the Helmholtz frequency of the system, taking the downstream plenum into account.