Axial Compressor

Axial compressor is utilized by most aircraft turbojet engines. Their main advantage is their ability to deliver high mass flow rates together with large pressure ratios at the same time. In a jet engine axial compressor, the air flows in a uniform direction, eliminating the need for turning the flow. Another advantage is that their smaller cross-section lessens aerodynamic drag of the engine nacelle.

Essentially, the axial compressor of an aircraft jet engine consists of four parts; 1) compressor front frame; 2) compressor casing with stator vanes; 3) rotor with rotor blades; 4) compressor rear frame.

1) The compressor front frame consists of an outer ring, an inner hub, and between 6 and 8 streamlined supporting struts. Thus, it is a ring-shaped, single-piece structure made of aluminum alloy or steel. It is usually cast and then machined. The function of the compressor front frame is to accomodate the rotor front bearing and to transfer rotor forces to the outer casing. The aft (back) facing flange of the front frame mates with the compressor casing to which it is tightly secured by bolts.

2) The compressor casing is tube-like and can be split lengthwise to make engine assembly and maintenance easier. It is usually light weight, since it is made of forged titanium. However, modern high performing engines employ alloy materials. The inner walls of the compressor casing are machined with circumferential (rounded) T-section grooves so it can retain the stator blades. Some modern engines are also fitted with variable inlet guide vanes, which direct flow alignment.

3) The most complex component of the compressor assembly is the rotor. Usually, the rotor is drum or disc-shaped. In a disc-type, the rotor blades are mounted on individual discs that are separately secured to the rotor shaft, with the discs being divided by spacer rings.

4) The compressor rear frame function is to guide and deliver the pressurized airstream to the combustion section. Therefore, the flow path design depends on the type of combustor employed. In the old can-type combustion chamber, the flow path through the rear frame had to be equally apportioned to each combustor. The rear frame is very important to the engine thrust forces; in fact, it is here (in most cases) where the main engine mounting is situated as thrust forces are transmitted to the airframe.

Schematic drawing of axial compressor assembly of a General Electric J79 turbojet engine