Centrifugal flow compressors have a single or double-sided impeller and occasionally a two-stage, single sided impeller is used, as on the Rolls-Royce Dart. The impeller is supported in a casing that also contains a ring of diffuser vanes. If a double-entry impeller is used, the airflow to the rear side is reversed in direction and a plenum chamber is required
Principles of operation
The Impeller is rotated at high speed by the turbine and air is continuously induced into the centre of the impeller Centrifugal action causes it to flow radially outwards along the vanes to the impeller tlp thus accelerating the air and also causing a rise In pressure to occur The engine intake duct may contain vanes that provide an initial swirl to the air entering the compressor.
The air on leaving the Impeller passes into the diffuser section where the passages form divergent nozzles that convert most of the kinetic energy into pressure. In practice
It is usual to design the compressor so that about half of the pressure rise occurs in the impeller and half in the diffuser.
To maximize the airflow and pressure rise through the compressor requires the impeller to be rotated at high speed therefore Impellers are designed to operate at tip speeds of up to 1 600 ft per sec. By operating at such high tip speeds the air velocity from the impeller is increased so that greater energy is available for conversion to pressure.
To maintain the efficiency of the compressor, it is necessary to prevent excessive air leakage between the impeller and the casing this is achieved by keeping their clearances as small as possible.
Construction
The construction of the compressor centres around the impeller, diffuser and air Intake system The impeller shaft rotates In ball and roller bearings and is either common to the turbine shaft or split in the centre and connected by a coupling, which is usually designed for each of detachment.
Impellers
The impeller consists of a forged disc with integral, radially disposed vanes on one or both sides (fig 3-5) forming convergent passages in conjunction with the compressor casing The vanes may be swept back. But for ease of manufacture straight radial vanes are usually employed. To ease the air from axial flow in the entry duct on to the rotating impeller, the vanes in the centre of the impeller are curved in the direction of rotation. The curved sections may be integral with the radial vanes or formed separately for easier and more accurate manufacture.
Diffusers
The diffuser assembly may be an integral part of the compressor casing or a separately attached assembly. In each instance it consists of a number of vanes formed tangential to the impeller. The vane passages are divergent to convert the kinetic energy into pressure energy and the inner edges of the vanes are in line with the direction of the resultant airflow from the impeller. The clearance between the impeller and the diffuser is an important factor, as too small a clearance will set up aero¬dynamic buffeting impulses that could be transferred to the impeller and create an unsteady airflow and vibration.
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