18-12-2012, 06:19 PM
Compressors
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Introduction
There exist a large number of fluid machines in practice, that use air, steam and gas (the mixture of air and products of burnt fuel) as the working fluids. The density of those fluids change with a change in pressure as well as in temperature as they pass through the machines. These machines are called 'compressible flow machines' and more popularly 'turbomachines'. Apart from the change in density with pressure and temperature, other features of compressible flow, depending upon the regimes, are also observed in course of flow of fluids through turbomachines. Therefore, the basic equation of energy transfer along with the equation of state relating the pressure, density and temperature of the working fluid and other necessary equations of compressible flow, are needed to describe the performance of a turbomachine. The names of a few compressible flow machines are, namely, compressors, fans and blowers. In practice two kinds of compressors: centrifugal and axial are generally in use.
Centrifugal Compressors
A centrifugal compressor is a radial outward flow rotodynamic fluid machine that uses mostly air as the working fluid and utilizes the mechanical energy imparted to the machine from outside to increase the total internal energy of the fluid mainly in the form of increased static pressure head. A centrifugal compressor essentially consists of three components. A rotating impeller as shown in Fig. 1 (a) which imparts a high velocity to the air resulting in increasing the centrifugal head of the fluid with a consequent rise in static pressure. The impeller may be single or double sided as show in Fig. 1 (b) and ©, but the fundamental theory is same for both. A diffuser consisting of a number of fixed diverging passages in which the air is decelerated with a consequent rise in static pressure.
Principle of operation:
Air is sucked into the impeller eye and whirled outwards at high speed by the impeller disk. During the flow of fluid through impeller vanes, the centrifugal head of fluid increases so that the static pressure head of the air increases from the eye to the tip of the impeller. The remainder of the static pressure head rise is obtained in the diffuser with diverging vanes, where the very high velocity of air leaving the impeller tip is reduced to almost the velocity with which the air enters the impeller eye.
Usually, about half of the total pressure rise occurs in the impeller and the other half in the diffuser. Owing to the action of the vanes in carrying the air around with the impeller, there is a slightly higher static pressure rise on the forward side of the vane than on the trailing face. The air will thus tend to flow around the edge of the vanes in the clearing space between the impeller and the casing. This results in a loss of efficiency and the clearance must be kept as small as possible.
Diffuser
The basic purpose of a compressor is to deliver air at high pressure required for burning fuel in a combustion chamber so that the burnt products of combustion at high pressure and temperature are used in turbines or propelling nozzles (in case of an aircraft engine) to develop mechanical power. The problem of designing an efficient combustion chamber is eased if velocity of the air entering the combustion chamber is as low as possible. It is necessary, therefore to design the diffuser so that only a small part of the stagnation temperature at the compressor outlet corresponds to kinetic energy. It is much more difficult to arrange for an efficient deceleration of flow than it is to obtain efficient acceleration. There is a natural tendency in a diffusing process for the air to break away from the walls of the diverging passage and reverse its direction. This is typically due to the phenomenon of boundary layer separation.