Mindblown: a blog about philosophy.
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CONSTRUCTIONAL FEATURES
The cross-section of a typical axial flow compressor is shown in Fig. 15.1. It consists of adjacent rows of moving and fixed blades. The moving blades are mounted on the rotating drum and the fixed blades are fixed to the casing or stator. One stage of the compressor comprises a row of moving blades, followed by…
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INTRODUCTION
In an axial flow compressor, the air flows throughout the compressor parallel to its axis. Axial flow compressor has many merits over centrifugal compressor and is particularly suitable for super charging of internal combustion engines, turbojets, and gas turbines.
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SURGING AND CHOKING
The delivery pressure v’s mass flow rate in a dynamic compressor is shown in Fig. 14.13. Suppose a discharge valve is put in the line for this compressor. The mass rate of flow will be zero when the discharge valve is closed and the static pressure developed is that delivered by the impeller to the air…
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PERFORMANCE CHARACTERISTICS
Let us consider the following variables for the performance of the compressor. p3 = pressure at the outlet of diffuser p1 = inlet pressure T1 = inlet temperature D = diameter G = mass rate of flow N = r.p.m of rotor Then, p3 = f (p1, T1, D, G, N) The following dimensions are chosen: M = mass (kg), L = length (m), t = time (s) Number of variables, n = 6 Basic…
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PRE-WHIRL
In order to avoid acceleration to sonic or supersonic velocities, the Mach number of flow entering the impeller eye should be kept below the value of 0.9. When the absolute velocity of approach is high enough or the static absolute temperature of the entering air is low enough to make the Mach number more than…
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EFFECT OF IMPELLER BLADE SHAPE ON PERFORMANCE
The following types of blade shapes are used for impellers of centrifugal compressors: The characteristics of these blades are shown in Fig. 14.8. Centrifugal effects on the curved blades create a bending moment and produce increased stresses which limit the maximum speed at which the impeller can run. Slightly backward-curved impeller blades (i.e., β2 < 90°) give optimum…
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LOSSES
There are two types of losses in a centrifugal compressor: internal and external. The internal losses sustained in the compressor are manifested by an increase of enthalpy of air. The internal losses which occur in compressor are as follows: The compressor sustains an external loss in the form of bearing friction and windage loss. The…
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SLIP FACTOR AND PRESSURE COEFFICIENT
In the ideal velocity diagrams, we had assumed that vw2 = u2. However, in actual operation, this condition is not satisfied in actual practice due to secondary flow effects. Actually, vw2 < u2. The difference (u2 – vw2) is called the slip. The slip factor is defined as the ratio of actual whirl component to the ideal whirl component and is denoted by ϕs.…
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VELOCITY DIAGRAMS
The velocity diagrams at inlet and outlet of the impeller for the centrifugal compressor are shown in Fig. 14.6(a). Figure 14.6 (a) Actual velocity diagrams Let u1 = mean blade velocity at entrance u2 = mean blade velocity at exit va1 = absolute velocity at inlet to rotor va2 = absolute velocity at outlet to rotor vr1 = relative velocity at inlet…
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Isentropic Efficiency
Isentropic efficiency may be defined as the ratio of isentropic temperature rise to actual temperature rise. where During compression process, work has to be done on the impeller. The energy balance then gives, cpT01 = cpT02 – W
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