UNIT 6
Centrifugal & Axial Compressor
Q1) Classify compressors.
Ans.
Classification of compressors
Q2) Explain construction of centrifugal compressor with neat sketch.
Ans.
Q3) Draw & explain velocity diagrams for centrifugal compressors.
Ans.
Let, Linear velocity of the moving blade at inlet (BA)
V = absolute velocity of the air entering the blade (AC)
Relative velocity of air to the moving blade at inlet (BC). It is vectorial difference between
velocity of flow at inlet
= angle which the relative velocity makes with the direction of motion of the Blade and
= Corresponding values at outlet
Let, m = mass of air compressed by the compressor in kg/s.
we know that according to Newton's second law of motion force in the direction of motion of blades (in Newtons)
F = mass of air flowing in kg/s × change in velocity of whirl in m/s
And work done in the direction of motion of the blades
Now power required to drive the compressor may be find out as usual by the relation,
The blades velocity at inlet or outlet, may be find out by the relation
Where D and are the internal and external diameters of the impeller
Q4) Explain flow process on T S diagram.
Ans.
Isentropic efficiency, = = =
Q5) Explain Euler’s work.
Ans.
Now, for an angular velocity of impeller rad/s
Theoretical work required /kg of air = T× w/kg
Above Equation is known as Euler's equation and gives your Euler's work
Further, if air enters radially
Theoretical work required per kg =
The theoretical work required /kg also represents the theoretical or virtual head developed in the absence of losses.
Thus, virtual of theoretical head, is given by
Q6) Explain performance characteristics of centrifugal compressor.
Ans. characteristics
Q7) Explain various losses occurred in centrifugal compressor.
Ans.
The fluid friction losses over the impeller are given by-
loss in the inducer where,
loss due to change of direction from axial to radial at the impeller inlet
where =loss coefficient= 0.1 to 0.2
loss due to friction in the blade passages
where loss coefficient 0.2 to 0.4
loss due to design deviations
change in relative velocity due to design deviation
= 0.54 to 0.60
= De friction loss in impeller
= 0.003 to 0.008
II. Losses in the Diffuser:
III. Frictional Losses
IV. Shock Losses
Q8) Explain working & construction of axial flow compressor.
Ans.
Q9) Draw & explain velocity triangles for axial flow compressor.
Ans.
Q10) Draw enthalpy entropy diagram for axial flow compressor.
Ans. Work required/kg =
Above Equation is represented graphically on h-S diagram in Fig
Q11) Explain various efficiencies involved in axial flow compressor.
Ans.
Cascade or diffuser efficiency
It is defined as the ratio of actual pressure rise to the isentropic pressure rise. Thus,
Friction in rotor and stator blade passages, shock losses, etc, reduce efficiency of the cascade (stage).
Polytropic efficiency:
stage isentropic efficiency =
Polytropic efficiency =
Q12) Explain various losses involved in axial flow compressor.
Ans. The pressure loss is due to three factors, namely:
(a) Profile losses On the Blade surface
(b) Skin friction on the annulus walls and
(c) Secondary flow losses
Figure illustrates various losses influencing the stage efficiency.
(a) Profile losses:
This is a pressure loss of two-dimensional cascade arising from the skin friction on the Blade surface and due to mixing of fluid particles after the blade.
(b) Skin friction losses on the annulus walls
The total pressure loss arise from the skin friction on the annulus walls and some secondary loss.
(c) Secondary flow losses
These losses occur due to combined effects of curvature and boundary layer.
2. Annulus losses. Because of adverse pressure gradient in compressors the boundary layer along the annulus walls taken as the flow progresses. This will reduce the area available for floor and lead to drop in axial velocity through the compressor.
3. Secondary losses. The loss occurring in the regions of flow near the end walls due to presence of unwanted circulatory or cross flows. This type of secondary flow develops due to turning of flow through the blade channel in the pressure of annulus wall boundary layers. Secondary flows in the cascade will also affect the profile and annulus losses.
4. Tip clearance losses. the loss arising due to the clearance between the moving blade and the casing is the clearance loss. The flow leaks from pressure side towards the suction side.
Q13) Explain performance characteristics of axial flow compressor.
Ans.
(a) Shows stagnation pressure rise vs non dimensional mass flow, isentropic efficiency is for fixed values on non dimensional speed from figure
(b) It is clear that the characteristics for fixed values of a much narrow range of mass flow then in the case of centrifugal compressor. At high rotational speeds the constant speed line become very steep and ultimately become vertical. The same limitations occur at either end of the due to surging and choking.