Lecture-1- Compressor and its Types.ppt

COMPRESSOR
Definition
 It is that part of a jet engine which
compresses the ingested air and then, feeds
it to combustion chamber.
 Power absorbing device.

Q. How a compressor works?
 Compressor is continuous flow machine which
function by imparting kinetic energy to the air by means
of a rotary part (rotor blades or impeller), subsequently
diffusing the velocity into static pressure rise.
WORKING OF A COMPRESSOR

ROLE OF A COMPRESSOR
 Compressor in a gas turbine engine is to
provide a maximum of high-pressure air.
Q. What is the use of high-pressure air?
 This air is then heated in a combustion
chamber and expanded in a turbine.

 Energy released in the combustion chamber is
proportional to the mass of the air consumed.
 Its efficient operation (maximum compression with
minimum temperature rise) is the key to high overall
engine performance.

Q. What is the importance of compressor efficiency?
 The compressor efficiency will determine the power
necessary to create the pressure rise and will affect the
temperature change in the combustion chamber.
 Present day compressors have compression ratios
over 25:1.
 Compression ratio.
 Compressors have efficiencies over 90 percent.
 Mass flow rate of air up to approximately 350 lbs/s
(158.8 kg/s).

With the addition of a fan,
total pressure ratios of more
than 25:1 and mass airflows
of 1000 lbs/s (453.6 kg/s)
have been achieved.
AMENDMENTS IN A JET ENGINE

TYPES OF A COMPRESSOR
Q. What are different types of a compressor?
 There are two types of compressors with respect to
airflow:-
Centrifugal compressor
Axial flow compressor

CENTRIFUGAL COMPRESSOR
The airflow is radial, i.e. the
flow of air is outward from the
centre of the compressor.

It is used extensively in the early days of gas turbines.
These are limited to the small gas turbine ‘gas
generators’ for engine air starters and missile engines.
Single Entry Impeller Double Entry Impeller

Turbojet Engine-Centrifugal Compressor

AXIAL FLOW COMPRESSOR
 Compressor in
which air flows axially
i.e. the flow of air is
maintained parallel to
the compressor’s
shaft.
 Used in turbofan
and turbojet engines.

COMPARISON OF COMPRESSORS
Centrifugal
compressors
generally need to
operate at much
higher rpm than
axial compressors.

COMBINATION OF BOTH
COMPRESSORS
Axial/centrifugal
compressor
combinations are
used extensively
in turbo-shaft
and turboprop
engines.

FACTORS AFFECTING THE PERFORMANCE
Q. What are those factors which affect the performance
of a compressor?
The principal factors are:-
Aerofoil sections,
Pitch angles and
The length/chord ratios of the blades
Clearance between the blade tips and the
compressor annulus

REQUIREMENTS OF A COMPRESSOR
Introduction
 SFC is very important in case of internal
combustion engines.
 The efficiency of a compressor is one of the
factors directly influencing the specific fuel
consumption (SFC).

HIGH PRESSURE RATIO
It is the ratio of pressure of the air at the outlet of
compressor to the inlet.
The thermal efficiency and the work output are both
proportional to the compressor pressure ratio.
The centrifugal compressor has a maximum pressure
ratio of about 4.5:1 for a single stage.
Raised to approximately 6:1 by using a two stage,
single-entry centrifugal compressor.

HIGH PRESSURE RATIO
 For axial compressors, it is a matter of stability and complexity.
 The current values are 10:1 for single-spool compressors.
 For multi-spool compressors, the pressure ratio is in excess of
25:1.
 Higher pressure ratios give higher engine efficiency due to an
improved SFC.

High Mass Flow
 Jet engine air mass flows are becoming much larger.
 Ram-compression
 Must be matched with the swallowing capacity of the
compressor.
 For transport aircraft, the required air mass flow at altitude
requires large diameter compressors.
 Larger diameter fans for dual flow engines are required (high by-
pass turbofan engines).

Stable Operation Under all Conditions
 Both centrifugal and axial compressors are liable to exhibit
unstable characteristics under certain operating conditions.
 The centrifugal type is less likely to stall and surge than the axial.
 But it is not capable of the high pressure ratios now required.
 In high pressure ratio axial compressors anti-stall/surge devices
are often a design requirement to guard against unstable
conditions.

COMPRESSOR DESIGN
 Compressor design in most engines is a compromise between
high performance over a narrow band of rpm, and moderate
performance over a wide band of rpm.
 Consequently, although it is possible for the compressor to be
designed so that very high efficiency is obtained at the highest
power.
 Any deviation from the designed conditions may cause significant
changes in the aerodynamic flow leading to a loss of efficiency
and unstable conditions within the engine.

Axial Flow Compressor-Single Spool

Axial Flow Compressor-Double Spool

Q. What is a compressor?
Q. What are different types of
compressors?
Q. Which compressor is used in T-37
Aircraft’s engine?

Definition
 1. The airflow is radial, i.e. the flow of air is
outward from the centre of the compressor.
Explanation
 2. (a) Centrifugal force
 (b) Limited to the small gas turbine (gas
generators) for engine air starters and
missile engines.

Purpose
3. To increase the mass flow rate through the
engine which results in generation of thrust.
Principle of Operation
4. To impart kinetic energy to the airflow and
then converts this kinetic energy into pressure
energy.

Main Components
5. (a) Compressor casing
(b) Impeller
(c) Diffuser

Compressor Casing
6. (a) Embodies the air inlet guide vanes
(IGVS) and outlet ports.
(b) Air entering the intake at atmospheric
temperature and pressure passes through the
fixed intake guide vanes.
(c) IGVS direct the air smoothly into the centre
of the impeller.
(d) Outlet ports direct the air into the
combustion chamber.

Impeller
7. (a) Rotating part
(b) Rotated at high speed by the turbine
(c) The incoming air smoothly fall at the centre
of the impeller.
(d) Designed to admit the air without excessive
velocity
(e) The air is picked up by the rotating guide
vanes of the impeller, and centrifugal force causes
the air to flow outwards along the vanes to the
impeller tip.

Impeller
(e) Air leaves the impeller tip approximately at
right angles to its intake direction.
(f) Impellers operate at tip speeds of up to 500
meter per second to give high tangential air
velocity for conversion to pressure.
(g) Impellers are of two types
(i) Single entry impeller
(ii) Double entry impeller

Single Entry Impeller
8. Type of an impeller in which air is
falling on one side (towards intake).
(a) Single Stage
(b) Double Stage
Advantages
9. (a) The single-sided, single- entry
impeller lends itself to efficient ducting.
(b) Makes more use of the ram
effect at high speeds than does
the double-entry arrangement.

(c) This leads
to an increase in
the overall
diameter of the
engine for a
given thrust.

Double Entry Impeller
10. (a) Type of an impeller in
which air is falling on
both sides of impeller.
(b) It can handle a higher
mass flow.
(c) The airflow to the rear
side is reversed in
direction.

(d) Penalties are
(i) Relatively
inefficient intake
ducting.
(ii) Preheating
imparted to the air
by virtue of its
reversal on entry.

Diffuser
11. (a) Used to convert
energy of the air
leaving the compressor to
pressure energy.
(b) Formed integral with
the compressor
casing or be bolted to it.
(c) The vane passages
are divergent to
convert the velocity energy
into pressure energy.

AXIAL FLOW COMPRESSOR
Definition
1. Compressor in which air flows axially
Purpose
2. Converts kinetic energy into static pressure
energy
- How it converts?
• Rows of rotating blades (rotors) impart kinetic energy
to the air
• Alternate rows of stationary diffusing vanes (stators),
convert the kinetic energy to pressure energy

Construction
3. Consists of an annular passage
Series of small blades of aerofoil section, alternately
rotating on a central shaft assembly or fixed to the outer
case
Each pair of rotor and stator rings is termed a stage

Typical gas turbine engine may have
between 10 and 15 stages on a single
spool
Divided between multiple spools
An additional row of stator blades may
be fitted to single spool-engines

Automatically adjusted to suit prevailing intake
conditions
Outlet guide vanes straighten the airflow into the
combustion stage
The cross-sectional area of the annulus is
progressively reduced
To maintain constant axial velocity with increasing
density

Types
4. (a) Single spool
(b) Multi spool

Axial-Centrifugal-Flow Compressor

COMPRESSOR STALL AND SURGE
Introduction
 A compressor is designed to operate between certain
critical limits of rpm, pressure ratio and mass flow.
 If operation is attempted outside these limits, the flow
around the compressor blades breaks down to give
violent turbulent flow.
 When this occurs, the compressor will stall or surge.
 The greater the number of stages in the compressor
more complex will be the situation.

Stalling of Compressor Rotor Blades
 These blades (which are like small
aero foils) stall in the same way as the
wings of an aircraft.
 Stalling of wing occurs at high angle of
attack when the flow separates from
the upper surface of wing.
 Since the pitch of the blades is fixed,
this condition is brought about by a
change in direction of the relative
airflow (V1).

 A reduction in the axial
velocity Va to a value Va1
while the rpm and hence
blade speed U remains
constant.
 Increases the angle of
attack.
 If the fall in Va is
sufficient, the blade
stalling angle will be
reached.

 The fall in Va at constant rpm is associated with a
reduction in mass flow from the stable value due to a
variety of causes.
 Generally, when the critical condition is approached due
to velocity gradients and local effects, a group of blades
will be affected first rather than the complete blade row.
 A stall cell would be generated.
 The stall “cell” moves around the blade row.
 There may be several “cells” which can stall the whole
row.

 Compressor Stall at Low Rpm
 Compressor Acceleration Stall
 Compressor Stall at High Rpm
TYPES OF COMPRESSOR STALL

Compressor Stall at Low Rpm
 With a reduction in mass flow rate at low rpm, the angle of
attack of the first low pressure stage is greater than that of
the high pressure stages.
 Due to this reason the low pressure stages will first stall.
 It is not necessary that the succeeding stages will be
affected.
 Stalling of the initial compressor stages may be indicated by
an audible rumbling (low or dull) noise and a rise in turbine
gas temperature (TGT).
 In the latter case, a further reduction in mass flow would
cause a successive breakdown of the remaining stages.

Compressor Acceleration Stall
 On starting or during rapid acceleration from low rpm,
there will be increase in mass flow rate of air and
additional fuel will be sprayed in the combustion
chamber.
 During combustion, there will be sudden increase in
combustion pressure which causes a momentary back
pressure.
 This affects the compressor performance and hence,
there will be reduction in the mass flow rate which causes
the same effects as described above.

Compressor Stall at High RPM
 At high compressor rpm, the angle of attack of all the
stages is almost same.
 If there is sudden reduction in mass flow then a
simultaneous breakdown of flow through all the stages is
caused.
 This type of stall is usually initiated by airflow interference
at the intake during certain maneuvers or gun firing.
 The compressor may be un-stalled by throttling fully back,
but in some cases it may be necessary to stop the engine.

COMPRESSOR SURGE
 In an axial compressor, surging indicates a complete
instability of flow through the compressor.
 Surging is a motion of airflow forwards and backwards
through the compressor, which is accompanied by audible
indications ranging from soft rumblings to an abrupt
explosion and vibration, depending on the degree of
severity.
 Surge is indicated by rapid rise in turbine gas temperature
(TGT ) and fluctuation or falling of rpm.
 Compressor surge causes very severe vibrations and
excessive temperatures in the engine.
 A compressor stall usually leads to a compressor surge.

Lecture-1- Compressor and its Types.ppt

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