Chapter 3 Compressor PDF

Chapter 3: Compressor Introduction  Compressor Performance Map / Qualities  Centrifugal Flow Compressor  Compression ratio  Construction of centrifugal compressor  Types of centrifugal compressor  Axial Flow Compressor  Compression ra...

Chapter 3: Compressor Introduction  Compressor Performance Map / Qualities  Centrifugal Flow Compressor  Compression ratio  Construction of centrifugal compressor  Types of centrifugal compressor  Axial Flow Compressor  Compression ratio  Construction of axial compressor  Advantage of multiple spool engine  Compressor stall / surge  Centrifugal and axial flow compressor  Fan Blade  Attachment type  Types of fan blade  Shingling 2 Compressor Performance Map / Qualities  Compressor pressure ratio  Gas flow (mass flow rate)  Rotational speed  Compressor adiabatic efficiency (no heat loss or gain within the system) 3 Types of Compressor  Centrifugal Flow Compressor  Axial Flow Compressor 4  Centrifugal Compressor 5 Centrifugal Flow Compressor 6 Centrifugal Flow Compressor 7 Centrifugal Flow Compressor - Advantages  Higher stage pressure ratio.  Simplicity and ruggedness of construction  Less drop in performance due to dust on blades  Shorter length for same overall pressure ratio  Wider range of stable operation between surging and choking limits 8 Centrifugal Flow Compressor - Disadvantages  Larger frontal area for a given flow rate  More than 2 stages are not practical because of the energy losses between the stages 9 Centrifugal Flow Compressor - Pressure & Velocity change  The centrifugal compressor efficiency is about 80% with a compressor ratio of 4 or 5 to 1. 10  Centrifugal Flow Compressor - Double Entry Impeller Disadvantage 1: Double entry faces implications in design to get the air to the rear impeller. Disadvantage 2: The volume of air input may not be uniform from both inlet duct due to aerodynamic flow of air Entry 1 Output Impeller Entry 2 Entry 1 Entry 2 Compressor Manifold 11 Centrifugal Flow Compressor - Two-Stage Compressor Higher compression ratios can be obtained to about 14/1 with two centrifugal stages. Air intake Diffusers Centrifugal compressors *Turbomeca Training notes 12 Centrifugal Flow Compressor - Two- Stage Compressor Disadvantage 1: using more than single entry compressor maybe impractical as energy lost in the airflow as it slows down to make turns from one impeller to another impeller. Disadvantage 2: Added weight, more power is used to drive the compressor 13 Centrifugal Flow Compressor - Tip Clearance  The tip clearance is highly important:  If the clearance is too small, vibration of the engine could result in abruption between the rotating impeller and the stationary diffuser.  If the clearance is too large, the efficiency of the compressor would be affected as part of the airflow will rotate with the impeller without entering the diffuser. 14  Axial Compressor 15 Facts of Axial Compressor  A row of rotating and stationary blades is called a stage.  The compression ratio for the axial compressor is approx 1.2 per stage. 16 Axial Compressor Assembly R S Turbine A compressor includes: - a rotor (r) (rotating blades) which impart motion to a mass of air, - a stator (s) (stationary vanes) which transforms part of the air velocity into pressure. Moreover the flow is straightened and directed onto the Compressor next stage at the correct angle. W1 V1 V2 - Inlet Guide Vane (IGV) which direct airflow to the 1st stage Rotor W0 V0 (no change to velocity and pressure) U Rotor Stator *Turbomeca Training notes 17 Axial Compressor Assembly 18 Chapter 5: Compressor Axial Compressor Assembly 19 Chapter 5: Compressor Velocity and Pressure  At Inlet Guide Vane: Velocity and Pressure remain constant  After IGV and before 1st stage Rotor: V increase, P increase  At 1st stage Rotor: V decrease, P increase  After 1st stage Rotor and before 1st stage Stator: V increase and P increase  At 1st stage Stator: V decrease, P increase  After 1st stage Stator and before 2nd stage Rotor: V increase and P increase 20 Chapter 5: Compressor Axial Compressor - Assembly 21 Axial Flow Compressor - Advantages & Disadvantages Advantages  Smaller Frontal Area  Flow Direction At Discharge More Suitable For Multi-staging  Higher Efficiency At High Pressure Ratios Disadvantages  the mechanical design is rather complex  Higher possibility of surge  More prone to foreign object damage 22 Axial Compressor - Tip Clearance  The clearance between the rotating blades and the outer casing is an important factor that affects the efficiency of the engine. 1. One method is to apply a special soft coat over the knife-edge tips on the rotating blades so that the blades will wear away and form its own clearance between the blades and the casing. 2. Another method is to coat the inner layer surface of the compressor casing with a soft material which can be worn away without damage to the blades. Method 1 Method 2 23 Advantages of Multi-spool engine: 1. Higher compressor ratios 2. Quick acceleration due to less inertia of spools during starting process 3. Low weight due to less number of stages and smaller pool 4. Better control of stall characteristics. 24 What is compressor Stall and Surge?  Compressor stall is a localized condition where one or few stages are affected by airflow disruption.  Compressor surge is a situation when the entire compressor suffer complete airflow breakdown. 25 Signs of Surge and Stall  Both Surge and stall can be identified by loss of power, vibration, rpm fluctuation, increase in EGT and loud noise.  Both Surge and Stall will have the same effect but in different magnitude. 26 Chapter 5: Compressor Compressor Stall 27 Factors that lead to Surge / Stall  High power operation in low air speed  High inlet temp, distortion of the inlet flow can result in a reduce air velocity causing surge.  Excessive fuel flow during sudden acceleration can cause the combustion chamber to increase in pressure causing the airflow to decrease and choke the compressor.  Abrupt changes in aircraft attitude  Encountering air turbulence  Deficiency of air volume, caused by atmospheric conditions  Rapid throttle movement 28 Mechanical Malfunctions that lead to compressor stall / surge  Variable stator vane (VSV) failure  Fuel control unit (FCU) Failure  Foreign object damage (FOD)  Variable exhaust nozzle failure 29 Effect of Surge / Stall  Increase of gas temperature which may damage the turbines  Severe vibration which may damage the bearings,  Abnormal violent noises,  smoke and flames from the exhaust  Power drop 30 Solution to Compressor Stall / Surge  Variable Guide Vane or Variable Stator Vane  Air Bleed Valve  Split spool  Variable exhaust nozzle 31 Stall / Surge Control – VSV  Place variable Stator vane at the compressor inlet and several variable stator vane stages in the front high pressure compressor stages, so that the angles-of-attack can be adjusted according to the speed of airflow. 32 Stall / Surge Control – VGV Bleed valves Variable inlet guide stages Air Diffusers intake Axial compressors *Turbomeca Training notes 33 Stall / Surge Control – Bleed Valve  Introduced a bleed valve into the middle or rear of the compressor and use it to bleed air and increase airflow in the front end of the HP compressor at lower engine speeds. As well as relieving of pressure at the HPC area. 34 Stall / Surge Control – Bleed Valve Compressor bleed valve Axial compressors Centrifugal compressor *Turbomeca Training notes 35  Centrifugal & Axial Compressor 36 Centrifugal & Axial Compressor Assembly  For some gas turbine engine, 2nd Diffuser the compression is built by several stages of axial Centrifugal compressor 1st Diffuser compressor followed by a centrifugal compressor to Wheel produce a higher compressor ratio. Axial compressor Axial wheel Diffusers  This type of engine is most used on a Turbo-shaft engine. *Turbomeca Training notes 37 Centrifugal & Axial Compressor Assembly Variable inlet Diffusers Centrifugal guide vanes compressor Axial compressors *Turbomeca Training notes 38  Compressor Fan Blade 39 Fan Blade Attachment  Generally there 3 methods of attachment used on engine: Dovetail root Solid root Fir tree root 40 2 Types of Fan Blades Narrow Fan Blade Wide-Chord Fan Blade 41 Shingling of Narrow Fan Blades 42 Shingling of Narrow Fan Blades  The special feature of a mid-span support (snubber or clapper) is required to prevent aerodynamic instability.  It also prevents the blade from bending.  The disadvantage is that the shroud blocks some of the airflow and cause fan blade shingling.  Fan blade shingling is the overlapping of the mid span shrouds of the fan blade. 43 Wide chord fan blade  Advantages of Wide chord fan blade: 1. Fewer FOD ingestion 2. Increase aerodynamic efficiency 3. Decrease opportunity for surge 4. Reduced frontal area thru increased flow per unit volume 5. Lower noise level 6. No weight penalty 44 Material of Blade  The wide chord fan blade is a sandwich construction of titanium honeycomb and titanium panels. This sandwich is subjected to vacuum.  The result is a superior fan design with no weight penalty. 45 Recap  Name the qualities / performance map of compressor blade  2 major types of compressor  Name the major parts of the centrifugal and axial compressor assembly  Advantage/disadvantage of the two types of compressors  Importance fact on tip-clearance  Compression ratio of centrifugal and axial compressor  Principal operation of axial flow compressor  Characteristics of stall and surge  Factors leads to surge and stall  Mechanical malfunction that leads to compressor stall/surge  Solution to prevent surge and stall  Advantages of multi-spool engine  Advantages of wide cord fan blade over narrow chord fan blade 46