Gas Turbine Blade Cooling Technique[1]

DEPARTMENT OF MECHANICAL ENGINEERING

PROGRAMME : M.TECHBRANCH : FLUID ENGINEERING

COURSE : GAS TURBINE & JET PROPULSION

PRESENTATION TOPIC BLADE COOLING TECHNIQUE

COURSE COORDINATOR : Dr. BIRESWAR PAUL PRESENTED BY : NEERAJ SHUKLA

THERMAL ISSUES• Gas Turbines are limited to lower operating temperatures due to the materials available for the engine itself.

• Operating at the lower temperature will decrease the efficiency of the gas turbine so a means of cooling the components is necessary to increase temperatures at which engine is run .

• Blade Temperature may vary along the blade surface from LE to TE by 200 to 300 K .

• Blade temperature may also vary from the root to the tip of a rotor .

• Maximum blade temperature is felt at the LE of the first stator –as the flow comes from C.C.

• HP turbine blades have maximum temperature and maximum temperature gradient across both the rotor and the stator .

• Blades are thermally loaded in cycles of operation .

•Turbine failure occurs mostly in creep (thermal fatigue)

TRENDS IN BLADE COOLING

TIMELINE OF BLADE COOLING

(1950 ) UNCOOLED BLADE

1000 TO 1100K

(1970) DISTRIBUTED INTERNAL

CONVECTION COOLING 1300 – 1500 K

(1990) FILM + IMPINGEMENT

COOLING 1600 – 1900 K

TEMPERATURE GRADIENT

BLADE COOLING

BLADE COOLING TECHNIQUE

WATER COOLING AIR COOLING

EXTERNAL COOLING

Film cooling

Transpiration or effusion

cooling

INTERNAL COOLING

Convection cooling

Impingement cooling

WATER COOLING

• Liquid cooling seems to be more attractive because of high specific heat capacity and chances of evaporative cooling but there can be problem of leakage, corrosion, choking, etc. which works against this method .

• It may overcool the blade ( due to its high heat transfer coefficient ) which is not desirable .

EXTERNAL COOLING• The external surface of the gas turbine blade is cooled by making use of compressed air from the compressor .

• The quantity of air required for this purpose is from 1 to 3 % of the main flow entering the turbine stage .

• Blade metal temperature can be reduced by about 200-300 C .

FILM COOLING• It is applied to local areas by drilling holes (up to dia. of 0.15 mm ) on precision cast blades by the process EDM ( Electric Discharge Machining ) .

• The cooling air flow out of these small holes forms a thin film over the blade surfaces .

• Besides cooling the blade surface it decreases the heat transfer from the hot gases to the blade metal .

TRANSPIRATION COOLING

• In this the air is allowed to effuse or sweat from the pores of the porous blade metal .

• This provides a blanket of cool air , insulating the metal of the turbine blade from the hot gas .

• Effusion of the coolant over the entire blade surfaces cause uniform cooling of the blade .

• Sintered wire composites are the most commonly used transpiring materials.

CONVECTION COOLING• It works by passing cooling air through passages internal to the blade .

• Heat is transferred by conduction through the blade, and then by convection into the air flowing inside of the blade .

• The internal passages in the blade may be circular or elliptical in shape .

• Cooling is achieved by passing the air through these passages from hub towards the blade tip.

• This cooling air comes from an air compressor .

• The fluid outside is relatively hot which passes through the cooling passage and mixes with the main stream at the blade tip

IMPINGEMENT COOLING• Impingement cooling, works by hitting the inner surface of the blade with high velocity air.

• This allows more heat to be transferred by convection than regular convection cooling does .

• Impingement cooling is used in the regions of greatest heat loads.

• In case of turbine blades, the leading edge has maximum temperature and thus heat load .

• Blades are hollow with a core.

• Cooling air enters from the leading edge region and turns towards the trailing edge .

EFFECT OF INCREASED INLET TEMPERATURE AND PRESSURE RATIO

EFFECTS OF COMBINATION OF COOLING TECHNOLOGIES

STATOR BLADE

ROTOR BLADE

REFERENCES

• WIKIPEDIA • NPTEL • GAS TURBINES BY V GANESHAN

THANK YOU