2722-semin sanuri-12. gas turbine system theory

8/6/2019 2722-Semin Sanuri-12. Gas Turbine System Theory

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GAS TURBINESSYSTEM THEORY

PREPARED BY: SEMIN SANURI

DEPARTMENT OF MARINE ENGINEERING


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OUTLINE:

1. GAS TURBINE OPERATING CYCLE2. COMPONENTS EFFICIENCIES

3. COMPRESSOR4. BURNER SECTION5. TURBINE

GAS TURBINES SYSTEM THEORY


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1. GAS TURBINE OPERATING CYCLE:


Gas turbine cycle is best depicted by the Brayton Cycle.The characteristics of the operating cycle are shown on thepressure-temperature map, the pressure-specific

volume map, and the temperature-entropy map


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The gas turbine, as a continuous flow machine, is bestdescribed by the first law of thermodynamics.


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In most gas turbine applications, the numericalmagnitude of the difference in potential energy is sosmall, relative to the other values in the equation, that it iscustomary to disregard it. This first law equation isrewritten as follows:


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For adiabatic processes (no heat transfer):

where HP is horsepower, C is the velocity of the air entering the compressor or air and

combustion products leaving the turbine, and gc is the gravitational constant 32.17 ftlbm/lbfsec2.


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Gas Turbine Horsepower Output:

where 0.707 (more exactly 0.7068) convertsBtu/sec to horsepower


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Gas Turbine Efficiency:

Thermal efficiency (t)of a gas turbine, considering the compression

and expansion processes as being irreversible, is defined as thework output divided by the fuel energy input. The work output isthe total turbine workminus the work on the compressor (notecompressor work is negative). Therefore;


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2. COMPONENTS EFFICIENCIES:


Compressor Efficiency:

Compressor efficiency (c) is directly proportional to the compressorpressure ratio and inversely proportional to the compressor dischargetemperature. The following equation more exactly defines compressorefficiency:


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Compressor Horse Power:

Compressor horsepower is the power that the compressor

consumes in compressing the air and moving it into thecombustor.

where Wa is the air flow entering the compressor in lb/sec.


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Turbine Efficiency:

Tracking turbine efficiency would be an excellent method to monitor thehealth of a unit. However, as turbine inlet temperatures (TIT) haveclimbed higher and higher, they have become virtually impossible tomeasure on a long term basis. In fact, many manufacturers measure anintermediate turbine temperature for gas turbine control. Where this isthe case the turbine inlet temperatures are calculated.


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Turbine Horse Power (Produced):

Total horsepower produced by the turbine. It includes the horsepower todrive the compressor and, for single shaft machines, the power used bythe driven load. For units with separate power turbines, this horsepowershould equal the power absorbed by the compressor plus losses.


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Power Turbine Efficiency:

Decreases in power turbine efficiency are primarily the result ofloss of material due to erosion, corrosion, or foreign object damage.


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Power Turbine Horse Power:

On free power turbines units, this is the horsepower generated to drivethe driven load.


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The internal pressure, temperature, and velocity variations within thegas turbine are shown in figure below:


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3. COMPRESSOR:


The compressor provides the high pressure, highvolume air which, when heated and expanded through theturbine section, provides the power output required by the

process (mechanical drive, generator drive, etc.).Compressor performance is generally shown as pressureratio plotted against airflow.

Two types of compressors are in use todaythey are theaxial compressor and the centrifugal compressor. Theaxial compressor is used primarily in medium and highhorsepower applications, while the centrifugalcompressor is utilized in low horsepower applications.


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3. COMPRESSOR:


The best way to illustrate airflow through a axial compressorstage is by constructing velocity triangles as figure below:

Air leaves the stator vanes at an absolutevelocity of C1 and direction 1. The velocity

of this air relative to the rotating blade isW1 at the direction 1. Air leaves therotating stage with an absolute velocity C2and direction 2, and a relative velocity W2and direction 2. Air leaving the secondstator stage has the same velocity triangleas the air leaving the first stator stage. Theprojection of the velocities in the axialdirection are identified as Cx, and thetangential components are Cu. The flowvelocity is represented by the length of thevector. Velocity triangles will differ at theblade hub, mid-span, and tip just as the

tangential velocities differ.


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3. COMPRESSOR:


Pressure rise across each stage is a function of the air density, ,and the change in velocity. From the velocity triangles thepressure rise per stage is determined:

This expression can be further simplified by combining thedifferential pressure and density, and referring to feet of head:

where AP/ is pressure rise across stage and Head is pressure

rise of the stage measured in feet head of the flowing fluid.


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3. COMPRESSOR:


The standard equation for compressor head is given below,where Zave is the average compressibility factor of air, andMW is the mole weight.


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3. COMPRESSOR:


Before proceeding further, we willdefine the elements of an airfoil:


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3. COMPRESSOR:


Considering the Centrifugal Compressor:

The centrifugal compressor, like the axial compressor, is a

dynamic machine that achieves compression by applyinginertial forces to the air (acceleration, deceleration, turning)by means of rotating impellers.

The centrifugal compressor is made up of one or more

stages, each stage consisting of an impeller and a diffuser.The impeller is the rotating element and the diffuser is thestationary element. Impellers may be either open, semi-enclosed, or enclosed design.


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3. COMPRESSOR:


Velocity diagrams for a centrifugal compressor:

Centrifugal force, appliedin this way, is significant

in development ofpressure. Upon exitingimpeller, air moves intodiffuser (flowdecelerator). Samedeceleration of flow or

diffuser action thatcauses pressure build-upin axial flow compressoralso occurs in centrifugalcompressor.


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3. COMPRESSOR:



Impeller is the only means of adding energy to the air and all

the work on the air is done by these elements. Thestationary components, such as guide vanes and diffusers,can only convert velocity energy into pressure energy (andincur losses).

Pressure from impeller eye to impeller outlet is representedby the following:


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3. COMPRESSOR:


represents the increase in kinetic energy contributed to

the air by the impeller. Absolute velocity C1 (entering theimpeller) increases in magnitude to C2 (leaving impeller).

measures pressure rise associated with the radial /

centrifugal field,


is associated with the relative velocity of the airentering and exiting the impeller.


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3. COMPRESSOR:



The ideal head is defined by the following relationship:


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4. BURNER SECTION:


The burner section is made up of the diffuser duct, thecombustor, fuel nozzle and the transition duct.

The fraction of the velocity head that is converted to staticpressure (diffuser efficiency) is a function of the area ratioand diffuser angle. Diffuser efficiency is defined as:

Diffuser efficiency:


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4. BURNER SECTION:


PROPERTIES OF A GOOD BURNER:

1. HIGH COMBUSTION EFFICIENCY.

2. STABLE COMBUSTION.3. LOW NOX FORMATION.4. FREEDOM FROM BLOWOUT.5. UNIFORM OR CONTROLLED DISCHARGE TEMPERATURE.

6. LOW PRESSURE LOSS.7. EASY STARTING.8. LONG LIFE.9. FOR LIQUID FUEL OPERATION MINIMUM CARBON

ACCUMULATION.


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4. BURNER SECTION:


COMBUSTORS MUST BE ABLE TO WITHSTANDVARIOUS CONDITIONS:

1. A WIDE RANGE OF AIR FLOW.2. FUEL FLOW.3. DISCHARGE TEMPERATURE.4. RAPID ACCELERATION AND DECELERATION.

5. VARIATION IN FUEL PROPERTIES.

Combustor efficiency is defined as:


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4. BURNER SECTION:


COMBUSTORS MUST BE ABLE TO WITHSTANDVARIOUS CONDITIONS:

1. A WIDE RANGE OF AIR FLOW.2. FUEL FLOW.3. DISCHARGE TEMPERATURE.4. RAPID ACCELERATION AND DECELERATION.

5. VARIATION IN FUEL PROPERTIES.

Combustor efficiency is defined as:


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THANKYOU

2722-semin sanuri-12. gas turbine system theory

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