Gas Turbine

OutlineComprehend the basic components of gas turbine engines and their basic operationComprehend the thermodynamic processes occurring in a gas turbineComprehend the types of gas turbine engines

*There are industrial gas turbines and there are jet engine gas turbines

Gas Turbines find their applications in

electric power generation, mechanical drive systems, supply of process heat and compressed air, pump drives for gas or liquid pipelines

jet propulsion, land and sea transport (infancy state)Industrial turbines or prime moversBackground

Brayton CycleUnlike diesels, operate on STEADY-FLOW cycleOpen cycle, unheated engine1-2: Compression

2-3: Combustion

3-4: Expansion through Turbine and Exhaust Nozzle

(4-1: Atmospheric Pressure)

*Output 22.3 MW Thermal Eff. 40%

Basic Components

Basic Components

Basic ComponentsCompressorDraws in air & compresses itCombustion ChamberFuel pumped in and ignited to burn with compressed airTurbineHot gases converted to workCan drive compressor & external load

Basic ComponentsCompressorDraws in air & compresses itCombustion ChamberFuel pumped in and ignited to burn with compressed airTurbineHot gases converted to workCan drive compressor & external load

Basic ComponentsCompressorDraws in air & compresses itCombustion ChamberFuel pumped in and ignited to burn with compressed airTurbineHot gases converted to workCan drive compressor & external load

CompressorSupplies high pressure air for combustion processCompressor typesRadial/centrifugal flow compressorAxial flow compressor

CompressorRadial/centrifugal flowAdv: simple design, good for low compression ratios (5:1)Disadv: Difficult to stage, less efficient

Axial flow Good for high compression ratios (20:1)Most commonly used

CompressorControlling Load on CompressorTo ensure maximum efficiency and allow for flexibility, compressor can be split into HP & LP sectionsVane control: inlet vanes/nozzle angles can be varied to control air flowCompressor StallInterruption of air flow due to turbulence

Use of Compressed AirPrimary Air (30%)Passes directly to combustor for combustion processSecondary Air (65%)Passes through holes in perforated inner shell & mixes with combustion gasesFilm Cooling Air (5%)Insulates/cools turbine blades

Combustion ChambersWhere air & fuel are mixed, ignited, and burnedSpark plugs used to ignite fuelTypesCan: for small, centrifugal compressorsAnnular: for larger, axial compressorsCan-annular: rarely used

TurbinesConsists of one or more stages designed to develop rotational energyUses sets of nozzles & bladesSingle shaftPower coupling on same shaft as turbineSame shaft drives rotor of compressor and power components

TurbinesSplit ShaftGas generator turbine drives compressorPower turbine separate from gas generator turbinePower turbine driven by exhaust from gas generator turbinePower turbine drives power coupling

Dual Shaft, Split Shaft

Gas Turbine SystemsAir SystemAir intakes are located high up & multiple filtersExhaust discharged out stacksFuel SystemUses either DFM or JP-5Lubrication SystemSupply bearings and gears with oil

Gas Turbine Accessory SystemsStarting SystemTo get compressor initially rotated, HP air used (can use electrical also)Once at certain RPM, fuel injected and spark ignitedPower Transmission SystemReduction gears used to transfer torqueWith split shaft, turbines can run @ different speeds

GTG vs SteamFor the same hP,Weight reduction of 70%Simpler (less maintenance, fewer components)Reduced manning automated controlQuicker response timeModular replacement

Engine Power TransferTurbojetThrust provided by reaction against expansion of exhaust gasesTurbofanThrust provided by reaction against expansion of large volumes of airTurbopropThrust provided by turbineSCRAMjet/RAMjet

*Characteristics and ApplicationsThe turbojet engine : Turbojet engine derives its thrust by highly accelerating a mass of air , all of which goes through the engine. Since a high " jet " velocity is required to obtain an acceptable of thrust, the turbine of turbo jet is designed to extract only enough power from the hot gas stream to drive the compressor and accessories . All of the propulsive force (100% of thrust ) produced by a jet engine derived from exhaust gas.

*The turboprop engine : Turboprop engine derives its propulsion by the conversion of the majority of gas stream energy into mechanical power to drive the compressor , accessories , and the propeller load. The shaft on which the turbine is mounted drives the propeller through the propeller reduction gear system . Approximately 90% of thrust comes from propeller and about only 10% comes from exhaust gas.

The turbofan engine : Turbofan engine has a duct enclosed fan mounted at the front of the engine and driven either mechanically at the same speed as the compressor , or by an independent turbine located to the rear of the compressor drive turbine The fan air can exit seperately from the primary engine air , or it can be ducted back to mix with the primary's air at the rear .

*The turboshaft engine : Turboshaft engine derives its propulsion by the conversion of the majority of gas stream energy into mechanical power to drive the compressor , accessories , just like the turboprop engine but the shaft on which the turbine is mounted drives something other than an aircraft propeller such as the rotor of a helicopter through the reduction gearbox . The engine is called turboshaft.

*

a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow. Ramjets/scramjet relies on high vehicle speed to forcefully compress the incoming air before combustion scramjet is supersonic throughout the entire engine. ramjet decelerates the air to subsonic velocities before combustionoperate efficiently at extremely high speeds: theoretical projections place the top speed of a scramjet between Mach12 (8,400mph; 14,000km/h) and Mach24 (16,000mph; 25,000km/h)RAMJET/SCRAMJET

*Gas TurbineminCpTin(min+mF)CpToutShaft power mFqRcombEnergy balance

Simplistic Gas Turbines working principles 1-2 Isentropic compression (in a compressor); h2-h1 = mCp(T2-T1)2-3 Constant pressure heat addition (in a combustor); h3-h2 = mCp(T3-T2)3-4 Isentropic expansion (in a turbine); h3-h4 = mCp(T3-T4)4-1 Constant pressure heat rejectionWnett = Wturbine - Wcompressor Wnett = Cp [ (T3-T4) - (T2-T1) ] Because T3-T4 > T2-T1, then Wnett > 0

OPEN CYCLECLOSED CYCLE

Based on the air supply

*Brayton Cycle: Ideal Cycle for Gas-Turbine EnginesGas turbines usually operate on an open cycle (Fig. 929). Air at ambient conditions is drawn into the compressor, where its temperature and pressure are raised. The high pressure air proceeds into the combustion chamber, where the fuel is burned at constant pressure. The high-temperature gases then enter the turbine where they expand to atmospheric pressure while producing power output. Some of the output power is used to drive the compressor.The exhaust gases leaving the turbine are thrown out (not re-circulated), causing the cycle to be classified as an open cycle.

*The open gas-turbine cycle can be modelled as a closed cycle, using the air-standard assumptions (Fig. 930). The compression and expansion processes remain the same, but the combustion process is replaced by a constant-pressure heat addition process from an external source. The exhaust process is replaced by a constant-pressure heat rejection process to the ambient air.Closed Cycle Model

*The thermal efficiency of an ideal Brayton cycle depends on the pressure ratio, rp of the gas turbine and the specific heat ratio, k of the working fluid.The thermal efficiency increases with both of these parameters, which is also the case for actual gas turbines.A plot of thermal efficiency versus the pressure ratio is shown in Fig. 932, for the case of k =1.4.Parameters Affecting Thermal Efficiency

*Actual Gas-Turbine CyclesSome pressure drop occurs during the heat-addition and heat rejection processes. The actual work input to the compressor is more, and the actual work output from the turbine is less, because of irreversibilities.Deviation of actual compressor and turbine behavior from the idealized isentropic behavior can be accountedfor by utilizing isentropic efficiencies of the turbine and compressor.Turbine:Compressor:

*Brayton Cycle With RegenerationTemperature of the exhaust gas leaving the turbine is higher than the temperature of the air leaving the compressor (T4 > T2)The air leaving the compressor can be heated by the hot exhaust gases in a counter-flow heat exchanger (a regenerator or recuperator) a process called regeneration (Fig. 9-38 & Fig. 9-39). The thermal efficiency of the Brayton cycle increases due to regeneration since less fuel is used for the same work output.Note: The use of a regenerator is recommended only when the turbine exhaust temperature is higher than the compressor exit temperature.

*Effectiveness of the regenerator,Effectiveness under cold-air standard assumptions,Thermal efficiency under cold-air standard assumptions,Effectiveness of the RegeneratorAssuming the regenerator is well insulated and changes in kinetic and potential energies are negligible, the actual and maximum heat transfers from the exhaust gases to the air can be expressed as

*Thermal efficiency of Brayton cycle with regeneration depends on: a) ratio of the minimum to maximum temperatures, and b) the pressure ratio. Regeneration is most effective at lower pressure ratios and small minimum-to-maximum temperature ratios.Factors Affecting Thermal Efficiency

*Brayton Cycle With Intercooling, Reheating, & RegenerationThe net work output of a gas-turbine cycle can be increased by either: a) decreasing the compressor work, or b) increasing the turbine work, or c) both.The compressor work input can be decreased by carrying out the compression process in stages and cooling the gas in between (Fig. 9-42), using multistage compression with intercooling.The work output of a turbine can be increased by expanding the gas in stages and reheating it in between, utilizing a multistage expansion with reheating.

*Physical arrangement of an ideal two-stage gas-turbine cycle with intercooling, reheating, and regeneration is shown in Fig. 9-43.

*The work input to a two-stage compressor is minimized when equal pressure ratios are maintained across each stage. This procedure also maximizes the turbine work output.Thus, for best performance we have,Conditions for Best PerformanceIntercooling and reheating always decreases thermal efficiency unless are accompanied by regeneration.Therefore, in gas turbine power plants, intercooling and reheating are always used in conjunction with regeneration.

*As you refer to T-s diagram, the higher the entropy at constant pressure, the higher the entahlpy, and it's not linear line, It follows certain power series. It means delta enthalpy(energy) of compressor and turbine will be different, thus There is some net work available. W_nett/m_dot = W_turbine - W_compressor, assume cold standard analysis valid. W_Nett/m_dot = Cp*[ (T3-T4) - (T2-T1) ] Because T3-T4 > T2-T1, then W_nett > 0

A turbine is an open thermodynamic system. Assuming it to be in a steady state, we get: Q - W = m ( h_e - h_i + (V_e^2 - V_i^2)/2 + g(z_e - z_i) ) For a turbine,we generally have Q = 0 (well insulated, no heat gain/loss), and the second and third terms on the RHS (representing changes in kinetic energy and gravitational potential energy) are negligible. Hence, we have: W = m (h_i - h_e) Now, if we have the fluid to be an ideal gas with constant specific heatsh (T) = Cp dTU (T) = Cv dT

, then we get: W = m Cp (T_i - T_e)

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