1.2 brayton cycle

Brayton or Joule cycle -P-V diagram and thermal efficiency. �Construction and working of gas turbine i] Open cycle ii] Closed cycle gas turbine, �simple circuit, �Comparison, �P-V & T-S diagram�Turbojet and Turboprop Engine and Application

1. 1.2 Brayton or Joule cycle -P-V diagram and thermal efficiency. Construction and working of gas turbine i ] Open cycle ii] Closed cycle gas turbine, simple circuit, Comparison, P-V & T-S diagram 1.3 Turbojet and Turboprop Engine and Application Subject: Heat Power Engineering – II Presented By Arup Kumar Sikdar Lecturer in Automobile Engineering Nalhati Government Polytechnic

2. Overview Understanding the operation of Brayton cycle with P-V & T-S diagram. Understand thermal efficiency and other performance terms related to Brayton cycle. Understand the turbojet and turboprop engine and its application.

3. Introduction of Brayton Cycle

4. Types of cycle Open Cycle Gas Turbine Power Plant In this type of plant the atmospheric air is charged into the combustor through a compressor and the exhaust of the turbine also discharge to the atmosphere. Closed Cycle Gas Turbine Power Plant In this type of power plant, the mass of air is constant, or another suitable gas used as working medium, circulates through the cycle over and over again. Open Cycle Closed Cycle

5. Working Principle of Brayton Cycle According to the principle of the Brayton cycle, air is compressed in the turbine compressor. The air is then mixed with fuel and burned under constant pressure conditions in the combustor. The resulting hot gas is allowed to expand through a turbine to perform work. Most of the work produced in the turbine is used to run the compressor and the rest is available to run auxiliary equipment and produce power.

6. Cycle The Brayton cycle consists of four internally reversible processes: 1-2 Isentropic compression (in a compressor) 2-3 Constant-pressure heat addition 3-4 Isentropic expansion (in a turbine) 4-1 Constant-pressure heat rejection Brayton cycle on P-V and T-S diagrams

7. The energy balance for a steady-flow process can be expressed as: ( q i n  q out )  ( w i n  w out )   h The heat transfer to and from the working fluid ca n be wr itt e n a s : q i n  h 3  h 2  c p ( T 3  T 2 ) q out  h 4  h 1  c p ( T 4  T 1 )

8. The thermal efficiency of the ideal Brayton cycle under the cold air standard assumptions becomes: P r o c esse s 1 - 2 an d 3 - 4 a re i se n t rop i c and P 2  P 3 and P 4  P 1 . The thermal efficiency of the ideal Brayton cycle under the cold air standard assumptions becomes :

9. Substituting these equations into the thermal efficiency relation and simplifying: The thermal efficiency of a Brayton cycle is therefore a function of the cycle pressure ratio and the ratio of specific heats.

10. Performance terms Pressure Ratio- Ratio of the cycle’s highest pressure to its lowest pressure. Work Ratio : Ratio of network output to the total work developed in the turbine. Air Ratio : kg of air entering the compressor inlet per unit of cycle net output, Kg/kWh Compression efficiency : Ratio of work needed for ideal air compressor through a given pressure range to work actually used by the compressor. Engine Efficiency : It is the ratio of the work actually developed by the turbine expanding hot power gas through a given pressure range to that would be yeilded for ideal expansion conditions Machine Efficiency : Collective term of engine efficiency and compressor efficiency of turbine and compressor. Combustion Efficiency : It is the ratio of heat actually released by 1 g of the fuel to heat that would be released by complete perfect combustion. Thermal Efficiency : It is the percentage of total energy input appearing as net work output of the cycle.

11. Difference between Open and closed cycle gas turbine S.no Closed Cycle Gas Turbine Open Cycle Gas Turbine 1. In this, first the air is compressed in the compressor and then heated in a heating chamber. As the air is heated by an external source, so the amount of the gas remains the same. Here the compressed air is heated in the combustion chamber. So the products of combustion gets mixed with the heated air and hence the amount of gas does not remain the same. 2. In closed cycle gas turbine, the gas that comes out from the gas turbine passes into the cooling chamber. In an open cycle, the gases coming out from the gas turbine is exhausted in the atmosphere. 3. The working fluid is circulated continuously. The working fluid is replaced continuously. 4. Any other fluid that possesses better thermodynamic properties like helium can be used. Here only air can be used as the working fluid. 5. No earlier wear of the turbine blades, because the enclosed gas does not get contaminated while flowing through the heating chamber, Earlier wear of turbine blades,  as the air from the atmosphere enters the combustion chamber it gets contaminated. 6. It is best suited for the stationary installation and marine uses because the air from the turbine is cooled by the circulating water. It is best suited for the moving vehicle because the air from the turbine is discharged into the atmosphere. 7. The maintenance cost of this  type of turbine  is high. Its maintenance cost is low. 8. More mass of installation per kW.  Less mass of installation per kW.

12. Difference Between I.C. Engine and Gas Turbine Mass/KW Developed Mass per KW development of a gas turbine is less than that of the I.C engine Installation Cost Installation cost of an I.C engine is much more than the gas turbine Running Cost Running cost of gas turbine is less than the I.C engine Efficiency Efficiency of gas turbine is much higher than the I.C Engine Balancing Gas turbines are perfectly balance while I.C engines are not perfectly balance Torque   Gas turbine can produce a uniform torque so there is no need of flywheel but I.C engine require flywheel because of non-uniform torque production Running Speed Gas turbine run at very high speed where I.C engines are design for medium level speed Pressure involvement Pressure required to run a gas turbine is very low while very high pressure is required for I.C engine Pollution Creation Gas turbines exhaust contain very small number of harmful gasses while I.C engine exhaust produce more pollution Starting  Starting of an I.C engine is simple while gas turbine starting is little complicated Application   Gas Turbines are suitable for air crafts while I.C engine are suitable for road vehicles

13. Turbojet engine A turbojet per see is the most basic type of a jet engine. In such a setup, the air taken in from the opening in the front of an engine is compressed to about 3 to 12 times its original pressure in the compressor. This air is then burned in combination with fuel in the combustion chamber to raise the temperature of the fluid mixture to about 600-to-700-degree Celsius. The resulting hot air is then passed through a turbine that in turn powers the compressor. If the turbine and compressor are efficient, the pressure coming to the turbine will be near twice the atmospheric pressure and this excess pressure is sent to the nozzle to produce a high-velocity stream of gas that produces the thrust. In addition to this, substantial thrust can further be created with the help of a second combustion chamber that is positioned after the turbine and before the nozzle. This results in an increased temperature and subsequently an increase of about 40 per cent in thrust. In a nutshell, the turbojet is a reaction engine where expanding gases push against the front of the engine. The turbojet sucks in air and either compresses or squeezes it. These gases bounce back and shoot out of the rear of exhaust, pushing the plane forward.

14. Turboprop Engines In case of a turboprop jet engine, the turbine at the back is turned by hot gasses and this in turns a shaft that spins the propeller. This type of engine is most common in small airliners and transport aircraft. Similar to a turbojet engine, a turboprop too has a compressor, a combustion chamber, and turbine. The air and gas are combined to run the turbine which then powers the compressor. As against a turbojet engine, a turboprop engine has better propulsion efficiency at speeds below 500 miles per hour. Modern turboprop engines come with a smaller diameter propeller with a greater number of blades for efficient operation at higher speeds. To cope up with the higher flight speeds, the blades are made in scimitar-shape with swept-back leading edges at the blade tips. Such kind of propellers is called propfans.

15. Differences between Turbojet and Turboprop engines S.no Turbojet engine turboprop engine 1. power produced by the turbine is used to drive compressor. Power produced by the turbine is used to drive the compressor and propeller. 2. Low take off thrust. High take off thrust . 3. Low propulsive efficiency. Good propulsive efficiency. 4. Reduction gear is not needed. Reduction gear needed 5. lower weight per unit thrust. Higher weight per unit thrust. 6. Low frontal area. Use of centrifugal compressor stages increases the frontal area

16. Review Question How many stages are there in gas turbine? (One/Two/Three/Four) Gas turbine engines are classified into _______ types.(5/4/3/2) Jet engines works on ______ cycle.(Brayton/dual/diesel/Otto) Internal energy in the fuel is converted into __________ of the exhaust in turbojets.(kinetic energy/pressure energy/kinetic & pressure energy/none of the mentioned) The Brayton cycle consists of a) two reversible isotherms and two reversible isobars b) two reversible isochores and two reversible adiabatic c) two reversible isotherms and two reversible isochores d) two reversible isobars and two reversible adiabatic For the same compression ratio, the efficiency of Brayton cycle is ____ the efficiency of Otto cycle. a) less than b) equal to c) greater than d) none of the mentioned  Which of the following statement is true? a) an IC engine is exposed to highest temperature only for a short period b) a gas turbine plant is always exposed to highest temperature used c) in a gas turbine plant, maximum temperature used should be less than that used in IC engine d) all the mentioned