Turbine with core working concept



Turbine with core working concept


GAS TURBINE



Before knowing gas turbine we should know , What is engine?


· Engine is a mechanical device which convert one form of energy into another useful from.


· For example- chemical energy stored in the fuel is converted into heat energy, heat is converted into shaft work etc.

















    Types of engine:



    1. IC Engine (Internal combustion engine)-


    In this combustion and expansion takes place at same location or fuel is the

    working fluid.


    2. EC Engine (external combustion engine)


    In this case combustion and expansion takes place at different location or product of combustion transfer their heat to


    the another working fluid which is utilized to producing some useful output.


    Turbine:



    Turbine is a mechanical device in which energy is transferred from the

    working fluid to rotor or called turbine blade.


    In this case of turbine expansion of the working fluid takes place which

    results decrease in pressure.


    Turbine is also called expansion device it means the work fluid comes

    with high energy and gets converted into shaft


    work with the help of turbine.





                 Block diagram of turbine.



    We can write SFEE (steady flow energy equation ) for Turbine







    Using SFEE,







    …(General Equation for Turbine)




    Assumption for some problem for understanding,


    We can say It’s a special case,


    1. Adiabatic turbine means heat interaction with surrounding is zero.


    i.e Q=0


    2. Elevations are same, hence Z1=Z2


    3. Neglecting change in Kinetic energy


    Hence, C1=C2


    Finally, Turbine work is ,


    WT=m(h1-h2) in Kj

    Discussion on Gas Turbine:


    Gas turbine basically work on Brayton cycle in which air is the working fluid(Remember we are talking about gas turbine).


    The major fields of application of gas Turbines are:


    1. Power generation


    2. Aviation


    3. Oil and gas industries


    4. Marine propulsion ,ete.


    Some advantage of gas turbine over IC Engine


    1 High speed can be developed due to the rotary engine as it is in IC engine has reciprocating piston cylinder arrangement.


    2 Easy balancing.


    3 Simple mechanism and compact design.









    Disadvantage of Gas turbine over IC engine


    1.cpmressor handles the gaseous phase of the working fluid which
    is not negligible incomparision to the Turbine work therefore.
    it reduces the net work output of the cycle.

    WorkNET = WT – Wc
    2. slow response to acceleration
    3. Piston stroke is very large in case of reciprocating compressor so
    it is not used in Automobiles applications.
    4. High speed reduction gears are required.
    5.Heat resistant material is required as in Gas Turbine the blades are subjected to higher temperature continuously.
    6. Required a starting motor because it is not self starting.
    Types of gas turbine






    Open cycle Gas Turbine- In the case of open cycle gas turbine

    every cycle fresh charge is introduced into the compressor
    and at the end of expansion exhausted to the atmosphere.











    Here,


    · At point 1 fresh air enters into the compressor and compressed upto


    a pressure P2 which is generally above the atmospheric pressure.


    · This process is done in isentropic manner, hence it is called isentropic process.


    · At point 2, is a combustion chamber is provided in such the manner it supplied


    head by use of fuel such as coal etc.


    · It is basically a heat exchanger device and it is operating at constant pressure.


    · At last point a turbine is provided in such a manner it is capable of expanding


    the working fluid or heat energy coming from the combustion chamber.


    · By expansion a shaft work is obtained which is known as Turbine work.


    · This process is done isentropically.


    · After the turbine it exhausted to the atmosphere because it is open cycle gas


    turbine and so forth again fresh air is taken to compressor for next cycle.


    T-S and P-V Diagram

















    Process 1-2: Isentropic compression


    Process 2-3: Heat addition at constant pressure


    Process3-4: Isentropic expansion


    Process-4-1 heat rejection to the atmosphere and fresh air is entering to the compressor inlet.





    Closed cycle Gas Turbine












    In closed cycle gas turbine the exhaust gas is recirculated again as we seen in


    case of open cycle gas turbine exhaust gas is emitted to the open atmosphere.


    Hence this is used to preheat the inlet air in order to increase the enthalpy of the gas.


    Here, basically four process are involve during the cycle.


    Process 1-2 Isentropic compression.


    Process 2-3 Constant pressure heat addition


    Process 3-4 isentropic heat rejection


    Process 4-1 Constant pressure heat rejection


    We can draw P-V and T-S diagram for Close cycle gas turbine:





    T-S and P-V Diagram



    m
















    As I have been already told about these four process through which this cycle produce work.


    It is basically Brayton cycle .


    Brayton cycle has two isentropic processes and two constant pressure processes.


    Here Air the working medium for the Brayton cycle.

    AIR STANDARD ASSUMPTION FOR THE BRAYTON CYCLE:



    · Air is treated an Ideal gas.


    · Cp, Cv and Ÿ are taken treated as constant with respect to temperature.


    · Heat is added through the external source at constant pressure.


    · It should be a closed cycle analysis.


    · Neglecting pressure, kinetic energy and potential energy drop.


    · All the process are internally reversible.

    Advantage of closed cycle gas turbin over open cycle gas turbine:


    · Wide variety of fuel is used because the product of combustion does not enter the turbine.


    · Better working fluid can be used like Argon, Helium having index 1.67.


    · It can operate even low atmospheric pressure.


    · Having maximum efficiency between the same maximum and minimum temperature limit.

    Disadvantage of closed cycle gas turbine over open cycle gas turbine:






    · A coolant is used to cooldown the turbine exhaust before entering the compressor where as in case of open cycle gas turbine atmosphere is acting as a sink.


    · Absolute leak proofing is very difficult to achieve.


    · System is bulky, complicated, costly therefor it is not suitable for air craft application,




    Basically we should know Brayton cycle and their efficiency.





    Mathematically , efficiency is the ratio of output to the input.




    i.e,







    for improving the efficiency there are some methods are used:


    1. Regeneration: In regeneration basically exhaust gas is recirculated through the combustion chamber hence total heat supplied is reduced as compared to simple cycle. It means tip of the compressor is heating by tip of the turbine by utilising exhaust gas.


    2. Reheating: Here exhaust gases further heated by using heat supplied means multiple time heating at constant pressure.


    3. Intercooling: It is just reverse to the reheating because in this compressed air is cool down at constant pressure in order to increase the cycle efficiency.


    The device used used for intercooling is known as intercooler.


    Similarly, superheater is used for superheating.






    Thanks to the reader to know the Mathematical derivation of Brayton cycle efficiency visit the uploaded image below.


    Brayton cycle efficiency image:






    (Thanks)



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