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  • Inside It : Brayton Cycle

    raj87verma88

    Member

    Updated: Oct 26, 2024
    Views: 1.2K
    During her KYCEan interview, CEan Durga mentioned that she would like something like "How Stuff Works" on CE. The idea sounded good and I have decided to write the first "Inside It" We currently do not have a seperate section for this and each "Inside It" would be posted under the appropriate engineerig section.

    Brayton Cycle
    (Source: Wikipedia, Thermodynamice by Yunus A. Cengel and Michael A. Boles, Thermal Engineering by P.L. Ballaney, Engineering Thermodynamics by P.K. Nag)
    ​

    Introduction


    Proposed by George Brayton an American Engineer for use in the reciprocating oil-burning engine he had developed in around 1870, though some sources claim that it was originally patented by an English John Barber in 1871.
    It is used for Gas turbines and to define modern aircraft engines. The Brayton Cycle is the only cycle which can be used for both External and Internal Combustion Engines.


    Working


    Usually it operates on an Open Cycle Fig 1(A). Fresh air at ambient conditions enters the Compressor, the temperature and pressure are raised. Air at high pressure enters the combustion chamber, where the fuel is burned at a constant pressure. The high temperature exhaust gases enter the turbine, where they expand to atmospheric pressure produckng power. The exhaust gases are then thrown out.
    This cycle can be modeled into a closed cycle by assuming standard air-assumptions. It also makes the thermodynamic analysis more easy. Fig 1(B) shows the closed loop Brayton Cycle. The compression and expansion processes are the same. Combustion is replaced by constant-pressure heat addition, and exhaust by constant pressure heat rejection process to the atmosphere.


    [​IMG]
    Open and Close Loop Brayton Cycle
    Image: #-Link-Snipped-#
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  • raj87verma88

    Member • May 23, 2009

    Derivation of Equation and Cycle Efficiency

    The T-S and P-V diagram of an ideal Brayton Cycle are as follows.

    [​IMG]
    Image: #-Link-Snipped-#


    As seen the isdeal cycle is made of 4 internally reversible cycles.
    1-2 Isentropic Compression (in compressor)
    2-3 Constant Pressure heat addition
    3-4 Isentropic Expansion (in turbine)
    4-1 Constant Pressure heat rejection

    Note: All four processes are executed in steady flow devices and should be analyzed as steady flow processes

    When change in Kinetic and Potential Energies are neglected, energy balance equation for steady-flow process can be expressed, on a unit mass basis as:

    (q in - q out) + (w in - w out) = h exit - h inlet

    Therefore, heat transfer to and from working fluid are

    qin = h3 - h2 = cp(T3 - T2)

    qout = h4 - h1 = cp(T4 - T1)

    Thermal efficiency(Neta) of ideal cycle,

    Neta(thermal brayton) = wnet / qin = 1 - [qout/qin] = 1 - [cp(T4 - T1)/cp(T3 - T2)]
    = 1 - [T1(T4/T1 - 1)/T2(T3/T2 - 1)]


    We know 1-2 and 3-4 are isentropic. P2 = P3 and P4 = P1



    T2/T1 = (P2/P1)^(k-1)/k = (P3/P4)^(k-1)/k = T3/T4

    Substituting these values in the efficiency equation

    Neta(Thermal Brayton) = 1 - [ 1/rp^(k-1)/k]

    where rp = P1/P2, it is called pressure ratio.

    k is specific heat ratio.

    The efficiency equation shows that thermal efficiency of a Brayton Cycle depends upon pressure ration of Gas Turbine and specific heat ratio of working fluids.
    Thermal efficiancy increases with both these factors.

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  • raj87verma88

    Member • May 23, 2009

    Thats it for today. The total working with all factors would be completed in a few days.
    As this is the first thread under "Inside It" series, this would be a rather haphazard. I would try to co-ordinate the other posts in a more proper manner.
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  • raj87verma88

    Member • May 23, 2009

    Function of Air in a Gas Turbine:-


    1. Provides oxidants for fuel combustion
    2. As a coolent to keep component's temperature within safe limits
    Application of Brayton Cycle


    • Electric Power Generation: The exhaust from the combustion chamber are at high speed. There kinetic energy is converted work i.e. rotation of the propeller connected to a shaft which is connected to a generator and produces electric power. They are also used in conjunction with Steam Power Plants. The high temperature exhaust is used as a heat source for generating steam.
    • Aircraft Propulsion: Gas turbine produces just enough power to drive a small generator and provide power to small auxiliary components. The high velocity exhaust gases provide the required thrust for aircraft propulsion.
    • It can also be used as a closed loop cycle in Nuclear Power Plant. Usually a gas with more desirable qualities like Helium is used as working fluid.
    • Propulsion of Ships: Majority of the Naval fleets in the west use Gas-Turbine engines for propulsion and electricity generation. Some modern ships use gas turbine combined with diesel engines. Diesel provides efficient low-power and cruise operation, gas turbine is used when high speeds are needed.
    Note: Ratio of compressor work to turbine work is called back work ratio. It is very high in gas turbines. Half of turbine work output is used to drive compressor. In contrast to steam turbine where back work ration is a few percent. is because in steam turbines a liquid is compressed.
    That is why Gas power plant turbines are larger than those of steam power plants of same net power output.

    Keep tuned for more. We have a few posts left before this cycle is complete.
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  • durga ch

    Member • May 23, 2009

    :shocked:

    that's a looott of info for people in your field 😁 awesome !!!I scanned through it ! 😁
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  • Ashraf HZ

    Member • May 23, 2009

    This is a great initiative, patty! Honestly, the title "Inside It" took a while to accept, but I think its arlight =)

    Looks like the rest of us can follow Patty's format and post their own "Inside It" articles.
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  • silverscorpion

    Member • May 23, 2009

    Looks cool. good initiative.

    I lost contact with thermodynamics years ago. So, I couldn't follow this article as well as I'd wish to. But still, I read through it. It was nice.

    I'll try to post some Inside it articles of my own, of course in electronics.
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  • ShrinkDWorld

    Member • May 23, 2009

    I am realy thank full man you do so much work for us
    You are explain in brief man!!!!! thanks.
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  • raj87verma88

    Member • May 24, 2009

    Deviation of Actual Cycle from Ideal Cycle


    1. There is some pressure drop during heat-addition and rejection processes
    2. Due to irreversibilities, actual work input to compressor is more and, actual work output from turbine is less
    The deviation can be accurately accounted for by utilizing isentropic efficiencies of turbine and compressor :-

    Neta(compressor) = ws/wa = (h2s - h1)/(h2a - h1)

    Neta(turbine) = wa/ws = (h3 - h4a)/(h3 - h4s)

    2a, 4a :- actual exit states at compressor and turbine.

    2s, 4s :- corresponding values for isentropic case.


    Blue Line shows actual turbine diagram and black shows the ideal case
    [​IMG]
    Picture: #-Link-Snipped-#


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  • raj87verma88

    Member • May 24, 2009

    Developments in Gas Turbine
    ​

    Gas turbine has progressed a lot since the 1930s when it was first successfully developed. Till 1950s the single cycle efficiency was 17% because of low turbine inlet temperature due to metallurgic limitation and low compressor/turbine efficiency.
    Efforts were taken to improve cycle efficiency:-


    1. Increasing turbine inlet temperature This was the primary approach to increase efficiency. The inlet temperature has increased from 500 C to 1425 C. This was possible due to development of new materials and innovative cooling techniques.
    2. Increasing efficiencies of turbomachinery components Use of computers in manufacturing, CAD/CAM made it possible to design the components aerodynamically with less losses
    3. Modifications to the Cycle Efficiency was almost doubled by adding regeneration, reheating and intercooling (discussed later). The negative side was the increase in initial and operating cost. There use can not be justified unless decrease in fuel cost offsets increase in others. But 40% increase in efficiency and relatively low fuel cost was in favour of this method.
    The first Gas Turbine for power generation was installed in Oklahoma in 1949. Built by GE, it produced 3.4 MW. In between there was a shift towards coal and nuclear power plants. But now many are using natural gas fired turbines, as they are more efficient, have low capital cost, shorter installation time, better emmision standard, natural gas is abundant. These turbines are used for both base load power production and during peaking.
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  • krishthd

    Member • May 24, 2009

    very very informative
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  • raj87verma88

    Member • May 24, 2009

    Thanks a lot for the appreciation, guys.
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  • shalini_goel14

    Member • May 24, 2009

    Hey raj, This was good information shared but I guess it would arouse more interest of people like me (who are not of your field) if you take any real life object and can tell us how these Cycle's work in it. I mean practical knowledge sharing has chances to beat theoretical knowledge.

    PS: Never mind ok. Just a feedback. 😀
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  • raj87verma88

    Member • May 25, 2009

    The application of the cycle will come up later. I was thinking something similar earlier. This thread looks more like technical paper and this is not what I had in mind. SS said that he was unable to follow as he had left Thermodynamics years ago, same may be said for others.
    I am thinking of creating all "Inside It" articles in a way, that they describe the History, Anatomy, Working and Application without going into deep technical details like I did here.

    This cycle is not yet complete and it will be, maximum by, day after tomorrow. I will continue this one as such but will change the format for other thread under said heading.
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  • raj87verma88

    Member • May 26, 2009

    Brayton Cycle with Regeneration

    #-Link-Snipped-#

    Image: #-Link-Snipped-#

    The temperature of the exhaust gases is usually very high. By incorporating a regenerator as shown, the high pressure gas from the compressor can be pre-heated via a counter flow heat exchanger called regenerator.
    This increases the thermal efficiency of the cycle as the rejected heat is put to use.

    Note: It should be made sure that the temperature of exhaust is greater than compressed air, otherwise the heat will flow in opposite direction and the efficiency would decrease.


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  • raj87verma88

    Member • May 26, 2009

    Brayton Cycle with Intercooling, Reheating and Regeneration

    #-Link-Snipped-#
    Image:#-Link-Snipped-#

    This is another way of increasing the efficiency of the Brayton Cycle.
    The work output is increased by incorporating multistage expansion and reheating without raising the maximum temperature of the cycle.
    The basic principle is: The steady flow compression or expansion work is proportional to the volume of fluid. Therefore, specific volume of fluid should be as low as possible during a compression process and as high as possible during expansion.


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  • g_rakesh2

    Member • May 26, 2009

    raj very good information i had not studied this when I was in college..

    I just dont remeber all these thanks good information...
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  • raj87verma88

    Member • Jun 12, 2009

    Application of Brayton Cycle

    This cycle is mos widely used in two fields


    • Aircraft propulsion
    • ElectricityGeneration
    Aircraft Propulsion or rather Jet propulsion is described in the following thread
    #-Link-Snipped-#
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  • raj87verma88

    Member • Jun 12, 2009

    g_rakesh2
    raj very good information i had not studied this when I was in college..

    I just dont remeber all these thanks good information...
    Thank You.
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  • yadavundertaker mohit

    Member • Sep 20, 2009

    thank you very much....it will help me in my gate preparations...
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  • blacrobous

    Member • Sep 21, 2009

    nice that helped me to brush up my basics 😁
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  • 4d1

    Member • Sep 24, 2009

    nice..keep up the good work 😁
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  • KKOSS

    Member • Jan 22, 2010

    I need to know "rule of thumb" energy balance for gas turbine engines. I am very familiar with reciprocating engine energy balance, which is used to design a the support systems for engine testing. For example, we use on most diesel engines:
    Radiated heat: 5%
    Jacket Water: 24%
    Dyno (Shaft): 35%
    Exhaust Heat: 28%
    Intercooler: 8%

    So for a 100HP diesel engine, (100HP/35%) - 286HP of fuel input it required. Is there similar energy balance for typical 100-500HP turbine engines?
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