Combustion chamber. Fig.1: Schematic for an open gas-turbine cycle.

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Raymond Lewis
7 years ago
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1 Open Ga urbine Cycle Fuel Combution camber urbine Saft Compreor W net Air Combution product Woring rincipal Fig.: Scematic for an open ga-turbine cycle. Fre air enter te compreor at ambient temperature were it preure and temperature are increaed. e ig preure air enter te combution camber were te fuel i burned at contant preure. e ig temperature (and preure) ga enter te turbine were it expand to ambient preure and produce wor. Feature: Ga-turbine i ued in aircraft propulion and electric power generation. Hig termal efficiencie up to %. Suitable for combined cycle (wit team power plant) Hig power to weigt ratio, ig reliability, long life Fat tart up time, about min, compared to r for team-propulion ytem Hig bac wor ratio (ratio of compreor wor to te turbine wor), up to 0%, compared to few percent in team power plant. Brayton Cycle Brayton cycle i te ideal cycle for ga-turbine engine in wic te woring fluid undergoe a cloed loop. at i te combution and exaut procee are modeled by contant-preure eat addition and rejection, repectively. M. Barami ENSC (S ) Brayton Cycle

e ig temperature (and preure) ga enter te turbine were it expand to ambient preure and produce wor. Feature: Ga-turbine i ued in aircraft propulion and electric power generation.

2 M. Barami ENSC (S ) Brayton Cycle e Brayton ideal cycle i made up of four internally reverible procee: - ientropic compreion (in compreor) - cont. preure eat-addition (in combution camber) - ientropic expanion (in turbine) - cont. preure eat rejection (exaut) Fig. : - and -v diagram for ideal Brayton cycle. ermal efficiency for te Brayton cycle i:,, r r tu Brayton t in out Brayton t were r i called te preure ration and = c p c v i te pecific eat ratio. Maximum reure Ratio Given tat te maximum and minimum temperature can be precribed for te Brayton cycle, a cange in te preure ratio can reult in a cange in te wor output from te cycle. Q L = Cont. = Cont. v Ientropic Ientropic Q L

ermal efficiency for te Brayton cycle i:,, r r tu Brayton t in out Brayton t were r i called te preure ration and = c p c v i te pecific eat ratio.

3 e maximum temperature in te cycle i limited by metallurgical condition becaue te turbine blade cannot utain temperature above 00 K. Higer temperature (up to 00 K can be obtained wit ceramic turbine blade). e minimum temperature i et by te air temperature at te inlet to te engine. Fig. : An optimum preure ratio exit for a given max., min. temperature ratio tat maximized te net wor output. Actual Brayton Cycle Irreveribilitie exit in actual cycle. Mot important difference are deviation of actual compreor and turbine from idealized ientropic compreionexpanion, and preure drop in combution camber. M. Barami ENSC (S ) Brayton Cycle

: An optimum preure ratio exit for a given max., min. temperature ratio tat maximized te net wor output.

4 reure drop a a reure drop c w w a Fig. : Actual Brayton cycle. a wa w a e Brayton Cycle wit Regeneration e ig preure air leaving te compreor can be eated by tranferring eat from exaut gae in a counter-flow eat excanger wic i called a regenerator. Regenerator Q Combution camber Combution product W net Saft urbine Compreor Air Fig. : Scematic for a Brayton cycle wit regenerator. M. Barami ENSC (S ) Brayton Cycle

tranferring eat from exaut gae in a counter-flow eat excanger wic i called a regenerator.

5 Q regen Regeneration Q regen Q L Fig. : - diagram for a Brayton cycle wit regeneration. In te ideal cae, te air exit te regenerator at te inlet temperature of te exaut gae,. One can write: regen, actual = regen, max = = e effectivene of regenerator i defined a: regen, act regen,max auming cold - air - tandard condition ermal efficiency of an ideal Brayton cycle wit regenerator can be found from: t, regen r p e Brayton Cycle wit Intercooling, Reeating, and Regeneration e net wor output of te cycle can be increaed by reducing te wor input to te compreor andor by increaing te wor output from turbine (or bot). Uing multi-tage compreion wit intercooling reduce te wor input te compreor. A te number of tage i increaed, te compreion proce become nearly iotermal at te compreor inlet temperature, and te compreion wor decreae. Liewie utilizing multitage expanion wit reeat (in a multi-turbine arrangement) will increae te wor produced by turbine. M. Barami ENSC (S ) Brayton Cycle

regenerator can be found from: t, regen r p e Brayton Cycle wit Intercooling, Reeating, and Regeneration e net wor output of te cycle can be increaed by reducing te wor input to te compreor andor by

6 0 Regenerator Compreor Combution camber Q Reeater Reeater W net Intercooler Compreor urbine urbine Q Intercooler Fig. 7: A ga-turbine engine wit two-tage compreion wit intercooling, two-tage expanion wit reeating, and regeneration. Wen intercooling and reeating are ued, regeneration become more attractive ince a greater potential for regeneration exit. e bac wor ratio of a ga-turbine improve a a reult of intercooling and reeating. However; intercooling and reeating decreae termal efficiency unle tey are accompanied wit regeneration. Q in Q in 8 Q out Q regen. 7 0 Q out 9 Fig. 8: - diagram for an ideal ga-turbine cycle wit intercooling, reeating, and regeneration. M. Barami ENSC (S ) Brayton Cycle

Wen intercooling and reeating are ued, regeneration become more attractive ince a greater potential for regeneration exit.

7 A own in Fig. 8: =, = In an ideal regenerator, = 9. In practice (actual regenerator), < 9. 8 =, 7 = 9 e net wor input to a two-tage compreor i minimized wen eual preure ratio are maintained acro eac tage. at i: i procedure alo maximize te turbine wor output M. Barami ENSC (S ) Brayton Cycle 7

Ideal Rankine Cycle T 1 2

Ideal Rankine Cycle T 1 2 Vapor Poer Cycle We kno that the Carnot cycle i mot efficient cycle operatg beteen to pecified temperature limit. Hoever; the Carnot cycle i not a uitable model for team poer cycle ce: he turbe ha to handle