TehniÄka termodinamika - Kemijsko-tehnoloÅ¡ki fakultet

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__________________________________Tehnička postrojenja za provedbu kružnih procesa Bez prigušenja, za komprimiranje bila bi potrebna mehanička energija proporcionalna površini a – 1 – 2 – d u p, v – dijagramu. To je minimalan iznos za kompresiju od tlaka p1 na tlak p2 pa je prema h, s – dijagramu w min. = h − h . Stvarno potrebna mehanička energija bit će veća i proporcionalna je površini b – 1’ – 2’ – c u p, v – dijagramu ili w = ′ 2 h 2 − h 1 prema h, s – dijagramu. Prividna krivulja kompresije predočena je također točkasto na slikama 86. i 87. između stanja 1 i 2’’. 1 6.1.2. Ekspanzija i kompresija u turbostrojevima Kroz turbostrojeve radna tvar struji neprekidno pa se njezino stanje ne mijenja periodički kao u stapnim strojevima, već se ta promjena zbiva kontinuirano uzduž puta kroz turbostroj. Na pojedinim mjestima stroja stanje radne tvari je točno definirano i nepromjenjivo. Zbog relativno velikih površina presjeka dovoda i odvoda, mogu se zanemariti prigušenja u njima, ali se mora računati na utjecaj trenja jer radna tvar struji velikom brzinom kroz turbostroj. Toplina proizvedene trenjem ostaje u radnoj tvari pa joj se povećava specifični volumen, a posljedica će biti pomak ekspanzijske krivulje u desno. U p, v – dijagramu na slici 88.a, ekspanzija bez trenja prikazana je adijabatom 1 – 2, a krivuljom 1 – 2’ ekspanzija s trenjem. Taj proces predočen je na slici 88.b i u h, s – dijagramu. Prividna krivulja ekspanzije u stapnim strojevima jednaka je stvarnoj krivulji u turbostrojevima. Za maksimalan mehanički rad vrijedi izraz a za stvarni mehanički rad w max. = h − h , 1 1 2 ′ 2 w = h − h . Analogno tome bit će i za kompresiju u turbostrojevima, slika 89. Minimalna potrebna energija za kompresiju određena je izrazom w min. = h − h , a stvarno potrebna relacijom w = h 2 ′ − h1 . 2 1 273
TEHNIČKA TERMODINAMIKA__________________________________________ Slika 88. Ekspanzija u turbostrojevima s gubicima zbog trenja u p, v – dijagramu i h, s – dijagramu Slika 89. Kompresija u turbostrojevima s gubicima zbog trenja u p, v – dijagramu i h, s – dijagramu 274
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TehniÄka termodinamika - Kemijsko-tehnoloÅ¡ki fakultet

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