Bazele termodinamicii - Curs 5

Cicluri termodinamice

Cicluri termodinamice

Ciclu termodinamic = o succesiune de procese termodinamice la sfritul crora sistemul revine n starea iniial.

Carnot: este imposibil producerea lucrului mecanic in mod continuu, fr un ciclu termodinamic.Principiul I

sistem nchis

QB1A = QSC > 0 , QSC Q schimbat cu sursa caldQA2B = QSR < 0 , QSR Q schimbat cu sursa rece

LC = QC = QSC + QSR = QSC - |QSR| ; QSC = LC + |QSR|

t = = = 1 - < 1 ; t randamentul termic

f =

- eficienta frigorifica

c = > 1- eficienta calorica

QC = LC = QSC + QSR = QSR - |QSC| |LC| = |QSC| - QSR

Ciclul Carnot direct si inversat

Ciclul Carnot direct (parcurs in sens orar)

-format din doua adiabate si doua izoterme

t= =

EMBED Equation.DSMT4

= =+=

EMBED Equation.DSMT4

EMBED Equation.DSMT4

===mRg TSC ln > 0 (V3 > V2)

===mRg TSR ln T0) si cea a mediului ambiant (T0).

Temperatura medie a sursei de caldura, TmQ

Q12=EQ12+ AQ12

AQ12=T0S12 Anergia calduriiPentru un ciclu termodinamic:

;

;

tc limita impusa de natura pentru transformarea caldurii in lucru mecanic

Ex randamentul exergetic - gradul de perfectiune a ciclului

EMBED Equation.3

S12

Q12

S

TmQ

2

1

EMBED Equation.3

n=1 (T1=ct)

TSC

1

EMBED Equation.3 EMBED Equation.3 q = Tds

(l = pdv in diagram p-v)

ciclu reversibil: EMBED Equation.DSMT4=0 EMBED Equation.DSMT4=EMBED Equation.DSMT4+EMBED Equation.DSMT4= 0 EMBED Equation.3

EMBED Equation.DSMT4= - EMBED Equation.DSMT4= EMBED Equation.DSMT4= EMBED Equation.3

Integrala lui Clausius pentru procese reversibile nu depinde de drum

1-2 - comprimare adiabatica

2-3 - incalzire izotermica de la sursa calda

3-4 - destindere adiabatica

4-1 - racire izotermica la sursa rece

1-2-comprimare adiabatica

2-3-cedarea de caldura izotermica la sursa calda

3-4-destindere adiabatica

4-1-incalzirea izotermica de la sursa rece

s[J/kg/K]

T[K]

1< n < k

n=k

v1=ct (nEMBED Equation.DSMT4)

p1=ct (n=0)

T

s [J/kg/K]

T[K]

2

1

TSC1

TSR1

TSCi

TSCn

TSRn

EMBED Equation.3 Q=0

EMBED Equation.3 Q=0

EMBED Equation.3 QSCi

TSRi

EMBED Equation.3 QSRi

V

P

TSR

EMBED Equation.DSMT4

EMBED Equation.DSMT4

QSR

QSC

LC < 0

V

P

3

4

1

2

TSR

TSC

EMBED Equation.DSMT4

EMBED Equation.DSMT4

QSR

QSC

Lc > 0

V

P

3

4

1

2

QSR

IF

QSC

LC

QSR

Mot

QSC

LC

T0

V

p

(Q < 0

(Q > 0

(Q = 0

Ciclul direct (in sens orar)

(>0) ( 0 ; L1A2 = ( pdV > 0

1A

QC = QA2B + QB1A = LC > 0

(0)

U12 = Q12 L12

UC = EMBED Equation.3 = 0

UC = QC LC = 0

QC = LC

Ciclu inversat (in sens trigonometric)

LC=L1B2+L2A1 < 0

>0 0 QA1B = QSC < 0

LC

QSC

QSC - |QSR|

QSC

|QSR|

QSC

QSR

PC

QSC

LC

PAGE 9

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