Thermodynamics filled in class notes_Part_10

Thermodynamics filled in class notes_Part_10 - 27 2.2...

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Unformatted text preview: 27 2.2. EXERGY So we get ψ = cP (T − To ) − To cP ln T To − R ln 1 + v · v + g (z − zo ). 2 P Po (2.94) For T ∼ To P ∼ Po , we can use Taylor series to simplify this somewhat. First, we recall the general Taylor series for a log function near unity. Consider y (x) = ln x, (2.95) for x ∼ 1. For a Taylor series near x = 1, we have y (x) ∼ y (1) + dy dx x=1 (x − 1) + 1 d2 y 2 dx2 x=1 (x − 1)2 + . . . . (2.96) Now for y = ln x, we have 1 d2 y = − 2, dx2 x 1 dy =, dx x (2.97) and thus y (1) = ln(1) = 0, dy dx d2 y dx2 = 1, x=1 x=1 = −1 (2.98) So 1 y (x) = ln x ∼ 0 + (x − 1) − (x − 1)2 + . . . 2 (2.99) We then expand ψ via the following steps: 1 P T + RTo ln + v · v + g (z − zo ), To Po 2 T k−1 1 T P + + v · v + g (z − zo ), − 1 − ln ln To To k Po 2 = cP (T − To ) − cP To ln (2.100) = cP T o (2.101) = ψ cP T o = 1 T −1 − To 2 T −1 − To 1 + v · v + g (z − zo ), 2 1T −1 cP T o 2 To T −1 To 2 + ... + k−1 k P − 1 + ... Po (2.102) 2 + ... + k−1 k P − 1+ ... Po 1 + v · v + g (z − zo ). 2 (2.103) Note that in the neighborhood of the ambient state, relative pressure differences are more effective than relative temperature differences at inducing high exergy. CC BY-NC-ND. 18 November 2011, J. M. Powers. 28 CHAPTER 2. CYCLE ANALYSIS 2.3 Rankine 2.3.1 Classical The Rankine cycle forms the foundation for the bulk of power generating devices which utilize steam as a working fluid. The ideal cycle is described by • 1 → 2: isentropic pumping process in the pump, • 2 → 3: isobaric heat transfer in the boiler, • 3 → 4: isentropic expansion in the turbine, and • 4 → 1: isobaric heat transfer in the condenser. Note that to increase cycle efficiency one can • lower the condenser pressure (increases liquid water in turbine), • superheat the steam, or • increase the pressure during heat addition. A schematic for the Rankine cycle is shown in Figure 2.3. The T − s plane for the Rankine 3 Turbine Boiler 4 2 Condenser Pump 1 Figure 2.3: Schematic for Rankine cycle. cycle is shown in Figure 2.4. Example 2.3 (adopted from Moran and Shapiro, p. 312) Consider steam in an ideal Rankine cycle. Saturated vapor enters the turbine at 8.0 M P a. Saturated liquid exits the condenser at P = 0.008 M P a. The net power output of the cycle is 100 M W . Find CC BY-NC-ND. 18 November 2011, J. M. Powers. ...
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This note was uploaded on 11/26/2011 for the course EGN 3381 taught by Professor Park-sou during the Fall '11 term at FSU.

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