1.1

ME 200 –Thermodynamics I

Lecture 43: Regenerative Gas Turbines with

Reheat and Intercooling

Yong Li

Shanghai Jiao Tong University

Institute of Refrigeration and Cryogenics

800 Dong Chuan Road Shanghai, 200240, P. R. China

Email : liyo@sjtu.edu.cn

Phone: 86-21-34206056; Fax: 86-21-34206056

1.2

Continue Brayton Cycle

Introduce “regeneration” to boost overall efficiency :

» Idea: reclaim “waste” heat normally exhausted to ambient.

Regenerative open Brayton cycle:

T-s diagram

1.3

Continue Brayton Cycle

» Heat transfer limitations:

l (length of heat exchanger)

“True” counterflow

Limiting states:

Tx ? T4

T2 ? Ty

Usually:

DTHX = 5 K

T T

DTHX

T4

Tx

Ty

T2

Tx < T4

T2 < Ty

1.4

Temperature distributions in counterflow heat exchangers.

(a) Actual. (b) Reversible.

1.5

Continue regenerative Brayton cycle

»Heat exchanger effectiveness:

reg

x 2

4 2

actual heat transfer

maximum heat transfer

h h

h h

y 1out

th,R

in 3 x

Overall cycle efficiency :

h hq1 1

q h h

4 y

4 2

h h

h h

4 1 4 y

3 2 x 2

(h h ) (h h )1

(h h ) (h h )

4 1 reg 4 2

th,R

3 2 reg 4 2

(h h ) (h h )1

(h h ) (h h )

1.6

Continue regenerative Brayton cycle

For a perfect heat exchanger,reg= 1.0

2 1th,R

3 4

h h1

h h

1

p 2 1 22th,R

4p 3 4 3

3

For constant specific heats:

T1

c (T T ) TT1 1

Tc (T T ) T 1T

4 1 reg 4 2

th,R

3 2 reg 4 2

(h h ) (h h )1

(h h ) (h h )

k 1 k 1

k k4 4 1 1

3 3 2 2

Also, assuming ideal gas and isentropic expansion and compression:

T p p T

T p p T

1.7

Continue regenerative Brayton cycle

k 1

k2 1 2 1 2

th,R

3 3 1 3 1

k 1

1 kth,R p

3

T T T T p1 1 1

T T T T p

T1 r

T

Note: for maximum th,R want T3 >> T2!

1

22th,R

43

3

T1

TT1

TT 1T

k 1 k 1

k k4 4 1 1

3 3 2 2

T p p T

T p p T

2p

1

pr

p

1.8

Brayton Cycle with Reheat

Two-Stage Expansion with Reheat: T-s diagram

1.9

Continue Brayton Cycle with Reheat

Continue Two-Stage Expansion with Reheating:

s

T Notes:

-For cycles with regeneration:

qin relatively constant

qin = (h3-hx)+(h3-hx) ~ h3-hxo

wnet increases (by 4-5-6-6o)

Reheater increases th,R

- For cycles without regen.:

qin increases by h5-h4 and

wnet increases (by 4-5-6-6o)

Reheater reduces th,R

3

1

7

x

2

xo

6o

5

T1

4 6

Increase

in work

Increase in temp.

difference available

for regeneration

1.10

Compression with Intercooling

Cooling a gas as it is compressed

would reduce the work

Practical alternative is to separate the

work and cooling

Use the heat exchanger ---- intercooler.

T-s diagram

1.11

Brayton Cycle with Intercooling and Reheating, For an internally reversible, steady flow process:

Notes: - Intercooler

reduces T4 which

improves regeneration.

- Reheater

increases T9 which also

improves regeneration.

T-s diagram

1.12

Continue Brayton Cycle with Intercooling and Reheating

Example :

T1 = 295 K (22oC), p1 = 0.95 bars, rp = p2/p1 = 6, TH = 1100 K

System th 1. Ideal Brayton Cycle 0.385

2.) Brayton cycle with C = 0.82 and T = 0.85 0.233

3.) System 1. with ideal regenerator (reg = 1.0) 0.562

4.) System 2. with real regenerator (reg = 0.7) 0.318

5.) System 4. with ideal intercooler and reheater 0.370

1.13

Continue Brayton Cycle with Intercooling and Reheating

Performance limit for gas turbine engines

Infinite stages of intercooling and reheating with ideal regeneration?

Ericsson Cycle!

s

T

TL

TH

1 2

4 3

1.14

Home work

Review

» All the contents we have learned in this semester

» Contact me or discuss with your classmates if you have any questions.

» Read through all the homework solutions to make sure you can solve

them by your self.