1.1

ME 200 –Thermodynamics I

Lecture 44: Review Thermodynamics I

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

What is Thermodynamics?

Science to study how one energy changes from one to

another

Thermodynamics = Therme(heat) + dynamis(force)

Energy exists in several forms, e.g., potential, kinetic,

chemical, thermal, electrical, nuclear among many others

During interactions in nature, energy simply changes from

one form to another; but the total energy remains constant

1.3

Basic Principles

First law of thermodynamics

» A statement of conservation of energy principle

» Energy is a thermodynamic property; quantifies energy

Second law of thermodynamics

» Energy has quality as well as quantity. Actual processes occur in

direction of decreasing quality of energy

» Establishes direction and possibility for process

» Provides means for measuring the quality of energy

» Determines theoretical limits regarding the performance

of engineering devices

1.4

Terms and Concepts

»» System – Thermodynamic system, Closed system, Open (flow) system

– Surroundings,

– System boundary, Adiabatic (insulated) , Rigid, Isolated

»» Property Intensive, Extensive Specific properties

»» State

»» Phases

»» Equilibrium, Thermodynamic equilibrium Mechanical Eq.----- Thermal Eq. -----Phase Eq.-----Chemical Eq.

»» Process Isothermal, Isobaric, Isochoric ,

Quasi-Equilibrium Process

1.5

First law of Thermodynamics Open System

1.6

Important Equipments

Turbines

compressors

pumps

Nozzles,

diffusers, Throttling

valve

Heat

exchanger

Turbines

compressors

pumps

1.7

Second Law of Thermodynamics

Clausius (C) statement

It is impossible for any system to operate in

such a way that the sole result would be an

energy transfer by heat from acooler to a

hotter body.

Kelvin–Planck (K-P) statement

It is impossible for any system to operate in a

thermodynamic cycle and deliver a net

amount of energy by work to its surroundings

while receiving energy by heat transfer from

a single thermal reservoir..

» Analytical form of the K-P statement

Irreversibility

Heat transfer through a finite

temperature difference

Unrestrained expansion of a gas or

liquid to a lower pressure

Spontaneous chemical reaction

…….

Reversible cycle

» there are no irreversibilities within the

system as it undergoes the cycle

» heat transfers between the system and

reservoirs occur reversibly.

Two Carnot corollaries

irrev rev rev1 = rev2

1.8

Thermal Efficiency

A reversible power cycle operating between two

thermal reservoirs.

Four internally reversible processes: two adiabatic

processes alternated with two isothermal processes.

1.9

Entropy

The integral of dQ/T gives S only if the integration is carried out along an

internally reversible path between the two states.

Entropy is a property, it has fixed values at fixed states. S

between two specified states is the same no matter

what path, reversible or irreversible.

1.10

Entropy Balance

Closed system entropy balance

Other forms of the entropy balance

Increase of entropy principle

» the entropy of an isolated system during a process always increases or, in the limiting case of a

reversible process, remains constant. In other words, it never decreases.

» Control volume entropy rate balance

Steady state

1.11

Triple point ::: the triple line of the three-

dimensional p–v–T surface projects onto a

point on the phase diagram.

water, triple point defined

at 0.01oC 0.6113 kPa

p-v-T Surface

Subcooled liquid=compressed liquid

Saturated Liquid

Liquid‐Vapor Mixture

Saturated Vapor

Superheated Vapor

water, pcr ~ 221 bar; Tcr ~ 374.1C

1.12

Incompressible Substance model

Incompressible Substance model::: An

idealization to simplify evaluations of liquids or

solids, the v () is assumed to be constant and the u

assumed to vary only with T.

v =const

Concepts

≈0

1.13

u, h, c of Ideal Gases

specific internal energy depends only on T

specific enthalpy depends only on T

Important relation

1.14

Entropy

cv and cp are constants

Ideal Gas

liquids and solids modeled as

incompressible.

Variable cv and cp

Compressed liquid liquid–vapor mixture

Saturated liquid to saturated vapor at constant T and p

1.15

Isentropic Processes of air (IG)

Isentropic process for air modeled as ideal gas

relative pressure. )(]/)(exp[ TpRTs ro

)(/)( TpRTTv rr relative volume.

reduced pressure.

1.16

Isentropic Processes of air (IG) with constant c

constant1 kvT

constantkvp

constant/)1( kkpT

1.17

Polytropic Processes on p–v and T–s Diagrams

cpvn

cpn 0

cvpn /1

11 cTcpvRTn

cscpvkn k

cvn

1.18

Isentropic Efficiencies

Isentropic Efficiencies ::: Comparison between the actual

performance of a device and the performance that would be achieved under

idealized circumstances for the same inlet state and the same exit pressure.

Turbine

isentropic turbine efficiency

h2 > h2s ηt

1.19

Expressions for the Work

Control

Volumes

One-inlet, one-exit

steady-state flow

Internally reversible

e

ee

ee

i

ii

iicvcvcv gz

Vhmgz

VhmWQ

dt

dE

22

22

2

1

revint

Tdsm

Qcv

)(2

)( 21

2

2

2

121 zzg

VVhh

m

Q

m

W cvcv

)(2

)( 21

2

2

2

121

2

1int

zzgVV

hhTdsm

W

rev

cv

vdpdhTds

2

112

2

1vdphhdsT

)(2

21

2

2

2

12

1int

zzgVV

vdpm

W

rev

cv

1.20

Analyzing Rankine Cycle---I

Turbine

Condenser

Pump

Boiler

Thermal efficiency of the power cycle

Back work ratio

1.21

Superheat and Reheat

Superheat :

» Reason: Increase average temperature for

heat

addition at a given boiler pressure

increase in performance

Reheat: High quality (or superheated vapor)

existing the turbine without large

superheat

For a given TH can increase Tb without

reducing quality

1.22

Refrigeration Cycle

T

s

2

1

3

4

Tcond

Tevap

TH

TL

subcooling

Tsc

superheat: Tsh

COP=

Evaporator:

The heat transfer rate is referred to

as the refrigeration capacity. ( kW).

» Another unit for the refrigeration

capacity is the ton of refrigeration, =

211 kJ/min.

Compressor

Condenser

Throttling process

1.23

Air Standard Cycles

Air standard cycles are idealized cycles based on the

following approximations:

A fixed amount of air modeled as an ideal gas (working fluid).

The combustion process is replaced by a heat transfer from an

external source. There are no exhaust and intake processes as in

an actual engine.

The cycle is completed by a constant-volume heat transfer

process taking place while the piston is at the bottom dead center

position.

All processes are internally reversible.

Cold air-standard analysis The specific heats are assumed constant at Ta.

1.24

Otto Cycle and Diesel Cycle

Air Standard Cycle for CI Engines:

3 BDC1c

2 2 TDC

V VVDefine : r "cutoff ratio" compression ratio r

V V V

3xp

2 2

ppr pressure ratio

p p

k

cth k 1

c

1

th,Diesel th,Otto

r 11Then, 1

r k(r 1)

Thus, for a given r : !

th k 1

11

r

net

max min

W net work for one cycleMEP

V V displacement volume

1.25

Brayton Cycle

4 5x 2

4 2 4 2

h hh hactual heat transfer

maximum heat transfer h h h h

1

p 2 1 22th,R

4p 3 4 3

3

k 1 k 1

k k4 4 1 1

3 3 2 2

For constant specific heats:

T1

c (T T ) TT1 1

Tc (T T ) T1

T

Also, assuming ideal gas and isentropic expansion and compression:

T p p T

T p p 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