me 221 chp iii-handouts

8/6/2019 ME 221 Chp III-Handouts

1/12

2/21/201

Basics of Mechanical Engineering

Dr Muhammad Sajid

PROPERTIES OF PURE SUBSTANCES

Chapter 3

21-Feb-11 Basics of Mechanical Engineering - Chp III 2


2/12

2/21/201

Objectives

Introduce the concept of a pure substance. Illustrate the P-v, T-v, and P-T property diagrams and

P-v-T surfaces of pure substances. Demonstrate the procedures for determining

thermodynamic properties of pure substances fromtables of property data.


Pure substanceA pure substance has a fixed chemicalcomposition throughout.

Water, nitrogen, helium, and carbon dioxide, etcA homogeneous mixture of various chemical elements orcompounds is also a pure substance.

Air (a mixture of several gases) is a pure substance.But a mixture of oil and water is not a pure substance.

A mixture of two or more phases of a pure substance isstill a pure substance.

A mixture of ice and liquid water is a pure substance.A mixture of liquid air and gaseous air is not a puresubstance.



3/12

2/21/201

Phase change processes

Saturated liquid: At 100C water is still a liquid, butany heat addition will cause evaporation A liquid thatis about to vaporize is called a saturated liquid.Saturated vapor: Any heat loss from water vapor at100C will cause condensation. A vapor that is about to condense is called a saturated vapor.Saturated liquidvapor mixture: The saturated liquidand vapor phases coexist in equilibrium.Superheated vapor: When vapor is so hot that a lossof heat lowers temperature without condensation. Avapor that is not about to condense is called asuperheated vapor .


2-1

T-v diagram for theheating process ofwater at constantpressure.



4/12

2/21/201

Property diagrams

The variations of properties during phase-changeprocesses are best studied and understood with thehelp of property diagrams.

Next, we develop and discuss the T-v, P-v, and P-T diagrams for pure substances.


T-v diagram ofconstant-pressurephase-changeprocesses of apuresubstance atvariouspressures(numerical valuesare for water).

The point at which

the saturated liquidand saturated vaporstates are identical.



5/12


6/12

2/21/201

Extending phase diagram to solids

The two equilibrium diagrams developed can beextended to include the solid phase as well as thesolidliquid and the solidvapor saturation regions.

The basic principles discussed in conjunction withthe liquidvapor phase-change process apply equallyto the solidliquid and solidvapor phase-changeprocesses.

Most substances contract during a solidification (i.e.,freezing) process. Others, like water, expand as theyfreeze.


P-v diagram of asubstance thatcontracts onfreezing.



7/12

2/21/201

P-v diagramof asubstancethat expandson freezing(such aswater).


P-T diagramof puresubstances.



8/12

2/21/201

P-v-T surface of asubstance thatcontracts onfreezing.


P-v-T surface of asubstance thatexpands on freezing(like water).



9/12

2/21/201

Property tables

The relationships amongst thermodynamicproperties are too complex to be expressed bysimple equations.

Therefore, properties are presented in the form of tables.

In the following discussion, the steam tables are usedto demonstrate the use of thermodynamic propertytables.


Enthalpy In the steam tables you will notice two new

properties: enthalpy h and entropy s. Entropy is a property associated with the second law

and we will not use it until it is properly defined.


In the analysis of processes, inrefrigeration and power, we

encounter a combination of u& Pv .For convenience, thiscombination is defined as a newproperty, enthalpy, and given


10/12

2/21/201

A partial listof Table A4.


A partial listingof Table A6.



11/12


12/12

2/21/201

Percentage oferror involvedin variousequations ofstate fornitrogen.


Ideal-gasconstant-pressurespecific heats forsome gases


me 221 chp iii-handouts

Documents