ME 221 Chp III-Handouts
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Transcript of ME 221 Chp III-Handouts
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Basics of Mechanical Engineering
Dr Muhammad Sajid
PROPERTIES OF PURE SUBSTANCES
Chapter 3
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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.
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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.
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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 .
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2-1
T-v diagram for theheating process ofwater at constantpressure.
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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.
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T-v diagram ofconstant-pressurephase-changeprocesses of apuresubstance atvariouspressures(numerical valuesare for water).
The point at which
the saturated liquidand saturated vaporstates are identical.
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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.
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P-v diagram of asubstance thatcontracts onfreezing.
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P-v diagramof asubstancethat expandson freezing(such aswater).
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P-T diagramof puresubstances.
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P-v-T surface of asubstance thatcontracts onfreezing.
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P-v-T surface of asubstance thatexpands on freezing(like water).
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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.
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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.
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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
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A partial listof Table A4.
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A partial listingof Table A6.
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Percentage oferror involvedin variousequations ofstate fornitrogen.
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Ideal-gasconstant-pressurespecific heats forsome gases
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