ThermCh03

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Thermodynamics An Engineering Approach Fourth Edition Yunus A. Çengel Michael A. Boles Professor H.K. Ma National Taiwan University Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Properties of Pure Substances

3.1 Pure Substance: 

3.1 Pure Substance A substance that has a fixed chemical composition throughout is called a Pure Substance. Pure Substance: - N2, O2, gaseous Air -A mixture of liquid and gaseous water is a pure substance, but a mixture of liquid and gaseous Air is not.

3.2 Phases of a Pure Substance: 

3.2 Phases of a Pure Substance Solid: -The molecules in a solid are kept at their positions by the large springlike intermolecular forces. -The attractive and repulsive forces between the molecules tend to maintain them at relatively constant distances from each other. Liquid: Groups of molecules move about each other. Gas: Molecules move about at random.

3.3 Phase-Change Processes of Pure Substance: 

3.3 Phase-Change Processes of Pure Substance Compressed liquid or a subcooled liquid: A liquid that is not about to vaporize. Saturated liquid: A liquid that is about to vaporize. Saturated vapor: A vapor that is about to condense. Saturated liquid-vapor mixture: the liquid and vapor phases coexist in equilibrium. Superheated vapor: A vapor that is not about to condense

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-1 FIGURE 3-11 T-v diagram for the heating process of water at constant pressure.

3.3 Phase-Change Processes of Pure Substance: 

3.3 Phase-Change Processes of Pure Substance Saturated temperature, Tsat: At a given pressure, the temperature at which a pure substance changes phase. Saturated pressure, Psat: At a given temperature, the pressure at which a pure substance changes phase. Latent heat: the amount of energy absorbed or released during a phase-change process. Latent heat of fusion: the amount of energy absorbed during melting. Latent heat of vaporization: the amount of energy absorbed during vaporization.

3.4 Property Diagrams for Phase-Change Processes: 

3.4 Property Diagrams for Phase- Change Processes The T-v diagram: -Critical point: the point at which the saturated liquid and saturated vapor states are identical. -Saturated liquid line: -Saturated vapor line: -Compressed liquid region: -Superheated vapor region: -Saturated liquid-vapor mixture region:

3.4 Property Diagrams for Phase-Change Processes: 

3.4 Property Diagrams for Phase- Change Processes The critical-point properties of water: - Pcr= 22.09MPa - Tcr= 374.14C - vcr= 0.003155 m3 /kg The critical-point properties of helium: - Pcr= 0.23MPa - Tcr= -267.85C - vcr= 0.01444 m3/kg

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-2 FIGURE 3-16 T-v diagram of constant-pressure phase-change processes of a pure substance at various pressures (numerical values are for water).

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-3 FIGURE 3-18 T-v diagram of a pure substance.

3.4 Property Diagrams for Phase-Change Processes: 

3.4 Property Diagrams for Phase- Change Processes The P-v diagram: - The triple line: three phases of a pure substance coexist in equilibrium, these triple-phase states forms a line. - The triple point: the triple line appears as a point on the P-T diagrams. For water, 0.01C & 0.06113 kPa -Sublimation: Passing from the solid phase directly into the vapor phase. The P-T diagram(phase diagram): The P-v-T surface:

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FIGURE 3-19 P-v diagram of a pure substance. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-4

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-5 FIGURE 3-21 P-v diagram of a substance that contracts on freezing.

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-6 FIGURE 3-22 P-v diagram of a substance that expands on freezing (such as water).

EVALUATING THERMODYNAMIC PROPERTIES: 

EVALUATING THERMODYNAMIC PROPERTIES The P-V-T Surface for Water

Description of thermodynamic state: 

Description of thermodynamic state

Experimental basis: 

Experimental basis One independent property for each way that the energy of the system can be varied independently.

Experimental basis: 

Experimental basis The number of independent properties that uniquely describes the state of the system is the number of relevant work interactions plus one.

Simple, compressible systems: 

Simple, compressible systems

Simple chemical systems: 

Simple chemical systems

Simple compressible systems: 

Simple compressible systems f(p,V,T) = 0 p = p(V,T) V = V(p,T)

State and Equilibrium: 

State and Equilibrium A simple compressible system: The state of a simple compressible system is completely specified by two independent, intensive properties. -two properties specified be independent to fix the state. -Once the two properties are fixed, all the other properties become dependent properties.

p-V-T surfaces...: 

p-V-T surfaces...

P-v-T Surfaces: 

P-v-T Surfaces

State Equations : 

State Equations f(p,V,T) = 0 p(V,T) = p v(p,T) = V T(p,V) = T f(U,V,T) = 0 U = U(V,T) V = V (U, T) T = T(U,V)

The p-V-T surface for water... : 

The p-V-T surface for water...

The P-v-T surface for water: 

The P-v-T surface for water

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At a total constant pressure.... p

The p-V-T surface for water: 

The p-V-T surface for water Solid in equilibrium with vapor (0 oC) Solid in equilibrium with vapor and liquid (0 oC) Liquid and vapor in equilibrium ( 0 < T < 100 oC) Saturated Vapor (T = 100 oC) Superheated Vapor (T > 100 oC)

The P-v-T Surface for Water: 

The P-v-T Surface for Water

The p-T plane for water, Phase diagram: 

The p-T plane for water, Phase diagram

Water: 

Water

Water: 

Water

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-7 FIGURE 3-25 P-T diagram of pure substances.

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FIGURE 3-26 P-v-T surface of a substance that contracts on freezing. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-8

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FIGURE 3-27 P-v-T surface of a substance that expands on freezing (like water). Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-9

3.5 Property Tables: 

3.5 Property Tables Table A-4: Saturated water-Temperature table Table A-5: Saturated water-Pressure table Table A-6: Superheated water Enthalpy, H H= U + PV (kj) h= u + Pv (kj/kg) Entropy, S

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FIGURE 3-30 A partial list of Table A–4. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-10

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FIGURE 3-40 A partial listing of Table A–6. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-11

Quality...: 

Quality...

Liquid-vapor equilibrium : 

Liquid-vapor equilibrium

Quality: 

Quality

The two-phase region for water: 

The two-phase region for water

Quality defined: 

Quality defined Quality, x, allows location of states inside the vapor-liquid region.

Water: 

Water

Quality: 

Quality

Internal energy in the vapor-liquid region: 

Internal energy in the vapor-liquid region

Key concepts and terms: 

Key concepts and terms Critical point P - v - T surface Quality Saturation pressure Saturation temperature State equation State surface The State Principle Triple point

ADDITIONAL ASPECTS OF THE P-V-T SURFACE FOR WATER: 

ADDITIONAL ASPECTS OF THE P-V-T SURFACE FOR WATER

The P-v-T surface for water: 

The P-v-T surface for water Liquid Vapor- Liquid Critical Point

Water: 

Water

At the critical point for water: 

At the critical point for water

Compressed liquids: 

Compressed liquids

3.6 The Ideal-Gas Equation of State: 

3.6 The Ideal-Gas Equation of State Equation of state: Any equation that relates the pressure, temperature, and specific volume of a substance. Gas: The vapor phase of a substance Vapor: A gas that is not far from a state of condensation

Definition of gas...: 

Definition of gas...

Fluids and gases: 

Fluids and gases

What is gas?: 

What is gas?

The ideal gas...: 

The ideal gas...

P-V-T relation for ideal gases: 

P-V-T relation for ideal gases

The ideal gas equation of state: 

The ideal gas equation of state This implies: (1) very little molecular interaction (p = 0), (2) molecules are point masses, i.e., zero volume.

The universal gas constant: 

The universal gas constant

The ideal gas state surface: 

The ideal gas state surface

The perfect gas: 

The perfect gas

3.7 Compressibility Factor- A Measure of Derivation from Ideal-Gas Behavior: 

3.7 Compressibility Factor- A Measure of Derivation from Ideal-Gas Behavior Compressibility Factor, Z Z = Pv/RT Z = vactual/videal Ideal gas: Z = 1 Real gases: Z > 1 Z = 1 Z < 1

3.7 Compressibility Factor- A Measure of Derivation from Ideal-Gas Behavior: 

3.7 Compressibility Factor- A Measure of Derivation from Ideal-Gas Behavior The normalization: -Reduced pressure, PR PR = P/ Pcr -Reduced temperature, TR TR = T/ Tcr The principle of corresponding states: The Z factor for all gases is approximately the same at the same reduced pressure and temperature.

Real gases...: 

Real gases...

Real Gases: 

Real Gases P

Reduced pressure and temperature: 

Reduced pressure and temperature Generalized Compressibility Chart (14.6)

The virial form for compressibility: 

The virial form for compressibility

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. FIGURE 3-49 Percentage of error involved in assuming steam to be an ideal gas, and the region where steam can be treated as an ideal gas with less than 1 percent error. 2-12

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-13 FIGURE 3-51 Comparison of Z factors for various gases.

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-14 FIGURE 3-60 Percentage of error involved in various equations of state for nitrogen.

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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. FIGURE Ideal-gas constant-pressure specific heats for some gases (see Table A–2c for Cp equations). 2-15

3.8 Other Equations of State: 

3.8 Other Equations of State Van der Waals Equation of state: Beattie-Bridgeman Equation of state: Benedict-Webb-Rubin Equation of state: Virial Equation of state:

Key concepts and terms: 

Key concepts and terms Fluid Gas Ideal gas Perfect gas Compressibility factor Compressibility chart Local equilibrium Reduced pressure Reduced temperature Thermodynamic pressure Virial equation of state