ThermCh03

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Information about ThermCh03
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Published on January 4, 2008

Author: Aric85

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Slide1:  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. Slide2:  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 Slide6:  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 Slide10:  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). Slide11:  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: Slide13:  FIGURE 3-19 P-v diagram of a pure substance. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-4 Slide14:  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. Slide15:  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 Slide29:  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 Slide35:  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-7 FIGURE 3-25 P-T diagram of pure substances. Slide36:  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 Slide37:  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 Slide39:  FIGURE 3-30 A partial list of Table A–4. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-10 Slide40:  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 Slide71:  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 Slide72:  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-13 FIGURE 3-51 Comparison of Z factors for various gases. Slide73:  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. Slide74:  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

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