2013-2-27 Learning Outcomes(1) Demonstrate understanding of key concepts including phase and pure substance,state principle for simple compressible systems,p-- T surface,saturation temperature and saturation pressure.two-phase liquid-vapor mixture,quality,enthalpy,and specific heats Apply the closed system energy balance with property data. 3-3 Learning Outcomes(2) Sketch T-v,p-v,and phase diagrams,and locate states on these diagrams Retrieve property data from Tables A-1 through A-23. Apply the ideal gas model for thermodynamic analysis,including determining when use of the model is warranted. 3-4 2
2013-2-27 2 Learning Outcomes(1) ►Demonstrate understanding of key concepts . . . including phase and pure substance, state principle for simple compressible systems, p-vT surface, saturation temperature and saturation pressure, two-phase liquid-vapor mixture, quality, enthalpy, and specific heats. ►Apply the closed system energy balance with property data. 3-3 Learning Outcomes(2) ►Sketch T-v, p-v, and phase diagrams, and locate states on these diagrams. ►Retrieve property data from Tables A-1 through A-23. ►Apply the ideal gas model for thermodynamic analysis, including determining when use of the model is warranted. 3-4
2013-2-27 Phase A quantity of matter that is homogeneous throughout in both chemical composition and physical structure. Homogeneity in physical structure means that the matter is all solid,or all liquid,or al vapor (gas). Examples: The air we breathe is a gas phase consisting of a mixture of different gases. Drinking water with ice cubes contains two phases of water:liquid and solid. Vinegar and olive oil salad dressing contains two different liquid phases. 3-5 Pure Substance A substance that is uniform and invariable in chemical composition. A pure substance can exist in more than one phase,but its chemical composition must be the same in each phase. Examples: Drinking water with ice cubes can be regarded as a pure substance because each phase has the same composition. A fuel-air mixture in the cylinder of an automobile engine can be regarded as a pure substance until ignition occurs 3-6 3
2013-2-27 3 Phase ►A quantity of matter that is homogeneous throughout in both chemical composition and physical structure. ►Homogeneity in physical structure means that the matter is all solid, or all liquid, or all vapor (gas). ►Examples: ►The air we breathe is a gas phase consisting of a mixture of different gases. ►Drinking water with ice cubes contains two phases of water: liquid and solid. ►Vinegar and olive oil salad dressing contains two different liquid phases. 3-5 ►Examples: ►Drinking water with ice cubes can be regarded as a pure substance because each phase has the same composition. ►A fuel-air mixture in the cylinder of an automobile engine can be regarded as a pure substance until ignition occurs Æ Pure Substance 3-6 ►A substance that is uniform and invariable in chemical composition. ►A pure substance can exist in more than one phase, but its chemical composition must be the same in each phase
2013-2-27 Phase Pure Substance 3-1 State Principle for Simple Compressible Systems ar imple co The intensive state of a simple compressible system at equilibrium is described by its intensive properties,including temperature,pressure, specific volume,density,specific internal energy, and specific enthalpy. Properties such as velocity and elevation are excluded because their values depend on arbitrary datum choices.such as zero values at the surface of the earth.For the state principle,these properties are not relevant. 3-8 4
2013-2-27 4 Phase & Pure Substance 3-7 State Principle for Simple Compressible Systems ►Systems of commonly encountered pure substances are called simple compressible systems. These substances include those in appendix tables A-2 through A-18, A-22, and A-23. ►The intensive state of a simple compressible system at equilibrium is described by its intensive properties, including temperature, pressure, specific volume, density, specific internal energy, and specific enthalpy. ►Properties such as velocity and elevation are excluded because their values depend on arbitrary datum choices, such as zero values at the surface of the earth. For the state principle, these properties are not relevant. 3-8
2013-2-27 State Principle for Simple Compressible Systems Not all of the relevant intensive properties are independent. Some are related by definitions-for example,density is 1/and specific enthalpy is u+pv. Others are related through expressions developed from experimental data. Some intensive properties may be independent in a single phase,but become dependent when there is more than one phase present. 3-9 State Principle for Simple Compressible Systems For a simple compressible system,values for any two independent intensive properties determine the values of al/other intensive properties.This is the state principle for simple compressible systems. Among alternative sets of two independent intensive properties,(T,and (p,are frequently convenient.We soon show that temperature and pressure are not always an independent set(within the two-phase regions p and T are not independent). 3-10 5
2013-2-27 5 State Principle for Simple Compressible Systems ►Not all of the relevant intensive properties are independent. ►Some are related by definitions – for example, density is 1/v and specific enthalpy is u + pv. ►Others are related through expressions developed from experimental data. ►Some intensive properties may be independent in a single phase, but become dependent when there is more than one phase present. 3-9 State Principle for Simple Compressible Systems ►For a simple compressible system, values for any two independent intensive properties determine the values of all other intensive properties. This is the state principle for simple compressible systems. ►Among alternative sets of two independent intensive properties, (T, v) and (p, v) are frequently convenient. We soon show that temperature and pressure are not always an independent set (within the two-phase regions p and T are not independent). 3-10
2013-2-27 STATE AND EQUILIBRIUM ynmic ea with Fauilibrium A state of balance In an equilibrium state there are no m2kg T1=209 =15 w. The Mechanical eauilibrium:If there is n at any point ofh 20C 23℃ 32 32℃ d when the 30 32 involves two ph is c level and stays there Acosed system reacn mica (a)Before (b)After change with tim thermal equilibrium. 3.11 The State Postulate The number of properties required to fix the state of a system is given by the state postulate: The state of a simple compressible system is Nitrogen T=25C properties. V=0.9 m/kg Simple compressibles syste a system involves no electrical. State of nitrogen is magnetic,gravitational,motion. and surface tension effects.(the fixed by two independent, only energy transfer by work is by volume change). intensive properties 3.12 6
2013-2-27 6 A closed system reaching thermal equilibrium. 3-11 STATE AND EQUILIBRIUM Thermodynamics deals with equilibrium states. Equilibrium: A state of balance. In an equilibrium state there are no unbalanced potentials (or driving forces) within the system. Thermal equilibrium: If the temperature is the same throughout the entire system. Mechanical equilibrium: If there is no change in pressure at any point of the system with time. Phase equilibrium: If a system involves two phases and when the mass of each phase reaches an equilibrium level and stays there. Chemical equilibrium: If the chemical composition of a system does not change with time, that is, no chemical reactions occur. A system at two different states. The State Postulate The number of properties required to fix the state of a system is given by the state postulate: The state of a simple compressible system is completely specified by two independent, intensive properties. Simple compressible system: If a system involves no electrical, magnetic, gravitational, motion, and surface tension effects.(the only energy transfer by work is by volume change). 3-12 State of nitrogen is fixed by two independent, intensive properties
2013-2-27 p-v-T Surface For pure,simple compressible systems,pressure can be determined as a function of temperature and specific volume p=p(T,v) The graph of this relation for water is indicated by the p-v-T surface shown y Single-phase regions on the surface include solid,liquid,and vapor. Two-phase regions are located between single-phase regions,where two phases exist in equilibrium:liquid. vapor,solid-vapor,solid-liquid. 313 p-v-T Surface The dome-shaped region composed of the two-phase liquid-vapor states is called the vapor dome. A state at which a phase change begins or ends is called a saturation state.Lines bordering the vapor dome are called the saturated liquid and saturated vapor lines. At the top of the dome,where saturated liquid and saturated vapor lines meet,is the critical point. Critical temperature(T)is the maximum temperature at which liquid and vapor phases can coexist in equilibrium. Critical pressure(p)pressure at critical point. 314 7
2013-2-27 7 ►For pure, simple compressible systems, pressure can be determined as a function of temperature and specific volume: p = p(T, v) The graph of this relation for water is indicated by the p-v-T surface shown Æ ►Single-phase regions on the surface include solid, liquid, and vapor. p-v-T Surface ►Two-phase regions are located between single-phase regions, where two phases exist in equilibrium: liquidvapor, solid-vapor, solid-liquid. 3-13 p-v-T Surface ►The dome-shaped region composed of the two-phase liquid-vapor states is called the vapor dome. ►A state at which a phase change begins or ends is called a saturation state. Lines bordering the vapor dome are called the saturated liquid and saturated vapor lines. ►At the top of the dome, where saturated liquid and saturated vapor lines meet, is the critical point. ►Critical temperature (Tc) is the maximum temperature at which liquid and vapor phases can coexist in equilibrium. ►Critical pressure (pc) pressure at critical point. 3-14
2013-2-27 Projections of the p-v-T Surface p-T projection (phase diagram) p-v projection (p-v diagram) 3-15 Projections of the p-v-T Surface Projection of the p-v-T surface onto the pressure-temperature Twoduce toies plane is a phase diagram. Saturation temperature designates the temperature at which a phase change Critica takes place at a given Solid Liquid poin pressure. L Saturation pressure Vapor designates the pressure at Triple poi which a phase change takes Temperature place at a given temperature. Within two-phase regions pressure and temperature are not independent. 3-16 8
2013-2-27 8 3-15 Projections of the p-v-T Surface p-T projection (phase diagram) p-v projection (p-v diagram) Projections of the p-v-T Surface ►Projection of the p-v-T surface onto the pressure-temperature plane is a phase diagram. ►Saturation temperature designates the temperature at which a phase change takes place at a given pressure. ►Saturation pressure designates the pressure at which a phase change takes place at a given temperature. ►Within two-phase regions pressure and temperature are not independent. 3-16 Two-phase regions reduce to lines
2013-2-27 Phase Diagram (p-T) Substances that expand and contract on freezing differ in the slope of the melting line LIQUID Triple point VAPOR 3-17 Substance Behavior on Freezing Expands Contracts on freezing (H2O) on freezing 3-18 9
2013-2-27 9 Phase Diagram (p-T) 3-17 Substances that expand and contract on freezing differ in the slope of the melting line Substance Behavior on Freezing 3-18 Expands on freezing (H2O) Contracts on freezing
2013-2-27 Substance Behavior on Freezing P-v-T surface of a P-v-T surface of a substance that expands substance that on freezing(like water). contracts on freezing. Projections of the p-v-T Surface Projection of the p-v-T surface onto the pressure- specific volume plane results in a p-o diagram MPa (204 Specifie volum Isobars Projection of the p-v-T Liguid-vapoe surface onto the temperature-specific c212 volume plane results in a T-o diagram. 3-20 10
2013-2-27 10 P-v-T surface of a substance that contracts on freezing. Substance Behavior on Freezing P-v-T surface of a substance that expands on freezing (like water). 3-19 Projections of the p-v-T Surface ►Projection of the p-v-T surface onto the pressurespecific volume plane results in a p-v diagram. ►Projection of the p-v-T surface onto the temperature-specific volume plane results in a T-v diagram. 3-20 Isotherms Isobars
2013-2-27 Phase Change-Liquid States Consider a closed system consisting of a unit mass (1Kg or 1lbs)of liquid water at 20C contained within a piston-cvlinder assembly This state is represented by I(highlighted by the blue dot). Liquid states such as this,where temperature is lower than the saturation temperature corresponding to the pressure at the state.are called compressed liquid states (or subcooled liquid.or just liquid) Unit mass of liquid at 20°C (68F)→ 3-21 Saturated Liquid As the system is heated at constant pressure,the temperature increases considerably while the specific volume increases slightly. Eventually,the system is brought to the state represented by f(highlighted by the blue dot) This is the saturated liquid state corresponding to the specified pressure =220丹MP%(3204n 0h/ O MP 0YC212 Liquid wate .22 1
2013-2-27 11 ●l Phase Change – Liquid States ►Consider a closed system consisting of a unit mass (1Kg or 1lbs) of liquid water at 20oC contained within a piston-cylinder assembly. ►This state is represented by l (highlighted by the blue dot). ►Liquid states such as this, where temperature is lower than the saturation temperature corresponding to the pressure at the state, are called compressed liquid states (or subcooled liquid, or just liquid). 3-21 Unit mass of liquid at 20°C (68F)Æ Saturated Liquid ● f 3-22 ►As the system is heated at constant pressure, the temperature increases considerably while the specific volume increases slightly. ►Eventually, the system is brought to the state represented by f (highlighted by the blue dot). ►This is the saturated liquid state corresponding to the specified pressure