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Chapter 3 Thermodynamic Properties

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Chapter 3 Thermodynamic Properties Chapter 3 Thermodynamic Properties 3.1 Phase and Pure Substance A phase is a quantity of matter characterized by both uniform physical structure and uniform chemical composition. A phase can be solid, liquid, vapor or gas. The atoms in a solid phase are fixed relative to other atoms in the solid. They are however can vibrate about this fixed position. Like a solid, molecules within the liquid phase are in close proximity to one another due to intermolecular forces. However, the molecules in a liquid are not fixed relative to other molecules in the liquid. They are constantly in motion, free to move relative to one another. More than one liquid phase can coexist such as oil and water. They are considered separate liquid phases since they are not miscible. Similarly, solids can coexist in different phases. For example, solid carbon can exist in the diamond phase or the graphite phase. Gas molecules move randomly to fill the entire volume of the container in which they are housed. The molecules continuously change direction as they colide with each other and bounce off the container surface creating the measurable pressure p. A substance that has a fixed chemical composition throughout the system is called a pure substance. Water, hydrogen, nitrogen, and carbon monoxide, for example, are all pure substance. A pure substance can also be a mixture of various chemical elements or compounds as long as the mixture is homogeneous. Air, a mixture of several compounds, is often considered to be a pure substance because it has a uniform chemical composition. “A mixture of two or more phases of a pure substance is still a pure substance as long as the chemical composition of all phases is the same. A mixture of ice and liquid water, for example, is a pure substance because both phases have the same chemical composition.”1 The state of a system at equilibrium is defined by the values of its thermodynamic properties. System of pure substances or simple compressible systems are defined by two intensive properties such as temperature and pressure. For any homogeneous, pure substance, all thermodynamic functions or properties may be expressed in terms of any two given independent properties. v = f(T, p) or v = v(T, p) u = u(T, p) or u = u(T, v) s = s(T, p) or s = s(T, v) The functional relations for these properties must be developed from experimental data and are dependent on the particular chemical molecules making up the system. Only expansion or compression works given by — pdV are permissible in simple compressible systems undergoes quasiequilibrium processes. 1 Cengel Y. A., Boles M. A., Thermodynamics: An Engineering Approach, Mc-Graw Hill, 1998, pg. 48. 3-1 Pressure can be expressed as a function of temperature and specific volume: p = p(T, v). The plot of p = p(T, v) is a surface called p-v-T surface. Figure 3.1 shows the p-v-T surface of a substance such as water that expands on freezing. Figure 3.1-1 p-v-T surface and projections for a substance that expands on freezing. (a) 3-D view (b) p-T diagram (c) p-v diagram2. The location of a point on the p-v-T surface gives the values of pressure, specific volume, and temperature at equilibrium. The regions on the p-v-T surface labeled solid, liquid, and vapor are single-phase regions. The state of a single phase is determined by any two of the properties: pressure, temperature, and specific volume. The two-phase regions where two phases exist in equilibrium separate the single-phase regions. The two-phase regions are: liquid-vapor, solid-liquid, and solid-vapor. Temperature and pressure are dependent within the two-phase regions. Once the temperature is specified, the pressure is determined and vice versa. The states within the two-phase regions can be fixed by specific volume and either temperature or pressure. The projection of the p-v-T surface onto the p-T plane is known as the phase diagram as shown in Figure 3.1 (b). The two-phase regions of the p-v-T surface reduce to lines in the phase diagram. A point on any of these lines can represent any two-phase mixture at that particular temperature and pressure. The triple line of the p-v-T surface projects onto a point on the phase diagram called the triple point. Three phases coexist on the triple line or the triple point. 2 Moran, M. J. and Shapiro H. N., Fundamentals of Engineering Thermodynamics, Wiley, 2008, p. 83 3-2 The constant temperature lines of the p-v diagram are called the isotherms. For any specified temperature less than the critical temperature, the pressure remains constant within the two- phase region even though specific volume changes. In the single-phase liquid and vapor regions the pressure decreases at fixed temperature as specific volume increases. For temperature greater than or equal to the critical temperature, there is no passage across the two-phase liquid-vapor region. Figure 3.1-2 T-v diagram for water (to scale). Figure 3.1-2 is a T-v diagram for water. For pressure greater than or equal to the critical pressure, temperature increases continously at fixed pressure as the specific volume increases and there is no passage across the two-phase liquid-vapor region. The isobaric curve marked 50 MPa in Figure 3.1-2 shows this behavior. For pressure less than the critical value, there is a two-phase region where the temperature remains constant at a fixed pressure as the two- phase region is traversed. The isobaric curve with values of 20 MPa or less in Figure 3.1-2 shows the constant temperature during the phase change. o 3 At 100 C, the saturated volumes of liquid and vapor water are 1.0434 cm /g and 1,673.6 cm3/g, respectively. The quality of steam is the mass fraction of water vapor in a mixture of liquid and vapor water. The specific volume of 100oC steam with a quality of 0.65 is given by v = (1 − 0.65)vL + 0.65 vV = (0.35)(1.0434) + (0.65)(1,673.6) = 1088.2 cm3/g 3-3 3.2 Phase Behavior We will consider a phase change of 1 kg of liquid water contained within a piston-cycinder assembly as shown in Figure 3.2-1a. The water is at 20oC and 1.014 bar (or 1 atm) as indicated by point (1) on Figure 3.2-2. Figure 3.2-1 Phase change at constant pressure for water3 Figure 3.2-2 Sketch of T-v diagram for water4 As the water is heated at constant pressure, the temperature increases with a slight increase in specific volume until the system reaches point (f). This is the saturated liquid state corresponding to 1.014 bar. The saturation temperature for water at 1.014 bar is 100oC. The liquid states along the line segment 1-f are called subcooled or compressed liquid states. When the system is at the saturated liquid state (point f in Figure 3.2-2) any additional heat will cause the liquid to evaporate at constant pressure as shown in Figure 3.2-1b. When a mixture of liquid and vapor exists in equilibrium, the liquid phase is a saturated liquid and the vapor phase is a saturated vapor. Liquid water continues to evaporate with additional heat until it becomes all saturated vapor at point (g). Any further heating will cause an increase in both temperature and specific volume and the saturated vapor becomes superheated vapor denoted by point (s) in Figure 3.2-2. For a two-phase liquid-vapor mixture, the quality x is defined as the mass fraction of vapor in the mixture mvapor x = (3.2-1) mvapor + mliquid 3 Moran, M. J. and Shapiro H. N., Fundamentals of Engineering Thermodynamics, Wiley, 2008, p. 87 4 Moran, M. J. and Shapiro H. N., Fundamentals of Engineering Thermodynamics, Wiley, 2008, p. 86 3-4 The properties of pure subtances can be obtained from tables such as Table 3.2-1 for saturated properties of water and Table 3.2-2 for properties of superheated water vapor. Thermodynamic properties can also be obtained from softwares, which are more convenient since interpolations for values not directly available in the tables are not necessary. Table 3.2-1 Thermodynamic properties of saturated water5. Specific Volume Internal Energy Enthalpy Entropy m3/kg kJ/kg kJ/kg kJ/kg·K Sat. Liquid Sat. Sat. Sat. Sat. Sat. Sat. Sat. Temp. Press. vf × Vapor Liquid Vapor Liquid Evap. Vapor Liquid Vapor Temp. 3 °C bar 10 vg uf ug hf hfg hg sf sg °C .01 0.00611 1.0002 206.136 0.00 2375.3 0.01 2501.3 2501.4 0.0000 9.1562 .01 4 0.00813 1.0001 157.232 16.77 2380.9 16.78 2491.9 2508.7 0.0610 9.0514 4 5 0.00872 1.0001 147.120 20.97 2382.3 20.98 2489.6 2510.6 0.0761 9.0257 5 6 0.00935 1.0001 137.734 25.19 2383.6 25.20 2487.2 2512.4 0.0912 9.0003 6 8 0.01072 1.0002 120.917 33.59 2386.4 33.60 2482.5 2516.1 0.1212 8.9501 8 10 0.01228 1.0004 106.379 42.00 2389.2 42.01 2477.7 2519.8 0.1510 8.9008 10 11 0.01312 1.0004 99.857 46.20 2390.5 46.20 2475.4 2521.6 0.1658 8.8765 11 12 0.01402 1.0005 93.784 50.41 2391.9 50.41 2473.0 2523.4 0.1806 8.8524 12 13 0.01497 1.0007 88.124 54.60 2393.3 54.60 2470.7 2525.3 0.1953 8.8285 13 14 0.01598 1.0008 82.848 58.79 2394.7 58.80 2468.3 2527.1 0.2099 8.8048 14 15 0.01705 1.0009 77.926 62.99 2396.1 62.99 2465.9 2528.9 0.2245 8.7814 15 16 0.01818 1.0011 73.333 67.18 2397.4 67.19 2463.6 2530.8 0.2390 8.7582 16 17 0.01938 1.0012 69.044 71.38 2398.8 71.38 2461.2 2532.6 0.2535 8.7351 17 18 0.02064 1.0014 65.038 75.57 2400.2 75.58 2458.8 2534.4 0.2679 8.7123 18 19 0.02198 1.0016 61.293 79.76 2401.6 79.77 2456.5 2536.2 0.2823 8.6897 19 20 0.02339 1.0018 57.791 83.95 2402.9 83.96 2454.1 2538.1 0.2966 8.6672 20 21 0.02487 1.0020 54.514 88.14 2404.3 88.14 2451.8 2539.9 0.3109 8.6450 21 22 0.02645 1.0022 51.447 92.32 2405.7 92.33 2449.4 2541.7 0.3251 8.6229 22 23 0.02810 1.0024 48.574 96.51 2407.0 96.52 2447.0 2543.5 0.3393 8.6011 23 24 0.02985 1.0027 45.883 100.70 2408.4 100.70 2444.7 2545.4 0.3534 8.5794 24 25 0.03169 1.0029 43.360 104.88 2409.8 104.89 2442.3 2547.2 0.3674 8.5580 25 5 Moran, M.
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