上游充通大￥ SHANGHAI JIAO TONG UNIVERSITY Engineering Thermodynamics I Lecture 12 Spring,2019 Prof.,Dr.Yonghua HUANG 强 R是n http://cc.sjtu.edu.cn/G2S/site/thermo.html 1日G
Engineering Thermodynamics I Lecture 12 Spring, 2019 Prof., Dr. Yonghua HUANG http://cc.sjtu.edu.cn/G2S/site/thermo.html
Evaluating Specific Internal Energy and Enthalpy A new property:Enthalpy,H Control volume (unit:J,KJ) ”=光 FIGURE 3-25 The combination u+Pv is frequently encountered in the analysis of control volumes. PropertyAll properties 0 For a unit mass,h (unit:J/g,kJ/kg) m'/kg I kJ/kg kJ/kg I kJ/kg K p=0.06bar=0.006MPa (T=36.16C) h u+pu Sat. 23.739 2425.0 2567.4 8.3304 80 27.132 2487.3 2650.1 8.5804 120 30.219 2544.7 2726.0 8.7840 per mole (unit:J/mol,kJ/kmol) 160 33.302 2602.7 2802.5 8.9693 200 36.383 2661.4 2879.7 9.1398 240 39.462 2721.0 2957.8 9.2982 h=u+p而 280 42.540 2781.5 3036.8 9.4464 320 45.618 2843.0 3116.7 9.5859 360 48.696 2905.5 3197.7 9.7180 400 51.774 2969.0 3279.6 9.8435 440 54.851 3033.5 3362.6 9.9633 500 59.467 3132.3 3489.1 10.1336 上游通大学 March 14,2019 2 SHANGHAI JLAO TONG UNIVERSITY
March 14, 2019 2 Evaluating Specific Internal Energy and Enthalpy A new property: Enthalpy, H Property All properties For a unit mass, h (unit: J/g, kJ/kg) per mole (unit: J/mol, kJ/kmol) (unit: J, kJ)
u and h in two-phase region The same way as the specific volume v Tables for sat.properties quality x u=(1-x)4r+g=4+x(ug-4） ↓ Sat.liquid Sat.vapor h (1 -x)hy xhg hr x(hg -h) Lhie Latent heat,vaporization heat 上游究通大学 March 14,2019 3 SHANGHAI JLAO TONG UNIVERSITY
March 14, 2019 3 u and h in two-phase region The same way as the specific volume v Tables for sat. properties + quality x Sat. liquid Sat. vapor Latent heat, vaporization heat + pv
Reference states and reference values Recall reference values for PE,KE u,h,s cannot be measured directly;p,T,v can be measured directly Changes in u,h,s computed using derived relations with p,T,v Note:only changes can be computed,not u,h,s,at a specific state But fortunately in 1st and 2nd laws only changes in these properties are needed arbitrary 国 We choose a convenient reference state and set properties to zero at that state ● For water,u=0,s=0,at 0.01C for Sat.Liq.state; h=u+pv;could have negative values relative to reference; For ammonia,propane,refrigerants,h =0 sat.liq.at-40C; u=h-pv Ch available in table but not u) Different ref.state for different substances;ref.state cancels out 上游通大学 March 14,2019 4 SHANGHAI JLAO TONG UNIVERSITY
March 14, 2019 4 Reference states and reference values Recall reference values for PE, KE u,h,s cannot be measured directly; p,T,v can be measured directly Changes in u,h,s computed using derived relations with p, T, v Note: only changes can be computed, not u, h, s, at a specific state But fortunately in 1st and 2nd laws only changes in these properties are needed We choose a convenient reference state and set properties to zero at that state For water, u = 0, s = 0, at 0.01ºC for Sat. Liq. state; h=u+pv; could have negative values relative to reference; For ammonia, propane, refrigerants, h =0 sat. liq. at -40ºC; u=h-pv (h available in table but not u) Different ref. state for different substances; ref. state cancels out arbitrary
Example12.1 Stirring water at constant volume T1=100℃ Sat.vapor Water 1.5 bars stir 0.25m3 1.014 bars P2=1.5 bar 100C Boundary insulated Find:(a)T2;(b)W Assumptions: 1.5 bars 1.The water is a closed system. 2.The initial and final states are at equilibrium. 1.014 bars There is no net change in kinetic or potential energy. 100°C 3.There is no heat transfer with the surroundings. 4.The tank volume remains constant. 上游充通大 March 14,2019 5 SHANGHAI JLAO TONG UNIVERSITY
March 14, 2019 5 Example12.1 Stirring water at constant volume insulated 0.25m3 p2=1.5 bar T1=100 ̊C Sat. vapor stir Find: (a) T2 ; (b) W