西北大学《工程热力学》课件-Chapter2THE FIRST LAW OF THERMODYNAMICS.PPTVIP

2-2 Energy Change of a System, ?E system ?E system = ? U ?E system =U2-U1 For Stationary System 2-3 Transfer and conversion of energy The only two forms of energy interactions associated with a system, are heat transfer and work. An energy interaction is heat transfer if its driving force is a temperature difference. 1) Doing work and heat transfer If the energy crossing the boundary of a closed system is not heat, it must be work. 2) Push work and flow work p,v Push work is the work done on the barrier by working fluid when the fluid flow forward. L In the figure on the right, the push work is Wp=mpistongL=pAL=pV (J) And, on a unit mass basis, w=pv (J/kg) Flow work 1 1 2 2 p1,v1 p2,v2 Flow work is the work done by a open system for maintaining a continuous flow through the system. The specific flow work can expressed as following: wf=p2v2- p1v1 (J/kg) 2-4 Enthalpy, Specific enthalpy Specific enthalpy of a working fluid, h, is a property of the fluid which is defined as: h=u+pv where, u— Specific internal energy P—Pressurev—Specific volume Total enthalpy of a system is defined as the mass of the system, m, multiplied by the specific enthalpy of the system, h i.e. H=m h =U+pV (J) Total Enthalpy Because the enthalpy is a property, it definitely is the function of two independent properties of the system. h=f(T, v) or h=f(T, p) or h=f(p, v) And, , 2-5 The energy equation of closed systems 1) Energy Balance Most closed systems in practice are stationary or relatively stationary, that is, they do not involve any changes in their velocity or elevation during a process, so, 2) Energy equation of closed systems The change in energy of the system The heat added to a system The work done by a system The energy entering the system The energy leaving the system 2) Energy equation of closed systems The equation above is the e