高等工程熱力學(xué)英文課件：Chapter-2-first-law

1、1,Chapter 2 First law of thermodynamics,21 Energy,2-2 The first law of thermodynamics,23 Energy equation for closed system,24 Energy equation for open system,25 Application of Energy equation,2,2-1 Energy,1. Types of energy,The total energy stored in a system,To measure the motion of objects (macro

2、and micro),Thermodynamic energy OR Internal energy (U),3,2. Internal energy (U),Internal energy (U) is a single-valued function of thermodynamic state and thus a state parameter. The internal energy of gas can be determined by two independent state parameters U=f(T, V) The absolute zero point of int

3、ernal energy doesnt exist as motion is eternal. Therefore, the change of internal energy (U) is the concern of thermodynamic study.,4,3. Enthalpy H,Definition：H=U+pV h=u+pv Unit： J（kJ） J/kg（kJ/kg） Enthalpy is a state property. It represents the energy flux caused by mass transfer through an open sys

4、tem.,Internal energy+push work The energy contained in flowing substance; Different from mechanical energy,For closed system U For open system H,5,2-2 The first law of thermodynamics,The core of the first law is energy conservation and energy transformation. Energy conservation principle: During a p

5、rocess, the net change in the total energy of a system is equal to the energy into the system minus the energy leaving the system. (energy can neither be generated nor fade away) There are three forms of energy transfer: heat transfer, doing work and mass flow.,6,The mathematic expression for energy

6、 conversation in a closed system,The first law is one of the great laws of physics. Its validity rests on experiments to which no exceptions have been seen.,The mathematic expression for energy conversation in a open system,7,E,E+dE,Energy input：,Energy output,The increase of energy: dE,For a proces

7、s between two states that are infinitely close to each other,8,or,Per unit time,differential form,integral form,Mass flow rate (kg/s),Generalized energy equation,23 Energy equation for closed system,9,Ignoring Ek, Ep,Analysis formula of the first law of thermodynamics The basic equation for the tran

8、sformation between heat and work,Per unit mass,Algebraic value,10,For a reversible process,For a cycle,11,2-4 Energy equation for open system,to keep the working medium to flow,12,qm1=qm2=qm; and h=u+pv,For a steady flow: The thermodynamic properties and mechanical properties of any point in the sys

9、tem dont vary with time.,ECV = 0,divided by qm,13,The total mechanical energy caused by heat transfer. equal to the expansion work in a closed system,Shaft work,Flow work,The increase of mechanical energy,（C）,h=u+pv,q-u=w for a closed system,14,technical work,（C）could be regrouped as (D),For a rever

10、sible process,The area covered by blue shadow,Normally, the change of mechanical energy is negligible,15,The second analysis formula,heat,The relationship between these two analysis formulas in reversible process,work + the change of working medium,If reversible,For open system For closed system,16,

11、The first law in four basic thermodynamic processes,(1) Adiabatic process Q=0, U=Q-W=-W,we consider a simple system: a fixed mass of an ideal gas enclosed in a container fitted with a movable piston (internal combustion engine). = 3 2 = 3 2 ,W0, T0 power stroke,W0 compression stroke,(2) Isobaric pro

12、cess W=pV,(3) Isochoric process V=0, W=0, U=Q,(4) Isothermal process T=0, U=0, Q=W,To elucidate the physical meanings,1.steam turbine, gas turbine,17,energy input：,energy output：,E=0 (steady flow),2-5 Application of energy equations,The system does work to the environment at the cost of enthalpy decrease,Energy conservation,2. compressor，pump (to increase the pressure of working medium),Energy input,18,Energy output,E=0,The work consumed by equipment,Algebraic Value,19,3. heat exchanger: boiler、heate