第2章节热力学第一定律幻灯片.pptVIP

* * * * * * * * * * * * * * * * 2-4-6 Various Forms of the First Law Relation for Steady-Flow System （不同形式） Differential form ? Q = d H + ? Wt （微分形式） ? q = d h + ? wt Reversible process Q = ? H - ?12 V d p （可逆过程） q = ? h - ?12 v d p Differential reversible process ? Q = d H - V d p （可逆微元过程） ? q = d h - v d p General Q = ?H + W t （一般形式） q = ? h + w t 根据热力学第一定律，任何循环的净热量等于该 循环的净功量。 热力过程中，工质向外界放热，其温度必然降低。 工质从同一初态出发，分别经历可逆过程和 不可逆过程达到相同的终态，则两过程中工质 与外界交换的热量相同。 工质所作的膨胀功与技术功，在某种条件下， 两者的数值会相等。 Q = ?H + W t，q = ? h + wt （?） （?） （x） （x） 功不是状态参数，热力学能与推动功之和也不是状态参数。 流动功的改变量仅取决于系统进出口状态，而与工质经历的过程无关。 （x） （?） 2-5 Some Steady-flow Engineering Devices（稳定流动能量方程式的应用） Energy balance for steady-flow systems Some common steady-flow devices Turbines and compressors (pump, fan)（汽轮机和压缩机） Heat exchanges（热交换器） Nozzles and diffusers （喷嘴和扩压器） Throttling valves（节流阀） 2-5-1 Heat Exchanges（热交换器） Energy Balance Notes: The heat transfer associated with a heat exchange may be zero or nonzero depending on how the system is selected. Heat exchanges are devices where two moving fluid streams exchange heat without mixing. Characteristics No work interactions, ws=0 Negligible kinetic and potential energy changes, ?cf2 =0, ?z =0 h2 h1 2-5-2 Turbines（涡轮，汽轮机） Characteristics Well insulated, Negligible, q=0 Negligible kinetic and potential energy changes, ?cf2 =0, ?z =0 Energy Balance Notes: Ws=Wt . The device that drives the electric generator in steam, gas, or hydroelectric plants. 2-5-3 Compressors（压缩机） Compressors, as well as pumps and fans, are devices used to increase the pressure of a fluid. Characteristics Well insulated, Negligible q=0 Negligible kinetic and potential energy changes, ?cf2 =0, ?z =0 Energy Balance No