工程热力学第二章热力学第一定律教案详解.ppt

* （２）The physical meaning of each item 每一项的物理意义 Q and W means the heat added to the system and work done by the system respectively. No moving boundary work is involved. (稳态稳定流动系统中不涉及移动边界功。） Q is positive (+), if the heat is transferred to the system. Q is negative (-), if it is transferred from the system. W is positive (+), if the work is done by the system, W is negative (-), if the work is done on the system. Physical interpretation for enthalpy. 方程直观地显示出焓的物理意义 In an open flow system, enthalpy is the amount of energy that is transferred across a system boundary by a moving flow which depends on the system states. This energy is composed of two parts: the internal energy of the fluid u the flow work pv associated with pushing the mass of fluid across the system boundary. (3) Technical work 技术功 The kinetic energy, potential energy and shaft work can be used easily. we define Technical work is equal to shaft work if kinetic and potential energy is negligible. 如果动能和势能可以忽略不计，那么技术功就等于轴功 Then, the energy balance equation can be rewritten as We know, for the flow fluid, Then This equation indicates the relationship between flow work, moving boundary work and technical work 并非所有的膨胀功在技术上都是可用的 Not all moving boundary work can be used technically. For example. For reversible process p v wt 1 2 Substitute it into We get 4. Some steady-flow engineering devices 工程中的几种稳态稳定流动装置 (1) Turbines and Compressors 气轮机与压缩机 The kinetic and potential energy difference between the inlet and outlet can be neglected as well as the heat rejection. The work done by a turbine is positive and that equals to the enthalpy decrease of the flowing fluid. 汽轮机中所做的功为正,且等于流动工质的焓降 Similarly, the work consumed in a compressor is equal to the enthalpy increase of the flowing fluid 类似地,压缩机所消耗的功量等于流动工质的焓增 (2) Nozzles and Diffusers (喷管和扩压管) 通过