一章节基本概念Chapter1BasicConcepts.ppt
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1、第一章 基本概念 Chapter 1. Basic Concepts,热力系统(Thermodynamic System) 系统的状态参数(Properties of A System) 状态及平衡状态(State and Equilibrium) 过程与循环(Process and Cycles),1.1 热力系统 Thermodynamic System,热力系统的定义(Definition ) 系统就是指被选做研究对象的物体或空间。 (A quantity of matter or a region in space chosen for study.) For example:,气缸-
2、活塞装置 Cylinder-piston equipment,汽轮机Steam turbine,外界(Surroundings) The mass or region outside the system (2)边界(Boundary) The real or imaginary surface that separates the system from its surroundings. (3) 系统与外界之间的质量及能量交换都必须穿越边界 The mass or energy exchange between system and its surroundings must cross
3、the boundaries.,2. 系统,外界和边界 System, Surroundings and boundary,(4) Characteristics of Boundary,边界是人为选定的 Boundaries are selected subjectively.,边界可以是固定的,也可以是可移动的Boundaries can be fixed or movable.,边界可以是真实的,也可以是假想的Boundaries can be real or imaginary.,热力系统选取的人为性,锅 炉boiler,汽轮机(turbine),发电机(generator),给水泵(
4、pump),凝汽器(condenser),过热器(reheater),只交换功,只交换热,既交换功 也交换热,真实、虚构,固定、活动,fixed 、 movable,real 、,imaginary,热力系 外界,物质交换 能量交换,物质流进和流出热力系统,传热和作功两种形式,3.热力系统分类 Types of systems,Energy can cross the boundary, in the form of heat and work.,(1) 以系统与外界关系划分:,有 无 是否传质 开口系 闭口系,是否传热 非绝热系 绝热系,是否传功 非绝功系 绝功系,是否传热、功、质 非孤立系
5、 孤立系,闭口系与外界无物质交换 A Closed system (a control mass 控制质量) consists of a fixed amount of mass, and no mass can cross its boundary. That is, no mass enters or leave a closed system. such as, Piston-cylinder device (汽缸-活塞装置),A. 闭口系统和开口系统 Closed system and Open system,An Open system (or a control volume 控制
6、体积)is a properly selected region in space. Both mass and energy can cross the boundary of a control volume. such as, A Water heater, a turbine and a compressor, etc,开口系与外界有物质交换,绝热系统与外界之间无热量交换 Adiabatic system is that no heat cross the boundary or heat is negligible compared with work cross the bound
7、ary 孤立系统与外界之间无任何物质和能量交换 Isolated system is a special case that no mass and energy cross the boundary.,B. 绝热系统和孤立系统 Adiabatic system and Isolated system,1 开口系,非孤立系相关外界 孤立系,1+2 闭口系,1+2+3 绝热闭口系,1+2+3+4 孤立系,(2)其它分类方式,其它分类方式,物理化学性质,均匀系(Even System) 非均匀系(Uneven System),工质种类,多元系(multi substance system),单元系
8、 (Single substance system),相态,多相(Multiple Phase),单相 (Single Phase),最重要的系统,只交换热量和一种准静态的容积变化功,容积变化功,压缩功膨胀功,简单可压缩系统 Simple compressible system,Moving Boundary Work,Compression Work,Expansion Work,1.2状态及状态参数 State and State Properties,状态:某一瞬间热力系所呈现的宏观状况 State: it reveals the physical condition of a syst
9、em. Consider a system not undergoing any change. At this point, all the properties can be measured or calculated throughout the entire system. It can be described by a set of properties,(1)状态参数:描述系统宏观物理状况的物理量,简称参数 Properties are used to depict any characteristic of a system. such as Pressure P (压力),
10、temperature T(温度), volume V(体积),mass m(质量), internal energy U(内能),enthalpy(焓), entropy(熵), viscosity(粘度), thermal conductivity(导热系数)。,2. Property (状态参数),(2)状态参数的特征 Characteristics of State Properties,A. 状态确定,则状态参数也确定,反之亦然 Properties of a state are determined by the state. If the state is specified,
11、its properties are fixed, or vise versa.,B. 状态参数的积分特征:状态参数的变化量与路径无关,只与初终态有关 The magnitude of the change in property is independent of the path (route), but just depend on the initial and final states.,状态参数的积分特征,数学上:,点函数、态函数,1,2,a,b,point function,可判断是否是状态参数,充要条件:,C. 状态参数的微分特征:全微分 Properties are func
12、tions of state, there exist differential for properties,After a series of change, the state returns to the original one, then the changes in its properties are 0,状态参数的微分特征,设 z =z (x , y),dz是全微分,充要条件:,可判断是否是状态参数,Total differentials,压力 p、温度 T、比容 v (容易测量) (1)密度和比容 Density and Specific Volume 密度指单位体积所含物
13、质的多少。 Density is defined as mass per unit volume.,3. 基本状态参数 Basic State Properties,比体积,单位质量的物质所占有的体积,用v表示。 Specific volume is the reciprocal of density and is defined as volume per unit mass.,比体积是表示物质内部分子疏密程度的状态参数。, 定义 Definition:垂直作用于单位面积上的力 It is defined as the force exerted by a fluid vertically
14、on a surface of unit area. (for solid is stress: person stand on foot) 压力的单位 Unit of pressure It has the unit of Newtons per square meter (N/m2) 1 Pa=1 N/m2 1 kPa= 103 Pa 1 MPa= 106 Pa,SI单位制,(2) 压力 Pressure,绝对压力,1 bar =105 Pa=100 kPa =0.1 MPa 标准大气压 Standard atmosphere 1 atm= 101325 Pa=101.325 kPa 工程
15、大气压 Engineer atmosphere 1 at=1 kgf/cm2 =9.807 N/cm2 =9.807*104 Pa 液柱高度 Height of liquid column 1 atm=760 mm Hg 1 at =10 m H2O,其他单位 Other units,1 mmHg=1gh =133.3 Pa,绝对压力指系统中工质的真实压力。 Absolute pressure is the actual pressure at a given position P . 相对压力反映绝对压力与当地大气压力的差值。 Relative pressure indicates the
16、difference between the absolute pressure and the local atmospheric pressure ., 绝对压力和相对压力 Absolute pressure and relative pressure,表压力:当高于大气压时,压力计显示的绝对压力超出大气压力的部分。 Gage pressure(表压力)is denoted as Pg 真空度表示绝对压力低于大气压力的量值 Vacuum pressure (真空度):Pressures below atmosphere pressure. It is the pressure differ
17、ence between atmospheric and system pressure when system pressure is lower than atmospheric And it is denoted as PVAC or H., 表压与真空度 Gage Pressure and Vacuum Pressure,表压力=绝对压力-大气压力,真空度=大气压力-绝对压力,绝对压力与相对压力,当 p pb,表压力 pg,当 p pb,真空度 pv,pb,pe,p,pv,p,relative pressure,absolute pressure,Gage pressure,Vacuu
18、m pressure,Attentions,如果大气压力发生变化,即使工质的绝对压力不变,测压计的对数也会变,所以只有绝对压力才是状态参数。 工程计算中,必须选取绝对压力,注意,测量压力的仪表通常处于大气环境中,不能直接 测量出绝对压力,显示的是绝对压力和当时当地大气压的差值。,Variation of pressure with Depths Pressure is a scalar quantity. (压力是一个标量) At any point in a fluid, Pressure is the same in all directions. Pressure in a fluid
19、increases linearly with depth. (液体中的压力随液体的深度线性增加) Pressure is the same at all points on a horizontal plane in a given continuous fluid at rest. (帕斯卡定律)., 压力p的测量 Pressure Measuring,绝对压力与环境压力的相对值 相对压力,示意图,U-tube manometer,Bourdon Tube,环境压力与大气压力,指压力表所处环境,环境压力 Environmental pressure,大气压力Atmospheric pres
20、sure,大气压随时间、地点变化,物理大气压 1atm = 760mmHg,1mmHg = gh = 133.322Pa,A.The Manometer (压力计) Manometers measure a pressure difference by balancing the weight of a fluid column between the two pressures of interest.,Pressure in a continuous static fluid is the same at any horizontal level so,For the left hand
21、arm,For the right hand arm,Example 1: U-tube manometer using multiple fluid column (采用多种液柱的U型管压力表),It is a mechanical pressure measurement device and consists of a hollow metal tube bent like a hook whose end is closed and connected to a dial indicator needle.,C-Type,B. Bourdon tube (弹簧管式压力表),As sho
22、wn in the following figure,it is known that pb=101325Pa, the height difference is H=300mm for murcury liquid. The gauge pressure of B is 0.2543MPa, Then what is the pressure for side A, and what is the value of Pg,A? 已知大气压pb=101325Pa,U型管内 汞柱高度差H=300mm,气体表B读数为0.2543MPa,求:A室压力pA及气压表A的读数pgA,Exercise.1
23、(练习1),解:,热力学第零定律(R.W. Fowler in 1931) 如果两个系统分别与第三个系统处于 热平衡,则两个系统彼此必然处于热平衡。,温度测量的理论基础 B 温度计,传统:冷热程度的度量。感觉,导热,热容量 微观:衡量分子平均动能的量度 T 0.5 m w 2,(3) Temperature (温度),The Zeroth Law of Thermodynamics,如果两个系统分别与第三个系统处于热平衡,则两个系统彼此必然处于热平衡。,If two bodies are in thermal equilibrium with a third body, they are al
24、so in thermal equilibrium with each other.,Two bodies are in thermal equilibrium if both have the same temperature reading even if they are not in contact. By replacing the third body with a thermometer (温度计), the zeroth law can be restated as.(即使两个物体不相接触,只要它们处于热平衡,则温度值相等,Equality of temperature is
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