热能与动力工程专业外语.doc

UNIT2热对流For both situation shown in Fig.1.2,we express the overall effect of convection,we use New-ton”s law of cooling: Q=Ha(Tw-T8横)在图1.2所示的两种情况，我们对对流的整体效果，我们使用的冷却新吨级“定律为: Q=Ha(Tw-T8横)UNIT3热辐射Thermodynamic considerations show that an idea thermal radiator ,or blackbody ,will emit energy at a rate proportional to the fourth power of the absolute temperature of the body and directly proportional to its surface area. Thus q(emitted)=AT4 (1-6) 热力学规律揭示出一个理想的热辐射体或黑体，所散发出能量的速度与其自身的绝对温度的四次方和其表面积成正比。即（公式） Where is the proportionality constant and is called the Stefan-Boltzmann constant with the value of 5.669*10-8 W/m2k4. Equation(1-6)is called the Stefan-Boltzmann law of thermal radiation, and it applies only to blackbodies. It is important to note that this equation is valid only for thermal radiation; other types of electromagnetic radiation may not be treated so simply. 其中是比例常数，值为5.669*10-8 W/m2k4称为斯蒂凡玻尔兹曼常数。公式（1-6）被称为热辐射斯蒂凡玻尔兹曼定律，它仅适用于黑体。重要的是要注意这个公式只适用于热辐射;其他类型的电磁辐射可不能如此简单对待。UNIT8气体燃料的燃烧机理Combustion of gaseous fuels occurs by the laws of branched chain reactions which where discovered by Soviet Academician N.N.Semenov and C.N.Hinshelwood.The conversion of the original substances to the final products passes through a sequence of reaction links which are connected in succession with one another and develop in the volume of a combustible mixture like the branches of a tree develop from its trunk. This results in the formation of the final reaction products and of even greater number of active centres which further ensure the development of the reaction in the confining volume.气体燃料燃烧时由苏联院士NNSemenov和CNHinshelwood发现支链反应法。通过反应与另一个连接在继承和发展的一个可燃混合气的体积，像一棵树的树枝从树干的链接序列的原始物质转化为最终产品。在最终的反应产物和更活跃的中心，进一步确保围体积的反应发展形成这样的结果。Let us consider the mechanism of branched chain reactions,taking as an example the combustion of hydrogen in air. By the stoichiometric equation让我们考虑支链反应的机制，作为一个例子，在空气中燃烧的氢。通过计量公式2H2+O2=2H2OThe rate of reaction between molecules of the combustible substance 的可燃物质的分子之间的反应率(Wh2o=K0e.)Cannot be very large .Actually ,however, combustion of hydrogen at temperatures above 500 is an explosive chain reaction proceeding at a very high rate. Indeed, according to N.N.Semenov, the beginning of the active reaction is preceded by the formation of active centres:其实不能非常大。然而，在500以上温度，氢的燃烧是一个爆炸性的连锁率非常高的反应程序。事实上，根据NNSemenov的积极反应开始之前活性中心的形成：(H2+M)(H2+O.)Where M and Q2 are active molecules which possess high energy levels in the volume其中，M和Q2拥有量高的能量水平的积极分子Atoms and radicals formed by this mechanism actively enter the reactions which surrounding molecules,i.e. chains of successive reactions develop which result in the formation of the final reaction products and ever greater number of active centres.这一机制所形成的原子和自由基积极进入反应，周围的分子，即连续反应链的发展，从而最终反应产物的形成和活动中心的数目越来越大的。UNIT9固体燃料与液体燃料的燃烧机理Fig.1.17 schematically shows the combustion of a liquid fuel droplet in stagnant air. A vapour cloud forms around the droplet and diffuses into the environment ,with the diffusion of oxygen of the air occurring in opposite direction .As a result ,the stoichiometric relationship between the combustible gases and oxygen is established at a certain distance rst from the droplet , i.e. the burning fuel vapours form a spherical combustion front around it. The magnitude of rst is equal to 4-10 droplet radii, i.e. rst = 4-10 rd, and depends heavily on the droplet size and the temperature in the combustion zone. In the zone where rrst ,fuel vapours prevail, but their combustion zone. The highest temperature is established in the reaction zone. Although at both sides of this zone the temperature decreases gradually, its decrease is more intensive in the inside direction, i.e. on approaching the droplet ,since some heat is spent there for heating fuel vapours.Fig.1.17示意图显示了一个停滞的空气中的液体燃料液滴燃烧。蒸气云周围形成液滴扩散到环境中，在相反的方向发生空气中的氧气扩散。因此，之间的可燃气体和氧气的化学计量关系是建立在一定的距离RST从液滴，即燃烧燃料蒸汽形成一个围绕它的球形的燃烧前。RST幅度等于4-10液滴半径，即RST =4-10路，很大程度上取决于墨滴的大小和燃烧区的温度。在R RST，燃料蒸气占上风，但其燃烧区的区域。最高温度是建立在反应区。虽然双方在此区域的温度逐渐降低，其跌幅更密集的方向在里面，即在接近液滴，因为有一定的热量是用于取暖燃料蒸汽。Thus, the burning rate of a liquid fuel droplet is determined by the rate of evaporation from its surface, the rate of chemical reaction in the combustion zone. And the rate of oxygen diffusion to this zone. As stated earlier, the reaction rate in a gaseous medium is very high and cannot limit the total rate of combustion. The quantity of oxygen diffused through the spherical surface is proportional to the square of sphere diameter, and therefore, a slight removal of the combustion zone from the surface of the droplet (under oxygen deficiency) noticeably increases the mass flow rate of supplied oxygen. Thus, the rate of combustion of the droplet is mainly determined by evaporation from its surface. The combustion rate of liquid fuels is increased by atomizing the fuel just before burning, which substantially increase the total surface of evaporation. Besides all this, as the size of the droplets decreases, the intensity of evaporation per unit area of their surface increases. Fine liquid fuel droplets suspended in an air flow move at low Reynolds numbers, Re4. In such cases, the heat flow through a spherical surface is determined solely by the conductivity 入 through the boundary layer, which is much thicker than the droplet diameter. Under such conditions, the heat-transfer co-efficient a is given by Sokolskys formula:Nu = ad/入=2 whence a = 2入/d = 入/r Where Nu is the Nusselt number.因此，一个液体燃料液滴的燃烧率是由从它的表面，在燃烧区域的化学反应速率的蒸发率。率氧气扩散到该区域。如前所述，在气体介质中的反应速度是非常高的，可以没有限制的燃烧率。通过球面扩散的氧气量是球体直径的平方成正比，因此，一个轻微去除表面的液滴（缺氧）从燃烧区明显增加提供氧气的质量流量。因此，液滴的燃烧率主要取决于其表面的蒸发。液体燃料的燃烧率增加了雾化的燃料，只是在刻录之前，这大大增加蒸发表面。除了这一切，为液滴的跌幅，其表面增加单位面积蒸发强度大小。精细液体燃料液滴，悬浮在空气流移动低雷诺数RE 4。在这种情况下，通过球形表面的热流仅由通过边界层，这是比液滴直径厚的导电入。在这种情况下，传热合作效率的一个是由Sokolsky的公式：Nu = ad/入=2 何处 a = 2入/d = 入/rAs follows from formula(1-13), the heat exchange between a droplet and the surrounding medium increases as the size of the droplet decreases, i.e. with a decrease in its mass. It turns out that the evaporation time of the droplet decreases, i.e. with a decrease in its mass. It turns out that the evaporation time of a droplet is proportional to the square of its initial diameter.按以下公式（1-13），液滴和液滴的大小减小周围介质之间的热交换，即减少其质量。事实证明，液滴降低蒸发时间，即减少其质量。事实证明，液滴的蒸发时间是其初始直径的平方成正比。When combined with air in a furnace, pulverized coal first passes through the stage of thermal preparation(Fig. 1.18), which consists in the evaporation of residual moisture and separation of volatiles. Fuel particles are heated up to a temperature at which volatiles are evolves are evolves intensively (400-600C) in a few tenths of a second. The volatiles are then ignited, so that the temperature around a coke particle increases rapidly and its heating is accelerated (III”). The intensive burning of the volatiles(II)takes up 0.2-0.5s. A high yield of volatiles (brown coal, younger coals, oil shales , peat), produces enough heat through combustion to ignite coke particles. When the yield of volatiles is low, the coke particles must be heated additionally form an external source(III”). The final stage is the combustion of coke particles at a temperature above 800-1000C(4).This is a heterogeneous process whose rate is determined by the oxygen supply to the reacting surface. The burning of a coke particle proper takes up the greater portion (1/2 to 2/3) of the total time of combustion which may constitute 1 to 2.5s , depending on the kind of fuel and the initial size of particles.当于一炉的空气相结合，煤粉首先通过热的准备阶段（图1.18），包括残留水分的蒸发和挥发物的分离。燃料颗粒被加热到温度下挥发物的发展是集中发展的第二个零点几秒（400- 600C）。然后点燃的挥发，使焦炭颗粒周围的温度迅速上升，并加速其加热（三）。密集的挥发物燃烧（二）占用0.2-0.5s的。一个高产挥发物（褐煤，年轻的煤，油页岩，泥炭），产生足够的热量通过燃烧焦炭颗粒点燃。当挥发物的产量低，焦炭颗粒必须加热此外形成外部源（三）“。最后一个阶段是焦炭颗粒的燃烧温度在800- 1000C（4）以上，这是一种异质性的过程，其速率是反应表面的氧气供应决定。焦炭颗粒适当的燃烧占据了较大部分的总燃烧时间可能构成1至2.5S（1 / 2至2 / 3），燃料种类和粒子的初始大小而定。UNIT11压力和压力测量Capacitive or inductance电容或电感The movement associated with one of the mechanical sensors already described can be used to influence an electrical property such as capacitance affecting a measured signal .For example ,under changing pressure a diaphragm causes a change in capacitance or inductance .与前面介绍的机械传感器之一相关的运动可以用来影响，如影响测量信号的电容的电气性能。例如，在不断变化的压力隔膜导致电容或电感的变化。Resistive, strain gauge电阻，应变计The electrical resistance of a metal wire depends on the strain applied to the wire .Deflection of the diaphragm due to the applied pressure cause strain in the wire, and the electrical resistance can be measured and related to pressure.金属丝的电阻取决于应用的隔膜由于所施加的压力线的原因应变线。偏转应变，电阻可以测量和压力有关。Piezoelectric压电A piezoelectric material, such as quartz, generates a voltage output when pressure is applied on it. Force can be applied by the diaphragm to a quartz crystal disk that is deflected by process pressure.压电材料，如石英，它适用于当压力产生的电压输出。队可应用于隔膜过程压力偏转石英晶体的磁盘.UNIT12流量计的简介 Thermocouple Theory热电偶理论 A thermocouple circuit has at least two junctions: the measurement junction and a reference junction. Typically, the reference junction is created where the two wires connect to the measure device. This second junction it is really two junctions: one for each of the two wires, but because they are assumed to be at the same temperature(isothermal) they are considered as one (thermal) junction. It is the point where the metals change-from the thermocouple metals to what ever metals are used in the measuring device-typically copper. 热电偶电路至少有两个路口：它们是测量交界处和参考结。通常情况下，我们把连接到测量设备的两条线定义为参考的交界处。这第二个路口，它才是真正的两路口：为每个两条线之一，但因为他们都被假定为在相同的温度（温），他们被认为是（热）交界处。它是金属的变化，从热电偶金属什么都在测量设备通常是铜使用的金属。 The output voltage is related to the temperature difference between the measurement and the reference junctions. This is phenomena is known as the Seebeck effect. The Seebeck effect generates a small voltage along the length of a wire ,and is greatest where the temperature gradient is greatest. If the circuit is of identical material, then they will generate identical but opposite Seebeck voltages which will cancel. However, if the wire metals are different the Seebeck voltages will be different and will not cancel. 输出电压是关系到测量和参考路口之间的温差。这是现象称为塞贝克效应。塞贝克效应产生一个小的电压，沿线的长度和最大温度梯度最大。如果电路是相同的材料，那么他们就会产生相同，但方向相反的塞贝克电压，将取消。但是，如果是不同的金属丝塞贝克电压将有所不同，并不会取消。 In practice the Seebeck voltage is made up of two components: the Peltier voltage generated at the junctions, plus the Thomson voltage generated in the wires by the temperature gradient, as shown in Fig.1.22. 在实践中，Seebeck电压由两部分组成：在交界处产生的电压的佩尔蒂埃，加上汤姆森电压导线所产生的温度梯度，在Fig.1.22所示。 The Peltier voltage is proportional to the temperature of each junction while the Thomson voltage is proportional to the square of the temperature difference between the two junctions. It is the Thomson voltage that accounts for most of the observed voltage and non-linearity in thermocouple response. 佩尔蒂埃电压是每个交界处的温度成正比，而汤姆森电压的两路口之间的温度差的平方成正比。这是汤姆逊电压占大部分观测到的电压和热电偶响应非线性。 Each thermocouple type has its characteristic Seebeck voltage curve. The curve is dependent on the metals, their purity ,their distribution in the wire is also important. These potential inhomogeneous characteristics of metal are why thick wire thermocouples can be more accurate in high temperature applications, when the thermocouple metals and their impurities become more mobile by diffusion. 每个热电偶类型有其特有的Seebeck电压曲线。曲线上的金属，其纯度，他们分布在电线也很重要。这些潜在的金属不均匀的特点是为什么粗丝热电偶，可以更准确地在高温应用中，当热电偶金属及其杂质的扩散变得更加移动。UNIT13流量测量Pitot tube and annubar皮托管和阿牛巴:The pitot tube, shown in Fig.1.26 below, measures the static and dynamic pressures of the fluid at one point in the pipe. The flow rate can be determined from the difference between the static and dynamic pressures which is the velocity head of the fluid flow. An annubar consists of several pitot tubes placed across a pipe to provide an approximation to the velocity profile, and the total flow can be determined based on the multiple measurements. Both the potit tube and annubar contribute very small pressure drops, but they are not physically strong and should be used only with clean fluids. Fig.1.26下面所示，皮托管，流体在一个管道中的点的静态和动态压力措施。流速可确定这是流体流动的速度头的静态和动态压力之间的差异。由阿牛巴放在整个管道提供了一个近似的速度剖面的几个皮托管，并可以多次测量的基础上确定的总流量。potit管和阿牛巴的贡献非常小的压力下降，但它们不是身体强壮，只应使用清洁的流体The following flow sensors are based on physical principles other than head.头比其他物理原理是基于以下的流量传感器。Turbine涡轮:As fluid flows through the turbine, it causes the turbine to rotate with an angular velocity that is proportional to the fluid flow rate. The frequency of rotation can be measured and used to determine flow. This sensor should not be used for slurries or systems experiencing large, rapid flow or pressure variation.作为通过涡轮的流体流动，它会导致涡轮的旋转角速度，流体流速成正比。可旋转的频率测量和用于确定流。该传感器不应该用于泥浆或遇到大的，快速的流量或压力变化的系统。UNIT14控制系统简介Controlled Variable and Manipulated Variable . The controlled variable is the quantity or condition that is measured and controlled. The manipulated variable is the quantity or condition that is varied by the controller so as to affect the value of the controlled variable. Normally ,the controlled variable is the output of the system. Control means measuring the value of the controlled variable of the system and applying the manipulated variable to the system to correct or limit deviation of the measured value from a desired value.控制变量和操纵变量。控制变量是测量和控制的数量或条件。操纵变量的数量或条件变化，从而影响控制变量的值是由控制器。通常情况下，控制变量是系统的输出。控制意味着测量系统的控制变量的值和调节变量的应用系统所需的值的测量值的偏差纠正或限制。In studying control engineering, we need to define additional terms that are necessary to describe control systems.在研究控制工程，我们需要定义来描述控制系统所必需的附加条款。UNIT15控制系统举例Open loop control systems.