发布时间 : 星期一 文章热能与动力工程专业英语更新完毕开始阅读
1.1.8 The Reheat cycle
It is apparent that when operating in a Rankine cycle with a high boiler pressure or a low condenser pressure it is difficult to prevent liquid droplets from forming in the low-pressure portion of the turbine. Since most metal cannot withstand temperatures above about 600℃, the reheat cycle is often used to prevent liquid-droplet formation: the steam passing through the turbine is reheat at some intermediate pressure, thereby raising the temperature to state 5 in the T-s diagram of Fig.1-6. The steam then passes through the low-pressure section of the turbine and enters the condenser at state 6. This controls or completely eliminates the moisture problem in the turbine. The reheat cycle dose not significantly influences the thermal efficiency of the cycle, but it does result in a significant additional work output, represented in the figure by area 4-5-6-4’-4 of Fig.1-6. The reheat cycle demands a significant investment in additional equipment, and the use of such equipment must be economically justified by the increased work output. If reheat is not used to avoid droplet formation, the condenser pressure must be quite high, resulting relatively low cycle efficiency. In that sense, reheat significantly increase cycle efficiency when compared to cycle with no reheat but with the higher condenser pressure.
1.1.8 再热循环
对于一个处于高锅炉压强和低凝汽器压强条件下的朗肯循环，显然，很难阻止液滴在汽轮机低压部分的形成。由于大多数金属不能承受600℃以上的高温，因此，通常采用再热循环来防止液滴的形成。再热过程如下：经过汽轮机的部分蒸汽在某中间压强下被再热，从而提高蒸汽温度，直至达到状态5，如图1-6所示。然后这部分蒸汽进入汽轮机低压缸，而后进入凝汽器（状态6）。再热循环方式可以控制或者完全消除汽轮机中的湿蒸汽问题，因此，通常汽轮机分成高压缸和低压缸两部分。虽然再热循环不会显著影响循环热效率，但带来了显著的额外的输出功，如图1-6中的面积4-5-6-4?-4所示。当然，再热循环需要一笔可观的投资来购置额外的设备，这些设备的使用效果必须通过与多增加的输出功进行经济性分析来判定。如果不采用再热循环来避免液滴的形成，则凝汽器出口压强必须相当地高，因而导致循环热效率较低。在这种意义上，与无再热循环且高凝汽器出口压强的循环相比，再热可以显著提高循环效率。
图1-6 再热循环
1.2 Fundamental of Fluid Mechanics
Fluid motions manifest themselves in many different ways. Some can be
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described very easily, while others require a thorough understanding of physical laws. In engineering applications, it is important to describe the fluid motions as simply as can be justified. This usually depends on the required accuracy. Often, accuracies of ±10% are acceptable, although in some applications higher accuracies have to be achieved. The general equations of motion are very difficult to solve; consequently, it is the engineer’s responsibility to know which simplifying assumptions can be made. This, of course, requires experience and, more important,
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a understanding of the physics involved.
1.2 流体力学基础
流体运动表现出多种不同的运动形式。有些可以简单描述，而其它的则需要完全理解其内在的物理规律。在工程应用中，尽量简单地描述流体运动是非常重要的。简化程度通常取决于对精确度的要求，通常可以接受±10%左右的误差，而有些工程应用则要求较高的精度。描述运动的一般性方程通常很难求解，因此，工程师有责任了解可以进行哪些简化的假设。当然，这需要丰富的经验，更重要的是要深刻理解流动所涉及的物理内涵。
Some common assumptions used to simplify a flow situation are related to fluid properties. For example, under certain conditions, the viscosity can affect the flow significantly; in others, viscous effects can be neglected greatly simplifying the equations without significantly altering the predictions. It is well known that the compressibility effects do not have to be taken into account to predict wind forces on buildings or to predict any other physical quantity that is a direct effect of wind. After our study of fluid motions, the appropriate assumptions used should become more obvious. Here we introduce some important general approaches used to analyze fluid mechanics and give a brief overview of different types of flow.
一些常见的用来简化流动状态的假设是与流体性质有关系的。例如，黏性在某些条件下对流体有显著的影响；而在其它条件下，忽略黏性效应的影响可以大大地简化方程，但并不会显著改变计算结果。众所周知，气体速度很高时必须考虑其压缩性，但在预测风力对建筑物的影响程度，或者预测受风力直接影响的其它物理量时，可以不计空气的压缩性。学完流体运动学之后，可以更明显地看出采用了哪些恰当的假设。这里，将介绍一些重要的用来分析流体力学问题的一般性方法，并简要介绍不同类型的流动。
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1.2.1 Lagrangian and Eulerian Descriptions of Motion
In the description of a flow field, it is convenient to think of individual particles of which is considered to be a small mass of fluid, consisting of a large number of molecules that occupies a small volume that moves with the flow. If the fluid is incompressible, the volume does not change in magnitude but may deform. If the fluid is compressible, as the volume deforms, it also changes its magnitude. In both cases the particles are considered to move through a flow field as an entity. 1.2.1 拉格朗日运动描述和欧拉运动描述
描述流场时，将着眼点放在流体质点上是非常方便的。每个质点都包含了微小质量的流体，它由大量分子组成。质点占据很小的体积，并随流体流动而移动。对不可压缩流体，其体积大小不变，但可能发生形变。对可压缩流体，不但体积发生形变，而且大小也将改变。在上述两种情况下，均将所有质点看作一个整体在流场中运动。
In the study of particle mechanics, where attention is focused on individual particles, motion is observed as a function of time. The position, velocity and acceleration of each particle are listed as s(x0,y0,z0,t), V(x0,y0,z0,t) and a(x0,y0,z0,t) and quantities of interest can be calculated. The point (x0,y0,z0) locates the starting point the name-of each particle. This is the Lagrangian description, named after Joseph L. Lagrange, of motion that is used in a course on dynamics. In the Lagrangian description many particles can be followed and their influence on one another noted. This becomes, however, a difficult task as the number of particles becomes extremely large, as in a fluid flow.
质点力学主要研究单个质点，质点运动是时间的函数。任一质点的位移、速度和加速度可表示为s(x0, y0, z0, t)，V(x0, y0, z0, t)，a(x0, y0, z0, t)，其它相关参量也可计算。坐标(x0, y0, z0)表示质点的起始位置，也是每个质点的名字。这就是拉格朗日运动描述，以约瑟夫?Ｌ?拉格朗日的名字命名，该描述方法通常用于质点动力学分析。拉格朗日法跟踪多个质点的运动过程并考虑质点间的相互作用。然而，由于实际流体包含质点数目巨大，因而采用拉格朗日法研究流体流动则非常困难。
An alternative to following each fluid particle separately is to identify points in space and then observe the velocity of particles passing each point; we can observe the rate of change of velocity as the particles pass each point, that is, ?V/?x，?V/?y，?V/?z and we can observe if the velocity is changing with time at each particular point,
that is, ?V/?t .In this Eulerian description, named after Leonhard Euler, of motion, the
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flow properties, such as velocity, are functions of both space and time. In rectangular, Cartesian coordinates the velocities expressed as V=V(x, y, z, t). The region of flow that is considered is called a flow field.
与分别跟踪每个流体质点不同的另一种方法是将着眼点放在空间点上，然后观察质点经过每个空间点时的质点速度，由此可以得到质点流经各空间点时的速度变化率，即?V/?x，?V/?y，?V/?z；还可以判断某一点上的速度是否随时间变化，即计算?V/?t。这种描述方法称为欧拉运动描述，以莱昂哈德?欧拉的名字命名。在欧拉法中，速度等流动参数是空间和时间的函数。在直角笛卡儿坐标系中，速度表示为V=V(x, y, z, t)。我们所研究的流动区域称为流场。
1.2.2 Pathlines and streamlines
Two different lines help us in describing a flow field. A pathline is the focus of points traversed by a given particle as it travels in a field of flow; the pathline provides us with a “history” of the particle’s locations. A photograph of a pathline would required a time exposure of an illuminated particle.
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1.2.2 迹线和流线
可采用两种不同的流动线来帮助我们描述流场。迹线是某一给定质点在流场中运动时所经过的不同空间点形成的轨迹，它记录了质点的“历史”位置。一定曝光时间下可以拍得发亮粒子的运动迹线。
A streamline is a line in the flow possessing the following property: the velocity vector of each particle occupying a point on the streamline is tangent to the streamline, that is, V×dr=0. Since V and dr are in the same direction; recall that the cross product of two vectors in the same direction is zero. A photograph of a streamlines cannot be made directly. For a general unsteady flow the streamlines can be inferred from photographs of short pathlines of a large number of particles.
流线是流场中具有这样特性的线：任一质点在流线上某点处的速度矢量与该流线相切，即V?dr=0。这是因为V和dr具有相同的方向，而具有相同方向的两个矢量的叉乘积等于零。同迹线相比，流线不能直接由相机拍摄获得。对于一般的非定常流动，根据大量质点的短迹线相片可以推断出流线的形状。
1.2.3 One-, two-, and three-dimensional flows
In the Eulerian description of motion the velocity vector, in general, depends on
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