Lesson 7 Measuring Rate of Fluid Flow 测量流体流速

Although there is a decrease in pressure at the narrowest pare of the venturi tube, the pressure is almost entirely recovered as the liquid passes through the gradual enlargement, so that added pumping costs are minimal. 虽然在文丘里管的 隘路段压力会有损失，但当液体通过 渐扩管段时压 力几乎完全恢复，所以流体附加的泵送费用是小的。
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The simplest form of such a flowmeter is k nown as an orifice plate meter ( Fig. 7-1). 最简单的流量计是孔 板 式流量计（图7-1）。
It consists simply of a plate with a hole in it, inserted into the pipeline, usually between two flanges. 孔板式流量计只有一个 带 有 孔 的 板， 插 入 管路中，通常插在两个法兰之间。
流出 浮子
被测的液体 通过一个带有较小锥
度的 锥形管 。在管内有一个 小金 属铅锥 ，也就是 浮子 （虽然叫浮 子，但它是 由密度比 液体 大 的 金 属制成）。
锥形管 流入
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As liquid flows up the tube, it lifts the float to a position where the weight of the float (a downward force) is exactly balanced by the lifting effect of the liquid in the space between the edge of the float and the inside of the tapered tube. 当 流体流过锥形管时，液体使 浮子处于一个特定的 位子，这时浮子和 锥形管之间的液体产生的浮力等于浮子的重力。
at the largest part of the venturi tube 直径最大
at the smallest diameter 直径最小
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These tubes overcome most of the problems of the orifice plate meter, but the venturi tubes themselves are rather expensive, particularly when they must be made from corrosion resistant materials such as stainless steel. 这种文丘里管 克服了孔板流量计的大多数缺点，但文丘里管的价格 太 高，尤其当文丘里管必须用耐腐蚀材料如不锈钢制造时。
为了改变由孔板引起的管子直径的突变，可采用渐缩管。
Special fittings ( called venturi tubes) are used, by which the pipe is gradually narrowed and gradually enlarged. 应用了一种特殊的装置叫文丘里管，
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orifice plate meter 孔板式流量计
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The hole in the plate is called an orifice and is smaller than the internal diameter of the pipe. 板孔的尺寸小于管子的内径。 The manometer reads the differential in pressure on both sides of the orifice and may be calibrated to read in terms of rate of flow. Small openings, called taps, are made in the pipe on both sides of the orifice plate, and these are connected to 压力计 读出孔板流量计 两侧 压力 的 变化 some kind of pressure-measuring 然后校正标定流量。 device most often a manometer. 小孔，又称排出孔，开在 孔板两侧的管路 上， 与某种 压力测量装置 通常是 压力计相 连。
An orifice plate also causes pressure losses and a consequent increase orifice plates in pumping costs. 孔板流量计也会导致压力损失从而增加了成本。
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Instead of an abrupt contraction in the pipe size, as is caused by an orifice plate, the contraction may be gradual.
HOMEWORK 作业
Copy the words 抄写单词
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0rifice plates cannot be used if the liquid being measured contains solids because they will be trapped by the plate.
孔板流量计不能用于 带有固体物质的被测液体，因为孔板会将固体 截留。 The trapped solids interfere with the calibration of the flowmeter and may even clog the pipe. 被截留的固体会干涉流量计的读数，甚至堵塞管路。
Lesson 7
Measuring Rate of Fluid Flow
测量流体流速
New words and expressions
New words and expressions
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The most common means of measuring rate of flow of a fluid is based on the principle that if there is a contraction (a section of reduced diameter) in a pipe, the pressure of fluid flowing downstream from the contraction will be lower than that upstream from the contraction. 最常见的测量流体流速的方法 是基于这样一个原理，即如果在 管路上 有断面收缩，则收缩断面下游流体压力比上游的要低。 This pressure differential is proportional to the rate of flow. 这种压力的变化与流体的速度成比例。
The ingenuity of engineers is great, and many other types of flowmeters have been invented and are in use. The types described above are simply the most common. 工程师的创造性是 巨大的，发明了很多种类的 流量计并且得到 应用。 上面介绍的仅仅是最常用的。
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Another type of flowmeter is the rotameter, or variable-area meter ( Fig. 7-2) .
另一种流量计是转子流量计（图7-2）。
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The lቤተ መጻሕፍቲ ባይዱquid to be measured
parsed upward through a tapered tube with the smallest part of the taper at the bottom. In the tube is a small metal plummet, or float ( Although usually called a float, it is made of metal and is denser than the fluid to be measured).
这种装置的管子直径大小是逐渐变化的。
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The pressure taps are placed at the largest part of the venturi tube ( before it narrows) , and at the smallest diameter.
压力检测点安置在文丘里管直径最大的和直径最小的地方。