水轮机的有效水头

水轮机的有效水头(中文5200字,英文2700字)
毛水头就是坝前水位与尾水管水位的差值。
    有效水头就是毛水头减去蜗壳前多产生的所有损失。如果尾水管的水位只能不准确的测量或者根本不能被测量，那么这部分发生在出口断面与被测点之间的损失应该加进那部分被扣除的损失内。
    虽然水轮机能利用的水头既能量水头不能被准确的定义为静水头或者净水头或有效水头，我们还是要确定能量水头因为要校验转轮保证效率。有效水头被定义为两个能量水头之差，既蜗壳进口断面的能量水头与尾水管出水口断面的能量水头之差，用表1的符号我们可以写出净水头的表达式：
                                            （1）
       = 毛水头
       = 水库中水流流速
    = 蜗壳前的损失总和（包括尾水管后的损失）
     =尾水管出口附近的流速
    正确的理解上面的公式时间里下面的假设上的。

EFFECTIVE HEAD OF WATER TURBINE
1. The gross head is the difference between headwater elevation immediately before the station and tail-water elevation immediately after the exit from the draft tube.
Fig.1  Net or effective head
2. The static head can be derived from the gross head by subtracting all the losses occurring in the passage before the scroll case. If the elevation of water immediately below the draft tube can be gauged only inaccurately, or not at all, the losses between the exit section and the site of actual gauging shall be added to the amount of losses to be subtracted.
3. Since the energy head actually available for the turbine cannot precisely be defined by the static head either, the net head, or effective head, shall also be determined in order to check the guaranteed wheel efficiency. The effective head is defined as the difference between two energy heads, viz., between the energy head at the entrance to the scroll case and the one at the exit of the draft tube. Using notations of Fig.1 we can write for the net head: