英语翻译

英语翻译
Because the vortex rope appearing in the draft
tube is a common phenomenon for most Francis
turbines in the part load zone,and the rotating
frequency of the rope is about 1/4 to 1/3 multiple of
runner rotation frequency
,it may be concluded
that the low-frequency component of vibration,
oscillation and pressure fluctuation existing in the
investigated turbine is induced by draft tube vortex
rope.Generally,it could not be avoided completely in
this part load zone.Some technical means should be
applied to control the unsteady features.This content
will be included in another article in the future.
In addition to the above low frequency,another
characteristic frequency,1.25 Hz (or 1.0f),was found
in the frequency spectrum of vibration and pressure
fluctuation,as shown in Figs.3 and 7.It is very
possible to correlate this frequency and the runner
passing frequency.
Considering the 5.7 Hz (or 4.56f) characteristic
frequency existeing at load 540 MW-542 MW,it is
not found in the numerical simulation results.It is
should be noticed that this special frequency exists
only at the head 68.3 m according to the experimental
investigation.The 4.56 multiple frequency exists
neither in draft tube pressure fluctuation nor in head
cover vibration when the head exceeds 68.3 m.It may
be concluded that the head cover vibration is mainly
resulted from high-frequency pressure fluctuation of
the draft tube.This vibration area may be classified as
a special vibration area,and the corresponding
frequency is recognized as a special vibration
frequency.
It should be mentioned that this special vibration
area appears in the designed normal running range.
Therefore,this vibration area should be avoided by
carefully allocating load.