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FALL INCREASER-
HYDRAULIC JUMP APRON WEB PAGE
There
are three, classic forms, of fall increasers.
Clemens Herschel (the inventor of the Venturi Meter) designed a head increaser
where the discharge end, of a vertical, conical draft tube, was inserted into
the throat of an enormous, horizontal, Venturi meter. The upstream end of the
Venturi protruded into the impoundment and was gated. Its exit was into the
tailrace. During normal flows, the entrance to the Venturi was closed, with its
gate. The turbine discharged through the draft tube, into the venturi and out
into the tailrace. During freshets, when the tailwater was high, the gate at the
mouth of the Venturi was opened. The excess floodwaters flowing through the
Venturi, created a vacuum in the throat of the Venturi. Since the turbine draft
tube was discharging into the Venturi throat, the turbine saw an increased
pressure drop across its runner. The lost head created by the elevated tailwater
was effectively recovered.
Moody
designed his ejector turbine. In Moody's design, the turbine draft tube
has an annular opening cut in the top of the draft tube. This opening is
surrounded by an annular guide vane. A cylinder gate seals the
annulus when it is not being used. During normal operations, the cylinder is
closed and the turbine discharges through the normal draft tube. During freshets, the
cylinder gate is opened and the discharge through the draft tube is increased. The
additional water increased the pressure drop across the runner. Henry Ford
installed one.
The
last design utilizes the water surface suppression created by an ogee curved
spillway. The powerhouse is designed so that it is adjacent to the spillway. The
powerhouse discharges into the trough of the ogee spillway. At all spillway
surcharge flows, the tailwater exiting the powerhouse, remains suppressed. The
total head across the runner remains constant.
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