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Hydraulic hysteresis phenomenon

  1. Sep 21, 2006 #1
    I have the following unexplained phenomenon:

    (please excuse incomprehensible expressions I´m not a native speaker)

    Experimental setup:

    A 6 meter long measurment pipe is filled with a hydraulic fluid and has a square cross section (30mm²). The fluid in the pipe is pressurerised to 40 bar and has a temperature of 20°C. At the end of the pipe a device is installed to bring in the same fluid at arbitrary pressures (up to 2500 bar). Note the higher the pressure is the higher the temperature of the inserted fluid is (at 1500 bar ~100°C). The insertion can be timed within 0,5 and 4 ms. In this time depending on the actual pressure and time span the inserted mass is about 180mg.

    The sense in that is to measure the progression of the timed mass insertion at high resolutions. For these purposes a pressure sensor is installed short after the insertion device. The additional inserted mass generates an excess pressure in the pipe which propagates with the speed of sound of the fluid (c(T)) (~1000m/s). At the other end of the pipe the additional inserted mass discharges through a restrictor.

    The progression (gradient, characteristics) of the insertion can be seen as an idealised rectangular devolution. So the measured excess pressure of the passing pressure wave is expected to have the same shape. This is not the case. At the End of the insertion the pressure signal doesn´t fall back to zero but dies away by a 1/x behaviour. This decay develops even stronger if the insertion is repeated frequently so that the temperature is rising in the pipe. By choosing longer insertions times this phenomenon develops even stronger. So it seems to be at least partially dependend on the temperature gradient and it has some cumulative characteristics. The speed of sound in the fluid should be sufficiently high enough to carry off the excess pressure so that this hysteresis phenomenon shouldn´t appear.

    A sample can be viewed here: http://www.bilder-hosting.de/img/WGOUG.jpg

    Considerations so far:

    1) calculation of cumulative reflections due to the developing temperature gradient
    -> has some similarities to this characterisitcs but is not strong enough and doesn´t explain this hysteresis even with a "cold" pipe.
    2) calculation of dissipative energy which heats up the local fluid volume and effects a pressure rise
    -> the effect is too small and only visible at the maximun pressure and not at the decay
    3) the reflected signal at the end of the pipe doesn´t reach the sensor in time so that it can´t overlay the source signal
    4) cavitation should show dropping pressures through the collapsing bubbles

    I would appreciate any suggestions for the source of this pheanomenon
  2. jcsd
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