Download USER'S MANUAL FOR CORHYD: AN INTERNAL DIFFUSER

should allow for riser velocities, which are bigger than the diffuser velocities, but smaller than
the port velocities to allow for a constant flow acceleration.
For design discharges a homogeneous distribution should be achieved and often only gravity
discharge should allow to drive the system. This can be done by either changing port
diameters along the diffuser or applying variable area orifices. In comparison with fixed
(constant or invariable) port diameters the effective open area of variable area orifices
(duckbill valves) changes with different discharges. Therefore, they are good for non or low
discharge scenarios, where intrusion has to be prevented. Decreasing fixed port diameter leads
to a more homogeneous discharge distribution but to increased losses and total head due to
higher velocities. Attached duckbill valves give almost homogeneous discharge profiles, due
to the discharge dependent open area.
By applying different diameters for CorHyd calculations it has to be considered, that pipes are
not available in all sizes and only diameters are applied which are given as internal diameters
in catalogues of pipe producers. Nevertheless often a few centimeters difference in the port
orifice diameter makes considerable differences if applied all along the diffuser or in
designated pipe sections. Furthermore changes of port diameters might be necessary during
lifetime of the diffuser to adopt for changing boundary conditions. Both can easily realized by
flanges at the diffuser itself or by flanges at the riser pipe an attached port pipe, if risers are
necessary. A tap with a hole of an intermediate size can then be fixed on these flanges and
easily replaced even as submarine work. Attention has to be paid to avoid abrupt diameter
changes and sharp edges to reduce the additional losses caused by these constructional details.
4.5 Additional local losses (sub-menu)
If complex geometries are applied, which are not automatically foreseen in the CorHyd loss
formulations, further loss values may be included for ports or risers. Fig. 17 shows the pop-up
window, which opens after clicking on additional local losses. In this window additional loss
coefficients ζ (related to the port velocity) can be given as well as jet contraction ratios Cc.
Furthermore it is possible to define here, if Duckbill valves are applied and which nominal
diameter they have. Also further studies can be done by introducing additional losses and
analyzing their effects to check the system-performance-sensitivity on loss formulations and
so far the necessity in doing laboratory studies for achieving more accurate loss formulations.
For risers additional local losses (related to the riser velocity) as well as additional bends or a
total riser length can be given to achieve more accurate results, if complex geometries are
applied.
For example flanges with taps fixed on a port pipe cause an additional loss and a contracting
jet. Both effects can be considered and evaluated by entering the loss coefficient (e.g. from
chapter 10.2 in the annex) and the contraction coefficient.
The data given in the sub-window is not saved with the overall result and has to be put again
after the calculation.
Institut für Hydromechanik, Universität Karlsruhe
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