Encyclopaedia Index

### Title : THERMAL SIMULATION OF WASTE WATER DIFFUSER

by : Dr N.J. Cavanagh, Senior Engineer, Consultancy

Date: 1997 - PHOENICS Version: 3.1

### Details:

- A CFD model was created of the local thermal and fluid flow around
the venturi effluent outfall system, in order to assess the effect of
discharging effluent at temperature above ambient on the resulting
temperatures increases in the surrounding environment.

- The simulation is for steady, incompressible, three-dimensional
turbulent flow.

- The venturi is defined as 2 co-axial conical funnel structures and the
geometry within the domain was created using PHOENICS-VR.

- The computational domain is 30m long by 8m wide by 10m high, with
an underlying Cartesian grid of 50*55*55 cells in the X,Y and Z
directions respectively. The Arbitrary Source (or Solid) Allocation
Procedure (ASAP) was used to map the geometry to a fine Cartesian grid.

- The domain was assumed to be filled with water, and the Special Purpose
Program (SPP) 'WAter Movements in Bays, Lakes and Estuaries' (WAMBLE)
was used for the simulation.

- The effects of gravity were included by way of sources of momentum
in the equations for the z component velocity using the built in
buoyancy facilities available in PHOENICS.

- The KE-model of turbulence was used to include the effects of
turbulence within the flow.

- The ambient temperature was assumed to be 15C, and discharge
temperatures are known to peak at a daily average of 38C, however
the absolute maximum short term effluent discharge temperature may
reach as much as 50C. The maximum hourly effluent rate is 770m***3/h,
giving the following selected boundary conditions:

Effluent discharge: 770m***3/h
Effluent Temperature: 50C
Tidal Velocity: 0.5m/s
Tidal direction: in-line with the venturi, against outfall
Ambient Temperature: 15C

The following illustrates the geometry and results.
Wire-frame view of local venturi waste water diffuser

Computational grid concentrated in the region of the venturi contraction and funnel where the flow field is most complex

Typical temperature contours through outfall centre-line

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