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

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

wbs