4.1.3.2.1 The fluid flow through the pipes should be equivalent to those in water
distribution system. It is however impossible to derive an average flow or flow pattern in such systems. Flow in water distribution systems will vary according to its position in network, by the time o f day and by the time o f year. For example the hydrodynamic conditions in a ring main in the morning during the summer would be very different to
that in a dead end main during the night in winter. During all stages of the experiment the pipe rig was run at a constant flow rate of 2.5 1/min.
4.1.3.2.2 As outlined in the literature review the flow of fluids can be split into two types, laminar and turbulent. Comparisons of flows are best made by use of the dimensionless coefficient, Reynolds number (Re). Generally when the Reynolds number < 2000 a laminar flow pattern occurs, and when Re > 4000 turbulent flow occurs. Between Re of 2000 and 4000 a transitional phase exists, the flow regime will depend on factors such as the surface texture, the previous change in fluid velocity or any vibrations etc.
4.1.3.2.3 Laminar flow is characterised by individual particles of fluid following paths which do not cross those of neighbouring particles. A particle of fluid near the pipe surface is retarded by friction force and viscosity between the particles hence a velocity gradient occurs across the pipe diameter. At the pipe surface the velocity is zero. The velocity profile for laminar flow in a circular pipe gives a paraboloid of revolution.
4.1.3.2.4 Turbulent flow consists of unpredictable random particle movements which combine together to give flow. No theory has been developed to analyse this flow; most analysis consists of empirical data.
4.1.3.2.5 It can be seen that the flow regimes will govern characteristics such as flow velocity magnitude and distribution, shear stress at the wall and thickness of the viscous sublayer. They also govern the transport mechanisms hence determine the deposition, adhesion and detachment of bacteria or particles between the bulk liquid phase and the substratum.
4.1.3.2.6 Previous work into discolouration (Turrell, 1991), assumed that the water flow in dead end mains was laminar with a mean velocity of 0.006 m/s. This value was an average of measurements taken from dead end mains in the Northampton district. Information received from Anglian Water Services regarding flow was significantly
different. The information, summarised below, received was for a 'typical' 3" main in the urban and rural environment.
Table 4.3. Reynolds numbers from the Peterborough system, (from Watnet Model).
Max
Reynolds number
Min Mean (over 24 hrs)
Rural network 4.9 xlO" 1.3x10* 2.7x10*
Urban network 2.8x10- 1.4x10' 2.1 xlO'
4.1.3.2.7 As can be seen all the flows are in the turbulent range (Re>4000). Donlan et
al. (1994) gives Re ranging from 120-2904 for different areas o f a distribution system. This data underlines the variability o f flows in distribution and hence it can be concluded that no one flow can be regarded to be an accurate model o f the distribution system.
4.1.3.2.8 The over-riding constraint on the pipe rig flow rate was the volume o f water produced by the plant. All streams are to be operated at the same velocity hence the
lowest flow rate had to be adopted by all streams. Streams la and lb both had a
theoretical maximum output 8 1/min whilst stream 2 had an output o f 16 1/min. All
streams were operated at a flow rate o f 2.5 1/min to ensure a constant feed to the pipe
when the plant was not operating at its maximum capacity. This also allowed each
stream o f the pilot plant to feed two streams o f the pipe rig. Table 4.4 gives the flow conditions in the rig. The rig was not run at maximum since it was important to have constant flow. It was also important that there was a one hour contact time between the
water and the disinfecting chemicals prior to entry to the rig. The lower flow rate
enables a reserve o f water to collect and feed the rig during periods when the pilot plant was not functioning or running at reduced capacity. The feed tanks also minimised any variation in water quality throughout operation as incoming waters mixed with those already present in the tank. It is likely that the tanks did not act in plug flow therefore disinfection contact times are seen as an average. This flow rate theoretically gives a laminar flow pattern with a Reynolds number o f 800 in summer and 500 in winter.
Reynolds number is a function of the water viscosity and hence dependant on temperature. It should be noted that the length of the pipe sections was probably not sufficient for the flow pattern to fully develop. The flow characteristics are likely to be that of the transitional phase with a laminar pattern in the core of the flow and a turbulent outer layer. The effect is likely to be increased due to the roughness of the pipe sections.
Table 4.4. Summary of flow conditions for all streams of the pipe rig throughout the project.
Flow Rate (1/min) 2.5
Diameter (mm) 76
Area of cross-section (m^) 4.54 X 10’^
Velocity (m/s) 0.01
Residence Time (min) 22
Temperature (°C) 2 3 .5 - 5 .0
Reynolds number 807 - 500