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Title: Mass transfer of solutes in turbulent wall bounded flows reacting with the conduit surface
Author: Sookhak Lari, Kaveh
ISNI:       0000 0004 2708 4523
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2011
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This thesis focuses on the decay of chlorine in pipes of drinking water distribution networks due to wall and bulk demand. Accurate prediction of chlorine decay is important, as both chlorine concentrations which are too low and too high pose serious health risks, the former due to pathogen formation and the latter due to the formation of disinfection by-products. Water quality models used for the prediction of chlorine decay make use of parameterisations for the wall demand in the form of Sherwood number Sh correlations, which couple the wall mass flux to a Reynolds number Re, Schmidt number Sc and wall roughness. These correlations are subject to significant uncertainty, particularly for turbulent flows. A combined analytical and numerical approach is taken to study in detail the interaction between flow, turbulence and mass transport, with the aim of improving the understanding and accuracy of wall demand parameterisations for chlorine. Simulations of the chlorine decay in an axisymmetric pipe with hydraulically smooth walls were performed for Re = 104 to 106 and Sc = 1000 using Reynolds averaged conservation equations. These values are typical for chlorine transport in distribution networks. The simulations confirmed that the assumptions made in water quality models for chlorine wall demand are valid. Asymptotic solutions for high Sc solutes were developed which are applicable both to linear and nonlinear wall reactions. Results showed that the Sh correlation is independent of the reaction type. For rough walls, the two main wall demand parameterisations are mutually inconsistent: one is valid for low and the other for high wall demand coefficients only. Numerical simulation of flow and high Sc mass transport over a dtype rough surface at Re = 2.5×105 showed that the inconsistency between the two parameterisations was caused by the geometry. For low wall demand coefficients, the existence of roughness elements causes higher wall demand than for a smooth wall. However, at high wall demand coefficients the maximum wall demand achievable in the cavities was much smaller than for the crests. Hence, the effective surface area and therefore the wall demand became lower than for a smooth wall. A parameterisation was developed which reproduced the solute mass decay over the entire range of wall demand coefficients. Most of the solutions and parameterisations developed in this thesis are on the same level of description as water quality models. The findings of this thesis can be used as supportive evidence for the validity of assumptions made for water quality models, and to inform how processes should be modelled when these assumptions are violated.
Supervisor: Van Reeuwijk, Maarten Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral