Large Structures CFD Simulations For Water & Wastewater Treatment Plants P. Fonseca, N. Marques, J. Azevedo, V. Rodrigues, M. Toïgo
Outline Introduction 1. Problem Overview 2. CFD modelling Strategy 3. Examples: 4. Channel distribution 1. Circular distribution chamber 2. Non-circular distribution chamber 3. Pressure system distribution 4. CFD and 1D analysis 5. STAR-CCM+ assessment 5. Conclusions 6. Large Structures CFD Simulations For Water & Wastewater Treatment Plants - STAR Global Conference 2014
Introduction The water treatment plant conception can be classified in the process industry. The treatments are of the physical and chemical type (filters, settlers, coagulation, biological tanks, etc). Most of the water treatment processes are standardized. as a result of many years of accumulated experience. CFD has for long been used on the R&D of water treatment processes. The use of CFD on every day design is less common on the market. Large Structures CFD Simulations For Water & Wastewater Treatment Plants - STAR Global Conference 2014
Problem Overview The focus here will be exclusively on these case-by-case applications and their relevance on the design of each treatment plant ( i.e. , not on R&D cases). All examples are the result of the close collaboration of a water treatment market leader company with a CFD specialist company. All structures are large (up to dozens of meters), but subject to the impact of small details (centimeters: orifices, distribution weirs, small baffles, etc.) and with conclusions sensible to a water surface definition (on the order of the millimeters). Complexity and optimization needs make the case for everyday use of the CFD. Most thumb rules no longer belong to the current highly competitive market. Large Structures CFD Simulations For Water & Wastewater Treatment Plants - STAR Global Conference 2014
CFD Modelling strategy And the liquid is… Given the application requirements before, and for the examples that will be presented, the recommended modelling approach is to use VOF (Volume of Fluid) multiphase approach Description of the CFD modelling strategy: Transient simulations, even if we aim to obtain a steady-state solution: due to 1. the computational overhead, this implies the iterative solution of coarser to finer grids; HRIC surface capturing method, with free surface CFL<0.5 to avoid dissipation 2. issues; Iterative anisotropic mesh refinement, using field functions, to finely capture 3. the free surface details (mesh size normal to the interface in the order of mm); Typical mesh sizes of 30~40 Mio cells; 4. Realizable k- ε two-layer turbulence model/Two-layer, all-y+ wall treatment; 5. Surface tension effects taken into account; 6. STAR-CCM+ v7.04 to v8.04 used; 7. Large Structures CFD Simulations For Water & Wastewater Treatment Plants - STAR Global Conference 2014
Examples : Distribution structures Split the flow rate between parallel lines of treatment. Typically made out of concrete, they can go up to several dozens of meters in diameter or length. Influenced by the process designs being applied upstream and downstream. The diversity of possibilities makes these structures very prone to case by case design. Static structures are preferred. Layout arranges go from circular to linear. Flow rate outlet is generally performed by free fall sharp crest weir: The weir outlet structure brings the independency of the distribution from the downstream system, The weir outlet is highly sensible to flow behavior. Three different cases are presented: channel distribution structure, and the cross validation of reduced complexity methods, circular distribution chamber, and its comparison with general rules of conception guidelines, rectangular distribution chamber, with a non typical improved conception. Large Structures CFD Simulations For Water & Wastewater Treatment Plants - STAR Global Conference 2014
Channel distribution 1/3 In its simplest form, distributes water through lateral weirs, placed on one or both sides of the channel. The example is based on the simplest form: no upstream orifice chambers, etc. Objective of the example: simple comparison between the typical dimensioning methods and the CFD modeling. Simulation Conditions : Operating Conditions: Trimmed mesh with a CASE A: Peak flow rate prismatic cell sublayer at inlet = 4.936 m³ /s wall boundaries; CASE B: Peak flow rate inlet A= 6.477 m³ /s Final mesh with 13 855 819 cells Water properties: ρ = 999 kg/m³; Inlet as "Mass flow" and μ = 1,15x10 -3 Pa-s outlet as "Pressure outlet" Large Structures CFD Simulations For Water & Wastewater Treatment Plants - STAR Global Conference 2014
Channel distribution 2/3 Velocity Magnitude Water Surface Elevation Large Structures CFD Simulations For Water & Wastewater Treatment Plants - STAR Global Conference 2014
Channel distribution 3/3 1D simulation based on proven methodology and on formulation of common use (conservation of energy, Manning-Strickler, DeMarchi, etc. ). 1D simulation CFD results Conclusion 1: Extremely good agreement between the 1D and the CFD Difference between methods: < 0.5% of flow rate distribution Conclusion 2: CFD is of no use! Well… for the simplest of the cases . Most real conceptions are not 1D reducible. Large Structures CFD Simulations For Water & Wastewater Treatment Plants - STAR Global Conference 2014
Circular distribution chamber 1/3 “smoking pipe” kind of geometry: arriving from the bottom, water is redirected to the surface, distributes through the peripheral weirs (the example being presented is based on the simplest form: no inner baffles, etc .) very sensible design. e.g. the 90º bend of the streamlines at the inlet Objective of the example: Verify if solution are possible well beyond the general rules of thumb Cost constraints force very small overall Operating Conditions: Flow rate= 18.065,45 m³ /h diameters and little to no re-alignment Water properties: height for the upwards bent stream. ρ = 999 kg/m³ μ = 1,25x10 -3 Pa-s The inlet jet has a natural tendency to hit the water surface deviated from the Simulation Conditions : center, i.e. closer to some particular Trimmed mesh with a weir. prismatic cell sublayer at wall boundaries; Being a weir a very sensible structure, Final mesh with 35 412781 this can originate a bad distribution. cells; Inlet as "Mass flow" and outlet as "Pressure outlet“. Large Structures CFD Simulations For Water & Wastewater Treatment Plants - STAR Global Conference 2014
Circular distribution chamber 2/3 General rules of thumb that could be applied on these particular designs: Relation of jet velocity (kinetic energy) vs weir water height or overall chamber diameter; minimum alignment vertical length; etc. The "secondary" phenomena going on the 3D domain can be quickly appreciated. These render all the typical rules far from optimum. A curling flow is clear on the inner chamber Large Structures CFD Simulations For Water & Wastewater Treatment Plants - STAR Global Conference 2014
Circular distribution chamber 3/3 This is only a particular case of the Conclusion 1: circular distribution chamber domain; Optimum relation between inlet pipe Some of this domain can actually be standardized. and the first chamber diameter, This un-strengthens the jet hitting the surface, greatly improving distribution Conclusion 2: Either for optimization purposes or due to the huge possible variation of the main parameters (including non aligned inlet layouts, etc), case by case analysis is a constant need. CFD, as a general design tool on the day to day work of the engineer, becomes an advantage. Large Structures CFD Simulations For Water & Wastewater Treatment Plants - STAR Global Conference 2014
Non circular distribution chamber 1/3 “smoking pipe” geometry short length chamber a wall effect near the two weirs in front of the entry. Helping the direction change with guiding vanes delicate on bad fluid quality processes Objective of the example : test the most simplified of the guiding vanes a simple entry baffle that opens the jet depth, dividing it into two vertical ones, mastering the wall effect. Operating Conditions: Flow rate= 3,76 m³ /s Water properties: The distribution ρ = 998 kg/m³ quality is possible μ = 1,31x10 -3 Pa-s to estimate without the entry baffle. The 3D Simulation Conditions : Trimmed mesh with a effect of the baffle prismatic cell sublayer at wall obliges to a CFD boundaries simulation Final mesh with 11 514 025 cells Inlet as "Mass flow" and outlet as "Pressure outlet“. Large Structures CFD Simulations For Water & Wastewater Treatment Plants - STAR Global Conference 2014
Non circular distribution chamber 2/3 Large Structures CFD Simulations For Water & Wastewater Treatment Plants - STAR Global Conference 2014
Non circular distribution chamber 3/3 Conclusions: It is visible the in-depth distribution created by the baffle. The wall effect (coanda) is only of 3 cm, having a small impact on the distribution (≈1%) Large Structures CFD Simulations For Water & Wastewater Treatment Plants - STAR Global Conference 2014
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