Decreasing Water Consumption in Toilets
Optimization of a sanitary discharge valve using ANSYS CFX
by Miguel Francisco and António Gameiro Lopes, Departamento de Eng. Mecânica, Univ. Coimbra, Portugal
and
Vítor Costa, Departamento de Eng. Mecânica, Univ. Aveiro, Portugal
Reducing water consumption is a major concern these days. Home water usage can be quite significant, especially in toilets: a four-person family makes, on average, 20,000 toilet flushes per year which represents approximately 120,000 litres of water. Trying to cope with the environmental demands, legislation in many areas has become more restrictive in terms of water usage per flush. This has lead to the need to optimize the performance of the flushing devices by increasing the average water discharge velocity and therefore optimize the washing efficiency. Engineers at the Department of Mechanical Engineering of the Universities of Coimbra and Aveiro, in Portugal, are helping Oliveira & Irmão S.A, a manufacturer of toilet flushing valves, to improve the performance of their products by using numerical simulation with ANSYS CFX coupled with experimental validation. Several shape studies were performed, aimed at achieving higher instantaneous flow rates leading to a better washing effect. |
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Three-dimensional model simulating ¼ of the cistern with the discharge valve.
The domain is made of 5 solids with both structured and unstructured meshes
connected through 4 GGI fluid-fluid interfaces.
The problem to be solved is a three-dimensional unsteady gravity-driven flow with a free surface. As a first approach in the optimization process, stationary CFD simulations without free surface were employed to estimate the pressure drop across the valve. The best geometries were then simulated with the full physics, to compute the volumetric flow rate. Benchmark of the numerical simulations was done through comparison with experimental data.
Height of the free surface as a function of time, during the discharge: numerical versus experimental data.
For a reliable numerical estimation, a good definition of the free-surface (water-air interface) is crucial. The challenge was to get the best model approach to the physical problem. Parameters such as boundary conditions, meshing types, modelling schemes and convergence criteria needed to be carefully tested to find the best solution. Grid dependence studies showed that a blend of structured and unstructured meshes presented a good compromise for a correct description of both the valve geometry and the location of the free surface. Near the valve, an unstructured mesh was employed. In the rest of the cistern, where a detailed description of the water free-surface is crucial, a structured mesh was adopted. The generalized grid fluid-fluid interface where both meshes join was placed in regions of low velocity gradients.
ANSYS CFX simulation of the free surface movement during the discharge.
The transient simulations with a compressive scheme using the k-ω based Shear Stress Transport (SST) turbulence model could predict the volumetric flow rate with a relative error below 5%, which is excellent for this kind of problem.
Water velocity vector field analysis inside the discharge valve.
ANSYS CFX has shown to be a reliable tool for the simulation of gravity-driven flows with free-surface, making it a great tool for the prediction of discharge processes. Using ANSYS CFX enabled the development of a better valve.
Acknowledgements: This works was funded by Fundação para a Ciência e a Tecnologia, Portugal, through the Research Project POCT/EME/46836/2002.