Solving the World’s Toughest CFD Problems

Modeling Gas-liquid Flows in Water Treatment Plants

Ozone disinfection is a process in water treatment plants for removing bacteria/viruses from the infected water, reducing the concentration of iron, manganese, and sulfur, and reducing taste and odor problems. Ozone disinfection is critical to ensuring that the water coming out of a treatment plant is of high quality.

Ozone is formed in water treatment plants using an ozone generator. The untreated water is passed through a venturi, which pulls the ozone into the water, creating an ozone-water mixture. The disinfection effectiveness of this mixture depends on the mixing, advection, and dissolution of ozone in water. Capturing the behavior of such a gas-liquid mixture requires a clear understanding of the effect of the local hydrodynamics and mixing. A good numerical solution can capture these complex physics and accurately model gas-liquid flows in water treatment facilities. Accordingly, the physical parameters of the ozone-water mixing equipment and the ozone formation characteristics can be optimized.

In this blog, I discuss FLOW-3D’s new gas mass dissolution model, which helps our users better understand the behavior of the ozone-water mixture in water treatment plants.

How the Gas Mass Dissolution Model Works

The gas mass dissolution model keeps track of the relative gas concentrations in a computational cell. If the concentration of gas in the cell is less than the saturation concentration of gas, then there is a mass transfer of gas from the gas bubbles to the water. The equation of gas-liquid mass transfer below summarizes this idea:

$latex J={{k}_{L}}a(C_{L}^{*}-{{C}_{L}}),$

where $latex \displaystyle {{k}_{L}}$ is the local liquid mass transfer coefficient, $latex \displaystyle \alpha$ is specific interfacial area, $latex \displaystyle C_{L}^{*}~~$ is the saturation concentration of dissolved gas and $latex \displaystyle {{C}_{L}}$ is the local concentration of gas. With this relatively simple governing equation, the FLOW-3D gas mass dissolution model does an excellent job of tracking gas dissolution into the neighboring fluid.

In a typical simulation that uses this model, the gas is generated and introduced into the fluid, where the dynamics of the gas-fluid mixture are tracked. Using the FLOW-3D particle model, the gas is generated as particles, which then dissolve in the fluid over time, increasing the concentration of gas in the fluid.

This simulation has a saturation concentration ($latex \displaystyle C_{L}^{*}~~$) of 0.0004 and a local liquid mass transfer coefficient ($latex \displaystyle {{k}_{L}}$) of 0.07. The particles are generated at a rate of 100 particles/s for 100 seconds. Particles move upwards purely due to buoyancy. At this level of saturation concentration and local liquid mass transfer coefficient, all the gas particles almost entirely dissolve in the fluid and barely make it to the free surface. The animation also shows the particle lifetime, total number of particles and the concentration of gas in the fluid, illustrating the gas dissolution over time.

This simulation has a mixture of fluid and gas particles that are released into a container containing the mixer blade. As the blade rotates, the gas particles are dissolved into the fluid. A turbulence model (k-ε) is activated in this simulation to enhance the mixing, in addition to the already accelerated mechanical mixing happening due to the rotating blades.

These examples highlight a technique for simulating the mass transfer of gas into a fluid in simple or complex environments, which is crucial in predicting gas-liquid flow in water treatment facilities. Along with the gas mass dissolution model, FLOW-3D’s chemistry model can be used to capture additional ozone-water mixture physics based on chemical reaction rates. We’ll have more on the chemistry model in the future blogs.

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