Computational fluid dynamics: The answer to mitigating mixing risks?

Today, Alex Loubenets, Manager CFD Biological Treatment at Xylem, discusses the role of computational fluid dynamics (CFD) and how advanced 3D modelling can not only resolve everyday mixing challenges but also prevent them.  

With ever-increasing pressure on operators to maximize performance, meet sustainability goals, and improve cost management, finding the optimal solution requires industry-leading analysis and insight. When it comes to mixing systems, there are a number of significant risks that can impact the system's ability to treat water in a safe and energy-efficient way with minimal cost.

Mixing risk mitigation

To minimize negative effects on operational efficiency, public health, and system sustainability, mitigating the risks associated with mixing is paramount. If left unaddressed, improper mixing can have serious consequences, including ineffective water treatment and downstream pathogens. Conversely, excessive mixing can result in unnecessary energy use and increased operational expenses. To help manage these risks and better understand mixing systems, Xylem uses computational fluid dynamics (CFD) to support its customers.

What is CFD, and how is it used?

CFD is a specialized field of fluid mechanics that uses numerical analysis and algorithms to solve and predict the behavior of fluid flows. It enables advanced modeling during the design and post-design stages of a treatment system to identify and address process issues.

At Xylem, our engineering team uses a combination of physical lab testing, application expertise and validated CFD models. The process to assess a system, such as mixing tanks, starts with a customer inquiry. The team then assembles data, creates a CAD of the system, establishes the scope of work, and then completes the analysis process and delivery.

Now, let’s take a look at three common areas where CFD can help to improve operational outcomes and minimize risks in mixing tanks – sedimentation, homogeneity, and air entrainment. 

Sedimentation

Sedimentation is the accumulation of solids at the bottom of the tank. Regardless of composition, these solids will impact the tank’s working volume and capacity. They can also create bad odors. 

The past decade has seen important advances in CFD, including much higher computational capacity and better models. These developments have allowed engineers to predict sedimentation risks more accurately in a much wider range of applications – even in complex tanks. 

More importantly, we can perform CFD analysis before the equipment is manufactured or installed, allowing us to address potential issues in advance. To ensure the accuracy of our models, we correlate our multi-phase simulations with physical tests conducted in our laboratory.

Homogeneity

Homogeneity is a measure of the uniform distribution of all substances within the water. Maintaining homogeneity throughout a water treatment processes is essential to keep properties consistent and administer effective treatment. Without homogeneity, portions of water may be overtreated (excessive use of resources) or undertreated (potential for significant health risks) – essentially, a loss of treatment volume. 

In the case of mixing tanks, the water exiting must be completely homogenized and reacted. There are two main homogeneity challenges in mixing tanks. The first is short-circuiting, where the in-flow is not “participating” in the mixing and exits the tank prematurely. The second risk is the creation of zones where the flow exchange is low and there is significant stagnation, decreasing performance and effectiveness. 

CFD allows engineers to identify these zones and determine the optimal mixer placement and required power to eliminate or reduce them. 

Air entrainment

Air entrainment is when air bubbles reach the mixing propeller, resulting in excessive vibrations and a loss of thrust. Not only does this cause a decrease in efficiency, but it can also cause damage to the propeller blades and the mixing station as a whole, leading to significant downtime. The most common error in mixing station design is having the propeller in close proximity to the aeration system.

At Xylem, we use CFD to ensure that air entrainment is avoided. This involves multi-phase flow modelling to simulate air bubble distribution and assess entrainment risk based on the air volume fraction within the system. CFD analysis also helps us identify potential solutions to air entrainment for specific tank designs or operational conditions, providing a detailed view of what’s occurring in each area of the system.