A manufacturer of burners optimizes the distribution of gases for combustion by using CFD tools embedded in its CAD software.
By Ad Heijmans
The mixing of gas and air is important in a wide range of applications. Gas mixing in flues is often critical to the operation of emissions control systems. In packed columns and other types of chemical reactors, gas mixing affects throughput and variability of the process. Gas mixing has a major impact on the performance of rotary kiln incinerators used to treat hazardous wastes. In respiratory airways, gas mixing influences the performance of aerosolized medications. An improvement of just a few percent in mixing efficiency can substantially reduce the energy consumption and emissions of a low-NOx burner.
Optimizing gas and air mixing to meet the requirements of a specific application is a challenging task that normally involves a very expensive and time-consuming process of building and testing prototypes. Up to now, large companies have been primarily limited to research or troubleshooting existing designs because of the considerable cost, time, and expertise required to use CFD technology.
But in the past few years, new CFD tools have become available that are fully embedded in the mainstream mechanical design environment so they are much easier, faster, and less expensive to use. The new tools provide the ability to evaluate the performance of a large number of potential alternatives in the early stages of the design process. Early stage analysis makes it possible to improve the performance of the product and resolve design problems quickly and before large sums have been spent on a design that must be changed.
Gas mixing for combustion is just one application for embedded CFD software. The same principles apply to optimizing gas mixing for a wide variety of other applications.
Competitive and regulatory pressures are forcing manufacturers of combustion equipment to improve energy efficiency, reduce emissions, improve control, and provide greater fuel flexibility. To meet this challenge, manufacturers must improve the performance of burners, which are an integral part of all combustion systems. Even small improvements in performance can have a major impact on systems that operate continuously and consume large amounts of energy.
Fuel and air mixing plays a critical role in the design of nearly every burner. A major design challenge in many applications is injecting the gases so that near-ideal mixing is achieved. Mixing is important because uneven concentrations of air and fuel can substantially increase emissions levels and reduce combustion efficiency. The very thorough mixing of gas and air eliminates the hot and cold spots in the flame that are responsible for NOx emissions.
A CHANGE IN DESIGN METHODS
Until recently, designing for proper gas mixing was considered to be more art than science. The traditional practice has been to build a prototype or modify an existing product, test it, and then, based on the results, modify the prototype or product, a process repeated until desirable results are achieved. The problem is that the approach of building, testing, and modifying the prototype is often expensive and may take considerable time. Another concern is the expense in shutting down a product for modification and testing if it is used in a continuously operating process such as power generation.
More recently, improvements in experimental and analytical tools have made it possible to replace physical prototypes with digital prototypes that accurately predict the performance of design alternatives. Engineers use CFD to simulate the operation of the product under conditions that are representative of its use in the field.
A CFD simulation typically provides far more information than can be obtained from physical testing, such as fluid velocity and direction, pressure, temperature, and species concentration values throughout the solution domain. As part of the analysis, a designer may change the geometry of the system or the boundary conditions and view the effect on fluid flow patterns. For these reasons, CFD enables the analyst to evaluate the performance of a wide range of different configurations in a shorter amount of time and at a lower cost than by testing physical prototypes.
The recent trend toward the use of CFD software that is embedded in the CAD system makes it possible to use simulation in the design phase in order to examine more design alternatives than would be practical with physical prototyping, while reducing the number of prototypes required. Embedded CFD uses native 3-D CAD data, automatically generates a grid of the flow space, and manages the flow parameters as object-based features. Engineers can focus on the fluid dynamics of the product that is their responsibility, rather than on the computational part of CFD.
Starting point: Simulation of the original Linnox burner design using medium-pressure air swirl and gas injection showed a good quality mixture.
The newest generation of CFD software contains sophisticated automatic control functions that make it possible to converge to a solution in almost every application without the need for manual tuning. Perhaps the most important function controls the quality of the mesh to avoid one of the biggest reasons for run divergence. As a result, the skills required to operate the CFD software are simply knowledge of the CAD system and the physics of the product, both of which the vast majority of design engineers already possess. Automating these steps also greatly reduces the time required for analysis, making it possible to deliver results before the design has changed.
Several best practices can help ensure the accuracy of CFD gas and air mixing simulation. The utilization of native 3-D data places a premium on the quality of the solid model. For an internal flow model with minimum mesh requirements the solids must form a sealed internal space with no leak paths outside the internal flow field. Minute details of the geometry should be eliminated wherever possible to reduce the number of cells in the CFD model. After the geometry is imported, it should be checked for problems using the “check geometry” feature in the CFD software. The engineer can check for highly skewed cells, caused by holes in a thin solid, by performing a trial mesh generation and visualizing the irregular cells with the post-processor. Irregular cells can be corrected by increasing cell density where they occur.
Turbulence models are important in mixing simulation because most companies cannot afford computers that are powerful enough to capture the minute details of turbulent flow. The key factor in selecting the right turbulence model is matching the flow features likely to be present in the application with the models available in your solver.
The standard model for turbulence is called k-epsilon. It is a semi-empirical model based on model transport equations for the turbulent kinetic energy and its dissipation rate. Specialized versions of the k-epsilon model have been developed for specific flow configurations.
Design engineers need to be able to verify that their models accurately predict the chemistry and physics of the actual mixing process. One approach is to model the current generation of the product and confirm that the model predicts its performance. At this point, the designer can modify the model with confidence that it will predict the performance of the new design. If it will be too costly to interrupt the operation of the current-generation product, then it may make sense to build a small-scale model of the product and compare its performance to a simulation model.
These methods were used to design the new-generation Eclipse Linnox burner. This burner was designed to substantially reduce the energy consumption of the fans that push air into the natural gas burner, and still provide energy efficiency and emissions control to equal existing designs. To achieve this goal, engineers needed to streamline the design to remove features that helped achieve high levels of mixing on earlier designs but still maintain the proportion of gas to air at 7.5 percent, +- 0.5 percent, throughout the entire mixture duct. Eclipse designers generated the initial burner designs in Inventor, the 3-D CAD software from Autodesk. They used FloEFD embedded CFD software from the Mentor Graphics Mechanical Analysis Division to simulate them. This is the same software marketed by Flomerics Group before the company was acquired by Mentor Graphics.
The simulation results on the initial model showed the concentration of air and fuel throughout the mixture duct, and highlighted the areas where mixing needed to be improved. Design engineers made a series of changes in the mixer design.
Next generation: Simulation of a new burner design using less energy for air swirl and gas injection indicates an improved quality mixture.
After each change, they reran the simulation to determine the impact of the change, paying particular attention to the species or chemical compound distribution throughout the chamber and the pressure drop. With each major variation, they also performed a series of parametric studies to evaluate the impact of changing key dimensions of the design.
Viewing the impact of these changes on the distribution of the two species, they gained an understanding of the design sensitivities that would have never been possible with physical testing. Engineers zeroed in on one of the designs and performed further optimizations. The simulation results showed that the final design provides a pressure drop of only 300 pascals, a 900 percent reduction from existing burners.
Only at this stage of the process did Eclipse build the first prototype of the new design. The performance of the prototype was very close to that predicted by the simulation, which greatly reduced the time and cost required to obtain the new design, which is still in development.
CFD simulation in the early stages in the design of products involving gas mixing can save time and money. Best practices tuned for the requirements of a particular industry can help design engineers avoid analysis mistakes. By following specific procedures, any engineer can optimize the design at a time when changes can be made at little or no cost.
Ad Heijmans is manager of engineering at Eclipse Combustion BV in Gouda, Holland.