John Zink selected FLUENT CFD software from Fluent Inc., Lebanon, N.H., because the software has comprehensive combustion modeling tools and has become the analysis standard in the combustion industry. In a wide range of problems, engineers have found the software to be capable of providing predictions that closely match experimental measurements.
John Zink already had some experience with simulation software. Engineers had used GAMBIT, FLUENTs preprocessor, to create a full-scale numerical model of the existing furnace and surrounding ductwork. A key advantage of FLUENT and GAMBIT is the ability to handle unstructured/hybrid meshes. This feature greatly reduces the time needed to create a mesh for the CFD model. The actual fluid physical properties were used in the simulation, and solution-based mesh adaptation tools in the FLUENT solver locally refined the grid in regions where the flow domain required higher resolution. It took about two weeks to completely model the furnace.
The CFD simulation of the original model closely matched the observations of the operating furnace in showing the flames from the individual burners coalescing into a single flame. This gave us confidence in the accuracy of the model. Looking at the flow velocities, pressure distributions and chemical species concentrations predicted by the simulation, we identified an unusually high proportion of unburned fuel within the inner circular area formed by the burners, indicating that there was poor mixing of the fuel and air.
The unmixed fuel didnt burn until it had migrated out toward the end of the flame, creating the flame definition problem. Based on this insight, we reduced the amount of fuel and increased the air to the inner circular area. We hoped that we could achieve a balance. Mechanically, this was accomplished by biasing the primary and secondary fuel injectors.
The first iteration showed a considerable improvement. We viewed the results and noticed that a fair amount of unmixed fuel still remained in the inner circle. Eliminating the fuel was the goal of the second iteration. After about a dozen virtual experiments, we modified the design and optimized fuel/air mixing throughout the furnace. The simulation predicted that the final design would provide excellent flame definition and temperature uniformity. We showed the results to the customer, who was very pleased.
The refiner wasnt looking forward to a long and disruptive experimental process to fix the problem. The customer approved the new design and John Zink produced the modified components. The retrofit was completed on the fly, avoiding furnace shutdown. Our new design worked exactly as predicted, reducing flame length to the desired levels and lowering NOx emissions to acceptable levels. Using CFD visualization required about 12 weeks to identify a solution, without disrupting current operations.
A bright future ahead
Where do we go from here? CFD software will make it possible to avoid customer headaches, to analyze burner changes in advance. These headaches include: shortened coil life from over-temperature excursions, poor material choices, combustion inefficiency, changes in fuel composition, and others. Although the initial cost of this type of study seems daunting for all but critical applications the price can be expected to decrease as those who use it become more familiar with it. Then again, the cost of lost production and the specter of safety and environmental issues resulting from furnace failure may outweigh concerns about the cost of simulation. For more information on CFD and combustion systems visit http://www.fluent.com and www.johnzink.com.
Roberto Ruiz is a vice president of technology and commercial development at John Zink Company in Tulsa, Okla.; e-mail him at email@example.com.