Astec engineers addressed this and other design issues by modeling the burner using FLUENT CFD software from Fluent Inc., Lebanon, NH. The initial concept design was generated using the SolidWorks computer aided design system. Engineers imported the geometry into the ICEM CFD preprocessor where they generated separate CFD models for the blower, mixing chamber, and nose. They created separate models of different sections of the burner in order to focus on one part of the design at a time while reducing CFD solution times.
Several models were run in succession. The results of one were used as boundary conditions for the next.
The goal of the fan simulation was to obtain an even velocity distribution over the output cross-section. This goal was accomplished by adding directional vanes.
In the mixing chamber simulation, engineers moved the injection holes in order to achieve a swirling flow pattern that provided excellent mixing while also controlling the size of the flame to avoid damage to the drum. In simulations of the nose, engineers focused on generating small recirculation zones to hold the flame in place to maintain stability.
While the CFD simulations were being performed, Astec built and tested a prototype in order to validate the CFD predictions. The numerical results were found to closely match the experimental measurements.
Quickly moving to an optimized solution
The ability to quickly modify and solve the CFD models without making hardware changes made it possible to complete about two design iterations per day. Just 23 days into the design process, Astec engineers concluded they had a saleable product.
At about this time, the engineers created a combustion model to analyze the emissions performance of the engine. The combustion model for a 90-degree sector made use of 2.5 million cells, and took two days to solve.
The results turned out to be surprisingly accurate. It even predicted where in the flame envelope the highest concentration of NOx would occur. Testing on the pad confirmed the prediction to be accurate. The success of the design process was validated by the fact the initial NOx numbers turned out to be well below the toughest regulatory standards, as designed and predicted.
Since the design featured a variable speed combustion air fan, engineers also looked at how different fan speeds affected system characteristics such as the velocity distribution at the fan output, the fuel-air mixing and swirling patterns, and the combustion stability. At low speeds, engineers paid close attention to the potential for flashback by comparing the flow velocity at the burner to the flame speed. They have since used CFD to develop five additional models in sizes ranging from 30 million to 125 million BTUs per hour.
The resulting Phoenix Talon burner is designed for the tough demands of today's efficient aggregate and hot mix asphalt operations. It makes use of premix gas burning technology and advanced air-atomized, nozzle mix oil burning technology to provide high efficiency and low emissions.
The operator has only to change the excess air level via the controls to operate in the lean-burn range, meeting emissions regulations that require no difference of configuration from the base unit. The use of a variable speed combustion air blower, instead of a traditional damper, results in lower power consumption and fewer parts to potentially break or get out of adjustment. The final product has been an unqualified success in the field.