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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.