Combustion simulation helps solve burner problem from further damage

The Company

An international process company (name has been withheld for confidentiality reasons).

The Challenge

This forward-thinking company is always looking to improve their internal processes, but a small change to the burner design caused a hitherto unseen consequence.  Localised bands of hot glowing metal were observed on the outer air rings of the burners.  The observed bands seemed to line up with the holes in the cone that have a small ignition port directly in line resulting in significant damage to the burners, but confirmation was needed before deciding on how best to fix the problem. 

Figure 1. Damage observed on one of the burner rings.

The Solution

Matrix used a numerical technique called Computational Fluid Dynamics (CFD) to simulate the flow of fuel and air in the burner and model the combustion process.  The results enabled Matrix to to predict the gas and metal temperatures with the ignition ports either aligned or not aligned with the combustion air ports, and the results were very interesting.   

Figure 2. Geometry of burner with ignition ports aligned with air ports. 

The Results

When the ignition ports were aligned, the metal temperatures soared and bright bands were observed identical to those observed in the field. 

Figure 3. CFD Predicted burner temperatures with and without aligned ports.
But why?

Figure 4. Gas velocity magnitude (m/s)
Fuel moves through the ignition port very fast and so it has a lot of momentum. When the small ignition port is aligned with the combustion air port, it flows right through it and starts to burn above the burner ring resulting in bands of high temperatures.  If the ports are not aligned then fuel hits the cone wall of the burner and mixes with the fuel exiting through the main fuel ports and the problem is avoided. 
"Ahhh yes this shows exactly what is on site!  Perfect! I am glad we now know what is causing the hot spots." - Process Engineer 

Figure 5. Observed hot spots