What are Combustion Dynamics?

Combustion dynamics are a significant issue in many combustion systems from natural gas turbines to rocket engines. The acoustic pressure oscillations created by combustion dynamics can cause devastating damage to the engine hardware bringing operations to a halt. In the gas turbine world, this kind of damage can result in the loss of millions of dollars as the units can’t generate any power while they are being repaired.

By design, present-day DLN combustors utilize highly unsteady flows to aid in flame stabilization and fuel mixing to operate in a lean, pre-mixed state. This unsteadiness produces local oscillations in the equivalence ratio inside of the combustor. As the local equivalence ratio varies, the burning rate of the fuel varies as well as the heat release of the fuel.

These unsteady flows also carry small pressure fluctuations of their own. Based on the design of the combustor, certain frequencies will be excited more strongly than others (much like blowing over the opening of a glass bottle produces a specific tone based on how much liquid is in the bottle).

As these acoustic pressure waves travel and reflect off of the walls of the combustor, they can interact with the incoming air and fuel, creating oscillations in the local equivalence ratio. If these acoustic pressure oscillations are in phase with the oscillations in equivalence ratio (and thus the heat release) of the fuel, then the oscillations in the heat release will be amplified. This creates a feedback loop where the oscillations in acoustic pressure and heat release drive each other to larger and larger amplitudes. If these oscillations are strong enough, the gas turbine will become quite loud and the oscillating loads will transfer to the structure of the gas turbine. If these loads are strong enough, catastrophic failures can occur.

What does Dynamics Plus do?

Dynamics Plus is a combustion dynamics monitoring algorithm that looks at current combustion dynamics and provides a series of performance metrics, called health values, indicating how a gas turbine is operating relative to “normal”. Not only can operators use these health values to know that there may be damaged hardware in their turbine, but they can also begin to diagnose combustor issues as they arise since health values are generated for each combustor can and frequency band combination.

Dynamics Plus utilizes a moving reference database to see how differently a particular can is operating from “normal” conditions. By using a moving reference database as opposed to a static reference database, Dynamics Plus is able to account for seasonal affects as well as can drift as various hardware wears over time. In this way, the Algorithm has a constantly updating “normal baseline operating condition” against which it calculates health values.

Dynamic Plus finds these values by comparing behavior across the cans in a gas turbine rather than comparing against time. It also looks at changes in behavior instead of absolute values of amplitude and frequency. By considering the combustion dynamics in this manner, much of the effects of different operational parameters (such as load or firing temperature) are removed from the health values since all cans should be behaving in approximately the same
manner for a given operating condition.