This IIT-Madras team has a way to kill the hum in gas engines

Undesirable oscillations experienced in certain type of common gas turbines used by power plants and aircraft engines have always worried their developers.

Globally, the gas turbine industry loses as much as $1 billion annually due to downtime for turbine inspection and replacement of damaged parts due to such thermo-acoustic oscillations.

Some of the early-generation rockets used for satellite launches or space travel exploded in space because of such ruinously large sound oscillations-triggered thermal fluctuations in combustors.

Now, a team of researchers, led by RI Sujith, Professor in the Department of Aerospace Engineering at the Indian Institute of Technology (IIT) Madras, may have found a way to ‘quench’ such humming.

“Inside a gas turbine using can combustors, the flickering flames produce a continuous sound, which travel as sound waves to the boundary of the container and reflect back to amplify the flames further. This continuous cross-talk between the sound waves and the flames over a period of time becomes unmanageable leading to temporary shutdown of the turbine. This elusive hum has been troubling the gas turbine industry for quite some time,” said Sujith.

Now, the IIT research team, which includes Sujith’s research students, has proposed a unique method to quench this thermo-acoustic oscillations. In a recent paper published in the journal Chaos, the researchers showed that when two combustors exhibiting thermo-acoustic oscillations are coupled through a single connecting tube of appropriate dimensions (that is, length and diameter), the cross-talking of these oscillations leads to the simultaneous quenching of their amplitudes, through a phenomenon known as amplitude death.

The absence of large amplitude oscillations during amplitude death is a desirable operating condition for the combustor, providing an environment for healthy operation, the scientists argued.

“Imagine two suspended bridges side by side. If vehicles are passing on these bridges continuously, the bridges may flutter, leading to a bumpy motion experienced by all passing vehicles. But these bridges can be connected in such a manner that they cancel each other’s oscillation,” said Sujith.

“Although the concept of amplitude death has been known, it has mostly been shown in theoretical studies and also in simple experiments involving oscillators such as metronomes. We are using this concept for the first time in a practical system,” the IIT- Madras Professor told BusinessLine.

Cost-effective solution

The study provides a simple and cost-effective solution for quenching thermoacoustic oscillations developed in multiple combustion systems, where the knowledge for the control of such oscillations remains limited.

Currently, mitigation of thermo-acoustic instability is achieved through active and passive control strategies. Active control involves the alteration of the system condition externally, causing an interruption in the coupling between the acoustic and the heat release rate fluctuations, leading to quenching of thermo-acoustic oscillations. On the other hand, passive controls involve modification of system geometry such that it increases acoustic damping or modifies the instability frequency in the system. Recent studies also focus on developing technologies to alert and thereby avoid the onset of instability.

The insights obtained from the experiments conducted on laboratory systems, known as the horizontal Rijke tubes, by the IIT scientists can be potentially used for the development of several reliable control strategy for practical combustion systems (especially, can or can-annular combustors in a gas turbine engine).

The findings are pertinent and complementary to numerous real-world applications beyond combustion systems, such as a variety of oscillatory instabilities experienced by bridges, skyscrapers, ecological and biological models where they are known to be hazardous.