Visitors at the lab of RI Sujith , professor of aerospace engineering at IIT Madras, get to observe deceptively simple experiments involving a set of candles. The lab is hightech, with a panoply of combustion chambers and measuring devices. But Sujith’s experiments with candles kept next to each other belies the sophistication of his research and its wide-ranging applications.The candle flames start dancing together soon after they are lit, and by changing the distances between the candles, Sujit and his students control the way they dance. Keep the candles apart at a certain distance, and the flames move up and down at the same time. Move them apart to another specific distance, and the flames go up and down one after the other, like a dancer waving hands alternately.The experiment is cheap, simple and elegant to watch, but it provides Sujith with insights into how burners interact in gas turbines. Sujith’s research interest is thermo-acoustic instability . In simple terms, he studies how flames interact with their surroundings.Stability of flames is critical to the function of aircraft engines and gas turbines, which are at the heart of so much modern machinery. Combustion instability forces manufacturers to test engines repeatedly at great expense. It damages hot components and adds to the maintenance costs of turbines.The cost of replacement of these parts in turbines runs up to more than $1 billion every year.Sujith demonstrates the cause of this instability with another simple experiment. When he lights a flame inside a glass tube, the tube starts humming. The flame produces acoustic waves, or ‘sound’, in plain English. Flames in a jet engine or gas turbine also produce sound, and the sound waves can reflect back and rock the flames. This is why the phenomena is called thermo-acoustic instability. It is a feedback not too different from the boom that we hear when a microphone is placed close to the speakers. Sujith runs one among the most high-profile labs in the world for studying thermo-acoustic instability.His work is so valuable to turbine companies that General Electric signed a long-term agreement to use his technologies in its test facility in the US. Engineers have had to deal with thermo-acoustic instability throughout the history of engines and turbines. The F1 rocket engines used in the Apollo Mission had faced it. Aircraft engines face it regularly during testing, and so are allowed operate only in severely restricted conditions during flight. Thermo-acoustic instability was one of the problems that delayed India’s indigenous jet engine Kaveri for so long.All space engineering organisations face it, but few acknowledge it openly.In the mid-1990s, Sujith set about finding how these instabilities happen. He wanted to know how, for example, a beautifully-burning flame suddenly starts oscillating back and forth vigorously. No one knew how the flame transitioned from burning serenely to behaving violently.Watching a flame burning well is somewhat like watching a stream flowing smoothly. Put a small bit of paper on the stream, and it goes smoothly forward, without turning back on itself. Instabilities in a flame are like turbulent flow in a stream. The bit of paper will trace eddies, as different parts of the stream starts flowing at different speeds.In his experiments, Sujith and his students noticed that small changes in inputs produce unpredictable effects. For example, a small change in the amount of fuel or air produced large pressure changes in the combustion chamber. It was a signature of what physicists call chaos.Chaos is difficult to predict. So the question that Sujith asked was this: Were there intermediate stages that one could pick up as early warning signs of instability?Physicists find turbulence hard to understand. It is even harder to understand inside a turbine or an engine, as complex chemical reactions happen in addition to turbulence.As Sujit and his students worked on the problem, they started noticing precursors of unstable behaviour. Soon, they were able to foretell instability just by listening to the sound of the flame.What they discovered was well known in chaos theory but not expected in a flame environment. Before the flows became fully turbulent, the flame produced brief pockets of instability that served as a warning sign. The physicists called it intermittency. It was well known to occur before a system became chaotic, but the theoreticians had never gone to a lab and listened to the sound of equipment.Sujit’s insight became valuable in predicting when a flame will become unstable in an engine, and his method was converted over many years into a piece of software, which was used by General Electric in some of its turbines to warn against some kinds of instability.At a deeper level, Sujith found something far more interesting. When the combustor is stable, the data from it produces a rich variety of self-similar patterns called fractals. Physicists call it multifractal behaviour. When the combustor becomes unstable, it loses this complex pattern and one kind of pattern begins to dominate.Complexity is nature’s way of ensuring stability. A healthy human heart produces complex multifractal patterns. A diseased heart loses this multifractal nature. There was a hidden commonality in the mathematics of combustion and of the human heart.