08 February 2020 19:54 IST

Studying flames of candle bunches, the team understood how combustors used in rockets work

Using stacks of candles tied together, and studying pairs, and quartets, of such candles, an IIT Madras team of researchers has come up with interesting inputs that will help in building combustors in rockets.

We know that Apollo 11 successfully landed men on the Moon in 1969. Stories of the failures that paved the way to this success are less known. One such failure was due to thermo-acoustic failure of the F1 engine of the rocket during a test stand in 1962. When NASA tried to test how to launch the rocket, it just blew apart. The reason was uneven burning of the fuel. Like a candle whose flame flickers due to uneven presence of oxygen around it, the flames inside the F1 engine flickered, only at a higher frequency - an instability that blew the rocket apart. This is called a thermo-acoustic instability, which is another name for high-amplitude pressure oscillations, in the combustor.

Combustors contain several flames due to the presence of multiple fuel injection systems. “The interaction of these flames with the acoustic field (pressure variations) collectively results in the onset of thermo-acoustic instability. In order to understand the interaction between multiple flames in a much simpler and economical environment, we started to study candle-flame oscillators,” says R. I. Sujith in whose lab these experiments were carried out. He is a Chair Professor in Department of Aerospace Engineering at IIT Madras.

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Candle oscillators

The thermo-acoustic instability problem, and others like it, can be understood, albeit in a scaled-down manner, by studying stacks of two or three candles tied together so that their flames merge. The flames of such candles, when placed beside other candles, oscillate in synchronicity and show a rich variety of phenomena. The IIT Madras researchers have studied such candle flame oscillators and shown experimental manifestations of some phenomena that have hitherto only been known theoretically in oscillators. The research on this has been published in the journals Scientific Reports and Physical Review E.

One such phenomenon is amplitude death, which is the complete quenching of oscillations due to a coupling between the different flames. Another phenomenon is phase-flip bifurcation, which is an abrupt change from in-phase synchronisation to out-of-phase synchronisation.

First study

“Our candle study is the first to experimentally prove the existence of both states in a single system. The coexistence of these two states in a single system brings the possibility of evading undesirable states in various other oscillators,” says Prof. Sujith an author of the papers, in an email to The Hindu. He explains how studies on neural diseases such as Alzheimer's and Parkinson's disease model these conditions as a consequence of the occurrence of the amplitude death state in the neural oscillators. On the other hand, there are systems such as thermo-acoustic oscillators and oscillations of bridges and skyscrapers, where amplitude death is actually a welcome thing.

“The coexistence of the states of amplitude death and phase-flip bifurcation in a single system gives rise to the possibility that a system in which amplitude death state is undesirable can transition to a state of phase-flip bifurcation by varying a system-specific control parameter,” says Prof. Sujith.

Useful inputs

The experiments gave the team useful inputs into the original problem they were interested in – thermo-acoustic phenomena in combustors: “We got very relevant information about placing the injectors or flame locations in the combustor such that they would be inherently stable (or in an amplitude death state),” says Prof. Sujith.

He adds that they believe the study on four candle flame oscillators would help in understanding the interaction of multiple thermo-acoustic systems (more than two), which is practically used in can or can-annular type of gas turbine combustor. “Thus, we will be able to simultaneously control the thermo-acoustic oscillations in all the combustors. We are planning to build a test-rig of this type,” he says.