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The thermo-acoustic instability arising out of coupling between the pressure fluctuation and the unsteady heat release, when grows sufficiently, is known to cause serious structural damage thereby reducing the life span of systems having combustor for example, jet engines, gas turbines and industrial burners. The present work involves experimental study of thermo-acoustic instabilities occurring in a Rijke tube and their suppression by means of diverting a very small fraction of the incoming air flow in the form of radial injection from the wall of the tube through micro jets. A horizontal quartz tube with pre-mixed burner is used as the test model of Rijke tube. The coefficient Rayleigh Index ( RI ) is estimated from simultaneous measurement of chemiluminescence using a PMT and pressure in the plane of burner head. Experiments were carried out at the equivalence ratio of Φ = 1 for a range of the burner position inside the tube, velocity of micro jets, and total mass flow rate through the tube. The control technique of micro jets was found to completely suppress the thermo-acoustics in a range of jet velocity depending on the burner position and the total air mass flow rate. For complete suppression of the thermos-acoustics, the phase difference between the pressure and chemiluminescence waves was found to significantly increase and the RI was found to reduce to nearly zero. In such case, the flame luminescence was observed to significantly reduce. The proposed technique of air injection into the flame through radial micro jets is simple yet very effective in controlling the thermos-acoustic instability. [ABSTRACT FROM AUTHOR]

• The test setup is unique to study thermo-acoustic instability and its control. • The proposed RI coefficient estimated by simultaneous measurement. • The proposed RI coefficient eliminate problem of arbitrary scale and units. • The proposed RI coefficient (0.5 –0) serves as an indicator of thermo-acoustics. This study proposes a coefficient of Rayleigh Index (CRI) obtained by normalizing the acquired data which can circumvent the problem of arbitrary scale and unit while quantifying the Rayleigh Index. CRI is estimated from the simultaneous measurements of luminescence of a flame as an indicator of unsteady heat release and pressure fluctuation near the burner head. Experiments were carried out for six different mixture equivalence ratios ranging from 0.75 to 1.0 and a burner positioned at x/ L = 0.2. The control technique of micro jets is found to completely suppress the thermo-acoustics by decoupling the pressure waves and unsteady heat release waves as indicated by increasing phase difference between them. The coefficient of Rayleigh Index is found to reduce to nearly zero with the use of radial micro jets for suppressing the thermo-acoustic instability. In such case, the flame luminescence was observed to significantly reduce. The proposed coefficient of Rayleigh index is very useful in quantifying the effectiveness of control technique for the thermo-acoustic instability. [ABSTRACT FROM AUTHOR]

The coupling between pressure fluctuations and unsteady heat release in a combustion systems results in acoustic oscillations inside the combustion system. These acoustic oscillations, when grow sufficiently, may cause serious structural damage thereby reducing the lifespan of jet engines, gas turbines, and industrial burners. The aim of the first part of study is to define acoustically stable and unstable regions. The second part is focused on studying the effect of change in pressure field near the flame on the amplitude and frequency of the oscillations of instability. This study is carried out for three-burner positions and equivalence ratio of 0.7 by varying heat supply and total flow rate. The results show two acoustically unstable regions for 0.1 and 0.2 burner positions and only one acoustically unstable region for 0.25 burner position. The effect of pointed injection and diffused injection over a premixed flame on the sound pressure level was studied. The results show for burner position ofx/L = 0.2 there is 25 dB suppression is possible using pointed injection at higher total flow rate. The experiment of diffused injection shows sound amplification more than 12 dB was observed. [ABSTRACT FROM PUBLISHER]