Experimental Investigation of Self-excited Thermoacoustic Instability Using Different Geometrical Configurations

In the present study, we have investigated three different combustor configurations to determine their effect on combustion instability. In particular, the combustion chamber and downstream boundary conditions are altered. The combustor used in the current investigation is a straight duct fixed with (i) an open end with exit area ratio of 1, (ii) an orifice with exit area ratio of 0.12, and (iii) a convergent section with gradual exit decrease in area ratio to 0.12. These three geometries mimic acoustic characteristics of combustor exit boundary conditions under specific operations. In current investigation, the equivalence ratio varied between 0.64 and 0.73. The open end exhibits relatively large amplitude thermoacoustic oscillation compared to the orifice and convergent section. In all the geometries, increasing the air flow rate leads to an increase in acoustic pressure fluctuations. To scrutinize further, we reconstruct the acoustic pressure distribution inside the combustion chamber using the two microphone method. The location of the pressure node varies with configurations and acoustic loss. The stochastic stability map shows a transition from unimodal to bimodal distributions for increasing airflow rates in the case of a convergent section. The observations made in the present study assist in understanding the thermoacoustic instability in practical combustors.