Entropy generation and CO2 emission in presence of pulsating oscillations in a bifurcating thermoacoustic combustor with a Helmholtz resonator at off-design conditions

In this work, we develop a 2D numerical model of a Y-shaped bifurcating combustor with a Helmholtz resonator attached. Propane (C3H8) is fueled and burnt with air by applying single-step eddy dissipation combustion model and k−ϵ−RNG turbulence model for simplicity. To validate the numerical findings, experimental measurements are conducted on a bifurcating Y-shaped thermoacoustic combustor with an off-design Helmholtz resonator implemented. It is found that the frequency and amplitude of the dominant mode as experimentally measured agree well with the numerical results. Further agreement is obtained between numerically and theoretically predicted mode-shapes. With the model validated, it is applied to gain insights on the entropy generation and nonlinearity of the pulsating oscillations and the flow fluctuations across the resonator neck. It is found that the nonlinearities are originated in the unsteady heat release rate of the premixed propane flame, and the mass flow rate across the resonator neck. In addition, the rate of the entropy production depends strongly on the temperature fluctuations. Approximately 99% of the entropy production rate involves with the temperature oscillations. Furthermore, it is non-uniformly distributed along the combustor. In addition, the energy conversion rate between total heat release rate and the acoustical energy production rate is less than 0.0001%. Finally, the production of CO2 is increased exponentially, and then reduced gradually in the axial flow direction. In general, the present work provides a low-cost numerical tool of a bifurcating thermoaocustic system. It could be applied to predict the acoustic signature of the combustor and to examine and evaluate the performance of the Helmholtz resonator on attenuating self-sustained thermoacoustic oscillations.