Combustion modes and driving mechanisms of pressure fluctuation in a novel vortex-tube combustor with quasi-steady and stratified properties

Abstract

The operating limit and pressure fluctuations of a localized stratified vortex-tube combustor (LSVC) are investigated experimentally by taking methane as fuel at the inlet temperature of 300 K and the combustor pressure of 1 atm over a range of equivalence ratios (0.1–1.0). The combustion modes are distinguished according to the properties of pressure fluctuations and the driving mechanisms of each combustion mode are explored in combination with numerical simulation. The results show that the LSVC can realize stable combustion with pressure fluctuation amplitudes always less than 4 kPa in a large operating range, and the flame front is always continuous. The operating flammability limit experimental range can be divided into five regimes with different combustion modes. The first and fifth combustion modes are steady combustion with pressure fluctuation amplitudes less than 1 kPa, and the second to fourth ones are quasi-steady combustion with pressure fluctuation amplitudes between 1–4 kPa. The spectrum analyses of pressure fluctuations show that there are one low-frequency peak around 300 Hz and one high-frequency peak around 1500 Hz, which are dominated by the first and third axial natural acoustic modes of resonant oscillation combustion, respectively. In the first and fifth combustion modes, the resonances are both weak due to the influences of low flow rate and laminarization respectively. In the second mode, the thermo-acoustic coupling oscillation and the resonance are excited simultaneously, yielding the highest pressure fluctuation amplitude in the entire operating range. The high-frequency resonance causes the high-frequency pressure fluctuation of the third mode. Both the unsteady heat release and flow field affect the pressure fluctuation in the fourth mode. The former produces the low-frequency fluctuation, which can resonate with the natural frequency and excite a weak thermal-acoustic coupling.