In spite of the merits of high transfer, low pollutant emission and compact combustion, direct and/or indirect application of pulsating combustion system is at a low ebb due to the strong noise and the lack of physical understanding. Knowledge on the physical natures of the heat addition process coupled with the acoustic wave, unsteady flame structure and the effect of heat input or bumer geometry on the change in the operating characteristics would be a key factor in determining a way for the effective control of the pulsating combustor.
The purpose of this study is physical understanding of the periodic combustion processes in the Helmholtz-type pulsating combustor. To realize it, pulsating combustion processes were observed form the following three standpoints: (1) operating mechanism, (2) mixing conditions for the achievement of stable pulsation and indirect control of the burner and (3) spatially distributed heat release rate for the explanation of stability.
Through the image processing of the direct visualization and the relationships between the fuel consumption rate and operating frequency or the moving velocity of the periodic flame, pulsating flame's behavior and the operating response will be explained. Four tried nozzles and the effects of the stabilizer on the frequency and pollutant emission are investigated for the mixing rate and the stabilization of the flame front and to suggest the way of the pulsating combustor control. Finally, the space and heat release rate suggested from the results of ion current measurements is introduced for the local stability test represented as a Rayleigh criterion.
The following conclusions can be made from this experimental study.
Volumetric expansion of the flame attached at the edge of the stabilizer occurs in the main reaction period which corresponds to the pressure increasing. Ignition occurs by mixing fresh reactants with a hot residual flame which has a shape of a sooty luminous flame. This continuous reaction as a type of residual flame can be also ascertained by measuring the ion currents emitted from the flame. The result of the Rayleigh stability criterion shows that, in spite of stable operation, there is unstable region in the upstream of the combustion chamber ; this is caused by the initial active reaction including the ignition at the negative gauge pressure.
The operation of the pulsating combustor is very sensitive to the mixing characteristics. Operating response becomes to change with the flowrate, the diameter of the stabilizer and the shape of the fuel nozzle. These phenomena are caused by the short reaction time, so the characteristic mixing time and mixing length must be regarded as having geat importance in the design of the pulsating combustor.
The operating frequency and the amplitude of fluctuating pressure have linear and square relationships with the actual fuel consumption rate, respectively. And the amount of consumed fuel for a cycle and the reaction volume is independent of the operating frequency or the fuel flowrate in a given geometry. It means that the operating fuel input range can be extended to the frequency at which the formation of a cycle would not be disturbed by the mixing time.
