Since coupling between the combustor oscillations and the unsteady combustion of the fuel spray drives combustion instabilities in liquid fueled combustors, "disruption" of this coupling by changing the fuel spray properties offers an approach for controlling these instabilities. This paper describes an experimental study that employed such an approach to control combustion instabilities in a liquid ftieled (n-heptane) combustor that used a novel internally mixed liquid fuel injector with a vaporizing element. This injector forms a fuel spray by mixing liquid fuel and a secondary air in a liquid/air mixing tube. The generated spray impinges upon a hot vaporizer plate, on top, as it flows towards the combustor, resulting in partial evaporation of the fuel in the spray. The resulting spray then enters the combustor through a slit between the vaporizer plate and the conical flame holder. Combustion occurs when the spray mixes with the primary air stream that enters the combustor through an annular region between the combustor wall and conical flame holder. The dependence of the characteristics of the instabilities upon the operating conditions of the injector (and, thus, spray properties) and acoustic properties of the setup were studied. The spray properties were varied by changing the flow rate of the secondary air stream into the injector's mixing tube and the combustor's acoustics were varied by changing the length of the primary air supply tube. The characteristics of the flow and combustion processes were studied from measured pressure and radicals radiation data. The results clearly showed that combustion instabilities could be controlled by changing the fuel spray properties and that the amount of attained damping depends on the frequency. In addition, emissions measurements show that the concentrations of CO, SO 2 and unburned hydrocarbons decreased as the size of the fuel droplets in the spray decreased. Interestingly, NO levels decreased as the amplitude of the instability increased.
This paper describes an experimental investigation of the feasibility of using “slow” active control approaches, which change liquid fuel spray properties, to suppress combustion instabilities. The objective of this control approach is to break up the feedback between the combustion process heat release oscillations and the combustor oscillations that drives the instability by changing the characteristics of the combustion process (i.e., characteristic combustion time). To demonstrate the feasibility of such control, this study used a proprietary fuel injector (Nanomiser™), which can independently vary its fuel spray properties, and investigated the dependence of acoustics-combustion process coupling, i.e., the driving of combustion instabilities, upon the fuel spray properties. The results of this study showed that by changing the spray characteristics it is possible to significantly damp combustion instabilities. Furthermore, using Abel’s deconvolution, this study showed that the instabilities were mostly driven in regions where the mean axial flow velocity was approximately zero, in the near field of the vortices that were generated in the combustor. The results of this study strongly suggest that a “slow” active control system that employs controllable injectors could be used to prevent the onset and/or damp detrimental combustion instabilities.
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