Daniel Allgood

Optimal control of a swirl-stabilized spray combustor using system identification approach

An optimal controller using the system identification (SI) method was developed for a swirl-stabilized spray combustor operating between 30 and 114 kW. The efficacy of the controller was tested with two different nozzle configurations. The first consisted of a dual-feed nozzle whose primary fuel stream was utilized to sustain combustion, while the secondarystreamwasused for active control. The second configuration used a single-feed nozzle with two different swirling air streams. An LQG-LTR (linear quadratic Gaussian-loop transfer recovery) controller was designed using the SI-based model to determine the active control input, which was in turn used to modulate the secondary fuel stream. Using this controller, the thermoacoustic oscillations, which occurred under lean operating conditions, were reduced to the background noise level. A time-delay controller was also implemented for comparison purposes. The results showed that the LQG-LTR controller yielded an additional pressure reduction of 14 dB compared to the time-delay controller in both configurations. This improvement can be attributed to the added degrees of freedom of the LQG-LTR controller that allow an optimal shaping of the gain and phase of the controlled combustor over a range of frequencies in the neighborhood of the unstable mode. This leads to the extra reduction of the pressure amplitude at the unstable frequency while avoiding generation of secondary peaks.

Active control of combustion instabilities using model-based controllers

An implementation of active control of thermoacoustic instabilities on a swirl-stabilized spray combustor is presented. Loudspeakers were used as control actuators in the closed-loop control scheme. Pressure transducers located in the combustion chamber produced the feedback signal in the control study. Experimental models of the combustor dynamics were developed using a nonparametric identification method. Linear quadratic Gaussian (LQG), LQG/loop transfer recovery, and H ∞ loop-shaping controllers were derived and tested in simulation as well as experimentally. Phase-delay control was used as a baseline method to compare the performance of these different controllers. The advantages of the model-based controllers over the baseline strategy are clearly presented.

Experimental Investigation of a Pulse Detonation Engine with a Two-Dimensional Ejector

A parametric study into the integration of a pulse detonation engine (PDE) with an ejector was performed. High-speed shadowgraph visualizations of the flow inside the two-dimensional ejector provided a qualitative method of determining the performance of the integrated system. The performance was observed to be sensitive to the inlet geometry of the ejector as well as its axial position relative to the exhaust plane of the PDE. Significant levels of entrainment were obtained when the ejectors inlet was contoured, whereas flow separation reduced entrainment efficiency in the ejector with a straight thin inlet lip

Performance Studies of Pulse Detonation Engine Ejectors

An experimental study on the performance of pulse detonation engine ejectors was performed. Time-averaged thrust augmentation produced by straight and diverging pulse detonation engine ejectors was measured using a damped thrust stand. The ejector length-to-diameter ratio was varied from 1.25 to 5.62 by changing the length of the ejector and maintaining a nominal ejector diameter ratio of 2.75. In general, the level of thrust augmentation was found to increase with ejector length. Also, the ejector performance was observed to be strongly dependent on the operating fill fraction. A new nondimensional parameter incorporating the fill fraction was proposed. When the pulse detonation engine ejector data were represented as a function of this new parameter, the ejector data were reduced to one representative thrust augmentation curve for ejectors of similar internal geometry. Straight pulse detonation engine ejectors compared well with the available data on straight steady-flow ejectors. Diverging pulse detonation engine ejectors produced nearly twice the thrust augmentation as their straight-ejector counterparts due to the additional thrust surface area the divergence provided. All pulse detonation engine ejectors tested were seen to be sensitive to the axial position of the ejector as well. The optimum ejector axial placement was found to be a function of fill fraction due to a tradeoff between the detonation wave induced drag and increased mass entrainment. Downstream ejector placements performed the best at the low fill-fraction operating conditions.

Acoustic Control of Thermoacoustic Instabilities Using Experimental Model-Based Controllers

This paper presents an implementation of active control of thermoacoustic instabilities for a swirl- stabilized spray combustor. Loudspeakers located upstream of the burner were used as control actuators in the closed-loop system. Experimental models of the combustor dynamics were developed using a non-parametric identification method. Several controllers were obtained based on optimal control strategies. These controllers were simulated and tested in the experimental setup showing good agreement. Phase-delay control was used as a baseline method to compare the performance of these different controllers. The advantages of the model-based controllers over the baseline strategy are clearly presented.Copyright

Infrared measurements of thermoacoustic instabilities in a swirl-stabilized combustor

An experimental study of the pressure and temperature fluctuations associated with thermoacoustic instabilities was performed on a 30-kW model gas turbine combustor. The combustor consists of an annular swirling air stream with a centrally located Parker Hannifin Research Simplex Atomizer that injects liquid fuel (ethanol) into a dump combustion chamber. A high-speed infrared (IR) imaging system was used to qualitatively visualize the fluctuations in the CO 2 thermal emissions (temperature) within the combustor. The IR measurements were acquired in conjunction with pressure measurements obtained from high-frequency response transducers to investigate the coupling between the thermal and acoustic fields. The IR visualizations showed that near the flame blowout regime, which corresponded to the higher-pressure oscillations in the combustor, the flame was compact and fluctuated at the instability frequency near the combustor dump plane. The effects of phase-delay active control on attenuating the instabilities were also studied using the IR imaging system. With phase-delay control, the magnitude of the temperature and pressure fluctuations was reduced. Time-averaged IR measurementsalso showed higher average temperatures and stronger temperature gradients near the base of the flame with phase-delay control.

Identification and active control of thermoacoustic instabilities

This paper presents an implementation of identification and acoustic control of thermoacoustic instabilities on a swirl-stabilized spray combustor. Loudspeakers located upstream of the burner were used as control actuators in the closed-loop system. Experimental models of the combustor dynamics were developed using a non-parametric identification method. LQG and H/sub /spl infin// Loop-Shaping controllers were derived and tested in simulation as well as experimentally.

Infrared Measurements of Thermoacoustic Instabilities in a Swirl-Stabilized Combustor

An experimental study of the pressure and heat release dynamics associated with thermoacoustic instabilities was performed on a 30kW model gas turbine combustor. The combustor consists of an annular swirling air stream with a centrally located Parker Hannifin Research Simplex Atomizer that injects liquid fuel (ethanol) into a dump combustion chamber. An infrared (IR) imaging system has been implemented to visualize the spatial and temporal dynamics of the heat release within the combustor in order to study the effects of active control strategies on suppressing the instabilities. The infrared measurements were acquired in conjunction with heat flux and pressure measurements obtained from high frequency response transducers to investigate the coupling between the heat release and acoustic fields. Time averaged IR emissions show stronger gradients and higher average heat release near the base of the flame with closed-loop control. The results also showed that flow conditions corresponding to higher pressure oscillations in the combustor were characterized by strongly fluctuating compact regions of heat release near the base of the flame.Copyright