Stabilization, swirl combustor

Stephens, J.R., S. Acharya, and E. J. Gutmark. 1997. Controlled swirl-stabilized spray combustor. AlAA Paper No. 97-0464. 

Swirl is used commonly to stabilize high-intensity combustion 3,4, 5,6,7), In general, there are two main types of swirl combustors. 

Figure 8, Stability limits of the modulated swirl combustor. Total air flow = 1070 LI min, equally distributed in the four points of inlets.

Active control studies on a swirl-stabilized spray combustor are presented. Significant improvements with model-based control over traditional time-delay control is demonstrated in the present work. These improvements are particularly noted with acoustic modulation. Future work in this area is directed toward using a proportional drive spray injector where the full amplitude/phase information from the model-based controller can be exploited. 

Campos-Delgado, D. U., K. Zhou, D. Allgood, and S. Acharya. 2003. Active control of a swirl-stabilized spray combustor using model-based controllers. Combustion Science Technology 175(l) 27-53. 

The present work involves detailed measurements of flow, pollutant emissions, and acoustics in model multiple-swirl, partially-premixed flames and combustors. Complementary computations are also planned, combining computational aeroacoustics approaches with combustion modeling. The focus is on how pollutant control and flame stabilization strategies, such as partial premixing and swirl, respectively, influence combustor noise sources and potential instabilities. Also of interest are flow and acoustics associated with diffuser-combustor interactions and the utility of the trapped-vortex combustor design. A better understanding of pollutant and acoustic sources and how to modify them will aid in the control of emissions, noise, and instabilities in modern swirl combustors. 

Y. Huang and V. Yang. Bifurcation of flame structure in a lean-premixed swirl-stabilized combustor Transition from stable to unstable flame. Combust. Flame, 136(3) 383-389, 2004. 

Pressure oscillations with RMS value up to 10 kPa in two models of lean-burn gas turbine combustors, with heat release around 100 kW, have been actively controlled by the oscillation of fuel flow. The flames were stabilized behind an annular ring and a step in one arrangement, and downstream of an expansion and aided by swirl in the other. Control was sensitive to the location of addition of oscillated fuel. Oscillations in the annular flow were attenuated by 12 dB for an overall equivalence ratio of 0.7 by the oscillation of fuel in the core flow and comprising 10% of the total fuel flow, but negligibly for equivalence ratios greater than 0.75. Oscillation of less than 4% of the total fuel in the annulus flow led to attenuation by 6 dB for all values of equivalence ratio considered. In the swirling flow, control was more effective with oscillations imposed on the flow of fuel in a central axial jet than in the main flow, and oscillations were ameliorated by 10 dB for equivalence ratio up to 0.75, above which the flame moved downstream so that the effectiveness of the actuator declined. The amelioration of pressure oscillations resulted in an increase in NOj, emissions by between 5% and 15%. 

Active control of a swirl-stabilized spray flame was experimentally studied. Three combustor configurations representing low swirl, moderate swirl, and high swirl were studied. The following main conclusions were noted. 

Pulverized coal is fed directly from a variable speed auger into the high velocity primary air stream which conveys it to the injector at the top of the furnace. The coal and primary air enter the combustor through a single low-velocity axial jet. Secondary combustion air is divided into two flows which enter the combustor coaxial to the primary stream. Part of the flow is introduced through a number of tangential ports to induce swirl which is necessary for flame stabilization. The remainder enters the combustor axially. The two secondary air streams are separately preheated using electrical resistance heaters. 

The conical coal injector was replaced with a blunt cyclin-der with a single axial jet so the flame could be stabilized at lower swirl numbers, thereby reducing the centrifugal deposition on the furnace walls. The radiation shield between the combustor and heat exchanger was removed to reduce particle losses further. The increased radiative transfer decreased the wall temperature substantially. The later experiments were also carried out at lower fuel-air equivalence ratios, i.e., (J> = 0.57. The combination of increased heat losses and increased dilution with excess air reduced the maximum wall temperature to 990°C for the experiments reported below. 

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