Swirling combustor

In practice it has been found that the diameter of holes in the primary zone should be no larger than 0.1 of the liner diameter. Tubular lines with about 10 rings of eight holes each give good efficiency. As discussed before, swirl vanes with holes yield better combustor performance. 

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. 

Meier, W., Duan, X. R., and Weigand, P, Investigations of swirl flames in a gas turbine model combustor—II. Turbulence-chemistry interactions. Combust. Flame, 144, 225, 2006. 

Anand, M. S., A. T. Hsu, and S. B. Pope (1997). Calculations of swirl combustors using joint velocity-scalar probability density function methods.. 47.4.4 Journal 35, 1143-1150. 

Current research on control of combustion is focussed not only to reduce combustion-induced pressure oscillations and instability but also to improve combustion performance. Attention is being paid to increased flame speed and improved flame lift-off limits. Flame speeds ranging from laminar to 3.5 times laminar values have been examined, using a Countercurrent Swirl Combustor... 

FLAME SPEED CONTROL USING A COUNTERCURRENT SWIRL COMBUSTOR... 

Figure 17.1 (a) Countercurrent Swirl Combustor experimental facility (6) isometric view of the burner and (c) cross-sectional view of the burner indicating relevant parameters... 

Figure 17.2 Twenty frame composite image of the flame within the Countercurrent Swirl Combustor (exhaust port is on the right)...

Figure 17.3 (a) Cross-sectional view of the Countercurrent Swirl Combustor (6) anticipated velocity profiles within the combustor... 

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%. 

Acoustic quarter-waves with an antinode at the upstream end of the combustor and RMS pressures up to 10 kPa have been shown to dominate the flows in the two combustors tested. The quarter-wave occupied the duct length upstream of the annular ring in the first arrangement and the entire duct length in the swirling flow. 

Axial and swirling air streams in the combustor issued from a circular chamber through a conical nozzle. The chamber was utilized both as an acoustic resonator and a settling chamber. It contained a honeycomb to straighten the how and two acoustic drivers to apply acoustic excitation to the jet. The nozzle exit diameter was 3.8 cm and the maximum Reynolds number based on this diameter and the exit velocity with and without air forcing was 4800 and 1400, respectively. The tests were performed with total air how rate of 85 1/min, and fuel how rate of 0.063 1/min. The swirl was applied with tangential air injection and the maximum swirl number tested was Ns = 0.30. 

Figure 20.2 Schematic drawing of (a) the large-scale combustor with a moderate swirl (configuration 2) and (6) conical preburner with high swirl (configuration 3)... 

Configuration 1 Small-Scale Coaxial Combustor (Low Swirl) Cold Flow Visualization... 

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. 

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

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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