Combustors annular

Larger tubular, or single-can, units usually have more than one nozzle. In many cases a ring of nozzles is placed in the primary zone area. The radial and circumferential distribution of the temperature to the turbine nozzles is not as even as in tubo-annular combustors. In some cases, high stresses are exerted on the turbine casing leading to casing cracks. 

Higher temperatures also require more cooling and, as temperatures increase, the single can or annular combustor design becomes more attractive. The tubo-annular combustor has a more even combustion because each can has its own nozzle and a smaller combustion zone, resulting in a much more even flow. Development of a can-annular combustor is usually less expensive, since only one needs to be tested instead of an entire unit as in an annular or single-can combustor. Therefore, the fuel and air requirements can Be as low as 8-10% of the total requirements. 

Figure 10-15. Straight-through flow-type can-annular combustors. Rolls-Royce Limited.)...

In most aircraft engines the tubo-annular combustors are of the straight-through flow type seen in Figure 10-15. The straight-through flow type... 

Figure 10-17. Industrial-type can-annular combustor. (Courtesy of Solar Turbines Incorporated.)...

Figure 12-9. Cross section of an annular combustor showing high dome flow configuration. (Courtesy of General Electric Company.)...

Fan Spray 100-1000 High-pressure painting/coat ing, Annular combustors Good atomization, Narrow elliptical spray pattern High supply pressure... 

Fan spray atomizers have been widely used in the spray coating industry (Fig. 2.5), in some small annular gas turbine combustors, and in other special applications that require a narrow elliptical spray pattern rather than the normal circular pattern. In particular, fan spray atomizers are ideal for small annular combustors because they can produce a good lateral spread of fuel, allowing to minimize the number of injection ports. 

For a typical case, an axisymmetric jet with a mean velocity of 100 m/s flows through the cylindrical inlet of diameter D into a cylindrical combustion chamber of twice the diameter. An annular or central exit at the end of the combustion chamber is modeled to produce choked flow. Particles are injected from the inlet-combustor junction with a streamwise velocity of 50 m/s and zero radial velocity. If the number of particles is small (that is, for low-mass loadings), the effect of the particles on the flow can be neglected. Still the flow has an effect on the particles that depends on parameters such as the size and density of the particles. Such systems are called one-way coupled systems and are discussed next. 

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%. 

Measurements were obtained with a ratio of flow rates (flow rate in the an-nulus/flow rate in the inner duct) of 9, so that the bulk mean velocities upstream of the annular ring were nearly equal to those in practical combustors. Deviations from this ratio were allowed where they provided insight into the nature of the oscillations. Results relating to the influence of the geometric and flow parameters on flame stabilization and combustion oscillations are presented first. 

The tendency of premixed flames to detach from the flame holder to stabilize further downstream has also been reported close to the flammability limit in a two-dimensional sudden expansion flow [27]. The change in flame position in the present annular flow arrangement was a consequence of flow oscillations associated with rough combustion, and the flame can be particularly susceptible to detachment and possible extinction, especially at values of equivalence ratio close to the lean flammability limit. Measurements of extinction in opposed jet flames subject to pressure oscillations [28] show that a number of cycles of local flame extinction and relight were required before the flame finally blew off. The number of cycles over which the extinction process occurred depended on the frequency and amplitude of the oscillated input and the equivalence ratios in the opposed jets. Thus the onset of large amplitudes of oscillations in the lean combustor is not likely to lead to instantaneous blow-off, and the availability of a control mechanism to respond to the naturally occurring oscillations at their onset can slow down the progress towards total extinction and restore a stable flame. 

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. 

---