Effects of geometry and gas composition on swirling flow

Lean premixed swirl stabilised combustion is regarded as one of the most successful technologies for flame control and NOx reduction in gas turbines. Important characteristics of these flows are good mixing, flame stability through the formation of a Central Recirculation Zone, and low emissions at lean conditions as a consequence of the low operating temperature. This project presents a series of experiments and numerical simulations using commercial software (ANSYS) to determine the behaviour and impact on the blowoff process at various swirl numbers, nozzle geometries and gas compositions at same power outputs using confined and open conditions. Experiments were performed using a generic premixed swirl burner. The Central Recirculation Zone and the associated turbulent structure contained within it were obtained through CFD analyses providing details of the structures and the Damkölher Number (Da) close to blowoff limits. The results show how the strength and size of the recirculation zone are highly influenced by the blend and nozzle geometry, with a shift of Da and turbulence based on carbon-hydrogen ratio, shearing flows and Reynolds number. The Central Recirculation Zone was also measured and correlated to the blowoff phenomenon. A trend was found between the CRZ size/strength, the different compositions of gases used and the burner nozzle. Chemical kinetic analyses were carried out using PRO-CHEMKIN to determine flame speeds and chemical properties needed for CFD calculations. Experiments were performed using Phase Locked PIV and High Speed Photography. The Central Recirculation Zone and its turbulence were measured and correlated providing details of the structure close to blowoff. It was found that the nozzle angle has a small effect on the LBO at low flow rates using all mixtures. During the tests, the Coanda effect was observed with some geometries, thus further research was carried out regarding the transition of this phenomenon. It was found that the process occurs at a particular geometry and step size, with a shift in frequency produced by the leading structure due to the entrainment of air and strength of the latter. Stability of the flow occurs after a Coanda Vortex Breakdown (COVB) has occurred, a process similar to the one observed in the central region of the flow under regular swirling open flames. As the step size is increased, the COVB will evolve into a slower Trapped Vortex (TV

Impacts on blowoff by a Variety of CRZs using various gases for gas turbines

Fuel flexibility will drive the energy demand in the near future. The use of different syngas compositions from various sources will play a major role in the global fuel mix. CO2 in the blends will also be added as a mechanism to improve carbon capture and storage technologies. However, this can trigger instabilities such as thermoacoustics, flashback, autoignition and blowoff. In terms of blowoff, the phenomenon is still not entirely understood. This project presents a series of experiments to determine the behaviour and impact on the blowoff process at various swirl numbers, nozzle geometries and gas compositions. The Central Recirculation Zone was analyzed just before blowoff. The results show how the strength and size of the recirculation zones are highly influenced by these parameters. However, it seems that the CRZ dimensions/strength does not play an important role in the blowoff, whilst the composition of the mixture shows high correlation. Nevertheless, the CRZ intensity using these compositions can increase residence time, important for combustion improvement of other blends

CFD predictions of swirl burner aerodynamics with variable outlet configurations

Swirl stabilised combustion is one of the most widely used techniques for flame stabilisation in gas turbine combustors. Lean premixed combustion systems allow the reduction of NOx coupled with fair flame stability. The swirl mechanism produces an aerodynamic region known as central recirculation zone (CRZ) providing a low velocity region where the flame speed matches the flow velocity, thus anchoring the flame whilst serving to recycle heat and active chemical species to the root of the former. Another beneficial feature of the CRZ is the enhancement of the mixing in and around this region. However, the mixing and stabilisation processes inside of this zone have shown to be extremely complex. The level of swirl, burner outlet configuration and combustor expansion are very important variables that define the features of the CRZ. Therefore, in this paper swirling flame dynamics are investigated using computational fluid dynamics (CFD) with commercial software (ANSYS). A new generic swirl burner operated under lean-premixed conditions was modelled. A variety of nozzles were analysed using several gaseous blends at a constant power output. The investigation was based on recognising the size and strength of the central recirculation zones. The dimensions and turbulence of the Central Recirculation Zone were measured and correlated to previous experiments. The results show how the strength and size of the recirculation zone are highly influenced by the blend and infer that it is governed by both the shear layer surrounding the Central Recirculation Zones (CRZ) and the gas composition

Blowoff propensity, CRZs and flow turbulent nature using various syngases for gas turbines

This paper presents a series of experiments and numerical simulations using commercial software (ANSYS) to determine the behaviour and impact on the blowoff process with various geometries and simulated syngas compositions at fixed power outputs. Experiments were performed using a generic premixed swirl burner. The Central Recirculation Zone and the associated turbulent structure contained within it were obtained through CFD analyses providing details of the structures and the Damkolher Number (Da) close to blowoff limits. The results show how the strength and size of the recirculation zone are highly influenced by the blend, with a shift of Da and turbulence based on carbon-hydrogen ratio, shearing flows and Reynolds number. Instabilities such as thermoacoustics, flashback, autoignition and blowoff are highly affected by the flow structures and chemical reactions/diffusivity. Moreover, it has been observed that turbulence close to the boundaries of the central recirculation zone, a region of high stability for swirling flows, is highly altered by the chemical characteristics of the fuel blends. In terms of blowoff, the phenomenon is still not entirely understood. As the process occurs, its theoretical limits do not match its real behaviour. Therefore, one possibility could be the difference in turbulence and Da numbers across the flame, being critical at the base of the flame where the system is stabilized

The use of CO2 to improve stability and emissions of IGCC combustors

The use of gas for power generation is likely to increase in the medium term. Also, the introduction of new fuels will ensure a higher generation with lower emissions under continuous operation. These scenarios lead to the conclusion that there will be a considerably more diverse range of fuel supply. However, the use of these new fuels contrasts with recent experiences of global operators who report increasing emissions and difficult combustion dynamics with even moderate variations in their fuel characteristics. Clearly there are significant challenges for fuel flexible gas turbines, particularly emission control, combustor dynamics and flame stability. Trials using a power derivative gas turbine combustor and a high hydrogen content fuel produced unusual flashback events, in that flashback was induced by either leaning of the fuel mixture by the increase of combustion air, or by a change in composition through the reduction of methane pilot fuel. The introduction of CO2 through the combustors pilot injector prevented flashback from occurring under these circumstances. The resulting reduction of temperature in the combustion zone, indicated by lower burner tip temperatures causes a reduction in the emissions of nitrous oxides, whilst there is minimal effect on the effective turbine inlet temperature, only a 2.3% reduction. Investigations using a ‘generic’, radial swirl burner and stereo PIV demonstrated how the flashback depended on a combination of flow structure augmentation and changes in mixture burning rate. The injection of methane or CO2 had differing effect on these parameters of the combustion zone, but both produced combinations that facilitated stability