Visualisation of turbulent flows in a swirl burner under the effects of axial air jets

Meeting emission regulations represents a real challenge in the power generation sector. Swirl combustors and their operation under lean premixed (LP) conditions are a step towards attaining low emissions, especially NOx formation, while ensuring high efficiency. However, performing modifications on combustors and reaching the requirements of efficient combustion systems is difficult due to many combustion problems such as extinction, low reaction rates, mild heat release, instabilities, and mixing issues. Thus, giving careful attention to the hydrodynamics design of the swirl burners with extensive testing methods in both experimental and numerical approaches is crucial to stabilise the combustion phenomena in gas turbines. As a result, this study employed the implementation of CFD simulations in the design of a 150 kW tangential swirl burner and considered the consequences of 50 LPM diffusive air injection at different positions on three-dimensional isothermal flow field characterizations, especially the turbulence, downstream the burner nozzle. Various mass flow rates from 600 to 1000 l/min were used at atmospheric conditions with a geometrical swirl number of 0.913. Experimental work was conducted with good correlation. It was found that using the air injection system could increase the flashback resistance by affecting the velocity defect downstream the burner nozzle. Moreover, the axial air jet reduces the flow field turbulence at the central recirculation zone (CRZ) tip and hence minimises the flow fluctuations and affect its size and position. CFD results show a very good agreement with Laser Doppler Anemometry (LDA) data acquired from the experimental work

Ammonia-hydrogen combustion in a swirl burner with reduction of NOx emissions

Recently, ammonia is being considered for fuelling gas turbines as a new sustainable source. It can undergo thermal cracking producing nitrogen, hydrogen and unburned ammonia, thus enabling the use of these chemicals most efficiently for combustion purposes. Ammonia being carbon-free may allow the transition towards a hydrogen economy. However, one of the main constraints of this fuelling technique is that although the combustion of ammonia produces no CO2, there is a large NOx proportion of emissions using this fuel. In this work, cracked ammonia obtained from a modified combustion rig designed at Cardiff University was used to simulate a swirl burner under preheating conditions via heat exchangers. The primary objective of this system is to find new ways for the reduction of NOx emissions by injecting various amounts of ammonia/hydrogen at different mixtures downstream of the primary flame zone. The amount of injected ammonia/hydrogen mixture (X) taken from the thermal cracking system was ranged from 0%-4% (vol %) of the total available fuel in the system while the remaining gas (1.00-X) was then employed as primary fuel into the burner. CHEMKIN- PRO calculations were conducted by employing a novel chemical reaction code developed at Cardiff University to achieve the goal of this paper. The predictions were performed under low pressure and rich conditions with an equivalence ratio ϕ =1.2 in a swirl burner previously characterised at output powers of ~10 kW. Ammonia and hydrogen blends were evaluated from 50% NH3 (vol %) with the remaining gas as hydrogen, continuing in steps of 10% (vol %) NH3 increments. Results showed that the minimum unburned ammonia and higher flame temperature were achieved at 60%-40% NH3-H2 when compared to other blends but with high NO emissions. These NO levels were reduced by injecting a small amount of NH3/H2 mixture (X=4 %) downstream the primary zone in a generated circulations promoted by the new design of the burner which affecting the residence time hence reducing the NO emission in the exhaust gas

Experimental Investigation on effects of bluff-body size and axial air injection on blowoff limits in swirl burners

The stability limits of swirl combustors have been considered as a crucial factor for obtaining a wide stability operation map. The present global consideration is towards using low-carbon emission fuel in gas turbine production sector and, many other combustion systems. However, the demands of introducing lowcarbon emission fuels impose a considerable modification in the combustor hardware; consequently, the variation of burner stability operation map. Blowoff and flashback are two parameters that determined the margins of stability operation in swirl burners, when correlated with equivalence ratio and inlet tangential or bulk velocity. This study investigates the effect of hardware modification with different bluff-body sizes (external diameter) and flow-field manipulation like using axial air injection on blow-off limits in swirl combustors. The first part of this study has demonstrated that variation of bluff-body diameter alters the blowoff limits significantly. Small central injector (bluff-body) diameter displaces blowoff limits towards leaner equivalence ratios with (Φ= 4 to 4.2); which is favourable for low emission demands. However, the stability map became narrower regarding inlet tangential velocities with (w=2.7 to 4.2), consequently reducing output power. In contrast, bigger injector diameter leads to having blowoff limits occur at a wider range in term of inlet tangential velocity(w=2.5-4.5) which means high output power, despite slight displacement to the rich region, Φ= 0.5 at high tangential velocity. The second part of this work has proposed, a new technique that can replace hardware (bluff-body) by axial air-jets which can simulate the physical shape of bluff-body. Using axial air jets results in wider operation map, the inlet tangential velocity range is (w=2-8 m/s) compare with bluff-body case (w=2.5-4.5 m/s), hence increasing the burner output power while keeping its size. The position of air-jet opening inside burner plenum alter blowoff limits, baseline Lo=0 and Lo=150 extend the range of inlet tangential velocity at which the blowoff occurs, almost (2-8 m/s). While the other three positions revealed less range of inlet tangential velocities, as the affected by aerodynamic perturbations arise from the clash between axial jets and inlet tangential flo

Effects of different nozzle configurations on swirl flow topology in tangential swirl burners

Flame flashback is one of the central combustion instabilities, especially when it appears in the form of boundary layer flashback (BLF) or combustion induced vortex breakdown (CIVB) flashback, some of the most common instabilities in swirl combustors. This paper focuses on mitigating the phenomenon of CIVB and BLF flashback mechanisms using different nozzle configurations while using central air injection. Studies were conducted on a 150-kW tangential swirl burner manufactured and previously characterised at Cardiff University. The effects of different nozzle heights (hn) with and without microstructure on the swirl flow characteristics were investigated experimentally by utilising an LDA system. Different strip heights (hm) of a wire woven mesh have been employed as a liner on the smooth nozzle to change its surface roughness. It was found that longer smooth nozzles (hn/Ro=2.3) led to promotion of stability in the swirl burner by minimising the axial velocity defect while decreasing turbulence downstream the dump plane. Moreover, the average measurements show that the burner nozzles with microstructured surfaces enable improvements in controlling the BLF flashback and hence reduces outflow drag. It was found that the microstructured mesh alters the flow structure near the wall by increasing the velocity adjacent to this region delivering further resistance to BLF. On the other hand, using both central air injection and the nozzle with and without the microstructured surface can affect the operability of the gas turbine combustors

Experimental investigation of the Effect of Air Diffusive injection on premixing swirl flames

In recent decades there has been a strong trend towards the use of lean premixed combustion in order to produce gas turbine systems that are compliant with air quality regulations. Additional growing interest in using alternative fuels has drawn many problems in terms of operational stability, thus there have been extensive investigations to achieve more stable and reliable combustion systems. Flame flashback has been one of the major instability problems that have the potential of causing considerable damage to the combustion system hardware in addition to significant increase of emissions such as CO and NOx. Swirl combustors are proven as effective flame stabilisers over wide range of operation conditions due to swirl structures which provide a low velocity region that enables flame anchoring. However the interaction between swirl structures and swirl burner geometries can considerably alter the stability regime downstream the burner exit plane. Using central injectors either as a central bluff body or to inject fuel diffusively have been used successfully to achieve wide stability limits and prevent upstream flame propagation. However, central injectors in swirl combustors can be subject to large amounts of heat, which can cause flashback, subsequently increasing maintenance cost and reducing the predicted operating life of the system. This paper present a series of experiments using a well-characterised tangential swirl burner to investigate the effect of using diffusive air injection on flow field characteristics and how it can affect the lower instability limits through altering the flashback mechanism by Combustion Induced Vortex Breakdown (CIVB). Results show that using diffusive air stream injection affects flashback trends significantly by providing a wider range of stability limits both in terms of equivalence ratio and mass flow rate which implies the possibility of working at higher power outputs compared to the case when using a central body injector. This technique can be considered a promising technology in terms of flexibility of operation because it enables switching to another air stream diameter while maintaining full load operation as opposed to the use of central body injectors

Experimental study to enhance resistance for boundary layer flashback in swirl burners using microsurfaces

Flame flashback has been one of the major instability problems in premixed gas turbine combustion with the potential to cause considerable damage to the combustion system hardware in addition to significant increase in pollutant levels. Swirl combustion has been proven as an effective flame stabilizer over a wide range of operation conditions, although swirling systems can be prone to various types of flashback under fuel premixed conditions. Unfortunately, using methodologies for the mitigation of one flashback mechanism will lead to another one in these systems. Therefore, this paper focuses on improving Boundary Layer Flashback (BLF) while trying to mitigate Combustion Induced Vortex Breakdown (CIVB) in a medium swirl combustion system. A new technique inspired by Biomimetic Engineering has been developed to use micro-surfaces for this aim. The use of these biologically designed shapes for successful flow stabilisation allows improved control of the boundary layer, thus reducing outflow drag while resisting the random propagation of flashback. Therefore, boundary layer flashback resistance using this concept was investigated numerically and experimentally in a 150 kW tangential swirl burner to determine the effects of using a micro-surface in swirling flows with and without central air injection. Various techniques were used, including Hot Wire Anemometry, LDA measurements, LES CFD, and RANS CFD. The results showed enhancement of the system resistance to boundary layer flashback, and a new combustion stability map was generated with a wider operational region when using central injection combined with micro-surfaces, thus avoiding two types of flashback mechanisms, i.e. BLF and CIVB