Fundamental characterisation of coherent structures for swirl combustors

swirl combustors have demonstrated that they can effectively stabilise flames across a wide range of operating conditions due to established, widely known but not fully understood swirl coherent structures. Unfortunately, their use in lean premixed (LP) modes as occurs with the introduction of alternative fuels, particularly blends with a relatively high hydrogen content can result in unstable combustion. An important such instability is a flame flashback which can cause considerable hardware damage to the combustion system as well as significantly increasing pollutant levels. Combustion Induced Vortex Breakdown (CIVB) and Boundary Layer Flashback (BLF) which are a result of interaction between swirl structures and burner geometry is an important modes of flashback instabilities because they can occur even when the velocity of the combustible mixture is greater than the flame speed. This project is part of the attempts to improve burner geometry and control swirl flows to increase resistance to these modes of flashback. This investigation used numerical and experimental methods to ascertain the effect of a range of burner designs on flame flashback processes. Experiments were carried out on a 150-kW tangential swirl burner operating in a premixed mode to demonstrate practically the effectiveness of the flame flashback resistance methods techniques for premixed fuels. The flow field characteristics were simulated by the ANSYS Fluent code. The experimental work was carried out using a 1D LDA system which provided the required measurements for the swirl flow. First hydrodynamic parameters were investigated with the intention of enhancing resistance against CIVB flashback. Initially, by replacing the central fuel injectors by axial air injection. The effects were assessed using ANSYS Fluent code. It was confirmed that axial air jets had good possibilities for improving flame stability, producing a wider range of stable operations than did central fuel injectors. In addition, the increase in stability occurred with equivalence ratio and tangential inlet velocity. The use of these air jets also promised lower maintenance costs because the working environment of the combustor would not be so harsh. Unfortunately, reducing the likelihood of CIVB can increase the likelihood of a different flashback mechanism, Boundary Layer Flashback (BLF). Thus, the second part of the research programme was to experimentally combine techniques that increased CIVB resistance (e.g., using central air injection) while at the same time evading BLF (e.g., by modifying the characteristics of the wall boundary layer). The former technique was achieved by applying a scalloped riblet geometry to the nozzle surface. Results confirm that combining these techniques is very promising regarding achieving a wider range of stable operations, enabling swirl combustors to burn a wider variety of fuel blends efficiently and safely. Then the work was extended to investigate the effects of different burner nozzle heights on the characteristics of the swirl flow. Finally, a new technique to reduce Boundary Layer Flashback (BLF) using biomimetic engineering methods has been established and tested. A stainless-steel woven wire mesh liner has been used with different heights to modify the internal surface roughness of the longest smooth burner nozzle. It was confirmed that inserting the mesh as a liner changed the structure of the flow adjacent to the burner wall, increasing resistance to Boundary Layer Flashback. It was demonstrated that the likelihood of Boundary Layer Flashback was reduced by using the designed micro-surfaces, the shorter the woven wire mesh liner, the better effect. It is suggested that by combining the different flow manipulation techniques, there is the potential for increasing the fuel flexibility of GTs

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

CFD simulation and validation of hydrodynamic instabilities onset in swirl combustors

The objective of this paper is to employ a numerical approach to model a 150kW tangential swirl burner to investigate the consequence of central air injection on the flashback mechanism. The effects of diffusive air injection on flow field characteristics and how these can affect the lower instability limits by altering the flashback mechanism via CIVB are analysed in both experimental and theoretical approaches. Simulations under isothermal conditions are carried out using both premixed and partially premixed species models to compare the flow field behaviour with and without air injection. The experimental data includes LDA measurements for the same burner geometry. CFD and experimental results demonstrated that using diffusive air affects flashback propensity significantly by expanding the stability region in terms of both equivalence ratio and mass flow rate that lead to greater operability at higher power outputs compared to using only a central body injector. The CFD results were verified and correlated to experimental findings with very good agreemen

Experimental and numerical investigation of the effect of diffusive air injection on turbulence generation and flashback propensity in swirl combustors

Combustion instabilities are considered one of the most serious challenges for developing combustion systems through the years. Undesirable issues linked to these phenomena represent a risk for such systems especially in gas turbines and propulsion devices where the propagation of these instabilities can even lead to considerable damages. Flame flashback from the combustion chamber into premixer represents one of the most important combustion instability issue in swirl combustors used in gas turbines. This study proposes an experimental and numerical approach to validate the use of a central air injection in swirl combustors to reduce flame flashback propensity via controlling the turbulence generation at the tip of the flame while pushing the CRZ, thus retarding the appearance of the CIVB, to mitigate the progression of combustion into the system. Results showed the potential of this technique to affect turbulence generation and pushing back the flame into the combustion chamber, increasing operability limits. Very good agreement was achieved between experimental and numerical results, demonstrating that the use of injection through the central core of the system not only controls the position of the recirculation zone but also affects turbulence and mitigates other forms of flame flashback

Enhancement flame flashback resistance against CIVB and BLF in swirl burners

Swirl combustors have proven as effective flame stabilisers over a wide range of operation conditions thanks to the formation of well-known swirl coherent structures. However, employment of swirl combustors to work on lean premixed combustion modes while introducing alternative fuels such as high hydrogen blends result in many combustion instabilities. Under these conditions, flame flashback has been considered as one of the major instability problems that have the potential of causing considerable damages of the combustion systems hardware in addition to the significant increase in pollutant levels. Combustion Induced Vortex Breakdown (CIVB) is considered a very particular mode of flashback mechanism in swirling flows as this type of flashback occurs even when the fresh mixture’s velocity is higher than the flame speed, consequence of the interaction between swirl structures and swirl burner geometries. Improvements of burner geometries and manipulation of swirl flows can produce good resistance against this type of flashback. However, increase flame flashback resistance against CIVB can lead to an increase in the propensity of another flashback mechanism, Boundary Layer Flashback (BLF). Thus this paper presents an experimental and numerical approach that allows the increase in CIVB resistance by using diffusive air injection and simultaneously avoid BLF by changing the wall boundary layer characteristics using microsurface grids as a liner for the nozzle wall. Results show that using those two techniques together has promising potentials regarding wider stable operation for swirl combustors, enabling them to burn a great variety of fuel blends safely