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 agreement. Introduction Most of the global energy generation processes are based on combustion. Thus, the sector has faced real challenges due to requirements to decrease CO2 emissions concurrent with low NOx and other pollutants [1,2]. Emissions regulation and energy efficiency are of high interest due to the associated issues of environmental pollution and energy consumption, respectively. Optimum design and improvement of combustors is a step towards the answer to those considerations. Currently, industrial combustors aim to operate under lean premixed conditions to reduce harmful emissions due to its potential of low NOx production and enhance combustion performance. Moreover, using syngas instead of traditional fuels [3,4] or developing systems for hydrogen blends [5,6] are among the main means of solving this type of challenges. However, the reliability of lean premixed systems is complex as they operate close to the lean stability limit and are more sensitive to combustion instabilities. From a design point of view, it is critical to recognise and predict such instabilities [1,2]. Swirling flows have been used for many decades in many areas of engineering applications. The most significant improvements to the gas turbine combustion system are represented by using swirl combustors due to their flame stabilisation capabilities over a wide range of equivalence ratios thanks to the formation of coherent structures. Their high level of swirl creates vortex breakdown phenomena that lead to the appearance of the central recirculation zone (CRZ) and a vortex with an offcentre core known as the precessing vortex core (PVC) [7]. Vortex breakdown phenomena enhance the high turbulence and shear associated with this kind of burners. These features induce very powerful mixing and therefore improve combustion efficiency by ensuring that all unburned fuel molecules have plenty of oxygen molecules floating around them. The recirculation recycles hot combustion gases to the incoming air hence aiding ignition [8]. Furthermore, wide flame stability limits can be acquired allowing the system to burn of different fuel [8–12]. However, such combustors are frequently subjected to various combustion instabilities upstream the flame, producing phenomena such as flashback propagation from the stable flame position in the combustion chamber towards the premixing zone [2,7]. Consequently, flashback can hinder stable operation, produce a critical damage in the burners, increase the maintenance cost and push up the level of pollutants. One mechanism of flashback propagation is through the Combustion Induced Vortex Breakdown CIVB, which is considered a fast-acting flashback mechanism that appears in swirl burners because of the formation of the CRZ [13]. This type of flashback receives special attention amongst other flashback mechanisms since it is one of the prevailing mechanisms in swirl combustors and represents an obstacle in developing combustion systems, especially those fed by high flame speed fuels such as high hydrogen blends [13,15]. Another mechanism that increases flashback trends is the appearance of highly turbulent combustion zones caused by fuel properties, flow turbulence and other barely understood phenomena [16,17]. These instabilities occur even when the incombustible mixture velocity is higher than the flame speed. Thus, they can have dramatic consequences when high turbulent flame speed fuels such as those based on highly hydrogenated blends are used [18,19]. Many techniques can effectively tackle CIVB and anchor CRZ downstream the burner nozzle either by doing some geometrical modifications or by raising flow field patterns. Firstly, using diffusive fuel injection to mitigate the flashback mechanism, many researchers found that the strong and coherent axial jet can effectively push downstream the vortex breakdown, consequently eliminating the possibility of CIVB [13,20]. Secondly, using bluff bodies as stabilisers to the jet and swirling flow is another option. However, despite the vitality of this flame stabilisation technique, it cannot totally mitigate flame flashback. Moreover, using central fuel injector