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.

Swirl combustors have proven to be effective flame stabilisers over a wide range of operation conditions thanks to the formation of well-known swirl coherent structures. However, their employment for lean premixed combustion modes while introducing alternative fuels such as high hydrogenated blends results in many combustion instabilities. Under these conditions, flame flashback is considered one of the major instability problems that have the potential of causing considerable damage to combustion systems hardware in addition to the significant increase in pollutant levels. Combustion Induced Vortex Breakdown is considered a very particular mode of flashback instability in swirling flows as this type of flashback occurs even when the fresh mixture velocity is higher than the flame speed, a consequence of the interaction between swirl structures and swirl burner geometries. Improvements in burner geometries and manipulation of swirling flows can increase resistance against this type of flashback. However, increasing resistance against Combustion Induced Vortex Breakdown can lead to augmentation in the propensity of another flashback mechanism, Boundary Layer Flashback. Thus, this paper presents an experimental approach of a combination of techniques that increase Combustion Induced Vortex Breakdown resistance, i.e. by repositioning a central injector and using central air injection, while simultaneously avoiding Boundary Layer Flashback, i.e. by changing the wall boundary layer characteristics using microsurfaces on the nozzle wall. Results show that using these techniques together has promising potentials regarding wider stable operation for swirl combustors, enabling them to burn a broader variety of fuel blends safely, while informing developers of the improvements obtained with the combined techniques.

No abstract

The natural heat transfer in ducts depend on generating an induced flow due to the difference in density between the heated fluid and it is environment. The effect of reduce cross sectional area of a vertical isothermal duct by adding half circular obstacles to the inner side of the duct. The effect of the induced flow and obstacles were study using COMSOL software and compared to heat through same duct with no flow. The results that obstacles increase induced flow by 16% compared to normal vertical duct, Also shows a reduction in value of the maximum temperature of the hot surface at the solar peak heat flux (3:00 PM).