A modelling and experimental study to reduce boundary layer flashback with microstructure

Trying to improve gas turbines to be flexible to use different fuels requires a big challenge for gas turbine designers when working with current operation stability issues. Flashback is the major challenge for low NOx premixed combustion of high hydrogen content fuels. Flashback in gas turbine combustors is usually assigned to four mechanisms; core flow flashback, combustion instability flashback, boundary layer flashback (BLF) and combustion induced vortex breakdown (CIVB). The last two mechanisms are most common in swirl combustors, which are used to have better operation stability and low emissions. Improvement of swirl combustors against CIVB has been studied extensively to improve the combustion stability. The most promising solutions of CIVB are limited because the improvement against CIVB worsens the system against boundary layer flashback. Boundary layer flashback is theoretically based on the Lewis von Elbe’s formula for laminar flame, with formulas also used in turbulent flames (with some reservation) by most recent studies. However, the majority of studies take the flame side of the formula, parameters such as fuel type and blends, pressure and preheat temperature, and try to improve the understanding of the boundary layer flashback hoping to find ways to reduce its onset. However, the effect of the burner nozzle has not been studied in many types of research, especially the internal nozzle surfaces. Therefore, this work aims to study the effect of regular surface roughness on the boundary layer flashback in a 150 kW tangential swirl burner. The first part of this study is a numerical simulation using the in-house code Hydr3D to simulate the flow over riblets with different geometries (blade, triangular, scallop, diamond, lotus and sharkskin). The numerical results demonstrate that the blade riblets were the best at reducing the boundary layer thickness and consequently showed the best drag reduction around 11% compared to the smooth surface while the sharkskin geometry was the worse in drag reduction with only 0.5%. vi Although the blade showed the best drag reduction, its weak structure and complex machine specifications make the scallop, lotus, and diamond and sharkskin riblet to be chosen for manufacturing on small discs. The scallop riblet on the nozzle was manufactured using wire electrical discharge machining (WEDM). The second part of the study was isothermal experimental tests for manufactured surfaces. The flow structure was measured using a 1D LDA. The results show that the riblets alter the flow structure near the wall and increase the velocity gradient which helps the flow to reach a velocity 0.99 from mainstream velocity at y+ 30 for a smooth surface. The third part of the study was obtained with combustion and isothermal experiments using two different stainless steel woven meshes that served as a liner for the nozzle burner. A 50μm and a 150μm wire diameter meshes were used. The isothermal test showed that the 150μm mesh denoted the best shift of the velocity gradient close to the wall. The combustion experiments showed that the two meshes help to improve system against the boundary layer with the 150μm being the best. Thus, it was demonstrated that BLF could be reduced using microsurfaces, which in conjunction with other techniques, have the potential of increasing optionality, an essential feature for fuel flexibility in Gas Turbines.