Fundamental Characterisation of Coherent Structures for Swirl Combustors

• Main Author: Alsaegh, Ali
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2022
• Subjects: Geometry; Mathematics

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.