As the global interest in environmental problems is constantly raising, decreasing NOx emissions have became an important issue. Therefore, combustions with higher efficiency and lower NOx emissions are considered as the primary objective of the gas turbine industry. In order to meet the requirements, the flow behavior inside the gas turbine combustors should be precisely investigated. The flow characteristics in the combustors are dominated by the swirling flow with the ability to enhance the combustion features, such as spray atomization, air/fuel mixing ratio and flame stabilization.
In this paper, the swirling flow motion was investigated by applying the counter-rotating swirler to generate the swirling flow in the model combustor, and the effect of the design and working parameters such as vane thickness of the swirler, aspect ratio of the combustor and Reynolds number on the swirling flow were investigated experimentally.
A 2D Particle Image Velocimetry (PIV) system was used to acquire images of swirling flow in the model combustor. Pure water was used as the working fluid and the polyamid seeding particles (PSP), which have sufficient ability to follow the fluid, were added to the working fluid and circulated around in the entire experimental setup. A CCD camera captured the images of the flow while Nd:YAG laser illuminated the seeded particles through optical windows placed on the front and left side of the model combustor.
The model combustor was simplified as a rectangular shape, replicated from an annular type combustor, constructed from transparent acrylic plates. The swirler is placed in the center of the model combustor, and the flare is attached at the end of the swirler, and bolted with flare holder. The experiments were carried out with 5 swirlers which have different vane thicknesses. All of the swirlers have the same geometrical parameters, except only the vane thickness changes from 1.0 to 2.7 mm. To examine the effect of Reynolds number, it was varied from 1.7×104 to 3.3×104, corresponding the air pressure drop of the actual gas turbine combustor. The Reynolds numbers of water was derived using velocity calculated from similarity between air and water. The effect of aspect ratio of the rectangular shaped model combustor was also examined. The aspect ratios were 2.81, 2.43 and 2.14, corresponding to a chamber width 64 mm, 74 mm and 84 mm, respectively. Contours of iso-velocity, axial and radial velocity distributions were provided to explain swirling flow behavior. Structure of swirling flow was investigated from vertical section views at various positions.
Basic structure of the swirling flow was obtained from 2D PIV images. The central toroidal recirculation zone (CTRZ), the corner recirculation zone (CRZ), and the wall flow and downstream eddies were detected. In the confined system, the size of wall flow or corner recirculation zone may results in the enlargement or shrinking of the central recirculation zone. The increase of Reynolds number expanded the wall flow longer and thicker, leading to the thinner CTRZ. The increase of vane thickness showed similar trend to the decrease of Reynolds number. Higher average velocity of CTRZ and longer wall flow were observed with the lower combustor aspect ratio. The swirling flow behavior in the rectangular duct through a comparison of vertical section planes before and after impinging point was investigated.