An experimental study was conducted to perform an analysis of the effect of the geometric modifications of the venturi on the non-reactive and reactive flow behavior using a counter-rotating radial-radial swirler. In the non-reactive flow tests, measurements were taken in a central vertical plane and horizontal (cross-sectional) plane at the exit of the swirler, using a High-Speed, Two Dimensional, Particle Image Velocimetry (2D PIV) system. The size of the swirler used in the non-reactive flow tests is a 4.76X scaled size of the swirler used in combustion. The 4.76X swirler models were tested in air flow seeded with olive oil at Re = 51,500, corresponding to the pressure drop across the 1X swirler models of 4% of atmospheric pressure at ambient conditions. Compared with the 1X swirler models, the 4.76X swirler models provide high spatial and temporal resolutions from the enhanced visibility of the flow characteristics and lower velocities at the same Re. Four swirler configurations of high swirl number (SN ≈ 1.0) were used, with no modification for the baseline configuration (configuration 1), and with the chevrons on the venturi for the straight chevrons configuration (configuration 2). The design of the inclined venturi was used for the converging venturi configuration (configuration 3), and chevrons were added on the converging venturi for the converging chevrons configuration (configuration 4). In the combustion tests, the 1X swirler models were tested using 478K preheated air at 4% pressure drop across the swirler, and different chamber lengths. Measurements were conducted using a regular camera to capture the flame image, and dynamic pressure transducers to obtain the acoustic pressure oscillations. Four configurations were studied and compared in the non-reactive and reactive flows with the objective of understanding the mechanisms responsible in reducing the extent of the combustion instabilities. Results of this study show that the converging venturi in configuration 3 appears to be the best design in eliminating the combustion instabilities in the fuel-lean region as compared to the other configurations. This indicates that the prevention of the frequencies coupling between the heat release rate and acoustic oscillations has been achieved by using the design of the converging venturi.
An experimental investigation was conducted to study the effect of chevrons on the dynamic behavior of the swirling flow generated by a counter-rotating radial-radial swirler. 3X models of a low swirl number swirler (SN ≈ 0.6) were used to achieve lower velocities for the same Reynolds number (Re) and enhanced visibility of the flow characteristics by enabling high spatial and temporal resolutions. Three swirler configurations were used, including the baseline with no chevrons. Configuration 2 features chevrons on the trailing edge of the primary swirler, and configuration 3 has chevrons on the trailing edge of both primary and secondary swirlers. The swirlers were tested in water flow at Reynolds number (Re) = 51,500 which corresponds to the typical operational pressure drop of 4% of atmospheric pressure for the corresponding 1X model of the swirler at ambient conditions. Water testing was used since it allows additional slowing down of the flow dynamic features so that they can be captured and analyzed. Measurements were conducted in a vertical plane passing through the swirler centerline, and two horizontal (cross-sectional) planes using a High-Speed, Two Dimensional, Particle Image Velocimetry (2D PIV) system to obtain the mean, turbulent and dynamic behavior of the flow. Results of this study introduce the concept of chevrons on swirlers as a promising approach to change the flow dynamic behavior and thus, affect combustion dynamics. The results show that the presence of chevrons break down the region of high modal energy into several smaller regions. However, configuration 2 has few regions of the highest modal energy among the configurations, whereas the modal energy values for configurations 3 has the lowest magnitudes. Thus, the secondary chevrons in configuration 3 play an important role to eliminate these high-energy local spots as well as meet the requirement to break down the large scale structures.
This work reports on flow behaviors near the wall and secondary flow structures in a circular-to-rectangular transition duct at Reynolds numbers of 10*. Flow visualization experiments were conducted under the conditions of uniform and swirling inlet flows.As found, flow structures observed at the rectangular exit under the swirling inlet condition appear very differently from those observed under the uniform inlet condition. In the former case the secondary flows are developed due to the detachments of the swirling flow from the contoured wall, while in the latter case the secondary flows are known due to the mechanism of meanflow straining in the corner regions.
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