Flame front structures studies of highly turbulent reacting flow over a backward facing step using KHz OH-CH planar laser induced fluorescence
In recent years, there has been a greater move in the scientific and political community to increase combustion efficiency and reduce emission levels; for this reason, it is crucial to gain a better understanding of combustion. Experimental work at relevant conditions and the data generated from these is needed in order to gain a better understanding of combustion; this data may also be used for validation and further refinement of computational models and simulations. The improvement of optical combustion diagnostics systems has opened new ways to explore these combustion processes. The testing of a high intensity turbulence combustion system is presented in this work. The primary research objective of the project hereby discussed, was to study the structures of backward-facing step stabilized premixed flames in high turbulence flow, with the aid of KHz-level optical laser diagnosis technology. Planar induced fluorescence experiments were conducted to visualize the flame front characteristics of the reacting flows under highly turbulent conditions over a backward facing step combustor. Air-methane premixed turbulent flames were studied at varying Reynolds numbers and flow turbulence intensity levels. An optically accessible combustion chamber was used to house premixed air-methane combustion at atmospheric pressure and stoichiometric conditions. OH-CH Planar Laser Induced Fluorescence (PLIF) was used to track the flame front location for the flames at the different Re and turbulence level conditions. Flows were tested at atmospheric pressure and temperature, and a constant equivalence ratio of 1 with the variation of flow length scales (LT) and turbulence velocity fluctuations (u’). Flow length scales were varied through the use of turbulence generating grids of blockage ratios of 46% and 63% and hole diameters of 1.5 and 3 mm. Velocity fluctuations were varied through the increase of bulk flow rate resulting in Reynolds numbers, based on step height, ranging from 15000, 32000, and 64000. Turbulence parameters were measured at the combustor step, using Particle Image Velocimetry. These parameters including velocity fluctuations (u’) and integral flow length scales (LT) along with estimated flame parameters resulted in flames that are located well within the thin reaction zones of the Peters-modified Borghi diagram for premixed turbulent combustion. In summary, flames with 0.8 < Da < 34, 0.8 < Ka < 95, 3 < u’/SL < 72, and u’ values ranging from 1-27 m/s were tested. Grid and Re effects on flame front characteristics were studied. A 10 kHz tunable PLIF system was used to map the flame front characteristics with a high temporal resolution and a spatial resolution of ~100 um/pixel. The high temporal resolution enabled the detection of the flame front characteristics in steps as low as 0.1 ms. OH-PLIF images revealed highly turbulent flames under the flow conditions, which were more noticeable with increasing Reynolds numbers. In general, flame fronts exhibited pockets of burned gases within the unburned gases, and vice versa. These burned gas pockets were seen to go through different processes including development on their own, separation from the main flame front core or branches of it, and eventual reattachment to the flame front. CH-PLIF at 5kHz was attempted by optimizing the dye laser system for detection of the CH C-X band at 314 nm. The CH detection was difficult due to low laser pulse power, UV light scattering, and other effects, which affected the CH signal quality. OH-PLIF images were used to estimate burning velocity ratios ST /SL and results for ST/SL ratios vs u’/SL are presented. These showed that the ratio of s T/sL increases with increasing u’/sL ratios.
Acosta-Zamora, Arturo, "Flame front structures studies of highly turbulent reacting flow over a backward facing step using KHz OH-CH planar laser induced fluorescence" (2016). ETD Collection for University of Texas, El Paso. AAI10251418.