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Evaluation of three-dimensional velocity field, mixing field, and flame-front in a model gas turbine combustor

Periodic Reporting for period 1 - 3DFlameGT (Evaluation of three-dimensional velocity field, mixing field, and flame-front in a model gas turbinecombustor)

Reporting period: 2018-03-19 to 2020-03-18

The project focuses on quantifying the instantaneous, three-dimensional mixing process between a central jet flow and a swirling annular coflow. Such flow configuration is commonly used to supply the fuel and the air in gas turbine combustors for aviation and power generation, since it facilitates rapid mixing that assists flame stabilization, flexible operation. Mixing is also crucial to the reduction of harmful pollutants, such as CO, particulates and NOx emissions. The present project uniquely quantified experimentally the physics of the three dimensional turbulent mixing process in swirling flows. The deduced understanding will assist the gas turbine combustion community to design new fuel-efficient and low pollution combustion technologies, which consequently will have a positive impact on health and the environment. The overall objective of the project is to develop a new laser-based experimental method that allows the measurement of instantaneous, three-dimensional flow with high resolution that allows the detection of the mixing process down to molecular scale. This new dual-plane laser-induced fluorescence technique was applied to a model burner geometry flow and quantified the physics of the mixing process down to the smallest scale. It is found that there is an optimum laser light-sheet separation distance for which the air/fuel mixing rate can be measured accurately. The unique results provide unique understanding that allows the development and evaluation of new computational models for energy efficient and environmentally clean combustor design.
Since most industrial flows are turbulent, it is necessary to have physical understanding of the flow and mixing process in three dimensions. In general, experiments are limited to two-dimensional planar imaging of the flow characteristics. This project developed a dual-plane imaging method to obtain quantitative information of the additional out-of-plane dimension of the mixing process, in addition to the planar information. Planar Laser-Induced fluorescence (PLIF) from acetone tracer molecule is utilized to mark the mixing of a central jet fuel stream with surrounding coaxial swirl coflow of air. The length and time scales of air/fuel mixing are characterized by the scalar dissipation rate (SDR). The quantification of the SDR requires the instantaneous measurement of the spatial gradients of fuel concentration in all three directions. This information is obtained using a dual-plane acetone-PLIF technique. In addition to the optical set-up, new data processing approaches are proposed to de-noise the PLIF image and to obtain thickness (length scale) of dissipation layers in the flow. Finally, the effect of flow swirl intensity on the mixing process is quantified. The dissemination of this work is in progress. There are three manuscripts in preparation, which focus on a) Development of the new experimental technique and corresponding image processing strategy, b) Fluid mechanics of turbulent mixing, and c) Effect of flow swirl number on turbulent mixing.
The project provides, for the first time, scalar dissipation rate (SDR) measurements in swirling flows relevant to combustors, which account for the three-dimensionality of the turbulence. New imaging methodology and image processing approaches are used to ensure the accuracy of the SDR deduction. Generally, light-sheet separation distance is not measured on an instantaneous basis in multi-sheet laser measurements. The research fellowship developed an experimental approach to obtain shot-to-shot separation distance between dual-planes laser sheets. The developed tool is used to examine the effect of swirl on the turbulent mixing process. Manuscripts detailing the main findings are in preparation. The outcome of this work will help the gas turbine combustion community to understand better the physics of air/fuel mixing and quantify the associated time- and length- scales. Subsequently, the knowledge will contribute to the improvement of computational models used for the design of low pollution gas turbine combustors for aviation and power generation sectors, and thus positively impact society.
Burner and Dual-plane PLIF. L: Laser, BS: beam splitter, FP: fluorescent plate, and C: Camera
Fuel mole fraction and two-component scalar dissipation rate