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Numerical Investigation of Turbulent Spray Combustion in Hot Diluted Co-Flow

Karimi Motaalegh Mahalegi, Hamed | 2020

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 53878 (45)
  4. University: Sharif University of Technology
  5. Department: Aerospace Engineering
  6. Advisor(s): Mardani, Amir
  7. Abstract:
  8. The Moderate or Intense Low-oxygen Dilution (MILD) combustion of liquid fuels has attracted attention to use its advantages in industrial burners and gas turbines applications. Here numerical investigation has been conducted on a research experimental MILD turbulent spray burner (Deft Spray in Hot Co-flow, DSHC). RANS approach has been adopted for modeling of the reactive turbulent flow field of continues phase with the Lagrangian approach for droplet modeling of pressure-swirl atomizer fuel spray. The EDC combustion model is used which has the ability to take into account the detailed chemical mechanisms. The accuracy of different turbulence and droplet injection sub-models are examined and the spray flame in atmospheric air co-flow and different conditions of preheat and oxygen dilution have been simulated. In the results section, various effective phenomena in the combustion flow field and flame structure are investigated. One of the main goals of this study is to survey the MILD combustion field from the chemical kinetic aspect which is achieved by dividing the reactive flow field into five distinct zones and perfectly stirred reactors were used to extract the ethanol reaction pathways in each zone. Comparing the different reactive zones relieved that the high co-flow temperature and distributed heat release can strengthen the endothermic direct decomposition route of ethanol to ethylene and weaken the exothermic production of C2H5O isomer radicals which are dominant in ordinary combustion. Furthermore, stable intermediates like ethylene, acetaldehyde, and methane accumulate at fuel-rich and moderately high-temperature conditions of the internally reacting region due to the not existence of loss routes in the chemical pathways. In addition, the structure of the MILD spray flame was compared to the non-MILD spray flame. Using the definition of the flame index, it was shown that the triple-flame structure in the conventional condition which consists of two outer and middle diffusion branches, and an inner premixed branch converts to a double-flame flame structure in MILD condition including an outer diffusion and an inner premixed branches. In another step, sensitivity analysis relative to the two parameters affecting the structure of the combustion field, temperature and oxygen concentration of the co-flow stream was performed independently (which was not possible in the experiment cases). The parameter of ignition delay time was used for the selection of parametric study cases. This study showed that increasing or decreasing each of the parameters of temperature and oxygen concentration didn’t improve all the effective aspects toward achieving the MILD condition and the effective parameters should be changed according to a specific and more precise goal. Another part of the study is dedicated to the investigation of the effect of dilution of liquid fuel (ethanol) on Mild combustion. Results indicated that a low-level fuel dilution up to 5 percent has tendency toward expansion of MILD region, but higher degrees fuel dilution within the range of 5–20 percent results in a reduction of MILD combustion regions due to domination of physical effect contribution of addition of water to ethanol fuel compared to its chemical effect. Finally, the spray structure and the flame-spray interaction are investigated. It was conceived from spray structure study that in flame with atmospheric co-flow the evaporation of droplets is controlled by molecular diffusion however in flame with preheated and diluted co-flow evaporation is dominated by convection of flow rather than diffusion
  9. Keywords:
  10. Moderate or Intense Low Axygen Dilution (MILD) ; Chemical Kinetic ; Ethanol ; Eddy Dissipation Concept (EDC)Model ; Turbulent Spray

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