Ph.D. Proposal
Manya Subbaramaiah
(Advisor: Prof. Adam Steinberg)
"RAMAN SCATTERING MEASUREMENTS TO INVESTIGATE LEAN, TURBULENT PREMIXED FLAMES AT ELEVATED PRESSURE"
Wednesday, June 11
10:00 am
Guggenheim Building 246
Abstract
In turbulent premixed combustion, understanding the evolution of scalar fields describing the thermo-chemical structure of the flame is vital. This work presents, for the first time, high speed planar Raman scattering measurements in reacting flows, which enables detailed analysis of the flame dynamics through relevant species-specific scalar fields. These measurements are used to investigate bluff-body stabilized turbulent premixed flames at pressure up to 40 bar.
Although there has been extensive research on turbulent premixed flames, there remain many unanswered questions due to the complex nature of the interactions between chemical processes and fluid dynamics across a wide range of length and time scales. Specifically, there are significant gaps in understanding the effect of pressure on turbulence-chemistry interaction and key structural aspects of the flame such as flame brush thickness, curvature and stretch effects. Although many practical combustors operate at elevated pressure, most experimental characterizations of turbulent flames in literature are limited to near atmospheric conditions, making experimental data at practical conditions valuable. Another aspect of premixed flames that needs further investigation are the mechanisms that lead to lean blowoff. Previous studies in bluff-body stabilized flames have proposed entrainment of cold reactants through the local extinction sites into the recirculation zone as an important factor leading to blowoff, but validation and quantification has not been achieved. Furthermore, computational models, a cost effective alternative to iterate across combustor designs, have difficulty predicting the mechanisms that lead to lean blowoff or completely capture pressure effects. Consequently, experimental data are required to quantify the driving mechanisms leading to LBO and understand the influence of pressure on the turbulent flame structure.
This work will explore the following research questions: (1) How does elevated pressure impact the thermo-chemical structure and turbulence-chemistry interaction in turbulent premixed flames? (2) Are local flame extinction and reactant entrainment into the recirculation zone precursors to LBO and can their quantification be used characterize blowoff phenomenon? High speed, 1D and 2D Raman scattering measurements of simultaneous, species-specific scalar information will be used to characterize the flame structure at elevated pressure. In addition to parameters such as the flame brush thickness and surface density, these measurements will also be used to compute multi-species scalar gradient correlations and cross-scalar statistics. Also, the spatial and temporal evolution of burnt and unburnt gases can also be used to identify local extinction sites and quantify reactant entrainment into the recirculation zone. This work provides the first quantitative chemical 2D scalar field data in turbulent premixed flames, which is valuable for simulation validation.
Preliminary work has explored bluff-body stabilized premixed turbulent flames operated between 20-40 bar for equivalence ratios ranging from 0.75 to 0.92. In addition to the Raman scattering measurements, velocity fields have been obtained using Particle Image Velocimetry. The Raman scattering measurements have indicated the occurrence of local extinction sites and reaction entrainment into the recirculation zone. Additionally, the measurements also provide insights into the scalar gradients between the burnt and unburnt gases at various pressures.
The proposed work seeks to analyze the experimental data that has been acquired to answer the questions discussed above. The Raman scattering measurements will be used to characterize parameters such as local temperature, local equivalence ratio, flame thickness, curvature, species-specific cross scalar statistics and reactant entrainment into the recirculation zone. This work will provide multi-species, quantitative experimental scalar data to further understand fundamental phenomenon pertinent to turbulent premixed flames.
Committee
• Prof. Adam Steinberg (advisor)
• Prof. Jerry Seitzman
• Prof. Ellen Yi Chen Mazumdar