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Nanduri, Jagannath RamchandraA COMPUTATIONAL STUDY OF THE STRUCTURE, STABILITY, DYNAMICS, AND RESPONSE OF LOW STRETCH DIFFUSION FLAME
Doctor of Philosophy, Case Western Reserve University, 2006, Fluid and Thermal Engineering Science
Diffusional thermal instabilities occur because of the imbalance between the diffusion of heat and the diffusion of species into the reaction zone. While the phenomenon of diffusional thermal instability (DTI) has been recorded in detail in the high stretch extinction regime of diffusion flames, fundamental understanding of the DTI near the low stretch radiation induced extinction regime is lacking. Low stretch flames are relevant to the fire safety characteristics in many practical engineering fields. In the current study we investigate the phenomena of DTI for low stretch diffusion flames with radiative heat loss and map the one-dimensional and two-dimensional flame structure, stability and dynamics. Low stretch nonpremixed combustion including radiative heat loss is modeled using a planar counterflow configuration (PCC) and an axisymmetric counterflow configuration (ACC) to highlight the effects of flow configuration on low stretch flame instabilities. It is found that the 1D low stretch diffusion flames in both ACC and PCC systems initially lose their stability to pulsations which lead to oscillatory extinction for stretch rates much higher than the 1D steady state extinction limit. The effects of the Lewis number and reaction rate on this 1D oscillatory instability is also presented along with a fast Fourier transform (FFT) analysis to map the frequency and amplitude characteristics of the oscillatory instability. The stability of the 2D flame is mapped using different initial profiles. The 2D low-stretch diffusion flame in both ACC and PCC systems was found to lose its stability to wavy flames very close to the 1D neutral stability point subsequently leading to steady/unsteady stationary/traveling cellular flames for PCC system and unsteady cellular flames for ACC system. These multi-dimensional flame phenomena are shown to extend the dynamic extinction limits predicted by the 1D model. Comparisons between the ACC system and the PCC system also show the effects of flow configuration on multi-dimensional flame phenomenon for low-stretch radiative diffusion flames. A 2D bifurcation of the flame solution close to the 1D low-stretch radiation induced extinction limit is identified.

Committee:

Chih-Jen Sung (Advisor)

Keywords:

Diffusion Flame Stability; Diffusional Thermal Instability; Flame Oscillations; Flame Radiation Interaction; Edge Flames; Cellular Flames

Wessel, Richard AllenSpectral element method for numerical simulation of unsteady laminar diffusion flames
Doctor of Philosophy, Case Western Reserve University, 1993, Mechanical Engineering
A numerical method was developed for simulation of unsteady flow with combustion. The spectral element method is used to solve the Navier-Stokes equations for conservation of mass, momentum, energy and species. The method by Korczak and Patera (1986) was extended to include variable transport properties and Arrhenius rate chemical reaction. A variable density formulation was developed, but was found to be numerically unstable using fractional time splitting and an implicit formulation of the pressure solution to satisfy the mass continuity constraint. Therefore, the choice of constant density flow was made for practical reasons rather than physical. Time accurate simulations of a diffusion flame were conducted for initially separate co-flowing streams of fuel and oxidant in a two-dimensional duct. Flow regimes and transitional states for a laminar attached flame, unsteady attached flame and blow-off were investigated. An increase in air velocity has the effect of making the flame shorter. Eventually the wake type flow becomes unstable and begins to oscillate. As vortices alternately form and shed from the fuel channel, the flow stretches the flame between eddies and compresses the flame within the eddies. Increasing fuel velocity effects fl ame detachment and subsequent blowoff as the streamwise velocity exceeds the laminar flame speed. Results are in qualitative agreement with experimental observations in the literature.

Committee:

James T'ien (Advisor)

Keywords:

Spectral element method numerical simulation unsteady laminar diffusion flames

O'Neil, Alanna R.Chemiluminescence and High Speed Imaging of Reacting Film Cooling Layers
Master of Science (M.S.), University of Dayton, 2011, Aerospace Engineering

The demand for more efficient and compact gas turbine engines has resulted in an increase in the operating temperatures and pressures and a decrease in combustor weight and size. These advances may result in incomplete combustion products entering the turbine section. The products can react with the air intended to cool the turbine vanes, and the resulting flame can cause damage to the engine. This study reports chemiluminescence measurements of flames and correlates these to heat release rate and the measured heat flux to a surface. To accomplish this, fuel rich combustion products are generated in a well-stirred reactor. The flow of products is directed over a flat plate with cooling air jets normal to the flow. Chemiluminescence data of the flames is obtained, along with high speed images, and temperature measurements of the flow inside the test section. Three film cooling geometries are studied: normal holes, fan shaped holes, and slot. Measurements are acquired at three equivalence ratios (1.3, 1.4, and 1.5) at three different blowing ratios (M = 1, 4, and 7).

It is found that the rate of heat release from the flame does not always trend the same as the heat transfer to the surface. It is also seen that a large reaction region does not always equate to high heat flux to the surface. If enough cooling air is present the surface is protected from the heat released from the flame.

Committee:

Dilip Ballal, PhD (Committee Chair); Scott Stouffer, PhD (Committee Member); David Blunck, PhD (Committee Member); Sukh Sidhu, PhD (Committee Member)

Subjects:

Aerospace Engineering

Keywords:

Film Cooling; Chemiluminescence; Reacting Boundary Layers; Flames; Image Analysis;

Cakmakci, ArdaSpatiotemporal Distribution of Soot Temperature for Fuel-Rich Flames under Unsteady Inlet Airflow Conditions
MS, University of Cincinnati, 2015, Engineering and Applied Science: Aerospace Engineering
Two pyrometric tools for measuring soot temperature response in flames under unsteady inlet airflow conditions and naturally-occurring combustion instabilities are developed. High-speed pyrometry using a high-speed color camera is used and its results are compared with those of global temperature measured using a multi-wavelength pyrometer. For the former, the pixel RGB values pertaining to respective bandwidths of red, green and blue filters are used to calculate temperature and for the latter, the emission from whole flame over narrow bandwidth (±5 nm) of wavelength centered at 660 nm, 730 nm and 800 nm is used to measure temperature. Detailed procedure for camera calibration and verification of accuracy are outlined. The combustor, running on Jet-A liquid fuel, achieves unsteady inlet airflow using a siren running at frequencies of 150 and 250 Hz and with modulation levels (RMS) 20-50% of mean velocity. Spatiotemporal response of flame temperature fluctuation measured by the high speed camera is presented by phase-averaged with average subtracted images and video fast Fourier transform at dominant forcing frequencies. The results of global temperature and temperature fluctuation show reasonable agreement between the 3-color pyrometer and high speed camera. Temperature discrepancies between the three measured temperatures of the 3-color pyrometer are within 200 K, with same correlation to global temperatures from high speed camera. While average global temperature remains steady under changes in forcing frequency and inlet air velocity fluctuation frequency, the temperature fluctuation is primarily shown to increase with increase in modulation levels of mean air velocity.

Committee:

Jongguen Lee, Ph.D. (Committee Chair); Peter Disimile, Ph.D. (Committee Member); Awatef Hamed, Ph.D. (Committee Member)

Subjects:

Aerospace Materials

Keywords:

Pyrometry;Combustion Dynamics;Fuel-rich flames;Soot temperature;Diagnostics