Joint numerical and experimental study of thermoacoustic instabilities

From small scale energy systems such as domestic boilers up to rocket motors, combustion chambers are often prone to combustion instabilities. These instabilities stem from the coupling of unsteady heat release rate and acoustic waves. This coupling is two sided: flame front perturbations generate acoustic waves while acoustic waves impinging on flame holders can disturb flames attached on them. Important pressure and velocity oscillations can be reached during unstable regimes, that can alter its efficiency or even damage the entire combustion chamber. One major challenge is to understand, predict, and prevent from these combustion instabilities. The objectives of this thesis are twofold: (1) take into account acoustic dissipation and (2)analyze flame/acoustic coupling to obtain Reduced Order Model (ROM) for combustion instabilities. This work is divided into three parts. First, the concept of ROM that gives the acoustic modes of a combustion chamber is introduced. This modeling strategy is based on the acoustic network theory and may take into account flame/acoustic coupling as well as acoustic dissipation. An efficient numerical algorithm dedicated to solve ROMs was designed on purpose and validated on several academical configurations. Second, an experimental rig was commissioned to study mean and acoustic pressure losses across a diaphragm and two swirl injectors. Results show that these two phenomena are linked and can be simply incorporated into ROMs. Finally, flame/acoustic coupling is investigated by using both direct numerical simulations and experiments: a lean premixed V-shaped laminar flame is anchored on a cylindrical bluff-body and we show that its temperature greatly influences the flame mean shape as well as its dynamics

Institutional Intervention in DAT Preparation Among Undergraduate Pre-Dental Students

The objectives of this project were to study the Dental Admission Test (DAT) preparation experience of undergraduate pre-dental students (UPDS) at the University of South Carolina (USC) and to determine actionable steps to raise DAT scores, increase diversity among pre-dental students, reduce financial burdens associated with the DAT, and improve wellbeing in this population. A quantitative survey was distributed to characterize and create figures about UPDS demographics, use of advising services, preparation experience, and support for university- sponsored preparation resources. Ten phenomenological interviews were conducted to gain insights about the needs of UPDS. Thematic analysis was performed to identify unifying themes and unique occurrences of phenomena. I mostly captured the experiences of White female UPDS. Stress resonated among all UPDS surveyed and interviewed, and concerns about cost impacted student planning. UPDS enumerated many resources USC could provide, including financial assistance, guidance from the USC Office of Pre-Professional Advising (OPPA), and a network of pre-dental and dental students. Respondents overall desired more support on behalf of USC. UPDS desired a wide array of resources to feel more supported by USC during their preparation process. The USC OPPA should concentrate its efforts on better equipping UPDS with the knowledge and resources vital to succeed on the DAT. With the resources I created, I intend to catalyze this change with seven measures I identified as most urgent, such as furnishing students with information about the DAT and connecting UPDS with dental students. Further research is necessary to determine actions to better support underrepresented communities and describe the experiences of other UPDS

On the experimental determination of growth and damping rates for combustion instabilities

This paper presents four experimental methods for the evaluation of growth rates of combustion insta- bilities. A systematic investigation is conducted on a laminar slot burner with five operating points (two stable and three unstable). The accuracy of the methods is assessed by cross comparison and the use of three different flow variables as input: velocity, pressure and heat release rate fluctuations. Finally, the experimental determinations of the growth rates are compared to the prediction of a low-order acoustic model fed with a Flame Transfer Function

Accounting for Acoustic Damping in a Helmholtz Solver

Thermoacoustic Helmholtz solvers provide a cheap and efficient way of predicting combustion instabilities. However, because they rely on the inviscid Euler equations at zero Mach number, they cannot properly describe the regions where aerodynamics may interact with acoustic waves, in the vicinity of dilution holes and injectors, for example. A methodology is presented to incorporate the effect of non-purely acoustic mechanisms into a three- dimensional thermoacoustic Helmholtz solver. The zones where these mechanisms are important are modeled as two-port acoustic elements, and the corresponding matrices, which notably contain the dissipative effects due to acoustic–hydrodynamic interactions, are used as internal boundary conditions in the Helmholtz solver. The rest of the flow domain, where dissipation is negligible, is solved by the classical Helmholtz equation. With this method, the changes in eigenfrequency and eigenmode structure introduced by the acoustic–hydrodynamic effects are captured, while keeping the simplicity and efficiency of the Helmholtz solver. The methodology is successfully applied on an academic configuration, first with a simple diaphragm, then with an industrial swirler, with matrices measured from experiments and large-eddy simulation

Regeneration of Raney®-Nickel Catalyst for the Synthesis of High-Value Amino-Ester Renewable Monomers

Aiming to synthesize high-value renewable monomers for the preparation of renewable specialty polyamides, we designed a new protocol. Amino-esters, produced via the hydrogenation of unsaturated nitrile-esters, are alternative monomers for the production of these polymers. A high monomer yield can be obtained using a Raney®-nickel catalyst despite the drawback of fast deactivation. The hydrogenation of 10-cyano-9-decenoate (UNE11) was tentatively reactivated by three different regeneration procedures: solvent wash, regeneration under hydrogen, and regeneration under sonication. Among these procedures, the in-pot catalyst regeneration (H2 30 bar, 150 °C) demonstrated complete activity recovery and full recycling

Joint experimental and numerical study of the influence of flame holder temperature on the stabilization of a laminar methane flame on a cylinder

The mechanisms controlling laminar flame anchoring on a cylindrical bluff-body are investigated using DNS and experiments. Two configurations are examined: water-cooled and uncooled steel cylinders. Comparisons between experimental measurements and DNS show good agreement for the flame root locations in the two configurations. In the cooled case, the flame holder is maintained at about 300 K and the flame is stabilized in the wake of the cylinder, in the recirculation zone formed by the products of combustion. In the uncooled case, the bluff-body reaches a steady temperature of about 700 K in both experiment and DNS and the flame is stabilized closer to it. The fully coupled DNS of the flame and the temperature field in the bluff-body also shows that capturing the correct radiative heat transfer from the bluff-body is a key ingredient to reproduce experimental results

Influence of flame-holder temperature on the acoustic flame transfer functions of a laminar flame

The occurrence of combustion instabilities in high-performance engines such as gas turbines is often affected by the thermal state of the engine. For example, strong bursts of pressure fluctuations may occur at cold start for operating conditions that are stable once the engine reaches thermal equilibrium. This observation raises the question of the influence of material temperature on the response of flames to acoustic perturbations. In this study, we assess the influence of the temperature of the flame holder for a laminar flame. Both experiments and numerical simulations show that the Flame Transfer Function (FTF) is strongly affected by the flame-holder temperature. The key factors driving the evolution of the FTF are the flame-root location as well as the modification of the flow, which affects its stability. In the case of the cooled flame-holder, the formation of a recirculation zone is identified as the main impact on the FT

Experimental X-ray characterization of Gekko-XII laser propagation through very low-density aerogels (2–5 mg/cc) creating multi-keV photons from a titanium solid foil

Describes measurements of laser propagations through low density aerogels