Browsing by Subject "Ablation"
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Item Ablation and ignition by impinging jet flows(2013-12) Kurzawski, Andrew Joseph; Ezekoye, Ofodike A.Two separate heat transfer problems that involve jet flows impinging on a reacting target are studied through modeling and experimentation. The first system is an ablating carbon-carbon specimen exposed to high heat fluxes from an oxy-acetylene torch which has applications in atmospheric re-entry vehicles. The second system involves the penetration of hot gases into the void space in a compartment. The fire protection stands to benefit from knowledge of this system, both in building component design and informing firefighting personnel. Both problems can be modeled as a jet flow impinging on a flat surface where hot gases from the jet lead to primarily convective heat transfer. Ablation experiments are outlined and a theoretical framework is developed. A serial inversion technique is tested for predicting the recession rate observed in the experiments. A novel inversion technique that takes advantage of parallel computing is developed to circumvent the shortcomings of the serial technique. These techniques are then compared to synthetically generated and experimental data for different data streams and error signals. Compartment-scale experiments were conducted to test hot gas penetration into void spaces. Anecdotal evidence was observed outside of the intended test section prompting further investigation into the mechanics of ignition in void spaces. A theoretical framework is established to predict possibility of ignition under varied environmental factors. A leakage-scale experiment is constructed to gain insight into conditions that result in ignition of materials in void spaces.Item Characterization of ablative properties of thermoplastic polyurethane elastomer nanocomposites(2010-12) Lee, Jason Chi-Sing, 1983-; Howell, John R.; Koo, Joseph H.; Ezekoye, Ofodike A.; Taleff, Eric M.; Paul, Donald R.The advancement of each component of aerospace vehicles is necessary as the continual demand for more aggressive missions are created. Improvements in propulsion and guidance system electronics are invaluable; however without material development to protect the vehicle from its environment those advances will not have a practical application. Thermal protection systems (TPS) are required in both external applications; for example on reentry vehicles, as well as in internal applications; to protect the casing of rockets and missiles. This dissertation focuses on a specific type of internal solid rocket motor TPS, ablatives. Ablatives have been used for decades on aerospace vehicles. To protect the motor from the hostile environment, these materials pyrolyze and char. Both of these mechanisms produce a boundary between the combustion gases and the motor as well as release the heat that the decomposed material has absorbed. These sacrificial materials are intended to protect the casing that it is attached to. With the development of polymer nanocomposites (PNCs) in the last couple of decades, it is of interest to see how these two fields can merge. Three different nanomaterials (carbon nanofibers, multiwall carbon nanotubes, and nanoclays) are examined to observe how each behaves in environments that simulate the motor firing conditions. These nanomaterials are individually added to a thermoplastic polyurethane elastomer (TPU) at different loadings, creating three distinct families of polymer nanocomposites. To describe a materials ablative performance, a number of material properties must be individually studied; such as thermal, density, porosity, char strength, and rheology. Different experiments are conducted to isolate specific ablative processes in order to identify how each nanomaterial affects the ablative performance. This dissertation first describes each material and the ablative processes which are characterized by each experiment. Then basic material properties of each family of materials are described. Degradation and flammability experiments then describe the degassing processes. Studies of the material char are then performed after full blown rocket experiments are done. These tests have shown that of the three nanomaterials, nanoclay enhances the TPU ablative performance the most while the CNF provides the least enhancement.Item An electrostatic approach for producing nanoparticulate membranes using laser ablation of microparticle aerosols(2011-08) Davis, Claire Elisabeth; Kovar, Desiderio; Becker, Michael F.The Laser Ablation of Microparticle Aerosols (LAMA) process produces nanoparticles by ablating microparticles that are entrained in an aerosol. Two of the main advantages of this process are that the particles produced are charged (preventing agglomeration) and bare (without a capping layer). Two different techniques are possible to collect the nanoparticles. In this work, the charged state of the particles formed was utilized to collect them electrostatically. This approach has the additional advantage that particles can be selected according to their size. The focus here was a particular application for gas separation. The nanoparticles produced were directly collected in a polymeric liquid, which was then irradiated with ultraviolet light to form a rubbery film. These membranes were tested for olefin/paraffin gas separation, a challenge that finds many applications, notably in the petroleum industry.Item Investigation and modeling of coupled thermochemical and thermomechanical erosion in thermally degrading systems(2012-05) Barr, Benjamin Witten; Ezekoye, Ofodike A.; Rodin, Gregory J.The coupled effects of thermochemical and thermomechanical erosion are investigated. A quasi-steady ablation model with finite rate surface chemistry is developed and applied to a solid carbon combustion scenario to investigate the system’s behavior in situations in which surface reactions are not in equilibrium. It is found that in this regime, the system can be described effectively in terms of the B number and the Damkohler number, and a useful algebraic relationship between these parameters is determined for nonequilibrium behavior. The thermochemical ablation model is then expanded by considering mechanical removal of thermochemically weakened material from the ablating surface. A model is developed for a randomly oriented carbon fiber preform material, like that used in the production of phenolic impregnated carbon ablator (PICA), and this model is incorporated into the previously developed ablation code. It is found that for PICA in realistic reentry scenarios, the removal of individual fibers from the ablating surface by mechanical erosion is not an important mass loss mechanism, although hypothetical situations exist where this mechanism for mechanical removal of material is non-negligible. The thermo-chemo-mechanical erosion mechanism is then extended to address brand generation in wildland fire scenarios. A model is developed to predict the size and number distribution of embers generated from a tree with fractal geometry. This model is coupled to a simple plume and propagation model similar to those existing in the literature, and a case study is performed for a realistic wildfire scenario. The presence of an optimal branch diameter for brand propagation is identified, and areas for future work in thermo-chemo-mechanical degradation are discussed.Item Oxide-metal nanoparticles using laser ablation of microparticle aerosols(2009-08) Nahar, Manuj; Kovar, Desiderio; Becker, Miachel F.We have studied a continuous aerosol process for producing oxide nanoparticles with sizes of 10-60 nm that are decorated with smaller 1-3 nm metallic nanoparticles. Such particles may be useful in a number of areas including catalysis and as contrast enhancement agents in biomarkers. To produce the oxide nanoparticle carriers, an aerosol of 1-10 [micrometer] oxide particles are ablated using an excimer laser. The resulting oxide nanoparticle aerosol is then mixed with 1-2 [micrometer] metallic particles and this mixed aerosol is ablated a second time. The oxide nanoparticles are too small to ablate but act as seeds for the nucleation of metallic nanoparticles on the surface of the oxide. The nanoparticle sizes can be varied by changing the gas type or gas pressure in the aerosol. We demonstrate the feasibility of such an approach using two oxides, SiO₂ and TiO₂, and two metals, Au and Ag.Item Photoacoustic image guidance and tissue characterization in cardiovascular applications(2016-12) Dana, Nicholas Pacheco; Suggs, Laura J.; Emelianov, Stanislav Y.; Dunn, Andrew; Tunnell, James; Bouchard, RichardCollectively, cardiovascular diseases continue to be the leading cause of death, across nations and across decades. Improved diagnostic imaging methods offer promise to alleviate the morbidity associated with these diseases. Photoacoustic (PA) imaging is one such method, poised to make a significant impact on cardiovascular imaging, both as a research tool, as well as a clinical imaging modality. Offering the potential of molecular imaging in real-time, PA methods have been demonstrated in proof-of-concept studies tracking myocyte calcium dynamics. These results open the door to non-invasive longitudinal assessment of cardiac electrophysiological function, with implications for drug and contrast agent development. PA image guidance has also been extended to the characterization of cardiac radiofrequency ablation lesions. This method has been demonstrated to utilize endogenous chromophore changes resulting from ablation for the generation of depth-resolved tissue characterization maps, capable of assessing lesion extent. The technique has been subsequently validated by assessing high-intensity focused ultrasound ablation lesions in myocardium, with the hope for offering thermographic capabilities as well. While PA imaging offers such promise in cardiac ablation procedures, it is also in the process of clinical translation for image guidance and characterization in coronary artery disease applications. Research has shown, using Monte Carlo optical modeling, that using a simple dual-wavelength PA imaging technique has great potential for successful visualization of atherosclerotic plaques across multiple tissue types and at clinically relevant multiple millimeters of depth. Collectively these results offer a suite of PA imaging tools with the potential for molecular and thermographic imaging across a broad range of cardiovascular applications.Item Quantitative measurements of ablation-products transport in supersonic turbulent flows using planar laser-induced fluorescence(2015-08) Combs, Christopher Stanley; Clemens, Noel T.; Danehy, Paul M; Ezekoye, Ofodike A; Raja, Laxminarayan; Varghese, Philip LA recently-developed experimental technique based on the sublimation of naphthalene, which enables imaging of the dispersion of a passive scalar using planar laser-induced fluorescence (PLIF), is applied to a Mach 5 turbulent boundary layer and a NASA Orion capsule flowfield. To enable the quantification of naphthalene PLIF images, quantitative fluorescence and quenching measurements were made in a temperature- and pressure-regulated test cell. The test cell measurements were of the naphthalene fluorescence lifetime and integrated fluorescence signal over the temperature range of 100 K to 525 K and pressure range of 1 kPa to 40 kPa in air. These data enabled the calculation of naphthalene fluorescence yield and absorption cross section over the range of temperatures and pressures tested, which were then fit to simple functional forms for use in the calibration of the PLIF images. Quantitative naphthalene PLIF images in the Mach 5 boundary layer revealed large-scale naphthalene vapor structures that were regularly ejected out to wall distances of approximately y/δ = 0.6 for a field of view that spanned 3δ to 5δ downstream of the trailing edge of the naphthalene insert. The magnitude of the calculated naphthalene mole fraction in these structures at y/δ = 0.2 ranged from approximately 1-6% of the saturation mole fraction at the wind tunnel recovery temperature and static pressure. An uncertainty analysis showed that the uncertainty in the inferred naphthalene mole fraction measurements was ± 20%. Mean mole fraction profiles collected at different streamwise locations were normalized by the mole fraction measured at the wall and a characteristic height of the scalar boundary layer, causing the profiles to collapse into one “universal” mole fraction profile. Two-dimensional fields of naphthalene mole fraction were also obtained simultaneously with velocity by using particle image velocimetry (PIV) and PLIF. The images show large-scale naphthalene vapor structures that coincide with regions of relatively low streamwise velocity. The covariance of naphthalene mole fraction with velocity indicates that an ejection mechanism is transporting low-momentum, high-scalar-concentration fluid away from the wall, resulting in the protrusions of naphthalene vapor evident in the instantaneous PLIF images. Lastly, naphthalene PLIF was used to visualize the dispersion of gas-phase ablation products on a scaled Orion capsule model at four different angles of attack at Mach 5. High concentrations of scalar were imaged in the capsule recirculation region. Additionally, intermittent turbulent structures were visualized on the heat shield surface, particularly for the 12° and 52° AoA cases.Item RF/microwave absorbing nanoparticles and hyperthermia(2009-12) Cook, Jason Ray; Emelianov, Stanislav Y.; Pearce, John A.The primary purpose of this work was to evaluate the capability of nanoparticles to transform electromagnetic energy at microwave frequencies into therapeutic heating. Targeted nanoparticles, in conjunction with microwave irradiation, can increase the temperatures of the targeted area over the peripheral region. Therefore, to become clinically viable, microwave absorbing nanoparticles must first be identified, and a system to monitor the treatment must be developed. In this study, ultrasound temperature imaging was used to monitor the temperature of deep lying structures. First, a material-dependent quantity to correlate the temperature induced changes in ultrasound images (i.e. apparent time shifts) to differential temperatures was gathered for a tissue-mimicking phantom, porcine longissimus dorsi muscle, and porcine fat. Then microwave nanoabsorbers were identified using an infrared radiometer. The determined nanoabsorbers were then injected into ex-vivo porcine longissimus dorsi muscle tissue. Ultrasound imaging frames were gathered during microwave treatment of the inoculated tissue. Finally, the ultrasound frames were analyzed using the correlation between temperature and apparent shifts in ultrasound for porcine muscle tissue. The outcome was depth-resolved temperature profiles of the ex-vivo porcine muscle during treatment. The results of this study show that magnetite is a microwave nanoabsorber that increases the targeted temperature of microwave hyperthermia treatments. Overall, there is clinical potential to use microwave nanoabsorbers to increase the efficiency of microwave hyperthermia treatments.Item Technique for imaging ablation-products transported in high-speed boundary layers by using naphthalene planar laser-induced fluorescence(2010-08) Lochman, Bryan John; Clemens, Noel T.; Raman, VenkatramananA new technique is developed that uses planar laser-induced fluorescence (PLIF) imaging of sublimated naphthalene to image the transport of ablation products in a hypersonic boundary layer. The primary motivation for this work is to understand scalar transport in hypersonic boundary layers and to develop a database for validation of computational models. The naphthalene is molded into a rectangular insert that is mounted flush with the floor of a Mach 5 wind tunnel. The distribution of naphthalene in the boundary layer is imaged by using PLIF, where the laser excitation is at 266 nm and the fluorescence is collected in the range of 320 to 380 nm. To investigate the use of naphthalene PLIF as a quantitative diagnostic technique, a series of experiments is conducted to determine the linearity of the fluorescence signal with laser fluence, as well as the temperature and pressure dependencies of the signal. The naphthalene fluorescence at 297 K is determined to be linear for laser fluence that is less than about 200 J/m². The temperature dependence of the naphthalene fluorescence signal is found at atmospheric pressure over the temperature range of 297K to 525K. A monotonic increase in the fluorescence is observed with increasing temperature. Naphthalene fluorescence lifetime measurements were also made in pure-air and nitrogen environments at 300 K over the range 1 kPa to 40 kPa. The results in air show the expected Stern-Volmer behavior with decreasing lifetimes at increasing pressure, whereas nitrogen exhibits the opposite trend. Preliminary PLIF images of the sublimated naphthalene are acquired in a Mach 5 turbulent boundary layer. Relatively low signal-to-noise-ratio images were obtained at a stagnation temperature of 345 K, but much higher quality images were obtained at a stagnation temperature of 380 K. The initial results indicate that PLIF of sublimating naphthalene may be an effective tool for studying scalar transport in hypersonic flows.Item A toolkit for characterizing uncertainties in hypersonic flow-induced ablation(2010-12) Anzalone, Reed Anthony; Ezekoye, Ofodike A.; Upadhyay, Rochan R.A one-dimensional, quasi-steady ablation model with finite rate surface chemistry and frozen equilibrium pyrolysis gases is developed and discussed. This material response model is then coupled to a film-transfer boundary layer model to enable the computation of heat and mass transfer to and from the ablating surface. A shock model is outlined, as well, and all three components are then coupled together to form a stand-alone ablation code. The coupled models in the code are validated with respect to arcjet experiments, and comparisons are drawn between the ablation code and the unsteady ablation code Chaleur, as well as other computations for a graphite ablator in an arcjet. The coupled code is found to compare very well to both the experimental results and the other calculations. It is also found to have unique computational capabilities due to the use of finite-rate surface chemistry. Finally, uncertainty propagation using the quadrature method of moments (QMOM) is discussed. The method is applied to a number of simplified sample problems, for both univariate and multivariate scenarios. QMOM is then used to compute the uncertainty in an application of the coupled ablation code using a graphite ablator. The results of this study are discussed, and conclusions about the utility of the method as well as the properties of the ablation code are drawn.