A research-scale solid-fuel ramjet test article has been developed to study the secondary combustion zone of solid fuel ramjets. Tests were performed at a constant core air mass flowrate of 0.77 kg/s with 0%, 15%, and 30% bypass ratios. The propulsive performance analysis results indicate that the 0% bypass case had the highest regression rate and fuel mass flowrate. The regression rate and fuel mass flowrate of fuel without carbon black was the lowest. The specific impulse with air mass flowrate included was highest for the 0% bypass case reaching 130 s and lowest for the 30% bypass case reaching 110 s. For specific impulse with air mass flowrate excluded, the 30% bypass case achieved 2,800 s while the 0% bypass case achieved 1,800 s. The characteristic velocity was greatest for 0% bypass reaching 1,025 m/s and lowest for 30% bypass reaching 900 m/s. The combustion efficiency was highest for the 15% bypass case with carbon black addition approaching 0.82. 50 kHz and 75 kHz CH* chemiluminescence imaging was performed. Analyzing thin slivers of the images over 40,001 frames with frequency-domain techniques showed that most of the high amplitude content occurred below 1-5kHz with small peaks near 20 kHz and 30 kHz. Dynamic mode decomposition (DMD) was performed on sets of 10,001 spatially-calibrated images and their corresponding uncalibrated, uncropped images. Most of the tests exhibited low-frequency axial pumping, transverse modes, and other mode shapes indicative of the secondary injection. The prominence of transverse and other jet-related modes over axial modes appeared to be related to increasing bypass ratio. High-frequency axial modes also appeared in a case thought to have high core-flow momentum that did not appear at these high frequencies for other cases. The DMD modes for 0% bypass were indiscernible due to high soot content. Most of the modes corresponding to the calibrated images also appeared in the uncalibrated images, however, with different mode amplitude rankings. PIV was performed at 5 kHz for one test at 15% bypass. The instantaneous vector fields for these tests displayed local velocities up to 600 m/s. The mean images showed velocities up to 250 m/s. The two-dimensional turbulent kinetic energies reached 200 m2/s2 in several regions throughout the flowfield. The turbulence intensity exceeded 0.20 near the bottom of the flowfield.

<p dir="ltr">Instantaneous fuel regression rate within a solid fuel ramjet combustor was characterized using X-ray radiography and ultrasonic transducer measurements. Experiments were performed with cylindrical, center-perforated hydroxyl-terminated polybutadiene (HTPB) fuel grains at three mass fluxes (407-561 kg/m2-s) with consistent inlet total temperatures and chamber pressures. Ultrasonic transducer measurements demonstrated changes of web thickness ranging from 7.50-9.85 mm and regression rate measurements ranging from 1.35-1.74 mm/s. Local maxima of change in web thickness due to flow reattachment and erosive burning were consistently measured with the ultrasonic transducers. Changes in port radius on the order of 8-9 mm and regression rates of approximately 1.25 mm/s were deduced from the X-ray radiography images. Structure of the flow reattachment region was evident in measurements from the X-ray radiography images captured near the combustor entrance while images captured at the mid-length of the combustor exhibited more uniform fuel regression profiles. Ultrasonic measurements of change in web thickness were consistently greater in magnitude relative to X-ray radiography measurements. X-ray radiography imaging allowed for the more accurate measurement of fuel regression with the greatest axial spatial resolution while ultrasonic transducer measurements yielded the greatest radial spatial resolution. The change in web thickness calculated with weight-based techniques yielded smaller magnitude measurements of change in web thickness relative to X-ray radiography.</p><p dir="ltr">Time-dependent measurements of web thickness and regression rate along the port of aluminum-loaded and boron carbide-loaded, hydroxyl-terminated polybutadiene (HTPB) fuel grains were measured in a solid fuel ramjet combustor with X-ray radiography. The combustor was operated at three mass flux conditions, ranging from 397-532 kg/m2-s, with consistent chamber pressures and upstream-of-combustor total temperatures of 1313 kPa and 748 K, respectively. A cross-correlation-based edge detection scheme was used to extract the fuel grain edges within X-ray radiography images collected at 15 Hz. Cross-section photographs of the post-combustion fuel grain surfaces exhibited evidence of flow reattachment and large aft-end regression. Aluminized fuel grains exhibited average weight-based regression rates of 1.29-1.48 mm/s, and boron carbide-loaded fuel grains yielded average regression rates of 1.21-1.38 mm/s. Head-end X-ray measurements of change in port radius indicated flow reattachment, particularly for the bottom (theta = 180) edge of the fuel grain. The absolute maximum of change in port radius, which ranged between 8.56-10.31 mm for aluminized fuel grains and 8.22-9.40 mm for boron carbide-containing fuel grains, did not always coincide with the flow reattachment location. Time-averaged regression rate profiles measured with X-ray radiography were relatively uniform along the port axis but smaller in magnitude compared to the weight-based measurements; 1.17-1.35 mm/s for the aluminum-loaded fuel grains and 1.07-1.24 mm/s for the boron carbide-loaded fuel grains. Pre-ignition fuel regression, on the order of 1.5 mm, was determined to be the cause of the over-prediction of regression rate by weight-based measurements compared to X-ray measurements.</p><p dir="ltr">The weight-based average regression rates measured in tests conducted with the axisymmetric solid fuel ramjet test article in its various configurations were compared to quantify the effects of average port air mass flux, bypass air addition, carbon black addition, and metal particle addition on regression rate. Baseline tests without an aft-mixing section or bypass air addition fuel grains containing carbon black yielded a regression rate coefficient of a = 5.33E-2 and an exponent of n = 0.50 for p4 = 1179-1298 kPa. Including an aft-mixing section without bypass air addition yielded regression rates of 0.94-1.04 mm/s due to the increased residence time. Bypass air addition of 14\% bypass ratio reduced the regression rate to 0.83-0.92 mm/s, and 30% bypass ratio reduced the regression rate to 0.80-0.82 mm/s. For otherwise equal tests, adding carbon black to the fuel grain increased the regression rates from 0.76-0.78 mm/s to 0.83-0.92 mm/s (6-21%). Aluminized fuel grains exhibited an increase in regression rate coefficient over the baseline fuel grains from a = 5.33E-2 to a = 6.30E-2 (18%), but the regression rate exponent remained at n = 0.50. Boron carbide (B4C) addition reduced the regression rate exponent to n = 0.46 but increased the regression rate coefficient to a = 7.55E-2; a 42% increase.</p><p dir="ltr">A simplified solid fuel ramjet combustion model which includes (1) turbulent heat convection, (2) radiation, (3) radiation-coupled surface blowing, (4) unsteady sub-surface heat conduction, (5) solid fuel regression, (6) gas-phase combustion, and (7) fuel port hydrodynamics was developed for regression rate prediction over a range of combustor geometries and operating conditions. Turbulent convection was modeled with empirical correlations relating non-dimensional boundary layer transport numbers. Radiative heat transfer was estimated using modified empirical correlations for radiation in a slab hybrid rocket combustor. Hybrid rocket combustion theory was used to model surface blowing. The condensed-phase heat transfer was modeled by solving the unsteady, variable thermophysical property, regressing surface heat equation with an explicit time-integration, finite volume scheme on a non-uniform grid. A general Arrhenius expression was used to estimate the fuel regression rate. Chemical equilibrium calculations for a stoichiometric HTPB/air diffusion flame were used to model the gas-phase combustion. The port gas dynamics were modeled with compressible flow ordinary differential equations. The results of these individual physical processes were examined in detail for a high mass flux (G_air = 561 kg/m2-s) case. Experiments performed in the axisymmetric solid fuel ramjet combustor were simulated in the model, which yielded a lower regression rate versus mass flux exponent of n = 0.39 compared to the experimentally-obtained n = 0.50. A larger parameter sweep of the model yielded a mass flux exponent of n_1 = 0.30, a pressure exponent of n_2 = 0.04, and an inflow total temperature exponent of n_3 = 0.39. These exponents are less than those observed in other works, but the model successfully captured the relative influence of mass flux, chamber pressure, and inflow total temperature.</p><p dir="ltr">A combustion diagnostic consisting of X-ray radiography and thermocouples embedded within the fuel grain was successfully applied and demonstrated in a solid fuel ramjet slab combustor. One representative test condition with an air mass flowrate of 1 kg/s, an upstream-of-combustor static pressure of 560 kPa, and an upstream-of-combustor total temperature of 639 K was examined. Changes in web thickness of approximately 4 mm and steady-state regression rates of 0.35 mm/s were measured at the thermocouple locations. Condensed-phase temperature measurements yielded fuel grain surface temperatures of 820 K and temperature profiles which were compared to theoretical Michelson profiles. The Michelson profile closely matched the thermocouple-measured temperature profile at one axial location. Sub-surface conductive heat fluxes of 0.35 MW/m2, heat fluxes required to vaporize solid fuel of 0.60 MW/m2$, and surface heat fluxes of 0.95 MW/m2$ were estimated using the condensed-phase temperature profiles.</p>

Basic experimental studies of the injection of liquid fuel into a two dimensional flowfield designed to represent a sudden-expansion "dump" combustor were performed under cold-flow conditions. Test conditions were as follows: 0.6 entrance Mach number, 25 PSIA total pressure, and nominally 75°F stagnation temperature. Two step heights were investigated, 1.0 in. and 0.5 in., corresponding to area ratios of 1.33 and 1.17. The investigation included Pitot and static pressure distributions, spark and streak shadowgraphs, surface flow visualization, direct photographs and videotape recordings. The backlighted streak and spark shadowgraphs were used to obtain jet penetration and break-up information. Oil drop surface flow studies showed details of the flow in the recirculation region behind the step. The injectant for these cold flow studies was selected as water, which was injected transversely to the air flow 1.0 in. and 0.5 in. upstream of the step at various flow rates. It was found that both the location of the injection port relative to the step and the step height had no measurable effect on jet penetration and break-up. Injectant accumulation on the combustor wall in the base-flow region was found to be substantial under some conditions, and the amount of accumulation was shown to be a strong function of initial liquid jet penetration height. / M.S.

La conception d'un statoréacteur bénificie aujourd'hui des progrès divers des outils numériques permettant par la même occasion d'alléger les différentes étapes préliminaires de tests en géométrie réelle nécessaires au développement de telle configuration. L'objectif de cette thèse est de développer une méthodologie s'appuyant sur la Simulation aux Grandes Echelles (SGE) afin de contribuer à la validation de ce nouvel outil numérique pour la simulation de statoréacteur et ainsi de contribuer à la compréhension des phénomènes mis en jeu dans ces chambres de combustion. L'outil numérique est tout d'abord adapté pour la simulation des écoulements réactifs fortement turbulents avec un accent mis sur la gestion des chocs avec des schémas centrés et la discrétisation de la convection pour la simulation Eulérienne de la phase dispersée. La configuration cible est le "Statoréacteur de Recherche'' étudié expérimentalement par l'ONERA. Sa simulation est réalisée de manière graduelle. Tout d'abord, il est montré que la simulation de la totalité de la configuration, y compris les diffuseurs d'entrée où se positionne un réseau de choc, est essentielle afin de considérer une géométrie acoustiquement close pour reproduire correctement les modes d'oscillation du statoréacteur. La pertinence du schéma cinétique est aussi étudiée et il est montré l'importance de bien reproduire l'évolution de la vitesse de flamme adiabatique pour une plage de richesse grande, en raison du régime de combustion partiellement prémélangé. Finalement, trois cas à richesse différente sont simulés et un excellent accord est trouvé avec l'expérience. La phénoménologie et les mécanismes pilotant la combustion sont alors étudiés pour ces trois cas. / Design of ramjets benefits today from the progress of numerical tools which relieve the various test stages of real engines that remain necessary for the development of such a kind of configuration. The objective of this dissertation is to develop a methodology based on the Large Eddy Simulation (LES) to contribute to the validation of this new type of advanced numerical tool for the simulation of ramjets and improve the understanding of combustion in these devices. The numerical tool is first adapted for the simulation of highly turbulent reacting flows with emphases on the management of shocks with centered schemes and the discretization of convection for the Eulerian simulation of the dispersed phase. The target configuration is the “Research ramjet” experimentally studied by ONERA. Simulation is carried out gradually. First, it is shown that the simulation of the entire configuration, including diffusers at the inlets where shocks appear is essential to consider an acoustically close geometry to properly reproduce the oscillation modes of combustion. The relevance of the kinetic scheme is also studied. It is shown that reproducing the evolution of the adiabatic flame speed for a wide range of equivalence ratio is critical because of the partially premixed combustion regime involved in this configuration. Finally, three different cases are simulated and excellent agreement is found with experimental data. The phenomenology and the different mechanisms governing combustion are studied for these three cases.

In this thesis, a simple moving boundary technique has been suggested, implemented and verified. The technique may be considered as a generalization of the well-known "ghost" cell approach for boundary condition implementation. According to the proposed idea, the moving body does not appear on the computational grid and is allowed to move over the grid. The impermeable wall boundary condition is enforced by assigning proper gasdynamic values at the grid nodes located inside the moving body close to its boundaries (ghost nodes). The reflection principle taking into account the velocity of the boundaries assigns values at the ghost nodes. The new method does not impose any particular restrictions on the geometry, deformation and law of motion of the moving body. / The developed technique is rather general and can be used with virtually any finite-volume or finite-difference scheme, since the modifications of the schemes themselves are not required. In the present study the proposed technique has been incorporated into a one-dimensional non-adaptive Euler code and a two-dimensional locally adaptive unstructured Euler code. / It is shown that the new approach is conservative with the order of approximation near the moving boundaries. To reduce the conservation error, it is beneficial to use the method in conjunction with local grid adaptation. / The technique is verified for a number of one and two dimensional test cases with analytical solutions. It is applied to the problem of supersonic inlet starting via variable geometry approach. At first, a classical starting technique of changing exit area by a moving wedge is numerically simulated. Then, the feasibility of some novel ideas such as a collapsing frontal body and "tractor-rocket" are explored.

[pt] Essa dissertação realiza uma análise do escoamento reativo e turbulento no interior do combustor de um estato reator a combustível sólido. Investiga-se diferentes modelos para prever a pirólise do combustível sólido. O modelo matemático é baseado na solução numérica das equações de conservação de massa, quantidade de movimento linear, energia e equações de transporte para quantidades escalares. O modelo de turbulência empregado é o (constante de Von Kármán -taxa de dissipação da energia cinética turbulenta) para altos Reynolds e na modelagem da combustão emprega-se o formalismo da fração de mistura/função densidade de probabilidade prescrita. Próximo às paredes, a lei da parede é usada, sendo a camada limite dividida em duas regiões, uma subcamada laminar e uma região totalmente turbulenta. As transferências de calor e massa para as paredes são calculadas utilizando-se a lei da parede modificada com uso de um parâmetro de transferência de massa. Os resultados obtidos através do modelo proposto foram comparados com os resultados obtidos anteriormente com outros modelos e com dados experimentais, verificando-se que os mesmos apresentam um boa concordância com os dados existentes na literatura, concluindo-se que o modelo é satisfatório para o problema proposto. / [en] This dissertation presents an analysis of reactive and turbulent flow field in a solid fuel ramjet combustor. The ability of different models to predict solid fuel pyrolysis is investigated. The mathematical model is based on the numerical solution of the conservation equations of mass, momentum, energy and transport equations for scalar quantities. The energia cinética turbulenta - taxa de dissipação da energia cinética turbulenta for high Reynolds turbulence model is employed and the combustion is modeled with the mixture fraction/prescribed probability density function formalism. Close to the walls the law-of-the-wall is specified, with the boundary layer divided into two regions, a viscous sublayer and a fully turbulent region. Heat and mass transfer at the walls are calculated using a modified law-of-the-wall based on a blowing parameter. The results obtained using the proposed model were compared with other earlier models predictions and with empirical data. It was verified that the results are in good agreement with literature data, allowing to conclude that the model presented is suitable for the prediction of the mas s transfer and flow field in a solid fuel ramjet combustor.

No description available.

<p dir="ltr">This dissertation describes efforts in the development of an optically-accessible solid fuel ramjet combustion experiment and the application, and requisite modifications, of multiple laser-based diagnostics. These measurements target the characterization of the complex turbulent reacting flow physics in a multi-phase combustion environment representative of conditions within a solid fuel ramjet.</p><p dir="ltr"><br>First, dynamic flow-flame interactions were investigated in an optically-accessible solid fuel ramjet combustor. Experiments were performed with a single hydroxyl-terminated polybutadiene fuel slab located downstream of a backward-facing step in a rectangular chamber. To emulate flight-relevant combustor conditions, unvitiated heated air was directed through the combustion chamber with an inlet temperature of ∼655 K, chamber pressures of 450–690 kPa, and port Reynolds number of ∼500,000. 20 kHz OH∗-chemiluminescence and 10 kHz particle imaging velocimetry measurements were used to characterize the heat-release distribution and velocity field. Comparison between the mean OH∗ chemiluminescence images acquired at three flow conditions indicates reduction in flame height above the grain with increasing air mass flow rate. Dominant heat-release coherent structures in the statistically stationary flow are identified using the spectral proper orthogonal decomposition technique implemented on time-series of instantaneous images. The spatial mode shapes of the chemiluminescence and velocity field measurements indicated that the flow-flame interactions were dominated by vortex shedding generated at the backward facing step in the combustor, at Strouhal numbers of 0.06 – 0.10.</p><p dir="ltr"><br>Following this effort, a coherent anti-Stokes Raman scattering (CARS) laser system was assembled and aligned for measurements of the Q-branch ro-vibrational energy level structure of nitrogen using a coannular phase-matching scheme and frequency-shifted probe beam. These measurements were demonstrated in the model SFRJ combustion chamber operated with an inlet air temperature of 690 K and pressure of 0.59 MPa. Over 300 single-shot spectra were collected and fit for temperatures ranging from the core air flow to the combustion gases with a probe location situated above the redeveloping boundary layer region diffusion flame. A skewed temperature distribution was reported at the probe location, as expected from a region only intermittently exposed to hot combustion gases. Temperatures of 500-2000 K were fit to theory, indicating a requirement for high dynamic range measurements.</p><p dir="ltr"><br>A handful of shortcomings were identified in the application of the shifted-CARS technique to the luminous SFRJ flow-field and thus modifications were made to the CARS system for improved dynamic range, signal-to-noise ratio and signal-to-interference ratio. A dual-pump system provided simultaneous measurements of the Q-branch ro-vibrational energy level structure of nitrogen and pure-rotational energy level structure of nitrogen and oxygen. These spectra possessed ample features for accurate comparison to theory at temperatures of 600-2500 K, a typical range at flame locations within the highly dynamic SFRJ reacting flow. Additionally, an electro-optical shutter (EOS), comprised of a Pockels cell located between crossed-axis polarizers, was integrated into the CARS system. The use of the EOS enabled thermometry measurements in high luminosity flames through significant reduction of the background resulting from broadband flame emission. Temporal gating ≤100 nanoseconds along with an extinction ratio >10,000:1 was achieved using the EOS. Integration of the EOS enabled the use of an unintensified CCD camera for signal detection, improving upon the signal-to-noise ratio achievable with inherently noisy microchannel plate intensification processes, previously employed for short temporal gating.<br></p><p dir="ltr">Using this system, temperature and relative oxygen concentration scalar fields were measured in an optically accessible solid fuel ramjet (SFRJ) combustion chamber using coherent anti-Stokes Raman scattering (CARS). Additionally, planar laser-induced fluorescence measurements of the hydroxyl radical (OH-PLIF) were performed to spatially characterize flame location and provide context to the temperature measurements. The combustion chamber was operated with an inlet air temperature of 670 K, mass flowrate of 1.14 kg/s, and pressure of 0.57 MPa, conditions relevant to practical device operation. The dual-pump CARS system provided simultaneous measurements of the Q-branch ro-vibrational energy level structure of nitrogen and pure-rotational energy level structure of nitrogen and oxygen. These spectra possessed ample features for accurate comparison to theory at temperatures of 600-2500 K, a typical range at flame locations within the highly dynamic SFRJ reacting flow<br>and inherently track the relative oxygen concentration within the measurement volume. A skewed temperature distribution was reported at various probe locations, as expected from stochastic probing of dynamic reacting vortex structures. Comparison between CARS and OH-PLIF measurements within the flow impingement region indicated that the high temperature regions closely align with regions of high OH-PLIF intensity while the temperature standard deviation better matches the flame-surface density. The signal intensity distribution within instantaneous OH-PLIF images indicates transport of combustion products toward the grain, supported by the near-wall peak temperatures. This process is critical for the transport of energy to the grain such that additional fuel can be volatilized and mix with the air to support the flame.</p><p dir="ltr"><br>Finally, an ultra-fast CARS system has been designed and aligned for 1 kHz one-dimensional measurements of temperature by targeting the ro-vibrational Q-branch transitions of nitrogen. This effort seeks to develop a technique that can capture the hydrodynamics that drive the combustion in SFRJ and provide an intuition for the energy transport near the solid fuel wall of the SFRJ combustor through capturing instantaneous temperature profiles. The designed system utilized a 9 W high-energy regenerative amplifier with 30 fs duration pulses.<br>For the CARS measurement, the 4 W 800 nm output from the external compressor is used as the Stokes beam and a 0.5 W, 675 nm ouput from the TOPAS optical parametric amplifier (OPA) split to and used as the pump and probe beams. A chirping rod placed in the beam path of the probe beam was used to generate the picosecond pulse. Preliminary measurements have been acquired within room air and a laminar H2-Air nonpremixed flame. A discussion of the experimental challenges and remaining work is presented in this document.</p>

In the present work, two phase reacting flow in a single cavity Trapped Vortex Combustor (TVC) is studied at atmospheric conditions. KIVA-3V, numerical program for simulating three dimensional compressible reacting flows with sprays using Lagrangian-Drop Eulerian-fluid procedure is used. The stochastic discrete droplet model is used for simulating the liquid spray. In each computational cell, it is assumed that the volume occupied by the liquid phase is very small. But this assumption of very low liquid volume fraction in a computational cell is violated in the region close to the injection nozzle. This introduces grid dependence in predictions of liquid phase in the region close to the nozzle in droplet collision algorithm, and in momentum coupling between the liquid and the gas phase. Improvements are identified to reduce grid dependence of these algorithms and corresponding changes are made in the standard KIVA-3V models. Pressure swirl injector which produces hollow cone spray is used in the current study along with kerosene as the liquid fuel. Modifications needed for modelling pressure swirl atomiser are implemented. The Taylor Analogy Breakup (TAB) model, the standard model for predicting secondary breakup is improved with modifications required for low pressure injectors. The pressure swirl injector model along with the improvements is validated using experimental data for kerosene spray from the literature. Simulations of two phase reacting flow in a single cavity TVC are performed and the temperature distribution within the combustor is studied. In order to identify an optimum configuration with liquid fuel combustion, the following parameters related to fuel and air such as cavity fuel injection location, cavity air injection location, Sauter Mean Diameter (SMD) of injected fuel droplets, velocity of the fuel injected are studied in detail in order to understand the effect of these parameters on combustion characteristics of a single cavity TVC.