Articles
Ph.D. Thesis
Ms.C. Thesis
J. Vargas, B. Lopez, and M. Lino da Silva, "Heavy Particle Impact Vibrational Excitation and Dissociation Processes in CO2"J. Phys. Chem. A., 2021.
A heavy particle impact vibrational excitation and dissociation model for CO2 is presented. This state-to-state model is based on the forced harmonic oscillator (FHO) theory, which is more accurate than current state-of-the-art kinetic models of CO2 based on first-order perturbation theory. The first excited triplet state 3B2 of CO2, including its vibrational structure, is considered in our model, and a more consistent approach to CO2 dissociation is also proposed. The model is benchmarked against a few academic zero-dimensional (0D) cases and compared to decomposition time measurements in a shock tube. Our model is shown to have reasonable predictive capabilities, and the CO2 + O ↔ CO + O2 reaction is found to have a key influence on the dissociation dynamics of CO2 shocked flows, warranting further theoretical studies. We conclude this study with a discussion on the theoretical improvements that are still required for a more consistent analysis of the vibrational/dissociation dynamics of CO2.
J. Vargas, B. Lopez, and M. Lino da Silva, "CDSDv: A Compact Database for the Modeling of High-Temperature CO2 Radiation", JQSRT 245, 106848, 2020.
The Carbon Dioxide Spectral Databank 4000 is tailored for radiative transfer applications relevant for Mars atmospheric entry studies. This is carried out through the refitting of the original database, encompassing individual rovibrational transitions, into a more compact form where rotational transitions for specific vibrational bands are obtained through traditional polynomial expressions, fitted to the levels and transitions of the original database. This originates a certain loss of precision since the fitted expressions do not always reproduce the original data with full accuracy, namely for the perturbed transitions. This is offset by a more compact database suited for wideband radiative transfer simulations. This CDSDv database also provides some minor advantages, such as enabling the thermodynamic use of two-temperature (T,Tv) models and the determination of vibrational Einstein coefficients Av'v" which may be used in state-to-state kinetic models.
M. Lino da Silva et al., "Qualification of the European Shock-Tube for High Enthalpy Research", AIAA paper 2020-0624, AIAA Scitech 2020 Forum, 2020.
We discuss the design, manufacturing, and qualification of the ESTHER shock-tube and its associated instrumentation. ESTHER is a new generation shock-tube funded by the European Space Agency and developed by an international consortium led by the Institute of Plasmas and Nuclear Fusion, an associated laboratory from the Instituto Superior Técnico of Lisbon. This facility aims at improving the European predictive capabilities for Spacecraft planetary entries, providing support for the next generation European exploration endeavours. The main drivers for this facility design have been (in no particular order) performance, repeatability, and cleanliness. These are enforced through an innovative design encompassing a laser-ignited, H2/He/O2 combustion driver capable of reaching pressures up to 600bar, and strict tolerances for the shock-tube interior surface state. A significant effort has also been put into setting-up state-of-the-art diagnostics. Besides the traditional streak-camera/spectrometer setups for carrying emission spectroscopy in the visible range, an additional VUV capable streak-camera/spectrometer setup is under development, complemented by a fast camera/MWIR spectrometer setup. ESTHER will be capable of carrying optical measurements in the extended 150nm–4.5μm range. Finally an in-house developed interferometer will be deployed for providing time-resolved electron density measurements.
L. Fernandes, B. Lopez, and M. Lino da Silva, "Computational Fluid Radiative Dynamics of the Galileo Jupiter Entry", Phys. Fluids 31, 106104 (2019); doi: 10.1063/1.5115264.
On December 7th, 1995, the Galileo descent probe entered Jupiter’s atmosphere at a relative velocity of 47.4 km/s. Flight data revealed an unforeseen recession profile: while the stagnation region had been significantly oversized, the shoulder almost completely ablated. In an attempt to understand why numerical predictions diverge from the flight data, several sensitivity studies were performed at the 180 km altitude point. The inaccuracy of the Wilke/Blottner/Eucken model at temperatures above 5000 K was confirmed. When applied to Galileo’s entry, it predicts a narrower shock with higher peak temperatures compared to the Gupta/Yos model. The effects of He and H2 line-by-line radiation were studied. Inclusion of these systems increased radiative heating by 9% at the stagnation point, even when precursor heating is unaccounted for. Otherwise, the internal excitation of H2 due to absorption of radiation originating from the highly emitting shock layer promotes H2 emission before dissociation occurs at the shock, yielding 196% higher radiative heat fluxes. This emphasizes the importance of H2 radiation not only on the recession experienced by Galileo but also for future entries in gas giants. Accordingly, thermal nonequilibrium resulted in 25% lower radiative heating when compared to an equilibrium solution, contrary to previous investigations that neglected H2. Ablation products absorption was shown to counteract the increased emission due to precursor heating of H2. However, the ablation layer temperature must be accurately predicted using a material-response code coupled to the flowfield since radiative heating has been shown to significantly depend on this energy-exchange interaction. Finally, the tangent-slab and ray-tracing models agreed to within 12%.
Ricardo Ferreira, "Development and Qualification of the European Shock Tube for High-Enthalpy Research", Ph.D. Thesis, 24th Apr. 2024
The European Shock Tube for High-Enthalpy Research (ESTHER) is a state-of-the-art shock tube facility capable of reproducing the plasma surrounding a spacecraft crossing the upper layers of planetary atmospheres at hypersonic speeds. ESTHER is a two-stage shock tube, divided into four main sections separated by metallic diaphragms. Its driver is a high-pressure combustion chamber operating a hydrogen-oxygen diluted mixture, ignited by a high-power Nd:YAG laser, whit filling pressures ranging from 10 to 100 bar. Two operational mixtures were considered to generate the shock waves. The first is an [He:H2:O2] 8:2:1.2-1.4 mixture tailored for high-speed entries. The second, a [N2:H2:O2] 10:2:1.4 mixture, is used for slower shocks. The theoretical performance map of ESTHER was computed, showing it is capable of generating shock waves between 4 and 14 km/s in air. A two-stage vacuum pumping system (comprised of a primary and two turbomolecular pumps )generates a high-vacuum level in the test section. The setup yields a leakage rate of 3.94x10-5 mbar Ls-1 and ultimate pressure of 9.8x10-7 mbar, meaning a sub-1% contamination rate of the test-gas can be achieved. ESTHER is equipped with an optical emission UV-VUV spectrometer and streak camera setup with a 0.5 nm resolution on a 1,200 gr/mm grating. A mid-IR spectroscopy setup was also designed to cover the 1-5 μm spectral region, and a pre-selection of the appropriate equipment was carried out. A modular-designed interferometer is finally used to probe the electron density behind the shock. It may also work as a reflectometer, and probe different plasma densities by switching the equipment front-end section.
Joao Vargas, "High-Temperature Non-Equilibrium CO2 Kinetic and Radiative Processes", Ph.D. Thesis, 13th Nov. 2020
CO2 kinetic and radiative vibrational state-to-state processes (STS) are explored in this work. Better insights into these processes may lead to more efficient spacecraft design for Mars atmospheric entries, and better understanding on the dynamics of physical-chemical processes for CO2 plasma sources. The kinetic models developed in this work include an extension of the Forced Harmonic Oscillator (FHO) theory to the collisional dynamics of triatomic molecules. The FHO does not suffer from many shortcomings such as First Order Perturbation Theories such as SSH, while remaining computationally affordable, making it a very attractive theory at contemporary level. Other improvements to the state of the art include a better accounting of the dissociation pathways and the modelling of the excited 3B2 state. Other important chemical processes in CO2 gases and plasmas were accounted for, and in particular the reaction CO2 + O ↔ CO + O2 was studied extensively, since it is found to significantly contribute to the decomposition of CO2 and may as well play a key role in recombination processes through the inverse CO + O2 reaction. Strong evidence of CO2 dissociation being a two-step process through this Zeldovich reaction is found, either from the bibliography research, and from the state-specific kinetic simulations carried out in this work. The obtained model was compared to available experimental shock-tube data, outperforming macroscopic chemistry models, and reproducing the observed experimental trends of decreasing characteristic dissociation times, while showing a reasonable agreement absolutevalues comparisons between experimental/synthetic decomposition times. Contemporary radiative databases for carbon dioxide are not tailored for full spectrum calculations necessary for atmospheric entry spacecraft design. To this end, the well-known rovibrationally specific CDSD4000 database for CO2 was refitted, using well known spectroscopic polynomial expressions, to obtain a vibrationally specific database. The presented method may be applied to databases for any arbitrary linear polyatomic molecule, yielding vibrationally specific data with the correspondingly compact data size, more performance friendly than detailed databases such as CDSD. The refitting procedure introduces some loss of detail since fitted polynomials cannot reproduce data with full accuracy. Once alternative methods for computing broadening effects are accounted for, the refitted database, dubbed CDSDv, is able to reasonably reproduce calculated and experimental spectra found in literature, particularly in the 4.3 μm spectral region the most significant contributor to the radiative features of CO2 Infrared radiation. In the last chapter of this work, several venues for improvement of both the kinetic and radiative models are discussed at length.
Mário Lino da Silva, "Simulation des propriétés radiatives du plasma entourant un véhicule traversant une atmosphère planétaire a vitesse hypersonique - Application à la planète Mars", Ph.D. Thesis, 4th Dec. 2004
L'entrée d'un véhicule spatial dans une atmosphère planétaire provoque la formation d'une onde de choc hypersonique puis d'un plasma à haute température et à basse pression. Ce plasma entourant le véhicule rayonne fortement sur une large gamme spectrale et le dimensionnement des protections thermiques du véhicule demande de quantifier ce rayonnement avec précision. L'étude présentée analyse le rayonnement émis par un plasma d'entrée atmosphérique sur Mars. Un code de calcul raie-par-raie a été développé pour simuler le rayonnement d'un plasma de type Martien, et ses propriétés radiatives à l'équilibre ont été simulées. Des mesures du rayonnement d'un plasma de type Martien ont ensuite été réalisées dans des moyens d'essai complémentaires. Les principaux systèmes moléculaires émissifs ont été identifiés et un modèle théorique d'excitation de ces systèmes est proposé. L'étude est complétée par une analyse numérique et expérimentale dans le moyen d'essai supersonique SR5.
Beatriz Oliveira, "High-Pressure He/H2/O2 Mixtures Combustion on the ESTHER Driver: Experiment and Modeling", Ms.C. Thesis, 29th Oct. 2021
The conditions of thermal, chemical and radiative non-equilibrium attained in a pure N2 gas subjected to a strong shock wave were quantified using vibronic-specific state-to-state models. The Forced-Harmonic-Oscillator model was employed in the computation of rate coefficients for vibrational transition and dissociation of N2 and N2+ by heavy particle impact. Thermal dissociation rate coefficients of N2(X) were obtained and compared with state-of-the-art experimental results, showing a good agreement. By fitting the curve that represents an exponential gap law to experimentally obtained values for rate coefficients values of several vibronic transitions of N2 reported in the literature, discrepancies of as much as one order of magnitude were obtained. Shots 19, 20 and 40 of the test 62 of the Ames Electric Arc Shock Tube (EAST) were simulated using the SPARK code. The experimental radiation variables were underestimated by one to two orders of magnitude by the ones obtained in Euler one-dimensional simulations. And sensitivity tests performed on the rate coefficients were not successful in getting a reasonable agreement. The shape of the radiative intensities profiles of the low speed shot was correctly predicted, but not the ones of the higher speed shots which revealed non-null plateaus proceeding peaks. These plateaus were not predicted at all. Strong evidence was found for such discrepancies resulting from the non-modelling of the precursor phenomena, the absorption of radiation emitted by the driver gas and the electric arc, and/or the conduction of heat due to downstream plasma being subjected to a stronger shock wave.
Elio Pereira, "State-to-state Modelling of High-Speed Nitrogen Shocked Flows", Ms.C. Thesis, 22nd Jul. 2021
The conditions of thermal, chemical and radiative non-equilibrium attained in a pure N2 gas subjected to a strong shock wave were quantified using vibronic-specific state-to-state models. The Forced-Harmonic-Oscillator model was employed in the computation of rate coefficients for vibrational transition and dissociation of N2 and N2+ by heavy particle impact. Thermal dissociation rate coefficients of N2(X) were obtained and compared with state-of-the-art experimental results, showing a good agreement. By fitting the curve that represents an exponential gap law to experimentally obtained values for rate coefficients values of several vibronic transitions of N2 reported in the literature, discrepancies of as much as one order of magnitude were obtained. Shots 19, 20 and 40 of the test 62 of the Ames Electric Arc Shock Tube (EAST) were simulated using the SPARK code. The experimental radiation variables were underestimated by one to two orders of magnitude by the ones obtained in Euler one-dimensional simulations. And sensitivity tests performed on the rate coefficients were not successful in getting a reasonable agreement. The shape of the radiative intensities profiles of the low speed shot was correctly predicted, but not the ones of the higher speed shots which revealed non-null plateaus proceeding peaks. These plateaus were not predicted at all. Strong evidence was found for such discrepancies resulting from the non-modelling of the precursor phenomena, the absorption of radiation emitted by the driver gas and the electric arc, and/or the conduction of heat due to downstream plasma being subjected to a stronger shock wave.
Joao Coelho, "Aerothermodynamic Analysis of Aerocapture and Ballistic Entry Flows in Neptune’s Atmosphere", Ms.C. Thesis, 25th Jan. 2021
Neptune is one of the Solar System’s planets that are still unexplored. A lot of similarities are assumed between the atmospheres of Neptune and Jupiter. The main components are believed to be molecular hydrogen and helium (in an approximate proportion of 80%/20%). However, Neptune’s atmosphere is also believed to have methane (CH4) in a small percentage (1.5%). This work aims to evaluate the exact Neptune chemical composition (including CH4) influence in the aerothermal environment of a capsule entering its atmosphere. Different capsule’s shapes are considered (60° and 45° sphere-cones), and two trajectory points for two different mission types are studied: a ballistic entry trajectory point (around 80km altitude at 18 km/s) an and aerocapture trajectory point (around 130km altitude at 29 km/s). For both capsules, different trajectory points and chemical compositions (with and without CH4) are considered for the performed aerothermodynamic analysis, including both the convective and the radiative wall heat fluxes through the capsule’s wall. The results show that, when the small percentage of methane is considered, the radiative wall heat fluxes increase significantly, particularly for the entry trajectory point. Finally, a brief aerodynamic analysis is performed for the aerocapture trajectory point,evaluating the aerodynamic coefficients for capsules with trim tabs. The capsule with a cone angle θ = 45° seems to present better performance, whereas θ = 60° is prone to aerodynamic instabilities.
Ines Cardoso, "Aerodynamic Analysis of a Scramjet Inlet and Isolator", Ms.C. Thesis, 20th Jan. 2021
This work uses an hypersonic CFD code to study the compression system of a scramjet engine designed for a trajectory point of Mach 10. An initial pre-analysis of a two-dimensional case study from the literature evidenced some discrepancies between the obtained values for pressure and those presented therein, but with a similar overall behaviour of the flow. It was found that lower wall temperatures benefit performance, whereas an adiabatic wall leads to impractically high temperatures. Chemical dissociation was found to be negligible, while the deployment of a 2T model showed that thermal non-equilibrium exists in a scramjet compression system, moderately impacting performance. Similar studies were then conducted for an axisymmetric geometry with the same compression and contraction ratios, with the main conclusion being that performance was worse for the axisymmetric case. Geometry parametric studies were conducted to verify how the number of ramps, the compression ratio, the isolator length, the contraction ratio and the expansion corner edge affected performance. An increased contraction ratio was found to favour inlet unstart for the two-dimensional geometry. Simulations at a trajectory point of Mach 7 were also conducted for both configurations and showed that it is possible to have a started inlet in this off-nominal regime, albeit at a decreased performance.
Francisco Afonso, "Projeto do Diafragma para o Tubo de Choque ESTHER", Ms.C. Thesis, 4th Dec. 2019
This thesis shows the Shock Tube ESTHER diaphragms design. This ESA project has as its object of study the re-entry of aircraft into the earth atmosphere and these diaphragms separate the combustion chamber that operates at extreme pressure levels from the shock tube. The main objectives of this thesis are: 1) diaphragm sizing and notch geometry; 2) determination of the opening time of the diaphragms; 3) manufacturing options; 4) Design of some tube components. In the analysis of the diaphragms, the conditions to which the diaphragm is subjected were verified and a structural analysis was performed through the Finite Element Method that allowed to verify the material to be used as well as the appropriate geometry for the diaphragms. In addition to the design of the diaphragms, some subsets of the shock tube were also designed and some modifications that were necessary during the course of the project. In conclusion, we present the diaphragm geometry and the shock tube in their final version in terms of design and manufacture.
Marilia Matos, "Aerothermodynamic Properties of an Upper Stage Rocket Equipped with Control Surfaces", Ms.C. Thesis, 22nd Nov. 2019
This work aimed to study the reentry of an upper stage rocket equipped with control surfaces by using the CFD SPARK code. The considered reentry velocity was 7.6 km/s for a 60 km altitude and two geometries for the nose. Aerodynamic coefficients and convective heat fluxes were computed for these parameters and for three flap deflection angles (10◦ , 20◦ and 30◦ ) and four flap configurations. The two transport models considered (Wilke and Gupta-Yos/CCS) showed a good agreement within the shock-layer. Park’s two-temperature model for thermal non-equilibrium was also analysed. The non-equilibrium state was particularly visible in the shock-wave region, whereas in the boundary layer, an almost perfect agreement exists between the two temperatures. The two geometries presented good aerodynamic similarity. The highest aerodynamic efficiency was found for the spherical nose geometry with a value of 0.19. The agreement between the results for the equilibrium and non-equilibrium cases supports the initial assumption of thermal equilibrium in the boundary layer.
Luis Fernandes, "Computational Fluid Radiative Dynamics of The Galileo Jupiter Entry at 47.5km/s", Ms.C. Thesis, 15th May. 2019
On December 7th, 1995, the Galileo descent probe entered Jupiter’s atmosphere at a relative velocity of 47.4 km/s. Data collected during flight revealed an unforeseen recession profile: while the stagnation region had been significantly oversized, the shoulder almost completely ablated. In an attempt to understand why numerical predictions diverge from the flight data, several sensitivity studies were performed at the 180 km altitude point. The inaccuracy of the Wilke/Blottner/Eucken model at temperatures above 5, 000 K was again confirmed. When applied to Galileo’s entry, it predicts a narrower shock with higher peak temperatures compared to the Gupta/Yos model. The effects of He and H2 line-by-line radiation were studied. Inclusion of these systems increased the radiative heating by 10% at the stagnation point, even when precursor heating is unaccounted for. Otherwise, the internal excitation of H2 due to absorption of radiation originating from the highly emitting shock layer promotes H2 emission before dissociation occurs at the shock, resulting in 209% higher radiative heat fluxes. This result emphasizes the importance of H2 radiation, not only on the recession experienced by Galileo, but also for future entries in Gas Giants. Accordingly, thermal non-equilibrium resulted in 25% lower radiative heating when compared to an equilibrium solution, contrary to previous investigations that neglected H2 . Ablation product absorption was shown to counteract precursor heating of H2, but the ablation layer temperature must be accurately predicted using a material-response code coupled to the flowfield. Finally, the tangent-slab and ray-tracing models agreed to within 13%.
Diana Luis, "Performance Design of Hypervelocity Shock Tube Facilities", Ms.C. Thesis, 25th Sep. 2018
This thesis focuses on the performance estimates made for ESTHER shock tube in both single and double stage configuration. The single stage configuration is expected to reach 8 km/s, satisfying the requirements for most Mars, Titan and LEO entries. For higher performance, using a double stage configuration, ESTHER is expected to reach up to 14.5 km/s, simulating Earth return missions and Venus entries. A drawback of the double-stage configuration is that it makes slower velocities difficult to achieve, and Mars and Titan entries have to be reproduced resorting to a few performance tweaks. More recently, there has been renewed interest in exploring Gas Giants. The atmosphere for these planets is mainly composed of molecular hydrogen with ten to twenty percent of helium and some trace elements. Although their small molecular weight means that the gas may be more easily accelerated, simulations in ground facilities are nevertheless difficult to achieve, due to high entry velocities imposed by their respective mass. Velocities up to 18.4 km/s are the fastest that can be expected to be reached by the ESTHER facility, hence, alternatives such as changing the helium and neon diluent fraction have been considered. These two changes increase the post-shock temperature, allowing important flight condition phenomena to be reproduced. Combining the high velocities obtainable with the short test times available for the experiments, an accurate trigger system is also essential for the appropriate operation of the facility. The devised ESTHER streak camera trigger system includes four sensors, an FPGA rapid development board, a FMC fast ADC converter and a signal conditioning system.
Ana Gomes, "State-to-State Transport in Hypersonic Entry Flows", Ms.C. Thesis, 13th Sep. 2018
The design of such vehicles and its thermal protection systems relies on the accurate modelling of transport phenomena. Dissociation and vibrational energy transfer relaxation processes are treated using a vibrational state-to-state kinetics approach. A numerical study is carried out to assess the impact of using state-specific transport coefficients in CFD simulations of hypersonic external flowfields. Two transport models have been considered in the code SPARK: the Gupta-Yos/CCS macroscopic model directly transposed to state-to-state species, and the same model considering enchanced state-dependent collisional cross-sections. The code was applied to a full Navier-Stokes simulation of a nitrogen flow past a sphere. The obtained results were compared against the following simulations: Euler state-to-state, Navier-Stokes one-temperature and two-temperature simulations. Accounting for transport phenomena in a state-to-state approach resulted in smoother variations of the flow properties, translated into a 15% lower peak temperature and a significantly thicker shock layer. Comparing to the two-temperature simulation, the state-to-state approach yielded, respectively, a 10% and 5% larger shock standoff distance and peak temperature. Nonetheless, the impact of the enchanced state-dependent cross-section model was found to be negligible. The effects of dissociation and vibrational excitation processes were confirmed to be emphasised for higher freestream temperatures.
Pedro Gonçalves, "Hypersonics: Laying the Road Ahead", Ms.C. Thesis, 9th Jul. 2018
This work provides a technology roadmap for hypersonic applications and use it to some extent in preliminary hypersonic aircraft design. A survey on barriers preventing hypersonic flight was conducted and a technology roadmap was created based on novel concepts which could address the critical requirements. Three engines (ramjet, scramjet and rocket) were numerically modeled in SUAVE, a framework tool developed by Embraer and Stanford University, to run low-fidelity hypersonic vehicle analysis. Since the tool lacked a detailed hypersonic background, simple aerothermodynamic and weight distribution capabilities were also added. These new models were compared against numerical codes and/or experimental data for validation before being implemented in SSTO and atmospheric cruise mission scenarios that assessed the usefulness of new technologies and the performance of its subcomponents. With this work, a new proposal for the future of hypersonics is presented and SUAVE is now able to run basic hypersonic simulations.
Marta Bruxelas, "CFD Simulations of Superorbital Reentry for a Phobos Sample Return Capsule", Ms.C. Thesis, 22nd Nov. 2017
A simulation of the reentry flow for a Phobos Sample Return Mission is presented. Initial reentry velocity is 11.6 km/s for the same forward body geometry as the Hayabusa capsule. The simulations are performed using CFD code SPARK, developed and maintained at IPFN. The heat fluxes along the capsule surface are analysed for several trajectory points, centered in the convective heating peak trajectory point, since the assessment of the total heat fluxes is paramount for the proper design of a TPS. A mesh generation process and mesh convergence study are presented. Two methods for calculating the transport properties, Wilke and Gupta-Yos, are applied and compared. The obtained results are also compared against a previous simulation performed using the same software considering the RAM-C II capsule reentry at 7.65 km/s. For the highest reentry speed, both models diverge more than for the lowest speed. An analysis of thermal nonequilibrium is performed applying Park’s two-temperature model to account for thermal nonequilibrium and compared against thermal equilibrium conditions. Two trajectories (steep and shallow) are considered for the capsule descent. The stagnation point convective heat flux is analysed at six key points for each trajectory and compared to the Sutton-Graves semi-empirical correlation used in the preliminary design.
Ricardo Ferreira, "Laser ignition of a High-Pressure H2/He/O2 combustible mixture", Ms.C. Thesis, 14th Nov. 2017
Laser ignition is an efficient method to ignite high pressure combustible mixtures. It consists on focusing a nano second laser pulse into a small spot size creating an electrical spark and igniting the mixture. The performed tests at the ESTHER shock-tube facility reveled that unfocused laser beam could ignite mixture with filling pressures of 30 up to 100 bar. The irradiance in this condition were about 10^8 W/cm2, 3 orders of magnitude lower than the average irradiance reported in the literature. We set to study how the gas absorbs laser at this high pressure conditions, which revealed to be < 1% of total laser energy and supports the seed electron theory providing breakdown in laser-induced spark ignition. Ignition is provided by the rapid ionization of volatile dust particles present in the mixture, exciting H2 molecules, forming highly reactive radicals and starting the chain reactions. Another experiment was done in CCRC at KAUST measuring the minimum pulse energy for H2-air mixture at atmospheric pressure, showing a lower pulse energy for stoichiometric mixture and difficulties in igniting mixture close to the lean flammability limit.
Jorge Carmona, "Statistical Methods Applied to High Pressure Combustion in ESTHER Facility", Ms.C. Thesis, 31st Oct. 2017
This paper presents a novel approach to study H2/O2 /He high-pressure combustion in ESTHER shock-tube facility. The ESTHER shock-tube provides filling pressures up to 100 bar, extreme initial conditions that have very few studies in the literature. Multiple Logistic Regression and Neural Networks are used to model different combustion regimes, among which detonation holds major interest for being non desirable in the shock-tube performance, as well as compromising for the integrity of the facility. The results of the statistical analysis of the sample of experiments available in ESTHER database shows that the dilution with inert gas, helium, is the leading parameter in detonation formation. The H2/O2 must be carefully diluted in order to avoid detonation risks but also to allow an efficient combustion. Another key factor in detonation phenomenon is the optical system configuration. The shock-tube uses an infrarred laser as external source of energy, which is also statistically decisive since one configuration provides a way sharper energy gradient. The results also provide regression models and a network structure to predict some of the variables involved in the combustion, such as pressure ratio or consumption velocity. These robust prediction tools can benefit ESTHER project allowing more efficient experiments, which will translate into time and expenses savings, along as improving themselves as statitiscal models thanks to each new experiment, which will feed the considered database in a feedback process.
Carlos Teixeira, "Gas-Surface Interaction Models in Hypersonic Flows", Ms.C. Thesis, 7th Dec. 2015
This thesis consists on the implementation of catalycity in SPARK. Catalyticy has an effect on the composition of the flow and plays a pivotal role on the heat flux into the space-ship. A model that describes catalycity macroscopically has been introduced. The recombination at the wall of two dissociated species is characterized by a single parameter that can be either constant or temperature dependent. That required a suitable improvement of the mass and energy balance equations between the fluid flow and the wall. The results from various simulations were compared with other numerical codes and experimental data. Furthermore, the first stage of the implementation of a more advanced model, termed FRSC, that takes into account ablation and pyrolysis phenomenon has been achieved. This initial formulation describes microscopically, and in great detail, the heterogeneous chemical reactions on the particular case of no gas flow; and serves as the foundation for the final implementation of the FRSC on a flow governed by the full reacting Navier-Stokes equations.
Daniel Loureiro, "High-Temperature Modeling of Transport Properties in Hypersonic Flows", Ms.C. Thesis, 19th Nov. 2015
This work studies non-equilibrium hypersonic plasma flows surrounding space re-entry spacecrafts or planetary probes. Two approximate models have been implemented in IPFN’s hypersonic CFD code SPARK for the computation of the transport coefficients in a weakly ionized gas: the Wilke/Blottner/Eucken and the Gupta-Yos/Collision Cross-Section. Additional models to compute mass diffusion fluxes were reviewed, leading to the proposal of a method with improved numerical consistency, for charged particles transport. The code was successfully applied to the CFD simulation of the RAM-C II re-entry vehicle, for which both transport models provided excellent correlation with experimental data for electronic density. Owing to the similar overall numerical efficiency of both models it was concluded that these are equally recommendable for this particular test case, although the Gupta-Yos will be more accurate for higher ionization levels, typically achieved for higher entry speeds.
João Vargas, "Improvement of state-resolved kinetic models applied to N2-CH4 hypersonic entry flows", Ms.C. Thesis, 16th Oct. 2015
Previous kinetic models for Titan's atmosphere, although successfully employed in the Huygens probe entry in 2004, are not physically consistent for high temperature flows. New developments have allowed to create a model which doesn't violate physical consistency. In this work, we propose some additions and modifications to this model to extend it for state-to-state kinetics and apply it to compute the radiation emitted by a shock-tube flow for different shock wave speeds. These treatment allows for comparison with experimental results from the X2 shock-tube. It is found that there is reasonable agreement with the experiments for low speeds and while there is some improvement for higher speeds, the results are not in agreement with the experiments.