Arc-jet Facility Research Articles

Two-Photon Laser-Induced Fluorescence Measurements of Atomic Oxygen Density in Hypersonic Plasma Flow

An experimental investigation of high-enthalpy air flow generated by L2K arc-jet facility of DLR is carried out by means of two-photon absorption laser-induced fluorescence (TALIF) measurements of atomic oxygen at transition. The experiment has been conducted with an arc current of 600 A, a mass flow rate of , and a total pressure of , resulting in both mass-specific enthalpy of and a reservoir temperature of 4200 K. Atomic oxygen has been characterized in terms of translational temperature and absolute number density either in the freestream or behind a shock wave upstream a blunt body. A calibration method based on TALIF measurements of xenon at transition has been used. The results highlight that in the freestream the fluorescence signal exhibits a peak activity lower with respect to that inspected in the shock layer. The measured lifetimes of and indicate that, in the shock layer, the depletion of the atomic oxygen excited state occurs mainly via quenching. The analysis of the line broadening of the spectral profile of atomic oxygen reveals an increment of the translational temperature in the shock layer with respect to that measured in the freestream, resulting in and , respectively. It is found that in the freestream the absolute number density attains and the presence of the shock wave induces an increment of the number density of , that is, 4.85 times higher than that detected in the freestream.

Frozen Sonic Flow Method Applied to Arc-Heated Facilities for Planetary Re-Entry Simulation

The most important parameter in low-density arc-heated flows for atmosphere entry simulations is the centerline total enthalpy Hcl which is strictly related to the average total enthalpy Hne (bulk enthalpy at nozzle exit). In this paper the “sonic flow method” widely used for Hcl calculation in mono-atomic and diatomic molecules gas mixtures, has been extended to polyatomic molecules gas mixtures. By means of a reduced-scale arc-jet facility and using Nitrogen N2 as hot feeding gas, three mixtures were investigated by adding cold gases like O2, CO2 and CH4 through a mixer just before the nozzle. Using bulk enthalpies ranging between 5 and 30 [M J/kg] the calculated enthalpy ratios were Hcl/Hne = 1.38 for N2 - O2 mixture, Hcl/Hne = 1.18 for N2 - CO2 mixture and Hcl/Hne = 1.85 for N2 - CH4 mixture.

Numerical-experimental analysis of a carbon-phenolic composite via plasma jet ablation test

Materials used in space vehicles components are subjected to thermally aggressive environments when exposed to atmospheric reentry. In order to protect the payload and the vehicle itself, ablative composites are employed as TPS (Thermal Protection System). The development of TPS materials generally go through phases of obtaining, atmospheric reentry tests and comparison with a mathematical model. The state of the art presents some reentry tests in a subsonic or supersonic arc-jet facility, and a complex type of mathematical model, which normally requires large computational cost. This work presents a reliable method for estimate the performance of ablative composites, combining empirical and experimental data. Tests of composite materials used in thermal protection systems through exposure to a plasma jet are performed, where the heat fluxes emulate those present in atmospheric reentry of space vehicles components. The carbon/phenolic material samples have been performed in the hypersonic plasma tunnel of Plasma and Process Laboratory, available in Aeronautics Institute of Technology (ITA), by a plasma torch with a 50 kW DC power source. The plasma tunnel parameters were optimized to reproduce the conditions close to the critical re-entry point of the space vehicles payloads developed by the Aeronautics and Space Institute (IAE). The specimens in study were developed and manufactured in Brazil. Mass loss and specific mass loss rates of the samples and the back surface temperatures, as a function of the exposure time to the thermal flow, were determined. A computational simulation based in a two-front ablation model was performed, in order to compare the tests and the simulation results. The results allowed to estimate the ablative behavior of the tested material and to validate the theoretical model used in the computational simulation for its use in geometries close to the thermal protection systems used in the Brazilian space and suborbital vehicles.

Arcjet Tests and Thermal Response Analysis for Dual-Layer Woven Carbon Phenolic

A new dual-layer carbon phenolic manufactured from a three-dimensional woven substrate was tested in three arcjet facilities over a range of environments up to heat flux of approximately at pressure of 5 bar. Sample geometries included both stagnation and wedge models. Based on property measurements and the observed material response, a midfidelity thermal response and ablation model was developed. The material and tests are described, and predictions of recession and temperature are compared with the arcjet test data.

Experimental analysis of spallation particle trajectories in an arc-jet environment

Spallation is a phenomenon in which solid particles are ejected off the surface of an ablative material in a high-enthalpy, high-shear flow field. The main contributor to this phenomenon in carbon-based heat shields is the mechanical erosion of carbon fibers weakened by oxidation decomposition. The dynamics of this phenomenon, which are poorly characterized in the literature, strongly affect the ablation rate of the material. In state-of-the-art codes, ablation by spallation is modeled using a “failure” ablation rate that is empirically determined. The present study aims at understanding the rate of ablation of low-density carbon materials. Results from a test campaign at the NASA Langley Hypersonic Materials Environmental Test System (HYMETS) arc jet facility are used to examine spallation. High-speed multi-camera imagery at 44,000 fps is used to generate velocity vectors of spalled particles emitted from carbon-fiber samples exposed to an arc jet airflow. The imagery recorded approximately 4×106 unique particles, indicating that spallation is a potentially non-trivial process. The velocities of the particles ejected from the surface were found to be between 10 m/s and 20 m/s, accelerating to velocities as high as 250 m/s further away from the sample surface. Although the particle diameters were not directly observable, estimates suggest anywhere from 0.06% to 5.6% of the mass loss from the sample occurred due to spallation.

Aero-thermo-chemical characterization of ultra-high-temperature ceramics for aerospace applications

Ultra-High-Temperature Ceramic (UHTC) materials, because of their high temperature resistance, are suitable as thermal protection systems for re-entry vehicles or components for space propulsion. Massive UHTC materials are characterized by poor thermal shock resistance, which may be overcome using C or SiC fibers in a UHTC matrix (UHTCMC).The University of Naples “Federico II” has a proven experience in the field of material characterization in high-enthalpy environments. A hypersonic arc-jet facility allows performing tests in simulated atmospheric re-entry conditions. The Aerospace Propulsion Laboratory is employed for testing rocket components in a representative combustion environment. Ad-hoc computational models are developed to characterize the flow field in both facilities and perform thermal analysis of solid samples.Current research programs are related to a new-class of UHTCMC materials, for rocket nozzles and thermal protection systems. The activities include design of the prototypes for the test campaign, numerical simulations and materials characterizations.

Thermo-chemical surface instabilities of SiC-ZrB2 ceramics in high enthalpy dissociated supersonic airflows

The response of three different SiC-ZrB2 ceramics obtained by hot-pressing was studied at typical conditions of thermal protection systems of a re-entry spacecraft. Button-like lab-scale demonstrators were manufactured and tested in high enthalpy dissociated supersonic airflows using an arc-jet ground facility. Under severe aero-heating of up to 21MJ/kg of specific total enthalpy and 3.5 MW/m2 of (cold-wall) heat flux the SiC-ZrB2 UHTC buttons endured rather well, though thermo-chemical surface instabilities started taking place for side wall surface temperatures of some buttons above 2050K. The experimental determinations of the surface temperature, correlated to the microstructure changes occurred during testing, allowed to interpret the observed phenomena. Potentials and limits of the oxidation-resistant SiC-ZrB2 system to withstand such extreme conditions were outlined.

Improvements on Infrared-Camera Calibration for Accurate Surface Temperature Measurements in Arcjet Facilities

Ground testing of thermal protection systems play an essential role for understanding the response of candidate materials for thermal shields. Nonintrusive surface temperature measurements are necessary to map the temperature distribution for the correct interpretation of the results obtained in wind-tunnel facilities. An accurate calibration of the infrared camera to be used for a planned experimental campaign on variable transpiration cooling is described in detail. A prototype carbon–carbon cone has been used for both the calibration procedure and experiments. For this application, standard calibration procedures, based on the combined use of blackbody radiators for the calibration of the camera sensor together with tests for emissivity characterization, would not allow for the determination of the accurate surface temperature distribution. This is due to the diversified surface finish and localized defect characteristics of a typical component. The calibration procedure described here, which is based on simultaneous measurements of the local temperature and radiance, allows determination of the local surface temperatures independent of the material emissivity. The calibration curves reveal similar surface thermal response. A higher temperature variance on the calibration curve related to the thermocouple located near the tip of the cone has been detected due to local surface defects.

Use of a Quasi-Steady Ablation Model for Design Sensitivity With Uncertainty Propagation

Sensitivity analysis and design calculations are often best performed using low-order models. This work details work done on adding complementary pieces to a low-order, quasi-steady-state ablation model to facilitate uncertainty propagation. The quasi-steady-state ablation model is a one-dimensional, quasi-steady-state, algebraic ablation model that uses finite-rate surface chemistry and equilibrium pyrolysis-gas-production submodels to predict surface recession rate. The material response model is coupled to a film-transfer boundary layer model to enable the computation of heat and mass transfer from an ablating surface. For comparison to arcjet data, a simple shock heated gas model is coupled. A coupled model consisting of submodels for the shock heated gases, film heat and mass transfer, and material response is exercised against recession rate data for surface and in-depth ablators. Comparisons are made between the quasi-steady-state ablation model and the unsteady ablation code, Chaleur, as well as to other computations for a graphite ablator in arcjet facilities. The simple models are found to compare reasonably well to both the experimental results and the other calculations. Uncertainty propagation using a moment based method is presented. 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.

Simultaneous Nitric Oxide/Atomic Oxygen Laser-Induced Fluorescence in an Arcjet Facility

Simultaneous nitric-oxide and atomic-oxygen laser-induced-fluorescence experiments were performed in the Hypersonic Materials Environmental Test System facility at the NASA Langley Research Center. The data serve as an experimental database for validation for chemical and thermal nonequilibrium models used in hypersonic flows. Measurements were taken over a wide range of stagnation enthalpies () using an Earth atmosphere simulant with a composition of 75% nitrogen, 20% oxygen, and 5% argon (by volume). These are the first simultaneous measurements of nitric-oxide and atomic-oxygen laser-induced fluorescence to be reported in literature for the Hypersonic Materials Environmental Test System facility. The maximum atomic-oxygen laser-induced-fluorescence mean signal intensity was observed at a stagnation enthalpy of approximately , whereas the maximum nitric-oxide laser-induced-fluorescence mean signal intensity was observed at a stagnation enthalpy of . The experimental results were compared to a fluorescence model that assumes equilibrium conditions in the plenum and frozen chemistry in an isentropic nozzle expansion (Mach 5). The equilibrium calculations were performed using CANTERA v2.1.1 with 16 species. The fluorescence model captured the correlation in mean atomic-oxygen laser-induced-fluorescence and nitric-oxide laser-induced-fluorescence signal intensities over a large range of stagnation enthalpies tested. The agreement between equilibrium calculations and the experimental laser-induced fluorescence signals was better for nitric oxide than atomic oxygen. Very weak correlations between single-shot atomic-oxygen laser-induced-fluorescence and nitric-oxide laser-induced-fluorescence intensities were observed in the experiments at all of the stagnation enthalpy conditions. It was found that the overall magnitude of nitric-oxide laser-induced-fluorescence fluctuations was much larger than atomic-oxygen laser-induced-fluorescence fluctuations.

Evaluation of Finite-Rate Gas/Surface Interaction Models for Carbon-Based Ablator

Two sets of finite-rate gas/surface interaction model between air and the carbon surface are studied. The first set is an engineering model with one-way chemical reactions, and the second set is a more detailed model with two-way chemical reactions. Each of these two proposed models intends to cover the carbon surface ablation conditions including the low-temperature rate-controlled oxidation, the midtemperature diffusion-controlled oxidation, and the high-temperature sublimation. The prediction of carbon surface recession is achieved by coupling a material thermal response code and a Navier–Stokes flow code. The material thermal response code used in this study is the Two-Dimensional Implicit Thermal-Response and Ablation Program, which predicts charring material thermal response and shape change on hypersonic space vehicles. The flow code solves the reacting full Navier–Stokes equations using the data parallel line relaxation method. Recession analyses of stagnation tests conducted in NASA Ames Research Center arc-jet facilities with heat fluxes ranging from 45 to are performed and compared with data for model validation. The ablating material used in these arc-jet tests is phenolic impregnated carbon ablator. Computational predictions of surface recession and shape change are in good agreement with measurement for arc-jet conditions.

Heat transfer in ultra-high temperature advanced ceramics under high enthalpy arc-jet conditions

Aim of this work is to analyze the response of an ultra-high temperature ceramic at typical heat flux conditions of thermal protection systems of a re-entry spacecraft. In particular, a ZrB2–SiC based ultra-high temperature advanced ceramic sharp leading edge demonstrator (1mm nominal radius of curvature) was manufactured and tested in a non-equilibrium high enthalpy supersonic airflow, 20MJ/kg of peak total enthalpy, by using an arc-jet ground facility. The surface temperature of the leading edge was monitored by infrared thermo-cameras coupled to a two-color pyrometer. The ultra-refractory advanced ceramic leading edge withstood stressful thermo-chemical loads successfully, without obvious failure. Ad-hoc computational fluid dynamics simulations rebuilt the adopted set-up and related experiment conditions: the numerical outputs matched fairly well the experimental in-situ determinations.

Development of the Mars Science Laboratory Heatshield Thermal Protection System

Early in the development of the Mars Science Laboratory thermal protection system on the heatshield, project management planned to use Lockheed Martin’s Super Light Ablator in honeycomb as the ablative material based on successful use on previous Mars entry heatshields and on stagnation arcjet tests at heating rates beyond the design levels. Because this heatshield would be the first to experience combined turbulent flow and high shear environments as it entered the Mars atmosphere, tests were performed in various arcjet facilities on flat-plate, wedge, and swept-cylinder specimen configurations in order to ascertain the effects of shear on the material. During the course of these tests, a set of conditions within the flight envelope was identified that resulted in catastrophic failure in the SLA-561V. Consequently, project management decided to replace the SLA-561V with the phenolic-impregnated carbon ablator, the material that had flown successfully on the Stardust mission and was undergoing intense testing and characterization for the Crew Exploration Vehicle Thermal Protection System Advanced Development Program. With only two years remaining before the expected launch date, and less than 18 months before the heatshield delivery date, the Mars Science Laboratory team developed and built NASA’s first tiled ablator flight heatshield that contributed to the outstanding success of the spacecraft’s entry, descent, and landing on 5 August 2012.

Synthesis and Characterization of Hyperbranched Polyazomethine Ablators for Space Exploration Applications

A novel series of ablative composites containing a hyperbranched polyazomethine synthesized inside a carbon fiber preform (HyPAZA) were prepared, which have similar density to phenolic impregnated carbon ablators (). A novel method of synthesizing strong hyperbranched polyazomethine thermosets has been developed, enabling polyazomethines to be studied in ablators for the first time. Several formulations of HyPAZA perform better than the phenolic impregnated carbon ablator in terms of polymer char yield, composite mechanical strength, laser ablation tests at heat fluxes of 550 and , and small-scale arcjet testing at a heat flux of . Char yields of hyperbranched polyazomethines were as high as 79% at 1000°C by thermogravimetric analysis. This is one of the highest char yields ever reported for a fully organic polymer. Some HyPAZA composites are over 10 times stronger than the carbon fiber preform, as determined by compression tests. Specimens were also tested in an arcjet facility at . Several formulations exhibited better performance than the phenolic impregnated carbon ablator in terms of ablation rate by a factor of 1.2 to 1.7. In addition, HyPAZA composites impregnated with aerogel show a lower rate of increase of back wall temperature, and therefore a lower thermal conductivity.

Nitric-Oxide Planar Laser-Induced Fluorescence Measurements in the Hypersonic Materials Environmental Test System

Planar laser-induced fluorescence of naturally occurring nitric oxide has been used to provide insight into baseline flow conditions of the Hypersonic Materials Environmental Test System 400 kW arc-heated wind tunnel at NASA Langley Research Center via radial and axial velocity measurements and instantaneous flow-visualization images. This represents both the first flow-tagging velocity measurements and the first application of nitric-oxide planar laser-induced fluorescence flow visualization in an arcjet facility. Results are presented at selected facility run conditions, including some in a simulated Earth atmosphere (75% nitrogen, 20% oxygen, 5% argon) and others in a simulated Martian atmosphere (71% carbon dioxide, 24% nitrogen, 5% argon) for specific bulk enthalpies ranging from 6.5 to . Flow-visualization images reveal the presence of large-scale unsteady flow structures and indicate nitric-oxide fluorescence signal over more than 70% of the core flow for specific bulk enthalpies below about but over less than 10% of the core flow for specific bulk enthalpies above about . Axial velocimetry was performed using molecular tagging velocimetry. Axial velocities of about were measured along the centerline. Radial velocimetry was performed by scanning the wavelength of the narrowband laser and analyzing the resulting Doppler shift. Radial velocities of were measured.

Effect of Nonequilibrium Surface Thermochemistry in Simulation of Carbon-Based Ablators

This study demonstrates that coupling of a material thermal-response code and a flow solver using a nonequilibrium gas/surface-interaction model provides time-accurate solutions for the multidimensional ablation of carbon-based charring ablators. The material thermal-response code used in this study is the two-dimensional implicit thermal response and ablation program, which predicts the charring-material thermal response and shape change on hypersonic space vehicles. Its governing equations include total energy balance, pyrolysis-gas mass conservation, and a three-component decomposition model. The flow code solves the reacting Navier–Stokes equations using the data-parallel-line-relaxation method. Loose coupling between the material-response and flow codes is performed by solving the surface mass balance in the flow code and the surface energy balance in the material-response code. Thus, the material surface recession is predicted by finite rate gas/surface-interaction boundary conditions implemented in the flow code, and the surface temperature and the pyrolysis-gas injection rate are computed in the material-response code. Two sets of nonequilibrium gas/surface-interaction chemistry between air and the carbon surface are studied. Coupled fluid-material-response analyses of stagnation tests conducted at the NASA Ames Research Center arcjet facilities are considered. The ablating material used in these arcjet tests was phenolic impregnated carbon ablator. Computational predictions of in-depth material thermal response and surface recession are in excellent agreement with the data for conditions in which carbon recession rate is limited by species diffusion.

On Laminar-to-Turbulent Transition of Arc-Jet Flow in the NASA Panel Test Facility

This paper provides experimental evidence and supporting computational analysis to characterize the laminar-to-turbulent flow transition in a high-enthalpy arc-jet facility at NASA Ames Research Center. The arc-jet test data obtained in the 20 MW Panel Test Facility include measurements of surface pressure and heat flux on a water-cooled calibration plate and measurements of surface temperature on a tile plate with reaction-cured glass-coating. Computational fluid dynamics simulations are performed to characterize the arc-jet test environment and estimate its parameters consistent with the facility and calibration measurements. The present analysis is composed of simulations of the nonequilibrium flowfield in the facility nozzle, test box, and flowfield over test articles. Both laminar and turbulent simulations are performed, and the computed results are compared with the experimental measurements, including Stanton number dependence on Reynolds number. Comparisons of computed and measured surface heat fluxes (and temperatures), along with the accompanying analysis, confirm that the boundary layer in the Panel Test Facility flow is transitional at certain arc-heater conditions.

Experimental and numerical simulation, by an arc-jet facility, of hypersonic flow in Titan’s atmosphere

This paper summarizes experimental and numerical results on high enthalpy hypersonic flows in nitrogen–methane mixtures. The experimental results have been obtained by the Small Planetary Entry Simulator (SPES), an arc-jet hypersonic wind tunnel in operation at the University of Naples “Federico II”. The facility has been used in the past to simulate entry conditions into Earth’s and Mars’ atmosphere and has recently been upgraded to simulate entry into Titan’s atmosphere. Numerical simulations of the flow field in the facility, for a nitrogen–methane mixture in chemical nonequilibrium have been performed. The chemical species and the heat flux distributions on a calorimeter located in the arc-jet test chamber have been computed.The ability to perform a numerical rebuilding of the facility, by comparing numerical and experimental results has been investigated. The paper also highlights the importance of the wall catalyticity for the characterization of the thermal response of materials in high enthalpy flow.

Aerothermal behaviour of a SiC fibre-reinforced ZrB2 sharp component in supersonic regime

Abstract The aerothermal behaviour of a SiC fibre-reinforced ZrB 2 composite machined in a sharp-shaped component with a tip curvature radius of 0.1 mm was investigated using an arc-jet facility. The specimen underwent 4 thermal cycles and the emissivity was evaluated in the temperature range 1000–1700 °C. The temperature profile along the specimen was calculated through computational fluid dynamic (CFD) modelling. The microstructural evolution was compared to a typical ZrB 2 composite containing the same amount of SiC in particulate form, which underwent similar aerodynamic conditions.

Processing, sintering and oxidation behavior of SiC fibers reinforced ZrB2 composites

Borides and carbides of early transition metals are considered a class of promising materials for several applications, the most appealing ones being in the aerospace and energy sectors. The present work is mostly focused on toughening of UHTCs through incorporation of SiC chopped fibers. Mechanical properties of reinforced composites are compared to those of un-reinforced, whisker- and particle-reinforced materials and the effect of different kinds of sintering aids is studied. Addition of fibers allows toughness to be increased from 3–4 MPa m1/2 (for un-reinforced materials) to 5.0–6.2 MPa m1/2. The high temperature behavior is also investigated both in air furnace and in arc jet facility. Eventually, a paragraph is dedicated to potential of UHTCs as sunlight absorbers for future solar concentrating systems operating in the high temperature regime.