High Enthalpy Plasma Flows Research Articles

Development of Cf/C-UHTC composite and study of its resistance to oxidation and ablation in high-speed high-enthalpy air plasma flow

This article contains the results of research on the development of a Cf/C-UHTC carbon fabric composite based on a viscose precursor and a combined matrix consisting of partially sintered ceramics in a system consisting of HfC–HfB2–NbC–NbB2–TiC–TiB2–B4C–SiC, amorphous carbon, and pyrocarbon. The SiC fraction does not exceed 8.5–9.0 wt%. In its initial state, the composite has open porosity, with apparent and true densities of 18–22%, 2.25–2.29 g/cm3 and 2.79–2.91 g/cm3, respectively. The bending strength and the elasticity modulus are 27.8 ± 0.7 MPa and 7.8 ± 0.2 GPa, respectively, and the fracture strain is 0.85 ± 0.05%. The tests for resistance to oxidation and ablation were carried out in a gas dynamic flow regime and non-equilibrium air plasma heating at flow rates of 4.5–4.8 km/s and breaking enthalpy of 45–50 MJ/kg. Heating was performed in the temperature range Tw = 1400–2700 °C at the critical point on the front surface of the samples. The average linear ablation rate and mass loss rate of the composite are 6.3 ± 0.3 μm/s and 6.22 ± 0.44 mg/s. The estimated value of the conductivity factor is 0.280–0.285 W/(m K). The performance ability of the composite arises from the formation and evolution of a passivating heterogeneous oxide film consisting mainly of titanium niobate Ti2Nb10O29, mixed solutions of Hf1−xTixO2, (Ti1−xHfx)1−yNbyOz and (Ti1−xHfx)NbO4 with broad homogeneity ranges, and also encapsulated carbide and boride particles. It is shown that the oxidation resistance of the composite increases as a result of the transition through a number of phases into a liquid state as the working temperature increases.

Oxidation Resistance of the ZrSi2–MoSi2–ZrB2–ZrC Coating on the C/C–SiC Composite in High-Speed High-Enthalpy Air Plasma Flow

Oxidation Resistance of the ZrSi2–MoSi2–ZrB2–ZrC Coating on the C/C–SiC Composite in High-Speed High-Enthalpy Air Plasma Flow

Physicochemical and structural investigations of Si<sub>3</sub>N<sub>4</sub> and AlN powder mixture product using plasma treatment

The paper presents research results of the β-SiAlON production from aluminum, silicon and urea nitrides using high-enthalpy thermal plasma flow. Physicochemical properties of β-SiAlON are studied by using the X-ray phase analysis, scanning electron microscopy, and differential thermal analysis. It is shown that at the spicified plasma jet parameters, a thermochemical reaction occurs with the formation of β-Si5AlON7. Three zones of the structure formation caused by the temperature gradient in the bulk material as a result of the plasma exposure, are identified and studied. The first zone is directly irradiated by the plasma. The second is the transition zone, which locates between the first and third zones. The third is the thermal heating zone provided by the heat transfer from the irradiated zone.

Three-dimensional unsteady model of arc heater plasma flow

Arc jets are unique facilities used to evaluate the performance of Thermal Protection Systems (TPS) for hypersonic vehicles. They produce high pressure and high enthalpy plasma flow to simulate the extreme heat encountered during atmospheric entry. The constricted arc heater part of an arc jet increases the test gas temperature by Joule heating. This study details the development of the three-dimensional unsteady plasma flow analysis tool, ARCHeS (ARC Heater Simulator). Coupled Navier-Stokes, radiative transfer and Maxwell equations yield current density, magnetism, radiation, and flow field solutions. The present work constitutes the first demonstration of an unsteady three-dimensional plasma flow simulation of high pressure and high enthalpy arc heater that captures kink instabilities of the electric arc. It is found that the arc attachment is mainly driven by upstream arc instabilities.

Oxidation Resistance of a Si–TiSi2–MoSi2–TiB2–CaSi2 Coating on a Cf/C–SiC Substrate in High-Speed High-Enthalpy Air Plasma Flows

The results of a study on the development and testing of a heat-resistant coating in a Si–TiSi2–MoSi2–TiB2–CaSi2 system to protect Cf/C–SiC composites from oxidation and erosional entrainment in high-speed flows are presented here. The coating was formed using firing fusion technology on the powder composition. Oxidation resistance tests were carried out under static conditions in air at 1650 °C and under conditions of interaction with high-speed air plasma flows, with Mach numbers M = 5.5–6.0 and enthalpy 40–50 MJ/kg. The effectiveness of the protective action of the coating was confirmed at surface temperatures of Tw = 1810–1820 °C for at least 920–930 s, at Tw = 1850–1860 °C for not less than 510–520 s, at Tw = 1900–1920 °C for not less than 280–290 s, and at Tw = 1940–1960 °C for not less than 100–110 s. The values of the rate of loss of the coating mass and the rate constant of heterogeneous recombination of atoms and ions of air plasma on its surface were estimated. The performance of the coating was ensured by the structural-phase state of its main layer, and the formation and evolution on its surface during operation of a passivating heterogeneous oxide film. This film, in turn, is composed of borosilicate glass with titanium and calcium liquation inhomogeneities, reinforcing TiO2 microneedles and in situ Si2ON2 fibers. It was shown that at Tw ≥ 1850–1860 °C, the generation of volatile silicon monoxide was observed at the “oxide layer–coating” interface, followed by the effects of boiling and breakdown degradation of the oxide film, which significantly reduced the lifespan of the protective action of the coating.

Low-surface-temperature jump behavior of C/SiC composites prepared via precursor impregnation and pyrolysis in high-enthalpy plasma flows

The response of C/SiC composites prepared via precursor impregnation and pyrolysis was investigated in a 1 MW plasma wind tunnel. Under a considerable aero heating of up to 26.2 MJ/kg of specific total enthalpy, the samples were exposed to heat fluxes exceeding 5.7 MW/m2 and low pressures of 4.5–6.6 kPa. The samples were able to withstand low heat fluxes and low stagnation pressures, and their carbon-rich nature improved the thermal conductivity, presenting a low steadystate surface temperature. However, a spontaneous jump in the surface temperature at around 1700 °C was observed at high heat fluxes and high stagnation pressures. The jump temperature was lower compared with that reported in previous studies, and was found to increase rapidly to temperatures above 2000 °C. This low-temperature jump phenomenon was associated with the evolution of microstructure during testing, and the underlying mechanism was revealed through the use of thermodynamics analysis.

Lowtemperature plasma generator for effective processing of materials

Low-temperature plasma is used in metallurgy for steel alloying by nitrogen, deoxidization of magnetic alloys, obtaining of steels with particularly low carbon content, metal cleaning of nonmetallic inclusions, desulfurization and other refining processes. The wide application of those technologies is restrained by absence of reliable generators of low-temperature plasma (GLP) with sufficient resource of continuous operation. As a result of studies, a universal generator of high-enthalpy plasma jet of various working gases was created. The generator has expanding channel of the output electrode with an efficiency of ~60 % for argon working gas and ~80% for nitrogen and air. It was shown that the developed generator of low-temperature plasma ensures formation of a weakly diverging (2α = 12°) plasma jet with a diameter D = 5–12 mm, an enthalpy of 5–50 kJ/g and a mass average temperature of 5–10 kK, at a full electric power of the arc discharge of 5–50 kW and a plasma-forming gas flow rate of 1–3 g/s. Results of the study of propane additions to the plasma-forming gas effect on the state of cathodes with inserts made of pure tungsten, lanthanum tungsten, and hafnium presented. It was shown that a small propane addition (1%) to the plasma-forming gas, results in reducing effect of the insert material. Study of the GLP operation at arc current 100A with addition to the working gas nitrogen maximum possible volume of propane, which don’t disturb stability of arc showed that for the developed plasma generator at the nitrogen flow rate ~0,45 g/s, the propane flow rate was ~0,33 g/s (not more than ~73 % of the plasma-forming gas). The created high-resource GLP with changeable electrodes enables to obtain at the exit a high-enthalpy plasma flow of various gases (argon, nitrogen, air) and can be a prototype of more powerful plasmotrons of various technological application, in particular for plasma metallurgy.

High frame rate emission spectroscopy for ablation tests in plasma wind tunnel.

This article describes a novel high frame rate emission spectroscopy setup developed for measurements in high enthalpy flow fields. The optical setup and the associated hardware arrangements are described in detail followed by test case data to demonstrate the capability of recording spectral images at 1 kHz frame rate. The new system is based on a classical Czerny-Turner spectrograph but with a particular setup for high frame rate detection using a Generation II intensifier coupled with a high-speed camera. The high frame rate spectral images acquired enable, for the first time, investigation of the spatial distribution and temporal tracking and evolution of molten droplets of an ablating sample. In this paper, an example is shown from ablating meteorite samples tested in a high enthalpy plasma flow field corresponding to a flight scenario at an altitude of 80 km. This new instrumental configuration allows emission spectroscopic analysis of transient phenomena simulated in the high enthalpy ground test facilities with kHz resolution. The particular feature of this system is the ability to measure very faint spectral lines at high temporal and spatial resolution.

Structure and properties of YBCO before and after the short-term exposure of the plasma flow

The results on the structure and electrical resistance of the porous YBa2Cu3O7–δ nanoceramics before and after plasma treatment are shown. It is shown that after exposure to high-enthalpy argon plasma flow, the structural, stoichiometrical and superconducting properties in the volume of this ceramics change slightly. An amorphous layer forms on the surface, the mechanical strength of the inter-granular bonds of which becomes higher than for the material in the bulk.

Electronic-state-resolved analysis of high-enthalpy air plasma flows.

In the present study, three different electronic state-to-state methods are proposed to analyze nonequilibrium air plasma flows behind a strong shock wave. In the first approach representing the conventional method, a two-temperature model combined with the electronic quasi-steady-state assumption is adopted. In the second and the third methods, atomic and molecular electronic master equations are coupled with a conservation equation to describe the electronic state-to-state kinetics. State-of-the-art electronic transition rates for atmospheric gas species are compiled with comparisons of existing data. A prediction of the measured nonequilibrium radiation is made for the flow conditions of recent electric-arc shock tube experiments. In a comparison with the measured spectrum, the present electronic master equation coupling methods are more accurate than the conventional approach when used to estimate the initial rising rate and peak value of the diatomic intensity and small amounts of atomic radiation when the diatomic nonequilibrium condition is dominant. Moreover, the spatial distributions of the intensity and electron number density are more accurately predicted by the present methods when the flow fields are dominated by atomic nonequilibrium.

Light-Field Imaging for Plasma Wind-Tunnel Application

Limited optical access and high apparatus complexity are the main challenges for true three-dimensional (3-D) measurements in high-enthalpy plasma flows. However, with the advent of plenoptic cameras, the one-shot and single-camera acquisition of light-field data has become possible, enabling the 3-D analysis of complex scenes with one single optical access and one single exposure. So far, this technique has mostly been applied to problems containing opaque objects or particle-loaded flows. In this paper, approaches to explore the potential of light-field analysis for the 3-D investigation of brightly luminous, high-velocity plasma flows are presented. Using gas flames from a solder torch, the feasibility of plenoptic imaging of optically thin scenes is shown. The complete structure of the flame is derived from a single exposure. The transition of this approach to plasma-flow visualization is shown with very first acquisitions of a plasma freejet, including spectral filters for the measurement of the atomic oxygen distribution.

Response of Silicon Nitride Ceramics under High-enthalpy Plasma Flows

Silicon nitride ceramics were prepared by means of a hot-press sintering method. The ablation behavior was studied by plasma wind tunnel tests. The data suggested that, under low heat flux and stagnation pressure conditions, the ablation process was controlled by the atomic oxidation of Si3N4, leading to the elimination of Si3N4. By contrast, the erosion was mainly produced by the decomposition of Si3N4 under high heat flux and stagnation pressure conditions. Under these conditions, a fraction of Si phase, formed upon decomposition of Si3N4, was volatilized. The remained Si had melted due to high temperatures and scoured away from stagnation point area, generating mushroom-shaped samples.

Subpixel Temperature Measurements in Plasma Jet Environments Using High-Speed Multispectral Pyrometry

A high-speed (2 kHz) near-infrared (1.0–1.65 μm) multispectral pyrometer was used for noninvasive measurements of the subpixel temperature distribution near the sharp leading edge of a wing exposed to a supersonic plasma jet. The multispectral pyrometer operating in the field measurement mode was able to measure the spatial temperature distribution. Multiple spectra were used to determine the temperature distributions in the measurement region. The spatial resolution of the multispectral pyrometer was not restricted to one “pixel” but was extended to subpixel accuracy (the temperature distribution inside one pixel in the image space corresponding to the point region in the object space). Thus, this system gives high-speed, multichannel, and long working time spatial temperature measurements with a small data stream from high-speed multispectral pyrometers. The temperature distribution of the leading edge of a ceramic wing was investigated with the leading edge exposed to extreme convective heating from a high-enthalpy plasma flow. Simultaneous measurements with a multispectral pyrometer and an imaging pyrometer verify the measurement accuracy of the subpixel temperature distribution. Thus, this multispectral pyrometry can provide in situ noninvasive temperature diagnostics in supersonic plasma jet environments.

Carbon fiber reinforced silicon carbide composite-based sharp leading edges in high enthalpy plasma flows

Abstract Carbon fiber-reinforced silicon carbide (C/SiC) and zirconium diboride-zirconium carbide (ZrB 2 -ZrC) modified C/SiC (C/SiC-ZrB 2 -ZrC) sharp leading edges (SLEs) were prepared in this study. Their ablation behaviors were tested in plasma wind tunnels. The results indicated that the ablation of C/SiC SLEs was controlled by the atomic oxidation of SiC, leading to the elimination of SiC. In contrast, C/SiC-ZrB 2 -ZrC SLEs showed little reduction in plasma flows. During ablation, a ZrO 2 skeleton was formed with silica filling inside by the atomic oxidation of SiC. The ZrO 2 skeleton suppressed the consumption of SiC, resulting in the improvement of the ablation resistance of C/SiC-ZrB 2 -ZrC SLEs.

The effect of high-enthalpy argon plasma flow on the structure and properties of YBa2Cu3O7 – δ nanoceramics

The structure and electrical resistivity of YBa2Cu3O7 – δ-based nanostructured ceramics before and after exposure to the high-enthalpy argon plasma flow are investigated. It is demonstrated that the plasma-flow processing can strengthen the intergrain bonds in the surface layer of the ceramics and change its oxygen stoichiometry index.

Determination of graphite sublimation rate in high enthalpy plasma flow using ‘laser knife’ method

The experimental method to study a behavior of heat-resistant materials by the action of high-enthalpy plasma jets is developed. The main attention is focused on heating and thermal destruction of graphite samples. The argon plasma jet is generated by a plasmatron with an expanding anode channel providing a slightly divergent high-enthalpy laminar flow with the axial speed of 400–500m/s, the diameter of 6–8mm and high temperature which is not less than 10,000K. The jet affects the isotropic graphite MPG-6 samples shaped in the form of a parallelepiped with the dimensions of 23mm×23mm×16mm. A description of the method developed by the authors on the basis of the “laser knife” technique to determine the sublimation rate of heat-resistant materials is presented and the relevant time change in the sample material sublimation rate is analyzed. The shape and evolution of craters on the sample surface are analyzed. A nonmonotonic behavior in the sublimation is observed in the experiments. The maximum of sublimation rate is recorded when the initial warming up stage of the graphite sample is completed, which indicates the emergence and propagation of dislocations in graphite structure as it is heated to a temperature close to 3000K.

Characterization of a Reentry Plasma Wind-Tunnel Flow with Vacuum-Ultraviolet to Near-Infrared Spectroscopy

This paper presents the analysis of optical emission spectroscopic measurements from vacuum ultraviolet (120 nm) to near infrared (960 nm) in a high-enthalpy air plasma flow corresponding to superorbital reentry conditions. The vacuum ultraviolet measurements have been realized with a new experimental setup allowing measurements through a bore hole in the sample. Using the commonly applied optical emission spectroscopy from the side, the radiation transport along the line of sight of the vacuum ultraviolet measurements has been assessed. This allowed the determination of the local ground state densities of atomic oxygen and atomic nitrogen from a branching ratio analysis and the blackbody limiting correction of the vacuum ultraviolet radiation. Through the analysis of the absorption in the borehole, the spectra have been corrected, resulting in a stagnation point radiative heat flux in the vacuum ultraviolet of .

Photogrammetric Surface Analysis of Ablation Processes in High-Enthalpy Air Plasma Flow

Results from an in situ analysis of ablative heat shield materials based on stereoscopic photogrammetry during plasma wind tunnel testing are presented. A carbon fiber preform material sample has been investigated in a high-enthalpy airflow corresponding to a Hayabusa reentry condition at a 78 km altitude. The applied photogrammetry uses two digital single lens reflex cameras triggered at to acquire images simultaneously from two different perspectives of the sample exposed to the high-speed flow. The photogrammetric analysis resolves the surface with , approximately 400 dpi, allowing the in situ analysis of the recession phenomena of ablators. Material inhomogeneities are identified during the ablation process, and surface recession is derived with an accuracy of . The method is a useful tool for the investigation of temporally resolved volumetric ablation phenomena in low-density carbon phenolic materials.

Local Mass-Specific Enthalpy Measurements with a New Mass Injection Probe

The results of local measurements of the mass-specific enthalpy in a high-enthalpy oxygen plasma flow are presented. The sensor measures enthalpy based on stagnation point heat flux measurements and its reduction by mass addition into the boundary layer. The efficiency of this boundary-layer cooling depends on the freestream enthalpy. Thus, an analysis of this heat flux reduction is a measure for enthalpy. The enthalpy probe is described and theoretically analyzed, and measured data are compared to locally resolved enthalpy measurements from optical diagnostic measurements. For the pure oxygen plasma flow, a simplified theoretical model neglecting chemical reactions and diffusion fits comparably well to the measurements based on optical diagnostics. The mass-specific enthalpy at the corresponding plasma condition applying optical diagnostics is , and the probe measures .

Study of High-Enthalpy Electrothermal Energetic Plasma Source Concept

The electrothermal (ET) energetic plasma source (ETEPS) is a different concept in which the ablation mechanism is forced inside of the open-ended capillary that has energetic liner, a propellant. The generation of the ET plasma results from Joule heating and radiant heat transport to the liner. The discharge initiates erosive burn of the propellant, and the mixed plasma-propellant gasification produces high-enthalpy energetic flow. In the ablation-dominated source, the eroded materials from the solid propellant liner are mixed inside the source before they flow out as a result of the large pressure gradient. The energetic ET source also has another concept in which no ablation occurs and the plasma is generated from the injection of energetic gasses or liquids into the confined open-ended capillary. The ablation-free source generates the plasma from the dissociation of the gaseous/liquid components, which in turn releases the chemical energy of these propellants and mixes the energy with the electrical energy of the plasma. This concept is different from ET chemical (ETC) sources; it generates the propellant or energetic flow without requiring a combustion chamber. It also provides mixing at the ionic level not available in current configurations of ETC launchers, igniters, or thrusters. The ET plasma code ETFLOW-EN was developed to computationally simulate the plasma generation and flow in energetic ET capillary discharges to predict the behavior of the energetic source with the use of lined solid propellants. Operation with liquid/gaseous energetic forms in a nonablative capillary is also a character of this concept and is part of the ETFLOW code. The results of using different forms of energetic materials in solid, liquid, and gaseous mixtures have shown the applicability of ETEPS to produce high-enthalpy energetic plasma flows with sufficient parameters suitable for ETC launch applications. Plasma and flow parameters at the capillary exit were investigated at different mixing ratios.