High-enthalpy Air Research Articles

Computational investigations into heat transfer over a double wedge in hypersonic flows

Recently developed OpenFOAM application hy2FOAM is employed to predict the aerodynamic heat transfer numerically and compared with the experimental data from the University of Illinois. Mach 7 nitrogen flow at 2.1 MJ/kg stagnation enthalpy, and Mach 7 nitrogen and air flows at 8 MJ/kg stagnation enthalpy over a double wedge geometry have been reproduced numerically assuming chemical and thermal non-equilibrium. Good agreement of mean heat transfer profiles has been observed, although none of the simulations achieved a steady-state. The reattachment heat transfer peak in the high enthalpy air case showed an improved agreement with the experimental data, which is due to the non-equilibrium in the flow field.

Heat Transfer and Thermophysics of Subsonic Dissociated-Air Jets in Flow past a Cylindrical Model in a High-Frequency Induction Plasmatron

Heat transfer in the subsonic high-enthalpy air flow in the VGU-4 induction plasmatron is experimentally and numerically investigated for three versions of the sectional discharge channel with the exit section diameters of 30, 40, and 50 mm. The model was a flat-nosed cylinder, 20 mm in diameter. Using a Gamma code the enthalpy he and the velocity at the jet axis are numerically reconstructed from the experimental data. The method for approximately determine the enthalpy he on the basis of the experimental data and the Fay—Riddell formula for the heat flux is proposed. The effective dimensionless model radius reff is introduced, which makes it possible to take account for the dependence of the flow at the outer edge of the boundary layer on the model and discharge channel geometries and the plasmatron operation regime. The value of reff is calculated for the VGU-4 operation regimes with a simple cylindrical discharge channel and a sectional channel. The values of the enthalpy he calculated using the “exact” and approximate methods are compared: the greatest difference in the he values amounts to 6%.

Experimental Investigation of Reacting Flow Characteristics in a Dual-Mode Scramjet Combustor

Abstract In this work, a hydrogen fueled dual-mode scramjet combustor was investigated experimentally. Clean and dry air was supplied to the combustor through a Mach 2 Nozzle with a total temperature of 800 K and a total pressure of 800 kPa. The high enthalpy air was provided by an electricity resistance heater. Room temperature hydrogen was injected with sonic speed from injector orifices vertically, and downstream the injector a tandem cavity flame holder was mounted. Except wall pressure profiles, velocity and temperature profiles in and at exit of the combustor were also measured using hydroxyl tagging velocimetry (HTV) and tunable diode laser absorption spectroscopy (TDLAS) respectively. Results showed that combustion occurred mainly at the bottom side of the combustor. And there were also an extreme disparity of the velocity and temperature profiles along the Y direction, i.e. the transverse direction.

Experimental and Numerical Study of Heat Transfer of High-Enthalpy Underexpanded Air Jets with Cylindrical Models

Experimental and Numerical Study of Heat Transfer of High-Enthalpy Underexpanded Air Jets with Cylindrical Models

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.

Experimental and Numerical Investigation of Heat Exchange between Underexpanded High-Enthalpy Air Jets and Cylindrical Models

Experiments on heat transfer in supersonic underexpanded high-enthalpy air jets are conducted on the VGU-4 induction plasmatron at the pressure in the compression chamber of 8.5 hPa. At the air flow rate of 3.6 g/s and the high-frequency generator powers of 45 kW(regime 1) and 64 kW (regime 2) the heat fluxes to the copper surface at the stagnation point of watercooled cylindrical models along the axes of dissociated air jets are measured. The models, 30 mm in diameter, could have a flat face or a hemispherical nose. In the same regimes, the stagnation pressures are measured using the Pitot tube in the shape of a cylinder, 30 mm in diameter, having either a flat face or a hemispherical bluntness with a receiving hole, 14 mm in diameter. For the experimental conditions calculations of flows in the plasmatron discharge channel and supersonic underexpanded jets issuing from the discharge channel are performed within the framework of the Navier–Stokes and Maxwell equations. The heat fluxes to the experimental models are computed and compared with the experimental data.

Methodology of experimental determining the combustion efficiency of fuel mixture flows in channels of variable cross-section

The physically validated approaches to the setting of an experiment are presented and the methodological aspects of an analysis of experimental data for establishing the distinctive features of the fuel mixture combustion in variable-cross-section channels are considered. The special features of determining the fuel combustion efficiency coefficient in subsonic and supersonic flows are considered with account for heat removal into the channel walls. The effect of the fuel combustion efficiency coefficient on the distributions of gasdynamic and thermodynamic parameters along the length of the internal ducts of channels of different configurations is analyzed. The results of the testing of the techniques proposed against an experimental investigation of gaseous hydrogen combustion in high-enthalpy air flows are presented. The results obtained and the mathematical models developed can be used at the stage of computational and experimental investigations of the working process in promising power plants.

Fabry–Perot spectroscopy for kinetic temperature and velocity measurements of a high enthalpy air plasma flow

The atomic translational temperatures and velocities of a low pressure, high enthalpy air plasma are measured using Fabry–Perot spectroscopy. The measurements presented here are the first measurements using this system at this enthalpy level. The sub-picometre resolution of the unique system has allowed accurate translational temperature and velocity measurements of the atomic species in the plasma. The detection system allows the Doppler broadening of multiple atomic nitrogen and oxygen lines to be measured simultaneously. Additionally, having two optical paths, one perpendicular to the flow and one at 45 deg. allows the Doppler shift to be measured.Measurements were taken during three different plasma wind tunnel tests. Mean atomic nitrogen temperatures of K and atomic oxygen translational temperatures of K were measured. The thermal non-equilibrium determined verified earlier measurements of the same phenomena, however, the mechanism behind this has not yet been determined. The mean measured flow velocity was and was consistent between the atomic species.The translational temperature and velocity contribute approximately 35% of the local enthalpy of the flow. The direct measurement of these parameters, removing previously required assumptions, increases the fidelity of the flow characterisation significantly. This allows high quality testing to be conducted in this flow field.

Theoretical study on the micro-scale oxidation of resin-infused carbon ablators

When subjected to high enthalpy air, phenolic-impregnated carbon-based ablative thermal protection systems undergo oxidation of their carbon fiber substrate and carbonized phenolic matrix. The oxidation process is governed by the competition between reaction and diffusion within the porous material and occurs at a range of depths depending on the flow conditions, the material chemical composition, and the material micro-structure. This study aims to examine the effects of the distribution of carbonized phenolic matrix on the oxidation behavior of carbon fiber materials, with the goal of guiding future material design. The oxidation is simulated on ideal geometries and on representations of actual fibrous carbon preforms obtained from X-ray microtomography. Diffusion is simulated using a random walk technique with a linear interpolation method for surface collisions. Oxidation reactions are simulated using a sticking probability law. It is shown that the oxidation characteristics, particularly the oxidation depth, are strong functions of the Thiele number and that the effect of the matrix distribution within the fibers is more pronounced at low Thiele regimes.

Параметрическое исследование взаимодействия частиц конденсированной фазы с высокоэнтальпийным потоком воздуха в прямоточной камере сгорания

Параметрическое исследование взаимодействия частиц конденсированной фазы с высокоэнтальпийным потоком воздуха в прямоточной камере сгорания

Experimental Simulation of Meteorite Ablation during Earth Entry Using a Plasma Wind Tunnel

Abstract Three different types of rocks were tested in a high enthalpy air plasma flow. Two terrestrial rocks, basalt and argillite, and an ordinary chondrite, with a 10 mm diameter cylindrical shape were tested in order to observe decomposition, potential fragmentation, and spectral signature. The goal was to simulate meteoroid ablation to interpret meteor observation and compare these observations with ground based measurements. The test flow with a local mass-specific enthalpy of 70 MJ kg−1 results in a surface heat flux at the meteorite fragment surface of approximately 16 MW m−2. The stagnation pressure is 24 hPa, which corresponds to a flight condition in the upper atmosphere around 80 km assuming an entry velocity of 10 km s−1. Five different diagnostic methods were applied simultaneously to characterize the meteorite fragmentation and destruction in the ground test: short exposure photography, regular video, high-speed imaging with 10 kHz frame rate, thermography, and Echelle emission spectroscopy. This is the first time that comprehensive testing of various meteorite fragments under the same flow condition was conducted. The data sets indeed show typical meteorite ablation behavior. The cylindrically shaped fragments melt and evaporate within about 4 s. The spectral data allow the identification of the material from the spectra which is of particular importance for future spectroscopic meteor observations. For the tested ordinary chondrite sample a comparison to an observed meteor spectra shows good agreement. The present data show that this testing methodology reproduces the ablation phenomena of meteoritic material alongside the corresponding spectral signatures.

Influences of Inlet Geometry Modification on Scramjet Flow and Combustion Dynamics

The influences of inlet geometry modification on flow and combustion dynamics are experimentally investigated in a model supersonic combustion ramjet (scramjet) installed in Mach 4.5 high-enthalpy air freestreams. The inlet upper-lip angle (6, 12, and 20 deg) and the intake contraction ratio (1.3 and 1.9) chosen as the generic inlet design parameters are varied. With the inlet geometries systematically modified, flowfields inside the model scramjet are visualized at cold conditions (room temperature) using a planar laser Rayleigh scattering method (wherein laser radiation is scattered by carbon dioxide particles). In addition, combustion dynamics and structures in the scramjet combustor are investigated in high-enthalpy flows with a total temperature of 2600 K capable of autoigniting injected fuels. High-frame-rate time-averaged chemiluminescence (broadband) imaging and hydroxyl planar laser-induced fluorescence are used for this investigation. Incident shock-wave strength, separated flow behavior, boundary-layer development, combustion structure/anchoring, and chemiluminescence intensity are found to be sensitive to the inlet geometry modifications.

Effects of Cavity Configurations on Flameholding and Performances of Kerosene Fueled Scramjet Combustor

AbstractIn this work, the effects of cavity flameholder configurations on flameholding and performances of kerosene fueled scramjet combustor were studied experimentally and numerically. For experiments, a directly connected ground facility was used and clean high enthalpy air, with a total temperature of 800 K and a total pressure of 800 Kpa, was provided by an electricity resistance heater. To investigate the effects of cavity configurations on flameholding capacity and reacting-flow characteristics, three different flameholders, one single cavity flameholder and two tandem cavity flameholders, were used in experiments. For the two combustors with tandem cavity flameholders, the location and configurations of its up-stream cavity were same with the single cavity flameholder, and the length-to-depth ratios for down-stream cavities were 9 and 11 respectively. The experimental results showed that stabilize kerosene combustion were achieved for combustor with tandem cavity flameholders mounted, and none for that with single cavity flameholder. The none-reacting and reacting flows of combustor models with tandem cavity flameholders were compared and studied with numerical and experimental results. The results showed that higher combustion efficiencies and pressure recovery ratios were achieved for the combustor with down-stream cavity length-to-depth ratio of 9.

Experimental Study of the Combustion Completeness of Two-Phase Gasification Products of Energetic Boron-Containing Condensed Compositions in a High-Enthalpy Air Flow

Experimental Study of the Combustion Completeness of Two-Phase Gasification Products of Energetic Boron-Containing Condensed Compositions in a High-Enthalpy Air Flow

Local modeling of the aerodynamic heating of the blunt body surface in subsonic high-enthalpy air flow. Theory and experiment on a high-frequency plasmatron

The concept of the local similarity of nonequilibrium boundary layers in high-enthalpy gas flows past blunt bodies is briefly described. The technical possibilities of the VGU-4 induction high-frequency plasmatron in modeling the aerodynamic heating of the hypothetical Pre-X (CNES) spacecraft in the vicinity of the stagnation point of a high-enthalpy air flow are presented. The engineering approach to quantitatively reproduce the thermochemical effect of a dissociated air flow on the vehicle surface in the high-heat region of the terrestrial entry trajectory is developed. In this approach the full-scale values of the total enthalpy, the stagnation pressure, and the velocity gradient at the stagnation point near the surface are reproduced in the experiment. The effective coefficients of O and N atom recombination on a silicone carbide (SiC) surface are determined under the conditions similar with those of the peak heating of the Pre-X vehicle surface in the vicinity of the flow stagnation point.

Tomographic optical emission spectroscopy of a high enthalpy air plasma flow.

A method is presented allowing for locally resolved emission spectroscopy using a tomographic setup. The approach presented in this work is applied to a high enthalpy air plasma flow. The resulting data sets allow for a three-dimensional (3D) representation of the non-symmetric flow field using photographs of the test section and 2D representation of the spectrally resolved radiance of the flow field. An analysis of different exposure times shows that transient fluctuations of the plasma can result in substantial asymmetry that approaches symmetry only for longer exposure times when the temporal averaging of the emission is significant. The spectral data allows the analysis of species selective excitation and emission. A non-equilibrium between atomic and molecular excitation temperatures is concluded for the investigated air plasma flow field. The spatial distribution of atomic electronic excitation temperatures are close to rotational symmetry while molecular rotational and vibrational temperatures exhibit asymmetric behavior.

Behavior of HfB2-SiC (10, 15, and 20 vol %) ceramic materials in high-enthalpy air flows

HfB2–SiC ceramic samples containing 10, 15, and 20 vol % silicon carbide were prepared by spark plasma sintering. The samples were characterized by X-ray powder diffraction, SEM, and other methods. Their densities and calculated porosities were determined. The behavior of the materials under heating by a subsonic dissociated air flow was studied on a VGU-4 high-frequency inductive plasmatron. The average surface temperatures of the 10 and 15 vol % SiC samples were shown to increase up to 2550–2675°C during heating, due to the generation of surface localities having temperatures of 2600–2700°C (the initial surface temperature was ~1700–1900°C) and the progressive growth of these regions in area. The overall time during which the average surface temperatures of these samples were higher than 2000°C, was about 31–32 min. For the 20 vol % SiC sample, heat removal (when the sample touched a water-cooled holder) was shown to influence the surface temperature and surface temperature distribution. The variation in gas-phase composition over the central area of the sample surface during an experiment was studied using emission spectroscopy. Explanations are proposed to the variation of boron and silicon concentrations in the course of exposure to high-enthalpy flows. The elemental and phase compositions were determined and the microstructures were studied on the surface and sections of samples after long-term (~40-min) exposure to high-enthalpy air flows.

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 .

A high-power low-temperature air plasma generator with a divergent channel of the output electrode

We have developed and studied a powerful high-enthalpy (H ≥ 20 kJ/g) air plasma jet generator with a divergent channel of the output electrode, which belongs to the class of dc plasmatrons with a thermionic cathode. The plasma generator possesses an efficiency of about 80% and ensures the formation of slightly divergent (2± = 12°) plasma jet with diameter D = 50 mm and a mass-average temperature of 6000–9000 K.

Production and study of megawatt air-nitrogen plasmatron with divergent channel of an output electrode

Megawatt generator of high-enthalpy air plasma jet (H ≥ 30 kJ/g) is constructed. Plasmatron belongs to the class of plasma torches with thermionic cathode, tangential swirl flow and divergent channel of an output electrode-anode. Plasma torch ensures the formation of the slightly divergent (2α = 12°) air plasma jet with the diameter D = 50 mm. The current-voltage characteristics of the plasma torch has virtually unchanged voltage relative to its current with enhanced (compared with arcs in cylindrical channels) stable combustion zone. Preliminary analysis of the obtained air plasma spectra shows that at a current of 1500 A near-axis zone of the plasma jet is characterized by a temperature of up to 15000 K, and the peripheral radiating area has a temperature of 8000-9000 K.