High Stagnation Temperatures Research Articles

Cold spraying of Al-aerospace alloys: Ease of coating deposition at high stagnation temperatures

Cold spray is the only solid-state coating technique to deposit thick metallic coatings. Deposition of metallic and metallic alloy powders is carried out at the temperature lower than the melting point. Since spraying of Al-based aerospace alloys are associated with nozzle clogging, successful deposition is restricted to nitrogen or helium as the main process gas with a polymer nozzle. Deposition of the alloys using air as process gas at elevated stagnation temperatures (up to 600 °C) has benefits in the repair and refurbishment sector, especially in aerospace. In this study, the effect of elevated stagnation temperatures (from 400 to 600 °C) using air as the main process gas on the deposition characteristics of aerospace grade Al-alloys (Cp-Al, Al-2024, Al-6061 and Al-7075) is studied. Deposition of the alloys at various stagnation temperatures is studied using FEM and correlated with the coating thickness. The deposition behaviour is studied using various parameters such as lattice misfit calculations and solid solution strengthening calculations. The ease of cold spray deposition of Al-aerospace alloys is systematically studied and listed as Al-6061 > Cp-Al > Al-7075 > Al-2024 for the first time.

Evaluation of the absorber temperature frequency function valid for evacuated flat plate collectors

The degradation of performances for solar thermal collectors is linked to the decrease of the absorber efficiency caused by diffusion process, strongly dependent on temperature. The Standard procedure for qualification of solar absorbers surface durability defines the absorber temperature frequency function as one of the necessary parameters to estimate the failure time for collectors, but it only refers to applications at low temperatures like domestic hot water production (DHW). Because of this deficiency of the Standard, in order to make aging predictions for selective solar absorbers mounted on Evacuated Flat Plate Collectors (EFPCs) used for Mid-Temperature applications (with stagnation temperature over 573 K), in this paper a procedure to reconstruct their specific temperature frequency function, considering the high thermal efficiency and stagnation temperature, is provided. A dynamic simulation model of a small plant with a TVP-Solar EFPC is implemented in Simulink environment to obtain the yearly absorber temperature history in operating conditions and experimental data are collected to obtain the temperature trend of the EFPC under stagnation. The absorber temperature frequency function valid for EFPCs is presented for different operating temperatures considering that stagnation occurs for 30 days every year, as the Standard prescribes. 30 days of stagnation in a year is an unrealistic assumption for thermal collectors used for industrial application and, in order to give an indication in this sense, the variation of aging parameters for different stagnation periods (fallowing periods) is also presented. Using the simulation model for service-time, experimental data for stagnation and assuming the exact fallowing period for the solar plant, a greater accuracy of EFPC failure time evaluation can be obtained.

Experimental investigations of indirect noise due to modulation of axial vorticity and entropy upstream of a choked nozzle

An experimental cold-gas study of the response of a choked convergent–divergent nozzle to swirl perturbations is presented. The perturbations were obtained by means of upstream unsteady tangential injections into initially steady flows with different values of steady background swirl. The swirl perturbations induced changes in the axial mass-flow rate, due to either their ingestion or evacuation by the nozzle. This in turn caused a downstream acoustic response. For low-intensity background swirl the responses were found to be similar to those obtained without steady background swirl. Perturbations of a high-intensity background swirl led to different effects. For long injection times, the negative mass-flow rate modulation occurred in two stages. The first stage was similar to that of the background-swirl free case. The second stage occurred after a short time delay, and induced a much stronger negative acoustic response. This unexpected behavior suggests that a significant part of the tangentially injected fluid flows upstream inducing an accumulation of swirl, which is – after tangential injection is ceased – suddenly cleared out through the nozzle. A scaling rule for the amplitudes of these acoustic responses is reported. Furthermore, quasi-steady models, based on steady-state measurements are proposed. These models predict the downstream acoustic response amplitude within a factor two. Additionally, preliminary empirical evidence of the effect of swirl on the downstream acoustic response due to the interaction of entropy patches with a choked nozzle is reported. This was obtained by comparison of sound produced by abrupt radial or tangential sonic injection, upstream from the choked nozzle, of air from a reservoir at room temperature to that from a reservoir with a higher stagnation temperature. Because the mass flow through the nozzle does not increase instantaneously, the injected higher-enthalpy air accumulates upstream of the injection-port position in the main flow. This eventually induces a large downstream acoustic pulse when tangential injection is interrupted. The magnitude of the resulting sound pulse can reach that of a quasi-steady response of the nozzle to a large air patch with a uniform stagnation temperature equal to that of the upstream-injected heated air. This hypothesis is consistent with the fact that the initial indirect-sound pulse is identical to one obtained with unheated air injection. The authors posit that – given all of the insight gleaned from them in this case – acoustic measurements of indirect sound appear to be a potentially useful diagnostic tool.

Nanophotonics-Enabled High-Efficiency Selective Solar Absorbers for Waste Heat Management

Selective solar absorbers (SSAs) harnessing solar energy as heat and converting it into thermal energy have gained much attention, specifically in solar thermoelectric generators and solar thermophotovoltaic systems. However, most of these SSAs suffer from low solar-to-heat conversion efficiency at moderately-high temperatures (100 – 400 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> C) and require complex nanofabrication techniques such as focused ion beam milling or electron beam lithography, hindering large-scale production. This article presents a large-area compatible and lithography-free design of SSA comprising a nanoscale multilayer stack of silica–titanium dioxide–silicon–germanium–chromium layers supported by a silicon substrate. We report 87% solar-to-thermal energy conversion efficiency with 889.4 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mathrm{Wm}^{-2}$</tex-math></inline-formula> net absorbing power at 100 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> C. At this temperature, the absorption coefficient achieved is 0.90, and the emission coefficient is reported to be 0.04. A perfect match between theoretically and numerically obtained spectral responses validates our findings. The fabrication tolerance study unveils that the proposed SSA design is expected to be robust and would be less susceptible to fabrication imperfections. This angle-insensitive and polarization-independent design offers a high stagnation temperature of 378 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> C, thus indicating that the proposed SSA can efficiently operate at high temperatures.

On jet-wake flame stabilization in scramjet: A LES/RANS study from chemical kinetic and fluid-dynamical perspectives

A three-dimensional hybrid LES (Large Eddy Simulation)/RANS (Reynolds-averaged Navier–Stokes) study dedicated to understanding the jet-wake flame stabilization under high inflow stagnation temperature in a hydrogen-fueled dual-mode scramjet was presented in this paper. The computational method features a finite-rate PaSR (Partially Stirred Reactor) turbulent combustion model with a detailed hydrogen reaction mechanism. The simulation results agreed well with the experimental results on overall characteristics of the jet-wake flame stabilization mode. Furthermore, quantitatively satisfactory predictions were attained for wall pressures. From the chemical kinetic perspective, the jet-wake flame stabilization can be divided into two regions. In the upstream region, both premixed- and non-premixed combustion are responsible for radical production, and the former produces more heat release. In the downstream region, turbulent non-premixed combustion dominates the flame stabilization. From the fluid dynamic perspective, the premixed flame is sustained by the counter-rotating vortex pair in the leeward side of the jet plume, which creates a local region with enhanced fuel/air mixing and reduced local flow velocity. Non-premixed combustion is found in the leeward side periphery of the fuel jet.

High-temperature hypersonic Laval nozzle for non-LTE cavity ringdown spectroscopy.

A small dimension Laval nozzle connected to a compact high enthalpy source equipped with cavity ringdown spectroscopy (CRDS) is used to produce vibrationally hot and rotationally cold high-resolution infrared spectra of polyatomic molecules in the 1.67 µm region. The Laval nozzle was machined in isostatic graphite, which is capable of withstanding high stagnation temperatures. It is characterized by a throat diameter of 2 mm and an exit diameter of 24 mm. It was designed to operate with argon heated up to 2000 K and to produce a quasi-unidirectional flow to reduce the Doppler effect responsible for line broadening. The hypersonic flow was characterized using computational fluid dynamics simulations, Pitot measurements, and CRDS. A Mach number evolving from 10 at the nozzle exit up to 18.3 before the occurrence of a first oblique shock wave was measured. Two different gases, carbon monoxide (CO) and methane (CH4), were used as test molecules. Vibrational (Tvib) and rotational (Trot) temperatures were extracted from the recorded infrared spectrum, leading to Tvib = 1346 ± 52 K and Trot = 12 ± 1 K for CO. A rotational temperature of 30 ± 3 K was measured for CH4, while two vibrational temperatures were necessary to reproduce the observed intensities. The population distribution between vibrational polyads was correctly described with Tvib I=894±47 K, while the population distribution within a given polyad (namely, the dyad or the pentad) was modeled correctly by Tvib II=54±4 K, testifying to a more rapid vibrational relaxation between the vibrational energy levels constituting a polyad.

Effects of Transverse Jet Parameters on Flame Propagation and Detonation Transition in Hydrogen–Oxygen–Argon Mixture

ABSTRACT Two-dimensional numerical simulation is performed with the open-source program AMROC to study the effects of transverse jets (act as fluidic obstacles within a detonation tube) on the flame acceleration and deflagration to detonation transition (DDT). The slot transverse jets have been studied and compared with conventional solid obstacles in tubes. The jet initial parameters, such as mixture composition, stagnation temperature, pressure, and mass flow rate, are investigated. The results demonstrate that a hydrogen-oxygen-argon reactive fluidic obstacle leads to the shortest DDT distance and time compared with solid obstacles and fluidic obstacles composed of pure oxygen or argon. The fluidic obstacles can induce more vorticities to accelerate flame propagation. The DDT distance and time decrease with the jet initial temperature, pressure, and mass flow rate rise, while a high jet initial stagnation temperature is counterproductive to shorting DDT distance and time. The local static pressure rise plays an important role in flame acceleration when increasing the initial pressure of the fluidic obstacle. Higher jet pressure and a wider jet induce more compression waves, which can make the initial flame front more unstable and accelerate the flame as well.

Experimental Testing of Hydrophobic Microchannels, with and without Nanofluids, for Solar PV/T Collectors

Solar energy can be converted into useful energy via photovoltaic cells or with a photothermal absorber. While these technologies are well-developed and commercially viable, significant benefits can be realised by pulling these two technologies together in photovoltaic/thermal (PV/T) systems which can provide both heat and electricity from a single collector. Emerging configurations in the PV/T field aim to incorporate micro and/or nanotechnology to boost total solar utilisation even further. One example of this is the nanofluid-based PV/T collector. This type of solar collector utilises nanofluids—suspensions of nanoparticles in traditional heat transfer fluids—as both an optical filter and as a thermal absorber. This concept seeks to harvest the whole solar spectrum at its highest thermodynamic potential through specially engineered nanofluids which transmit the portion of solar spectrum corresponding to the PV response curve while absorbing the rest as heat. Depending on the nanoparticle concentration, employing nanofluids in a flowing system may come with a price—an efficiency penalty in the form of increased pumping power (due to increased viscosity). Similarly, microchannel-based heat exchangers have been shown to increase heat transfer, but they may also pay the price of high pumping power due to additional wall-shear-related pressure drop (i.e., more no-slip boundary area). To develop a novel PV/T configuration which pulls together the advantages of these micro and nanotechnologies with minimal pumping power requirements, the present study experimentally investigated the use of nanofluids in patterned hydrophobic microchannels. It was found that slip with the walls reduced the impact of the increased viscosity of nanofluids by reducing the pressure drop on average 17% relative to a smooth channel. In addition, flowing a selective Ag/SiO2 core–shell nanofluid over a silicon surface (simulating a PV cell underneath the fluid) provided a 20% increase in solar thermal conversion efficiency and ~3% higher stagnation temperature than using pure water. This demonstrates the potential of this proposed system for extracting more useful energy from the same incident flux. Although no electrical energy was extracted from the underlying patterned silicon, this study highlights potential a new development path for micro and nanotechnology to be integrated into next-generation PV/T solar collectors.

Experimental Investigations of Film Cooling in a Conical Nozzle Under Rocket-Engine-Like Flow Conditions

The current work presents an experimental parametric study on tangential, supersonic film cooling in the expansion part of a conical nozzle. For simulating rocket-engine-like hot gas conditions, the nozzle is attached to a detonation tube providing hydrogen-oxygen combustion gas at high stagnation pressures and temperatures. The Mach number of the hot gas at the point of injection is about , whereas the coolant is injected at Mach numbers of . Within the presented study the coolant injection conditions, the hot gas conditions, and the coolant gases are varied to determine the influence of different parameters on film cooling behavior. The results indicate that film cooling effectiveness massively depends on the blowing ratio and slot height. Film cooling efficiency is also found to depend on the coolant gas heat capacity, molar mass, and the density and velocity ratio of coolant and hot gas at the point of injection. To compare the experimental data the film cooling efficiencies are calculated using the measured wall heat fluxes and a newly developed, simple approach for the calculation of heat transfer coefficients. For a theoretical investigation of the experimental results the common Goldstein model is extended for axisymmetric flows. Finally, the gained experimental data are correlated by the extended Goldstein model.

A Facile and General Strategy to Deposit Polypyrrole on Various Substrates for Efficient Solar‐Driven Evaporation

AbstractSolar‐driven evaporation for clean water production is currently considered to be one of the most effective and sustainable strategies to alleviate water shortages because of the sustainability, eco‐friendliness, and inexhaustibility of solar energy. Plenty of effort has been paid to developing easy‐to‐follow methods to fabricate the high‐performance photothermal materials for efficient water evaporation. In this manuscript, a facile and general strategy named chemical vapor deposition polymerization (CVDP) to deposit dark PPy coating layer onto various substrates, which acts as a light absorber to efficiently harvest and convert light to heat for solar‐driven interfacial water evaporation, is reported. The obtained PPy‐coated membranes can achieve an outstanding light absorption and show a high stagnation temperature up to 82.3 °C under 1 sun illumination (1 kW m−2). Furthermore, the water evaporation can be accelerated to 1.41 kg m−2 h−1, corresponding to 81.9% on solar conversion efficiency which is comparable to the results of the state‐of‐the‐art photothermal membranes. Due to the moderate reaction conditions, the PPy coating layer can also be deposited onto different porous substrates to meet different water environments. Such novel and effective CVDP strategy for high‐performance photothermal membrane fabrication contributes to the widespread applications in sustainable clean water production.

Computational realization of multiple flame stabilization modes in DLR strut-injection hydrogen supersonic combustor

Inspired by the existence of multiple flame stabilization modes in cavity-assisted supersonic combustor, multiple flame stabilization modes of DLR hydrogen-fueled strut injection supersonic combustor were numerically realized and analyzed for a wide ranges of inflow stagnation temperature from 607 to 2141 K and overall equivalence ratio from 0.022 to 0.110. Finite-rate chemistry large eddy simulation with detailed hydrogen mechanism was employed to capture unsteady flow characteristics and the effects of chemical kinetics. Two typical flame stabilization modes were identified and presented in a regime nomogram, which shows the dominant influence of the stagnation temperature and the secondary influence of overall equivalence ratio. At relatively low stagnation temperatures, the flame is stabilized in an “attached flame” mode, which requires a low-speed recirculation zone behind the strut for radical production and a high-speed intense combustion zone for heat release. At relatively high stagnation temperatures, the flame is stabilized in a “lifted flame” mode, in which the effect of the low-speed recirculation zone is negligible, rendering most reactions take place in supersonic flow. At intermediate stagnation temperatures, blow-out was always observed and flame cannot be stabilized in the combustor even with initially forced ignition.

Combustion stabilization modes in a hydrogen-fueled scramjet combustor at high stagnation temperature

Combustion stabilization modes were studied experimentally under a Mach 2.52 supersonic inflow at a high stagnation temperature of 1629 K. Optical observations including the high-speed flame luminosity and schlieren images were utilized to capture the combustion stabilization characteristics. Effects of the equivalence ratio and the injection distance on the combustion stabilization mode transition were discussed. Two supersonic combustion stabilization modes are identified clearly in the scramjet combustor, as the cavity stabilized mode and the jet-wake stabilized mode. At a relatively low equivalence ratio, the flame is stabilized in the cavity stabilized mode, and it will transfer to the jet-wake stabilized mode with the increase of the equivalence ratio. The flame stabilization mode is also affected by the injection distance. The critical equivalence ratio for mode transition reduces with the decrease of the injection distance. In the cavity stabilized mode, the shear layer of the cavity stabilizes the flame. In the jet-wake stabilized mode, the intense combustion induces high pressure, and leads to a large boundary-layer separation zone that exists in front of the flame. It contributes to the flame stabilization. In addition, a triple-reaction-zone mechanism is proposed to further explain the transition process from the cavity stabilized mode to the jet-wake stabilized mode, and the large boundary-layer separation zone also contributes to the transition process.

Theoretical and Numerical Study of a Preheated Ludwieg Tube with Adiabatic Compression

This paper describes a novel hypersonic facility to reproduce the high stagnation pressures and temperatures necessary for the accurate simulation of hypersonic flows in the range Mach 5–7, while providing test times sufficiently long to study unsteady flow effects. The facility uses a preheated Ludwieg tube together with a piston-compression stage to generate reservoir temperatures difficult to achieve with electrical heating alone, but also avoiding the introduction of vitiation contaminants. The absence of shocks in the generating flow is expected to lead to increased flow quality while also providing longer test times compared with shock-driven facilities of comparable size. The operational concept is described and a condition for optimal operation is defined based on minimizing the dimensions of the heated section for a given set of desired flow conditions. A simple theoretical treatment shows that this condition constrains the ratio of the nozzle exit and Ludwieg tube diameters, assuming accurate simulation of flight conditions. A method-of-characteristics solver is used to determine the constrained facility parameters more accurately. Modeling of the unsteady adiabatic compression cycle is performed with quasi-one-dimensional finite volume computations, and various configurations are explored to minimize the pressure oscillations generated during compression.

Influence of condensation on heat flux and pressure measurements in a detonation-based short-duration facility

Detonation-based short-duration facilities provide hot gas with very high stagnation pressures and temperatures. Due to the short testing time, complex and expensive cooling techniques of the facility walls are not needed. Therefore, they are attractive for economical experimental investigations of high-enthalpy flows such as the flow in a rocket engine. However, cold walls can provoke condensation of the hot combustion gas at the walls. This has already been observed in detonation tubes close behind the detonation wave, resulting in a loss of tube performance. A potential influence of condensation at the wall on the experimental results, like wall heat fluxes and static pressures, has not been considered so far. Therefore, in this study the occurrence of condensation and its influence on local heat flux and pressure measurements has been investigated in the nozzle test section of a short-duration rocket-engine simulation facility. This facility provides hot water vapor with stagnation pressures up to 150 bar and stagnation temperatures up to 3800 K. A simple method has been developed to detect liquid water at the wall without direct optical access to the flow. It is shown experimentally and theoretically that condensation has a remarkable influence on local measurement values. The experimental results indicate that for the elimination of these influences the nozzle wall has to be heated to a certain temperature level, which exclusively depends on the local static pressure.

A PVT Collector Concept with Variable Film Insulation and Low-emissivity Coating

Hybrid photovoltaic-thermal (PVT) collectors co-generate solar electricity and heat in a single component with an optimum utilization of space. Spectrally selective but transparent low-emissivity (low-e) coatings are a proven method to reduce thermal losses. However, overheating and stagnation are critical issues for these collectors due to material degradation, thermal stress, and low electrical efficiency.This paper presents a PVT collector concept with variable film insulation as overheating protection. An inflatable glass-film cushion regulates thermal losses. Performance and stagnation tests were carried out with a prototype. During normal operation the collector achieves a high thermal efficiency. In periods of standstill, the deflated cushion has a high heat dissipation rate by deactivating the low-e coating. Stagnation temperatures are thus limited to 95 ̊C. To conclude, the PVT collector combines the advantages of glazed and unglazed PVT collectors which are a high thermal efficiency and low stagnation temperatures.

Modelling of the process of fragmentation and vaporization of non-reacting liquid droplets in high-enthalpy gas flows

The intensification of the fragmentation and vaporization of liquid droplets in two-phase flows with the gas stagnation temperature Тg = 800−2500 K is an important scientific and technological problem. One should note that despite a high practical importance the mechanism of the vaporization of droplets with their preliminary gas-dynamic fragmentation in high-enthalpy flows has been studied insufficiently completely and requires additional research. The paper presents a mathematical model and the results of the computations of the fragmentation and vaporization of liquid droplets in subsonic and supersonic flows with a high stagnation temperature. A comparison of the obtained data with the experiments of other authors has been done. The extension of the regions of the gas-dynamic fragmentation and droplet vaporization in flow ducts with a variable distribution of parameters has been estimated. The found peculiarities may be used at the design of energy installations of the promising samples of the aerospace technology and gas-dynamic pipes.

Influence of vibrational relaxation on perturbations in a shock layer on a plate

The influence of excitation of molecular vibrational degrees of freedom on the mean flow and perturbation development in a hypersonic (M = 6–14) viscous shock layer is studied. The layer originates on a plate placed in a flow of air, carbon dioxide, or their mixture at high stagnation temperatures (2000–3000 K). The mean flow and pressure pulsation on the surface of the plate are measured in an IT-302M pulsed wind tunnel (Khristianovich Institute of Theoretical and Applied Mechanics, Siberian Branch, Russian Academy of Sciences). Numerical simulation is carried out in terms of a model of a thermally perfect gas using the ANSYS Fluent program package based on solving nonstationary two-dimensional Navier-Stokes equations. External flow perturbations are introduced into the computational domain in the form of plane monochromatic acoustic waves using UDF modules built in the computational code. It is shown that the excitation of vibrational degrees of freedom in carbon dioxide molecules considerably influences the position of the head wave and intensifies perturbations in contrast to air in which the fraction of vibrationally excited molecules is low at the same parameters of the oncoming low. The influence of the excitation of vibrational degrees of freedom is studied both for equilibrium gas and for a vibrationally nonequilibrium gas. Nonequilibrium vibrational degrees of freedom are simulated using a two-temperature model of relaxation flows in which the time variation of the vibrational energy is described by the Landau-Teller equation with regard to a finite time of energy exchange between vibrational and translational-rotational degrees of freedom of molecules. It is found that the vibrational nonequilibrium has a damping effect on perturbations.

Optical Analysis of a Hexagonal 42kWe High-flux Solar Simulator

A 42-kWe high-flux solar simulator with hexagonal reflector symmetry has been designed, built and optically characterized at IMDEA Energy Institute, Spain. This facility makes possible the lab-scale generation of a quasi-uniform high radiation flux density and high stagnation temperatures and it will be used as a platform for analyzing processes under concentrating solar energy conditions; for instance, central receivers in concentrating solar power plants and solar fuel production process in thermochemical reactors. The high-flux solar simulator consists of seven reflector-lamp pairs arranged in the center and vertices of a regular hexagon. The 6-kWe Xe short arc lamps are allocated in the primary focus of the corresponding truncated ellipsoidal reflector. This hexagonal symmetry provides compactness and quasi-uniform spatial distribution of the radiation at the system common focal plane.This work presents the experimental characterization of the solar simulator optical performance. Preliminary measurements indicate an average flux density at the focal plane of 3.5 MW/m2 that means 3,500 suns (1 sun = 1kW/m2) and stagnation temperature of approximately 2,800K.

Modeling of Two-Stage Ejector for High-Altitude Testing of Satellite Thrusters

The performance of a two-stage ejector during high-altitude testing of large-area-ratio satellite thrusters is numerically investigated. Since theflowrate of the exhaust from the satellite thruster is very low, self-ejector action of the exhaust is quite weak; therefore, a two-stage external ejector is required to create the desired low vacuum in the test chamber. The present work attempts to investigate the effects of various operational and geometric parameters on the performance of the two-stage ejector. Predicted results show that the downstream ejector (E2) operation is more critical for maintaining the required vacuum. However, for optimal performance, it is possible to tune the parameters such that both ejectors deliver the same suction effect.Maximumperformance of each ejector is obtained when the primary jet expanding from the nozzle smoothly seals the mixer throat against backflow. Employing a low molecular weight fluid and high stagnation temperature helps in reducing the quantity of fluid required for test facility evacuation. Useful correlations have been derived to quantify the suction performance of the two-stage ejector, and the predicted results compare well with in-house experimental data.

Time-Resolved Visualization of Instability Waves in a Hypersonic Boundary Layer

LAMINAR-TURBULENT transition in hypersonic boundary layers remains a challenging subject. This is especially true of the hypervelocity regime, in which an intriguing phenomenon is the possible damping of second-mode disturbances by chemical and vibrational nonequilibrium processes. To generate flows with sufficiently high enthalpy to investigate such effects, the use of shock-tunnel facilities is necessary; furthermore, it is now generally accepted that direct measurements of the instability mechanisms active within the boundary layer, together with a characterization of the freestream disturbance environment, are required, as simple measurements of transition locations can lead to ambiguous conclusions. However, as difficult as the accurate measurement of instability waves in conventional hypersonic facilities can be, in shock tunnels it is appreciably more so. For identical unit Reynolds numbers, the higher stagnation temperature in a shock tunnel means that the dominant second-mode disturbances lie at even higher frequencies (typically hundreds of kHz or higher); moreover, because of the destructive testing environment, hot-wire techniques, a staple for instability measurements in conventional tunnels, cannot be used. Fast-response pressure transducers are an obvious alternative, but recent experiments have highlighted the challenging nature of interpreting data from mechanically sensitive sensors in the high-noise environment of a shock tunnel, especially without accompanying stability computations. Measurements with recently developed atomic-layer thermopile (ALTP) heat-flux sensors show promise, though their use has yet to be demonstrated in shocktunnel facilities.