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An experimental investigation is carried out to study the effects of Mach number on the acoustic resonance characteristics of a rectangular cavity with and without multiple stores. Measurements are conducted for a cavity with a length-to-depth ratio of 4.5 at Mach numbers of 2, 3, 4, and 5. Dynamic pressures are measured within the cavity and on the store surfaces. An increase in Mach number results in a decrease in overall sound pressure levels at all measurement locations within the cavity. Pressure spectra indicate that increasing the Mach number decreases both the peak and broadband amplitudes of the cavity pressure fluctuations. At Mach 5, the amplitude of the cavity modes is significantly reduced in the upstream half of the cavity, and the modes are either indistinguishable from broadband levels or absent in the downstream half. The addition of stores reduces cavity acoustics, with two stores offering greater reduction than a single store. The effect of Mach number and the presence of stores on the aeroacoustic feedback loop in a cavity align with literature. Additionally, the pressure fluctuations on the windward store surface scale with Mach number in a similar trend as the cavity acoustics. [ABSTRACT FROM AUTHOR]

A pitot probe is commonly used for the disturbance quantification of a hypersonic freestream, but the pitot results cannot reflect the actual disturbance amplitudes due to the turbulence/shock-wave interactions as well as the disturbance resonances. In this work, we characterize the disturbance resonance in the postshock zone of a blunt body using both experimental and theoretical approaches. A one-dimensional interference model with four traveling sound waves is proposed to simulate the resonance phenomenon due to the reflection between the detached shock and the wall of the pitot probe. Subsequently, the total pressure fluctuation on the surface and the density fluctuation in the space along the centerline are investigated using a pressure sensor and a noninvasive focused laser differential interferometer. Both the experimental results and theoretical analysis show the resonance mechanism on the surface is dominated by the interference between the initial diverging sound wave and the shock-reflected sound wave. In contrast, the stationary wave formed by the initial diverging sound wave and the body-reflected wave plays a leading role in the space. Finally, an equation is proposed to approximate the destructive resonant frequency in the space. [ABSTRACT FROM AUTHOR]

The study is aimed at measuring the gas flow temperature by thermocouples whose time of reaching the equilibrium temperature is comparable with the measurement time, and heat release to structural elements of the transducer can be rather large. The results of numerical simulations of the gas flow in the temperature transducer used for measuring the stagnation temperature in high-enthalpy hotshot wind tunnels are presented. A coupled problem of the air flow around the temperature transducer is solved, and the flow field inside the stagnation chamber is calculated with allowance for heat losses to input wires and structural elements of the transducer. The data obtained are considered as the results of a virtual experiment and are treated by methods of experimental aerodynamics. The retrieved results are compared with the initial numerical values of the stagnation temperature in the flow impinging onto the transducer. The sources of uncertainties arising in temperature measurements are determined, and the applicability of experimental methods for determining the stagnation temperature in short-duration wind tunnels, including those with parameters decreasing during the run, is justified. It is shown that the method of "two thermocouples" can be successfully used to determine the stagnation temperature even if the heat losses to transducer elements are comparable with heat input from the gas flow. The values of the retrieved stagnation temperature correspond to the flow temperature in the transducer within 1.2–3 % depending on the initial temperature of the thermocouple. [ABSTRACT FROM AUTHOR]

Concentrated collectors offer a broad variety of solar energy uses, including heating, cooling, power production, and water desalination. This study was conducted to construct and test a concentrated parabolic solar dish water heater. The aperture size of the dish is 4.556m², and a copper absorber has a surface area of 0.2278m², a volume of 0.015m³, and a concentrating ratio of 20. The water is heated up to 120°C at a solar radiation intensity of 700W/m² and a 20°C ambient temperature. The absorber's stagnation temperature reached 246°C in roughly 1500 seconds. The thermal efficiency of the system was found to be 46%. [ABSTRACT FROM AUTHOR]

High-speed impinging jets issued from the exhaust of a launch vehicle are highly oscillatory and create hazardous conditions for the vehicle structure, affecting the sensitive payloads and personnel due to intense acoustic loads. Therefore, to reduce the unsteadiness of a jet and mitigate its adverse effects, a fundamental understanding of the associated flow and acoustic field is of utmost importance. The present experimental study is one such step to characterize the aeroacoustic properties of a high-temperature jet issued from an overexpanded rocket nozzle. Further, the effectiveness of microjet-based active flow control in reducing the flow unsteadiness and near-field noise levels is also explored. The experiments were performed at different jet stagnation temperatures, and the results were quantified at different impingement heights simulating the takeoff and landing environment of reusable launch vehicles. Mean pressure measurements were mapped on the ground plane using discrete pressure taps. The time-varying pressure component was measured using an unsteady pressure transducer located at the impingement point, and near-field acoustic measurements were carried out using a microphone. In addition, the velocity field was mapped using planar particle image velocimetry. The near-field pressure spectra are broadband, and the magnitude of noise content is a function of jet stagnation temperature. Furthermore, the injection of microjets resulted in a significant attenuation of near-field noise, impingement point pressure fluctuations, and turbulent kinetic energy. [ABSTRACT FROM AUTHOR]

Hypersonic boundary-layer stability has significant importance in vehicle design and successful operation. This paper investigates the stabilization effects of local wall cooling on the hypersonic boundary layers over a 5 deg half-angle blunt cone with a nose radius of 0.0254 mm. We employed a high-order-accurate flow solver to calculate the steady flow for a freestream Mach number of 6.0 and a unit Reynolds number of 25.59×106/m. In simulations, we considered partial wall cooling, entire wall cooling, and adiabatic wall scenarios. Furthermore, we examined partial cooling parameters such as strip location, length, and temperature profiles. We calculated the growth rates, phase speed, and N-factor diagrams using a linear stability analysis. The results showed that complete wall cooling destabilizes the boundary layer. However, the cooling strip upstream of the synchronization point stabilized the boundary layer by damping the disturbances. The longer cooling strip further stabilized the boundary layer. The cooling strip placed downstream of the synchronization point destabilized the boundary layer. [ABSTRACT FROM AUTHOR]

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The present study investigates a novel control strategy to create a sweeping oscillatory jet using fluidic actuation principle. A supersonic underexpanded primary jet issued into a suddenly expanding duct is oscillated by giving cyclic pulsed secondary jet blowing in the top and bottom step regions of the suddenly expanding duct. The current study employs the bistable nature of jet expansion seen in a suddenly expanding duct for the jet oscillation. The cyclic perturbations (pulsed secondary jet blowing) given in the streamwise direction switch the direction of the asymmetric jet expansion of the underexpanded primary supersonic jet to the top and bottom wall alternatively, thereby creating an oscillatory supersonic jet. It is found that the switching of the direction of the primary jet to any one wall is caused by the Coanda effect produced by the secondary jet issued in the corresponding wall side. The results show that the primary jet quickly responds to the secondary jet perturbations and that the supersonic primary jet oscillation frequency nearly matches secondary jet perturbation frequency. A parametric study to investigate the jet oscillation characteristics with various secondary jet oscillation frequencies and mass flow rate ratios are also being carried out in the present study. It is found that the jet oscillation frequency increases with increase in the secondary jet pulsation frequency. The jet deflection is found to increase with increase in the secondary jet to primary jet mass flow ratio (blowing ratio) up to a critical limit, after which the increment in jet deflection ceases due to the restrictions imposed by the duct geometry. [ABSTRACT FROM AUTHOR]

In this study, combustion processes in a cavity-based scramjet combustor are experimentally studied based on the inflow conditions of different stagnation pressures (P0=1.0, 1.3, and 1.6 MPa), which correspond to a Mach 6 flight at trajectories of different dynamic pressures of 0.25, 0.34, 0.43 atm, respectively. Three combustion modes are identified according to flame distribution, flowfield structure, and wall pressure. Cases at the same equivalence ratio and combustion mode with different inflow stagnation pressures are selected to compare flame characteristics. For each inflow condition, flame structure, combustion intensity, and coherence between upstream and downstream flames are analyzed based on CH* chemiluminescence and schlieren imaging measurements. Combustion instability is examined according to statistical results, frequency domain and spatial distribution. It is found that the linearly increased inflow stagnation pressure causes irregularity variance in the combustion oscillation spectra, indicating the quasi-linearly enhanced combustion induces complicated aerodynamic and thermodynamic variances. Chaos of the combustion oscillation is quantitatively evaluated by a classical nonlinear analysis. The dynamic mode decomposition analysis presents that the high-frequency oscillation induced by the self-sustaining shear-layer instability exist mainly in the combustor upper region while the low-frequency oscillation that could be caused by the injector-flame feedback is predominant in the cavity region. In addition, thermo-acoustic couplings, which are in connection with these two kinds of oscillations, have been analyzed. It is further revealed that the variance of the shock waves caused by the backpressure destroys the existing thermo-acoustic feedback related to the high-frequency oscillation, which is the main reason for the chaotic phenomena shown in the transitional state between the scramjet mode and the ramjet mode. [ABSTRACT FROM AUTHOR]

The influence of the flat-plate leading-edge shape on laminar-turbulent transition is investigated. Experiments were carried out at a Mach number of M8=5; a unit Reynolds number Re18 from 1.5×107 to 9×107 m-1; and a Reynolds number Reb8, based on the leading-edge thickness b, up to 7×105. Four leading-edge shapes were investigated: cylindrical, elliptical, flat face, and "smoothed" cylindrical. It has been shown that for Reb8<0.4×105, these shapes practically do not affect transition; whereas at large Reb8, the laminar run can be lengthened or shortened by the leading-edge shaping. The influence of bluntness thickness on the flow parameters at transition onset is analyzed by solving the Navier-Stokes and boundary-layer equations. [ABSTRACT FROM AUTHOR]

• Overheating limitation of solar collectors by controlling heat pipe fluid. • Efficiency of all glass evacuated tubes with heat pipes. • Stagnation temperature of all glass evacuated tube can be controlled by dry out limit. • Heat transfer is prevented above a certain temperature in solar water heaters. Overheating is a major problem in all glass evacuated tube with heat pipe when hot water in a pressurized thermosiphon solar water heater is not consumed. Stagnation temperature of a system can be limited in a controllable manner without significantly degrading operating temperature performance. In this approach the dry out limit is used, which can be reached when the filled amount of heat pipe fluid is quite small. In this study, it is aimed to prevent the overheating problem by controlling the heat pipe fluid while almost keeping the efficiency at the same level up to 80 °C with the existing one. In order to stabilize the temperature at 95 °C, an efficiency reduction above 80 °C is utilized on the summer days which are the hottest and have the highest radiation. As the water temperature in the tank increases, the heat transfer of all glass evacuated tube to the water decreases. Another important factor to achieve this balance is nocturnal cooling of the water in the tank during the night. Three different liquids which are ethanol, methanol, and acetone are used as the heat pipe fluid in the experiments in addition to the existing one. Daily conversion efficiency for new designed all glass evacuated tube containing ethanol is equal to 77.5%, based on collector aperture area, up to water temperature of 80 °C in field test. It is observed that the experimental results are consistent. The all glass evacuated tube designed in this study has been started to be used commercially in pressurized thermosiphon solar water heaters, taking into account the results obtained from field experiments. [ABSTRACT FROM AUTHOR]

Methodological aspects of stagnation-temperature measurement in high-enthalpy supersonic air flows using a thermoelectric transducer (thermocouple) with shielded measuring junction are analyzed. Specific features of thermo- and gas-dynamic processes under the conditions of interest, leading to a substantial deviation of temperatures measured using traditional measuring methods from the true temperatures, are demonstrated. Refinements to the method of determining the temperature of high-enthalpy supersonic air flows allowing one to reduce the deviation from the true temperature are introduced. A mathematical apparatus developed for analysis of experimental data and for quantitative estimation of temperature-measurement error is described. Approbation of the proposed approaches and validation of the developed mathematical models are performed using the results of experimental studies of high-enthalpy supersonic flows. A satisfactory agreement between the results of indirect temperature measurements and specified temperature values is shown. The results of the study can prove useful in the development of techniques and methods, and in the analysis of physical experiments with high-enthalpy flows. [ABSTRACT FROM AUTHOR]

The aim of this work is to develop a calculation model based on the method of characteristics making it possible to study the effect of the stagnation pressure of the combustion chamber on the 2D and axisymmetric minimum length nozzle design giving a uniform and parallel flow at the exit section. The model is based on the use of the real gas approach. The co-volume and the intermolecular interaction effect are taken into account by the use of the Berthelot state equation. The effect of molecular vibration is considered in our model to evaluate the behaviour of gas at a high temperature. In this case, the stagnation pressure and the stagnation temperature are important parameters in our model. The resolution of the algebraic equations is done by the finite difference corrector predictor algorithm. The validation of the results is controlled by the convergence of the critical section ratios calculated numerically as obtained by the theory. The mass and the thrust are evaluated to improve the efficiency of the nozzle. The comparison is made with the high temperature and perfect gas models. The application is made for air. [ABSTRACT FROM AUTHOR]

Abstract 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. [ABSTRACT FROM AUTHOR]

The presented research deals with an experimental and numerical evaluation of the thermal characteristics of a novel prototype polymer solar collector design. The experimental part comprises an alternative approach for determining the optical characteristics of polymer materials and measurements of thermal efficiency. Functional dependency of thermal efficiency on solar radiation, working fluid and air temperature is computed. In order to validate the numerical model built in the ANSYS FLUENT software package, simulations are performed on a segment of the polymer solar collector, and the findings are correlated with the experimental ones. The efficiency curve is determined for a whole collector consisting of eight analysed segments. The obtained efficiency of the proposed polymer collector design is 20% lower relative to the state-of-the-art flat plate collector during the typical summer operating regime. A parametric numerical analysis of a polymer solar collector is carried out to evaluate the influence of design and operating parameters on thermal performances and to provide design improvement guidelines. In addition, stagnation temperature measurements are conducted in accordance with EN ISO 9806:2017 when a stagnation temperature of 125.1 °C is recorded after the application of overheating protection measures. • Novel design of polymer solar collector is presented. • Computed thermal efficiency is 20% lower than FPC. • Parametric analysis of thermal performances is carried out. • Stagnation temperature measurements are conducted. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Highlights • The stagnation temperature of solar circuits is significantly reduced by heat pipes. • The power shut-off is affected by dry-out and superheated steam in the heat pipe. • The superheating effect is theoretical described by several fluid parameters. • The model is successfully validated by comprehensively experimental investigations. • Dry-out and superheating is demonstrated by collector performance measurements. Abstract Heat pipes in solar thermal collectors enable to reduce the temperature loads in the solar circuit during stagnation periods by exploiting their dry-out limit. With this approach vapour formation in the solar circuit can be completely avoided, which is essential to reduce costs of solar thermal systems by simplified and more reliable solar circuits. The design of “deactivating” collector heat pipes with a desired maximum temperature requires a comprehensive understanding of the heat transfer processes in the heat pipe, in particular when dry-out takes place. We developed a model, which allows calculating the maximum fluid temperature in the collector for various working fluids. Compared to existing approaches, the effect of superheated vapour in the heat pipe during stagnation is additionally considered. The paper describes the theoretical model in detail and its extensive experimental validation. The results show that the model is able to predict the maximum fluid temperature with an accuracy better than 5 K. Based on parametric studies with different working fluids, we analyse and discuss the temperature limitation and its effect on the collector performance. [ABSTRACT FROM AUTHOR]

Concentrating Photovoltaic Thermal (C-PVT) solar collectors produce both thermal and electric power from the same area while concentrating sunlight. This paper studies a C-PVT design where strings of series-connected solar cells are encapsulated with silicone in an aluminium receiver, inside of which the heat transfer fluid flows, and presents an evaluation on structural integrity and performance, after reaching stagnation temperatures. Eight test receivers were made, in which the following properties were varied: Size of the PV cells, type of silicone used to encapsulate the cells, existence of a strain relief between the cells, size of the gap between cells, and type of cell soldering (line or point). The test receivers were placed eight times in an oven for one hour at eight different monitored temperatures. The temperature of the last round was set at 220 °C, which exceeds the highest temperature the panel design reaches. Before and after each round in the oven, the following tests were conducted to the receivers: Electroluminescence (EL) test, IV-curve tracing, diode function, and visual inspection. The test results showed that the receivers made with the transparent silicone and strain relief between cells experienced less microcracks and lower power degradation. No prototype test receiver lost more than 30% of its initial power, despite some receivers displaying a large number of cell cracks. The transparent and more elastic silicone is better at protecting the solar cells from the mechanical stress of thermal expansion than the compared silicone alternative, which was stiffer. As expected, larger cells are more prone to develop microcracks after exposure to thermal stress. Additionally, existing microcracks tend to grow in size relatively fast under thermal stress. EL imaging taken during our experiment leads us to conclude that it is far more likely for existing cracks to expand than for new cracks to appear.