High-temperature Air Flow Research Articles

Investigation of CN in the Reacting Boundary Layer of Ablating Graphite using PLIF

Planar Laser Induced Fluorescence (PLIF) of CN is used to study graphite ablation during exposure to an atmosphericpressure, high-temperature air flow. The 5000 K to 6000 K air flow is produced by the 50 kW Inductively Coupled Plasma (ICP) Torch at The University of Texas at Austin. Temporally- and spatially- resolved PLIF imaging is used to offer insight into reaction layer behavior. Two-line LIF thermometry in the reaction layer is also used, where the temperature is inferred from the ratio of signal intensities resulting from pumping two rotational lines, while collecting the broadband fluorescence. PLIF results are compared to s imultaneously-collected optical emission spectra. This work represents a first s tep towards building a better understanding of the thermo-chemical environment and the chemical kinetics of high-temperature material ablation, and provides insights into the production of CN during the ablation of carbon-based ablators in hypersonic applications.

Simulation, fabrication, and characteristics of high-temperature, quick-response tungsten–rhenium thin-film thermocouples probe sensor

In this study, a tungsten–rhenium thin-film thermocouples (TFTCs) is fabricated via magnetron sputtering on silicon carbide ceramics. Cuboid TFTCs probe is sealed with stainless steel shell to make the TFTCs probe sensor. This sensor has an average Seebeck coefficient of 27.2 µV °C−1 at 1167 °C (temperature difference). The thermal volatilization characteristics, electromotive force behavior, and response time of this tungsten–rhenium TFTC sensor are investigated. The real engine experimental results show that the TFTCs probe sensor can work normally under the high temperature and high-speed air flow environment of 4 Mach and 900 °C. In addition, the correlation between the response speed, substrate thickness, and film thickness is discussed based on simulations of the response to carbon dioxide laser pulses. As indicated by the dynamic experiment results, the laser response time is from 0.49 to 1.51 ms, which is much shorter than that of traditional armored C-type thermocouples (for tungsten–rhenium thermocouples, the response time is around 1 s). Tungsten–rhenium TFTCs probe sensor is a suitable alternative to conventional thermocouples to meet the requirements of measuring high temperatures with a quick response.

A novel FEM-FVM coupled method of evaluating the explosion shock wave of a thermobaric bomb on the internal and external flow characteristics of aircraft

The high-temperature and high-pressure shock waves generated after the explosion of a thermobaric bomb have a considerable impact on the aerodynamic characteristics of aircraft. Taking a typical thermoelastic model as the object, the explosion process was calculated by LS-DYNA software. The obtained pressure–time history curve data were written into a UDF function and coupled with Fluent software. At the same time, the afterburning effect of the combustion agent was considered. The influence of the explosion effect on the flow field characteristics of an M6 wing and the inlet of a turboprop engine was evaluated. Thus, an FEM-FVM co-simulation method was established. The results showed that when affected by the explosion shock wave of a thermobaric bomb, the maximum changes in the lift, drag, and pitch moment of an M6 wing in a very short period of time are 29, 13, and 7 times those of the original, respectively, and the effect is mainly reflected in the first shock wave. For the flow field in the intake duct, the total pressure recovery coefficient of the outlet section is reduced by up to 68%, and the distortion rate is increased by up to 325%. This change may even lead to engine surge and shutdown. Moreover, regardless of internal or external flow, the high-temperature airflow after detonation also affects the thermal protection ability of the wing or the inlet profile. All these factors need to be considered comprehensively in the design phase of the aircraft.

Functional Montmorillonite/Polymer Coatings

AbstractFunctional montmorillonite can be dispersed in polymer coatings and organic species and polymers can be intercalated into the interlayer space or grafted onto the surface of the functional montmorillonite. The addition of functional montmorillonite into polymer-based coatings can significantly improve anti-corrosion, refractory, super-hydrophobicity, antibacterial activity, and absorption of solar radiation by the resulting montmorillonite/polymer coatings. Montmorillonite can be functionalized for this purpose by ion exchange, intercalation, exfoliation, or combinations of these treatments. The rigid montmorillonite layers interspersed within the polymer matrix inhibit the penetration of corrosive substances, minimize the impact of high-temperature airflow, and thereby lead to strong resistance of the coating to corrosion and fire. The combination of polymers and dispersed montmorillonite nanolayers, which are modified by metal ions, metal oxides, and hydrophobic organic species, allows the resulting composite coating to have quite a rough surface and a much smaller surface free energy so that the montmorillonite/polymer coating possesses superhydrophobicity. The interlayer space of functional montmorillonite can also host or encapsulate antibacterial substances, phase-change materials, and solar energy-absorbing materials. Moreover, it can act as a template to make these guest species exist in a more stable and ordered state. Literature surveys suggest that future work on the functional montmorillonite/polymer coatings should be targeted at the manufacture of functional montmorillonite nanolayers by finding more suitable modifiers and tuning the dispersion and funtionalities of montmorillonite in the coatings.

Development of Manufacturing Technology of Nozzles from Composite Materials during Radiation Cooling

Carbon-carbon composite materials (CCCM) are a new class of structural materials designed to create light, durable, and space-based rigid structures for aerospace, hypersonic, gas turbine engines, parts of nozzle blocks, rocket combustion chambers, and transition trusses. They have the unique ability to maintain high strength and stiffness with 2500°C. For the manufacture of thermally stressed parts of nozzles, carbon fiber fabrics are used. Antioxidant coatings based on tantalum and silicon are applied to nozzles with adhesive (made based on tantalum carbide, which provides high mechanical strength of adhesion of erosion-resistant antioxidant coating with CCCM), antioxidant and erosion-resistant layers. To deposit a layer of tantalum carbide on CCCM, we used the technology of vacuum ion-plasma deposition of tantalum film followed by carbonization with carbon. As a result, an adhesive coating of tantalum carbide is formed on the surface of the packing. Obtaining an erosion-resistant antioxidant coating on the adhesion layer of tantalum carbide is obtained by applying a slip of silicon carbide powders and soot. The study of the performance of antioxidant and erosion-resistant protective coating in high-temperature airflow was carried out on plasmatron (it is a plasma generator in which an electric current is used to form a plasma; for cooling it, channels washed by water are used). This article shows that the studied packings have a unique ability to maintain high strength and rigidity with 2500°C. There is the expediency of manufacturing such nozzles. The physical and mechanical characteristics of the material were determined.

Effect of Introducing Long Chain Branching on Fiber Diameter and Fiber Diameter Distribution in Melt Blowing Process of Polypropylene

Abstract In the melt blowing process, the molten polymers extruded from nozzles are elongated by high-velocity and high-temperature air flow. In this study, with the aim of stabilizing the melt blowing process for producing nonwoven webs with fine diameter fibers, the effect of the control of polymer rheology by the introduction of either low melt flow rate (MFR) polypropylene (PP) or long chain branched PP (LCB-PP) to regular high MFR PP was investigated. Introduction of low MFR PP into regular PP increased shear viscosity and fibers of larger diameter were produced in the melt blowing process, while introduction of low MFR LCB-PP suppressed the elongational viscosity reduction with the increase of strain rate, and eventually spinning was stabilized. It was found that the blending of an optimum amount of LCB-PP to regular PP caused the stabilization of the melt blowing process. As a result, the formation of nonwoven webs consisting of fine fibers of rather uniform diameter distribution could be achieved.

A dual-flow choked nozzle based precise pressure controller for high-temperature gas systems

A novel dual-flow choked nozzle based pressure controller is developed to achieve high-precision pressure control. The primary air flow is mainly used for offering the required mass flow to the test section, and the secondary air flow is used for regulating the total mass flow through the choked nozzle to achieve required pressure levels. The test results show that the precision of the stabilization of the pressure can reach ± 0.005 bar for cold-state environments with air flow at ambient temperature, and ± 0.015 bar for hot-state environments with air flow temperature in the range of 797.1–931.5 °C. Besides, this pressure controller has fast response. A new pressure steady state can be reached within 23.1 s for air flow at ambient temperature and 70 s for high-temperature scenario. Since no moving component exposed to the high-temperature air flow, it is very suitable for the pressurized test rigs with extremely high-temperature gas flow.

Experimental testing of a solar air cavity-receiver with reticulated porous ceramic absorbers for thermal processing at above 1000 °C

Concentrated solar energy can be used as the source of high-temperature heat for industrial processes, but the challenge is to design a solar receiver that can effect such a thermal conversion efficiently. This study reports on the engineering design and experimental testing of a 5 kW solar cavity-receiver containing a reticulated porous ceramic (RPC) structure that can absorb high-flux radiation volumetrically and heat up, by convection, an air flow serving as the heat transfer fluid. The thermal performance, characterized by the thermal efficiency and the air outlet temperature, was determined experimentally for four parameters, namely: RPC material (silicon-infused silicon carbide or SiSiC, alumina, and ceria), mean pore size (range 0.8–2.5 mm, corresponding to 10–30 pores per inch or PPI, at 0.90 porosity), solar concentration ratio (range 1965–3900 suns over a 4 cm-diameter cavity aperture, supplied by a high-flux solar simulator), and air mass flow rate (range 2–10 kg/h). Thermal efficiencies between 0.22 and 0.69 were obtained at steady-state air outlet temperatures ranging from 1160 to 450 °C. Larger pores enhance heat transfer while variable porosity across the RPC can reduce temperature gradients and potentially contribute to the design optimization. The highest efficiency of 0.69 was achieved by the SiSiC 10 PPI cavity at an air outlet temperature of 1133 °C and air mass flow rate of 9.9 kg/h. The solar receiver design proved to deliver a high-temperature air flow (>1000 °C) with a reasonably high thermal efficiency (>0.65).

Simulation and Experimental Analysis of the Temperature Field of Flow of Airport Snow Thrower Based on CFD Theory

It is a hot issue for airport users and researchers to pay attention to the influence of the hot air flow generated by the hot blowing snow thrower on the durability and service life of the pavement. In order to clarify the influence of hot air flow generated by hot blowing snow throwers on the temperature distribution of airport runway, this paper studies the temperature distribution of airport runway under the action of hot air flow generated by hot blowing snow throwers by CFD simulation and design of field measurement experiments. The results show that the maximum temperature of the surface obtained by the simulation method is 554.21K, the limit temperature of the surface is 539.75K and the average temperature difference is 22.64K, which verifies the accuracy of the simulation model. In view of the research results, in order to promote the application of this type of snow thrower in the airport, it is necessary to verify whether the airport roadway can withstand the action of high temperature airflow of 539.75K.

Numerical Analysis of Airflow Fields from New Melt-Blowing Dies for Dual-Slot Jets.

The melt-blowing process uses high-speed and high-temperature airflow from the die head to draw polymer melt into micron-sized fibers. In this work, to reduce the diameter of the melt-blowing fibers, three new slot dies have been designed based on the common slot die. With computational fluid dynamics technology, the two-dimensional flow fields from these new types of slot dies were numerically calculated. To verify the validity of the calculation, the simulation data was compared with the experimental data. The numerical result shows that the internal flow stabilizers could increase the velocity peak and the pressure peak on the centerline of the flow field and could reduce the reverse velocity, temperature decay, and maximum value of turbulence intensity near the die head. Compared with the common slot die, the slot dies with cuboid bosses could increase the air velocity and temperature on the spinning line in most areas and reduce the air pressure within 1.5 cm below the die. The slot dies with internal flow stabilizers and cuboid bosses have the optimal flow field performance and would be beneficial to the production of thinner fibers.

Impact of Residential Building Layouts on Microclimate in a High Temperature and High Humidity Region

Microclimatic condition is a fundamental indicator for evaluating outdoor space livability and vitality. Research has shown that poorly designed building layouts can lead to discomfort; however, the mechanisms influencing outdoor microclimate based on residential building layout are unclear for high temperature and high humidity regions. This study explores the relationship between residential building layouts and the outdoor wind and thermal environment at the pedestrian level in Wuhan, a city renowned for high temperatures and high humidity. Six typical residential building layouts were simulated, using the ENVI-met numerical model, to determine the spatial distribution of wind speed and air temperature. The Universal Thermal Climate Index was adopted as a comprehensive index with which to assess spatial and diurnal variations in microclimates surrounding each building layout. Results showed that parallel building layouts formed a ventilation corridor that increased wind speeds by approximately 0.3 m/s. A staggered building layout, in line with the prevailing wind direction, facilitated airflow in the ventilation corridor and further increased wind speeds. Windward buildings blocked high-temperature airflows and reduced air temperatures by approximately 1 ℃ in parallel layouts, and 1.4 ℃ in enclosed layouts. However, the cooling effect of windward buildings on high-temperature airflow was weaker than the warming effect caused by the wind shadow effect and direct sunlight. Additionally, the performance of the thermal comfort of the enclosed type layout was significantly better, for most of the day, than the parallel type layout.

Humidity measurement of high temperature and high humidity air flow by TDLAS

Humidity measurement of high temperature and high humidity air flow by TDLAS

The Benefit of Using Multiple Thin Tanks Versus a Short Big Tank for Thermal Storage in Ceramic-Sphere Packed Bed With Airflow

Abstract To make a better thermal storage system that uses air as the heat transfer fluid flowing through a packed-bed of ceramic spheres (Al2O3) as thermal storage materials, the present work studied cases using multiple small-diameter thin tanks to replace a large-diameter big tank while keeping the same total volume and the same air flow rate. Performance analysis of thermal storage has been conducted for comparison and optimization. The long flow passage and faster flow velocity of air in the small-diameter tanks was found to significantly benefit thermal storage performance compared with that of a short big tank. It resulted in a longer duration of discharge of high-temperature airflow, if the same operation time is applied to the two situations. The faster airflow enhances the heat transfer between air and thermal storage material, although it incurs larger pressure loss. Overall, the energy storage efficiency of using several thin tanks can be significantly better than that of using a big short tank if the height-to-diameter ratio in the multiple thin tanks is properly optimized. The optimization methodology and results are of great significance to the development of thermal storage systems that use air as heat transfer fluid and rocks or ceramic spheres as the packed-bed material for thermal storage.

Influence of the High-Temperature Effects in a Nozzle with a Nonstationary Local Power Supply on the Gas Flow in it

A simulation of a high-temperature air flow in a nozzle with a nonstationary local power supply has been performed. The thermodynamic processes in this flow were simulated using an approximate model of equilibrium air thermodynamics. Results of a numerical simulation of one-dimensional flows in nozzles with a moving powersupply zone, used in alternating-current plasma generators, are presented. On the basis of the numerical-simulation data obtained, the qualitative patterns of the gasdynamic and thermal processes proceeding in such a nozzle with a nonstationary power supply were analyzed. The results of calculations performed on the basis of the ideal-gas and real-gas models with regard for the physicochemical processes occurring in a real gas at a high temperature were compared.

Recent progress in thermal/environmental barrier coatings and their corrosion resistance

Thermal/environmental barrier coatings (T/EBCs) play important roles in jet and/or gas turbine engines to protect the Ni-based superalloys and/or ceramic matrix composite substrates from the high-temperature airflow damage. Great efforts have been contributed to searching for enhanced T/EBC materials to improve the efficiency of the engines, which is the key of improving thrust-to-weight ratio and energy saving. The practical candidates, rare earth-contained materials, are widely used for T/EBCs in gas turbines due to their excellent properties such as low thermal conductivity, high melting point, high-temperature strength and durability as exhibited in yttria-stabilized zirconia, pyrochlore oxides and rare earth silicates. In addition to the intrinsic properties, the microstructures obtained by different synthesis processes and the service performances, as well as the underlying failure mechanism, are also significant to this specific application. However, the main challenges for T/EBCs developments are T/EBC materials selection with balanced properties and their anti-corrosion performances at higher operating temperature. In this review, we summarized the progress in their fabrication techniques and mechanical/thermal properties of typically rare earth-contained T/EBCs, together with their anti-corrosion performance under the condition of molten salts or oxides (such as, Na2SO4, V2O5 and NaVO3), calcium–magnesium–alumina–silicate (CMAS) and high-temperature water vapor.

Characteristics of the Mechanism of Thermal Power Destruction of Carbon Materials in a Supersonic High-Temperature Air Flow

An experimental simulation is performed for the mechanism of the destruction of carbon heat-shielding materials (polycrystalline graphite and carbon composites) with a rough surface passed by a supersonic high-temperature air flow. At high velocities of the oncoming flow, the roughness of such materials may correspond to the thickness of the boundary layer. Such properties of the surface contribute to a more intensive destruction of materials due to the mechanical carryover of mass caused by the gas dynamics of the flow and the thermochemical processes on a rough surface. This paper analyzes the methods and means of research under the ground conditions of the mechanism of the destruction of carbon heat-shielding materials that can be used in the design of thermal protection for rocket-space and high-speed aviation equipment.

Temperature and velocity fields inside a hanging droplet of a salt solution at its streamlining by a high-temperature air flow

Salt solutions are widely used in many industrial technologies, including those associated with high heat fluxes. The main constraint to the introduction of many promising high-temperature gas-vapor technologies with using salt solutions is that the mechanisms of interrelated processes of heat and mass transfer and phase transformations of such solutions under intense heating have not been sufficiently studied yet. In most modern devices, blocks and setups, intensive convective heating is realized both through gas and liquid media. As a result, the study of these processes under high-temperature convective heating is topical. The most significant results may be obtained by using the low-inertia non-contact measurement methods. This article presents the results of experimental studies of the processes of heating of drops of typical salt solutions (LiBr, CaCl2, LiCl, NaCl) placed in the flow of heated air on the holder. The temperature range was chosen from 20 °C to 600 °C to cover a large group of gas–vapor technologies, primarily evaporation (boilout) and burning (burnout) of impurities from water. Thus, the main attention was paid to the study of convective heat exchange of drops of salt solutions with a heated gas medium. The optical method Planar Laser Induced Fluorescence (PLIF) was used to control the temperature field of the droplet. The convection velocity in the droplet was registered using the optical method of Micro Particle Image Velocimetry (Micro PIV). The main attention was paid to the effects of crystallization, processes of heating and evaporation of droplets, taking into account different properties and component composition of salt solutions. The influence of these factors and processes is shown at different heat-up temperatures. The differences in the rates of heating and evaporation of salt solution drops under intense convection heating have been established. Recommendations on the application of research results for the development of promising gas–vapor technologies have been formulated.

A new strategy for the preparation of flexible macroscopic graphene fibers as supercapacitor electrodes

Graphene fibers (G-Fs) have attracted consistent attention due to the fascinating properties of graphene materials and their promising applications in supercapacitors, wearable electronics, smart textiles, sensors, and so on. In this research, flexible macroscopic graphene oxide fibers (GO-Fs) and G-Fs are prepared by dry spinning with high temperature airflow. The high temperature airflow induces immediate self-assembly of graphene oxide nano-sheets into dried state GO-Fs. Compared to thermal reduction, chemical reduction contributes to maintaining the flexibility and strong intensity of G-Fs. After chemical reduction, the produced G-Fs exhibit excellent mechanical properties. The tensile strength is about 127.5MPa and the elongation at break reaches up to 14.8%, one order of magnitude greater than that of previously reported neat graphene nanoribbon fibers. Moreover, G-F is directly used as supercapacitor electrode and the specific capacitance is 78.8F/m2 at the current of 20μA, displaying extraordinary capacitance and outstanding stability and indicating promising use as flexible supercapacitor.

Performance assessment of multi-stage thermoelectric generators on hypersonic vehicles at a large temperature difference

Thermoelectric generator (TEG) is one of the most promising electricity generation technologies, which converts temperature difference directly into electrical energy. Considering the large temperature difference caused by the special thermal environment of hypersonic vehicles, it is quite a potential application for the thermoelectric technology. In this article, a power generation scheme suitable for hypersonic vehicles has been proposed, which is based on the multi-stage thermoelectric generator. The hot fuel heated by aerodynamic heat or combustion heat dissipation, rather than the high-temperature and high-speed air flow is applied as the heat source of thermoelectric module. An optimal TEG model considering the flow in both heat source and cold source has been developed to predict the thermoelectric performance under a large temperature difference condition. The maximum conversion efficiency of 18.38% is obtained benefiting from the application of advanced thermoelectric materials. Besides, the influences of the geometry and operating parameters on maximum power output and corresponding conversion efficiency have been investigated. The results indicate that the cascade utilization of large temperature difference through multi-stage structure improves the thermoelectric performance of TEGs greatly. Both the maximum power output and the corresponding conversion efficiency increase with the inlet temperature of the heat source. This research provides a new method of the power generation on hypersonic vehicles.

An Experimental Study of Combustion and Emissions of Wheat Straw Pellets in High-Temperature Air Flows

ABSTRACTThe present work is performed to investigate the combustion and emission behavior of wheat straw pellets of different sizes and shapes in a high temperature air flow. Prior to the combustion experiments, thermogravimetric analysis of the pellet was carried out to understand the thermo-chemical characteristics of the wheat straw pellet. A sequential method was used to determine the kinetic parameters. A combustor using preheated air is used to investigate the effect of different parameters, such as pellet size, starting temperature, and primary air flow velocity on combustion and emission characteristics of biomass pellets. The surface temperatures as well as the time-dependent mass loss rates are recorded to study the ignition behavior of pellets. The results showed that the increase in starting temperature, and air flow velocity and decrease in pellet size leads to a reduction in combustion time and increase in the total combustion rate. The results also showed that the concentration of CO formed increases as the heating time increases and the formation of CO2 increased with increasing the starting air temperature and air flow velocity inside the combustion chamber. It was found that the combustion efficiency ranged from 99% to 100% and the combustion efficiency reached 100% in some experiments due to complete combustion.