High-temperature Wind Tunnel Research Articles

Experimental investigation of seeping gas film cooling effect at downstream wall in hypersonic flow

Generating a seeping gas film through porous material to cover the downstream wall can achieve a large-area thermal protection from aerodynamic heating for the windward surfaces of hypersonic vehicles. This study explores the cooling efficiency of the seeping gas film downstream of the porous wall through experimental research. It introduces a continuous high-temperature wind tunnel heat flux calculation method and investigates the influence of cooling gas injection rate and blowing length on downstream wall cooling efficiency. Additionally, an engineering evaluation of the thermal protection effect of the seeping gas film on the downstream wall of the flat plate boundary layer is conducted. Experimental findings demonstrate that cooling efficiency decreases rapidly along the flow direction, and an increase in the cooling gas injection rate enhances the cooling efficiency downstream of the porous wall, while the attenuation rate of cooling efficiency in the downstream region accelerates with higher injection rates. Moreover, a rise in injection rate results in a reduction of the effectiveness-cost ratio in the upstream region, with the seeping gas film’s effectiveness-cost ratio being relatively high at an injection rate of 0.2 %. Longer blowing lengths enhance the cooling efficiency at the end of the porous wall but accelerate the attenuation rate of cooling efficiency along the flow direction. Conversely, under constant blowing length, a higher injection rate contributes to a faster attenuation rate of cooling efficiency downstream. Assessment of the thermal protection effect for different combinations of blowing lengths (20 ∼ 60 mm) and injection rates (0.1 %∼0.5 %) reveals that layouts featuring shorter blowing lengths and higher injection rates demonstrate superior cooling effects downstream of the porous wall when supplied with the same mass flow rate.

Ablation evolution of a new light weight silicon based thermal protection material in high temperature gas flow

High silicon oxygen/phenolic aldehyde (VSF/PR) ablative heat-proof material is widely used as resin-based heat-proof system for long-term aircraft. Understanding the generation and flow process of the surface droplets during high temperature is key to improving the thermal stability, and prior to which, a direct visualization of the surface evolution is required. Here we use optical imaging technique to in-situ and real time record the surface evolution of a flat plate subjected to thermal ablation at 1700 °C in wind tunnel. Based on the experimental results processed using image processing method and by combining the fluid theory in high temperature wind tunnel, we explained the distribution of the surface temperature and surface liquid droplets, the velocity field, and the movement of the liquid droplets on the plate surface. Results show that the distribution of the liquid droplets was correlated to the temperature distribution. A verification experiment for the merging process of two liquid droplets shows that droplet with high viscosity would generate a great friction force and cause the droplet to move forward in a rolling manner. The chasing and merging process of the liquid droplets with high viscosity is rather an entrainment process than a spontaneous merging process normally induced by surface tension. These findings help to understand the flow behavior of the liquid droplets during thermal ablation.

Design and construction of an open loop subsonic high temperature wind tunnel for investigation of SCR dosing systems

The requirement of high-fidelity experimental data of microscopic properties as well as the whole spray development is fundamental since they are determinant in the understanding of atomization and evaporation process in a variety of multiphase flows. The Selective Catalytic Reduction (SCR) system is one of the industrial applications where understanding the whole spray atomization and development process is essential in developing new systems to cope with stringent emission regulations. A new subsonic heated open-loop wind tunnel has been designed and manufactured specifically with a 0.25 × 0.25 m cross-section and 1.59 m long optically accessible quartz glass side panels. The purpose of the wind tunnel was to investigate Selective Catalytic Reduction (SCR) sprays using Phase Doppler Anemometry (PDA). The wind tunnel is capable of reaching a flowrate equating to 50 m/s in the test section when operating at standard room conditions (25°C and 101,325 kPa). A 145 kW heater is mounted downstream the blower to achieve air temperatures of up to 400°C with maximum air mass flow of 1200 kg/h. The diffuser, flow conditioning and contraction cone sections of the wind tunnel have been designed to minimize velocity and temperature gradients within the test section while minimizing flow disturbances. A diffuser with a cone angle of 5° and an area ratio of 3.95 and a contraction cone with a contraction ratio of 4.05 was selected. The test section has been designed to give complete visible access through top and both sides, while the bottom is made of stainless steel. The two sides are made of quartz glass to provide the best signal transmission quality for usage with any optical equipment e.g. PDA. The top of the test section has sections made of Robax glass with detachable windows for easy mounting and dismounting of any equipment subject to be tested in the flow section. The streamwise turbulence intensity in the test section was found to be less than 2% in the core with similar values for the cross-stream turbulence intensities. The complete design and construction of each section of the wind tunnel has been presented and discussed in this paper.

Investigation of the bulging mechanism of C/SiC coating through in situ optical observation and numerical simulation

Carbon fibre-reinforced silicon carbide composites (C/SiC) are a type of high-temperature structural ceramics that can be widely applied in aerospace, hypersonic aircraft, and military fields. However, the practical application of such materials in some complex high-temperature environments is still limited owing to drawbacks such as the bulging problem. In this study, the bulging of a C/SiC coating in a high-temperature wind tunnel is investigated. The surface data of specimens in the high-temperature arc wind tunnel were obtained using optical visualisation equipment, and the entire process of the formation of coating bulges, and their evolution and fusion is illustrated. Then, a fully coupled thermal fluid-solid simulation model is established, and the influence of defects on the internal heat transfer of materials is analysed. Further, the thermal fluid-solid coupling simulation technology is also used to analyse the bulging process of the coating to understand the mechanism of chemical reactions in defects between the coating and substrate. This work can serve as a guide for the screening and improvement of the material coating process.

Precise infrared thermometry with considering background radiation for gas turbine air cooling application

The turbine inlet temperature keeps increasing as high-efficient gas turbines have been developed for the last decades. The increase of turbine inlet temperature makes the turbine blades being exposed in thermally severe conditions, leading to thermal damage. The thermal management with advanced cooling techniques is thus necessary to protect turbine blades from thermal deformations. For properly evaluating the air-cooling effects on turbine blades, the more accurate surface temperature measurement must be preceded. Infrared thermometry has often been employed for high-temperature turbine blades due to its convenience. However, background radiation must be carefully considered if the surrounding environment is at a higher temperature than the surface to be measured, as in the case of a gas turbine blade. In addition, the spectral emittance of an object should also be known a priori. In this study, we propose an accurate measurement scheme of infrared thermometry by properly considering the background radiation from the high-temperature environment as well as the spectral emittance of an object. The accuracy of infrared thermometry was examined for two exemplary surfaces with low and high emittance in a high-temperature wind tunnel. The proposed measurement scheme was found to be in good agreement with the thermocouple measurements within 5% for both surfaces.

Machine vision system for high-temperature wind tunnel: Principle, design and application

In aerospace and related fields, simulation tests in the high-temperature wind tunnel have been playing an important role in evaluating and optimizing the thermal performance of key materials/structures of the aircraft. Based on the characteristics of the test environment, the development of machine vision system for the high-temperature wind tunnel can obtain the wide ranging and comprehensive process information of the tests, which is of great significance to the development of high-temperature ground simulation tests. Thus, this paper reviews the theoretical principles, key technologies and equipment design of the machine vision system for high-temperature wind tunnel. Meanwhile, several application examples are also introduced along with the relevant research results of our research group in this field. Finally, the development direction and application prospects of the machine vision system for the high-temperature wind tunnel are also forecasted with the future-oriented needs of the simulation tests.

In-situ 3D shape and recession measurements of ablative materials in an arc-heated wind tunnel by UV stereo-digital image correlation

In-situ three-dimensional (3D) ablation shape and recession measurements of ablative materials of hypersonic vehicles in high-temperature wind tunnel tests are essential to understand the ablation performance of these materials, which can provide necessary information for better thermal protection system (TPS) design. In this work, a novel ultraviolet stereo-digital image correlation (UV stereo-DIC) technique is proposed for in-situ measurements of 3D ablation shapes and recession in ablation tests. Specifically, a self-developed UV stereo-DIC system, combing a monochromatic UV illumination, two bandpass filters and two UV cameras, is first established. Using the UV stereo-DIC system, strong thermal radiation emitted from the heated sample and the heating device can be effectively suppressed. Natural textures formed from illuminating the rough sample surface by UV light are used as speckle patterns in stereo-correlation. In-situ 3D ablation shapes of a blunt cone subjected to arc heating were measured with a temperature range of 1000 °C to 1868 °C, and the UV stereo-DIC measurements showed good agreement with post-test measurement of the recession of the test articles. Based on the experimentally measured 3D shapes, several key parameters, such as full-field recession, dynamic recession rate, were calculated. The results demonstrate the efficacy and potential of UV stereo-DIC for 3D shape and recession measurements in ablation tests.

Oxidation and erosion resistance of multi-layer SiC nanowires reinforced SiC coating prepared by CVD on C/C composites in static and aerodynamic oxidation environments

A multi-layer SiC nanowires reinforced SiC (SiCnws-SiC) coating was prepared in-situ on carbon/carbon (C/C) composites by three chemical vapor deposition (CVD) processes. The microstructure and phase composition of the nanowires fabricated on the first-layer SiCnws-SiC coating and the coatings were examined by SEM, TEM, and XRD. The bamboo-like SiC nanowires with a 50 nm diameter and a length of several tens of micrometers are straight, randomly orientated and distributed like a net on the first-layer SiCnws-SiC coating. The growth direction is [111], and the growth mechanism is VS. The multi-layer SiCnws-SiC coating has three layers: the thickness of the first-layer is roughly 400 µm, and the outer two layers are about 200 µm. Each layer has a sandwich structure. The isothermal oxidation and erosion resistance of the multi-layer SiCnws-SiC coating were investigated in an electrical furnace and a high temperature wind tunnel. The results indicated that the weight loss of the multi-layer SiCnws-SiC coated C/C composites was only 1.8% after oxidation in static air at 1773 K for 361 h. Further, the coated sample failed due to fracture of the coating at the clamping position (i.e. 80 mm) after erosion at 1873 K for 130 h in the wind tunnel. The weight loss of the coated C/C composites occurred due to the formation of penetrating cracks in the coating during the oxidation thermal shock. The maximum bending moment and the larger clamping force caused the coating fracture and resulted in intense oxidation of the substrate and the failure of the specimen.

Numerical Simulation of Non-Equilibrium Plasma Discharge for High Speed Flow Control

Numerical simulation of hypersonic flow control using plasma discharge technique is carried out using an in-house developed code CERANS-TCNEQ. The study is aimed at demonstrating a proof of concept futuristic aerodynamic flow control device. The Kashiwa Hypersonic and High Temperature wind tunnel study of plasma discharge over a flat plate had been considered for numerical investigation. The 7-species, 18-reaction thermo-chemical non-equilibrium, two-temperature air-chemistry model due Park is used to model the weakly ionized flow. Plasma discharge is modeled as Joule heating source terms in both the translation-rotational and vibrational energy equations. Comparison of results for plasma discharge at Mach 7 over a flat plate with the reference data reveals that the present study is able to mimic the exact physics of complex flow such as formation of oblique shock wave ahead of the plasma discharge region with a resultant rise in surface pressure and vibrational temperature up to 7000 K demonstrating the use of non-equilibrium plasma discharge for flow control at hypersonic speeds.

Experimental Investigation of Heat Transfer and Pressure Drop Characteristics of H-type Finned Tube Banks

H-type finned tube heat exchanger elements maintain a high capacity for heat transfer, possess superior self-cleaning properties and retain the ability to effect flue gas waste heat recovery in boiler renovations. In this paper, the heat transfer and pressure drop characteristics of H-type finned tube banks are studied via an experimental open high-temperature wind tunnel system. The effects of fin width, fin height, fin pitch and air velocity on fin efficiency, convective heat transfer coefficient, integrated heat transfer capacity and pressure drop are examined. The results indicate that as air velocity, fin height and fin width increase, fin efficiency decreases. Convective heat transfer coefficient is proportional to fin pitch, but inversely proportional to fin height and fin width. Integrated heat transfer capacity is related to fin efficiency, convective heat transfer coefficient and finned ratio. Pressure drop increases with the increase of fin height and fin width. Finally, predictive correlations of fin efficiency, Nusselt number and Euler Number are developed based on the experimental data.

DETERMINATION OF MATERIAL CATALYCITY AT HIGH TEMPERATURES IN THE VAT-104 HYPERSONIC WIND TUNNEL

Nitrogen and airflows in a duct of the VAT-104 high-temperature wind tunnel are modeled by means of the numerical solution to the unsteady two-dimensional Navier−Stokes equations in wide ranges of the total pressure (1−40 kPa) and stagnation temperature (5500−8000 K). Parametric numerical studies of nonequilibrium heat transfer on the end face of a thermally insulated circular cylinder at different cylinder surface catalycity values are performed. For each flow regime, the dependence of the surface temperature on the heterogeneous recombination rate constant Kw is obtained and the derivative α = dTw /dKw is determined. The catalycity of the C−SiC thermoprotective material in nitrogen and air plasma flows at surface temperatures of Tw = 1500−1900 K is determined in the calculations and experiments based on the measured temperature differences of the reference and examined materials.

Instability characters and suppression method of a pressure regulator

With the increasing interest in hypersonic propulsion, National University of Defense Technology in China sets up a large high-temperature wind tunnel. A pilot-operated pressure regulator is developed to supply gas with a given pressure to the wind tunnel. Although the regulator works normally and stably during the working stage, severe oscillations with maximum amplitude of 3 MPa occur during the pressurization stage. To solve the problem, a numerical approach is developed to model the supply system, especially the pressure regulator. The approach is validated by comparing numerical results with experimental results. It turns out that the frequency of the oscillation is mainly determined by the inlet pipe, while the amplitude is affected by the downstream volume, the seal chamber and the seal orifice. To suppress the instability, a connecting pipe is adopted to let the gas in the seal chamber come from outside of the regulator. The method is quite simple but the effect is dramatic, as is attested feasible by both simulations and experiments.

The design and performance of a high temperature rise combustor for wind tunnel

A high-temperature-rise combustor that can be used in high-temperature wind tunnel is introduced in this study. Aviation kerosene is used in this type of combustor, with division combustion scheme and evaporator fuel-supply device adopted. In the performance test under atmospheric pressure, when the inlet temperature is 500K and air flow is within the range of 1.5–3.0 kg/s, the outlet temperature can be precisely regulated within the range of 1050K–2100K. Moreover, higher uniformity of outlet temperature distribution and higher combustion efficiency can be achieved. After the long-time working in the wind tunnel, various components of the combustor, especially the combustor liners are checked without finding any anomaly such as thermal deformation.

The Use of a High Temperature Wind Tunnel for MT-SOFC Testing—Part II: Use of Computational Fluid Dynamics Software in Order to Study Previous Measurements

Micro-tubular solid oxide fuel cells (MT-SOFCs) are a much smaller version of larger tubular SOFCs. They are operational within seconds and allow a higher power density per volume than the larger version. Hence they are a potential technology for automotive, auxiliary and small scale power supply devices. In this study a commercially available computational fluid dynamic (CFD) software program was used to predict a MT-SOFCs performance when located inside a high temperature wind tunnel experimental apparatus. In Part I, experimentally measured temperature profiles were recorded via thermo-graphic analyses and I/V curves. These measurements were used in this study to establish the predictability and validity of the CFD code and furthermore understand the MT-SOFC attributes measured in Part I. A maximum 4% I/V curve deviation and 6 K temperature deviation between the experimentally measured and model predicted results was observed. Thus, the model predicted the MT-SOFCs performance in the experimental environment very accurately. A very critical observation was the current density and temperature profile across the MT-SOFC that was strongly dependent on the distance from the hydrogen/fuel inlet. Not only was the model validated but also a grid and quantitative solution analysis is explicitly shown and discussed. This resulted in the optimum grid density and the indication that a normally undesirable high grid aspect ratio is acceptable for similar MT-SOFC modeling. These initial simulations and grid/solution analysis are the prerequisite before performing a further study including multiple MT-SOFCs within a stack using different fuels is also envisaged.

C/SiC/MoSi 2–Si multilayer coatings for carbon/carbon composites for protection against oxidation

To improve the oxidation resistance of carbon/carbon (C/C) composites, a C/SiC/MoSi 2–Si multilayer oxidation protective coating was prepared by slurry and pack cementation. The microstructure of the as-prepared coating was characterized by scanning electron microscopy, X-ray diffraction and energy dispersive spectroscopy. The isothermal oxidation and erosion resistance of the coating was investigated in electrical furnace and high temperature wind tunnel. The results showed that the multilayer coating could effectively protect C/C composites from oxidation in air for 300 h at 1773 K and 103 h at 1873 K, and the coated samples was fractured after erosion for 27 h at 1873 K h in wind tunnel. The weight loss of the coated specimens was considered to be caused by the formation of penetration cracks in the coating. The fracture of the coated C/C composites might result from the excessive local stress in the coating.

The Use of a High Temperature Wind Tunnel for MT-SOFC Testing—Part I: Detailed Experimental Temperature Measurement of an MT-SOFC Using an Avant-Garde High Temperature Wind Tunnel and Various Measurement Techniques

The purpose of the first part of this study was to compare four different temperature measuring methods. The application of these tools for possible temperature monitoring or calibration of monitors of microtubular solid oxide fuel cells (MT-SOFCs) is explored. It was found that a thermographic camera is very useful to visualize the temperature gradient on the outside of a cell, while an electrochemical impedance spectroscopy method was useful for estimating the core temperature of a test cell. A standard thermocouple was also used in combination with the previous two methods. Furthermore, an inexpensive laser guided thermometer was also tested for MT-SOFC temperature measurement. This initial study has opened up a range of questions not only about the effect of the experimental apparatus on the measurement results but also about the radial temperature distribution through a MT-SOFC in a working mode. Both these topics will be further investigated in part II of this study through a computational fluid dynamics study. This should provide additional interesting information about any differences between testing single cells and those within a bundle of cells. The discussed results are expected to be mainly temperature related, which should have direct consequences on power output and optimized gas inlet temperatures.

Effect of Pre-Oxidation Treatment on the Thermal Shock Resistance of Thermal Barrier Coatings in a Combustion Gas Environment

Thermal barrier coatings (TBCs) were deposited by an Air Plasma Spraying (APS) technique. The TBC coating comprised of 92 wt.% ZrO2 and 8 wt.% Y2O3 (YSZ), CoNiCrAlY bond coat, and MarM247 nickel base super alloy. After APS of YSZ two batches of TBC specimens were tested, one batch of which was pre-oxidised in air for 10h at 1080 oC. Both types of the specimens were directly pushed into a combustion gas at 1150 oC for 25 min and then out to the natural air for quenching. The combustion gas was produced by burning jet fuel with high speed air in a high temperature wind tunnel, which simulates the real service conditions in an aeroengine. Results show that TBCs prepared by the APS had good thermal shock resistance in the combustion gas. The pre-oxidation treatment of the TBC had a significant effect on its thermal shock life. The as-oxidised TBCs always had worse thermal shock resistance than the as-sprayed ones after thermal shock cycles.

Oxidation and erosion resistance of MoSi2–CrSi2–Si/SiC coated C/C composites in static and aerodynamic oxidation environment

A MoSi2–CrSi2–Si multi-composition coating was prepared on the surface of SiC coated carbon/carbon (C/C) composites by pack cementation in argon. The crystalline structure of the multi-composition coating was measured by X-ray diffraction. The morphology and element distribution were analyzed by scanning electron microscopy and energy dispersive spectroscopy, respectively. The isothermal oxidation and erosion resistance of the multi-layer coating were investigated in electrical furnace and high temperature wind tunnel, respectively. These results indicated that the weight loss of the coated C/C composites was only 0.8% after oxidation in air at 1873K for 500h, and the sample was fractured after erosion at 1873K for 32h in wind tunnel. The weight loss of the coated C/C composites was considered due to the excessive depletion of the outer coating and the appearance of defects in the coating. The mismatch of thermal expansion coefficient between the multi-layer coating and C/C composites caused excessive local stress and resulted in the fracture of the coated C/C composites.

Relationship of damage evolution of ceramic matrix composites between static and dynamic oxidizing gas environments

The present work aims toward the application of an innovative methodology for testing environmental performance of CMC materials with an integrated testing platform including a simulated equivalent environments system (ES) and a high temperature wind tunnel environments system (WS). The similarity and the mapping relationship of damage evolution of CMCs between static ES and dynamic WS with the same contents of oxidizing atmosphere were analyzed and compared. The multiplication factor between ES and WS was identified, and correlated experimentally and theoretically with the major environmental variables such as temperature, stress, and gas velocity.

CVD Diamond Coating for Erosion Protection at Elevated Temperatures

Small solid particles entrained in a gas-fluid turbomachinery flow can cause degradation of the component surfaces containing the flow by erosion and corrosion processes. As diamond is the hardest known material, much work has been done to use polycrystalline diamond (PCD) as a protective coating on parts operating in a hostile and abrasive environment. Little attention is given in the literature to the high-temperature erosion behavior of chemical vapor deposition (CVD) diamond on different substrates. The objectives of this research were to develop CVD diamond erosion barriers for surface protection of cemented tungsten carbide at high temperatures and to study the erosion behavior of the coatings. Microwave plasma chemical vapor deposition (MPCVD) was used to apply diamond films on WC-6%Co. The erosion behavior of the coated specimens was investigated experimentally by exposing them to abrasive particle-laden flow in a high-temperature wind tunnel. The obtained results show the effects of impingement angle, temperature, and particle dose on the erosion rate. The data demonstrate that uncoated substrates suffer 6–7 times higher wear compared to diamond-coated samples at elevated temperatures up to 538 °C when exposed to alumina particle flow. This study indicates that polycrystalline diamond is emerging as a promising erosion protective coating for high-temperature applications.