High-temperature Air Flow Research Articles

Improving thermal environment and ventilation efficiency in high-temperature excavation tunnels via an innovative heat insulation and cooling baffle

An increasing number of tunnels inevitably encounter high-temperature environments induced by elevated geotemperatures. The energy demand for ventilation and cooling in excavation tunnels increases significantly as the temperature of the surrounding rock increases, thereby hindering the sustainable exploitation of deep resources. This paper presents an optimization method for airflow organization in excavation tunnels that integrates thermal insulation and cooling (TIC) baffles. The proposed approach significantly improves the cooling effect of the auxiliary ventilation system while reducing energy consumption. This method offers the advantages of simplicity, convenience, and cost-effectiveness. A multifield coupling model using COMSOL software was developed and validated to analyze the system, and the application scenario was explored. Several ventilation scenarios were examined, highlighting that TIC baffles effectively reduce the heat release from the surrounding rock and lower the airflow temperature in personnel areas in the excavation tunnel. By implementing TIC baffles, the average airflow temperature in the excavation tunnel decreases by 1.5 °C to 1.8 °C, resulting in a saving of approximately 5.11 kW in cooling energy. Increasing the circulating water flow in the heat-exchange pipe of the TIC baffles or reducing the initial circulating water temperature can enhance the cooling capacity of the TIC baffles and lower the tunnel airflow temperature to a certain extent. The distance between the TIC baffles and surrounding rock significantly affects the airflow temperature between the left and right TIC baffles. An excessive length of TIC baffles (L > 7 m) leads to heat accumulation and high-temperature airflow in localized areas. In This study, a method for optimizing the thermal environment and saving energy in high-temperature excavation tunnels is proposed.

Experimental investigation on the effects of complex pressure gradients on the film cooling effectiveness of a serpentine nozzle

Serpentine nozzles more easily deform and damage due to high-temperature airflow because of their multi-bend channel configuration. Therefore, film cooling technologies should be applied to serpentine nozzles. Complex pressure gradients in serpentine nozzles lead to different film cooling characteristics from combustion chambers, turbine blades, and other exhaust systems. This study experimentally investigated the effects of complex pressure gradients and film hole inclination angles on a serpentine nozzle’s film cooling effectiveness (FCE) using pressure-sensitive paint (PSP). The flow mechanisms were obtained via numerical simulations. The experimental nozzle design needs to consider the needs of PSP observations and the normal flow of serpentine nozzles. However, the complex structure of serpentine nozzles makes those increasingly difficult. The studied pressure gradients include the pressure gradient mutation (PGM), strong adverse pressure gradient (SAPG), weak adverse pressure gradient (WAPG), and favorable pressure gradient (FPG). The results show that complex pressure gradients change the coolant coverage and adhesion and may induce a recirculation zone in the SAPG region. This affects the development of the vortex construction and leads to different FCE distributions. The FCE under the effects of the PGM is the largest, followed by the SAPG, WAPG, and FPG. A larger inclination angle weakens the coolant adhesion but may enhance its downstream and lateral flow treads. This is coupled with the effects of complex pressure gradients, where larger inclination angles bring better film cooling effects in some cases. After accounting for the four blowing ratios, the area-averaged FCEs under the effects of the SAPG, WAPG, and FPG are 28.9 %, 42.8 %, and 52.3 % lower than that under the PGM, respectively. The area-averaged FCEs under the conditions α = 45° and 60° are 6.6 % and 10 % lower than that under α = 30°.

Comparative Ablation Behaviors of 2D Needled C/SiC and C/SiC-ZrC Composites

To investigate the effect of ZrC on the ablative properties of C/SiC composites in a high-temperature environment, the oxidative ablation of C/SiC and C/SiC-ZrC composites at high-temperatures was examined through ablation tests. In this study, two ceramic matrix composites, C/SiC and C/SiC-ZrC, were prepared by chemical vapor deposition and precursor impregnation pyrolysis. The ablation properties of the materials were tested and analyzed using an oxyacetylene flame to simulate a high-temperature environment. The results revealed that the line ablation rate of C/SiC-ZrC was 8.48% and 20.81% lower than that of C/SiC at 30 s and 60 s, respectively. At the same ablation time, the depth of the crater resulting from erosion of the C/SiC material by the high-temperature airflow was deeper than that of C/SiC-ZrC. The traces of the fibers subjected to erosion were more prominent. In a longitudinal comparison, the mass ablation rate of C/SiC-ZrC material decreased with the increase in time, while the line ablation rate initially increased rapidly and then decreased. From 30 s to 90 s of ablation, the line ablation rate and mass ablation rate decreased by 55.62% and 89.5%, respectively. The overall trend for both rates was a decrease with the increase in time. Under the same ablation time, the ablation rate of C/SiC-ZrC was generally lower than that of C/SiC. This is because the generated molten ZrO2 was more viscous and denser than SiO2, effectively blocking oxidizing gases from penetrating the interior of the material. The molten ZrO2 provided better protection for the substrate in the high-temperature environment.

Properties of shape memory polyimide composites with continuous “brick-and-mortar” layered structure: High flame retardancy, ablation resistance, and high mechanical properties

Shape memory polymer (SMP) is a type of smart materials mostly working below 200 ℃, which is difficult to be applied in the ultra-high temperature, even 1000 ℃. Herein, we have prepared shape memory polyimide composites (SMPIC) inspired by nacre “brick-and-mortar” layered structure. The SMPIC has the flame-retardant grade of V0 and the limiting oxygen index of 53.1% when the flame retardants amount is 20%. Besides, it has excellent resistance to high temperature airflow erosion, and the mass ablation ratio is 0.0053 g/s. After rapid thermal processing at 1000 ℃, the mechanical strength is 100 MPa, and the strength retention ratio is 103.3%. Also, the simple actuator can overturn iron blocks that are more than 8 times of its own weight, thus, the smart composite has huge application prospects in the high temperature environment, such as active morphing re-entry parachute, which provides reference for lightweight smart structures.

Experimental study of photothermal conversion of heat absorbers filled with metal foams of different pore densities

High output temperature and photothermal conversion effectiveness were achieved with the absorber platform structure. A novel solar receiver was manufactured to integrate pre-heating and thermal conversion, aiming to enhance heat utilization and output temperature. This work is based on the engineering design and experimental testing of a solar cavity-receiver containing a porous copper foam that can volumetrically absorb high-flux radiation and heat up, through convection with air flow. The air outlet temperature, outer wall temperature, thermal performance, and efficiency were experimentally determined by pore density, air mass flow rate and solar irradiance. Additionally, the temperature growth of unit incident power (?),the unit volume efficiency growth rate (?) and output temperature were employed to evaluate the thermal conversion characteristics of the endothermic body(copper foam).The results indicated that the air outlet temperatures can reach 500?C with lower input power. Furthermore, it was found that under a pore density of 30 pores per inch and a flow rate of 60 L?min-1, the photothermal conversion efficiency of the absorber with copper foam reached as high as 87.61%, which is 35.04% significantly higher than that of an absorber without copper foam. The manageable solar receiver design proved to deliver a high-temperature air flow (approximately 500?C) with a reasonably high thermal efficiency(over 85%).

Experimental analysis of smoke dispersion in belt fires by using a real-size model of the underground space

It is essential to investigate the dispersion of a large amount of toxic and harmful smoke in coal mine network during belt fires. A real-size model of an underground space was used to study the additional thermal resistance of single roadway and the dispersion characteristics of CO in bending structure, as well as the effect of air velocity on CO dispersion. The results show that the resistance of high temperature air flow is related to the ventilation resistance of the roadway under normal airflow, and is proportional to airflow temperature. The additional thermal resistance increases first and then decreases with time, and curve trend is similar to multiple “humps” with the increase of distance, which indicates that temperature is dominant factor. The distribution of CO is V-shaped at 90° corner, and the distribution pattern is unaffected by corner number. The increase of the air velocity destroys the CO distribution at the initial corner, and the air vortex zone appears at the bottom of the roadway. Meanwhile, the smoke flow diffused to the working face shifts from the outer wall to the inner wall of the working face. The results are helpful for selecting emergency evacuation areas and implementing safe measures.

Preparing dry powder inhalation formulation of salbutamol sulfate using an ultrasonic atomizer device

Inhaled particles must possess certain morphological characteristics to ensure effective drug delivery to the targeted site in the lungs. A modified version of ultrasonic spray pyrolysis was employed to prepare salbutamol sulfate in the form of dry powder. A solution of salbutamol sulfate was atomized using ultrasonic nebulization, and the droplets were transformed into solid drug particles through exposure to high temperature airflow. The engineered salbutamol sulfate samples underwent physical characterization, including particle size, morphology, thermal behavior, and crystallinity analysis. The aerodynamic particle size distribution (APSD) and in vitro deposition of the dry powder inhalation formulation were assessed using the Next Generation Impactor (NGI). The salbutamol sulfate dry powder prepared by the ultrasonic atomizer exhibited an aerodynamic diameter ranging from 1 to 5 μm, as supported by the SEM images. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) results showed a significant drop in the crystallinity of the engineered particles. Aerosolization performance studies demonstrated a fine particle fraction (FPF) value (below 5 μm) of 25% for the engineered salbutamol sulfate produced using the ultrasonic atomizer technique, which is acceptable for inhalation purposes. Based on the observed results, this newly introduced method appears to be suitable for producing dry powder formulations of different drugs, with a minimized need for the use of surfactants or stabilizers in the formulation.

Predictive modelling of the cross‐sectional reduction of smoke extraction ducts under fire resistance test

AbstractIn‐service performance of single compartment smoke extraction ducts is evaluated in the fire resistance test with the method described in EN 1366‐9. Failure due to cross‐sectional reduction is caused by the deflections of the steel walls which are exposed to a differential pressure and a high temperature airflow coming from the furnace.This study presents a methodology for the prediction of the deformation of smoke extraction duct steel walls using 1‐way transient coupling between computational fluid dynamics (CFD) airflow simulation and mechanical finite element analysis (FEA). The resulting deformation is compared with a full‐scale experimental test. The degradation of the steel mechanical behavior with temperature is calibrated using experimental data. This coupled analysis provides insights into the tools and methodology developed for the digitalization of fire resistance testing.

Oxidation and Ablation Behavior of Particle-Filled SiCN Precursor Coatings for Thin-Film Sensors.

Polymer-derived ceramic (PDC) thin-film sensors have a very high potential for extreme environments. However, the erosion caused by high-temperature airflow at the hot-end poses a significant challenge to the stability of PDC thin-film sensors. Here, we fabricate a thin-film coating by PDC/TiB2/B composite ceramic material, which can be used to enhance the oxidation resistance and ablation resistance of the sensors. Due to the formation of a dense oxide layer on the surface of the thin-film coating in a high-temperature air environment, it effectively prevents the ingress of oxygen as a pivotal barrier. The coating exhibits an exceptionally thin oxide layer thickness of merely 8 μm, while its oxidation resistance was rigorously assessed under air exposure at 800 °C, proving its enduring protection for a minimum duration of 10 h. Additionally, during ablation testing using a flame gun that can generate temperatures of up to 1000 °C, the linear ablation rate of thin-film coating is merely 1.04 μm/min. Our analysis reveals that the volatilization of B2O3 occurs while new SiO2 is formed on the thin-film coating surface. This phenomenon leads to the absorption of heat, thereby enhancing the ablative resistance performance of the thin-film sensor. The results indicate that the thin-film sensor exhibits exceptional resistance to oxidation and ablation when protected by the coating, which has great potential for aerospace applications.

Performance of supercritical carbon dioxide heat transfer in several enhanced tubes under external high-temperature airflow

A precooler is installed in the Synergetic Air-Breathing Rocket Engine to ensure the compressor operates properly at high Mach numbers. To better fit the actual precooler scenario, the present study simulated the flow and heat transfer fields of supercritical carbon dioxide (sCO2) in a horizontal tube in which hot air counterflows outside. In the coolant channel, two types of ribs were selected and added, including rectangular ribs as well as isosceles triangular ribs. The geometry parameters of the rib and its configuration on the flow and heat transfer characteristics of sCO2 were examined in detail. Results show that six out of eight cases revealed an increase in Nut over 100% in the triangular rib, making it the optimal enhanced structure. Compared to rectangular ribbed tubes, triangular ribbed tubes display a different variation trend in Nut with rib height due to the combined effects of fluid disturbance and recirculation zone. Both types of ribs increase their Nut first and then decrease it with the rise of rib pitch, which may be due to the fact that high turbulence zones require several distances to develop. This research can provide theoretical foundations and data support for the design of high-efficiency air-precoolers in the future.

Case study: Noise attenuating performance of perforated corrugated lined straight-through perforated pipe-resistant muffler

Noise of internal combustion engine is the main source of noise pollution, and mufflers are the main means to reduce exhaust noise of internal combustion engine. In the process of improving the acoustic performance of the exhaust muffler based on the three-dimensional numerical method, the influence of the high speed and high temperature airflow discharged by the engine in the actual working process on the acoustic performance of the exhaust muffler is often ignored, which leads to the obvious deviation between the predicted results and the actual situation. In this paper, based on the structural optimization design of the straight-through perforated pipe resistance muffler, the noise characteristics of the straight-through perforated pipe resistance muffler with perforated corrugated lining was analyzed considering the airflow velocity and temperature inside the muffler. The influence of temperature and velocity fields on the acoustic performance of muffler was studied by using numerical simulation method, which takes the solution results inside muffler as boundary conditions of sound field analysis. The influence of changing the internal structure of muffler on the aerodynamic performance of muffler was discussed, and the pressure loss is analyzed. The research has shown that adding perforated corrugated lining inside muffler could effectively improve the transmission loss of muffler and the noise reduction performance of prototype straight-through perforated pipe-resistant muffler.

Numerical study on droplet evaporation and propagation stability in normal-temperature two-phase rotating detonation system

A numerical study is carried out on the droplet-laden two-phase rotating detonation wave of kerosene/oxygen-enriched air at normal temperature. Two types of combustors without and with the inlet mixing section are constructed to illustrate the effect of inlet mixing section on the combustion characteristics of two-phase rotating detonation wave. In the inlet mixing section, a preheating zone is formed after the reverse oblique shock wave induced by the rotating detonation wave, and its important role on the droplet evaporation is analyzed. The parameter sensitivity of rotating detonation wave propagation stability to the average droplet diameter d0 is further discussed. Results show that the droplets mainly evaporate after the detonation front in the combustor without inlet mixing section, and the reaction heat release is completed in a short distance, which propels continuous propagation of the detonation wave. When d0 gradually increases, the droplet evaporation distance increases, and the coupling between the incident shock and reaction is continuously weakened, finally resulting in the detonation quenching. In the combustor with inlet mixing section, a preheating zone is induced close to the contact surface by the reverse oblique shock wave. A large number of droplets evaporate in this zone, and generate sufficient mixture of fuel vapor and oxidizer in front of detonation wave to maintain the detonation propagation. Priority to the combustor without inlet mixing section, the droplet evaporation relies less on the inlet high-temperature airflow with the assistance of preheating zone, and thus the wave propagation stability can be enhanced and the rotating detonation wave can sustain for a wider range of d0. The present analysis provides a new understanding of two-phase rotating detonation systems.

Microstructure and High-Temperature Ablation Behaviour of Hafnium-Doped Tungsten-Yttrium Alloys

W is a widely used refractory metal with ultra-high melting point up to 3410 °C. However, its applications are limited by poor ablation resistance under high-temperature flame and air flow, which is crucial for aerospace vehicles. To improve the ablation resistance of W under extreme conditions, W-Y alloys doped with different Hf mass fractions (0, 10, 20, and 30) were prepared using the fast hot pressing sintering method. Microstructure and ablation behaviours at 2000 °C were investigated. Results showed that adding an appropriate amount of Hf improved the properties of the W-Y alloy evidently. In particular, the hardness of the alloy increased with the increased content of Hf. The formation of the HfO2 layer on the surface during ablation decreased the mass and linear ablation rates, indicating enhanced ablation resistance. However, excessive Hf addition will result in crack behaviour during ablation. With a Hf content of 20 wt.%, the alloy exhibited high stability and an excellent ablation resistance.

A thermal environment prediction method for a mine ventilation roadway based on a numerical method: A case study

Thermal environment prediction has become increasingly significant in recent years with its promise of wide application in underground structures. The current work presents a numerical method for the feasible and effective analysis of the airflow and surrounding rock temperatures in ultralong mine ventilation roadways to understand their dynamic heat transfer. To reduce the modeling effort and computation time with satisfactory accuracy, the ventilation roadway is approximated as a one-dimensional (1D) line element. Still, the surrounding rock of the roadway remains three-dimensional (3D). An equivalent heat transfer coefficient calculates the dynamic heat transfer in the radial direction of the roadway. A case analysis of the Sanhejian coal mine ventilation roadways in China is performed, and a comparison between the simulated results and field measurements indicates that the predicted airflow temperature shows good agreement. The change in the underground thermal environment concerning ventilation time is systematically investigated in detail. The surrounding rock of the roadway adjusts the underground thermal environment through heat absorption or heat release, and the temperature distribution of the surrounding rock presents a "V" shape in winter and a "W" shape in summer. The self-compression of air contributes to a remarkable heat source for the increase in airflow temperature in the air intake shaft. A roadway with a low initial temperature of the surrounding rock after long-term ventilation can effectively cool the high-temperature airflow underground in early summer. The reduction in ventilation volume will increase the cooling degree of airflow by the surrounding rock in long-term ventilation roadways in summer. Still, the airflow will be heated more significantly in short-term ventilation roadways. In addition, the increase in the average annual ambient temperature will result in a linear rise in underground airflow temperature.

Experimental Study of the Thermally Grown Oxide and Interface of Thermal Barrier Coatings Using TEM In-Situ Heating.

Thermal barrier coating (TBC) materials play important roles in gas turbine engines to protect the Ni-based superalloys from high-temperature airflow damage. In this work, the nano-mechanism of TBC failure is analyzed. A scanning transmission electron microscopy-energy dispersive spectrometer (STEM-EDS)-based analysis method was used to study the influence of element migration on the deformation behavior of the bond-coat (BC) layer during heating. The content of elements in the same region varied greatly at different temperatures, which could prove the contribution of element migration to the deformation of the BC layer. TEM in-situ heating experiments were designed and carried out to study the deformation behavior near the ceramic topcoat (TC)/thermally grown oxide (TGO) and the TGO/BC interface. The TC/TGO interface was deformed violently during heating, and obvious deformation occurred at 100 °C, while the TGO/BC interface was relatively stable. A subset geometric phase analysis method was used for full field-strain measurement. The strain value near the TGO/BC interface was relatively small and did not change significantly at lower temperatures. The TC/TGO interface is more unstable and easier to deform than the TGO/BC interface. The stress and strain evolution in the internal region of TGO at high temperatures was quantitatively analyzed. The TGO layer has a tensile stress of GPa magnitude along the interface direction at the peak position, and the shear stress is small.

A simple model for predicting dispersion characteristics of high-temperature airflow and particle distribution during smelting process in a thermally stratified foundry shop

The flow-field characteristics of high-temperature airflow and particle distributions generated from the running furnace during smelting process in foundry shop were studied by on-site measurements and theoretical analysis in this work. A theoretical model was proposed to describe the dispersion characteristics of the high-temperature buoyant jet flow and plume induced by the furnace with two typical opening modes. The coupling effects of the heat source features and environmental factors were emphatically discussed by comparing the trajectory, cross-section diameter, velocity and temperature decay, and volumetric flow rate during the dispersion process. Results show that in the large foundry shop with natural ventilation, there is a vertical temperature gradient of 0.1–0.4 °C/m, which can make the pollutant locked in the workshop and weaken the entrainment of the thermal flow. The crossflow is a non-negligible factor, which causes the dispersion trajectory of the pollutant flatter and induces a long travelling distance along the horizontal direction. The indoor particle concentration when the furnace opens concentration seriously exceeds the limits in the existing indoor air quality standard of China. This work is expected to give suggestions on ventilation designs and arrangements of the exhaust port or device in industrial environments.

Numerical Analysis of Fiber/Air-Coupling Field for Annular Jet.

Melt-blowing technology is an important method for directly preparing micro-nanofiber materials by drawing polymer melts with high temperature and high velocity air flow. During the drawing process, the melt-blowing fiber not only undergoes a phase change, but also has an extremely complex coupling effect with the drawing airflow. Therefore, in the numerical calculation of the flow field, the existence of melt-blowing fibers is often ignored. In this paper, based on the volume of fluid method, a numerical study of the flexible fiber/air-coupling flow field of an annular melt-blowing die is carried out with the aid of computational fluid dynamics software. The results show that the pressure distribution in the different central symmetry planes of the ring die at the same time was basically the same. However, the velocity distribution may have been different; the velocity on the spinning line varied with time; the pressure changes on the spinning line were small; and velocity fluctuations around the spinning line could cause whiplash of the fibers.

Reliability evaluation of thermal barrier coatings for engine combustion chambers based on Monte-Carlo simulation

Due to their outstanding high temperature resistance, thermal barrier coatings are commonly applied in hot section such as aero-engine combustion chambers to protect components from high-temperature airflow. However, its material properties are insecure, the service environment is harsh, and the service life is uncertain. This paper uses a Monte-Carlo simulation-based dependability evaluation method to resolve this uncertainty. To begin with, the failure criteria for thermal barrier coatings were defined, as was the limit state equation. Second, using the Monte-Carlo simulation approach, the failure probability was calculated. Finally, the TBCs' dependability in a thermal gas flow environment was assessed and compared to actual measurement results. The thermal barrier coating in the area where the surface temperature of the combustion chamber is above 1100 °C begins to fail after 4000 h of service under the action of the hot air flow, with a failure rate of more than 60 %. The service life of the thermal barrier coating at surface temperatures of 1000, 1100, and 1197 °C was 6327, 3125, and 1642 h, respectively, based on the time it took to attain 60 % failure probability under the impact of hot air flow. The numerical relative error of the actual failure probability PT of the thermal barrier coating of the combustion chamber obtained by hole detection technology is less than 10 % (except for 3000 h), demonstrating the dependability and applicability of the reliability assessment technique. The reliability of thermal barrier coatings is influenced by temperature and TGO development.

Study of oxidation behavior and tensile properties of candidate superalloys in the air ingress simulation scenario

Air ingress incidents are major safety accidents in very-high-temperature reactors (VHTRs). Air containing a high volume fraction of oxygen may cause severe oxidation of core components at the VHTR, especially for the significantly thin alloy tube wall in the intermediate heat exchanger (IHE). The research objects of this study are Inconel 617 and Incoloy 800H, two candidate alloys for IHE in VHTR. The air ingress accident scenario is simulated with high-temperature air flow at 950 °C. A continuous oxide scale was formed on the surfaces of both the alloys after the experiment. Because the oxide scale of Inconel 617 has a loose structure, whereas that of Incoloy 800H is denser, Inconel 617 exhibited significantly more severe internal oxidation than Incoloy 800H. Further, Inconel 617 showed a significant decrease in ultimate tensile strength and plasticity after aging for 200 h, whereas Incoloy 800H maintained its tensile properties satisfactorily. Through control experiment under vacuum, we preliminarily concluded that serious internal oxidation is the primary reason for the decline in the tensile properties of Inconel 617.

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.