High-velocity Air Flow Research Articles

A microwave discharge in high-velocity flows initiated by a half-wave antenna

A microwave discharge in high-velocity (150—250 m/s) air flows induced on a half-wave vibrator is studied. A cw magnetron microwave generator with a frequency of 2.45 GHz and an output power of up to 5 kW was used for initiation of the microwave discharge. The high-speed video imaging was used for studying the discharge structure, determining the diameter and length of the plasma channel as a function of flow velocity and pressure. Electron concentration and temperature, along with characteristic gas temperature, were determined based on the optical spectra. The possibility of using this microwave discharge for ignition of hydrocarbon–air mixtures in combustion chambers of ramjet engines is proved experimentally.

Study on Temperature Field Distribution of a High-Speed Double-Helical Gear Pair with Oil Injection Lubrication

The temperature field distribution of high-speed double-helical gears under oil injection lubrication is investigated by obtaining heat flux density and convective heat transfer coefficients through theoretical calculations and CFD (computational fluid dynamics) simulations. Based on the CFD method, fluid simulations are performed to obtain the distribution of lubricating oil on the surface of the double-helical gears, the velocity streamline diagram of the lubricating oil, and the convective heat transfer coefficients of different surfaces of the gears. The friction heat flux density is calculated using Hertzian contact theory and theoretical formula of heat generation. The double-helical gears’ steady-state temperature field simulation uses this heat flux density as a boundary condition. The correctness of the calculation method is verified through experiments. The study shows that increasing the jet velocity allows the jet to reach the tooth surface more effectively, improving the cooling effect and reducing the maximum gear temperature. However, the relationship between the jet velocity and the minimum gear temperature is non-linear. Within a certain range, increasing the jet diameter makes the jet wider, and the area covered by the lubricating oil becomes larger as the jet spreads around the gear teeth, enhancing the cooling effect. An increase in gear speed leads to an increase in frictional heat flux density; moreover, the high-velocity airflow generated by the increased speed reduces the amount of lubricant entering the mesh zone, which in turn causes the maximum temperature of the gears to continue to rise.

Effect of airflow velocity in vessel-pipelines on dust explosion during pneumatic conveying

In order to investigate the flame propagation and pressure characteristics of dust particles during airflow transportation, a dust explosion experimental apparatus simulating dust handling processes was designed. The system is supplied with a stable airflow by a high-pressure fan, capable of continuously transferring dust from the vessel to the pipeline at a controllable speed. Dust explosion experiments were meticulously executed by applying 1 kJ of ignition energy to the transported dust particles at airflow velocities of 5, 10, and 15 m/s. This study examines the effects of airflow velocity, pipe diameter, and ignition position on explosion characteristics and delves into the mechanisms of these effects through a detailed analysis of experimental results. For the maximum explosion pressure, an increase in airflow velocity causes the maximum explosion pressure to rise. As the pipe diameter increases, the maximum explosion pressure first rises and then falls. The maximum explosion pressure occurs in the case of bottom ignition. The range of maximum explosion pressure values is expected to provide a reference for explosion detection and explosion venting. For flame propagation, the flame undergoes five processes within the vessel, including ignition of dust particles, spherical flame development, flame stretching towards the mouth of the pipe, complete combustion of particles, and near exhaustion of particle combustion. The higher the airflow velocity and the larger the pipe diameter, the more pronounced the flame stretching and development towards the pipe opening become. Within the pipe, two distinct flame propagation behaviors were observed under conditions of high and low airflow velocities. Compared with the experimental results of high-pressure pneumatic powder spraying forming a dust cloud within a vessel-pipeline, this study measures the maximum explosion pressure at specific turbulence levels and establishes a quantitative relationship between airflow velocity and explosion characteristics. Utilizing this correlation can provide a reference for predicting industrial-grade dust explosion characteristics at different airflow velocity levels, thereby offering a more optimized design basis for the design and safety protection of industrial equipment.

Predicting extinction limits of concurrent smoldering spread by a reduced analytical model

Smoldering is a flameless combustion mode occurring on the surface of charring fuels, such as wood and cigarettes. Although the smoldering process is slow and has a low temperature compared to flaming, it is easy to be initiated by a weak heat source and persists under poor oxygen conditions. Extensive work has been done for flame extinction to develop scaling models to predict the limiting oxygen concentration (LOC), but limited work is available for the smoldering extinction. This study develops a reduced analytical model to predict the extinction limits of smoldering. The model simultaneously solves smoldering propagation rate, surface temperature, and surface oxygen mass fraction as part of the solutions. The extinction limit is determined as the critical condition where solutions satisfying all governing equations cease to exist. The model provides a qualitative description and captures the essential characteristics of a previous experiment. The smoldering rate decreases with increasing fuel diameter, and a larger-diameter fuel is easier to extinguish. The mechanisms of the extinction process are investigated, showing the dominant role of radiative heat loss in the smothering limit at low airflow velocities and convective heat loss near the blowoff limit at high airflow velocities. Further analysis of the effect of oxygen concentration shows an increasing trend of LOC with fuel diameter, and the smothering branch cannot be predicted without considering the heat loss through radiation from the solid surface. Novelty and Significance StatementThe novelty of this research is the prediction of smoldering propagation rates and extinction limits across various fuel diameters using a reduced analytical model. The model is modified to accommodate a thin fuel configuration and high airflow velocity condition, where convective heat and mass transfer play a more important role. The model's significance is that it not only provides essential insights into the mechanisms but also has the capability to simultaneously determine key parameters such as spread rate, reaction temperatures, and surface oxygen concentration as part of the solutions. Additionally, this study demonstrates the model's ability to reproduce the limit conditions, including smothering/blowoff limits and limiting oxygen concentration (LOC).

Deformation of Sessile Droplets under a Gentle Shear Airflow.

Deformation of sessile droplets under shear flow is widespread in both nature and industry. Previous research focuses on the shedding process of sessile droplets under shear airflow, with insufficient attention paid to the droplet deformation before shedding. In this work, experimental studies on the deformation behaviors of sessile droplets under shear airflow are conducted to investigate the effects of airflow velocity and droplet volume on the tangential and normal droplet deformations. Scaling laws of the droplet deformations are established. The results show that the profile of sessile droplets changes under shear airflow with the topmost point exhibiting periodic oscillations in both tangential and normal directions. The oscillation period of the tangential deformation exceeds that of the normal deformation. The average tangential deformation of droplets increases with the increasing airflow velocity and droplet volume. The average normal deformation of droplets increases with the increasing airflow velocity and is influenced by the droplet volume at a higher airflow velocity. The contact angle on the windward side oscillates periodically, and its average value significantly decreases. The contact angle of droplets on the windward side decreases as the airflow velocity and droplet volume increase, while the contact angle on the leeward side remains almost unchanged. The average deformation of droplets in the tangential and normal directions is linearly related to the effective Weber number and the square of the effective Weber number. These findings could be used to predict the deformation of sessile droplets under shear airflows.

Numerical Simulation and Analysis of the Airflow Field in the Crushing Chamber of the Hammer Mill.

The airflow dynamics within hammer mills' crushing chambers significantly affect material crushing and screening. Understanding the crushing mechanism necessitates studying the airflow distribution. Using a self-built crushing test platform and computational fluid dynamics (CFD) simulations, we investigated the impact of screen aperture size, rotor speed, hammer-screen clearance, hammer quantity, and mass flow rate on airflow distribution within the rotor region, circulation layer, and screen apertures. Results indicated generally uniform axial static pressure distribution within the rotor region, with radial gradients. Increased rotor speed improved radial static pressure gradients, while higher mass flow rates reduced them. The highest airflow velocity within the circulation layer reached approximately 83.46% of the hammer tip's tangential velocity. Greater rotor speed and hammer quantity intensified circulation airflow, whereas increased mass flow rate decreased it. Eddies formed within screen apertures with higher rotor speeds and hammer quantities but diminished with larger apertures and higher mass flow rates. Static pressure differences across screen apertures increased with mass flow rate and rotor speed but decreased significantly with larger apertures. This systematic examination provides insights into airflow distribution within hammer mill crushing chambers, offering a theoretical foundation for improving and designing hammer mills.

Simultaneous PIV/LIF Measurements Of Velocity And Oxygen In An Advecting Air-Water Channel Flow

We investigate oxygen transfer across an air-water interface using an integrated Particle Image Velocimetry (PIV) and Laser-Induced Fluorescence (LIF) system. This setup allows for the simultaneous measurement of velocity and dissolved oxygen (DO) concentration fields, alongside the visualization of the air-water interface. The imaging process begins with zero DO concentration and continues until oxygen saturation in the water is achieved. Calibration of the oxygen LIF intensity field is performed using these images, benchmarked against measurements from an optical oxygen point probe, to ensure accurate conversion to a physically relevant unit. The air channel generates an air flow with a center-line mean velocity of 8.84 m/s, corresponding to a Taylor Reynolds number of 100. This high-velocity air flow results in the formation of wavy structures on the water interface creating situations similar to those found in nature (such as rivers and oceans). The wavy structures create reflections from the air-water interface; these reflections are mitigated during post-processing using a Wavelet-Fast Fourier Transform (W-FFT) method to remove high intensity noise. Additionally, the wind shear generated by the air flow induces high turbulence intensity and vortices near the air-water interface, significantly enhancing the dissolution of oxygen into the water. This turbulence and the associated vortical structures play a crucial role in the mixing process and the overall oxygen transfer efficiency. The study presents instantaneous dissolved oxygen structures overlapped with velocity structures in a flow with deformable interface, providing some insights into the oxygen transfer and mixing processes.

Starch powder in short air contact time: Material moisture change, stickiness and deposition at different air relative humidity and temperature

Starch powder can get sticky and cause deposition in process, resulting in downtime and performance deterioration. Glass transition temperature (Tg) is a key aspect of semi-crystalline material stickiness, being function of material water content, and the surpass of material temperature above Tg is related to its stickiness. During pneumatic transport and process, in short air contact time, powders can exchange heat and water, which might change stickiness. Dairy powders were already assessed on stickiness tests replicating these conditions, but not starches. Potato and corn starch powders are investigated and stickiness, deposition and water content change are measured. Water content changed linearly with relative humidity. Deposition occurred on a plate at high and low air flow velocities and the deposition patterns correlate with the flow velocity profile. Corn starch presented higher stickiness and more deposition. Stickiness can be achieved by the combination of different air temperatures and relative humidities.

Study on the effect of wall roughness on the flow characteristics of liquid film in airflow

During the flight, water flows under the action of shear force to form the overflow liquid film and freezes to form overflow ice. Overflow ice will change the aerodynamic shape of the aircraft and affect the safety and stability. In this paper, the three-dimensional structure of the liquid film in airflow was measured transiently with the digital image projection technology. The effects of different flat wall roughnesses, airflow velocities, and liquid film flow rates on the flow characteristics of the liquid film were analyzed. The experimental results show that the lower the wall roughness, the more pronounced the liquid film contraction toward the central axis. When the liquid flow rate drops to 0.228 l/min, the continuous liquid film transforms into rivulets on the surface of a large contact angle or low roughness. The average thickness of the liquid film on the flat is greater than that on the rough flat at low airflow velocity and low liquid flow rate, but smaller than that on the rough flat at high airflow velocity and high liquid flow rate. The wave near the inlet has an obvious acceleration process, and after reaching the peak speed, it gradually decreases and stabilizes near a certain value. The smoother the surface, the faster the acceleration of the wave at the inlet, and the greater the final stable wave speed.

Experimental Study of the Airflow Field and Fiber Motion in the Melt-Blowing Process.

The melt-blowing process involves high velocity airflow and fiber motion, which have a significant effect on fiber attenuation. In this paper, the three-dimensional airflow field for a melt-blowing slot die was measured using the hot-wire anemometry in an experiment. The fiber motion was captured online using a high-speed camera. The characteristics of the airflow distribution and fiber motion were analyzed. The results show that the melt-blowing airflow field is asymmetrically distributed. The centerline air velocity is higher than that around it and decays quickly. The maximum airflow velocity exists near the die face, in the range of 130-160 m/s. In the region of -0.3 cm < y < 0.3 cm and 0 < z < 2 cm, the airflow has a high velocity (>100 m/s). As the distance of z reaches 5 cm and 7 cm, the maximum airflow velocity reduces to 70 m/s. The amplitude of fibers is calculated, and it increases with the increase in air dispersion area which has a significant influence on fiber attenuation. At z = 1.5 cm, 2.5 cm, 4 cm, and 5.5 cm, the average fiber amplitudes are 1.05 mm, 1.71 mm, 2.83 mm, and 3.97 mm, respectively. In the vicinity of the die, the fibers move vertically downward as straight segments. With the increase in distance from the spinneret, the fiber appears to bend significantly and forms a fiber loop. The fiber loop morphology affects the velocity of the fiber movement, causing crossover, folding, and bonding of the moving fiber. The study investigated the interaction between the fiber and airflow fields. It indicates that the airflow velocity, velocity difference, and dispersion area can affect the motion of fiber which plays an important role in fiber attenuation during the melt-blowing process.

Study on characteristics and mechanism of re‐entrainment in intake filtration components of the gas turbine

AbstractAiming at the problem that the droplet re‐entrainment in the intake filtration components affects the gas intake quality and the safe operation of gas turbines, the characteristics and mechanism of the re‐entrainment in a marine double‐hook inertial stage filter are studied, and the distribution of liquid film and re‐entrainment under different inlet air velocities and droplet size distributions are compared. The results show that, due to the influence of inertia and turbulent dispersion, droplets tend to deposit on draining hooks and the lower surface of the leeward side of the blade to form a liquid film. Film stripping is the main form of re‐entrainment under ship‐driving conditions. Due to the high airflow velocity and the large film thickness, hook peaks are the most prone to film stripping, and the critical airflow for film stripping velocity is 14.8 m/s. With the increase of the inlet air velocity, the film thickness at the draining hooks increased first and then decreased, and the film stripping positions were extended from the hook peaks to the rear. Different droplet size distributions mainly affect the liquid film thickness at the draining hook and leeward side of the blade, which in turn affects the film stripping mass.

Flow Characteristics of Liquid Jet in Transverse Shear Crossflow

The numerical simulation method was used to investigate the deflection and deformation process of a circular lubricating oil jet in transverse shear airflow. The numerical model was compared and validated against the experimental data. The physical parameters of Mobil jet Oil II were utilized in this simulation with the nozzle diameter ranging from 0.5 to 2.5 mm, the maximum liquid/gas momentum ratios varying from 10.35 to 165.50, and the injection angle ranging from 0 to 30° in the opposite airflow direction. The results show that an increase in the nozzle diameter decreases the degree of jet deflection. The higher airflow velocity causes more fluctuations in the oil-jet trajectory, while the higher oil-injection velocity reduces fluctuations in the trajectory. The parabolic curve equations were used to derive the trajectory equations for the jet column’s pre-disintegration under both vertical incidence and a small angle of reverse airflow. The nozzle diameter and maximum oil/air momentum ratio were used to obtain a formula for the trajectory curve of the lubricating oil. Additionally, a formula for fitting the trajectory curve of oil injected in the opposite airflow direction regarding the injection angle was developed.

Numerical simulation of airflow temperature field and cooling in high temperature coal seam working face

With the continuous extension of coal mining depth, the risk of heat exhaustion from high temperatures gets worse. In this paper, we first identified the main heat sources of the high-temperature coal seam working face. The effects of air supply volume on the temperature evolution of working face were then investigated by numerical simulation. The air supply volume to effectively cool the working face was calculated using a new method, sigma heat difference. A mechanical cooling method, an air cooler in the intake airway, was proposed, and the accuracy of the sigma heat difference method was confirmed through numerical simulation and field measurements. The results show that: (1) The aggregate thermal energy emitted by the heat sources located in the mining working face is 688.2177 kW, with mechanical equipment heat dissipation accounting for the main part; (2) The numerical simulation shows that the temperature of the airflow of the roadway steadily drops as the airflow velocity from the inlet airway increases. Yet extremely high inlet airflow velocities lead to narrower change ranges of the air temperature of the working face; (3) The air supply volume required to effectively cool the working face is determined by the sigma heat difference method to be 1972.2 m3/min, which provides a basis for numerical simulation of inlet air velocity for cooling; (4) The Field measurement and numerical modelling demonstrate that the sigma heat difference method is reasonable. The research conclusions provide theoretical guidance for dealing with heat hazards in coal mining faces.

Simultaneous Enhancement of Welder Health and Aluminum Weld Joint Quality Using Controlled Welding Room Condition

Aluminum alloy is crucial for lightweight and fuel-efficient vehicles due to its strength, lightness, and corrosion resistance. However, welding aluminum vehicle parts poses challenges, particularly porosity issues caused by trapped hydrogen gas in the weld metal. This study aimed to investigate the impact of the welding room environment on the health and properties of aluminum welding joints. To achieve this, an isolated room was created, where variations in airflow velocity (1.1 m/s, 1.6 m/s, and 2.1 m/s) and temperature (19 °C, 27 °C, and 35 °C) were implemented. The fume condition of the room was assessed to determine its impact on health aspects, while bead appearance and macrostructure were evaluated to assess weld joint quality. Results revealed that higher airflow velocity and temperature reduced fume concentration in the welding room, indicating a healthier environment. However, these conditions also led to increased porosity defects and influenced the performance of the shielding gas. Additionally, higher ambient temperatures increased hydrogen solubility in the molten aluminum, exacerbating porosity issues. For optimal welder comfort and high-quality weld joints, it is recommended to maintain a low temperature and airflow velocity in the welding room, ensuring a healthier working environment while minimizing porosity defects.

Variation Of Air Speed On Energy-Saving Stoves With Waste Coconut Oil Frying Getas

Waste coconut oil (WCO) from brittle draining can be used as fuel for multifuel stoves and can be used to boil water. Combustion air velocity is one of the factors that affect the combustion process. Air velocity in a multi-fuel stove with WCO fuel greatly affects the character of the fire produced. The research was conducted with the aim of obtaining the character of the fire and the temperature of the water boiling process at variations in air speed and the length of the WCO combustion flame. This research process was carried out with a multifeul stove by measuring the temperature and boiling time of 0.5 liters of water using variations in air velocity of 10 m / s, 12 m / s and 14 m / s by using 20 ml of WCO for each test. The results at a speed of 14 m / s tejadi the highest flame this occurs high air flow velocity causes a faster temperature rise. The greater the air supplied will accelerate the oxidation process, so that the fuel decomposition process will be faster.

Electrowetting assisted shedding of droplets by a shearing airflow in dropwise condensation

Condensation is a necessary part of most industrial processes dealing with phase change. Efficient condensate removal (shedding) increases the systems’ performance. This study investigated the influence of electrowetting on droplet shedding and heat transfer enhancement during humid air condensation on a horizontal substrate. A mini closed-looped wind tunnel was used to simulate the condensation environment and control condensation parameters. AC electric field with the frequencies of 1 and 10 kHz was applied and compared with the control case (no voltage). The transient inverse heat conduction method was used to determine the heat transfer coefficient, and heat transfer measurements were synchronized to a high-resolution camera to monitor the condensation dynamics. The results show that the area-weighted average droplet radius was increased by 61 % and 84 % for 1 kHz and 10 kHz, respectively, compared to the control case for 1 m/s airflow. At 10 kHz, the shedding time was reduced, particularly at higher airflow velocities. Additionally, the heat transfer coefficient (HTC) exhibited a significant increase at 10 kHz when shedding was present. Notably, the difference in HTC became pronounced at an airflow velocity of 15 m/s, where shedding was prominent. In this scenario, the HTC rose by an impressive 35 % compared to the control case. These findings provide a promising foundation for utilizing EW to enhance heat transfer and improve condensate shedding under shear flow conditions.

Deconstructing plasma fog collection technology: an experimental study on factors impacting collection efficiency

Water scarcity is a global challenge that hinders human development. In recent years, electrostatic fog collection technology has emerged as a promising technology to alleviate this issue. Although electrostatic fog collectors based on a variety of electrode structures have been developed previously, there has been less research into other factors affecting the efficiency of electrostatic fog collection (e.g. electrical factors, environmental factors, etc), which has delayed the commercial application of the technology. In this paper, we experimentally investigate the effects of power supply polarity, voltage, airflow direction, airflow velocity, fog concentration and temperature on collection efficiency using a typical wire-mesh electrode fog collector as an example. The results show that both electrical and environmental factors influence the collection efficiency by changing the charge and the electric field force of the droplets. Negative polarity corona and high voltage are more favorable for fog collection. High velocity airflow and high fog concentration increase the amount of water collected due to the ability to bring more droplets into the electric field. However, the collection efficiency is reduced by the weakening of the corona discharge. High temperature accelerates the evaporation of fog droplets, which is not favorable for fog collection. In conclusion, this work will not only contribute to revealing the underlying mechanisms of the electrostatic fog collection but also will guide the development of highly efficient fog collectors.

Improvement of fuel mixing of single ejected 2-lobe fuel injector using shock generator at supersonic flow

In present paper, computational fluid dynamic is applied to evaluate hydrogen mixing impacts of the single extruded lobe injector at supersonic cross flow. The aim of this investigation is to introduce efficient injection system by elongation of the extruded nozzle at high-velocity air flow. The shock waves and temperature contour for the two nozzle heights are compared. The importance of vortex produced inside the domain on the fuel diffusion is also explained. The usage of annular and coaxial air and fuel jet for the improvement of the proposed configurations are also investigated. Obtained results shows that elongation of extruded nozzle form 2 mm to 4 mm improves the fuel mixing about 50%. However, the injection of the inner air jet results in 85% higher fuel mixing in combustion chamber of the scramjet engine.

Optimization of Vibrating Mesh Nebulizer Air Inlet Structure for Pulmonary Drug Delivery

The vibrating mesh nebulizer (VMN) has gained popularity for its compactness and noiselessness. This study investigates the impact of different air inlet structures on the deposition fraction (DF) of droplets generated by VMNs in an idealized mouth–throat (MT) airway model. Three homemade VMNs with semi-circular inlet, symmetrical four-inlet, and multiple-orifice inlet structures were evaluated through simulations and experiments. The changes in droplet DF of 0.9% w/v concentration of nebulized sodium chloride (NaCl) droplets as a function of inertial parameters were acquired under different inhalation flow conditions. Additionally, flow field distributions in models with different inlet structures were analyzed at a steady inspiratory flow rate of 15 L/min. The results indicate that optimizing the VMN’s air inlet structure significantly enhances droplet delivery efficiency. The multiple–orifice inlet structure outperformed the other designs, directing the airflow from the inlet position to the center of the mouthpiece and then into the oral cavity, achieving a DF of up to 20% at an inhalation flow rate of 15 L/min. The region of high airflow velocity between the mouthpiece and oral cavity proved to be a favorable VMN inlet optimization, reducing direct droplet–wall collisions and improving delivery efficiency. These findings offer insights for VMN design and optimization to enhance pulmonary drug delivery effectiveness and therapeutic outcomes.

Research on airflow convergence patterns and air distribution characteristics under multi-column attachment ventilation

Multi-column attachment ventilation (M-CAV) is usually delivered to the occupied zone by floor-attached airflows in large spaces. However, after the floor-attached airflow converges, the original air distribution will change, which will have an important impact on the traditional air distribution design. In this paper, airflow convergence patterns and air distribution characteristics in the most common double-column and tri-column layouts are studied by experiments and computational fluid dynamics (CFD) simulations. A 1:3 scaled-down model was established for velocity experiments and flow field visualization, and the results were used to verify and optimize the CFD model. The results indicated a seagull-like convergence flow pattern under the symmetric double-column layout. Under the axisymmetric layout with different column spacings, the floor-attached airflow in the half-open air outlet mode exhibited full convergence with the merging point (point MP) and combined point (point CP), partial convergence with point MP, and non-convergence without point MP and point CP. Under the centrosymmetric layout, an M-shaped velocity distribution was formed, and no airflow converged. For equal-velocity conditions, the dimensionless convergence axial velocity distributions for different air supply velocities was approximately consistent. In the fully-open air outlet mode, the floor-attached airflow could be divided into the collision-dominated region, the combined collision and convergence-dominated region, and the convergence-dominated region based on the forms of airflow interaction. For unequal-velocity conditions, the high-velocity airflow restricted the diffusion of the low-velocity airflow.