Air Jet Research Articles

Decoupling exhaust and return vents of supply momentum-driven stratified thermal environment with reinforced supply air jet entrainment from occupied zone

The supply momentum-driven stratified thermal environment created by advanced air distribution is a promising solution for sustainable and livable indoor environments. This study proposes a strategy to decouple the exhaust and return air for the supply momentum-driven stratified thermal environment. Experimentally validated Computational Fluid Dynamics simulations are used to verify the effectiveness of the strategy regarding energy efficiency improvement in a stratum-ventilated office. The proposed decoupled strategy positions the return vent below the exhaust vent and above the supply vent. The proposed decoupled strategy has two mechanisms for energy efficiency improvement, i.e., 1) reducing the cooling load of return air by lowering return air temperature relative to exhaust air temperature, and 2) advancing the airflow pattern with improved cooling efficiency of the occupied zone by reinforcing and suppressing the entrainments of the supply air jet from the occupied zone and the upper zone, respectively. The advanced airflow pattern reduces the cooling loads of both return air and fresh outdoor air and increases the Coefficient of Performance. Decreasing the height of the return vent strengthens Mechanism 1, but nonmonotonically affects Mechanism 2. By properly determining the height of the return vent, results show that the proposed decoupled strategy saves energy by 13.5% and 11.9% in thermally neutral and slightly warm environments, respectively. This study contributes to advancing air distribution for energy saving with thermal comfort.

Effect of Flow Swirling on the Subsonic Air Jet in the VGU-4 HF Plasmatron

Effect of Flow Swirling on the Subsonic Air Jet in the VGU-4 HF Plasmatron

Study of the effect of forced precipitation of working fluid on a vineyard sprayer for weed control

BACKGROUND: A distinctive feature of the sprayer presented is the possibility of more efficient penetration of the working solution into the middle and lower tiers of weeds, as well as on the lower (abaxial) side of the leaf. This is ensured by the use of an air-liquid jet, which significantly increases the effectiveness of the applied chemical method of weed control. AIM: Analysis of the effect of air jets on the deposition of chemicals on the surface of leaves of weeds. The penetration of the drug into the middle and lower tiers of plants, as well as deposition on the adaxial and abaxial sides of the leaves, is taken into account. METHODS: To combat unwanted vegetation, a special device was applied, which was used in field research both with and without air support. With the help of the developed method, spraying was carried out on indicator cards fixed on the upper, middle and lower tiers of the plant RESULTS: For the first time, an innovative technology based on the use of a flexible air distribution sleeve as part of a herbicide sprayer was proposed to combat weeds in vineyards. This sleeve ensures the precise supply of airflow from the fan to the working fluid spraying area. The result of such an air-liquid flow is improved efficiency and monodispersity of herbicide droplets, which, in turn, ensures uniform coverage of the lower tiers and the abaxial surface of the leaves of weeds. This innovative solution opens up new prospects in the fight against unwanted vegetation in the vineyards. CONCLUSION: The practical value of the proposed weed control technology lies is increasing the quality of the technological operation due to the forced precipitation of the working solution.

Hybrid heating in the fused filament fabrication process

AbstractAltering the heating regime of the polymer during the fused filament fabrication (FFF) process can lead to changes in both the behaviour of the polymer and the characteristics of the printed product. This study proposes replacing the traditional resistive heating system with two hybrid systems that introduce an additional temperature of 120–160 °C: one combining resistive and hot air jet heating, and the other combining resistive and infrared radiation heating. The samples printed using these hybrid systems were analysed using differential scanning calorimetry (DSC) and visually inspected. Commercial ABS and PLA filaments were used in the experimental programme. A model to evaluate the polymer’s melting during the printing process was proposed and experimentally validated. Visual testing revealed that the printed lattice structure had smaller voids, characterised by depositions that were flattened rather than circular in cross-section due to the extended time in a viscous/partially molten state. The elongation viscosity and storage modulus decreased by approximately 10%, with a slightly smaller decrease observed for the infrared radiation heat source. The glass transition temperature remained unchanged, and the molecular mobility was not affected by the additional heat. Similarly, the energy required for crystal formation was unaffected by the supplementary heat. The mechanical behaviour of the printed pieces during compression tests was also influenced by the addition of a second heat source. For both materials, a decrease in deformability was observed as the temperature of the hot air jet increased.

Development of a Novel Tailless X-Type Flapping-Wing Micro Air Vehicle with Independent Electric Drive

A novel tailless X-type flapping-wing micro air vehicle with two pairs of independent drive wings is designed and fabricated in this paper. Due to the complexity and unsteady of the flapping wing mechanism, the geometric and kinematic parameters of flapping wings significantly influence the aerodynamic characteristics of the bio-inspired flying robot. The wings of the vehicle are vector-controlled independently on both sides, enhancing the maneuverability and robustness of the system. Unique flight control strategy enables the aircraft to have multiple flight modes such as fast forward flight, sharp turn and hovering. The aerodynamics of the prototype is analyzed via the lattice Boltzmann method of computational fluid dynamics. The chordwise flexible deformation of the wing is implemented via designing a segmented rigid model. The clap-and-peel mechanism to improve the aerodynamic lift is revealed, and two air jets in one cycle are shown. Moreover, the dynamics experiment for the novel vehicle is implemented to investigate the kinematic parameters that affect the generation of thrust and maneuver moment via a 6-axis load cell. Optimized parameters of the flapping wing motion and structure are obtained to improve flight dynamics. Finally, the prototype realizes controllable take-off and flight from the ground.

Experimental study on stable combustion of pulverized coal stream in a confined small space with air jet

Experimental study on stable combustion of pulverized coal stream in a confined small space with air jet

Enhancing Bulb Turbine Performance Assessing Air Injection for Vibration Mitigation and Hydrofoil Cavitation

Small hydro technology is playing a crucial role in advancing sustainable, clean energy policies as part of the global hydro development strategy. Its contribution to social and economic development is becoming more prominent, particularly in ensuring electricity access for rural communities and supporting industrial expansion. The main causes of bulb turbine failures under high operating conditions are frequently attributed to variations in pressure in the draft tubes, which are aggravated when a spinning vortex rope is formed under load operation. Different fluid injection techniques, namely compressed air and water jet injection, address these issues and reduce the negative results of cavitation. The investigation covers the flow visualization on the suction side of a single hydrofoil utilizing a cavitation tunnel and a bulb turbine. This study assesses the effectiveness of compressed air injection in reducing vibration generated by cavitation in bulb turbines. Positive results of the experimental studies suggest a decrease in noise and vibration by air injection that prevents oscillations of the vortex rope. This research also considers how the hydrofoil design of bulb runner blades influences flow characteristics. Hence, it provides knowledge on cavitation structures in diverse cavitation numbers. Different studies that compare the original and modified hydrofoil designs reveal remarkable improvements that may be due to changes in the key parameters of the hydrofoil, such as later cavitation initiation and reduced intensity. To obtain the optimal output of a bulb turbine by considering air injection for vibration reduction and hydrofoil design changes to limit the negative effect of cavitation, the Reynolds number and cavitation number are to be defined. This multidisciplinary approach possesses enormous potential to increase the reliability and efficiency of bulb turbines in challenging operating conditions.

Air-jet impact craters on granular surfaces: a universal scaling

Craters form as the lander's exhaust interacts with the planetary surfaces. Understanding this phenomenon is imperative to ensuring safe landings. We investigate the crater morphology, where a turbulent air jet impinges on granular surfaces. To reveal the fundamental aspect of this phenomenon, systematic experiments are performed with various air-jet velocities, nozzle positions and grain properties. The resultant crater morphology is characterized by an aspect ratio. We find a universal scaling law in which the aspect ratio is scaled by a dimensionless variable consisting of the air velocity at the nozzle, the speed of sound in air, the nozzle diameter, the nozzle-tip distance from the surface, the grain diameter, the density of the grains and the density of air. The obtained scaling reveals the cross-over of the length scales governing the crater aspect ratio, providing a useful guideline for ensuring safe landings. Moreover, we report a novel drop-shaped sub-surface cratering phenomenon.

Exploring Neuromuscular changes in the larynx: Insights from rat ultrasonic vocalizations

The study of rat ultrasonic vocalizations (USVs) provides a unique window into understanding the neuromuscular changes in the human larynx that occur with aging. Rats communicate with each other using USVs to convey positive and negative affective states in a variety of communication situations, such as juvenile play, mating, and alerting one another to threats (Brudzynski, 2013). USVs are produced using a glottal constriction to generate a jet of air which creates a whistle (Hakansson et al., 2022). The frequency of this whistle is modulated using the same muscles that humans use to modulate our vocalization frequency (the cricothyroid and thyroarytenoid) (Riede, 2011). The vocal fold cover is not engaged in vibration during USVs production. However, the neuromuscular similarities between rat USVs and human vocalizations make USVs a good model to investigate laryngeal neuromuscular mechanisms, such as the laryngeal muscle fibers and neuromuscular junctions (NMJs; the connections between peripheral nerves and muscle fibers).

The influence of unconventional assembly techniques on the comfort indicators of waterproof materials

The present paper aims to highlight the correlations between the comfort indicators of waterproof cotton-based fabrics, and the specific parameters of unconventional assembly technology. The materials considered include layers of Gore-Tex film, serving as substitutes for waterproof materials, intended to manufacture outerwear products such as protective equipment (jackets and overalls), which are breathable waterproof membranes. The comfort indicator values were obtained through standardised methods, and data processing and representation in two-dimensional (2D) and three-dimensional (3D) systems using correlation/graphing methods were carried out in Excel software, and using Jandel Table Curve 3D v2. Although experimental research can be extended to include specific functions related to durability indicators, this study focused theoretically and experimentally on how the variation in temperature of the low-pressure hot air jet can lead to changes in the vaporization coefficient, air permeability index, and thermal conductivity coefficient. The relationship between the comfort indicator values (thermal conductivity coefficient, air permeability index, and vapour permeability coefficient) of the considered materials and the treatment parameters of the installation (pressure, temperature, velocity) is expressed through statistically adequate mathematical models, highlighted by a correlation coefficient R = 0.983, with significance certified by the Fischer test. The study of comfort indicators was conducted under optimized treatment parameters, ensuring durability and presentation value functions in the treatment zone remain unchanged compared to those of the base materials.

Investigation of unswept ramp system with lobe shape nozzle for fuel mixing of hydrogen jet at a scramjet engine

The role of efficient fuel mixing and a stable flame holder is crucial in enhancing the performance and capabilities of scramjet engines for high-speed flight. The present research paper has tried to disclose the fuel mixing efficiency of 3-lobe annular nozzle on the mixing mechanism of the fuel jet behind the strut. In addition, using internal air jet flow for increasing the circulation strength and fuel mixing behind the strut is also examined in this study. Numerical simulation of the flow and fuel jet behind the strut is done to reveal the main physics related to the mechanism of fuel mixing inside the combustor with the proposed injection system. The results of our simulation show that using annular 3-lobe fuel jet improve the fuel mixing via production of the multiple vortex pairs within the combustor behind the strut. The use of internal air jet also enhances the fuel mixing efficiency up to 90% in combustor of scramjet engine.

Directional pollutant extraction—the mathematics of the two-dimensional Aaberg exhaust hood

By deflecting unpolluted air away from the exhaust inlet by means of directional air jets, the Aaberg exhaust hood can focus the extraction flow it produces directly onto a source of contaminant and, thereby, achieve directional extraction of pollutant at high concentration. While significantly outperforming conventional hoods, such ‘jet-reinforced’ flows are notoriously challenging to control in practice and if operating incorrectly disperse, rather than extract, airborne pollutants. To provide a rapid predictive capability and insight into the role of device geometry, a mathematical model for the airflow created by the Aaberg exhaust hood is developed. Modelling the induced flow as potential flow, conformal mapping techniques are used to develop analytical solutions for the stream function and airspeed, from which we identify the region of pollutant capture. Investigating these flows as the directionality of the air jets and the ratio of jet-to-suction strengths are varied, our analytical solutions offer insights into the effective deployment of jet-reinforced exhaust hoods to improve indoor air quality in industrial workplaces.

Experimental and numerical investigations of forced convection impingement heat transfer on plate-fin heat sinks using three different electronic cooling fans

An experimental and numerical study was conducted for plate-fin heat sinks subjected to impinging air jets produced by electronic cooling fans. One axial and two different radial fans were used and their distance to the heat sink was varied. The aluminum heat sinks had either plain fins or fins with sinusoidal surface structures, were heated by heat sources either distributed over the entire base plate or concentrated in the center. In total, temperature measurements for 264 settings were performed, and numerical simulations were carried out for 19 selected cases. The results show that the axial fan outperforms the radial fans for the investigated fan distances, that the structured fins lead to lower base plate temperatures and that significant asymmetries in the heat sink temperature distribution can occur in certain cases. The numerical simulations are used to gain a better understanding of the trends observed in the measurements. Particularly, the impact of the velocity profiles at the fan outlets and the influence of certain geometrical details of the fan design on the heat flux distributions and the airflow into the fin channels are explored. The results highlight practical problems that need to be considered when optimizing heat sink-fan combinations.

A new track for high-efficiency marine diesel engines: Initiative air jet and post-split injection combined

In pursuit of adequate mixing with a high volume of fuel injection, a re-intake scheme is proposed to promote the diffusion of combustibles and flames by using the disturbance of high-pressure air jets after the pressure peak. The results indicate that during the power stroke, an additional air jet enhances combustion by penetrating the spray and disturbing the flame front. The high-pressure air jet's obstruction of the atomization of local sprays achieves a longer combustion duration, while intensifying the vortex and temperature at the cylinder center, which enhances the evaporation of fuel near the nozzle. However, excessively high fuel injection pressure brings in more cold air, causing a temperature drop. It is necessary to have sufficient flame expansion before re-intake to eliminate the impact on pressure. By adjusting the re-intake pressure and timing, the air jet, when it reaches half the cylinder diameter, can exert its maximum advantage. By introducing post-injection and increasing the pressure of the first fuel injection, the air-spray impingement duration is extended, thereby improving thermal efficiency. This work, at 190 MPa injection pressure without changing the duration, and combined with a 5°CA re-intake, achieved a 1.82 % increase in thermal efficiency.

Computational study of the coaxial air and fuel jet through a 3-lobe strut injector for efficient fuel mixing in a supersonic combustion chamber

In this article, comprehensive simulations of the compressible air stream inside the combustion chamber are done to disclose the mixing and circulation performance of the 3-lobe injector behind the strut. The computational approach is employed to visualize the jet flow with/without the inner air jet released at supersonic flow. The main focus is the importance of lobe shape nozzle on the fuel diffusion into the main stream behind the strut. The role of the shock waves and their contact with the fuel jet flow is also investigated in the present work. Free stream velocity inside the combustion chamber is Mach = 4 while the hydrogen fuel jet is injected via sonic speed. A comparison of single and 3-lobe nozzles indicates that the lobe configuration of the nozzle increases the circulation behind the nozzle. Besides, the usage of the internal air jet increases the contact of the fuel jet with the shear layer initiated from the edge of the strut.

The Effect of the Impinging Angle of Multi-Rectangular Air Jets on the Performance Improvement of Passive Solar Photovoltaic-thermal Systems

The Effect of the Impinging Angle of Multi-Rectangular Air Jets on the Performance Improvement of Passive Solar Photovoltaic-thermal Systems

Spreading of rotating liquid annular ring with hysteresis effects

The spreading of a liquid, subjected to centrifugal forces or an air jet, leads to a lowering of the liquid height at the center. If the system is partially wetting, experiments show a break up of the liquid and the appearance of a dry patch at the center. In this article, the spreading of a liquid droplet or a liquid film, featuring a dry patch at the center, is investigated. Via a generalized Tanner’s law, we allow for a contact-angle hysteresis in partially wetting systems. By means of the lubrication approximation, an analytical quasi-steady solution can be derived in the limit of small capillary numbers. We find a power-law regime for the spreading, in which at least one of the contact lines follows a power law in time, almost independent of both static contact angles. The influence of the static contact angles remains restricted to the beginning of the spreading and to transition periods. Four different types of spreading can be identified, namely spreading (i) with a very thin central liquid layer, (ii) as annular ring with central dry patch, (iii) into a static equilibrium, and (iv) with closing of the central dry patch. In a flow map, these types of spreading are mapped as function of both static contact angles. Moreover, the dependency from gravitational and centrifugal forces is investigated and included in the flow map. For a perfectly wetting system, a disjoining-pressure correction in combination with Tanner’s law prohibits negative liquid heights at the center. Hereby, the magnitudes of the Hamaker constants have a negligible influence and arbitrary-small values can be used, since the dynamics of the contact line remains controlled by Tanner’s law.

Experimental investigation of edge preparation for cemented carbide profile cutting tools using flexible abrasive jet polishing

This paper investigates the impact of a novel Flexible Abrasive Jet Polishing (FAJP) method on the edge preparation of profile cutting tools based on Hertz contact theory. FAJP employs flexible rubber particles encapsulating diamond micro-powders as abrasives for air jet polishing. The results indicate a significant improvement in material removal rate with FAJP compared to traditional drag finishing methods, along with superior surface quality near the tool edge. The duration of abrasive usage has the greatest impact on FAJP, depending on the specific requirements of the tool edge, with options ranging from 400 to 200 h. There exists a correlation between jet pressure and jet time, with a recommended jet pressure of 225 kPa, and different coating effects achievable by adjusting the jet time. A tool speed of 280 rpm is recommended.

Flow and heat transfer behavior of acoustically excited pulsating air jet impinging on a flat surface

Experiments are carried out to understand the flow and thermal behavior of a pulsating jet. The pulsating jet is generated using acoustic excitation. The study considered variations in Reynolds number (Re = 2800, 4900, and 6800), Strouhal number (St = 0–0.84), pulsation amplitude (A = 0–60 %), and nozzle-to-surface distance (z/d = 1–8). Findings revealed that the potential core length of the pulsating jet is shorter compared to the steady jet. The potential core length initially decreases with an increase in St up to 0.42, then begins to increase. Pulsating jets improve thermal performance in the wall jet region due to greater entrainment and mixing from the surrounding fluid. Results demonstrated that pulsating jets could increase average heat transfer rate by up to 58 % at Re = 2800 compared to the steady jet. Although heat transfer rates are higher in pulsating jets, changes in pulsation frequency or amplitude had a minimal effect. The enhancement in average heat transfer rate diminishes as the Reynolds number increases for the same Strouhal number. Each tested Reynolds number showed at least a 10 % improvement in heat transfer in pulsating jet over steady jet. The improved thermal performance of the acoustically pulsating chamber offers the potential for enhanced thermal management in various applications.

Co-flow jets application for occupant targeted ventilation: Focus on thermal comfort and air quality

Co-flow air jets can be used in occupant-targeted ventilation, such as personalized ventilation, stratum ventilation, aiming at improving air quality in the breathing zone of occupants. However, these have not been studied sufficiently. The velocity field and cleanness of co-flow jets comprising inner jet of clean air encircled by outer jet of room air were studied under isothermal conditions by physical measurements and CFD simulations at numerous combinations of initial diameter and velocity of the inner and outer jets appropriate for practical application. The co-flow jet had a lower axial velocity decay along the jet direction than that of the single jet. The magnitude (turbulence intensity) and frequency of the velocity fluctuations in co-flow jets were lower compared to those in a single jet. The results revealed that with regard to thermal comfort co-flow jets may provide more cooling compared to single jets, especially when supplied toward to the user. The level of cleanness depended on the initial velocity of the inner and outer jets as well as their diameters. The air on the jet axis of the co-flow was up to 40 % cleaner compared to the single jet. The increased area ratio of outer nozzle to inner nozzle from 1.0 to 3.0 improved the cleanness of the air by at least 10 % to the distance from 8 to 14 initial diameters compared to a single jet. The co-flow jet with velocity ratio of 1.5 had longest concentration core length, Lu, (up to 6 initial diameters to the inner jet) with clean air. The results can be used for design of occupant targeted ventilation.