High-velocity Air Flow Research Articles

Comparison of Local and Averaged Air-Side Heat Transfer Coefficients on Fin-and-Tube Heat Exchangers Obtained With Experimental and Numerical Methods

Abstract Numerical methods are often used to obtain two-dimensional air-side heat transfer coefficients (HTCs) on heat exchanger (HX) fin surfaces. The model's accuracy is usually verified through averaged HTCs by comparing with published experimental results. However, substantial disagreement is not uncommon and can hardly be explained by averaged HTCs. This study focuses on comparing experimental, local air-side HTCs to numerical ansys fluent results. A mass transfer experimental method was employed to obtain HTC distributions on the fin-and-tube HX fin surfaces. Therefore, disagreements between the experimental and numerical results can be explained in detail. There are several significant findings: inaccurate predictions of local HTCs are observed even though the averaged HTCs from the numerical method may agree with the averaged experimental results under some conditions. The models fail to capture horseshoe vortices, underestimate the HTCs in the wake region of the tubes, and overpredict row-by-row HTC degradation. Moreover, the accuracy of the numerical model decreases when the complexity of geometry increases. For the flat plate, numerically obtained HTCs agree with the experimental results within 10%. However, the error is more than 30% for the eight-row HX. Nonetheless, the model's accuracy becomes worse at higher airflow velocities. Oversized fin-and-tube HXs with multiple tube rows are often selected as a result of using the underpredicted averaged air-side HTCs from the numerical computational fluid dynamics (CFD) simulations. Thence, the authors have proposed a corrective method to improve the accuracy of the numerical model.

Thermal and Computational Fluid Dynamics (CFD) Analysis of a Modified Two Stroke Spark Ignition Engine Block

An engine block is the main supporting structure for other components of the engine. Poor heat dissipation of the engine block causes excessive thermal expansion and frictional wearing of the engine components and makes engine oil to lose its lubricating power. Therefore, there is a need to modify the design of the engine block in order to improve its effectiveness. In this analysis, a two stroke spark ignition engine block was initially used as a reference model for the development of the modified model. Solidworks (2013) simulation software was used to model and conduct a thermal and computational fluid dynamics (CFD) analysis on the two models. The thermal analysis results shows that, the maximum temperature gradient and heat flux obtained during the steady state and transient thermal analysis were, 2.237×107 K/m, 3.6817×104 K/m, and 1.066×107 W/m2, 1.661×106 W/m2 for the reference model, while 1.771×106 K/m, 4.913×104 K/m, and 7.970×107 W/m2, 2.211×106 W/m2 for the modified model respectively. The results shows that the modified model has an improved heat dissipation rate than the reference model. The CFD analysis shows that, the reference model is subjected to high pressure and air resistance than the modified model. This increases the drag force acting on the reference model. While the modified model has high air flow velocity round the engine block, than the reference model. This is due to fins modification of the modified model. This makes the engine block model to have low air restriction. It is hereby recommended that further validation should be carried out to ascertain the effectiveness of the modified model. The performance of the modified model can also be improved by converting its fins into triangular fins geometry.

Coughed droplet dispersion pattern in hospital ward under stratum ventilation

This study aims to investigate the characteristics of coughed droplet dispersion in hospital wards under stratum ventilation (SV). The dynamic dispersion process of the coughed droplets and the spatial distribution of the droplets in a two-bed ward under SV are investigated, considering the initial size and evaporation of the droplets. For SV, the supply air grilles are on the wall opposite the patients with the centre height at 1.5 m above the floor, the exhaust diffusers are on the lower part of the wall near the patients, and the air change rate is 12 ACH. For comparison, displacement ventilation (DV) and mixing ventilation (MV) are also discussed. The RANS-based RNG k-ε model is applied to solve the flow field. The Lagrangian approach is applied to solve the droplet motion. The exposure risk in the ward is less under SV because (a) The horizontal airflow leads to a strong deposition at the initial dispersion stage that decreases droplet concentration. With the infector in the supine and sitting positions, 68% and 49% of the coughed droplets are deposited on indoor surfaces in 10 s, respectively. (b) The air jet directly supplied to the breathing zone dilutes local droplet concentration. Compared with DV and MV, SV has better control over the 50 μm diameter droplet because the higher airflow velocity under SV could increase the deposition of the 50 μm diameter droplet. Controlling the droplet dispersion in the initial stage is more efficient to improve the infection control performance of SV.

Effect of high-rise buildings on the surrounding thermal environment

High-rise buildings degrade the thermal environment of their surroundings through wind turbulence, cooling, and excessive heating. This study identifies the role and relationship of geometric (form, orientation, dimension) and material (type, volume) parameters in conditioning the thermal environment around high-rise buildings. We performed the field measurements at four widely separated high-rise apartment buildings at different elevations (675–1050 m above sea level) in Bandung, Indonesia. Temperature, air velocity, and humidity were measured within a 150 m radius around each building three times daily (08–10 a.m., 12–04 p.m., and 06–08 p.m.) for three days in February. Airflow and shading patterns were analysed with Ansys-Fluent CFD and SketchUp software, respectively. Both direct observation and simulations showed that different orientations of buildings relative to the sun and wind can heat or cool the surrounding areas. The sail effect of a high-rise building exposed to direct sunlight can cause hot spots behind the building; choice of building material and use of shadowing can reduce this effect. The wind-tunnel effect can cause cold paths, especially if the area is shaded. Air turbulence and high-velocity airflow can result in uncomfortable conditions in the surroundings, based on the building height. Thus, high-rise buildings profoundly affect local thermal conditions. The results of this study are therefore essential for designing new buildings (and mitigating environmental quality around existing ones) in tropical cities.

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

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

Air curtain dust-collecting technology: Influence factors for air curtain performance

Air curtains are extensively used to isolate and control dust, heat, and smoke. Because of the high airflow velocity, columnar air curtains have several shortcomings, such as the nonuniform distribution of air, deviation from the axis, and the lack of a basis for determining the volume of the air curtain unit. Existing research cannot systematically explain the factors that directly influence these shortcomings. Therefore, this study employed a single factor design for investigating the effects of the spacing between guide vanes in the air curtain unit, cross sectionof the air curtain unit, and angle of the guide vanes on the air curtain uniformity, deviation, and volume of the air curtain unit, as well as the jet attenuation. The results show that the guide vanes can be used for shunting and diverting the airflow mechanically, the guide vanes with smaller spacing exhibit considerable mechanical diversion, which can lead the air flow out of the supply air slot in a timely manner, as well as reduce the downstream air flow and the overall static pressure difference of the air curtain unit to maintain a uniform air velocity. The cross section of the air curtain unit influences the uniformity of the air curtain. A wedge-shaped air curtain unit can ensure that the velocity of the internal airflow remains constant when the flow of air in the duct decreases gradually. This causes a drastic decrease in the pressure gradient in the air curtain unit, and an improvement in the overall uniformity of the air curtain unit. The deflection of the air curtain is affected directed by the angle of the guide vane, and the deflection is determined by the combined velocity direction of the mechanical deflection due to the guide vane and the non-mechanical air flow driven by the static pressure. The angle between the inside and outside of the symmetrically installed guide vane is 136°, and a jet of air curtain with a small inclination (≤8°) can be formed. The attenuation law of the air curtain jet may be approximately represented by a new correlation, and helps in the selection of air curtain velocity and range in different industrial applications.

Effects of Plate Edge Thickness on Droplet Generation Caused by Water Film Breakup at the Plate Edge

Abstract Droplets generated at trailing edges of low-pressure steam turbines strike the leading edge of moving blades, resulting in severe damage by erosion. Although the trailing edges of stationary blades employed in the steam turbine industry are often sharpened to produce fine droplets, this process requires further investigation. In this study, water film flows on a plate and breakup patterns are observed and measured to investigate the breakup behavior and the effect of the plate edge thickness. First, to determine the general characteristics of droplet formation of water film flows, the droplet diameter frequency distributions, which significantly affect erosion damage, are measured for several flow conditions. Profiles of frequency distribution of the droplet diameters exhibit on approximately linear in a semilog graph. Water lumps formed on the plate edge are expressed as a spanwise wavelength and initiate the primary breakup of the water film, while fine droplets whose diameters are similar to the critical diameter are generated and are assumed to contribute significantly to erosion damage. The effect of edge thickness on droplet diameter frequency distribution and the spanwise wavelength are observed. The plate edge thickness does not affect the frequency distributions of the droplet diameter. However, it does affect the spanwise wavelength, which is related to the intermittent discharge of water from the edge. The spanwise wavelength and discharged water period become longer when the plate edge thickness increases. Based on an experiment involving the highest airflow velocity, the discharged water frequency is similar to that generated by a general turbine rotation speed.

Тепловое состояние аэродинамического профиля летательного аппарата, обтекаемого высокоскоростным потоком воздуха

One of the features of high-velocity atmospheric aircraft is the presence of thin aerofoils with edges characterised by a small blunt radius, subjected to high-temperature aerodynamic heating at temperatures of up to 2000 -- 2500 °C. In order to ensure correct operation of both the power plant producing thrust in such vehicles, assumed to be a supersonic combustion ramjet, and respective aerodynamic controls, the components subjected to high-velocity air flows must retain their geometric stability. A way to ensure their performance is to use methods and means of thermal protection, as well as materials that are resistant to high temperatures in an oxidising atmosphere, while one of the promising trends is employing refractory oxide materials such as oxides of aluminium, zirconium and hafnium. Since this class of materials has low thermal conductivity, large temperature gradients develop in the vicinity of the surface being heated, resulting in temperature stresses, all of which designers should take into account. We analysed the temperature state in a model of an acute zirconium oxide wedge featuring a small blunt radius, subjected to a high-velocity air flow. To reduce the edge temperature and temperature gradients, we propose a design solution implemented as a thermally conductive core lined with a thin layer of zirconium oxide. We consider using aluminium oxide and hafnium boride as core materials

Droplet dynamic characteristics on PEM fuel cell cathode gas diffusion layer with gradient pore size distribution

Understanding of droplet dynamic characteristics on gas diffusion layer (GDL) of polymer electrolyte membrane (PEM) fuel cell is of great importance for cell performance improvement. This study comprehensively investigated droplet dynamic characteristics on fuel cell cathode GDL with gradient pore size distribution (GPSD). The influence of different pore sizes, GPSD and wettabilities of GDL on droplet dynamic characteristics is numerically evaluated by using the volume of fluid (VOF) method through the analysis of the interaction among the forces over droplet. Results indicate that droplet has a relatively short detachment time and a smaller detachment radius on superhydrophobic and superhydrophilic GDLs compared with on a moderately hydrophobic GDL regardless of pore size. Moreover, large pores can facilitate droplet detachment, but increase droplet detachment radius and pressure drop. Compared with uniform (U)PSD, for transverse (T)-GPSDs with a small pore distance at a relatively low airflow velocity or a large pore distance at a high airflow velocity, or for the longitudinal (L)-GPSD with a large pore in the upstream and a small pore in the downstream at a large pore distance, the droplet detachment time decreases by 34.3% and 25.2% with a reduction of pressure drop, respectively.

Stretchable and Compressible Si3 N4 Nanofiber Sponge with Aligned Microstructure for Highly Efficient Particulate Matter Filtration under High-Velocity Airflow.

Particulate matter (PM) is one of the most severe air pollutants and poses a threat to human health. Air filters with high filtration efficiency applied to the source of PM are an effective way to reduce pollution. However, many of the present filtration materials usually fail because of their high pressure drop under high-velocity airflow and poor thermal stability at high temperatures. Herein, a highly porous Si3 N4 nanofiber sponge (Si3 N4 NFS) assembled by aligned and well-interconnected Si3 N4 nanofibers is designed and fabricated via chemical vapor deposition (CVD). The resulting ultralight Si3 N4 NFS (2.69mg cm-3 ) processes temperature-invariant reversible strechability (10% strain) and compressibility (50% strain), which enables its mechanical robustness under high-velocity airflow. The highly porous and aligned microstructure result in a Si3 N4 NFS with high filtration efficiency for PM2.5 (99.97%) and simultaneous low pressure drop (340Pa, only <0.33%of atmospheric pressure) even under a high gas flow velocity (8.72 m s-1 ) at a high temperature (1000°C). Furthermore, the Si3 N4 NFS air filter exhibits good long-term service ability and recyclability. Such Si3 N4 NFS with aligned microstructures for highly efficient gas filters provides new perspectives for the design and preparation of high-performance filtration materials.

Droplet dynamic behaviors on gas diffusion layer surface of various wettabilities in a PEMFC gas flow channel

ABSTRACT Droplet dynamic behaviors in a gas flow channel (GFC) of proton exchange membrane fuel cell (PEMFC) with gas diffusion layer (GDL) of various wettabilities are observed by using the volume of fluid (VOF) method in this work. Droplet deformation, detachment, breakup, and multiple droplets coalescence are investigated via an analysis of forces over the droplet. Results show that droplets tend to detach from a hydrophobic GDL surface at a high airflow velocity; while exhibiting significant deformation even breakup on a hydrophilic GDL surface. Besides, the interaction among the pressure force, shear force, wall adhesion force (arising from surface tension), and lift force over the droplet is examined to qualitatively elucidate the droplet dynamic behaviors. It is found that a hydrophobic GDL with a moderate airflow velocity can facilitate water removal from the GFC and maintain a relatively small spreading area; however, too high or too low an airflow velocity can cause a breakup or a slow drainage of the droplet, especially on a hydrophilic GDL surface. Thus, the design and operation of the PEMFC should consider both the wettability of GDL surface and the airflow velocity to achieve effective water management.

Mach number effect on droplet phase angular distribution behind a nozzle under near-wall liquid film outflow with co-current gas flow into vacuum

The paper is devoted to experimental study of near-wall ethanol film outflow with high-velocity co-current air flow from sonic and supersonic nozzles into vacuum. Using the developed measurement techniques (droplet phase flow structure visualization, deposition of droplets on paper substrates, spectrophotometry method), the effect of the nozzle Mach number on angular distribution of droplet phase in the central and the peripheral flow regions behind the nozzle exit is established.

Numerical Analysis of the Activated Combustion High-Velocity Air-Fuel Spraying Process: A Three-Dimensional Simulation with Improved Gas Mixing and Combustion Mode.

Owing to its low flame temperature and high airflow velocity, the activated combustion high-velocity air-fuel (AC-HVAF) spraying process has garnered considerable attention in recent years. Analyzing the velocity field, temperature field, and composition of AC-HVAF spray coatings plays a vital role in improving the quality of coatings. In this study, an actual spray gun is adopted as a prototype, and the radial air inlets are introduced to improve the reaction efficiency so that the chemical reaction can be completed in the combustion chamber. Furthermore, a complete three-dimensional (3D) model is established to examine the effects of radial inlets and porous ceramic sheet on the combustion and flow fields. The hexahedral cells are used to discretize the entire model for reducing the influence of false-diffusion on the calculation results. The gas flow field is simulated by the commercial Fluent software, and the results indicate that the porous ceramic sheet effectively reduces the turbulent dissipation of the airflow with a good rectification effect (the ceramic sheet ensures a consistent airflow direction). The radial inlets and the porous ceramic sheet promote the formation of vortex in the combustion chamber, increase the residence time and stroke of the gas in the combustion chamber, and improve the probability of chemical reactions. In addition, it is observed that the stability of velocity for the airflow is strongly related to the airflow density.

Computational Fluid Dynamics study of a corrugated hollow cone for enhanced dew yield

Dew condensation is the result of cooling by radiative deficit between a substrate and the atmosphere. Dew yield can be enhanced in hollow structures like hollow cones where the influence of wind is lowered. Corrugation increases the local tilt angle with horizontal and makes dew drop grow faster on edges, then drops detach sooner and act as natural wipers. However, corrugation increases the heat exchange with surrounding local air, which may reduce cooling.In order to evaluate these effects on cooling and dew yield, a corrugated, W-shaped hollow cone is compared to the same, smooth structure (S-cone) by Computational Fluid Dynamics. Two softwares were used: Ansys CFX for a pre-study concerning detailed aerodynamics where the computational domain is modeled to obtain a fully developed wind profile assuming an unobstructed inlet and COMSOL Multiphysics for aerodynamics coupled with heat flux, including radiative exchange surface-to-sky and surface-to-surface. Local temperatures can be obtained, which can be related to the dew yield. Turbulence is seen at all speeds but stagnation of the flow is also observed, which limits the convective heat exchanges and facilitates dew formation. At low wind speed, convective heat exchange is similar for both smooth and corrugated surfaces, and corrugation increases cooling. At higher air flow velocities, convective heat exchange is larger for the W-cone but cooling is only slightly smaller than found on the S-cone. Corrugated W-cone should thus give larger yield than the corresponding smooth S-cone.

CFD investigation on gas-solid two-phase flow of dust removal characteristics for cartridge filter: a case study.

The filter cartridge dust collector has been widely used in industry, but the influence of its internal structure on its operation effects is rarely studied. FLUENT software was used to simulate the influence of different air volume and permeability values on the gas-solid two-phase flow of dust removal characteristics for a filter cartridge. The results show that when the air volume of the fan was greater than 1600m3/h, the increase in the dust reduction rate was not obvious, and the high-velocity airflow filled the entire dust removal chamber, which was conducive to the filter using the largest effective filtration area to remove dust; the optimal air volume was 1600m3/h. Furthermore, the dust removal effect gradually became worse when the porosity was higher than 0.65, but the fluidity of the internal air was poor when it was lower than 0.65. The optimum porosity was 0.65. A simulated validation analysis was conducted using the above optimal parameters. As the proportion of particles below 2μm increased, the dust removal effect worsened.

Liquid droplet detachment and dispersion in metal foam flow field of polymer electrolyte membrane fuel cell

Porous metal foams have gained much attention as the reactant distributor in polymer electrolyte membrane (PEM) fuel cell due to their high porosity, good distribution capacity, and excellent electrical and thermal conductivity. Water management is a major concern hindering their wide adoption in PEM fuel cells. In this study, two different pore size metal foam flow fields are digitally reconstructed based on their morphological analysis. Then, the two-phase flow field microstructures are simulated using the volume of fluid method (VOF) to investigate liquid droplet removal and dispersion. Various droplet diameters and ligament surface wettability are investigated. It is found that a comparable sized liquid droplet to the foam pore requires a high critical air velocity for droplet removal. Liquid dispersion in the metal foam flow field improves liquid removal, especially for large droplets. Moreover, increasing the ligament surface hydrophobicity or reducing the foam pore size helps liquid removal at a high-velocity air flow. The proposed method can be used to optimize the contact angle and pore size in the metal foam design for water removal in porous media flow fields.

Experimental Investigation on a Novel Airfoil-Based Piezoelectric Energy Harvester for Aeroelastic Vibration.

This paper presents a novel airfoil-based piezoelectric energy harvester (EH) with two small square prisms attached to an airfoil. This harvester can achieve a two degree-of-freedom (DOF) plunge–pitch motions. Several prototypes of energy harvester were fabricated to explore the nonlinear aerodynamic response and the output performance in a wind tunnel. The experimental results showed that the longer the flexible spring was, the lower the critical velocity and frequency of the harvester were, and the better aerodynamic response and output performance could be achieved. The initial disturbance, the following limit-cycle oscillation, and the ultimate chaos of nonlinear response occurred, as increasing airflow velocity was increased. The overall output performance of the harvesters with a flexible spring having a thickness of 1 mm outperformed than that of the harvesters with a flexible spring having a thickness of 0.5 mm at a higher airflow velocity, while the tendency was opposite at a lower velocity. An optimum output voltage of 17.48 V and a power of 0.764 mW were harvested for EH-160-1 at 16.32 m/s, which demonstrated it possessed better performance than the other harvesters. When the capacitor was charged for 45 s and directly drove a sensor, it could maintain working for 17 s to display temperature and humidity in real time.

Study on the air curtain dust control technology with a dust purifying fan for fully mechanized mining face

An air curtain dust control technology for a fully mechanized mining face was developed, using a method that combined numerical simulations with field measurements in this paper. The results show that a triangular air curtain was formed between the shearer and the footway by the high velocity airflow field generated at the outlets of two fans after the technology was applied, which can effectively prevent dust from entering the driver's working area. Before the dust control technology was applied, the dust concentration of the shearer driver's working area exceeded 2000 mg/m3. However, after the dust control technology was applied, the dust concentration of the shearer driver's working area was reduced to 300 mg/m3. After verification through field measurements, the dust control rate of each measuring point, apart from the advancing support, exceeded 40%, and that of the shearer driver's working area was over 90%.

Polyacrylonitrile/polyimide composite sub-micro fibrous membranes for precise filtration of PM0.26 pollutants

Particulate matter (PM) pollution has enormously threatened ecosystem and public health. Among various air filtration medium, fibrous ones are very attracting and promising, with an array of advantages such as high specific surface area, and good internal connectivity. Even so, the large-scale fabrication of fibrous filtration materials still remains challenging. Here, three-dimensional polyacrylonitrile/polyimide (PAN/PI) composite sub-micro fibrous membranes were fabricated facilely via free surface electrospinning for precise filtration of PM0.26 pollutants, where the waste PI short fibers were utilized as raw material. The resultant composite fibrous membranes, featuring thin fiber diameter (~150 nm), low areal density (<0.8 g m−2), large porosity, and highly tortuous airflow channels with uniform poresize distribution, possessed excellent mechanical property with tensile strength of 4.95 MPa (twice that of pristine PAN), high thermal durability as well as remarkable filtration performance for ultrafine NaCl aerosol particles (≤0.26 µm) even after multiple filtration tests at high airflow velocity of 14.1 cm s−1. The deepened aperture channels inside three-dimensional sub-micro fibrous membranes are tortuous enough for capturing ultrafine PMs from the airstream mainly via diffusion, interception, and impaction mechanisms, and the reported large-scale fabrication of cost-effective homogeneous PAN/PI fibrous filter media is promising for industrial production and commercial applications.

Inhalability of micro-particles through the human nose breathing at high free-stream airflow velocities

In this study, the inhalability of microparticles through a human nasal passage under various ambient airflow conditions is calculated. As megacities are becoming polluted and being exposed to the polluted windy outdoor environments is inevitable nowadays, from the toxicological point of view, it would be wise to investigate the impact of air pollution on human health. Though, as few studies have attempted to address this concern, especially for high free-stream velocities comprehensively, the main emphasis was placed on assessing the impact of both inhalation and Total Deposition Fraction (TDF) of particles. Several free stream airflow velocities from 1 to 70 km/h are simulated to cover almost all stormy outdoor conditions that could typically happen. RANS models and stochastic models for turbulent particle dispersion were examined to simulate the particle motions properly in such turbulent conditions. The impact of high ambient airflow velocities on the aspiration efficiency and the TDF of micro-particles were assessed for a wide range of particle sizes. The results reveal that TDF of the integrated airway is mostly higher than that of the detached airway system due to the non-uniformity of both airflow field and particle distribution which happens at the nostrils, similar to the work of Naseri et al. (Anderson and Anthony, 2013) [24]. Yet, contrary to that study in which the flow field was laminar, and there were no notable differences between the TDF of these two cases at different breathing flow rates, the differences become more notable when the breathing regime becomes fully turbulent.