Air Jet Flow Research Articles

Scalar transport in the near field between two coaxial square air jets

The scalar transport in the near field of two square coaxial free air jet flow has been experimentally investigated. In order to study the entrainment and mixing processes taking place between the two jets, heat was used as a scalar. A triple sensor probe was used consisting of an X-wire probe for the point measurement of two velocity components and a single cold wire probe, for the simultaneous recording of instantaneous temperature. From the velocity and temperature signals the momentum and heat flux components were calculated. Eddy diffusivities of heat and momentum were directly evaluated from the experimental data. These diffusivities for all streamwise positions reach a maximum in the interaction region between the two jets; the maximum values increase with the downstream distance in the near field. The quadrant splitting technique applied to turbulent fluxes of momentum and the scalar revealed that the positive contributions to these transport terms are much larger than the negative ones. The quadrant splitting data confirmed that the flow is dominated by large scale coherent structures, which are intermittent in character and permitted the association of the scalar transport with vortical structures occurring in the near field of the coaxial jet flow. A proposed simplified mechanistic flow picture, which shows the passage of a vortex series through the mixing layer in the near field of the jets, is in agreement with the dominant characteristics of the flow.

High-speed visualization of soap bubble blowing and image-processing-based analysis of pinch-off dynamics

Soap bubble blowing has long been an amusement for humans, and the process involves pinch-off similarly to liquid drops and gas bubbles. To visualize the pinch-off process of soap bubble blowing, we built an apparatus consisting of air jet flow and thin soap film on a circular ring, and replicated human soap bubbling. High-speed videography captured growing soap film tube and following pinch-off, and the minimal neck radius of the tube was measured based on image processing. Scaling law analyses show that regardless of the ring diameter, the scaling exponent of soap bubble pinch-off is about 2/3, which is similar to that of soap film bridge. Also, the speed of the airflow into the tube was evaluated based on volume calculation of the soap film tube, and the Reynolds number of the airflow was estimated to be 1060–2970, which suggests that soap bubbling may involve Bernoulli suction effect.

Experimental and numerical study of heat transfer from a heated flat plate in a rectangular channel with an impinging air jet

Thermal control of electronic components is a continuously emerging problem as power loads keep increasing. To solve these thermal problems impinging jets are generally used in engineering, science and industry for its good heat transfer performance. In this study; cooling of a flat plate under constant heat flux condition, inside a rectangular channel, consisting of one open and three blocked sides was experimentally and numerically investigated using a single air jet flow. The impingement surface was a copper plate of the same width as the jet nozzle. Inlet flow velocities were measured using a Laser Doppler Anemometer (LDA) system. Temperature measurements were performed using thermocouples. Local and mean Nu numbers were determined as a function of two parameters, (a) Jet-to-plate distance (H/D h) in the range of 4–10 and (b) Reynolds number based on the hydraulic diameter of the slot nozzle in the range of 4000–10,000 (corresponding to an exit jet velocity from 6.5 to 16.2 m/s). Numerical investigation was also performed. Low Re number k–e turbulence model was used during investigations. The results are presented in the form of graphs showing the variation of the local Nusselt number as a function of these parameters. The effect of Re number on local Nu number was examined for q″ = 1000 W/m2 and H/D h = 6. In general, it was observed that heat transfer is sensitive to the Reynolds number and increases with increasing Re number. Average Nusselt number increases of 49.5 % from Re = 4000 to Re = 10000. Effect of H/D h on heat transfer under impinging air jets was examined with local Nusselt numbers. Local Nusselt number was less sensitive to H/D h in the range of H/D h = 4–10. Average Nusselt number decreases of 17.9 % from H/D h = 4 to H/D h = 10. The highest average Nu number was achieved for Re = 10,000 and H/D h = 4. The numerical results agreed well with the experimental data, including local and average Nusselt numbers. Correlations are proposed to predict the local Nusselt number as a function of Re number and H/D h.

Experimental and Numerical Investigations of Impingement Air Jet for a Heat Sink

In this study, impingement air jet heat and flow characteristics of hexagonal finned heat sink which was named as OHS-2 were determined experimentally and analysed numerically by Ansys-Fluent CFD programme. The heat sink was optimized by using Taguchi method in the wind tunnel according to L18(21×37) orthogonal array in earlier study [1]. Therefore, the optimized heat sink was used instead of another heat sink. Six of most commonly used turbulence models in the numerical analysis of heat and mass transfer with impinging jets were examined and k-ɛ reliable turbulence model was chosen the most suitable for experimental study. The experimental and numerical studies were carried out for a heat sink with a nozzle diameter, two different Y/d distances, 6 different flow rates and 3 different fin heights. The variations of the Nu-Re and Cpx,y- l/(l0/2) were analysed and compared. Finally, heat transfer correlations were produced from experimental and numerical results of impingement jet heat transfer. Also the numerical and experimental results were observed to be in a good agreement.

フィラメント運動の高速度ビデオ画像を用いたインターレース糸生成過程の解析

In order to make a generation mechanism of interlaced yarn clear, an air jet was blown at a second to polyurethane yarn 400tex/3f, which was fixed to chucks at both ends, in an enlarged interlacer, and motion of each filament was photographed with a high speed video camera. Then we discussed twists, which create entanglements, by using an example of braids and a generation mechanism of entanglement from an example of filament motion photographed. The generation mechanisms of twist and entanglement obtained are as follows: (1) Filaments open at the cross-section of yarn duct positioning at the center axis of air jet nozzle: an opening part of interlaced yarn is generated. A twist is formed when a filament winds around another filament with this opening of filaments. Twists of different type, which are formed one after another, generate tangling parts at both sides of the opening part. (2) A high speed air jet collides with the yarn duct wall opposite to the air jet nozzle and flows along the wall in the direction of the exit of air jet nozzle to form a twin vortex. It would be a typical pattern of twist generation that a filament with this twin vortex winds around another filament, which stays at a relatively low speed near the wall opposite to the exit of air jet nozzle.

Jet flow issuing from an axisymmetric pipe-cavity-orifice nozzle

An axisymmetric air jet flow is experimentally investigated under passive flow control. The jet issues from a pipe of the inner diameter and length of 10 mm and 150 mm which is equipped with an axisymmetric cavity at the pipe end. The cavity operates as a resonator creating self-sustained acoustic excitations of the jet flow. A mechanism of excitations is rather complex – in comparison with a common Helmholtz resonator. The experiments were performed using flow visualization, microphone measurements and time-mean velocity measurements by the Pitot probe. The power spectral density (PSD) and the sound pressure level (SPL) were evaluated from microphone measurements. The jet Reynolds number ranged Re = 1600–18 000. Distinguishable peaks in PSD indicated a function of the resonator. Because the most effective acoustic response was found at higher Re , a majority of experiments focused on higher Re regime. The results demonstrate effects of the passive control on the jet behavior. Fluid mixing and velocity decay along the axis is intensified. It causes shortening of the jet transition region. On the other hand, an inverse proportionality of the velocity decay ( u ~ 1/ x ) in the fully developed region is not changed. The momentum and kinetic energy fluxes decrease more intensively in the controlled jets in comparison with common jets.

압축공기에너지저장 시설에서 발생 가능한 압축공기 유출 및 화재 시나리오 분석

본 연구에서는 압축공기에너지저장 설비를 운영함에 있어 위험성이 크다고 분석된 '내조시스템 파손에 따른 압축공기 유출'과 '접근갱도 내 화재 발생' 리스크를 대상으로 시나리오를 작성하여 분석을 실시하였다. Bernoulli 방정식과 운동량 방정식을 결합하여 압축공기 분출에 따른 충격력을 계산하기 위한 식을 유도하였다. 이 식을 바탕으로 시나리오를 작성하여 충격력을 계산한 결과 충격력은 균열 직경의 제곱 및 압축공기의 압력에 비례하는 것으로 나타났다. 계절의 변화 및 화원의 위치에 따른 연기 확산 거동을 분석하기 위하여 4가지의 화재 시나리오를 작성하였다. 저장 공동 벽면 근처에서 화재가 발생한 경우는 10 m 떨어진 지점에서 화재가 발생한 경우보다 연기가 호흡한계선까지 하강하는 데 소요되는 시간이 더 짧은 것으로 나타났는데 이는 연기 파선단 전파로 인해 발생한 것으로 예측된다. 갱내로의 공기 유동 방향에 따라 연기의 확산 거동이 달라지기 때문에 겨울에 화재가 발생한 경우는 여름에 발생한 경우보다 연기의 확산속도가 빠르고 더 멀리까지 연기가 퍼지는 것으로 나타났다. 피난 시뮬레이션 해석 결과 여름과 겨울 화재에서의 피난요구시간(RSET)은 각기 262, 670 s로 분석되었다. In this study, scenarios based on the leakage of highly compressed air and fire occurrence turned out to be high risks in an operation stage of CAES facility were constructed and estimated. By combining Bernoulli equation with momentum equation, an expression to calculate an impact force of a jet flow of compressed air was derived. An impact force was found to be proportional to the square of diameter of fracture and the pressure of compressed air. Four types of fire scenarios were composed to evaluate an effects that seasonal change and location of fire source have on the spread behavior of smoke. Smoke from the fire ignited in the vicinity of CAES opening descended more quickly below the limit line of breathing than one from the fire occurred 10 m away from CAES opening, which is expected to occur due to a propagation of wave front of smoke. It was shown that a rate of smoke spread of the winter fire is faster than one of the summer fire and smoke from the winter fire spreads farther than one of the summer fire, which are dependent on the direction of air flow into access opening. Evacuation simulation indicated that the required safe evacuation time(RSET) of the summer and winter fires are 262, 670 s each.

Parametric analysis of a round jet impingement on a heated plate

In this study, the flow and heat transfer characteristics of a round air jet have been experimentally investigated in details using two techniques: Particle Image Velocimetry (PIV) and the Laser Doppler Velocimetry (LDV). The measurement of the mean velocity components are compared, and agree well with the experimental data obtained by Baydar (1999). The distributions of the velocity, turbulence quantities and temperature profiles are analyzed in the main characteristic regions of the jet where the heat transfer occurs. Parametric variations were conducted to produce information about the influences of the Reynolds number (Re = 1000, 2000, 3000), the distance between the pipe exit and the flat impingement plate (h/d = 1 and h/d = 2) and the temperature of the plane (Tp = 22 °C, 54 °C, 96 °C) on the impinging jet flow field.

Investigation of impingement heat transfer for air‐sand mixture flow

AbstractHeat transfer between a heated flat plate and normal impinging gas‐solid two‐phase jet flow was investigated. A single jet from a nozzle of 10 mm diameter at nozzle‐to‐plate distance/nozzle diameter ratio in the range of 2–8 was used. Natural sand particles with average diameters of 220, 350, and 550 μm were used as a solid phase. The effects of particle size and loading ratio (mass of sand/mass of air) at different jet velocities on impingement cooling characteristics of the flat plate are investigated. The numerical simulations were performed with ANSYS Fluent 14.7 for a steady, three‐dimensional, incompressible turbulent flow using Eulerian simulation for the gas phase and Lagrangian simulation for sand particles. The experimental results show that the existence of sand particles decreases the Nusselt number compared to air jet flow. The single and two‐phase flow experimental results are close to predictions when the particle reflection option is used in the simulation. The discrepancy in local values near the stagnation point can be attributed to the complex nature of the two‐phase flow at the stagnation point that includes reflection of sand particles at different angles.

Numerical modeling and experimental investigation of fiber diameter of melt blowing nonwoven web

Purpose – The purpose of this paper is to varify, the air drawing model and the air jet flow field model of dual slot shape die for a polymer in a melt blowing process were established, by the experimental results obtained with experimental equipment. Design/methodology/approach – The air jet flow field model is solved by introducing the finite difference method. The air drawing model of polymers in the melt blowing process was studied with the help of the simulation results of the air jet flow field. Findings – The higher air initial velocity and air initial temperature can all yield finer fibers and causes the fibers to be attenuated to a greater extent. Originality/value – The predicted fiber diameter agrees well with the experimental result, which verifies the reliability of these models. At the same time, the results also reveal the great potential of this research for the computer-assisted design of melt blowing technology.

The Research of the Characteristics of Secondary Equipment Device and Airflow Jet

In large space and span building, secondary airflow with nozzle's jet could be used to improve the uniformity of temperature and velocity distribution of indoor thermal environment, as well as the occupant's comfort in the occupied zone. Therefore, it is very important to reveal the characteristics of the secondary airflow equipment with nozzle's jet. In this paper, the characteristics of secondary airflow equipment and its air jet were researched through experiment. The experiment results on secondary airflow equipment performance showed that air volume, air static pressure, power and sound pressure level of the equipment would increase along with the increase of rotation speed under the same air nozzle diameters. And the results on air jet track illustrated that when the horizontal distance increased, the axis velocity of secondary airflow decreased both in isothermal jet and non-isothermal jet conditions. Through this experiment, the characteristic parameters of the secondary airflow equipment were provided for actual construction design.

An experimental study of the flow of subsonic flat mini and micro air jets

We have experimentally studied subsonic submerged air jets emitted from flat mini and micro nozzles with characteristic dimensions from 22 to 600 μm in a range of Reynolds numbers 70–2600. The point of laminar-turbulent transition (jet penetration range) was determined using the flow visualization technique. It is established that the penetration range of micro jets can reach 100–300 nozzle calibers. The Reynolds number for the transition to turbulence in flat mini and micro jets reaches high values (1000–2600), which are two to three orders of magnitude greater than the Reynolds numbers for the loss of stability (3–10). Available experimental data are summarized and generalized based on the Reynolds number determined for the jet penetration range.

ICONE23-1467 Behavior of Entrainment Droplet formed by High Velocity Air Jet Flow in Stagnant Water

ICONE23-1467 Behavior of Entrainment Droplet formed by High Velocity Air Jet Flow in Stagnant Water

AN OPTIMIZATION STUDY OF HEAT TRANSFER ENHANCEMENT DUE TO JET IMPINGEMENT OVER POROUS HEAT SINKS USING THE LATTICE BOLTZMANN METHOD

In this paper, an optimization study involving five variable parameters that are critical in evaluating the thermal performance of the air jet flow impingement over porous heat sinks in a mixed convection regime is presented. The variable parameters include the porous block height, channel height, jet width, porosity, and Darcy number. The lattice Boltzmann method for porous media is used as the numerical tool to simulate the objective functions in terms of the Nusselt number and pressure drop. The Kriging method combined with a genetic algorithm is used to generate the optimal Pareto plot. Three optimal cases from the Pareto plot showing the maximum, minimum, and optimum Nusselt numbers and the pressure drop are considered. The optimized results show that the porosity and Darcy number are relatively invariant along the Pareto surface. Thus, the geometry of the channel and porous block are the most significant parameters governing the Nusselt number and pressure drop values. The results from optimization corresponding to the three optimal cases are analyzed in detail. The local variation in the Nusselt number for these cases is also plotted to understand the effect of the channel and porous heat sink parameters on heat transfer.

2D-PIV Experimental Study on the Air Distribution with Natural Convection Effect of Passengers in an Air Cabin Mockup

Many previous studies have mentioned the critical effect of natural convection caused by passengers heat transfer on the air distribution inside the air cabins, but No study focused on it to make a further experimental or numerical analysis. In fact, the air flow field inside the air cabin with narrow interior space and passengers seated intensively is the result of interaction between natural convection from the passengers and forced convection from the supply air diffusers, including the air jet which plays the most essential part on the flow pattern. This study has measured the air flow jet in a 7-row cabin mockup with 2D-PIV (two dimension Particle Image Velocity) measurement system, and to make comparison of the air jet under isothermal and cooling conditions to analyze qualitatively the impact of natural convection inside the cabin mockup, which is to increase the air jet entrainment and weaken the attaching effect which can also enhance the velocity distribution uniformity. With the variety of air flow volume in a reasonable range of design parameters, we measured the different air velocity fields to quantify the effects of natural convection on the air jet. It can be concluded that the air jet decay rate becomes slower with the enhancement of natural convection.

HfB2-SiC (45 vol %) ceramic material: Manufacture and behavior under long-term exposure to dissociated air jet flow

Ultra-high-temperature composite materials HfB2-SiC containing 45 vol % SiC were prepared by spark plasma sintering. The behavior of a sample of this composition under exposure to a subsonic jet of dissociated air of a high-frequency induction plasmatron was studied; the total time was more than 30 min. Under certain test conditions, some regions of the sample were found to experience a rapid increase in temperature up to 2700°C. So, most of the surface area of the sample experienced exposure to temperatures up to 2500–2700°C for more than 15–18 min, while the rest of the surface had a temperature of 1700–1800°C during almost the entire duration of the experiment. The joint use of optical microscopy, scanning electron microscopy (with EDX analysis), X-ray powder diffraction, and X-ray computed microtomography enabled us to study the microstructure and composition of a structurally complex oxidized layer.

Turbulence measurements of a personal airflow outlet jet in aircraft cabin

The jet airflow field of MD-82 aircraft cabin's personal airflow outlet (PAO) is measured by constant-temperature anemometry system with the resolution higher than the smallest time-scale of turbulence. The turbulence statistics at different radial and axial positions is analyzed. The airflow field of the PAO jet is similar to a single jet beyond the point of reattachment. However, the airflow diffuses stronger than the single jet, turbulence intensity is higher, each order turbulence statistics and characteristics scales get turbulent equilibrium earlier. The inertial sub-range is found by spectrum analysis, by which the energy is cascaded from the energy-containing large scale to the small scale, and finally dissipated into heat. Moreover, wavelet analysis is employed to perform multi-scale decomposition of instantaneous turbulence fluctuating velocity signals, in order to accurately distinguish the time scales which dominate the airflow field.

An unexpected route for otolaryngology bacterial contamination with a Venturi atomizer.

The Venturi-principle atomizer is a commonly used device in otolaryngology practices. The purpose of this study is to evaluate the possible route of bacterial contamination from the nasal vestibule to the atomizer tip through the jet airflow created during the use of the Venturi atomizer. Thirty nostrils from 15 enrolled volunteers were tested. The aerosols generated by spraying sterilized saline into the nostrils were collected using a specially made aerosol-collecting nozzle cap. The collected samples were sent for bacterial culture, and nasal vestibular swab cultures were performed for comparison. In the aerosol-exposed group, 18 out of 30 samples (60%) were positive for bacterial growth, confirming the bacterial contamination from the nasal vestibule to the atomizer tip through the reverse jet airflow. The bacteria species in 8 of the 18 positive samples were identical to those from the nasal swab culture results from the same nostril. In ordinary otolaryngology practices, there are significant risks for bacterial contamination from the nasal vestibule to the tip of the Venturi atomizer even without direct contact. Clinicians must be more aware of this pattern of contamination, which has not been reported in the existing literature.

Numerical study of air jet flow field during a loss of vacuum

Air leakage into tokamaks vacuum vessel during plasma burning or maintenance operations may lead to the fast pressurization of the vacuum vessel. A fraction of the dust inventory present in the vacuum vessel can be mobilized threatening the safety of staff and workers on site, the local population and the environment. A numerical analysis of the physical phenomena involved in such accidents is necessary in order to predict the thermal-fluid dynamics into the vacuum vessel after air ingress and consequent dust mobilization. Accuracy of the numerical results is also required in order to provide a sufficient margin in the design of the safety systems. The numerical simulation of Loss of Vacuum Accident (LOVA) scenarios is a challenging task for today numerical methods and models because it involves large volumes, multiphase flows ranging from highly supersonic to nearly incompressible and contemporary heat transfer. The drag force exerted on the dust by a moving fluid due to the viscous surface shear stress and pressure distribution around the dust particles depends mainly on the Reynolds number, i.e. property of the fluid (kinematic viscosity), its mean velocity and characteristic length of the geometry. For a fixed geometry, the key parameter for the dust mobilization is the velocity field of the continuous phase, and its thermodynamics properties, inside the vacuum vessel. In this contribution, the authors present and discuss the results of numerical simulations of air jet flow field during a LOVA with particular attention to the comparison with the experimental data and differences arising from the use of different types of grid resolution and turbulence models (Zero-Equation, k–ω and SST).

Computational fluid dynamics simulation and theoretical study the air jet flow field of a wide slot positive pressure drawing assembly in the spunbonding nonwoven process

The polymer air‐drawing model of polyethylene terephthalate spunbonding nonwovens and the air jet flow field model in wide slot positive pressure spunbonding process have been established. The influence of the density and the specific heat capacity of polymer melt at constant pressure changing with polymer temperature on the fiber diameter have been studied, which is solved by introducing the numerical computation results of the air jet flow field of attenuator. It is simulated by means of the finite difference method. The predicted fiber diameter agrees with the experimental data. The effects of the processing parameters on the fiber diameter with the help of the image analysis method have been investigated. A higher inlet pressure, smaller slot width, and smaller jet angle will all cause higher z‐axis position of air velocity and air pressure, which are beneficial to the air drawing of the polymer melt and thus to reducing the fiber diameter. The experimental results show that the agreement between the results and experimental data is better, which verifies the reliability of these models. The results present great prospects for this research in the field of computer assisted design of spunbonding process, technology, and equipment. POLYM. ENG. SCI., 55:231–242, 2015. © 2014 Society of Plastics Engineers