Plane Air Jet Research Articles

Interaction of a droplet spray with a turbulent plane air jet impacting a wall: Application to the confinement of atmospheres contaminated with particles by air curtain.

Experiments are carried out to study the interaction of a spray of spherical micronic oil droplets with a turbulent plane air jet impacting a wall. The context is the separation of a contaminated atmosphere with passive particles from a clean atmosphere by using a dynamical air curtain. A spinning disk is used to produce the spray of oil droplets close to the air jet. The diameter of the produced droplets varies between 0.3 and m. The jet and particulate Reynolds numbers and the jet and Kolmogorov-Stokes numbers are, respectively, equal to Re , , and . The ratio of the jet height to nozzle width is . The flow properties in the experiments are measured by particle image velocimetry and are in good agreement with large eddy simulation results. The droplet/particle passing rate (PPR) through the air jet is measured by an optical particle counter. The PPR decreases with the increase in the droplet diameter for the studied droplet size range. Whatever the droplet size is, the PPR increases with time due to the presence of two large vortices on each side of the air jet that bring the droplets back to the jet. The accuracy and repeatability of the measurements are verified. The present results can be used to validate Eulerian/Lagrangian numerical simulations on the interaction of micronic droplets with a turbulent air jet.

Dynamic modeling and simulation of bottom outlet air flow field in wide slot positive pressure spunbonding drawing process

The air jet flow has an important influence in wide slot positive pressure spunbonding process that not only includes the filament fiber diameter, crystallinity, and birefringence, but also the fiber web evenness. In this work, the air drawing model and the air jet flow field model of bottom outlet in spunbonding process are established and studied. The characteristics and regularities of the plane air jet flow in wide slot positive pressure spunbonding process are also demonstrated. It is simulated by means of the finite difference method. The numerical simulation computation results of distribution of the air velocity match quite well with the experimental data. The air drawing model of polymer is solved with the help of the distribution of the air velocity. The predicted filament fiber diameter, crystallinity, and birefringence agree well with the experimental data. It was found that higher initial velocity and initial temperature of air can yield finer fibers, and its effect was very significant. The results also reveal the great potential for this research in the computer-assisted design of spunbonding technology and equipment.

Similarity analysis of the momentum field of a subsonic, plane air jet with varying jet-exit and local Reynolds numbers

A similarity analysis is presented of the momentum field of a subsonic, plane air jet over the range of the jet-exit Reynolds number Reh (≡ Ubh/υ where Ub is the area-averaged exit velocity, h the slot height, and υ the kinematic viscosity) = 1500 − 16 500. In accordance with similarity principles, the mass flow rates, shear-layer momentum thicknesses, and integral length scales corresponding to the size of large-scale coherent eddy structures are found to increase linearly with the downstream distance from the nozzle exit (x) for all Reh. The autocorrelation measurements performed in the near jet confirmed reduced scale of the larger coherent eddies for increased Reh. The mean local Reynolds number, measured on the centerline and turbulent local Reynolds number measured in the shear-layer increases non-linearly following x1/2, and so does the Taylor microscale local Reynolds number that scales as x1/4. Consequently, the comparatively larger local Reynolds number for jets produced at higher Reh causes self-preservation of the fluctuating velocity closer to the nozzle exit plane. The near-field region characterized by over-shoots in turbulent kinetic energy spectra confirms the presence of large-scale eddy structures in the energy production zone. However, the faster rate of increase of the local Reynolds number with increasing x for jets measured at larger Reh is found to be associated with a wider inertial sub-range of the compensated energy spectra, where the −5/3 power law is noted. The downstream region corresponding to the production zone persists for longer x/h for jets measured at lower Reh. As Reh is increased, the larger width of the sub-range confirms the narrower dissipative range within the energy spectra. The variations of the dissipation rate (ɛ) of turbulent kinetic energy and the Kolmogorov (η) and Taylor (λ) microscales all obey similarity relationships, $\varepsilon h/U_{\rm b}^3 \sim Re_h^3$ɛh/Ub3∼Reh3, η/h ∼ Reh−3/4, and λ/h ∼ Reh−1/2. Finally, the underlying physical mechanisms related to discernible self-similar states and flow structures due to disparities in Reh and local Reynolds number is discussed.

Flow Characteristics of a Plane Jets with Deflectors inside the Nozzle

The present paper describes the flow characteristics of a plane air jet with deflectors. The divergent or convergent deflector was installed symmetrically inside of the plane nozzle. The effects of the angle and the length of deflectors on the mean and fluctuating velocities, and the velocity ratio of the inner jet to outer jet at the nozzle exit were examined by the hot-wire measurement and the flow visualization. In the case of the jets with the divergent deflector, the outer jet was accelerated and the inner jet was decelerated. The spreads of jet with the divergent deflector increased in the near field of the jet. In the case of the jets with convergent deflectors, the outer jet was decelerated and the inner jet was accelerated. The spreads of plane jet with the convergent deflector was smaller than the other jet. The relation between the velocity ratio of the nozzle exit, the area ratio, the pressure drop, the spreading rate and the virtual origin of a plane jet with divergent or convergent deflectors was found.

Heat Transfer Through Vertically Downward-Blowing Single-Jet Air Curtains for Cold Rooms

One of the major sources of heat gain in refrigerated storage rooms is the infiltration of warm ambient air through doorways. Air curtains reduce this amount of heat transfer by blowing a plane air jet in the doorway while allowing an easy passage of the traffic. An air curtain device installed at the doorway of a cold room in a supermarket was studied in detail. Thermographic images were taken, recording the temperature field across the doorway. Tracer gas decay measurements were used to estimate the airflow rate through the door. These measurements were then used to validate a computational fluid dynamics (CFD) model of the air curtain. With this CFD model the impact of some important air curtain parameters, such as the jet velocity and the jet nozzle width, on the heat transfer rate through the opening is determined. Finally, an expression to estimate the heat transfer rate through the air curtain is proposed.

S054082 ノズル内部に偏向板を設置した平面噴流の流れ特性([S05408]噴流,後流,およびはく離流れの基礎と応用:最前線(8))

The flow characteristics of a plane air jet have been investigated experimentally. Two deflectors were installed inside of plane nozzle. The effects of the slant angle of deflectors on the mean and fluctuating velocities were examined by means of hot-wire measurements. The operating velocity of the jet was fixed at 7.4 m/s. The slant angles of deflectors (divergent deflector, convergent deflector and parallel plate) were changed at a = ・-, 0- and 6-. In the case of the plane jet with the divergent deflector, the outer jet was accelerated and the inner jet was decelerated inside of the nozzle. The spread of plane jet with the divergent deflector increased in the near field of jet. On the other hand, in the case of the plane jet with the convergent deflector, the outer jet was decelerated and the inner jet was accelerated. The spreads of plane jet with the convergent deflector was smaller than the other jet.

Modelling and simulation of the near-wall velocity of a turbulent ceiling attached plane jet after its impingement with the corner

At present, ceiling-mounted diffusers are very popular for indoor air distribution, particularly in offices, owing to greater efficiency in the distribution of the air supply and a more comfortable indoor environment. The objective of this study is to construct an effective model to design the indoor airflow of an attached plane jet after its impingement with the corner in a room. In this study, a full-scale test facility was set up to obtain detailed experimental data. One commercial CFD tool, CFX 11.0, was used to simulate the air velocity distribution of an attached plane air jet bounded by the ceiling and an insulated wall. One semi-empirical model was also constructed to predict the impingement jet velocity. The results show that bout the semi-empirical model and CFX 11.0 were able to predict the maximum velocity of an impinging jet at low Reynolds numbers, 1000 and 2000, with an inaccuracy of ±11%. However, the semi-empirical model could be more conveniently used to predict the maximum jet velocity decay after its impingement the corner in a room than CFD simulation in terms of accuracy and the time required to design the indoor airflow pattern.

RANS modelling and LES of a single-phase, impinging plane jet

Reynolds-averaged Navier–Stokes (RANS) modelling and large eddy simulation (LES) of a plane air jet impinging orthogonally on a flat surface is described. Comparisons of predictions are made with the data of Yoshida et al. for a Re = 10,000 flow, with good agreement found in both the free and wall jet regions. Some under-prediction, particularly by the RANS approach, of turbulence quantities is apparent, and overall the LES is clearly in closer agreement with data, particularly in zones where large turbulence structures dominate. Differences in LES and RANS for the particular flow considered are not large, however, and this may be attributed to the significant effort already expended in the development of second-moment turbulence closures with wall reflection effects for impinging flow application. The results presented go some way towards validating LES for application to impinging flows, although uncertainties in the data set used for comparison purposes demonstrate a requirement for further experimental work.

Educing coherent eddy structures in air curtain systems

The work reported here comes within a broader research program dealing with volume separation or confining by means of air curtains (plane air jets). The process is studied experimentally by particle image velocimetry (PIV). In this paper, the emphasis is put on the flow structure in the impingement region of such jet systems insofar as it is where transfers occur preferentially. More precisely, a vortex eduction method was implemented under the Matlab environment enabling both the automatic detection of 2D coherent patterns embedded in PIV velocity vector maps, and a statistical analysis of the topological and energy features of these structures. First, the approach is explained in detail. The second part of this paper is devoted to its application in the case of plane turbulent impinging simple- and twin-jets for various jet exit velocities. Results about the size, the shape, the spatial distribution and the energy content of the detected vortices are provided. Although many questions still remain open, new insights into the fashion these structures might form, organize and evolve are given providing an original picture of the plane turbulent impinging jet.

Behavior of an Air Curtain Subjected to Transversal Pressure Variations

The mass transfer and the influence of sudden external perturbations on the behavior of an air curtain are evaluated by means of experimental and numerical studies. The air curtain, consisting of a plane air jet, is combined with a laminar flow unit. They constitute an “open” protection device intended to protect a localized area against any external airborne contamination. The external perturbations are characterized by fast pressure changes, such as drafts which can be experienced in a room. Numerical and experimental results both show that the air jet is strongly perturbed by the pressure gradient in the lateral direction. The jet is subjected to a brutal back and forth motion, breaks, and generates a swirl which is responsible for pollution entrainment inside the protected area. Gas concentration measurements suggested that there is an inlet jet velocity for which the pollution is minimal. These experimental results, corresponding to a strongly unsteady flow, were qualitatively well-predicted by the s...

Heat transfer from plastic film heated by thermal radiation

The heat transfer characteristics of a plastic film were experimentally studied using a two-dimensional plane model apparatus of a double bubble tubular blown film process, which is one of the most important methods of polymer processing to produce a biaxially oriented film. In the model apparatus, a polyethylene film was heated by infrared heaters and cooled by plane air jets. The radiation absorptivity of the film was estimated on the basis of the temperature and irradiation measurements of the film that was cooled by natural convection, i.e., without the air jets. The convective heat transfer coefficient on the film cooled by the air jets was evaluated using the estimated absorptivity of the film. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(4): 265–278, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20059

Heat Transfer from Plastic Film Heated by Thermal Radiant

Using two dimensional plane model apparatus of double bubble tubular blown film process which is one of the most important polymer processing to produce biaxially oriented film, the heat transfer characteristics on the plastic film were experimentally studied. In the model apparatus, a polyethylene film was heated by infrared heaters and cooled by plane air jets. The radiation absorptivity of the film was estimated from the measurements of the temperature and irradiation on the film cooled by natural convection without the air jets. The heat transfer coefficient on the film cooled by the air jets was evaluated using the estimated absorptivity of the film.

Experimental investigations of an air curtain device subjected to external perturbations

Although plane air jets are often used as dynamic barriers to separate two environments, only a few works have explored their sensitivity to perturbations. We investigated the influence of sharp changes of pressure on the flow field of a device designed to avoid air-borne contamination. Laser tomography and tracer gas experiments clearly indicate that the air curtain is strongly sensitive to perturbations such as draughts. The results highlight that the control of air curtains used in open protection devices should be further investigated.

A turbulent plane jet impinging nearby and far from a flat plate

Plane air jets presenting an impact find applications in many industrial devices. They can be found in installations of heating, cooling or drying, cleaning, pulverization, or containment of polluted environments. Other applications can be found in the ventilation of buildings. The correct design of these kinds of installations requires thorough knowledge of the structure of the jet from the cinematic point of view. With this intention a test bench with variable geometry was developed. Then, using laser Doppler velocimetry (LDV) and particle image velocimetry (PIV), it is possible to analyze the development of the jet for various geometrical and cinematic configurations. It appears that the development of the jet is independent of the Reynolds number, and the velocity decrease in the developed and impinging zones can be characterized by using very simple laws. Furthermore, by PIV visualization of the impinging zone, it has been possible to highlight the causes of mass transfer through the jet.

Numerical study of a heated cavity insulated by a horizontal laminar jet

In this work, we present a numerical study of the thermal insulation of a heated two dimensional cavity limited on its superior part by a horizontal plane air jet. The lower horizontal wall is isothermal, while the two vertical walls are adiabatics. A finite difference method based on the stream function–vorticity formulation is developed to solve the dimensionless Navier–Stokes and energy equations resulting from some assumptions. The results allowed us to point out two flow configurations: if natural convection prevails, the hot jet issuing from the nozzle diffuses upwards, and consequently, the cavity cannot be insulated correctly. However, the use of an aspiration zone can then improve the insulation. When forced convection predominates, the hydrodynamic barrier is conserved, and the enclosure is also thermally well confined.

Design of air curtains used for area confinement in tunnels

Air curtains' devices, i.e., plane air jets, are used as virtual screens to reduce the heat and mass transfer from one zone to another subjected to different environmental or climatic conditions. An air curtain is a plane air jet blown through an opening. It produces a pressure drop that forbids transversal flow through the opening. The principal advantage of such installations is to facilitate the transit of people, vehicles or material through doorways of buildings and other enclosures. The purpose of this research is twofold: (i) to characterize the efficiency of air curtains; (ii) to establish how scaled down models could be used to set up full-scale installations.

Comparison of flow characteristics in the near field of two parallel plane jets and an offset plane jet

Two parallel plane air jets and offset air jets have several common features such as the existence of a subatmospheric pressure region and the formation of a flow recirculation zone adjacent to the nozzle plate. In particular, the symmetry plane that exists between two parallel plane jets may appear to affect the flow field in much the same way as a solid wall does in a reattaching offset jet. It is obvious, however, that there are significant differences far downstream from the nozzles because the two parallel plane jets will combine to form a single free jet while the offset jet will develop into a wall jet. Differences in the near field between these two jet configurations with a separation ratio of 2.125 are examined here under identical initial flow exit conditions through laser Doppler anemometer measurements of the mean velocity components, turbulence intensities, and Reynolds shear stress. The results indicate that the wall exerts significant retarding and turbulence suppression effects on the offset jet in the flow development region. Through the use of comparisons, the interaction of two inner shear layers on both sides of the symmetry plane in two parallel plane jets results in a much more turbulent near field than that of the offset jet.

Confinement thermique d'une enceinte par jet d'air en convection forcée

A global model is proposed for the heat transfer through a plane air jet which is used as a thermal seal of a case at temperatures higher than the ambient one. The model considers that forced convection predominates over the natural convection and uses an analogy between heat and mass transfers. The heat balance calculation is based on an estimation of the velocity and temperature profiles at returning slot of the jet. Hydrodynamically, the jet is assimilated to a free jet. Thermally, a new formulation for the temperature profile in the jet is proposed for the most general case whatever the temperatures at the blowing slot and on each side of the jet — room and case —. The predictions by the model are in good agreement with experimental results from the point of view of: • - the changes in longitudinal and transversal temperature in the jet; • - the flux exchanged between all compartments; • - the energy consumption required to assure a given difference between the case temperature and the ambient one.

Turbulence measurements from a plane air jet with buoyancy induced curvature

An experimental investigation of the turbulence structure of a heated plane air jet discharged at various angles into quiescent surroundings is described. Hot-wire anemometry was used to obtain the profiles of mean and turbulent velocities and temperature normal and tangential to the curved path of the flow. Measurements in the buoyancy induced curved region of the jet show the relative influence of the stability induced by both buoyancy and jet curvature on the turbulence structure.

Mean flow characteristics of a turbulent plane jet with buoyancy induced curvature

An experimental investigation of heated vertical and inclined plane air jets discharged into quiescent surroundings is described. A unique feature of this data is that Pilot tube measurements were used to define the mean trajectory of the inclined jets so that subsequent hot-wire traverses could be made normal to the curved path. While the mean velocity and temperature profiles are self-similar for the range of exit conditions studied, other aspects of the mean jet development depend on the exit Reynolds and Froude numbers, or the discharge angle. It is noted that variations between this study and other published data suggest further measurements of this flow situation are needed, with particular attention to specific features of the jet apparatus and ambient surroundings, and to the exit Reynolds number.