Wall jet


Jets discharging dose to the plane of the ceiling or wall are common in ventilation practice. The presence of an adjacent surface restricts air entrainment from the side of this surface. This results in a pressure difference across the jet, which therefore curves toward the surface. The curvature of the jet increases until it attaches to the surface. This phenomenon is usually referred to as a Coanda effect. The attached jet or, as it is commonly called, wall jet, can result from air supply through an outlet with one edge coincident with the plane of the wall or ceiling fFig. 7.27). Jets supplied at some distance from the surface or at some angle to the surface can also become attached (Fig. 7.28)  [c.469]

Studies of wall jets show that they have two layers a turbulent boundary layer close to the wall and an outer shear layer. The thickness of the boundary wall layer can be neglected for practical purpose. Accordingly, to compute the maximum velocity in the wall jet, researchers - - apply the method of images by treating the wall jet as one-half of a free let. Application of this method gives a relationship between the characteristics of a wail jet and a free jet, which results in a correction factor equal to J2. This approach has some inaccuracy even with linear and radial jets, m a For a three-dimensional wall jet, the procedure is even more approximate. Discussion by Etheridge and Sandberg of some previous studies of attached jets indicates some loss of momentum in an attached jet due to the friction against the surface. The authors compiled information from previous stud-  [c.470]

When a jet is supplied at some distance from the surface, the attachment occurs when the distance between the outlet and the surface is below a critical distance otherwise the jet will propagate as a free jet. If the jet attaches to the surface, the flow downstream of the attached point is similar to that of a wall jet. For a compact isothermal jet, the critical distance for jet attachment to the surface is = 6Aq For < 6Al - the velocity decay coefficient Kj  [c.471]

Compact wall jet -1 - a Nielsen  [c.472]

Many different equations describe jets. Some of these are used in Chapter 7. All jets must be placed in such a way that the mixing of surrounding air is not restricted. If it is restricted the jet can behave in an unpredictable manner. For example, a wall close to a plane jet nearly always makes the jet attach to the wall and changes the jet from a free jet to a wall jet.  [c.920]

There are many possible combinations of supply and exhaust air. For example, a line jet could be used as a shield in an opening, as a stripping system on surfaces, for blowing contaminants into an exhaust, etc. An enclosure could be designed with a line jet in the opening, with a wall jet inside to increase efficiency, or with a low-momentum jet inside or outside the opening to replace the room air supply. In this section, only some basic combinations are described.  [c.935]

FIGURE 10.70 Simplified model of a wall jet combined with an exhaust flow.  [c.946]

Collectively, for the sake of brevity, we refer to Eqs. (10.92) to (10.96) as the original Verhoff formulae. A numerical analysis of the wall jet in the push-pull situation suggests that the Verhoff formulae fit the numerical data more closely if the following constants are taken  [c.947]

With these values of the constants, we refer to the wall jet formulae as the modified Verhoff formulae.  [c.947]

The assumptions that the exhaust flow has a negligible effect and that the offset jet can be treated as an equivalent wall jet were tested by Robinson and Ingham - and found to be reasonable over the majority of the surface of the tank, except close to the jet nozzle and exhaust hood. Far from the surface of the tank, the exhaust flow has a more noticeable effect.  [c.947]

Having established that these assumptions are reasonable, we need to consider the relationship between the parameters of the actual offset jet and the equivalent wall jet that will produce the same (or very similar) flow far downstream of the nozzle. It can be shown that the ratio of the initial kinematic momentum per unit length of nozzle of the wall jet to the offset jet,, and the ratio of the two nozzle heights,, depend on the ratio D/B, where D is the offset distance betw een the jet nozzle and the surface of the tank, and h, is the nozzle height of the offset jet. The relationship, which because of the assumptions made in the analysis is not valid at small values of D/hj, is shown in Fig 10.72.  [c.947]

We thus have a means of describing, albeit approximately, the fluid flow induced by the system in terms of the Verhoff formulae and a graphic relationship between the offset jet parameters and the equivalent wall jet parameters. We now wish to be able to calculate the movement of the contaminant in the system  [c.947]

FIGURE 10.71 The ratio of the horizontal component of the fluid velocity to its local maximum u/ u, as a function of t), at x = 0.75 m for the experimental results, the numerical results obtained using the commercial package for the offset and equivalent wall jet models, and the original and modified Verhoff empirical formulae.  [c.948]

Ingham. - This gives the required minimum value for the momentum ol the equivalent wall jet we must also recall the relationship shown in Fig. 10.72 to determine the required momentum of the offset jet in the push-pull system.  [c.953]

Wall Jet>Enhanced Exhausts  [c.978]

FIGURE 10.93 Design of exhaust with wall jet for a kitchen hood with partly shielded sides and circular grease filter.  [c.981]

The connection to the exhaust duct (the exhaust opening) is usually a horizontal surface with suction vertically up, and this connection should be situated above the jet inlet and the source generation point. Sources with heat generation should be on or slightly above the surface and between the jet inlet and the exhaust opening. For a vertical wall jet, the source should be as close as possible to the jet.  [c.982]

The designs that have been published are all empirical. However, it should be possible to use the wall jet equations from Chapter 7 and the equations for velocities outside exhaust openings (Section 10.2.2) to design these types of hoods.  [c.983]

An important design parameter is the jet angle. Normally the jet should be parallel to the table or the inner back wall and can thus be treated as a normal wall jet. If the jet has a small angle upward from the table, the wall jet equations may be unsatisfactory and experiments may be necessary.  [c.983]

A. Verhoff. The Two-Dimensional Turbulent Wall Jet with and without an External Stream. Report 626, Princeton, NJ Princeton University, 1963.  [c.1010]

Often the inlet device (air supply) in a ventilated room is geometrically complicated. To resolve the flow around such a device would require a very fine grid. Instead of trying to resolve the complex flow near the inlet device, one can choose to use the box method or the prescribed velocity method.Both methods are based on the observation that downstream of the inlet, the flow behaves like a wall jet. Thus it is important that the bound-  [c.1042]

With LES we get much more information than with traditional time-averaged turbulence models, since we are resolving most of the turbulence. In Fig. T1.15 the computed u velocity is shown as a function of time in two cells one cell is located in the wall jet (Fig.. 15a), and the other cell is in the middle of the room (Fig. ll.lSh). It is found the instantaneous fluctuations are very large. For example, in the region of the wall jet below the ceiling where the time-averaged velocity u)/l] ) is typically 0.5, the instantaneous velocity fluctuations are between 0.2 and 0.9. In the middle of the room, which is a low-velocity region, the variation of u is much slower, i.e., the frequency is lower.  [c.1049]

The probability density function of u is shown for four points in Fig. 11.16, two points in the wall jet and two points in the boundary layer close to the floor. For the points in the wall jet (Fig. 11.16<2) the probability (unction shows a preferred value of u showing that the flow has a well-defined mean velocity and that the velocity is fluctuating around this mean value. Close to the floor near the separation at x/H = I (Fig. 11.16f ) it is hard to find any preferred value of u, which shows that the flow is irregular and unstable with no well-defined mean velocity and large turbulent intensity. From Figs. 11.15 and 11.16 we can see that LES gives us information about the nature of the turbulent fluctuations that can be important for thermal comfort. This type of information is not available from traditional CFD using models.  [c.1049]

FIGURE 12.26 Velocity decay in a wall jet along the ceiling in a room and in a model.  [c.1182]

Coanda effect When a jet becomes and remains attached to a surface due to static pressure differences, as in the case of a wall jet.  [c.1422]

Ignition sources may be either soft or hard. Open flame, spark, or hot surfaces are examples of soft ignition sources, while jet and high explosives are categorized as hard ignition sources. Ignition intensity has almost no influence on flame speed for soft ignition sources confinement, obstacles, and fuel reactivity are most important here. By contrast, ignition intensity is the most important variable if a hard ignition source is present.  [c.124]

The results of studies by Kerka of jets supplied through rectangular outlets with and without an adjacent surface indicate an increase of 1,27 to 1.45 times in the velocity decay coefficient for wall jets compared with free jets from the same outlets. The angle of divergence of the wall jet in the direction perpaidicular to the wall was slightly less than one-half of a free jet, while the angle of spread of the jet along the wall was greater than the divergence of a free jet.  [c.469]

It is not uncommon to supply air into the room with jets attached both to the ceiling and to the wall surfaces. Air jets can be parallel to both surfaces or be directed at some angle to one or both surfaces (Fig. 7.28). Studies of compact wall jets supplied parallel to both surfaces reported by Grimitlyn show that the correction factor value is in the range from 1.6 to 1.7, which means that restriction of entrainment from two sides reduces velocity decay by 20% to 30% compared to the case of a wall jet.  [c.471]

Schwarz, W. H., and W. P. Cosart. 1960. The two-dimensional turbulent wall jet. Journal oj Fluid Mechanics, vol. 10, pp. 481-49.5.  [c.510]

Bakkc, P. 1957. An experimental investigation of a wall jet. Journal of Fluid Mechan cs. vol. 2, pp. 467-472.  [c.510]

More recently, in the middle 1990s, the UK s Health and Safety Executive (HSE) also reviewed the push-pull system. Hollis and Fletcher offer a comprehensive literature review on push-pull ventilation and note that the main conclusions of previous work on push-pull ventilation of tanks are that the control is primarily supplied by the inlet jet, forming a wall jet along the surface of the tank, and that the main purpose of the exhaust hood is to remove the air and contaminant contained within the push jet.  [c.945]

Turbulent wall jets have been extensively studied, and perhaps the most straightforward representation is given by a combination of the work of VerhofP and Launder and Rodi who together show that the fluid flow in a wall jet can be represented by  [c.946]

We have thus introduced two main simplifications first that the offset jet can be modeled as a simple wall jet, and second that the exhaust flow has a negligible effect. To demonstrate that these assumptions are reasonable, Fig. 10.71 shows the normalized horizontal component of the fluid velocity, U, as a function of the similarity variable r/ for numerical results for both the wall jet and offset jet, and the original and modified Verhoff formulae, and the experimental results of Huebener and Hughes. It shows reasonable agreement between all the different methods. A comparison of the other significant variables, such as the width of the jet and the local maximum velocity as a function of distance from the jet nozzle, also show good agreement.  [c.947]

From Fig. 10.72 we can see that the ratio of the momentum of the equivalent wull jet to that of the offset jet is typically about 0.46 and we have taken this value in the sample results presented here.  [c.953]

Consider an inlet region as in Fig. 11.10. In the box method all dependent variables are prescribed along surfaces a and b. The variables are not solved for inside the box. Along surface a, the mean flow quantities such as the streamwise velocity component U and the temperature are set from wall jet data. The turbulent kinetic energy k is also taken from wall jet data, and together with a prescribed turbulent length scale the dissipation e can be found. Along boundary b zero gradient d /dy is prescribed. Special care must be taken to conserve mass and energy in the box. For more details, see ref. 29.  [c.1043]

We have used H = 3 m, = 0.455 rnys. The inlet (height h) is located at the left-side wall adjacent to the ceiling, and a wall jet is formed below the ceiling. The exit is located at the right-side wall near the floor (height t).  [c.1049]

As the new centui y unfolds. Airbus and Boeing are heavily engaged in market studies to determine the scale and scope of the next generation of airliners. Air travel is steadily growing domestically and internationally, though the rate of growth is slowing. Through 2020, the U..S. Department of Energy projects that the growth in jet fuel consumption should outpace that of all other liquid fuels. The Annual Energy Outlook for 1999 foresees that by 2020 the consumption of gasoline will drop from 65 to 61 percent, diesel from 18 to 14 percent, while jet fuel use will rise from 13 to 17 percent.  [c.63]


See pages that mention the term Wall jet : [c.1933]    [c.1938]    [c.471]    [c.472]    [c.731]    [c.946]    [c.963]    [c.979]    [c.1043]    [c.47]    [c.137]   
Industrial ventilation design guidebook (2001) -- [ c.469 , c.473 , c.1454 ]