Confined air jet


Detailed experimental data were obtained by Sadovskaya on a physical model in isothermal conditions. She has found that the confined air jet has two critical cross-sections (Fig. 7.38). In the first cross-section, where the ratio of jet cross-sectional area to the area of ventilated space equals 0.24, the jet  [c.478]

In the latter case, the jet reaches the opposite wall/ceiling and follows room surfaces until it reaches the occupied zone (Fig. 7.7a). If the combination of room sizes (height, length, and width) allows such an airflow pattern, this room is considered to be short.The room where an air jet dissolves before it reaches the opposite wall is considered to be long. In such rooms, the occupied zone is ventilated by reverse flow, and secondary and tertiary vortexes (Figure 7.7h, c).Buoyant forces, e.g., when supply air is heated, can significantly affect the airflow pattern created by supply jets (Fig. 7.6). Applying proper design principles prevents warm air from rising to the upper zone of the room without heating the occupied zone. More detailed discussion of airflow created in confined spaces with mixing-type air supply can be found in Section 7.4.5.  [c.435]

The results of different analytical and experimental studies of the confined horizontal jet described above are presented in Table 7.17. The main reason for the differences in the analytical results is different approximations of reverse flow velocity profiles.  [c.488]

Plasma sources utilized for the production of materials and their modification are similar to those used to effect chemical reactions. Low temperature, glow-discharge, and r-f devices are employed to coat and heat the surfaces of soflds and to alter them by ion bombardment sputtering. Higher temperature plasmas are used in materials processing. Plasma torches are produced by confining the heating by r-f fields or arcs to a chamber through which gas flows at high velocity. Temperatures in excess of 10,000 K are attained in the plasma, which cools as it is swept along to form a jet.  [c.116]

Dry-sieving is typically performed using a stack of sieves having openings diminishing in size from the top downward. The lowest pan has a soHd bottom to retain the final undersize. Powders are segregated according to size by placing the powder on the uppermost sieve and then shaking the stack manually, using a mechanical vibrator (19,20), or with ak pulses of sonic frequency (21,22) until all particles fall onto sieves through which they are unable to pass or into the bottom pan. The unit, powered by sonic energy shown in Figure 6, confines the sample using very flexible diaphragms, ensuring against loss of fines. In another device, sieves are employed one at a time within a container from which passing particles are captured by a filter. Agitation on the sieve is provided by a rotating ak jet (23). The material retained by the sieve is recorded and recycled to the next coarser sieve until all the powder is exposed to the desked series of sieves or all material passes.  [c.130]

The first zone of the jet can be described using equations for velocity and temperature decay as well as jet trajectory with a coefficient accounting for jet confinement. The impingement zone can be characterized by a significant change in the static pressure and great curvature of the air current lines. After the impingement, the radial flow is formed as if it is supplied from the side surface of the truncated cylinder with a uniform initial velocity of U. In the basement of the cylinder, there is a particular line that crosses the quasi source of the radial flow. Equations provided in the paper can be used to evaluate velocities along the branches with maximum airflow, minimum airflow, and along the particular line.  [c.493]

Jet interaction should not be taken into account when the jets are closely adjacent to each other, are propagated in confined conditions, and entrainment of the ambient air is restricted. This may be the case for concentrated air supply when air diffusers are uniformly positioned across the wall and the jets are replenished by the reverse flow, which decreases the jet velocity. This effect should be taken into consideration using the confinement coefficient discussed in Section 7.4.5. For the same reason, jet interaction should not be taken into consideration when air is supplied through the ceiling-mounted air diffusers and they are uniformly distributed across the ceiling.  [c.496]

When it is necessary to confine an air volume from the ambient environment and simultaneously have access for operators or machinery, plane air jets offer a possible and simple solution. Air jets (plane and round) are described in Chapter 7. This section describes plane air jets combined with exhaust openings. In principle, they are similar to the air jets described in Chapter 7 and Section 10.3, but the combination with an exhaust opening makes it necessary to consider the influence of the exhaust on the jet. Usually these curtains are used in large doors to shield the interior from the exterior when the door is open. For example, experimental results have shown that from the moment a door is opened, a short time interval, less than 1 minute, is sufficient to get complete development of the airflow through the door. An air curtain allows a reduction of the overall flow through the door. The principles and use of air curtains are described in many textbooks.Some basics of air curtains are described here.  [c.936]

As can be seen from this table, the MESG values for a specific substance are quite often different depending on the source, due to the use of different experimental apparatus. The most notable difference is in the case of acetylene, whose USCG value is more than an order of magnitude smaller that that listed for the British or NEPA 497 value. The MESG values cited in the USCG Regulations for Marine Vapor Control Systems (33 CER Part 154, Subpart E) are primarily taken from lEC Standard 79-lA (1982). The table also shows that the AIT is not the only factor governing MESG differences between the Westerberg and European test apparatus. Chemicals such as hydrogen, carbon monoxide, vinyl chloride, and epoxides give smaller MESG values in the Westerberg apparatus despite their high AITs. All of these have unusually wide flammable ranges, implying a fast rate of combustion over a wide range of fuel concentrations. The flammable range is another gas sensitivity parameter that might be considered when attempting to identify gases whose Westerberg MESG values are significantly lower than the European MESG values. Since hot gas exiting a DDA approximates to a back-mixed jet with minimal entrainment at its base, neither type of test properly simulates DDA operation. However, owing to the greater confinement produced in its receptor chamber, the Westerberg apparatus should be better able to resolve gas sensitivity differences with respect to DDA performance.  [c.104]

Several experiments with ethylene and hydrogen investigated the effects of jet ignition on flame propagation in an unconfined cloud, or on flame propagation in a cloud held between two or more walls (Figure 4.11). Such investigations were reported by Schildknecht and Geiger (1982), Schildknecht et al. (1984), Stock and Geiger (1984), and Schildknecht (1984). The jet was generated in a 0.5 x 0.5 x 1-m box provided with turbulence generators for enhancing internal flame speed. Maximum overpressures of 1.3 bar were observed following jet ignition of an ethylene-air cloud contained on three sides by a plastic bag. In a channel confined on three sides, maximum pressures reached 3.8 bar in ethylene-air mixtures. A transition to detonation occurred in hydrogen-air mixtures.  [c.86]

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]

This is often fixed for the type of atomizer. Stability is either one of two basic principles bluff body with some aeration/cooling, and swirler types which are generally confined to twin fluid atomizers of large size. The stabilization process is achieved in both cases by recirculation of vortices spilling off the baffle in the case of bluff bodies and by a full recirculation flow pattern in swirl stabilizers. Important items also fitted to the register are ignition system (most commonly high-voltage spark in the case of pressure-jet burners and gas/electric in all other types) and the flame-supervision system. This is normally infrared for oil burners and ultraviolet light sensitive in the case of gas and dual-fuel burners. Smaller gas burners utilize the flame-rectification principle.  [c.377]

Characteristics of the air jet in the room might be influenced by reverse flows, created by the jet entraining the ambient air. This air jet is called a confined jet. If the temperature of the supplied air is equal to the temperature of the ambient room air, the jet is an isothermal jet. A jet with an initial temperature different from the temperature of the ambient air is called a nonisother-mal jet. The air temperature differential between supplied and ambient room air generates buoyancy forces in the jet, affecting the trajectory of the jet, the location at which the jet attaches and separates from the ceiling/floor, and the throw of the jet. The significance of these effects depends on the relative strength of the thermal buoyancy and inertial forces (characterized by the Archimedes number).  [c.446]

FIGURE 7.38 Schematic of air jet in confined space proposed by N. N. Sadovskaya. Reproduced from Grimitlyn.  [c.480]

The investigations of horizontal and inclined air jet trajectory, velocity, and temperature decay under buoyancy discussed in the previous section were conducted with free (nonconfined) jets. Only limited research data is available describing the behavior of inclined jets in confined spaces. Studies by Regenscheit of horizontal cooled air supply from linear and rectangular openings can be related to this topic. Graphs in Fig. 7.47 show how the relative distance Xq/L from the supply opening to the point of jet impingement with the floor surface is influenced by the modified Archimedes number Ar  [c.491]

Air supplied in confined space by downward vertical jets creates a similar flow pattern as in the case of air supply by horizontal nonattached jets. With vertical air supply, the occupied zone is ventilated directly by air jets. Grimitlyn suggests that the area of occupied zone ventilated by one jet be sized based on the jet s cross-sectional area at the point it enters the occupied zone. The jet cross-sectional area and configuration depend upon the height of the air supply, the type of air jet, and diffuser characteristics ( K, and K, ).  [c.494]

A typical electron energy-loss spectrometer is shown in figure Bl.6.5. The major components are an electron source, a premonocln-omator, a target, an analyser and an electron detector. For gaseous samples, the target may be a gas jet or the target may be a gas confined in a cell with small apertures for the incident beam and for the scattered electrons. The target may be a thin film to be viewed m transmission or a solid surface to be viewed in reflection. The analyser may be rotatable about the scattering centre so the angularly differential scattering cross section can be measured. Most often the detector is an electron multiplier that pennits scattered electrons to be counted and facilitates digital processing of the scattered-electron spectrum. In low-resolution instruments, the scattered-electron intensity may be sufficient to be measured with a sensitive electrometer as an electron current captured in a Faraday cup .  [c.1313]

The third example illustrates the capabiUty of the k-Z model in simulating confined swirling flows having internal circulation 2ones, which are encountered in many process appHcations where intense mixing is needed. Figure 19a shows the configuration of a confined double concentric jet which expands suddenly into a cylindrical chamber (26). The simulation swid number (dimensionless ratio of angular momentum to linear momentum) and  [c.104]

Until 1992, tokamak experiments were performed using deuterium or hydrogen only. The use of radioactive tritium gready compHcates the operation of experimental faciUties, impeding the pace of research. Certain experiments, however, such as those directiy involving D—T fusion, cannot be done without the use of tritium. A European research team in 1992 produced neady 2 million watts of fusion power for about one second in the JET device, and opened the modem frontier of D—T fusion experiments (13). Only about half of the JET fusion energy release came from fusion in the thermal plasma, at temperatures of 15—20 keV. The other half came from fusion of the injected tritium beams striking the deuterium in the plasma. The ratio of tritium to deuterium was about 2% in JET. If a 50 50 mixture of tritium and deuterium had been used instead, an amount of fusion energy would have been released roughly equal to the energy required to heat and sustain the plasma, giving an energy gain, Q, of about unity. In December 1993, scientists at the Princeton Plasma Physics Laboratory initiated a series of experiments on the Tokamak Eusion Test Reactor (TETR), introducing D—T fuel into the machine and producing over 6 MW of fusion power. Eor the first time in a tokamak experiment an approximately 50 50 mixture of deuterium and tritium was used as the fusion fuel. Preliminary analysis of the first 100 experimental mns indicated that the confinement in a D—T fuel mixture was better than in a pure deuterium plasma, the ion and electron temperatures were higher, and the plasma stored energy longer. No enhanced loss of alpha particles (the product of D—T fusion reactions) was observed as the fusion power was increased. These results are encouraging for tokamak-based power generation.  [c.154]

Current mixing-type air distribution methods typically consider occupied zone ventilation with jets intercepting its upper boundary. These methods include air supply with vertical jets through ceiling-mounted air diffusers and air supply with inclined jets. They also include air supply with vertical upward-directed jets or horizontal jets along room surfaces. In the latter case, the jet reaches the opposite wall/ceiling and follows room surfaces until it reaches the occupied zone (Fig. 7.35). If the combination of room sizes (height, length, and width) allows such an airflow pattern, this rottm is considered to be short. The room in which air jets dissolve before reaching the opposite wall is considered to Ire long. In such rooms, the occupied zone is ventilated by reverse flow. Initially, studies of jets in confined spaces were carried out for mining, chemical, and mechanical engineering applications. 32,S4 current chap-  [c.476]

The effect of supply air temperature on jet behavior in confined spaces was studied by Miillejans. Studies of cooled air jets were conducted in rooms with a size from 1.0 m x 1.0 m x 1.6 m to 2.27 m x 3.33 m x 5.31 m with an air supply through the slot (b = or rectangular opening (h B. . Numerous smoke photographs were taken reflecting supply situa-  [c.488]

Steam jet systems. These are used to smotlier some fires in closed containers or in confined spaces.  [c.221]


See pages that mention the term Confined air jet : [c.511]    [c.479]    [c.488]    [c.493]   
Industrial ventilation design guidebook (2001) -- [ c.446 ]