Parallel jets

Air circulation with a parallel jet supply is illustrated in Fig. 7.45. Jets are located at distance t from each other, and each jet forms return flow similar to that induced by a single jet in the room with a width B = t. Thus, in the case of M parallel jets, the room should be considered divided into several zones with a width B = B /N, separated from each other by airtight walls. 

FIGURE 7A4 Schemes of room ventilation with parallel jet supplied from the same wall (o), from opposite walls (b), and in a fan-type manner (c). Reproduced from Baharev and Troyanovsky. ... 

FIGURE 7.45 Room ventilation by parallel jets. Reproduced from Regenschelt. (1975)... 

Air velocity at any point (X, Y,Z) of the flow created by interaction of two parallel jets supplied from outlets located at a distance 2a from each other (Fig. 7.50) can be described by... 

For the most common practical situation, when air is supplied by parallel jets from several diffusers placed in one plane and having the same outlet area Aq and discharge velocity Vq, the resulting velocity on the axis of the coalesced flow V.. can be found ... 

There are only a few studies of air jets supplied at some angle a (0° < a < 90°) to each other. To predict characteristics (trajectory, velocity decay, etc.) of the flow resulting from interaction of two jets supplied at some angle toward each other, Hudenko proposed to sum momentums of interact ing jets as in the case with parallel jets. He has estimated that the error of predic tion will be smaller at a smaller... 

If a particularly parallel beam is required in the chamber into which it is flowing the beam may be skimmed in the region of hydrodynamic flow. A skimmer is a collimator which is specially constructed in order to avoid shockwaves travelling back into the gas and increasing 7). The gas that has been skimmed away may be pumped off in a separate vacuum chamber. Further collimation may be carried out in the region of molecular flow and a so-called supersonic beam results. When a skimmer is not used, a supersonic jet results this may or may not be collimated. 

In a skimmed supersonic jet, the parallel nature of the resulting beam opens up the possibility of observing spectra with sub-Doppler resolution in which the line width due to Doppler broadening (see Section 2.3.4) is reduced. This is achieved by observing the specttum in a direction perpendicular to that of the beam. The molecules in the beam have zero velocity in the direction of observation and the Doppler broadening is reduced substantially. Fluorescence excitation spectra can be obtained with sub-Doppler rotational line widths by directing the laser perpendicular to the beam. The Doppler broadening is not removed completely because both the laser beam and the supersonic beam are not quite parallel. 

It has been experimentally verified that initially jets spread roughly linearly in the similarity region, ie, (Sa x jet. For both axially symmetric and plane parallel (slot) jets, the jet width parameter, 5, is given by... 

The crystal stmcture of PPT is pseudo-orthorhombic (essentially monoclinic) with a = 0.785/nm b = 0.515/nm c (fiber axis) = 1.28/nm and d = 90°. The molecules are arranged in parallel hydrogen-bonded sheets. There are two chains in a unit cell and the theoretical crystal density is 1.48 g/cm. The observed fiber density is 1.45 g/cm. An interesting property of the dry jet-wet spun fibers is the lateral crystalline order. Based on electron microscopy studies of peeled sections of Kevlar-49, the supramolecular stmcture consists of radially oriented crystaUites. The fiber contains a pleated stmcture along the fiber axis, with a periodicity of 500—600 nm. 

Fig. 2. Parallel arrangement for (a) explosion cladding and subsequent collision between the prime and backer metals that leads to (b) jetting and formation...

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. 

Linear jet attachment to a plane not parallel to the supply direction was studied by Katz. The critical angle, 9, of the plane to the jet supply direction, as indicated in Fig. 7.33, was found to be dependent on the supply velocity (Reynolds number). It also depends on the distance of the plane edge from the supply outlet (see Fig. 7.34). 

Air can be supplied in rooms by one or several jets. Air supply openings can be located along one wall—parallel air jet supply (Fig. 7.44a)—and/or on opposite walls—contrary-directed jets supply (Fig. 7.44f>). In special cases air can be supplied in a fan-type manner (Fig. 7.44e). 

Kuzmina, L. V. 1968. On the Interaction of Parallel Supply Jets Collected Papers of the Occupational Safety Institutes under the VTsPS. Profizdat, Moscow. 

This section deals mainly with side push-pull ventilation. Center push-pull ventilation is also sometimes used, where two jets of air are blown from a central pipe towards two parallel exhaust hoods at opposite ends of the tank. Much of what vve say about side push-pull systems is equally valid to center push-pull. 

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. 

The maximum overpressure appeared to rise substantially when the jet was partially confined between 2-m-high parallel walls and obstructed by some 0.5-m-diameter obstacles. 

The most widely used amperometric detectors are based on the thin-layer and wall-jet configurations (Figure 3-22). The thin-layer cell relies on a thin layer of solution that flows parallel to the planar electrode surface, which is imbedded in a... 

Tubular Planar (parallel flow) Thin-layer cell Planar (perpendicular) Wall-jet detector 1= 1.61 nFC(DA/r)2/3U123 i = 0.68 nFCD2l3v- 6(A/b)V2UV1 i = 1.47 nFC(DA/b)2/3U123 i = 0.903nFCD2f3v- 6A3/4u 2 i = 0. mnFCD2/3v-5/ua-V2A3/tU3/4... 

Figure 2.40 shows the unsteady flow upstream of the ONE in one of the parallel micro-channels of d = 130 pm at = 228kW/m, m = 0.044 g/s (Hetsroni et al. 2001b). In this part of the micro-channel single-phase water flow was mainly observed. Clusters of water appeared as a jet, penetrating the bulk of the water (Fig. 2.40a). The vapor jet moved in the upstream direction, and the space that it occupied increased (Fig. 2.40b). In Fig. 2.40a,b the flow moved from bottom to top. These pictures were obtained at the same part of the micro-channel but not simultaneously. The time interval between events shown in Fig. 2.40a and Fig. 2.40b is 0.055 s. As a result, the vapor accumulated in the inlet plenum and led to increased inlet temperature and to increased temperature and pressure fluctuations.

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