Jet velocity

For times below about 5 msec a correction must be made to allow for the fact that the surface velocity of the liquid in the nozzle is zero and takes several wavelengths to increase to the jet velocity after emerging from the nozzle. Correction factors have been tabulated [107, 108] see also Ref. 109. 

In a free jet the absence of a pressure gradient makes the momentum flux at any cross section equal to the momentum flux at the inlet, ie, equations 16 and 17 define jet velocity at all points. For a cylindrical jet this leads to a center-line velocity that varies inversely with (x — aig), whereas for slot jets it varies inversely with the square root of (x — Xq As the jet proceeds still further downstream the turbulent entrainment initiated by the jet is gradually subordinated to the turbulence level in the surrounding stream and the jet, as such, disappears. 

J ct Spra.y, The mechanism that controls the breakup of a Hquid jet has been analy2ed by many researchers (22,23). These studies indicate that Hquid jet atomisation can be attributed to various effects such as Hquid—gas aerodynamic interaction, gas- and Hquid-phase turbulence, capillary pinching, gas pressure fluctuation, and disturbances initiated inside the atomiser. In spite of different theories and experimental observations, there is agreement that capillary pinching is the dominant mechanism for low velocity jets. As jet velocity increases, there is some uncertainty as to which effect is most important in causing breakup. 

Bo = slot height Do = circular nozzle opening q = total jet flow at custance x qo = initial jet flow rate / = radius from circular jet centerhne y = transverse distance from slot jet centerline = centerline velocity Vr — circular jet velocity at /... 

Pinczewski and Fell [Trans. Inst. Chem E/ig., 55, 46 (1977)] show that the velocity at which vapor jets onto the tray sets the droplet size, rather than the superficial tray velocity. A maximum superficial velocity formulation that incorporates ( ), the fractional open area, is logical since the fractional open area sets the jet velocity. Stichlmair and Mers-mauu [Int. Chem. Eng., 18(2), 223 (1978)] give such a correlation ... 

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. 

Koestel, A. 1957. Jet velocities from radial flow outlets. ASFIVE Transactions, vol. 6.3, p. 505-526. 

It is quite easy to disturb the flow in a jet either by inserting a small surface at right angles close to the outlet or by using a small air velocity far from the outlet. Usually the latter needs a large flow rate, but a small flow rate with high velocity may also change the direction of the jet, especially as jet velocity decreases. 

If the hood s exhaust flow rate is too small (smaller than the sum of the initial jet flow rate and the induced flow rate) the jet will carry some contaminants into the room. The same condition will occur with a jet flow rate or jet velocity that is too large. 

Two or more plane jets can be placed above and outside the rim (all sides) of a canopy hood and directed downward. Fhe exhaust flow into the hood makes the down-directed jets turn inward and upward when the jet velocity has slowed down enough to be influenced by the exhaust flow. In many cases, the aim is to diminish the general supply airflow rate into the room and sometimes to use the jets as separators. lliis method is quite often used on large kitchen hoods to increase their capture efficiency. If the jet is directed toward the front of the fireplace and just reaches the front before turning inward, a high capture efficiency can be achieved. 

Turbine designers had been experimenting with machines in which the blades were driven by a high-velocity steam jet that expanded through them in a single stage. It had been shown that steam-jet velocities of the order of 1000 m/s were required. 

Figure 7-74. Approximate flame distortion due to lateral wind on jet velocity from flare stack. Reprinted by permission, American Petroleum Institute, API RP-521, Guide for Pressure Reiieving and Depressuring Systems, 3rd Ed., Nov. 1990 [33].

Table 4-42 shows the jet velocity. Table 4-43 shows the diameters and areas of various nozzle sizes. 

Optimal hydraulics is the proper balance of hydraulic parameters (flowrate and equivalent nozzle size) that satisfy chosen criteria of optimization. Hydraulic quantities used to characterize jet bit performance include hydraulic horsepower, jet impact force, jet velocity, Reynolds number at the nozzle, generalized drilling rate or cost per foot drilled. While designing the hydraulic program the limitations due to cuttings transport in the annulus and pump performance characteristics must be included. 

A stationary lifted flame will be stabilized at (xV )/ where Yp=yps, and u = S. The liftoff height Hl=x and the limitation on jet velocity can be derived from Equation 4.3.3 as follows ... 

Note that the RHS of Equation 4.3.5 should be positive, which limits the jet velocity Ug. 

The relevance of nonpremixed edge flames to turbulent nonpremixed flames can be described in two aspects. One is the mechanism of turbulent nonpremixed lifted flames and the other, the flame-hole dynamics. For turbulent lifted flames in nonpremixed jets, the liftoff height is linearly dependent on jet velocity. There have... 

Liftoff height with jet velocity in free jet [10] (H attached flame length, quenching distance, Hp liftoff height, L premixed flame length). 

Correlation between liftoff height and jet velocity with partial premixing of air to fuel stream. (From Lee, B.J., Cha, M.S., and Chung, S.H., Combust. Sci. Technoh, 127, 55,1997.)... 

System temperature and pressure affect the momentum of grid jets via the gas density (see Ch. 2). The momentum of the gas jets is p AC/A. When the temperature is increased, the gas density decreases. For the same gas jet velocity this decreases the momentum of the jets and, therefore, decreases the jet penetration and the attrition at the grid. Similarly, when system pressure is increased, gas density increases, gas jet momentum increases and, therefore, the jet penetration and the attrition at the grid are increased. 

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