Round jets

Fig. 18. Jet trajectory of a round jet in bounded cross flow where J = Pj V j p (a) flow geometry, ratio of height of tunnel to diameter of injection tube HID) = 12 and (b) flow streamlines where the data points are experimental deterrninations and the lines correspond to calculated predictions for (—)...

For the circular jet, Regenscheit obtained empirical equations ior the maximum velocity in the downward and upward vertical jets of heated and cooled air. For a compact (round) jet,... 

I he round jet is most common in general ventilation and is used in local ventilation for spot cooling, for cleaning surfaces, or to direct air and contaminants in specific direction, e.g., into a large canopy hood or away from a contaminant generation point. The radial jet is not used much in local ventilation. 

The theory for plane jets is similar to descriptions of circular jets (see Section 7.4) and many derived equations describe both two-dimensional (plane) and three-dimensional (round) jets. The principle is to generate such high air velocity that a shield against pressure difference, temperature difference, and wind velocity is sustained. Howeveg it is not possible to have complete separation by an air curtain. The main reason for this, is that the jet entrains air... 

S. Ghosal and L. Vervisch, Stability diagram for lift-off and blowout of a round jet laminar diffusion flame. Combust. Flame 124 646-655,2001. 

Normal Pulsating Axisymmetric Rayleigh-type Sheet forms a round jet No breakup WfN <15... 

Linear-eddy modelling of turbulent transport. Part 3. Mixing and differential diffusion in round jets. Journal of Fluid Mechanics 216, 411 —4-35. 

Liepmann, D., and M. Gharib. 1992. The role of streamwise vorticity in the near-held entrainment of round jets. J. Fluid Mechanics 245 643-68. 

The jet is directed along the x-axis, u is the velocity, P1 is the momentum of the jet per unit length of the slit, and P2 is the full momentum of the round jet. The functions f1 and /2 are found by integration of the ordinary differential equations. The result is in satisfactory agreement with experiment. 

In all the above derivations in this section, the influence of viscosity is neglected so that analytical solutions for velocity and pressure profiles can be obtained. When the viscosity of fluid is taken into account, it is difficult to obtain any analytical solution. Kuts and Dolgushev [35] solved numerically the flow field in the impingement of two axial round jets of a viscous impressible liquid ejected at the same velocity from conduits with the same diameter and located very close to each other. The mathematical formulation incorporated the complete Navier-Stokes equations transformed into stream and velocity functions in cylindrical coordinates r and z, with the assumption that the velocity profiles at the entrance and the exit of the conduit were parabolic. The continuity equation is given by Eq. (1.22) and the equations for motion in dimensionless form are ... 

Popiel, C. O. and Trass, O. (1991). Visualization of a free impinging round jet. Experimental Thermal and Fluid Science, (4) 253-264. 

Marple and Rubow (1986) state that the jet Reynolds number in an impactor can be expressed in terms of the air mass flow rate m. Show that for a round jet impactor Re = mW/p, and for a rectangular jet impactor Re = 2mW/p. Marple and Rubow point out that this is a useful form for the Reynolds number since m is constant for all stages of the impactor. 

Example 7.4 A round jet impactor is operated such that the jet Reynolds number is 3000. Using Fig. 7.36, find the particle diameter (unit-density sphere) that will be collected with 50 percent efficiency if the jet diameter is 0.3 cm. 

In the first stage of the Lundgren impactor, air issues at a flow of 85 L/min through a number of round jets 0.82 cm in diameter at a Reynolds number of 3700. 

Standard k-s The most widely used model, it is robust, economical, and time tested. The Reynolds stresses are not calculated directly, but are modeled in a simplified way by adding a so-called turbulent viscosity to the molecular viscosity. Its main advantages are a rapid, stable calculation, and reasonable results for many flows, especially those with a high Reynolds number. It is not recommended for highly swirling flows, round jets, or flows with strong flow separation... 

RNG k-E A modified version of the ks model, this model yields improved results for swirling flows and flow separation. It is not well suited for round jets and is not as stable as the standard k-s model... 

Realizable k-s Another modified version of the ks model, the realizable ks model correctly predicts the flow in round jets and is also well suited for swirling flows and flows involving separation... 

Figure 6.9 Schematic diagram of jet impactor efficiency showing Stk corresponding to 50 impaction efficiency. For round jets, the lower tail of the cfticicncy curve may not exist (Miirpk and Liu, 1974),...

A round jet impacior is to be designed to sample a flowing aerosol. For a maximum jet velocity of 200 m/sec and a flow rate of 1 liter/min, determine the minimum particle size (aerodynamic diameter) that can be collected. Assume that the pressure throughout the impactor is approximately atmospheric and the temperature is 20 0. Be sure to account for the slip correetton factor (Chapter 2). 

The simplest example of a free furbulenf round jet is provided by a nonreacting (isothermal) flow stream expanded at high velocity from a cylindrical nozzle of diameter d. It is assumed that the turbulent round jet flows into a quiescent ambient of the same fluid, as shown in Figure 31.8. 

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