Isothermal Free Jet

For different types of free jets and air diffusers there are similarities in the resulting flows. Four major zones are recognized along a free jet. These zones, as described by Tuve,- may be roughly defined in terms of the maximum or center core velocity that exists at the jet cross-section being considered (see Fig. 7.20a). 

Zone 1 is a short zone, extending about two to six diffuser diameters (for compact and radial jets) or slot widths (for linear jets) from the diffuser face. In this zone, the centerline velocity of the jet remains nearly equal to the original supply velocity throughout its length. 

Zone 2 is a transition zone, and its length depends upon the diffuser type. For a compact jet the transition zone typically extends to eight or ten diameters from the outlet. Within this zone, the maximum velocity may vary inversely with the square root of the distance from the outlet. Some researchers 3-5 suggest use of a simplified scheme of the jet (Fig. 7.20b) with a transition cross-section for practical purposes. 

Zone 3 is the zone of fully established turbulent flow. It has major engineering importance, since it is usually in this zone that the jet enters the occupied region. The length of this zone depends on the air jet shape, the type and size of supply air diffuser, the initial velocity of the air jet, and the turbulence characteristics of the ambient air. 

Zone 4 is a terminal zone in which the residual velocity decays quickly into large-scale turbulence. Within a few diameters, the maximum velocity subsides to below 0.25 m/s. Though this zone has been studied by several researchers, its characteristics are still not well understood. 

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