Development of a Turbulent Jet

It was mentioned earlier that the sources of fluid flow in rotary kilns are the primary air, secondary air, combustion products, and infiltration air. Usually the combustion system is such that the primary air issues from a burner nozzle as a jet into an open tube or into a tube surrounded with secondary air prior to combustion. The primary air nozzle tip velocities can be anywhere between 20m/s and 100 m/s depending on the firing rate. The secondary air into which the primary air is discharged is at a relatively lower velocity, typically in the range of 5-15 m/s (Table 3.1). 

For round jets, particularly pertinent to primary air discharge in rotary kilns, Ricou and Spalding derived the following equation. 

Equation 3.32 is applicable to all values of Reynolds numbers at the nozzle greater than 2.5 x 10, and holds only for x/dg 6. At shorter distances from the nozzle Ricou and Spalding expect that entrainment mjtho — 1) proceeds at a lower rate of travel along the jet axis and increases progressively until it stabilizes at a constant value. 

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