| OCR Text |
Show downstream of the source, when the jet becomes fully developed. In the fully developed region of the jet, velocity and concentration profiles attain a shape which is similar at all subsequent sections. They are said to be self-preserving. The jet at this stage may be described as a fully developed self-preserving system. Provided that the Reynolds numbers are high enough to ensure a turbulent stream in this zone, all free jets with the same cross-section are regarded as similar from both the dynamic and fluid conservation point of view. When a jet is placed in an enclosed space, as is often the case in furnace systems, it undergoes some basic changes. The flow in such cases is constrained, and as a result the pressure gradients prevent the existence of an undisturbed surrounding stream, and velocity is everywhere dependent upon position. The flow pattern is governed by the chamber geometry and the input of mass and momentum. The turbulent jet entrains fluid to satisfy its entrainment capacity. In cases when the entrainment capacity of the jet exceeds the amount of fluid available for entrainment, the phenomenon of recirculation takes place as fluid is caused to flow in a reverse direction around the downstream periphery of the jet to a point nearer the source, where it is then entrained into the jet, figure 2. The enclosed jet, like the free jet, can be divided into three regions. The first part consists of the primary potential core and the discontinuity layer, as in the case of a free jet. However, an additional high shear zone near the jet boundary is initiated by the confinement, and as the flow develops the shear forces increase. By the start of the mixing zone, this enlarged shear layer has consumed the potential core and it continues to grow with the boundary layer until it either merges or all the surrounding fluid is consumed. It is in this region that the recirculation phenomenon occurs. In order to satisfy the jet entrainment capacity, the fluid from downstream is returned to the source, giving rise to the differences between the free and enclosed jets. -6- sj |
