Dilution Of Circular Wall Jet In Crossflow

Abstract

Dilution of turbulent circular wall jets discharging perpendicular to crossflow ambients has not received much attention compared to studies on free turbulent jets in crossflows. An effluent diffuser designed as a circular wall jet is an efficient means of enhancing dilution of waste discharges from both industrial and municipal treatment plants under river conditions. The outfall is located at I or near the bed in order to utilize the whole flow depth in mixing and therefore this increases the efficiency of effluent dilution within the mixing zone. The present study considers the dilution characteristics of a circular wall jet in river-like crossflows and its comparison with the free jet discharging perpendicularly from the bed. The location of the jet takes into account the limited depths available for mixing in most shallow rivers. This thesis presents the results of an experimental study on the dilution produced in circular wall jets discharging perpendicular to a freestream. The deflected wall jets were explored f,~r.dilutions up to 100: l for downstream distance x up to about 200 times the jet nozzle diameter. Dilution characteristics of the jet were studied by first performing a similarity analysis on -the_3-dimensional concentration distributions. Non-dimensional concentration profi.les were found to be similar both in the vertical and the transverse directions. Minimum dilutions were considered along the axial distance of the jet for"the flow regimes defined by the momentum dominated near field, the far filed and the passive .plume regions. For practical purposes, a mean equation was developed to describe the minimum dilution in the mixing region. The correlation was found to be of similar magnitude with the results of the free jets. The growth rate of the jet, both in the vertical and the transverse directions was also investigated. The results indicated that the lateral growth of the deflected jet is about twice that in the vertical direction. This unique characteristic of wall jets takes advantage of the available mixing space since most rivers have widths several times larger than the flow depth, and consequently increase the effluent dilution. Effect of the flow depths on the tracer dilution was also studied. The results indicated significant reduction in tracer dilution for free stream flow depths less than 10 times the jet nozzle diameter. This problem prevails for effluent discharged into shallow rivers due to early surfacing.