Stability and mixing of submerged turbulent jets at low Reynolds numbers

Abstract

An experimental study is made of the variation of volume and centerline dilution as a function of Reynolds number in non- buoyant and buoyant round jets discharged vertically from a submerged nozzle. The jet Reynolds numbers covered the laminar- turbulent transition with values ranging from Re = u D/v = 100 to 20,000 where u = jet exit velocity, D = jet diameter, and V = kinematic viscosity. Measurements of jet temperature profiles are obtained by using both fast and slow thermistor probes. Turbulent dilution is found to be independent of Reynolds number for non-buoyant jets above a critical Reynolds number of about 1,500. For buoyant jets (densimetric Froude numbers in the range 25 to 50), the critical Reynolds number is about 1,200. Reasonable agreement is obtained with the results of previous investigators for dilution values at high Reynolds numbers. Dye studies of transition Reynolds numbers are compared with a study by A.F. Pearce (1966) and good agreement is found. The results are useful in determining the minimum length scale ratio for hydro-thermal model studies, especially those of submerged multiport diffusers. It is concluded that modeling of turbulent jets is acceptable provided the model Reynolds number is larger than the critical Reynolds number and provided no other constraint becomes binding. In addition, the model jet's laminar length, if any, must be insignificant when compared to the total length of the path of the jet.