Dilution and Penetration of Vertical Negatively Buoyant Thermal Jets

Abstract

Five series of experiments were performed to study the penetration and dilution properties of vertical negatively buoyant thermal jets (thermal fountains). The flow was turbulent, and the densimetric Froude number F based on the radius of the discharge varied from 4.7 to 24. The experiments were conducted in the laboratory by discharging hot water vertically downward into a colder-water environment that had a temperature greater than 15°C. Under these conditions, the water equation of state was practically linear, and the downward negatively buoyant thermal jet was dynamically similar to an upwards negatively buoyant dense jet of equal densimetric Froude number. The temperature fields associated with the negatively buoyant jets were measured with arrays of fast responding thermocouples and were used to study the jet penetration and dilution properties. Detailed analyses of the temperature data revealed large fluctuations of jet penetration in the vertical direction. The mean and maximum vertical jet penetrations obtained in this study using temperature data were consistent with the results of previous studies based on visual data. In contrast, smaller fluctuations of jet penetration occurred in the horizontal direction, and the maximum horizontal penetration of the return flow, at the level of the source (z=0), was δm=1.40 r0F. This value is about one-half of the mean vertical jet penetration. On the other hand, the minimum dilution of the returning fluid at the source height just outside of the nozzle was μmin=0.58F.