The effect of density on the near field of a naturally occurring oscillating jet.

Abstract

A major component of the world’s ever increasing energy demand is supplied by combustion. Despite concerns of the enhanced greenhouse effect, primarily due to the emission of CO₂, fossil fuels will remain a major energy source for the foreseeable future. One approach to help to combat the enhanced greenhouse effects of combustion is to design highly efficient burners that achieve low levels of pollution. The fluidic precessing jet (FPJ) and the related triangular oscillating jet (TOJ) burners have shown such benefits when used in the cement and lime industry. As a result, they have been studied at the University of Adelaide for many years. Despite these investigations there are still significant gaps in the understanding of how they work. Addressing these gaps will allow their design to be improved. This work focuses on improving the understanding of the TOJ, and also provides insight into the understanding of the FPJ. The benefits that can be provided by the FPJ and TOJ nozzles include fuel savings of up to 10% and NOₓ reduction between 40-70%. This is due to the flows they produce. These flows are unsteady, creating large scale unique eddies that alter the mixing of the fuel and air, and hence the combustion. Many nozzle parameters, such as the nozzle expansion ratio and chamber length to diameter ratio, influence the nature of these unsteady flows. The influence of such parameters is well understood when the density ratio between the nozzle fluid and the ambient is unity. However, no previous investigations of the effect of density ratio on FPJ or TOJ flows have been performed. Density ratio has been previously shown to alter mixing in simple jets, and will therefore also affect the mixing of an unsteady flow. Therefore an understanding of how the jet-toambient fluid density ratio affects the flow from the TOJ is required to further our knowledge and improve its design. To gain an understanding of the effects of density ratio, the TOJ nozzle has been investigated under cold flow conditions over a broad range of density ratios. Particle image velocimetry (PIV) and oscillation frequency data have been collected to assess any density ratio effects on the near field of the flow emerging from the TOJ nozzle. Along with the oscillation frequency, key flow parameters measured were the mean jet spread, the mean jet decay and the instantaneous jet deflection angle. The role of density ratio (jet fluid/ambient fluid), and its relative influence is assessed with the nozzle chamber length fixed. The effect of density ratio is also investigated with the chamber length as a variable and in a more industrially relevant configuration, in which a co-annular flow surrounds the TOJ flow. Although the sensitivity to density ratio is less significant when the density ratio is greater than unity, it was found that increasing the density ratio leads to an increase in the mean spread, decay rate and the instantaneous jet deflection angle, and a decrease in the frequency of oscillation. At any given density ratio, increasing the nozzle chamber length within the investigated range resulted in an increase in the mean spread, decay rate and instantaneous jet deflection angle as well as an increase in the frequency of oscillation. While no measurements of the flow were taken within the nozzle chamber, frequency measurements suggest a decrease in the density ratio is analogous to an increase in the chamber length with respect to the influence on the internal flow. The results from this study shed new light on the flow in the near field region of the TOJ nozzle. The knowledge gained will allow future designs for industrial use to be better tailored to use in rotary kilns, and contribute to improved efficiency and reduced emissions.