Control and optimisation of mixing and combustion from a precessing jet nozzle.

Abstract

The present study seeks to examine the effects of co-flow, confinement and a shaping jet on the mixing and combustion characteristics of a precessing jet flow. In particular, scientific analysis is used to investigate the physical mechanisms by which the control and optimisation of heat transfer and pollutant emissions from natural gas burners for rotary kilns can be achieved. To achieve these aims, a range of experimental techniques in reacting, nonreacting, confined and unconfined conditions have been employed. The precessing jet, in conjunction with a shaping jet, is shown to provide continuous control of mixing characteristics and corresponding combustion characteristics. Hence the optimum mixing characteristics for the maximum heat transfer and minimum emissions and the conditions under which the precessing jet nozzle produces such mixing characteristics are determined. A scaling procedure is also proposed for the precessing jet nozzle that, for the first time, provides a method to relate the results of small-scale isothermal mixing experiments to operating rotary kilns. Flow visualisation using a two colour planar laser-induced fluorescence technique in an unconfined, isothermal environment is used to demonstrate that a central axial jet is the most effective form of shaping jet for controlling the mixing from a precessing jet nozzle. The characteristics of the combined jet flow are shown, by a semi-quantitative image processing technique, to be controlled by the ratio of the central axial jet momentum to the combined jet momentum, denoted by Γ [subscript]CAJ =superscript]G [subscript]CAJ/(G[subscript]PJ+[superscript]G [subscript]CAJ). The flow visualisation results also demonstrate that, when the momentum ratio is in the range 0≤ Γ [subscript]CAJ ≤0.2, corresponding to low proportions of flow through the central axial jet, the combined flow field visually appears to be “precessing jet dominated”. For momentum ratios in the range 0.23< Γ [subscript]CAJ ≤1, the flow appears visually to be dominated by the features of the central axial jet. The effect of a central axial jet on the characteristics of a precessing jet flame is assessed in an unconfined environment by recording the visible flame luminescence photographically. The results demonstrate that a significant change in the flame volume, length and width is achieved by varying the proportion of central axial jet to total flow rate and hence the momentum ratio, Γ [subscript]CAJ. These parameters were correlated with changes in the global residence time, radiant fraction and NOx emissions based on scaling criteria from the literature. These correlations suggest that, consistent with the flow visualisation results, the momentum ratio, Γ [subscript]CAJ, controls the combustion characteristics, which in turn change significantly in the precessing jet and central axial jet dominated flow regimes. Confined combustion experiments are undertaken in a pilot-scale cement kiln simulator to quantify the heat flux and NOₓ emission characteristics as a function of the combined precessing jet and central axial jet flows and to compare them with that of a conventional burner in a well controlled, confined facility. These experiments demonstrate that the central axial jet provides good control over the heat flux profile, consistent with the experience in industrial installations. Furthermore, the heat transfer from a precessing jet burner is shown to be enhanced relative to a conventional burner and the NOₓ emissions and process interaction is taken into account. To quantify the mixing characteristics of each of the above flows and so to provide insight into the characteristics of relatively “good” and “bad” mixing for the optimisation of combustion in rotary kilns, concentration measurements are performed in a confined, isothermal environment. The effect of co-flow, confinement and the central axial jet on the mixing from a precessing jet nozzle are also assessed. The experiments are performed in a water-tunnel using a quantitative planar laser-induced fluorescence technique to provide measurement of a conserved scalar. The effect of the central axial jet is quantified with respect to its influence upon concentration decay, concentration fluctuations, jet width and probability distribution functions. The effect of co-flow and confinement are also quantified by measurement of the concentration decay, concentration fluctuations, jet width and probability distribution functions. The data is used to develop equations relating the flow conditions and geometry to the mean concentration on the jet axis and jet spread. These equations can be used to describe the entire mean concentration distribution in the far field of the precessing jet flow. Based on the modelling equations a scaling procedure is proposed that provides a method to scale the precessing jet flow, i.e. to relate isothermal laboratory scale investigations to full scale plant. The scaling procedure is based on a first order assessment of the separate effects of confinement, velocity ratio and mass flow ratio on the scalar mixing. The final scaling parameter represents an additional correction to a modified form of the well known Thring-Newby scaling criteria which distorts the mixture fraction ratio, i.e. the air-fuel ratio, in the model from that in the industrial scale. This correction enables similarity of the jet mixing characteristics to be preserved while correcting for the geometric distortion of the confinement ratio. The new scaling procedure is used to show that the isothermal concentration measurements are representative of the mixing conditions within the pilot-scale combustion facility and hence that the scaling procedure is appropriate for the precessing jet nozzle. The optimum combustion characteristics of the precessing jet nozzle, defined as the maximum heat transfer and minimum NOₓ emissions, are shown to occur at the maximum momentum ratio that still generates a flow characterised as precessing jet dominated. The mixing characteristics associated with high radiation and low NOₓ emissions are shown, by the quantitative mixing experiments, to be associated with the maximum mean concentration and the widest range of instantanteous concentrations measured on the jet axis of any flows produced by the combined precessing jet and central axial jet flows. This suggest that such mixing characteristics are desired from any natural gas burner for the maximum heat transfer and minimum emissions in a rotary kiln. The optimal mixing characteristics for the maximum efficieny and lowest emissions from a gas-fired rotary kiln are hence shown to be generated by the precessing jet-central axial jet nozzle at a momentum ratio of 0.17≤ Γ [subscript]CAJ ≤0.23.