Phase averaged velocity field within a fluidic processing jet nozzle

A nozzle consisting of a round inlet orifice expanding suddenly into a short axisymmetric chamber is the basis of an industrial burner which produces lower levels of NOx pollution than other, more conventional burners. The jet flow from the inlet orifice reattaches asymmetrically and precesses around the wall of the chamber. Phase-averaged measurements of axial velocity component in the chamber have shown that the length of the potential core of the inlet flow is about half that of an unconfined turbulent jet and the velocity decay rate is about twice as large. Entrainment by the reattaching flow induces a reverse flow in the nozzle. The measurements indicate that this reverse flow has a speed in the order of 30% of the forward flow speed. Work by other researchers on the effect of counter-flow on jets suggests that interaction between the reverse flow and the reattaching forward flow is largely responsible for the high spreading and decay rates of the latter. Introduction The Fluidic Precessing Jet (FPJ) nozzle (Figure 1) consists of a cylindrical chamber with a small axisymmetric inlet at one end and an exit lip at the other. The inlet flow separates at the abrupt inlet expansion and reattaches asymmetrically to the wall of the chamber [8]. The reattaching flow from the inlet precesses around the inside wall of the chamber and thus produces a precessing exit flow. The lip and large transverse pressure gradients near the outlet together steer the exit flow through a large angle, towards the axis and across the face of the nozzle outlet. In the cement and lime industries, the FPJ nozzle has demonstrated significant reduction in NOx emissions and improvement in product quality [4, 3]. These benefits are a result of differences between the combustion in the flow field of traditional axial-jet burners and the combustion produced by the FPJ [9] — which in turn depends on the differences in the turbulentjet flow field. Since the discovery of the FPJ by Nathan [7] and its first application as an industrial burner, a significant research effort has been directed at optimising the geometry of the FPJ nozzle and at understanding the FPJ flow. The early research on the FPJ consisted of flow visualisation studies, combustion trials and experiments with pressure transducers and uncalibrated hot-wire inlet flow reverse induced air flow