In this paper different flow patterns of an annular jet with a stepped-conical nozzle as well as the transition between these patterns are numerically investigated as a function of the swirl number S which is the ratio of tangential momentum flux to axial momentum flux. The Reynolds number of the jet based on the axial velocity and the nozzle hydraulic diameter is 180. The 3D Navier Stokes equations are solved using the direct numerical simulation. Four different flow patterns are identified and their associated flow structures are discussed. Starting from an annular jet at zero swirl, spinning vortices around the central axis originate with increasing swirl. As the swirl is further increased, the onset of vortex breakdown occurs, followed by jet attachment to the nozzle. Decreasing the swirl number back from this flow pattern, the Coanda effect near the nozzle outlet creates a wall jet. This wall jet remains till the decreasing swirl number equals to zero, showing hysteresis in flow patterns between an increase and a subsequent decrease in swirl. The determined flow states are experimentally validated. Potential applications related to these flow patterns and their hysteretic behavior are also briefly discussed.
The increase in the LED junction temperature is an important problem since it directly affects the lumen output and the colour characteristics of the LED chips, resulting in low performance in LED-based lighting systems. In this study, commercial computational fluid dynamics software is used with the JEDEC's tworesistor compact thermal model to analyse the temperature distribution in two selected luminaires. It is aimed to show that use of the two-resistor model and measurement combination is a fast and easy way of predicting thermal behaviour of the system and LED junction temperatures instead of trial and error designs and thus ensures a better estimation for the total luminous flux of the luminaire and lowers the costs and time required for the prototyping processes.
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