Effects of noncircular collars on an axisymmetric jet

Abstract

An experimental investigation was carried out to study the effects of noncircular collars on an axisymmetric jet using surface flow visualization and hot-wire anemometry. Circular, square, and triangular collars with expansion ratios of 1.20, 1.35, and 1.54, respectively, with collar lengths of up to two jet diameters were used. Flow visualization shows that circular collars led to equidistant flow reattachments along the collar wall, while square and triangular collars resulted in the formation of a pair of counter-rotating vortex-pairs on each side of the collar wall. These vortex-pairs are caused by the presence of the three-dimensional velocity gradients between locations of minimum and maximum step-heights, which drove fluid from the collar wall centerlines towards the corners. Time-averaged velocity measurements show that the circular collar required the shortest collar length to achieve maximum centerline velocity decay, followed by square and triangular collars. Centerline turbulence intensity and velocity spectra results reveal that all three collars were able to suppress vortex-pairing events when they were sufficiently long with the triangular collar being the most effective. Furthermore, the triangular collar also produces the widest overall jet-spread, ahead of square and circular collars, respectively, even though it demonstrates significantly different jet-spreads along planes of minimum and maximum step-height. Self-excitation frequencies from all three collars could be distinguished into distinct frequency bands with changes in the collar lengths. Within each band, the circular collar results in the largest variation of excitation frequency over the square and triangular collars, respectively. Lastly, square and triangular collars require wider ranges of collar lengths for each of the frequency bands, as compared to the circular collar.

Free jet issuing from a pipe with a triangular collar: A numerical simulation

“…The physical problem consists of a turbulent jet of air issuing from a pipe with a triangular collar and discharging into still air, as prescribed in New et al [19]. The circular nozzle and the equilateral triangular collar are shown schematically in Figure 1(a).…”

“…It was reported that these vortex pairs can have a significant impact on jet mixing [5]. Another recently proposed passive method for increasing jet mixing and spreading uses a collar, which is a sudden expansion of the nozzle [1,[16][17][18][19]. Husain et al [16] studied an elliptic whistler (i.e.…”

“…They also reported that the increased turbulence intensities persisted even in the far-field region. New et al [19] examined collared jets with relatively small expansion-ratios. They studied the influence of non-circular collars on an axisymmetric jet.…”

“…Therefore, in the present work, the main characteristics (e.g. jet decay and spreading) of a turbulent jet issuing from a pipe with a triangular collar measured experimentally by New et al [19] is simulated numerically. Results are reported only for the standard Reynolds stress model (RSM) and the standard k −ε model, as a previous study by the present authors revealed that the RSM produced the best predictions among the RANS models for simulating a turbulent jet issuing from an equilateral triangular orifice [2].…”

Experimental Investigation on Jets issuing from Angular Cut Nozzles

“…Although there is a reduction in efficiency jet mixing plays a prominent role in combustion, noise reduction and also serves in many other applications. The researches [1][2][3][4][5][6] even carried out in the bevel nozzles and notched circular nozzles in which effective jet mixing has taken place. They also specified that the mixing properties also depend upon the aspect ratio of the nozzle.…”