Studies on the flowfield of multijet with square configuration

Abstract

MEASUREMENTS of mean velocity, spread rate, and merging point location were made to investigate the flowfield generated by four identical jets of air in a square configuration, issuing from axisymmetric nozzles in a common end wall and mixing with the ambient air. The effect of stagnation pressure ratios as well as the nozzle spacings on the flowfield behavior were investigated. The effect of nozzle configurations was also studied. The results show that the nature of the mean velocity profile as well as the mean velocity decay are unaffected by the pressure ratio. The nozzle spacing effect on the mean velocity decay is only marginal. The four jets interact with axis switching at the midpoint between them, which implies better mixing. Contents It is well known that the multijet has many advantages over a single jet. The advantages include better mixing and noise reduction.1 Recently, many investigators have used a non circular nozzle2 (triangular, elliptic, etc.) to improve the jet mixing process. The elliptic jets were shown to be capable of entraining large amounts of surrounding fluid relative to that entrained by a circular jet.3 The main advantage for using these geometries is the axis-switching phenomenon. However, the fabrication of such geometrical shapes is somewhat difficult. The circular nozzle fabrication is easier, and an attempt was made to identify the advantage of the multijet (good mixing) and the noncircular jet (axis switching) by keeping the four nozzles in a square configuration, with the arrangement as shown in Fig. 1. The nozzle exit diameter de was 4.2 mm, and the nozzle spacings S were chosen as S = 12, 16, 18, and 22 mm. The nozzles' axes were aligned parallel to the Xaxis of the three-dimensional traversing system. The stagnation pressure was varied from 1.25 to 2.5 atm. The total pressure in the jet was measured by a three-hole pressure probe of 0.3 mm i.d. All measurements were made in the X-Y plane at two positions in the Z direction. The total pressure reading was taken from the axis of the top jet or the symmetric axis between the two base jets to the distance at which the mean velocity became zero in the Y direction. This step was repeated at several positions downstream (up to 5Qde) from the nozzle exit plane. The mean velocity profiles in the X- Y plane of the top jet at x/de = 18.5 is shown in Fig. 2. Earlier measurements4 of single and triple jets also appear on the plot for comparison. The center line mean velocity un decay of the top jet as a function of the stagnation pressure ratio P&/PQ is presented in Fig. 1. As shown, the jet velocity decay as well as the potential core length have a fairly weak dependence on the