Mixing characteristics of transverse jets in an annular supersonic flow

The supersonic combustor with an annular cross section has the potential to enhance fuel/air mixing because the height of an annular flow path is relatively low. However, the efforts that have been dedicated to this topic are exiguous. In this study, numerical simulations were conducted to reveal the fuel/air mixing process in annular flow paths. The Mach number, total temperature, and total pressure of the entrance flow were 2.35, 300 K, and 101.325 kPa, respectively. The height of the annular flow path (H) varied from 7.5 to 12.5 mm, but the equivalent diameter was 37.74 mm for all the cases. In the cases with a jet orifice number (Nj) of 8, an orifice diameter of 0.5 mm, an equivalence ratio of 0.25, and a flow path height of 7.5 mm, the low flow path height significantly enhanced the shock reflection and fuel air mixing in the annular flow path. Therefore, the mixing efficiency in the annular flow path was higher than that of the circular flow path. However, when the equivalence ratio increased to 0.50, the penetration height of the fuel jet was limited by the height of the annular flow path. As a consequence, the mixing efficiency in the circular flow path overpassed the annular flow path. The influence of the annular flow path height (H = 7.5∼12.5 mm) was studied under the condition with an equivalence ratio of 0.5 and Nj = 8. As the flow path height increased from 7.5 to 12.5 mm, the jet penetration height enjoyed a growth but the shock wave reflection was weakened. Consequently, the mixing efficiency in the annular flow path experienced an increase followed by a decrease and reached the maximum at H = 10 mm. The investigation of Nj (from 8 to 32) suggested that the increase in Nj could distribute the fuel jet more evenly along the circumference of the annular flow path but it also reduced the jet penetration height. Therefore, there was an optimum Nj for a certain annular flow path. Under the present condition, the optimum Nj for the case with H = 7.5, 10, and 12.5 mm were 32, 20, and 16, respectively. The optimum mixing efficiencies of these three cases were higher than the circular flow path. Compared to the annular flow path height and orifice number, the impact of the orifice diameter (d = 0.25 to 1.41 mm, H = 10 mm) on mixing efficiency was not significant.