Basic studies of multiple free and confined jets

An experimental investigation was carried out to study the mixing characteristics of a moderate aspect ratio rectangular free and confined jet; and free and confined multiple jet. The main parameters were the primary jet pressure ratio (stagnation pressure/ambient pressure) and the ejector mixing duct to width ratio. The pressure ratio varied from 1.5 to 5; the mixing duct to width ratio varied from 8 to 24, which is equivalent to the cross sectional area ratio from 11 to 34. It is found that an underexpanded jet inside a duct generates intense discrete tones (screech tones) similar to a free underexpanded jet. For a given pressure ratio, the variation of the dominant tone with duct widths shows staging behavior, and the frequency varies continuously with duct width in each stage. Similarly, for a given duct width, the variation of the dominant discrete frequency with pressure ratio shows staging behavior. It is found that enhanced mixing and improved performance of the ejector was related to the Strouhal number of the ejector screech tone. For all the duct widths tested, it is found that when the screech tone Strouhal number lies within the range 0.1 to 0.15, the primary jet spreading is increased and the ejector flow is better mixed at the exit.