AN EXPERIMENTAL INVESTIGATION OF AN UNDEREXPANDED RECTANGULAR JET EJECTOR

An experimental investigation was carried out on a rectangular ejector (constant area mixing duct) with an underexpanded rectangular jet as the primary flow. The purpose of this investigation is to study the mixing behavior of the ejector flow in general and to identify the effects of the screech tones on the mixing and performance of the ejector in particular. The quantities measured include frequency and amplitude of the screech tone, surface pressure on the ejector duct wall, and the mean flow velocity at the ejector exit in the two central planes of the primary jet. Schlieren flow visualization was made in the plane containing the short dimension of the primary nozzle.; It has been found that the screech tone frequency of the ejector depends not only on the primary jet pressure ratio but also on the ejector duct width. The variations of the screech tone frequency with both the pressure ratio and the duct width show staging behavior. For a given duct width, each screech tone stage matches with one of the transverse modes of the duct. The ejector performance, as determined from the static pressure distribution on the walls, shows irregular variation with pressure ratio; and it is found to be related to the screech tone stages. A better performance was found to lie within one screech tone stage. During this stage the primary jet has a larger spreading rate. As a result, a better mixing is achieved between the primary and secondary streams. For all the ejector duct widths tested, the screech tone stage during which the flow is well mixed corresponds to a Strouhal number of about 0.13, which agrees with the most unstable Strouhal number of the free underexpanded rectangular jet. By incorporating a phase-locked technique with schlieren flow visualization, standing waves excited by the screech tones inside the ejector mixing duct were photographed.