Identification of longitudinal acoustic modes associated with pressure oscillations in ramjets

Abstract

A one-dimensiona l, isentropic analytical method is presented for calculating the longitudinal acoustic modes in an idealized dump-type ramjet engine. The analysis accounts for the complex acoustic admittances of the choked exit nozzle and of the normal shock in the supercritical inlet. The method assumes that all mean flow properties such as inlet and combustor temperatures, densities, and average Mach numbers are known. The predicted longitudinal modes are compared with data acquired from three different types of experiments. The first experiment concerned a wind tunnel model of a missile which utilized a ramjet with two side-dump inlets. The wind tunnel tests were primarily intended to determine the behavior of the supersonic inlets in the presence of downstream pressure oscillations. A mechanical device was used to excite periodic pressure perturbations in the simulated combustor exit. The second experiment involved connected pipe combustion tests of a side dump combustor. These tests included systematic changes in combustor length and in the entrance and exit boundary conditions. The third experiment utilized connected pipe tests of a coaxial dump combustor. The geometric length of the combustor was varied in order to examine the effects on combustion instability.