Combustion Characteristics of Pressurized Swirling Spray Flame and Unsteady Two-Phase Exhaust Jet

Abstract

The effects of flame enclosure and combustor pressure on the combustor flowfield and structure of turbulent spray flames have been investigated. The exhaust jet from the combustor was directed into water to simulate underwater propulsion applications. Twophase interactions between the exhaust jet from the pressurized combustor and liquid water in an attached mixing chamber have been examined to address issues associated with the operation of a submerged combustor for underwater propulsion applications. Enclosure of the flame, in the absence of pressurization, was found to affect the flame structure and dynamic behavior significantly. At normal pressure, the combustor flow attached to one side of the exit port to result in large-scale, low-frequency flame oscillations. In contrast the pressurized flame, obtained by constricting the exhaust flow from the combustor, was not found to display the same large-scale distortions associated with the enclosed, unpressurized case. The global structure of the pressurized and unenclosed flame cases was similar; however, the flame in pressurized case was found to be shorter. Axial and radial velocities of the air flowfield in the combustor were examined using particle imaging velocimetry. The interaction of the gaseous combustor exhaust jet with water in a downstream mixing chamber was also examined. Unchoked and choked isothermal exhaust jets at two combustor pressures were examined using two exhaust nozzle geometries. The primary parameter influencing the behavior of the isothermal exhaust jet was found to be the pressure drop across the exhaust port. At relatively low combustor pressures the exhaust jet was found to emerge as a series of distinct bubbles. The inertia of the liquid phase and buoyant forces acting on the bubbles appear to dominate the dynamic behavior of the jet. Swirl imparted to the combustion air did not affect the behavior of the jet significantly, since the momentum of the jet is insignificant in comparison to other factors associated with the two-phase jet interaction. The exhaust nozzle geometry affects the shape of the bubble during emergence and detachment. However, the frequency of the bubble formation and detachment was not influenced strongly by exhaust nozzle geometry. The Strouhal number associated with the cycle in the two-phase flow case was found to be two orders of magnitude smaller than that associated with large-scale jet mixing instabilities in single-phase mixing. The maximum diameter of the large-scale bubble structure was found to be effectively independent of combustor chamber pressure. The structure and dynamics of the exhaust jet from a reacting flow case, issuing from the pressurized combustor, were also examined. The structure of the heated jet was found to be more chaotic than the isothermal flow, displaying events occurring across a wide range of time scales. A distinct, repeatable bubble-formation mode was not observed. The exhaust jet associated with the reacting flow displayed a greater tendency to produce extremely small as well as large bubbles, with diameters ranging from microns to a few millimeters.