Improved Injector Metrics for Supersonic Combustors

Abstract

A S MACH number increases, an airbreathing device requires supersonic through flow to preclude excessive pressure loss, air static temperature, and device weight. Losses that are negligible in subsonic combusting propulsion systems become drivers of supersonic combusting cycle design. In a subsonic system, the performance loss associated with incomplete combustion is great enough to make this a primary design driver. At supersonic combustor velocities, entropy generation from Rayleigh effects, mixing, and friction become of first-order importance [1]. An optimal combustor designwill achieve high performance (Rthrust and ISP) by balancing, subject to thermal constraints, combustion efficiency with entropy generation, and weight. Heat load, weight, and drag all make a short combustor duct desirable. These same issues also make a low aspect ratio, or low surface area to cross-sectional area, combustor desirable. Therefore, the injector must provide a uniform fuel distribution across the relatively large transverse extent of the duct and high combustion efficiency with minimum length. The trouble with stating the goal in this fashion is that it ignores additional loss associated with the mixing process and hardware. It is clear that losses associated with mixing and combustion are of a first-order importance to overall engine performance [1], mixing and combustion efficiencies approaching unity are not generally desirable [2], and that incremental improvements to mixing or combustion efficiency do not necessarily equate to improved specific impulse or thrust [3]. The accounting of thrust lost due to a given entropy generation is subject to the current state of the gas and concurrent lossmechanisms, but entropy generation does necessarily relate to lost thrust [4]. Therefore, mixing or combustion enhancement needs to relate to enhancement per some loss metric. A system designer is not necessarily interested in achieving a high combustion efficiency as much as maximizing thrust or specific impulse, which requires low loss in addition to high combustion efficiency. Historically, the relative merits of multiple injection schemes are compared on the basis ofmore rapidmixing or combustion. Common practice is to present mixing or combustion efficiency data and loss data as a function of distance along the combustor. However, comparison of these data in a space with the metric of interest (e.g., efficiency) as the ordinate and an appropriatemetric of loss (e.g., total pressure deficit) as the abscissa provides a unique approach for examining performance; we will refer to this as a loss-enhancement space. This technique elucidates the cost of improved performance between injection schemes. Any computational study of mixing or combustion should have the requisite data for this, and many experimental studies also have these data. Depiction of data in this fashion is rare [5,6]. This technique is useful for clarifying combustor component data; quantifying system level impact necessitates the addition of a nozzle process calculation that relates to an engine cycle.