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Top1. Introduction
In recent years, a significant amount of high speed combustion research is directed towards understanding the complex flow phenomena inside a scramjet combustor over a range of operating conditions. Studies – analytical, experimental and numerical – are focused on different aspects of the flow field in the various components of scramjet engines viz., intake, combustor, nozzle etc. The components have also been coupled to make a complete scramjet engine, and the flow fields of the engine with different fuel injection systems have been subjected to numerical and experimental exploration. Curran (2001) reviewed comprehensively the status of scramjet engine in first 40 years and identified two emerging scramjet applications namely (1) Hydrogen fueled scramjet engine to access space and (2) Hydrocarbon fueled scramjet engine for an air launched missile. To increase the flight envelope of the air breathing vehicle, Billig (1993) and Cockrell et al. (2002) introduced Dual-mode Ramjet Scramjet (DMRJ) concept which integrate the advantageous capabilities of both ramjets and scramjets into one flow path.
In a ramjet, the flow is subsonic by the time it gets to the combustor. In a scramjet, the flow remains supersonic through the combustor. The dual mode scramjet bridges the gap between the ramjet and scramjet. It uses the same combustor geometry for both the ramjet and scramjet modes, but operates with a thermal throat in ramjet mode. This combination may enable a vehicle to operate from Mach 3 to Mach numbers approaching 20 with only minor engine geometry changes. At the lower limit of this envelope, the DMRJ operates in ramjet mode and combustion occurs at subsonic speeds. In this mode, the addition of heat can be used to drive the supersonic inflow to sonic conditions and achieve a thermal choke and a precombustion shock train forms in the DMRJ isolator. The shock train consists of a series of normal or oblique shocks, which terminate with a normal shock that drives the flow to subsonic conditions. The pre-combustion shock train aids flame stabilization by increasing the static pressure and temperature and decelerating the flow. In the dual-mode scramjet engine, a constant area diffuser (isolator) is placed upstream of the combustor to reduce the interaction of the combustor and intake flow field and to prevent the intake un-start. The position and strength of three-dimensional pre-combustion shock train and combustor heat release distribution are strongly coupled. However, as described in Heiser and Pratt (1994), at speeds approaching Mach 6, pressure losses associated with choking the flow increase and operational efficiency decreases. At M>6, the level of heat release may be reduced by flowpath geometry modification and/or reducing the fuel-flow rate. The DMRJ operates in scramjet mode, in which combustion occurs at supersonic speeds. Transition from subsonic to supersonic combustion is obtained by controlling the heat released due to combustion such that the thermal choke is alleviated. Once the heat release is reduced by a sufficient amount, the flow is no longer choked and the flow through the combustor remains largely supersonic. The schematic of flow field in DMRJ is shown in Figure 1.
Figure 1. Schematic of DMRJ operation