Combustion Experiments for a Liquid Hydrocarbon Fueled Mach 5 Scramjet Engine Model

S INCE the concept of a supersonic-combustion ramjet, i.e. scramjet, was introduced, it has always been considered to be a candidate power plant for a hypersonic vehicle for use as a civil transport or a portion of a space launch system. The classical concept of a scramjet engine envisions it to use hydrogen as the fuel and to operate highly efficiently in Mach numbers of 6 and above [1–3], producing a large flight range as well as reaching the destination fast. In practice, many problems have been found with the hydrogenburning scramjet idea: to achieve high fuel efficiency, the combustor must operate at a high supersonic Mach number. Such a high combustor Mach number prevents fuel from mixing with air. The volume of the tank holding hydrogen becomes very large. The skin temperature becomes so high that no existingmaterial could sustain a cruising condition. Gaseous hydrocarbon fuels, such as ethylene or methane, have also been investigated [4,5]. These fuels need tanks that are much smaller than those needed for hydrogen. But these fuels must still be cryogenically stored. For this reason, kerosene has been studied extensively as an alternate fuel for the past several decades [6,7]. Kerosene is a liquid at room temperature and requires a much smaller tank volume than that required for hydrogen. Although ignition and flame holding become difficult for a liquid fuel, the problems are solved by applying advanced fuel atomizing techniques [7] or using additives [8]. Kerosene, as well as other hydrocarbon fuels, can be used for vehicles having a flight Mach number of only 3.5 to 5. Flying at such a relatively lowMach number, the combustor entranceMach number becomes low supersonic. At such a low supersonic Mach number, fuel–air mixing will improve greatly over a vehicle flying at Mach 6 or higher. Besides, the skin temperature becomes so much lower. Heat flux is proportional to the square of flight speed, and therefore there is a large difference between theMach 4 andMach 6 vehicles in the skin temperature. In this study, a kerosene-burning scramjet engine model is designed and tested to obtain an overall insight into such an engine. A blowdown, free-jet-type continuous facility is used. A scramjet engine, in an integrated form of an intake, isolator, and combustor, is used as the test model. Ignition and fuel atomization parameters as well as the fuel equivalence ratio and the geometry parameter such as combustor length are changed for respective experiments. Through this experiment, some aspects of the relationship between the scramjet engine configuration and the operational characteristics such as pressure rise, thermal choking, supersonic-to-subsonic transition, and intake unstarts are discovered.