Scramjet Ignition Research Articles

A free radical relay combustion approach to scramjet ignition at a low Mach number

Turbine-based combined cycle (TBCC) engines, which consist of turbine and scramjet engines, are a key power system for the acceleration, powered horizontal landing, and reuse of hypersonic vehicles. Low Mach number ignition of kerosene-based scramjet fuel is expected to enable the practical application of TBCC engines. However, ignition under these conditions is extremely difficult to achieve. In this study, free radical relay combustion (FRRC), motivated by multi-catalyst relay catalysis, was proposed to solve the problem of scramjet ignition at low Mach numbers. The ignition temperature of modified kerosene was significantly reduced from 620 to 150 °C, and the hydroxyl radicals (OH·) signal value of the modified kerosene at 210 °C was found to be 538 times higher than that of pure kerosene at 680 °C. The FRRC-based ignition of the modified kerosene presented a flat droplet evaporation. This significantly reduced the ignition delay time of the fuel to 3 ms at 220 °C, which was comparable to or better than that of hydrogen. In addition, the FRRC strategy described in this study achieved direct-connect supersonic combustor ignition at an inflow of Ma 1.5. This study provides a reference for the design of scramjet fuel for low Mach number ignition.

Synergistic promotion of multi-component RP-3 kerosene low-temperature ignition with addition of radical activator

Scramjet engines play a vital role in hypersonic vehicles, but they suffer from the ignition issue due to the low reactivity of hydrocarbon fuel when flying at low flight Mach numbers. In this work, the low-temperature ignition performance of multi-component kerosene was significantly improved by only adding 3% borane-dimethyl sulfide (BDMS), resulting in the low-temperature ignition reducing greatly from 620 to 160 °C. This indicates that BDMS was an effective activator for multi-component kerosene. To analyze the low-temperature efficient activation mechanism of multi-component kerosene, paraxylene, ethylcyclohexane, n-decane, and n-tetradecane (each with added 3% BDMS) were studied as they are essential components in kerosene, respectively. The low-temperature ignition performance and the ignition process of n-alkane were significantly improved and changed for a single component of hydrocarbon. There was a synergistic promotion effect for multi-component hydrocarbons with the addition of activator BDMS. BDMS could significantly improve the low-temperature ignition performance and oxidation of ethylcychohexane in the presence of n-tetradecane. The addition of paraxylene promoted the evaporation of the hybrid fuel of n-tetradecane/ethylcyclohexane. The results obtained from this study could help to gain a better understanding of the low-temperature ignition of multi-component kerosene and provide an effective method to solve the scramjet ignition problem at low flight Mach numbers.

A High-Temperature MEMS Surface Fence for Wall-Shear-Stress Measurement in Scramjet Flow.

A new variant of MEMS surface fence is proposed for shear-stress estimation under high-speed, high-temperature flow conditions. Investigation of high-temperature resistance including heat-resistant mechanism and process, in conjunction with high-temperature packaging design, enable the sensor to be used in environment up to 400 °C. The packaged sensor is calibrated over a range of ~65 Pa and then used to examine the development of the transient flow of the scramjet ignition process (Mach 2 airflow, stagnation pressure, and a temperature of 0.8 MPa and 950 K, respectively). The results show that the sensor is able to detect the transient flow conditions of the scramjet ignition process including shock impact, flow correction, steady state, and hydrogen off.

Experimental Investigation on the Plasma Torch Used for Scramjet Ignition Enhancement

A high frequency arc discharge plasma torch was specially designed for ignition enhancement in scramjet combustor. At first, the process of plasma injection into quiescent air was investigated experimentally through CCD camera and schlieren technology. Then, the energy property characterization of active particle distribution was measured by emission spectrometry. Several kinds of working gas under different injection pressures were compared. Finally, the typical supersonic flow-field structure with plasma cross-injection was obtained. The results show that plasma jet energy is concentrated near the jet axis, which has the maximum attenuation in the downstream as far as 2 cm from the outlet. The working gas and injection pressure have great effect on emission spectrometry and the process of jet expansion. The case with N2 under higher injection pressure shows better performance of energy exchanging process when comparing with air and argon. From the emission spectroscopy, we can see that plasma from nitrogen consists of nitrogen and oxygen atom mainly, whose intensity decreases with increasing distance from the nozzle, while it increases with the increase of pressure. When plasma was vertically injected into supersonic flow-field, bow shock wave and mixing layer structure were formed with thicken mixing layer, which helps enhance the mixing process between active particle and incoming air.

Plasma-Assisted Ignition in Scramjets

This study assesses the prospect of main-fuel ignition with plasma-generating devices in a supersonic flow. Progress from this study has established baseline conditions for operation, such as the required operational time of a device to initiate a combustion shock train as predicted by computational fluid dynamics computations. Two plasma torches were investigated: a direct current constricted-arc design and an alternating current unconstricted-arc design based on a modified spark plug. Both plasma torches are realistic in size and operate within the same current and voltage constraints, although differing substantially in orifice geometry. To compare the potential of each concept, the flow physics of each part of the igniter/fuel-injector/combustor system was studied. To understand the constraints involved with the ignition process of a hydrocarbon fuel jet, an experimental effort to study gaseous and liquid hydrocarbons was conducted, involving the testing of ethylene and JP-7 fuels with nitrogen and air plasmas. Results from individual igniter studies have shown plasma igniters to produce hot pockets of highly excited gas with peak temperatures up to 5000 K at only 2 kW total input power. In addition, ethylene and JP-7 flames with a significant level of the hydroxyl radical, as determined by planar laser-induced fluorescence, were also produced in a Mach 2 supersonic flow with a total temperature and pressure of 590 K and 5.4 atm. Information from these experiments is being applied to the generation of constraints and the development of a configuration with perceived high ignition potential in full scramjet combustor testing.

Use of silane-methane mixtures for scramjet ignition

Calculations have been made to demonstrate the use of silane-methan e liquid mixtures as an ignition promoter and flame stabilizer in hydrogen- (or other) fueled scramjet applications. The calculations show that mixtures initially nonflammable undergo rapid methane evaporation to produce drops capable of spontaneous inflammation. This technique may be expanded to other spontaneously flammable materials. It was assumed that the liquid silane-methane mixture was sprayed into the combijstion chamber upstream of the main fuelflow positions. Drop size, flow velocity, temperature, and initial liquid mixture composition were varied.