Development and verification of a supersonic nozzle with a rectangular cross section at a Mach number of 2.8 for a scramjet model combustor

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Forced ignition is important for the scramjet engines in supersonic flights. When compared to other forced ignition devices, burned-gas torch igniters are considered suitable for scramjet combustors because it is easy to control the input energy of burned-gas torch igniters. In the conventional burned-gas injection methods, the injection gas total temperature is estimated by assuming a discharge coefficient in the range of 0.90-0.95. However, the assumption has not been validated and it may cause misestimations when analyzing combustion in scramjet engine in supersonic flight. Therefore, a validated temperature estimation method is needed. In this paper, a new total temperature estimation method was proposed and investigated experimentally and numerically. Additionally, a novel hydrogen/air burned-gas torch igniter, which can control the injection gas total temperature by controlling the overall equivalence ratio, was also developed. The experimental results showed that the newly-developed hydrogen/air burned-gas torch igniter can successfully control the injection gas total temperature by changing the overall equivalence ratio in the range of 1.0-10.0. Also, the chemical equilibrium assumption for estimating injection gas compositions was validated by O2 mole fraction measurement. The numerical results showed that the estimated discharge coefficient results in a maximum underestimation of 7.02% due to wall heat loss, which means that the maximum uncertainty of the estimated total temperature is 14.7%. This, in turn, indicates that the influence of wall heat loss on the discharge coefficient should be considered to estimate the injection gas total temperature more accurately.

This study clarifies the flow field and flame structure of a cavity flameholder with pylons in a Mach 2.8 airflow. A burned hydrogen/air gas mixture, rich in fuel, was injected in supersonic combustion experiments because self-ignition of fuel is difficult in a mainstream with low enthalpy. Experimental data were collected using the shadowgraph method, direct photography of the flame, wall pressure measurement, and OH Planer Laser-induced Fluorescence (OH-PLIF) measurement. In addition, three-dimensional numerical simulation was conducted. When one pylon was installed upstream of the jet flow, the penetration height of the jet increased, and a flame was formed in the mainstream center. When two pylons were installed at the front edge of the cavity, the flow field inside the cavity differed depending on the distance between the pylon and jet flow. The ignition and combustion of the burned-gas were suppressed at a close distance between the pylon and jet flow. For a large distance between the pylon and jet flow, the ignition and combustion of the burned-gas was enhanced. When hydrogen was injected as the main fuel from the upstream of the cavity, ignition of the main fuel was successful only for a large distance between the pylon and jet flow.

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