Two-stage plasma torch ignition in supersonic airflows

Abstract

A two-stage plasma torch, which has two plasma injection ports in a row, has been proposed to enhance the ability as an ignitor used for a supersonic combustion. This paper reports on the ignition performance and characteristics of the two-stage plasma torch examining ignition limit of methane fuel in main airflow of Mach 2. Combution tests were conducted by changing the distance between two plasma injection ports L, the total and ratio of upstream and downstream feedstock flow rate. The experimental results indicate that increase of the total or the upstream feedstock flow rate improves ignition ability, more specifically that the two-stage plasma torch of L = 20 mm operated at 1:1 of upstream and downstream feedstock flow rate ratio could ignite methane fuel with 20 % less electrical input power than a coventional plasma torch case. Introduction The supersonic combustion ramjet engine is one of the candidates as an engine for hypersonic propulsion in the next generation. In this engine, the speed of the air flowing from the air inlet is so high, and accordingly, the residence time of the airflow in the engine is very short. Therefore, it is very difficult to ignite and complete the combustion in the engine. Furthermore, it is necessary to ensure the ignition for the wide conditions of the airflow, since the aerial in* Graduate Student t Researcher * Associate Professor, Senior member AIAA Copy right ©2001 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. flow condition to the engine widely changes with an increase of the flying speed and altitude. Therefore, some means for compulsorily ignition or augmentation for combustion is necessary.' Quite a few researches are being carried out on the plasma torch which contains high temperature gases and highly active species and jets them into fast main air stream, and the effectiveness has been confirmed in supersonic combustion tests. In these combustion tests, from the viewpoint of excellence in reactivity, hydrogen is mainly used as fuel, but methane has some advantage in a practical aspect. With methane fuel, however, ignition delay is much longer than with hydrogen fuel, and chemical reactivity is inferior as well, so methane fuel requires higher temperature of the mainstream in comparison with the hydrogen fuel in order to obtain selfignition.' From such reasons, the development of some ignition method superior to the performance by the plasma torch is desired in order to use methane as fuel for the supersonic combustion ramjet engine. It is known that ignition ability by plasma torch greatly depends on penetration height of its plasma jet to the mainstream, temperature and concentration of the active species.' In this paper, a two-stage plasma torch the jet of which is ejected from two holes perpendicularly to the mainstream is proposed for the purpose of improvement in the ignition ability of a conventional plasma torch. By emitting two plasma jets to the mainstream, high-temperature region of the plasma is expected to spread in the mainflow direction which would raise the performance. In this work, the ignition limit of methane fuel in supersonic airflow American Institute of Aeronautics and Astronautics c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.