A Theoretical Investigation on the Kinetic and Radiative Extinction of Spherical Diffusion Flames in Microgravity

Thrust and fire safety are among NASA’s major concerns in the fulfillment of its mission on Mars Exploration, especially the missions that planned to bring astronauts to and back from the Red Planet. Responding to these concerns, the research studied the burning characteristics and extinction of diffusion flames in space, which are different from those on Earth due to the lack of gravity. Because microgravity combustion experiments require special facilities available only in NASA and are very costly, the investigation is theoretical that includes analytical and computational contents. In the analytical study, a mathematical model was formulated to identify the flame location, flame temperature, and extinction condition. The computational study was performed using an existing flame code that incorporated detailed chemistry and transport properties. The analytical study focuses on the steady burning while the computational study on the transient behavior of flames. Spherical flames stabilized by a porous burner were used in this research. It was observed that Lewis number of fuel in the ambient has a profound impact on the kinetic extinction state. From the numerical analysis, it was observed flames with a higher flow rate extinguish quicker due to the radiative heat loss . INTRODUCTION In recent years, the primary interest of NASA is focused on the exploration of Mars, which includes launch of spacecraft carrying astronauts to and back from Mars. Thrust and fire safety are among NASA’s major concerns to fulfill this most challenging mission. In a space travel, as on earth, thrust is provided by the reaction between the fuel and the oxidizer. To provide thrust, the flame is required to burn as strong as possible and extinction should be avoided. During the trip, fire outbreak can also occur from various ignition sources such as radiation from the Sun. Such accidents need to be prevented because external assistance is unavailable when a fire burst out and the result could be disastrous. If a fire accident happens, the flame needs to be extinguished as fast as possible to minimize the damage. In most of practical combustion systems, including those used to provide thrust in space travels and related to undesired fire incidents, the flames are non-premixed. The fuel and oxidizer are supplied from separated sources and transported through convection and molecular diffusion to the reaction region within which they meet and react to produce products and heat. The flame is also known as diffusion flame. Realizing the needs, the research studied the burning characteristics and extinction conditions of diffusion flames stabilized by a spherical porous burner in microgravity. By employing this type of burner, different factors that affect the flame can be separated in order to study the fundamental behavior of diffusion flame extinction. With this geometry, reactant is supplied uniformly from the burner into the ambient filled with other reactant. After the ignition,