Flame Photography Discerns Peculiarity in Ramjet Ignition

Amid pictures of dazzling auroras and satellite passes, pictures of a flame may seem boring in comparison. However, a Chinese team’s recent photography of flames igniting in a high speed engine (Technical note, AIAA Journal of Propulsion and Power) captured an unexpected result.

Hiding their cameras behind a quartz window and taking pictures at a rate of 10,000 frames per second, the team photographed how a flame ignites in subsonic and supersonic conditions. Understanding combustion at different speeds is important to developing efficient ramjets and scramjets, which react atmospheric air with a fuel to accelerate the next generation of supersonic airplanes and space-planes. Despite our computing power, our knowledge of how air reacts in these high-speed, high temperature environments is limited. More insight into how flames ignite in this intense environment can lead to better ramjets and scramjets in the future.

Ramjets and scramjets use an inlet to swallow air at high speeds, which the engines mix with fuel and then ignite to provide thrust. The primary difference between a ramjet and scramjet is the speed at which the mix is ignited; ramjets combust at subsonic speeds and scramjets ignite at supersonic speeds. The fuel-to-air ratio influences whether combustion is subsonic or supersonic. In fact, the Chinese team was able to induce either subsonic or supersonic combustion simply by changing the fuel to air ratios. A lower fuel-to-air ratio produced supersonic combustion and a higher ratio allowed subsonic combustion. The speed of the heated air forced into the inlet never changed during the experiment.

By igniting a slow stream of oxygen and a kerosene fuel at different fuel-to-air ratios, the Chinese team was able to photograph how flames look in their infancy. The flame ignited at subsonic levels danced and transitioned through three distinct states before stabilizing at a steady glow. Conversely, the flame ignited at supersonic speeds (and a lower fuel-to-air ratio) stabilized more quickly. Through the photography, the Chinese team showed that the subsonic flame was affected by a counterflow, where the air moved toward the inlet instead of the exit.

Identifying the counterflow in the subsonic flame is an insight into how air moves and reacts after flame ignition. Better understanding of phenomena like this leads to accurate modeling of this extreme environment and development of more effective ignition sources. These pretty pictures may help in the design of the next space-plane.

 

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Technical Notes – Improvements in air-breathing propulsion pave the way to space

Aside

This section is geared those who have a background in aerospace engineering:

The concept of  multiple rocket exhaust areas was based upon the Strutjet concept, which used multiple rockets in an individual air duct. The mixing effects of multiple rockets entrained more air for combustion, leading to greater efficiency. The team wanted to examine having multiple rocket exhaust ares without multiple heavy thrust chambers, so they built a annular nozzle with three major circular arcs and small circular air entrainment tubes in between each arc. For comparison, the team also used a circular nozzle.

To enable accurate comparison, the mass flow and Mach number was kept constant across the two nozzles. The experiment was also set up have the maximum amount of entrained air at the duct exit. Pressure sensors were arranged around the nozzle and exit plane of the duct. To replicate high speed environments, ambient air was injected into the duct at high pressures, replicating up to Mach 2 speeds (after expansion of the flow). No fuel was injected.

Initial results showed the annular nozzle entrained more air than the circular nozzle at lower pressures. The pressures taken at the duct exit plane also showed that the air pressures were more uniform in the annular nozzle configuration, suggesting more mixing of the air had taken place. These results showed an average Mach number 58% higher in the annular configuration than the circular configuration.

Please see the article “Experimental Investigation of an Alternative Rocket Configuration for Rocket Based Combined Cycle Engines” in the July-August edition of the AIAA Journal of Propulsion and Power for more details.