Study Shows Hydrogen Droplet Size Affects Mechanism of Burning Process

Modern rockets and their launch vehicles mostly depend on hydrogen-oxygen mixtures as propellant. However, this combination is considered to be highly explosive. The 1986 Challenger space shuttle catastrophe is linked with self-ignition of such mixtures.

The evaporation and burning of droplets present within hydrogen-oxygen aerosol mixtures lead to the mitigation of explosion risks. A similar evaporation and burning process is utilized inside the rocket combustion chambers, and a strong understanding of the behavior of these processes is important in order to analyze the safety and effectiveness of hydrogen-oxygen liquid engines.

In the research framework motivated by the Challenger disaster, the Applied Physics Group at NASA's Ames Research Center partnered with researchers from the Supercomputing Division at NASA Ames Research Center and also with physicists from New Jersey Institute of Technology, Newark, and South Dakota School of Mines and Technology, to study all the potential mechanisms of igniting explosions of liquid oxygen and liquid hydrogen in similar scenarios. The results exhibited new mechanisms, which lead to the need to pursue an in depth understanding of how droplets of hydrogen burn within these mixtures.

Physicists from the Applied Physics Group at NASA's Ames Research Center currently report in Applied Physics Letters, by AIP Publishing, that their previous findings motivated them to move on and explore evaporation and burning scenarios of hydrogen droplets, caused by infrared radiation discharged from hot gas. This hot gas is produced during combustion of a hydrogen-oxygen mixture.

This work is known to be the very first detailed study examining the intense situations of evaporation and burning of hydrogen-oxygen aerosol mixtures. The research group includes Viatcheslav Osipov, Marina Marchenko and Michael Khasin. All of the researchers explored a complicated combination of evaporation, radiation, extremely low critical temperature of liquid hydrogen, and negative thermo-diffusion (referring to negative thermo-diffusion passing from cold to hot regions) of gaseous hydrogen in a gaseous oxygen-hydrogen mixture.

The key physical processes in this kind of burning are an explosive and rapid evaporation of liquid hydrogen droplets as a response to radiative heating from the hot ambient gas, which is then followed by a gradual burning of the gaseous hydrogen.

We were quite surprised by the extreme time-scale separation. These are the processes that were likely responsible for the Challenger explosion, and they are expected to occur within the liquid engine combustion chamber of rockets using liquid hydrogen-oxygen fuel.

Michael Khasin, Senior Researcher, Applied Physics Group, NASA Ames Research Group

Khasin and colleagues also discovered that varied mechanisms of burning take place based on the size of the hydrogen droplets.

"One of the applications for this finding is that it now enables optimization of spray systems within combustion engines of liquid hydrogen-oxygen-powered rockets," Khasin said.

A wide variety of issues related to the evaporation of cryogenic fluid droplets and burning will also stand to benefit from the exceptionally precise numerical method created by the team during the research work.

To move forward, the group plans to examine "different scenarios of the evaporation of hydrogen droplets while spray cooling the cryogenic tank in microgravity conditions," Khasin said. "This process is important for safe and efficient cryogenic fuel management in deep space missions."

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