Flammability limits of hydrogen/oxygen/helium mixtures in confined spaces

The flammability limits of hydrogen, oxygen, and helium mixtures in confined spaces were determined in an experiment designed to simulate the volume around the shaft that connects the turbine and turbo-pump of a liquid rocket engine. Flammability limits were determined for gap sizes of 0.2, 0.1, and 0.025 inches. Without the helium diluent, the lower flammability limit (LFL) ranged from 6% to 13% and 92.7% and the upper flammability limit (UFL) ranged from 88% to 93%. The LFL with helium increased by approximately 3% volume of hydrogen for all helium concentrations and gap sizes. The UFL shows a large decrease with helium, which is primarily due to replacing the excess hydrogen with helium.