In this work, buoyancy effects on hydrogen jet flames confined in a small tube without air co-flow were numerically investigated. The results show that the extinction limit of fuel velocity under buoyant condition is much lower than that without buoyancy. Moreover, hydrogen flames under buoyant condition attatch the nozzle exit for all fuel velocities investigated; however, the flames without buoyancy surround the lower wall at low fuel velocity. In addition, combustion is nearly complete in the presence of buoyancy, whereas the combustion efficiency under non-buoyant condition is below 45%. Furthermore, flame temperature under buoyant condition is much higher compared to the counterpart under non-buoyant condition at low and moderate fuel velocities. Analysis reveals that in the case without buoyancy, the negative gauge pressure in the annular space is unable to entrain sufficient air from the ambient. Hence, hydrogen has to diffuse downwards to sustain the flame and complete combustion is unrealizable. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.