<p>Understanding the effects of gravity on hydrogen-air flame propagation dynamics in straight and curved tubes is crucial for mitigating the fire and explosion risks associated with hydrogen. The impact of gravity on flame propagation is expected to depend on the angle between the gravity and flame-propagation directions, but this relationship remains poorly understood. In this study, we numerically investigated the propagation characteristics of hydrogen/air flames in straight and curved open-ended tubes, focusing on how tube placement angle affects flame propagation. The results reveal that variations in tube placement angle, together with the effects of gravity and curvature on pressure gradients, can significantly influence unburned-gas flow and thereby alter flame propagation, particularly in curved tubes. Flame propagation could be substantially influenced by gravity. The evolution of the flame front is analyzed using the vorticity equation and baroclinic torque, showing that in straight tubes, the flame tilt direction changes with tube placement angle. In contrast, the bend of curved tubes consistently directs the flame front toward the inner wall. Gravity induces two distinct flame structures in the transitional regime of curved tubes, thereby affecting flame-front evolution and propagation through the stretch rate and displacement velocity. These findings highlight the crucial role of tube orientation in flame propagation, underscoring the potential fire hazards associated with hydrogen utilization.</p>