Abstract: The gas-liquid two-phase flow in the up leg of the Ruhrstahl-Hereaeus (RH) unit is one of the main momentum sources of the whole device, and it affects the flow state of the molten steel in the ladle. A physical model of 300 t RH in 1:6 ratio was set up to simulate the bubble behavior process and to measure the change of the RH circulation flow in the up leg and in the vacuum chamber. The gas-liquid fraction and the movement velocity of bubbles were measured to assess the residence time of the bubbles in the vacuum chamber. In addition, the formation of the bubbles at different values of the RH circulation flow and liquid-level height in the vacuum chamber were recorded by a high-speed camera. One of the main factors influencing the bubble formation is the increase of the lifting gas flow in the vacuum chamber. With the increase of blowing gas, the large independent bubbles undergo multiple collisions, break into small bubbles, and finally small and large irregular-sized bubbles coexist. When the liquid height is>80 mm, the residence time of the bubbles in the vacuum chamber achieves a stable value and cannot be further affected by the increase of the liquid-level height in the vacuum chamber. At a lifting gas flow of 3000 L·min^-1, a weak decreasing trend of the residence time of bubbles is observed, and the bubbles start polymerizing in the vacuum chamber. In conclusion, for the 300 t RH physical model, the liquid height in the vacuum chamber is recommended to be 80 mm, whereas the lifting gas flow should be set at 3500 L·min^-1. After these optimization steps, the decarburization time decreases from 21.4 to 17.5 min.