A typical cryogenic upper stage mission requires a reliable liquid-gas phase separation prior to engine restart in space, and auxiliary engines are usually used to provide positive acceleration effect to satisfy vapor-free liquid supplement purpose. To assist thruster logic design for an upper stage, bubble distribution inside a delivery tube and bubble spilling behaviors should be known previously. In the present paper, a CFD approach is adopted to numerically study the bubble generation and bubble movement in liquid hydrogen (LH2) delivery tube under microgravity, and the bubble spilling upward behaviors under different situations are significantly analyzed. The results show that space heat from surroundings brings about liquid evaporation inside the delivery tube, and weak convection effect induced by bubble generation process could result in random bubble distribution in the whole tube range. The maximum bubble reaches the size of tube inner diameter. Under positive acceleration effect, the bubbles spill upward and bring about gas amount decrease in the delivery tube range. Moreover, the bubble spilling upward is not a continuous process, and only the big bubble discharge exerts a significant contribution to the gas amount decrease. In addition, the tube layout and the initial gas amount have slight influence on the bubble discharge performance, but the space acceleration level plays the primary effect on this process. For the present objective, the acceleration with 10- 3g0 could drive the bubble to be discharged within 700 s. When the acceleration increases to 10- 1g0, the required time correspondingly decreases to less than 100 s. Under the acceleration level of 10-4g0, however, the bubble could not spill upward any more but a vapor amount increase phenomenon is observed. Generally, the present work provides an opportunity to understand the bubble properties inside the cryogenic delivery tube in microgravity, and the results on acceleration influence as well as acceleration period are beneficial to the sequence design of space engine restart.
