Analysis of bubble distribution and energy gradient in impeller of multiphase pump

To elucidate the mechanisms of bubble breakup, transport, and size evolution in gas–liquid two-phase flow within multiphase pumps, this study employs a numerical approach based on the population balance model to investigate the axial and radial distributions of gas volume fraction under various inlet gas volume fractions and flow rate condition. Flow instability and bubble breakup hotspots are analyzed using the energy gradient theory. Results show that, regardless of operating conditions, the volume fraction of small bubbles consistently increases along the spanwise direction, while medium and large bubbles gradually decrease, indicating a continuous breakup process from larger to smaller bubbles within the impeller passage. This radial gradient in bubble size evolution is identified as a fundamental characteristic of multiphase flow in such pumps. At impeller outlet section, the proportion of small bubbles consistently follows the pattern of 0.9Q > 1.0Q > 1.1Q, and this trend remains consistent across the entire gas volume fraction range. Furthermore, analysis based on the energy gradient reveals that bubble breakup primarily occurs near the blade leading edge, after which bubbles migrate toward flow-stable regions.