PIV-based experiments on reverse flow characteristics in an equal-height-difference passive heat removal system for ocean nuclear power plants

•A fully visualized experimental facility combined with PIV system was designed to study the reverse flow phenomenon in the EHDPHRS.•Single-phase reverse flow velocity is proportional to heating power and inversely proportional to subcooled-water temperature.•Two-phase reverse flow velocity increases first and then decreases with heating power.•Buoyancy lift induced by density difference and pressure difference induced by direct contact condensation are the driving force for reverse flow.•Flow inertia force caused by forward flow is the restraining force for reverse flow.