Application of improved two-phase flow theory in air–water pipe flushing: Based on air-to-water ratio and cement paste rheological properties

This research presents a comprehensive theoretical and practical framework for optimizing air–water pipe flushing in cemented paste backfill (CPB) systems by integrating improved two-phase flow theory with rheological analysis. Key advancements include the following: (1) establishing a novel correlation between CPB slump height and viscosity to quantify adhesion effects, (2) developing a frictional pressure drop model incorporating an equivalent diameter to account for CPB adhesion-induced pipeline roughness, and (3) identifying bubbly flow as the optimal flow pattern through rigorous analysis of air-to-water ratio impacts on shear stress and void fraction. Experimental and numerical validation demonstrates that increasing the air-to-water ratio reduces homogeneous density by up to 75% (from 1000 to 253.9 kg/m3) while enhancing flow velocity by 33% (from 6.93 to 9.24 m/s). The proposed model achieves a 10% improvement in pressure drop prediction accuracy compared to traditional methods. Field implementation in an African copper mine validated that air-to-water ratios of 2:1 reduce water consumption by 61.63% (75–28.78 m3) while maintaining flushing efficiency, addressing incomplete flushing and environmental concerns. A redesigned air–water system with sealed valve controls and pressure-adaptive compressors further ensures operational reliability. This study bridges theoretical gaps in two-phase flow modeling for non-Newtonian fluids and provides actionable strategies for sustainable mining practices.