Variable diameter configurations influence propane air deflagration and pressure rise in industrial pressurized pipelines

Variable-diameter sections are widely used in industrial gas transmission, exhaust, and collection pipelines; however, their effects on deflagration-induced pressure loading remain insufficiently understood. In this study, a validated large eddy simulation (LES) scheme integrated with a premixed turbulent combustion model was adopted to investigate the effects of different variable-diameter configurations on propane-air deflagration and transient pressure loading. Four typical configurations were investigated, including contraction-then-expansion, expansion-then-contraction, continuous expansion, and continuous contraction, and their influences on flame propagation dynamics, transient pressure-rise characteristics, and turbulent flow-field evolution were comprehensively examined. The results demonstrate that pipeline geometric parameters dominantly control deflagration hazard evolution via jet flow generation, shear layer intensification, flow recirculation, and vortex-flame interaction. In contraction-then-expansion pressurized pipelines, the contraction segment markedly exacerbates early-phase deflagration. Across the LML and LSL contraction-expansion configurations, the peak flame velocity increases from 44.03 to 112.56 m/s, while the contraction-stage peak pressure-rise rate increases from 10.37 to 61.37 MPa/s. The highest overall pressure-rise rate, 67.48 MPa/s, occurs in the MSL configuration, where the downstream expansion receives the high-momentum flow generated by the upstream contraction. In expansion-then-contraction pressurized pipelines, the expansion segment enhances initial flame fluctuation, whereas the downstream contraction restrains late-phase flame propagation and pressure accumulation. Among the continuous configurations, continuous expansion generates persistent vortical disturbances and induces the most severe overall deflagration hazard, whereas continuous contraction sustains persistent flow confinement and reduces the peak pressure-rise rate to 6.89 MPa/s. These results reveal that expansion-dominated configurations can significantly elevate transient explosion-induced pressure loads, whereas contraction-dominated configurations exhibit superior performance in attenuating pressure-rise intensity. This study offers key engineering guidance for configuration optimization and hazard risk control of industrial pressurized pipelines in explosion-prone process facilities.