Study on the influence of explosive structure constraints on shock wave and explosive fireball in tunnel with thermobaric explosives

To investigate the propagation characteristics of blast shock waves and thermal effects of fireballs in tunnel explosions involving thermobaric explosives, numerical simulations were conducted using OpenFOAM. The simulation accuracy was validated through comparative analysis with experimental data from tunnel explosion tests. The effects of axial distance along the tunnel and explosive mass on shock wave propagation characteristics and fireball thermal effects were systematically studied. The results demonstrated that under identical charge mass conditions, when the axial distance exceeds 1/3 times the equivalent tunnel diameter, the attenuation law of shock wave overpressure peak and planar wave formation distance remain independent of axial position. After planar wave formation, the impulse initially increases with axial distance before stabilizing. At equivalent axial distances, the planar wave formation distance increases with explosive mass. Post planar wave formation, the attenuation pattern of the shock wave overpressure peak remains unaffected by charge mass, while the impulse exhibits a growth trend proportional to the increase in charge mass. Under the influence of the tunnel portal energy dissipation effect (tunnel effect), explosion-induced fireballs exhibit a consistent propagation tendency toward the proximal tunnel portal. The confinement imposed by tunnel walls restricts the lateral expansion of the fireball perpendicular to the tunnel axis, while facilitating the formation of a high-temperature tip along the longitudinal axis. Especially, the temperature distribution along the tunnel axis maintains axial symmetry despite directional propagation biases. A fitting formula was established to characterize the relationship between the maximum axial propagation distance of explosion fireballs at different temperatures and the explosive mass, enabling the prediction of axial spread limits for fireballs at specific temperatures in typical thermobaric explosive detonations within tunnel-confined environments.