Effect of initial pressure on methane/air deflagrations in the presence of NaHCO3 particles

To investigate the effect of initial pressure on the suppression of gas explosions, deflagration experiments on stoichiometric methane/air mixtures at different initial pressures (0.7-1.8 atm) inhibited by the sodium bicarbonate (NaHCO3) were carried out in a 36 L spherical vessel. The explosion pressure, characteristic times, flame temperature, and chemical kinetic parameters of the gas explosion process in the presence of NaHCO3 particles were evaluated. The results indicate that the critical solid concentration required for a successful explosion suppression increases gradually with increasing initial pressure. The corresponding critical concentrations are 100, 200, and 350 g/m3 for the initial pressures of 0.7, 1.0, and 1.4 atm, respectively. In contrast, an explosion can still occur at initial pressures greater than 1.6 atm even if the NaHCO3 concentration exceeds 800 g/m3. Under a fixed inhibitor concentration, a lower initial pressure gives a higher suppression effectiveness. The increase of initial pressure decreases the drop ratios in both the maximum explosion pressure and the maximum rate of pressure rise, and it also shortens the explosion induction time and the time to peak pressure. In addition, a thermal-equilibrium equation for the heat-absorbing decomposition of NaHCO3 is developed to explore the initial-pressure effect on the heat losses during gas deflagrations. The dependence of the flame temperature on the initial pressure and inhibitor concentration is discussed. Moreover, the calculation of laminar burning velocity (LBV) is performed by a kinetic model together with a power-law equation. It is found that the principal mechanism of the LBV reduction switches from the chemically-dominated inhibition to the physically-driver effect as the inhibitor concentration increases. Furthermore, this transition concentration at which the leading inhibition mechanism changes gradually increases with increasing initial pressure. The mechanism of methane explosion suppression by NaHCO3 at different initial pressures is revealed.