Quasi-one-dimensional numerical simulation study on the effect of nitrogen isolation on flow field characteristics in the shock tube of a hydrogen-driven shock tunnel

To address the adverse effects of contaminants generated by mixing and combustion in the hydrogen-air contact region of the shock tube on both the effective test time and test gas quality in a hydrogen-driven shock tunnel, a nitrogen isolation operating mode was proposed. Numerical simulations were conducted to investigate the flow field under typical operating conditions of the shock tunnel. The quasi-one-dimensional thermochemical non-equilibrium Navier-Stokes equations were solved, where a cubic equation of state was employed to evaluate the thermodynamic properties of high-temperature gases. The wall friction and heat transfer along the tube, as well as the thermochemical non-equilibrium effects, were comprehensively considered. The flow field characteristics under two configurations, namely direct hydrogen-air contact operation and isobaric nitrogen isolation operation, were compared, covering incident shock Mach number, onset and evolution of mixing-induced combustion, as well as pressure, temperature and species distribution in the stagnation chamber of the shock tube. On this basis, the influence of the nitrogen isolation mode on the flow field characteristics of the shock tunnel was clarified. The results show that nitrogen isolation has little effect on the incident shock Mach number at the end of the shock tube, and exerts no notable impact on the tunnel’s capability to reproduce total temperature and total pressure in terms of shock strength. The oscillation amplitude of the gas pressure curve in the stagnation chamber is reduced, and the pressure stabilizes earlier, which slightly prolongs the effective test time. The addition of a nitrogen isolation section effectively suppresses combustion inside the shock tunnel. This is mainly manifested in a significant reduction in both the concentration and axial distribution width of water species along the shock tube, as well as a marked decrease in water species concentration at the monitoring point in the stagnation chamber at the end of the shock tube. Meanwhile, it effectively inhibits the sudden rise in the stagnation chamber temperature curve caused by combustion heat release. Under the typical condition of hydrogen at 50 MPa driving air at 0.1 MPa, setting the nitrogen isolation length to approximately one-third of the total length of the low-pressure driven section can achieve satisfactory suppression of hydrogen-oxygen combustion while avoiding excessive loss of available test air caused by an overly long nitrogen isolation section. Keywords: shock tunnel; shock tube; quasi-one-dimensional numerical simulation; mixing and combustion