Effect of computational constraints on zero-dimensional computations for the nanosecond-order ignition process of the $\mathrm{CH}_4$/air mixture

Zero-dimensional computations of nanosecond-order ignition using a nanosecond discharge are performed with two constraints. The effects of these constraints are assessed to study the experimental rapid pressure change properly at the initial stages. The computations are carried out with the following constraints: constant internal energy and volume (U&V) and constant enthalpy and pressure (H&P), revealing differences between the two solutions. As the pressure remains constant under the H&P constraint, the total number density of all species decreases during ignition. In this case, $\mathrm{O}$ radicals are less generated and consumed. The progression of all reactions and temperatures increases under the H&P constraint less intensely than under the U&V constraint. Significant differences are found between the results calculated under the U&V and H&P constraints. Therefore, large discrepancies with real phenomena can be caused if the loss due to pressure reduction is not treated well.