An improved potential energy surface and multi-temperature quasiclassical trajectory calculations of N2 + N2 dissociation reactions

Abstract

Accurate modeling of high-temperature hypersonic flows in the atmosphere requires consideration of collision-induced dissociation of molecular species and energy transfer between the translational and internal modes of the gas molecules. Here, we describe a study of the N2 + N2⟶N2 + 2N and N2 + N2⟶4N nitrogen dissociation reactions using the quasiclassical trajectory (QCT) method. The simulations used a new potential energy surface for the N4 system; the surface is an improved version of one that was presented previously. In the QCT calculations, initial conditions were determined based on a two-temperature model that approximately separates the translational-rotational temperature from the vibrational temperature of the N2 diatoms. Five values from 8000 K to 30,000 K were considered for each of the two temperatures. Over 2.4 × 10(9) trajectories were calculated. We present results for ensemble-averaged dissociation rate constants as functions of the translational-rotational temperature T and the vibrational temperature T(v). The rate constant depends more strongly on T when T(v) is low, and it depends more strongly on T(v) when T is low. Quasibound reactant states contribute significantly to the rate constants, as do exchange processes at higher temperatures. We discuss two sets of runs in detail: an equilibrium test set in which T = T(v) and a nonequilibrium test set in which T(v) < T. In the equilibrium test set, high-v and moderately-low-j molecules contribute most significantly to the overall dissociation rate, and this state specificity becomes stronger as the temperature decreases. Dissociating trajectories tend to result in a major loss of vibrational energy and a minor loss of rotational energy. In the nonequilibrium test set, as T(v) decreases while T is fixed, higher-j molecules contribute more significantly to the dissociation rate, dissociating trajectories tend to result in a greater rotational energy loss, and the dissociation probability's dependence on v weakens. In this way, as T(v) decreases, rotational energy appears to compensate for the decline in average vibrational energy in promoting dissociation. In both the equilibrium and nonequilibrium test sets, in every case, the average total internal energy loss in the dissociating trajectories is between 10.2 and 11.0 eV, slightly larger than the equilibrium potential energy change of N2 dissociation.

Rovibrational grouping for N2(1Σ+ g)-N2(1Σ+ g) energy transfer using state-to-state model

“…[8] who used the same PES adopted in this work. : Global N 2 ( 1 Σ + g )-N 2 ( 1 Σ + g ) dissociation rate coefficient obtained using the proposed sampling method compared with results of Bender et al [30] and with the global N 2 ( 1 Σ + g )-N( 4 S u ) dissociation rate coefficient.…”

“…The result of this sampling is shown in Fig. 3 compared with the N 2 ( 1 Σ + g ) − N 2 ( 1 Σ + g ) dissociation results of Bender et al [30] and the N 2 ( 1 Σ + g ) − N( 4 S u ) dissociation rates of Panesi et al [23]. The global dissociation rate coefficient obtaining using the proposed sampling method is in good agreement with the results of Bender as well as with the N 2 ( 1 Σ + g ) − N( 4 S u ) global dissociation rate obtained by using the method outlined by Jaffe in Ref.…”

Assessment of Vibrational Non-Equilibrium Effect on Detonation Cell Size

“…The distance between the shock front and the position at which H 2 is half consumed is defined as the half-reaction thickness, δ. At 0.1 atm, when the collision frequency is low, the two-temperature model seems to overestimate the vibrational relaxation, as mentioned by Bender et al (2015) and Panesi et al (2014). These halfreaction thickness features are further demonstrated in Figure 3.…”

A Coupled Vibration-Dissociation Model for Nitrogen from Direct Molecular Simulation

“…Due to the coupling between rotation and vibration modes, for a given v level, only certain number of j levels are allowed. For N 2 considered in this work, v varies from 0 − 54 and j = 0 − 300 [38]. v is function of v and is written as v instead of v (v), only for the sake of brevity.…”

“…The finding that nonequilibrium vibrational energy distributions account for a variation in the dissociation rate off 3−4 times compared to the assumption of Boltzmann distributions, is accurately captured. Model predictions are compared with both DMS [14,15,16] and QCT [38] results.…”