  The simulation of thermochemical nonequilibrium for the atomic and molecular
energy level populations in plasma flows requires a comprehensive modeling of
all the elementary collisional and radiative processes involved. Coupling
detailed chemical mechanisms to flow solvers is computationally expensive and
often limits their application to 1D simulations. We develop an efficient
Lagrangian diffusive reactor moving along the streamlines of a baseline flow
simulation to compute detailed thermochemical effects. In addition to its
efficiency, the method allows us to model both continuum and rarefied flows,
while including mass and energy diffusion. The Lagrangian solver is assessed
for several testcases including strong normal shockwaves, as well as 2D
axisymmetric blunt-body hypersonic rarefied flows. In all the testcases
performed, the Lagrangian reactor improves drastically the baseline
simulations. The computational cost of a Lagrangian recomputation is typically
orders of magnitude smaller with respect to a full solution of the problem. The
solver has the additional benefit of being immune from statistical noise, which
strongly affects the accuracy of DSMC simulations, especially considering minor
species in the mixture. The results demonstrate that the method enables
applying detailed mechanisms to multidimensional solvers to study
thermo-chemical nonequilibrium flows.
