It is based on element conservation in steady-state in a control volume close to the wall as sketched in Fig. 3 and expressed in Eq. (17). The underlying hypothesis is that over a time increment Dt (corresponding to the numerical time step in the material response code), the equilibrium chemistry problem in the control volume is quasi-steady (p, T, m_ pg , and zpg
variations may be neglected), allowing decoupling pyrolysis gas injection (material response) and boundary layer mass transport.