Simplified combustion modeling of double base propellant: Gas phase chain reaction vs. thermal decomposition

Simplified combustion modeling of nitrocellulose (NC), nitroglycerin (NG) d ouble base propellant is considered. Two models with simple bur rational ch emistry are compared: the classical thermal decomposition, high gas activat ion energy (E-g/RT >> 1) Denison-Baum-Williams (DBW) model, and a new chain reaction, Low gas activation energy (E-g/RT << 1) model recently proposed by Ward, Son, and Brewster (WSB). Both models make the same simplifying ass umptions of constant properties, Lewis number unity, single-step, second or der gas phase reaction, and single-step, zero order, high activation energy condensed phase decomposition. The only difference is in the gas reaction activation energy E-g which is asymptotically large for DBW and vanishingly small for WSB. The results show that within the same set of assumptions (t hose listed above plus constant heat release and radiative heat feedback) t he WSB model more accurately predicts the steady-state gas phase temperatur e profile, burning rate sensitivity parameters and oscillatory combustion r esponse (pressure- and radiation-driven) for NC/NG propellant than the DBW model. The oscillatory results support our earlier finding that for accurat e unsteady predictions it is necessary to use the full AEA decomposition ex pression of Lengelle and Ibiricu/Williams, with its inherent negative Jacob ian parameters (n(s,q) < 0), rather than the usual constant-prefactor Arrhe nius pyrolysis relation (n(s,q) = 0). All thermophysical, thermochemical, a nd chemical kinetic parameters are consistent with what is known about deta iled chemistry of NC/NG combustion. initiation in the condensed phase, thou ght to occur by CO-NO2 bond homolysis, is represented by E-c = 40 kcal/mol. The primary name zone, thought to be dominated by NO2/aldehyde reactions w ith on effective activation energy of E-g similar to 5 kcal/mol, is better represented by a vanishingly small value of E-g (WSB) than an asymptoticall y large value (DBW). The main implication of this finding is that the impor tant (regression rate determining) gas reaction zone near the surface has m ore the character of chain reaction;than thermal decomposition. An importan t consequence is that the burning rare may be more sensitive to condensed p hase decomposition kinetics than previous simplified models have allowed. A lso, the BDP monopropellant model is shown to be essentially equivalent to the gas kinetically controlled DBW model and a quantitative BDP-DBW link is demonstrated.