Numerical Investigation of the Blowing Effect on Laser Rayleigh Scattering Temperature Measurements via a Coupled Method of Lines Formulation

Laser Rayleigh Scattering (LRS) serves as a critical non-intrusive diagnostic for boundary layer thermometry; however, its accuracy is significantly compromised near pyrolyzing surfaces due to transient shifts in gas composition. In Poly(methyl methacrylate) (PMMA) environments, the rapid efflux of high-molecular-weight fuel vapor displaces the inert calibration gas, leading to a systematic bias that is frequently misinterpreted as a physical temperature decrease. This research resolves this diagnostic ambiguity by developing a transient, one-dimensional coupled thermo-kinetic framework. The model integrates solid-phase Arrhenius degradation kinetics (E = 230 kJ/mol) with gas-phase species transport equations, utilizing the Method of Lines (MOL) to solve the resulting stiff system of partial differential equations. Quantitative results demonstrate that the dis placement of the helium tracer by Methyl Methacrylate (MMA) monomer increases the effective Rayleigh scattering cross-section by a factor of 3.1. It is shown that failing to account for this compositional shift leads to a temperature underestimation of approximately 650 K during the quasi-steady gasification phase (TS ≈ 593.4 K). Furthermore, the simulation confirms that the characteristic “temperature dip” observed in raw LRS experimental data at the onset of ignition is a compositional artifact rather than a thermal phenome non. This work establishes a physics-based protocol for de-biasing optical measurements through dynamic correction factors (α), providing a scalable methodology for high-fidelity thermometry in variable composition pyrolyzing systems.