  A three-dimensional numerical simulation is conducted for a complex process
in a laser-material system, which involves heat and mass transfer in a
compressible gaseous phase and chemical reaction during laser irradiation on a
urethane paint coated on a stainless steel substrate. A finite volume method
(FVM) with a co-located grid mesh that discretizes the entire computational
domain is employed to simulate the heating process. The results show that when
the top surface of the paint reaches a threshold temperature of 560 K, the
polyurethane starts to decompose through chemical reaction. As a result,
combustion products CO2, H2O and NO2 are produced and chromium (III) oxide,
which serves as pigment in the paint, is ejected as solid parcels from the
paint into the gaseous domain. Variations of temperature, density and velocity
at the center of the laser irradiation spot, and the concentrations of reaction
reactant/products in the gaseous phase are presented and discussed, by
comparing six scenarios with different laser powers ranging from 2.5 kW to 15
kW with an increment of 2.5 kW.
