The microwave transmittance of glass fiber-reinforced plastic (GFRP) slabs subjected to continuous-wave laser ablation was studied in the framework of continuum mechanics. First, a one-dimensional physical model involving laser absorption, heat conduction, resin pyrolysis, thermal radiation, and convection heat transfer was established to obtain the temperature field. An experiment-based absorption coefficient was proposed to capture the bulk-to-surface absorption transition during laser ablation. Second, the complex dielectric constant was modeled using a solid-state kinetic model describing the graphitization of pyrolysis products. The microwave reflectivity and transmittance were calculated based on the dielectric constant distribution. The agreement of the temperature and microwave transmittance with the experimental results suggests the feasibility of the model. The influence of laser power density, material thickness, and tangential airflow velocity on microwave transmittance was studied based on the model. The microwave transmittance changed nonmonotonically with increasing slab thickness owing to the competition between different physical mechanisms. The existence of tangential airflow reduced the decrease in microwave transmittance, particularly for weaker lasers. This study provides a useful physical model for predicting the microwave transmittance performance of GFRP in extreme heat environments.
Thermal blooming effect of inner optical path remarkably affects far-field beam quality and energy distributions which should be taken into account in high energy laser (HEL) system. A physical model of thermal blooming is established. Based on the model, numerical simulations are carried out to study both the influences of absorptions of laser energy and tube structures on laser propagation in a closed tube. The natural convection of gas is numerically simulated by computational fluid dynamics (CFD) method. Gas temperature distributions, additional phase differences (APDs), variations of beam quality and drifts of mass center in far-field under different absorptions of laser energy and tube structures (Z-shaped and U-shaped) are compared, respectively. By analysis of numerical simulation results, the switch time of heat conduction and heat convection in gas is distinguished, which significantly affects the variations of beam quality and drifts of mass center in far-field. In addition, it also indicates that less absorption of laser energy improves beam quality and delays the switch time of beam quality between two heat transfer mechanisms. Therefore, it is significant to control the absorptions of laser energy for HEL system in practice. Different tube structures owning different beam paths change the distributions of APDs and thus influence beam quality. APDs of the two horizontal sections are the same (superposition effect) for Z-shaped tube while inverse (compensation effect) for U-shaped tube. It is shown that drifts of mass center in far-field are greatly suppressed for U-shaped tube than that of Z-shaped tube and beam quality is also improved.
Numerical simulation and analysis of the temperature distributions in multilayers under laser irradiation have been reported. Using finite element method (FEM), we have developed 2D and 3D programs, and calculated the temperature distributions under irradiation of immovable or laterally movable laser beams. The simulated results show that the maximum temperature rise appears at where the most laser energy deposition is. The results also show that when the moving velocity of laser beam is not so fast, the maximum temperature rise would not descend much compared with immovable irradiation.
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