Laser Beam Optimization for LAS Tomography by Ergodic Evaluation

Laser absorption spectroscopy (LAS) technique is widely applied in flow field diagnosis for temperature and concentration imaging. The optimization of beam layouts is important for the improvement of imaging quality for LAS tomography. An ergodic evaluation of fan-beam layouts was conducted in this work for beam optimization. The influences of the sensor shape, laser source number, detector number, and divergence angle on the imaging quality were studied by calculating normalized mean absolute errors of reconstructed images. Distributions of temperature and water vapor concentration of a flat flame from computational fluid dynamics (CFD) simulation were reconstructed to determine candidate layouts for optimization. Distributions of a counterflow flame from CFD simulation and a bimodal distribution were used for synthetical evaluations of candidate layouts. The optimal beam layout was then determined in the balance of the imaging quality and system complexity. In the numerical verification, flat flame distributions contaminated by random noise were reconstructed respectively by the optimal layout and a pentagonal layout derived from the real sensor. Temperature and concentration errors of the optimal layout were respectively 57.4% and 53.7
^%
smaller than those of the pentagonal layout at 40 dB, which verified the effectiveness of the optimization.