  This paper develops a hybrid-size meshing scheme for target-dependent imaging
in Chemical Species Tomography (CST). The traditional implementation of CST
generally places the target field in the central region of laser sensing, the
so-called Region of Interest (RoI), with uniform-size meshes. The centre of the
RoI locates at the midpoint between the laser emitters and receivers, while the
size of the RoI is empirically determined by the optical layout. A too small
RoI cannot make the most use of laser beams, while a too large one leads to
much severer rank deficiency in CST. To solve the above-mentioned issues, we
introduce hybrid-size meshes in the entire region of laser sensing, with dense
ones in the RoI to detail the target flow field and sparse ones out of the RoI
to fully consider the physically existing laser absorption. The proposed scheme
was both numerically and experimentally validated using a CST sensor with 32
laser beams. The images reconstructed using the hybrid-size meshing scheme show
better accuracy and finer profile of the target flow, compared with those
reconstructed using the traditionally uniform-size meshing. The proposed
hybrid-size meshing scheme significantly facilitates the industrial application
of CST towards practical combustors, in which the combustion zone is bypassed
by cooling air. In these scenarios, the proposed scheme can better characterise
the combustion zone with dense meshes, while maintaining the integrity of the
physical model by considering the absorption in the bypass air with sparse
meshes.
