Near-infrared tomographic thermometry for three-dimensional flame based on multi-level parallel defocusing

In this work, a near-infrared tomographic thermometry method based on the multi-level parallel defocusing is proposed, which can measure the three-dimensional temperature of combustion flame. By applying Fourier optics to establish the object-image mapping relationship, three-dimensional gray level model of the combustion flame is analyzed, and the combustion temperature is calculated by utilizing Planck's radiation law. The thermometer is designed with different imaging channels, each focused on different axial sections of flame. It consists of integrated beam splitter, synchronous control circuit, imagers and mechanical packaging. Firstly, the flame radiation is distributed to different output channels through an integrated splitting prism as the beam splitter. A field programmable gate array (FPGA) circuit is used to drive the imagers arranged at different channel ports for the acquisition of transient combustion flame information, and the mechanical structure is employed to fabricate the overall package. Secondly, the coaxial optical path of thermometer, the point spread function and the photoelectric mapping are calibrated for higher precision measurement. After the calibration, the effectiveness of tomography is verified by the four-layer acrylic plate with an axial 15 mm interval. The section information at different spatial positions along the axis direction can be separated. After screening the flame spectral information, the appropriate measurement band is selected. Finally, the temperature measurement experiment of the tail flame of the solid burner is carried out. The experimental results show that between the theoretical value and the measured value, the maximum measurement error is within 10 %.