Flow Visualization of Heated CO_2 Gas using Thermal Imaging

摘要

Walk-through portal detection systems are being developed to screen passengers for the presence of explosives in support of homeland security. These portals utilize a series of air-jets to remove the explosive particles for detection using ion mobility spectrometry. In this work, we describe the use of a thermal imager to visualize the flow from the nozzles with heated, pure CO_2 gas for enhanced emission. The thermal imaging is performed using an LN_2-cooled, InSb focal-plane array with a germanium lens. Since CO_2 gas at 300 K has a strong absorption centered at 4.3 μm which is isolated from other absorbing gases, a spectral filter centered at 4.4425 μm with a full-width half maximum bandwidth of 0.18 μm was used to detect the CO_2 emission. To increase the radiance from the gas, pure, heated CO_2 was ejected from the nozzle. The concentration of CO_2 in standard atmosphere is < 0.05 %, and thus the atmosphere is effectively transparent under laboratory conditions. As the temperature of the CO_2 is increased above room temperature, the emission increases according to Planck radiance law and also broadens to longer wavelengths, thus enhancing the collected signals. The thermal images were corrected for both spatial uniformity of responsivity and detector linearity with constant and variable-integration times using a large-area variable-temperature blackbody with known emissivity and temperatures. The correction algorithm using the blackbody at many different temperatures will be described. Corrected, thermal videos under both laminar and turbulent flow conditions are shown. Fine details such as residual CO_2 swirls cooled slightly below the ambient background are visible because of improved non-uniformity correction enabled by a differential imaging extension of the algorithm.