Investigation of the effect of thermal radiation on a fully developed magnetohydrodynamic (MHD) convective flow of a Newtonian, incompressible and electrically conducting fluid in a vertical microchannel bounded by two infinite vertical parallel plates with constant temperature walls through a lateral magnetic field of uniform strength is presented. The Rosseland model for the conduction radiation heat transfer in an absorbing medium and two plates with slip-flow and no-slip conditions are assumed. In addition, the induced magnetic field is neglected due to the assumption of a small magnetic Reynolds number. The non-dimensional governing equations are solved numerically using Runge–Kutta–Fehlberg method with a shooting technique. The channel is optimized based on the Second Law of Thermodynamics by changing various parameters such as the thermal radiation parameter, the temperature parameter, Hartmann number, Grashof to Reynolds ratio, velocity slip length, and temperature jump.

The OH radical concentration was measured by applying tunable diode laser absorption spectroscopy, which is an in situ optical method. An optical absorption region (P7.5ff transition at 1502.7 nm) of the OH radical was selected in the near-infrared range to measure the OH radicals quantitatively in premixed CH₄/air flames. An improved direct absorption spectroscopy (DAS) method based on wavelength division multiplexing was proposed to extract the H₂O absorption signal that interfered with the OH light absorption signal, and the integral intensity of OH* chemiluminescence was compared to the measured OH radical concentration based on the improved DAS method.

It is important to monitor the temperature and H₂O concentration in a large combustion environment in order to improve combustion (and thermal) efficiency and reduce harmful combustion emissions. However, it is difficult to simultaneously measure both internal temperature and gas concentration in a large combustion system because of the harsh environment with rapid flow. In regard, tunable diode laser absorption spectroscopy, which has the advantages of non-intrusive, high-speed response, and in situ measurement, is highly attractive for measuring the concentration of a specific gas species in the combustion environment. In this study, two partially overlapped H₂O absorption signals were used in the tunable diode laser absorption spectroscopy (TDLAS) to measure the temperature and H₂O concentration in a premixed CH₄/air flame due to the wide selection of wavelengths with high temperature sensitivity and advantages where high frequency modulation can be applied. The wavelength regions of the two partially overlapped H₂O absorptions were 1.3492 and 1.34927 μm. The measured signals separated the multi-peak Voigt fitting. As a result, the temperature measured by TDLAS based on multi-peak Voigt fitting in the premixed CH₄/air flame was the highest at 1385.80 K for an equivalence ratio of 1.00. It also showed a similarity to those tendencies to the temperature measured by the corrected R-type T/C. In addition, the H₂O concentrations measured by TDLAS based on the total integrated absorbance area for various equivalent ratios were consistent with those calculated by the chemical equilibrium simulation. Additionally, the H₂O concentration measured at an equivalence ratio of 1.15 was the highest at 18.92%.

In a combustion reaction of hydrocarbon fuel, carbon monoxide (CO) is a gas species that is closely related to air pollution generation and combustion efficiency. It has a trade-off with nitrogen oxide and increases rapidly in case of incomplete combustion or in fuel-rich (Φ > 1) environments. Therefore, it is essential to measure CO concentration in order to optimize the combustion condition. In the case of a steel annealing system, the combustion environment is maintained in a deoxidation atmosphere to prevent the formation of an oxide layer on the steel sheet surface. However, it is difficult to measure the CO concentration in a combustion furnace in real-time because of the harsh environment in the furnace. Tunable diode laser absorption spectroscopy, which has the advantages of non-invasiveness, fast response, and in situ measurement-based optical measurement, is highly attractive for measuring the concentration of a certain gas species in a combustion environment. In this study, a combustion system of a partially premixed flamed burner was designed to control the equivalence ratio for fuel-rich conditions. CO concentration was measured using a distributed feedback laser with a wavenumber of 4300.7 cm⁻¹ in the mid-infrared region. The results showed that the CO concentration measured at an equivalence ratio of 1.15 to 1.50 was 0.495% to 6.139%. The detection limit in the combustion environment was analyzed at a path length of 190 cm and an internal temperature of 733 K. The ranges of the peak absorbance were derived as 0.064 and 0.787, which were within the theoretical bounds of 10⁻³ and 0.80 when the equivalence ratio was varied from 1.15 to 1.50.

In urban atmospheric chemistry, NOx and NH₃ in the atmosphere are major species participating in the secondary aerosol formation process, causing severe environmental problems such as decreased visibility and acid rain. In order to respond effectively to particulate matter problems, the correlation of precursors should be identified in detail. This study used UV-C light to convert gaseous substances into particulate substances in the small-scale smog chamber to simulate the photochemical reaction. The effects of several operating variables, such as UV-C light intensity, relative humidity, and initial concentrations of O₂, NO, and NH₃, on the NH₄NO₃ formation were investigated. Since atmospheric gas species are short-lived, they require a measurement technique with an ultra-fast response and high sensitivity. Therefore, the concentrations of NO and NH₃ were measured using Direct Absorption Spectroscopy techniques with the wavenumber regions of 1926 and 6568 cm^-1, respectively. NO and NH₃ were precisely measured with an error rate of less than 3 % with the reference gas. The results show that NO and NH₃ were converted over 98 % when UV-C light intensity was 24 W and relative humidity was about 30 % at 1 atm, 296 K. It also showed that higher UV-C light intensity, O₃ concentration, and relative humidity induced higher conversion rates and secondary aerosol generation. In particular, it was experimentally confirmed that the secondary aerosol generation and growth process was greatly influenced by relative humidity.