In the present work, the spectral properties of gaseous ozone (O) have been investigated aiming to perform quantitative concentration imaging of ozone by using a single laser pulse at 248 nm from a KrF excimer laser. The O molecule is first photodissociated by the laser pulse into two fragments, O and O. Then the same laser pulse electronically excites the O fragment, which is vibrationally hot, whereupon fluorescence is emitted. The fluorescence intensity is found to be proportional to the concentration of ozone. Both emission and absorption characteristics have been investigated, as well as how the laser fluence affects the fluorescence signal. Quantitative ozone imaging data have been achieved based on calibration measurements in known mixtures of O. In addition, a simultaneous study of the emission intensity captured by an intensified charge-coupled device (ICCD) camera and a spectrograph has been performed. The results show that any signal contribution not stemming from ozone is negligible compared to the strong fluorescence induced by the O fragment, thus proving interference-free ozone imaging. The single-shot detection limit has been estimated to ∼400 ppm. The authors believe that the presented technique offers a valuable tool applicable in various research fields, such as plasma sterilization, water and soil remediation, and plasma-assisted combustion.
The stabilization characteristics and local extinction structures of partially premixed methane/air flames were studied using simultaneous OH-PLIF/PIV techniques, and large eddy simulations employing a two-scalar flamelet model. Partial premixing was made in a mixing chamber comprised of two concentric tubes, where the degree of partial premixing of fuel and air was controlled by varying the mixing length of the chamber. At the exit of the mixing chamber a cone was mounted to stabilize the flames at high turbulence intensities. The stability regime of flames was determined for different degree of partial premixing and Reynolds numbers. It was found that in general partially premixed flames at low Reynolds numbers become more stable when the level of partial premixing of air to the fuel stream decreases. At high Reynolds numbers, for the presently studied burner configuration there is an optimal partial premixing level of air to the fuel stream at which the flame is most stable. OH-PLIF images revealed that for the stable flames not very close to the blowout regime, significant local extinction holes appear already. By increasing premixing air to fuel stream successively, local extinction holes grow in size leading to eventual flame blowout. Local flame extinction was found to frequently attain to locations where locally high velocity flows impinging to the flame. The local flame extinction poses a future challenge for model simulations and the present flames provide a possible test case for such study.
IntroductionPartially premixed flames, defined as flames where the compositions of the mixture vary from fuel-rich to stoichiometry and fuel-lean [1], are found in many engineering applications. In modern internal combustion engines using multiple injections of fuel to control emissions, partially premixed charge are typically formed before ignition. Due to the presence of rich, lean and stoichiometric mixtures in partially premixed flames, lean and rich premixed flame fronts exist in the leading front followed by the main diffusion flame [2][3][4]. The combustion characteristics of partially premixed flames are not well understood and modeling of partially premixed flames is challenging [1,2,5].Local flame quenching and re-ignition in laminar and turbulent flames is an issue that has attracted the attention of recent research. Once a non-premixed flame is locally quenched, edge flame exists at the extremity of the reaction zones, where the flame progressively evolves to partially premixed flames [6]. The critical strain rate for quenching of partially premixed flames is influenced by the degree of partial premixing. For laminar flames, experimental and theoretical studies [7,8] using a counter flow configuration showed that partial premixing of fuel to the air stream can increase the quenching strain rate, whereas partial premixing of air to the fuel stream can decrease the critical quenching strain rate. In high Reynolds number turbulent flames, the quenching process is more complex. In an
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.