Two-dimensional OH-thermometry in reacting flows using photofragmentation laser-induced florescence

Abstract

In the present work, the possibility of using photofragmentation laser-induced fluorescence (PFLIF) for thermometry in different reacting flows is investigated. Hydroxyl (OH) fragments are created by UV-laser (266nm) fragmentation of hydrogen peroxides (H2O2 and HO2), whereupon the fluorescence, induced while scanning the wavelength of a second laser across the A2Σ+(v = 1)–X2Π(v = 0) absorption band (around 282nm) of the generated OH fragments, is collected and detected. The temperature is determined by fitting simulated OH-excitation spectra of different temperatures to the experimentally recorded spectrum. In combustion, hydrogen peroxides are intermediate species formed during the low-temperature oxidation of the fuel, and hence they are present in a region covering a wide temperature span, ranging from unburnt to burnt gas temperatures. Thus, LIF of OH photofragments stemming from hydrogen peroxides allows for thermometry covering a wider temperature range than LIF of naturally present OH radicals. There is another important advantage of the concept in that the temperature sensitivity of OH excitation spectra is greater at lower temperatures. The method is demonstrated for two-dimensional (2-D) thermometry in three different measurement situations, namely a free flow of vaporized H2O2 at room temperature, a preheated mixture of CH4/N2/O2/O3 at intermediate temperatures (300–600K), where the OH fragments stem from photodissociation of O3 followed by chemical reactions, and in an optical homogeneous charge compression ignition (HCCI) engine prior to ignition, i.e. at elevated pressures and temperatures. It is found that the technique performs well in all three cases, with measured temperatures in good agreement with thermocouple readings, for the two first cases, and with temperatures calculated based on the ideal gas law using measured pressure traces as input for the engine measurements. The quantitative 2-D temperature images acquired in the engine experiments reveal inhomogeneous temperature distributions, clearly illustrating the capacity of the technique to yield crucial experimental input to engine modelers and designers. The accuracy of the technique in the temperature range 300–600K is lower than 23K. For the room temperature case the precision is 4.3%, corresponding to 12K.