Unburned gas temperature measurements in a surrogate Diesel jet via two-color toluene-LIF imaging

Non-intrusive temperature measurements of the unburned fuel/air mixture in vaporized Diesel jets have been performed using two-color toluene laser-induced fluorescence (LIF). This diagnostics technique exploits the temperature-dependent spectral shift of the LIF signal which occurs after ultraviolet (UV) excitation of toluene that is added as tracer to a non-fluorescing base fuel. The method requires the determination of the ratio of LIF intensities collected by two detectors separate spectral bands. In the current study, measurements were performed in a high-pressure, high-temperature cell capable of reproducing the thermodynamic conditions in the combustion chamber of a Diesel engine during the injection event. Various aspects of the experimental set-up and the data evaluation were optimized. The temperature sensitivity of the measurement strategy is optimum at temperatures below 700 K. Temperature data acquired from two-color LIF thermometry were compared to single-color toluene-LIF measurements using an adiabatic mixing model. The latter is determined from toluene LIF-based fuel concentration measurements, the evaporation enthalpy, and thermocouple measurements of the bath-gas/ambient cell temperature prior to fuel injection. Based on simultaneous measurements with two cameras using identical optical filters a methodology to optimize the image superposition and to minimize the statistical error was developed. These measurements also allowed to determine the 1 − σ precision of the two-color LIF measurement to be in the 20–40 K range.