Quantitative 2-D Fuel Vapor Concentration Measurements in an Evaporating Diesel Spray using the Exciplex Fluorescence Method

To experimentally investigate evaporating sprays under conditions experienced in high speed direct-injection (HSDI) diesel engines, the exciplex LIF technique with the TMPD / naphthalene dopant system was applied in a combustion-type constant-volume spray chamber. The chamber allows spark ignition of a slightly rich C₂H₂-air mixture, and subsequent fuel injection into the high temperature and pressure products. A detailed set of calibration experiments has been performed in order to quantify the TMPD fluorescence signal. It has been demonstrated that the TMPD fluorescence intensity is directly proportional to concentration, is independent of the chamber pressure, and was not sensitive to quenching by either water vapor or carbon dioxide. Therefore, the temperature dependence of the TMPD fluorescence was the only correction factor required for quantitative measurements. Using a dual heated-jet experiment, the temperature dependence of TMPD fluorescence up to 1000 K was measured. The temperature field in the spray images was determined using a simple mixing model, and an iterative solution method was used to determine the concentration and temperature field including the additional effects of the laser sheet extinction and laser sheet profile variations. The integrated fuel vapor concentration compared favorably with the measured amount of injected fuel for times when all of the liquid fuel had evaporated. The data indicate that early in the injection event liquid and vapor coexist at the spray leading edge, however the liquid length reaches a terminal value and the vapor phase continues to penetrate. The vapor concentration past the intact liquid length has an equivalence ratio from 2 - 3, similar to previous research. At temperatures higher than 1000K, the exciplex technique was found to have limited application due to thermal decomposition.