Laser-Based Measurements of Surface Cooling Following Fuel Spray Impingement 2018-01-0273
A major source for soot particle formation in Gasoline-Direct-Injection (GDI) engines are fuel-rich zones near walls as a result of wall wetting during injection. To address this problem, a thorough understanding of the wall film formation and evaporation processes is necessary. The wall temperature before, during and after fuel impingement is an important parameter in this respect, but is not easily measured using conventional methods. In this work, a recently developed laser-based phosphor thermography technique is implemented for investigations of spray-induced surface cooling. This spatially and temporally resolved method can provide surface temperature measurements on the wetted side of the surface without being affected by the fuel-film. Zinc oxide (ZnO) particles, dispersed in a chemical binder, were deposited onto a thin steel plate obtaining a coating thickness of 17 μm after annealing. Following pulsed UV excitation, a temperature-dependent luminescence signal (< 1 ns) is captured by two CCD cameras, equipped with different spectral filters. The change in the ratio of this pair of luminescence fields is used to infer the change in temperature. The coated plate was homogeneously heated to a set temperature of 353 K. UV-grade n-hexane was injected using a current GDI, solenoid driven, 6-orifice injector. The third harmonic of a Nd:YAG laser (355 nm) at 10 Hz was used as excitation source. Precise surface temperature readings were performed for different times, 5 ms and 8 ms, after the end of injection. The impact of the rail pressure (50 bar, 150 bar, 300 bar) and injector energizing time (0.5 ms, 1.5 ms, 3.0 ms and 6.0 ms) on the impingement-induced cooling were investigated as well. Effective cooling area and surface temperature evolution after impingement results are used to compare and analyze the influence of the aforementioned parameters.