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Time and Spatially Resolved Measurements of the Interaction of Combusting Diesel Spray and Walls with Elevated Temperatures
Abstract:
The interaction between a combusting diesel spray and a wall at temperatures of 700K and 735K was investigated in a combustion chamber using optical measurement techniques. The temperatures were chosen as they appear in the range of the maximum piston surface temperatures of the latest production engines. Combustion was investigated with a dual camera setup, which is designed to take simultaneous pictures of the UV flame luminosity (FL_UV) and the visible flame luminosity (FL_VIS). The FL_UV is used to measure lean or stoichiometric combustion. The FL_VIS is capable of detecting the thermally excited soot. Mie scattering is used to study the liquid fuel phase.
It was found that there is almost no FL_VIS signal visual in the 700K case, but a very strong signal in the 735K case. In general, one might expect that higher wall temperatures lead to an improved mixture formation and, consequently, lower soot production. However, the opposite was detected. The reason for this is that the ignition delay is much larger at the colder wall and, therefore there is more time for premixing. Also the FL_OH signal is much weaker at the lower wall temperature. This can be explained by the fact that the OH-radical is not only a marker of lean combustion, but also a marker of soot oxidation.
In addition, the ignition delays are compared at different measurement settings. It can be assumed that the ignition delays are shorter in the hot wall cases and also shorter at the higher ambient gas pressures. The comparison between unaffected spray and wall-influenced spray shows that ignition delays for the unaffected spray cones are always smaller or equal to those for wall-influenced ignition. Moreover, the differences between ignition delays tend to be smaller at the higher wall temperatures.