The accuracy of laser Doppler and phase Doppler measurements in a very dense spray can be affected by the fact that the signals are very noisy and thus the achievable data rate is very low. Indeed, in some cases only a small fraction of the drops passing through the measurement volume will be detected and validated and furthermore, those drops which are validated may also be non-representative of the total drop population, for instance larger drops may be preferentially validated because of their higher scattering intensity.
To systematically investigate some of the influencing parameters in such situations, an experimental set-up comprising a common rail Diesel system, a single hole nozzle injector and a commercial Dual-PD system (Dantec) has been assembled. The injection duration was purposely kept very long, up to 8 milliseconds, to better analyze the main injection period. It is this phase of the injection which is generally more difficult to investigate, while the beginning and the end of the injection often yield very satisfactory data rates. This is probably due to the overall density of the spray, which is lower during these phases and thus provide better optical access.
The influence of the laser power, of the photomultiplier high voltage and of the detected burst SNR validation level on the number of measured drops per injection (data rate), on their average velocity and on their diameter was studied. The optimal parameter value ranges were found, the uncertainty in drop diameter measurement was evaluated and a characterization of the spray was performed to study also the influence of the measurement location inside the spray on the achievable results. Some further tests and data processing elucidated the physical reasons for these influences and also led to compensation procedures for some situations. Using the optimized system parameters, up to 1500 single drop measurements per injection were possible at distances of 50mm from the nozzle exit. This high temporal resolution of the spray revealed velocity oscillations throughout the injection period, which in the past have not been observed.