SP Fire Research AS carried out in 2006 and later investigations on the effect of Deluge systems on oil- and gas fires, partly as generic experimental work and also as baseline for development of computational models.
To be able to calculate how water from the Deluge systems were heated and evaporated, the droplet sizes were crucial to know. Laser Doppler Anemometry, LDA, which was quite new at that time, was used to characterize several Deluge nozzles, and some unexpected results appeared. Most of the droplets that were measured had a diameter less than 1000 µm (1 mm). This was not the common knowledge of droplet sizes for this type of nozzles.
To be able to evaluate if the LDA-measurements gave trustable results, SPFR carried out parallel measurement s of droplet sizes at two laboratories, with a LDA-technique and a photographic technique based on Laser light.
The result of the research was that the photographic method showed far more large droplets, and a closer look at the earlier measurements showed that a non-optimal optic system had been used to analyze the droplet sizes. By using a different type of optics in the LDA system, the difference between the two types of measurement was reduced. The difference was largest for droplet sizes above 1200 µm.
The conclusion from the project was that there are differences in results for droplet size distribution, by using different measurement techniques. A significant source of error was the use of non-optimal optics in the LDA measurements, leading to that the largest droplets were not measured. One cannot conclude definitely which of the measurement techniques that gives correct results, since the photographic technique also showed to have some sources of error with regards to which droplet that were disregarded in the counting routine.
The report about these measurements is now made public, and may be downloaded here.