from Part III - Spray Formation and Impact onto Surfaces
Published online by Cambridge University Press: 13 July 2017
A description of collision phenomena involving drops and/or sprays requires a characterization of the drops before and after the collision as well as information about possible liquid films if impact on a solid surface is involved. The present chapter is devoted to the various techniques used to visualize and characterize drops, sprays and films. Independent of the measurement technique employed, collision phenomena are often described in terms of statistical quantities and Section 7.1 provides some fundamental definitions in common use. The remainder of the chapter, dealing with measurement techniques for drops and sprays, is divided into three sections: non-optical measurement techniques (Section 7.2), direct imaging techniques (Section 7.3) and non-imaging optical techniques (Section 7.4). The measurement of liquid films on a surface is treated separately in Section 7.5.
Very general reviews of measurement techniques for drops and sprays can be found in textbooks (Lefebvre 1989, Liu 1999), handbooks (Crowe 2005) and review articles (Bachalo 1994, Chigier 1983, Jones 1977); however, many techniques discussed have been superseded by more recent developments of imaging and non-imaging optical methods. The field of optical diagnostics has developed rapidly in recent years, primarily due to improvements in illumination technology (LEDs, solid-state lasers, etc.) and camera/detector technology, offering higher temporal and spatial resolution visualization of transient phenomena. Perhaps for this reason more recent review articles and handbook entries addressing spray measurement technology concentrate more on developments of optical techniques, e.g. (Bachalo 2000, Fansler and Parrish 2015, Tropea 2011).
Fundamentals
In this section some basic relations expressing the most common quantities necessary to describe impacting drops and sprays onto surfaces – input and outcome – will be presented, with special attention on how these quantities are derived from experimental measurements. The most important fundamental quantities to be acquired are
• flux density distributions (e.g. number or diameter flux density)
• local concentration (e.g. number or mass concentration)
• local probability density function (PDF) of particle properties (e.g. diameter, velocity, and their moments).
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