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Measurement Techniques for Droplet Properties and Intelligent Control of Droplet Processes

Measurements of droplet properties are required in many areas of scientific research and engineering applications. For example, accurate and detailed measurements of distributions of droplet properties in sprays are needed for understanding spray processes and validating spray modeling. The droplet properties to be measured include droplet size, velocity, mass flow rate (flux), temperature, deformation history, and final splat dimensions, among others. Accurate spray diagnostics require developments in instrumentation to increase the span of droplet sizes down to submicron scale and to measure distributions and flux quantities in sprays with significantly higher number densities than current normal levels. It is also desired to measure droplet temperature, composition (for multi-component fuels), and non-sphericity (for liquid breakup), as well as other properties, jointly with the measurements of droplet size and velocity in high number-density, highly turbulent sprays.

Various measurement techniques have been developed and applied with different degrees of success. Some techniques, that are well applicable to the measurement of single droplet properties, find difficulties in the application to measurements of droplet properties

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398 Science and Engineering of Droplets

in practical sprays. The difficulties may be caused by the large number of droplets, wide range of droplet properties (such as wide droplet size range, high and varying droplet velocities and temperatures), and/or simultaneous phase changes that lead to variations of droplet properties with time through evaporation, solidification, chemical reactions and/or coalescence. In many spray combustion processes, for example, dramatic variations in droplet properties may occur due to the high pressures and temperatures in the sprays. Such severe conditions further increase the difficulties in accurate and reliable spray diagnostics. These are among the factors to be considered when selecting a measurement technique to meet the requirements of a specific application.

For the measurement of droplet properties in a spray, no intrusive techniques are often desired. The measurement should not create disturbance to the spray pattern. An ideal measurement technique should have a large range of capability to measure both spatial and temporal distributions. Such technique should tolerate wide variations in droplet properties at some extreme conditions present in sprays in various engineering applications. An appropriate technique should also be able to acquire sufficient representative samples to ensure a reasonable measurement accuracy. Rapid sampling and data processing means are hence needed for the analysis of measurement results. The sampling, data acquisition and processing system must be fast enough to record every droplet passing through a measurement volume when measuring the number density of droplets in a spray. The system should be able to reproduce in real time the mean droplet properties, their distributions and standard deviations in histogram form. Although these features are often required in practical applications, it is virtually impossible for a single measurement system to provide all these functions.

Many laser-based droplet diagnostic techniques have evolved from the fields such as spray combustion and spray drying. PhaseDoppler particle analyzer is now recognized as the most successful and advanced diagnostic instrument for spray characterization. Other proven diagnostic techniques include laser velocimetry and