Ultrasonic Atomization: New Spray Characterization Approaches

In particle engineering, spray drying is an essential technique that depends on producing sprays, ideally made of equal-sized droplets. Ultrasonic sprays appear to be the best option to achieve it, and Faraday waves are the background mechanism of ultrasonic atomization. The characterization of sprays in this atomization strategy is commonly related to the relation between characteristic drop sizes and the capillary length produced by the forcing frequency of wavy patterns on thin liquid films. However, although this atomization approach is practical when the intended outcome is to produce sprays with droplets of the same size, drop sizes are diverse in real applications. Therefore, adequate characterization of drop size is paramount to establishing the relations between empirical approaches proposed in the literature and the outcome of ultrasonic atomization in actual operating conditions. In this sense, this work explores new approaches to spray characterization applied to ultrasonic sprays produced with different solvents. The first two introduced are the role of redundancy in drop size measurements to avoid resolution limitation in the measurement technique and compare using regular versus variable bin widths when building the histograms of drop size. Another spray characterization tool is the Drop Size Diversity to understand the limitations of characterizing ultrasonic sprays solely based on representative diameters or moments of drop size distributions. The results of ultrasonic spray characterization obtained emphasize: the lack of universality in the relation between a characteristic diameter and the capillary length associated with Faraday waves; the variability on drop size induced by both liquid properties and flow rate on the atomization outcome, namely, lower capillary lengths produce smaller droplets but less efficiently; the higher sensibility of the polydispersion and heterogeneity degrees in Drop Size Diversity when using variable bin widths to build the histograms of drop size; the higher drop size diversity for lower flow rates expressed by the presence of multiple clusters of droplets with similar characteristics leading to multimodal drop size distributions; and the gamma and log-normal mathematical probability functions are the ones that best describe the organization of drop size data in ultrasonic sprays.

Investigation on Flat-Fan Spray Characterization in High-Speed Air Coflow for Gas Turbine Online Water Washing Application

Fouling in compressor blades due to dirt deposition is a major issue in land-based gas turbines as it impedes the compressor performance and degrades the overall engine efficiency. The online water washing approach is an effective alternate for early-stage compressor blade cleaning and to optimize the time span between offline washing and peak availability. In such case, typically a series of flat-fan nozzles are used at the engine bell mouth to inject water sprays into the inflowing air. However, optimizing the injector operating conditions is not a straightforward task mainly due to the tradeoff between blade cleaning effectiveness and material erosion. In this context, the knowledge on spray characteristics prior to blade impingement play a vital role, and the experimental spray characterization is crucial not only to understand the basic process but also to validate numerical models and simulations. The present paper investigates spray characteristics in a single flat-fan nozzle operated in the presence of a coflowing air within a wind-tunnel. A parametric investigation is carried out using different air flow velocity in the tunnel and inlet water temperature, while the liquid flow rate was maintained constant. The spray cone angle and liquid breakup length are measured using back-lit photography. The high-speed shadowgraphy technique is used for capturing the droplet images downstream of the injector exit. The images are processed following depth-of-filed correction to measure droplet size distribution. Droplet velocity is measured by the particle tracking velocimetry (PTV) technique. As both droplet size and velocity are known, the cross-stream evolution of local droplet mass and momentum flux are obtained at different axial locations which form the basis for studying the effectiveness of the blade cleaning process due to droplet impingement on a coupon coated with foulant of known mass.

On the Statistical Characterization of Sprays

The statistical characterization of sprays is an essential way of organizing data on drop size and velocity to provide reliable information on the spray dynamics. A clear presentation of data using statistical tools provides evidence of a clear research question underlying the spray characterization. In this article, a review of the best practices to build histograms is presented, as well as three relevant details on spray characterization: (i) the application of information theory to assess if we have enough information (not data); (ii) the link between mathematical probability distributions and the physical interpretation of spray data; (iii) and introducing, for the first time, the concept of drop size diversity, with the quantification of the polydispersion and heterogeneity degrees. Finally, the view presented is applied to the characterization of nanofluid sprays for thermal management.