摘要：

Liquid spray and atomization systems are widely used in industrial, commercial, and household applications. For example, pressure jet and swirl atomizer are found in liquid dispensing systems from fuel injectors in large scale power generation engines to small scale liquid sprayers, mainly due to their good atomization characteristics, low costs, and simple geometries. Therefore, it is of practical importance to understand the breakup and atomization processes in these systems.;The liquid-air interaction, surface tension, and viscous forces are the primary factors that govern the liquid breakup processes. The stability of the ejected liquid sheet from swirl atomizers plays an important role in spray development, liquid breakup, and atomization. Liquid properties directly influence the surface stability of liquid sheets, which consequently affects the downstream atomization quality. The effect of fluid viscosity and surface tension on spray structures, cone angle, and breakup length were studied experimentally in this work. High speed visualization was applied to capture the spray images. A MATLAB image processing program was developed to analyze the spray characteristics. The wave frequencies for fluids with a range of different viscosities were calculated by Fast Fourier Transform (FFT) in order to study the temporal wave propagation characteristics. A laser diffraction technique was used to measure the droplet size and its distribution at different locations along the spray developing trajectory. Results showthat liquid viscosity plays an important role on the spray primary breakup process. Spray cone angle is found to decrease as the viscosity increases, while the breakup length gives an opposite trend. Fluid surface tension is not a critical parameter that determines the primary breakup process, but it affects the downstream droplet size and distribution significantly, namely the secondary breakup or atomization process.;Liquid film thickness inside the swirl atomizer nozzle plays an important role in controlling the initial sheet thickness and primary breakup. In order to study the internal flow of swirl atomizers, a transparent swirl atomizer was designed. The liquid film thickness was measured using a high speed camera under different injection pressures. Fluids with different viscosities were applied to study their influences on the film thickness. Results show that higher pressures lead to thinner initial film thickness and as the viscosity increases, film thickness becomes thicker.;In addition, there are many practical processes involving liquid evaporation during the breakup and atomization process. In order to study the effect of evaporation on liquid jet breakup, experiments were conducted using circular and rectangular orifice nozzles under low pressures. Five ambient temperatures and ten injection pressures were used to study the liquid jet breakup length and wavelength on the jet surface. Correlations between nondimensional number and liquid jet characteristics were developed. This work reveals that the ambient temperature has a strong influence on liquid jet breakup length under low jet velocity conditions. But as the jet velocity increases, its influence becomes weak. The surface wavelength was found in a linear relationship with jet velocity under different ambient temperatures.;In summary, the external and internal characteristics of low pressure jets and sprays were studied in this dissertation. The quantitative measurements contribute valuable experiment data to support further understanding of the fundamental physical processes of liquid breakup and atomization. The results in this dissertation also provide more accurate initial inputs for spray model validations.

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