This work examines and compares two proven techniques for assessing key characteristics of liquid sprays for combustion applications: shadowgraphy and time-averaged X-ray computed tomography (CT). Atomization has key applications in combustion as it can improve fuel efficiency, increase heat release, and decrease pollutant emissions. To improve the design of fuel injection nozzles, the ability to conduct accurate analyses of sprays is crucial. Key characteristics of the liquid spray, such as mean particle diameter, spray-cone angle, mass distribution, and penetration length give insight into the effectiveness of a nozzle. Shadowgraphy is a relatively inexpensive method that produces a two-dimensional, instantaneous image of the spray particles or spray called a shadowgram. Shadowgrams can be used for analyzing mean particle size, spray-cone angle, and location of breakup regions. X-ray CT measures the time-averaged X-ray absorption of two-dimensional projection images of spray to produce a three-dimensional reconstruction of the spray. X-ray CT can provide valuable insight into the symmetry and mass distribution of a spray; however, X-ray absorption diminishes rapidly with increased distance from nozzles, thereby limiting analysis to the regions near the nozzle. A detailed comparison of the overall effectiveness and insights yielded by the two methods illustrates the unique uses, benefits, and shortcomings of each method. The results confirm that X-ray CT scanning proves more effective in the dense, near-nozzle spray region. Shadowgraphy effectively complements the X-ray CT analysis through particle analysis. It also allows for relatively simple spray cone analysis, though it cannot provide quantitative mass distribution analysis.

Characterizing the near-nozzle region of liquid sprays is a challenge for spray system designers due to the lack of optical accessibility in this dense region of the spray using traditional imaging techniques. A novel diagnostic technique, utilizing standard medical X-ray computed tomography equipment, is developed to investigate the time-averaged mass distribution of the liquid phase of the spray within 50 diameters downstream of the nozzle exit. Reconstructed images provide the three-dimensional concentration of the liquid phase of the spray. Analysis of several iodine-based liquids was conducted based on physical properties and non-dimensional parameters. To compare a general procedural approach for obtaining the three-dimensional concentration field from an X-ray CT system, a traditional optical analysis was conducted using Mie Scattering and Shadowgraphy. Results for a full-cone pressure swirl atomizer with an orifice of 0.79 mm are presented for both water and Povidone Iodide. Qualitative and quantitative results gave insight into the spray cone angle, atomization of the spray, and the droplet size and velocity distribution 50 diameters downstream. The water (Re ≈ 20,000) atomizes further upstream than the Povidone Iodide (Re ≈ 6,100) with a larger spray cone angle. The images obtained from the traditional optical techniques were qualitatively compared to the new diagnostic technique. The three dimensional mass distribution of Hypaque from a 0.79 mm nozzle has a number of similarities with the water spray, measured with an isotropic spatial resolution of 200 μm and noise per voxel less than 0.02 g/cm3.