A comparison of shadowgraphy and X-ray computed tomography in liquid spray analysis

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.