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Jun 06, 2023

Meeri Kim

The experimental setup for the new 3D imaging method includes a single high-speed camera, two xenon lamps and a series of fiber bundles. This equipment is all relatively affordable compared with the more complex and specialized setup used in other techniques. [Image: Q. Lei, Northwestern Polytechnic University]

Schlieren imaging is an optical technique capable of visualizing invisible flow structures, such as those in gases, air and other transparent media. It can capture shock waves from a trumpet, heat rising off a human hand or a jet of warm air from a hair dryer.

Now, researchers in China say they have developed a high-speed 3D schlieren approach that can image fundamental turbulent flame properties during combustion (Opt. Lett., doi: 10.1364/OL.496333). The technique improves upon previous 3D schlieren methods by using only a single high-speed camera instead of multiple, as well as boosting the temporal resolution.

Traditional schlieren imaging employs light from a single collimated source shining on or behind a target object. Any spatial variations in density caused by factors like pressure or temperature lead to changes in the refractive index, distorting the beam and resulting in a 2D image of fluid flow.

Recently, progress has been made in extending schlieren measurements to three dimensions. Most approaches up to this point have required several cameras to capture flow information from different perspectives, followed by tomographic reconstruction to create a 3D distribution of flow properties. However, disadvantages to these methods include limited temporal and spatial resolutions, difficulties in processing data and the high cost of equipment.

In the latest research, Qingchun Lei and his colleagues demonstrated a new 3D schlieren technique that combines fiber imaging, traditional schlieren imaging and computed tomography (CT). With their system, which only includes a single high-speed camera, they could simultaneously capture the schlieren images of turbulent flames from seven orientations with a frame rate beyond tens of kilohertz.

The complex behavior of the turbulent flames produced during combustion. Shown on the left are two cross sections of the 3D density measurement; the horizontal slice is at Z = 16 mm and the vertical slice at X = 0 mm. On the right is the 3D isosurface of the largest density gradient between the mixture and burned product. It depicts turbulent wrinkles and flame pockets. [Image: Q. Lei, Northwestern Polytechnic University]

“The high-speed imaging approach we developed provides detailed insights into flame dynamics, ignition processes and combustion behavior,” said study author Lei, Northwestern Polytechnic University, in a press release accompanying the research. “This can provide insights into combustion efficiency, pollutant emissions and the optimization of energy production processes that could be used to improve the design and operation of power plants, engines and other combustion devices, leading to reduced environmental impact and enhanced energy efficiency.”

The light source consisted of two xenon lamps, two fan-out fiber bundles and seven collimating lenses. The fiber bundles split the light into seven individual rays, after which the lenses expanded and guided the light to pass through the flame area. On the detection side, the imaging setup included seven convergent lenses, seven knife edges to block some of the incoming light, a bifurcate imaging fiber bundle and a CMOS high-speed camera.

Lastly, the researchers used CT reconstruction and postprocessing to obtain 3D schlieren images, along with 3D density and velocity information. The system successfully measured both turbulent and stable laminar premixed flames, as well as the transient, dynamic ignition process, at a lower cost and higher speed than previous methods.

“The detailed understanding of flame behavior and ignition processes facilitated by this technique can also contribute to more effective fire safety measures by providing information on how fires spread, develop and can be suppressed,” said Lei. “This can be used to enhance fire prevention strategies, improve building designs and develop more efficient fire suppression systems that could ultimately help save lives, protect property and improve overall fire safety standards.”

Publish Date: 03 August 2023