High-pressure sprays are widely used in various field of industry such as combustion or rocket engines and paint applications. However, quantitative imaging of atomizing sprays is particularly challenging due to the presence of a variety of irregular liquid structures such as ligaments, liquid blobs, droplets, liquid sheets and a possible liquid core. This is why the only measurements of the liquid mass have been obtained using X-ray radiography on synchrotron, thus limiting the number of studies. We report here the first radiography of a fuel-port injection spray using betatron X-rays emitted from a laser plasma accelerator (see attached file).
For this experiment, we used the High Intensity Laser system at Lund University that provides on target 800mJ, 38fs laser pulses. It was focused down to 12µm focal spot onto a gas jet (99% He, 1% N2) flowing from a 1mm supersonic nozzle. The betatron radiation emitted had a critical energy of 2.3keV and propagated outside of the vacuum chamber where an X-ray camera recorded the shadow of the spray.
The spray flowed from a commercially available port fuel injection system used in small diesel engines with 4.6 bars of backing pressure. The liquid consisted either of water and 0.1% of fluorescein either of a mixture of water and 10% of an X-ray contrast agent (KI).
We have found that we reached a liquid mass sensitivity of 60µm in a single shot for pure water and 30µm for water with KI, i.e. about two times better than the latest synchrotron measurement. Moreover, this sensitivity was achieved while observing a four times larger field of view.
In parallel to that, a small fraction of the laser pulse (approx. 10mJ) was send directly on the spray and focused with a cylindrical lens where it induced fluorescence via a 2 photon process (2p-LIF) in a dye (fluorescein) added to the liquid. The 2p-LIF images provides a great level of details on the size and shape of the liquid structures, optically sectioned by the light sheet, while the integrated liquid mass is extracted from the X-ray radiography.
This is making the two imaging techniques highly complementary for the characterization of spray systems as well as for further understanding the physics related to liquid atomization. We believe that this new application will greatly benefit industry and hydrodynamic simulation.
Attached image: Images of the liquid jet recorded with X-ray (top) and fluorescence (bottom) taken simultaneously 350µs after the beginning of the injection. On the right is a shadowgraph of the same jet taken at for a different event. The red rectangle represents the Light sheet use for two photon LIF.
|Working group||Secondary radiation generation & applications|