Aim: Determining the influence of turbulence on the dispersion of sprays
The breakup and droplet dispersion of sprays is an important aspect of the combustion process, where it influences the efficiency and pollution of engines. It is hard to measure this due to the high velocities (close to the speed of sound) and small scales (droplets smaller than the head of a pin) of these sprays.
The shadow of a spray coming from a 0.2 mm nozzle hole, similar to the sprays coming from diesel injection nozzles (leftmost image source: Bosch). While the breakup happens over a large area, the droplets themselves are small.
To overcome this we use a phosphorescent fluid. This is a fluid that glows for a relatively long time after hitting it with a laser. We can then follow droplets made from this fluid with a high speed camera (20 000-140 000 frames per second) and measure how far and fast they travel (which indicates their dispersion and velocity). By tagging a small part of the spray with a laser sheet, we can measure the dispersion over time.
After tagging a small amount of fluid (left), it is possible to determine how much this fluid has changed 0.000067 seconds later. From this fluid velocities can be determined.
The dispersion itself is, however, influenced by many different effects: sudden changes in pressure, air that is sucked into the spray, or the design of the spray nozzle. To measure the effect of turbulence, we use a turbulence box. In this box we create turbulent by manipulating local pressure with eight large speakers. By adding droplets to the box with the same phosphorescence as in the sprays, we can see them move and disperse. In this way we can measure the effect of turbulence in spray dispersion, without the other effects.
Left: The box in which we create turbulence using speakers to force air jets. It has several windows for the laser and the camera. Right: Snapshot of glowing droplets inside the turbulence box.
Dennis van der Voort, Rudie Kunnen, Willem van de Water, Herman Clercx, GertJan van Heijst
Nico Dam (Department of Mechanical Engineering)