It is important for the different applications of fluid atomisation to understand the pattern and dimensions of the sprays. For example, spray (cone-) angle and penetration are of importance to the design of the IC engine manufacturer to assist the ignition setup and possible wetting in the combustion chamber walls.
Two different imaging methods can be used in the spray pattern investigation. The first is by using a (diffused) backlight aligned with a camera sensor. The camera in this case captures the image of the spray shadow. This method is called “shadowgraph” imaging, and it is relatively simple and low cost, as low power lasers can be used. The main problem in this method is that the produced shadowgraph of the spray is an even (solid) pattern, with no representation of the spray density in the space. This is because in the shadow image all droplets are treated equally (binary image). The experiments show that sprays with different flow rates could have similar overall pattern.
The second method is the light scattering imaging, where a laser sheet is used for illuminating the spray mist. The camera sensor in this case captures the image of the scattered light by the spray droplets. Droplets with different sizes scatter the light in different angles and amplitudes. Therefore, more information about the spray distribution can be obtained.
If the spray images were acquired in different test conditions, spray density maps could be generated for the evaluation of the operation conditions. The density map is a statistical representation of the distribution of the fluid surface area within the light sheet plane. So, the average of a number of images per test condition should be calculated, then visualised as a colour coded map as shown in the figure below. Fuel spray pattern was generated against the differential pressure, ranging between 1 to 5 bar.