Particle Image Velocimetry (PIV) for Micro-Droplets in Still Air

Images of JP8 (kerosene) fuel sprays were acquired using microscopic shadowgraphs in the University of Warwick . The fuel was injected by a (5 bar) gasoline injector (for shadowgraph images of a similar experiments see http://z-nee.co.uk/home/?p=36).   The tip penetration of a spray can be calculated dirctly by image processing such as image enhancement and edge detection. However, for a better understanding of the development process of a spray, a more comprehensive representation of the velocity within the different parts of spray is required.

Particle Image Velocimetry (PIV) technique produces velocity vector maps which can be generated during different stages of the injection incident. PIV is an image correlation technique that estimates the displacement between an image elements by dividing them into small windows, and then calculating the maximum correlation (magnitude and direction)  using successive image-frames. Smaller correlation (interrogation) windows usually generate a higher resolution vector map. However, a smaller correlation window requires a higher concentration of the seeding particles, which cannot be always controled in the case of fluid sprays. The seeding particles here are the fluid droplets themselves which vary in size and shape.

Laser Imaging of Large Droplets in Fuel Sprays

drop

Kerosene droplet (approx. 2.2 mm in diameter) photograph; demonstrating the light vectors behaviour inside a fluid droplet (in air).

 Fuel droplets, with diameters larger than 200 times the laser’s wavelength, were detected in great quantity during our experiments on fuel injection systems equipped with heating matrixes. A large number of particles condensed on the heater grid, forming giant droplets, which were later released with the flow. Laser imaging technique was performed to visualise fuel sprays. For small droplets (<100 microns), only droplets located inside the light sheet area and within the sensor sensitivity region were detected. For large droplets, several illumination patterns and light intensity levels were identified.Large spheres are often dealt with in literature as a special type of optics, since light diffraction is not the only dominant effect in this case. When a light beam encounters an obstacle, the obstacle scatters the light energy into different angles. The term “light scattering” describes a number of mechanisms in which a droplet/particulate emits electromagnetic waves as a secondary source of energy. For particles with a certain degree of transparency, the refracted light through the particle generates secondary scatters, in addition to the primary edge scatters, due to a set of internal reflections inside that particle . Beside the light diffraction and interreflections, large particles show a big deal of light reflection off the external surface (see the figure). The latter effect becomes much more dominant in particles much larger than the wavelength, and for high refractive indices.

Fuel Combustion Fundamentals in IC Engines

Fuel combustion

A hydrocarbon fuel is an organic compound consisting of hydrocarbon molecules. A molecule is a group of atoms tied together by chemical bonds. A hydrocarbon fuel basically contains only C and H atoms, mixed with O and N from air. A fuel could also contain a small amount of sulphur, oxygen, nitrogen, and impurities such as water and sand. Depending on the carbon to hydrogen ratio, petroleum fuels are divided into families as follows: Paraffin (CnH2n+2 such as Methane, Ethane, Propane, and Butane), Olefin and Naphthene (CnH2n, such as Hexene and Cyclopentane), and Aromatic (CnH2n-6 such as Benzene). An internal combustion (IC) engine is a device which transforms the chemical energy of a fuel into thermal energy and utilize this thermal energy to perform useful work. A fuel is heated up either by an electric spark or by compression (diesel engines).  A four-stroke engine completes its power cycle in four stages (180° x 4= 720°): 1-suction  2- compression (ratio 6-10) 3-exansion (power stroke) 4- exhaust. The four stokes can be expressed through a pressure-volume diagram (p-V).  There is only one power stroke for every two revolutions of the crankshaft.  Fuel combustion effectively takes place between compression and expansion. Its lifetime is about 10% of a rotation (typically between 1 and 3 ms). There is time needed for the ignition lag and for the propagation of flame.

Spray Density Map Using Imaging Method

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.

Evaporation Rate of Kerosene (Jet) Fuel

An isolated fuel drop in a hot oxidizing atmosphere is a source of fuel vapour which is surrounding the droplet surface within a limited distance called “flame reaction zone” or flame radius. The reaction zone forms a flame envelope around each droplet where the fuel vapour defuses into the oxidant and reacts with it. In other words, the reaction zone is the “sink” where both the fuel substance and oxygen are consumed and turned into thermal energy [1]. Understanding the single drop combustion is important in the overall spray combustion mechanism.  the “sensitivity” of the air-fuel mixture to explode increases as the fuel drop size decreases. Recent publications show strong relationship between particle size and soot formation . Large particles do not completely burn, and thus they turn into particulate matters (soot) to be later exhausted into the air.

Static Drop Formation

A fluid static drop is a drop formed under the influence of the gravity force only. The surface tension force tends to maintain the integration of the fluid bulk, and keep it attached to the tube surface.  A drop with a specific mass is formed once the gravity force exceeds the surface tension force, and this happens with no external pressure. The effect of the surrounding air in further drop breakup is neglected here. Although this type of fluid disintegration action seems to be basic and impractical for atomisation applications, the analysis of static drop formation may give an important indication of the influence of fluid physical properties, such as density and surface tension, on the process (or ability) of atomisation using different nozzle diameters.

Microscopic particles imaging

Microscopic characteristics of fluid spray are investigated using microscopic imaging. This includes particle size distribution and particle velocity. The microscope provides a close-up view to a small section of the spray as small as 1 mm2. Double frame images have been produced with a 5- 30 µs time separation between frames. Those images could be used to generate velocity vector maps. A histogram of particle size distribution can be obtained for evaluating the atomization performance of the injector. Shadowgraphy method along with Nd-YAG lasers are used for micro-imaging of the fluid droplets size between 4 and 50 micrometer in diameter.

microscopic view of high-speed kerosene particles. Average particle diameter is 17 micrometer

Fuel Spray Visualisation

Our broad research interests include digital imaging and image processing and their applications in fluid characterisation. Digital imaging system was developed with the purpose of performing LASER diagnostics in high-speed fuel spray jet.

Fuel jet with 4 ns duration of laser illumination (by R. Zakaria, University of Warwick)

A fuel injection system have been evaluated prior to application in a rotary “Wankel” engine; the objective being to improve combustion efficiency and engine performance. Incomplete combustion produces various air pollutants such as carbon monoxide, nitrogen oxide, and nitrogen dioxide. Factors such as particle size distribution and injection- sparkling timing play an important role in improving the thermal efficiency of an engine. Fuel particle size is directly related to the design and quality of the atomiser. The more effective a fuel atomiser is, the smaller droplets it produces, thus increasing the fuel surface and evaporation rates. Increasing the burning range leads to a higher released power rates and lower exhausted pollutant emissions. High-power lasers and high-resolution CCD cameras are employed to perform the experimental work. Image processing is applied to investigate the spray behaviour and characteristic including spray profile, spray-cone angle, spray tip penetration and velocity, and spray development process. Microscopic characteristics of the fuel spray have been investigated as well using long distance microscopes. This includes particle size distribution and particle velocity (PIV).