Fuel injection System

Spray Characterization

Spray Characterization

In order to verify that we have successfully designed a spray system that achieves the desired droplet size we must characterize the spray. Spray characterization will include determination of droplet size and distribution at various locations within the flow field. The method that we believe will be the cheapest and allow us to utilize existing equipment is a laser illumination system.  This system consists of a pulsed laser illumination source, beam diffuser (beam expander) and a high speed camera. This configuration allows us to capture a 2-D slice of a 3-D flow. This will permit us to quickly determine the ideal location for the placement of an ignition source based on droplet size. It should be noted that the flow velocity will have some effect on the flame front depending on the type of flame initiation and initial energy input.

The target values as stated by the Air Force is a location within the spray where the mean droplet diameter can be determined to be at the range desired for each test run. Using the flow visualization techniques suggested previously the entire flow can be quickly evaluated to locate the desired conditions. In addition to the target size droplets being identified we can also identify areas of the spray where ignition may more easily occur. Such areas where fine mist and large droplets coexist may be easier to ignite than areas weighted by a larger number of large droplets.

In order to keep the characterization of the nozzle repeatable variability in the test area must be eliminated. Standard test procedures for spray characterization should account for the following environmental factors. To minimize flow disruption we must ensure that there are no external sources of wind in the test area. The test area air will be at approximately room temperature and all testing should be done at a relatively constant temperature. The temperature of the fuel will greatly affect the rate of fuel droplet break-up by decreasing the vapor pressure of the fuel, thus allowing the fuel to atomize more freely. Therefore the fuel temperature will need to be kept constant. The testing will need to cover a number of different fuel delivery pressures to find the ideal operating conditions for each nozzle.

 
Minimal System

This system is most effectively used with a laser and diffuser screen to create a laser light sheet. Spreading of the beam can also be accomplished my directing the laser through an optical quality glass rod. Also it is necessary for this laser source to be pulsed in order for us to utilize a strobe-like effect to capture the still image. The imaging system which is available to us through the AFRL is capable of a FPS (frames per second) speed of 30000 which is much faster than necessary for our purposes. If the frame rate can be reduced to 100 FPS then we can significantly increase picture quality and image resolution which will aid in manual droplet sizing.

 
Test System

Figure: “Oxford lasers: Imaging Division”

Typically the system setup is simple. The components include a laser light source in a visible spectrum of about 500 nanometers in wavelength. Commonly used is a green Nd:YAG laser between 200W and 1000W peak output. A beam spreader/light sheet generator is used to diffuse the beam into a flat sheet. The optical system could be a high speed film camera, CCD camera, film camera, or a digital camera. In order to minimize cost we will attempt to utilize a high speed camera provided by our sponsor, which is capable of frame rates as high as 30,000 FPS. However, due to the degradation of picture quality at 30k FPS we will operate at approximately 30 FPS and if necessary we could increase the rate until we achieve the appropriate image quality best suited for the flow characterization.

In order to process the image to find the target droplet size the images must be manually evaluated. This will include setting up the test area with reference grids. Then counting and sizing the droplets for each area of interest. This approach is recommended only when testing involves a few events and where the droplet size is large enough to be visually compared to a reference.

Suggested System(s)

Alternatives to manual count and measure system include both imaging and non-imaging systems. Imaging systems utilize programs that count the number of droplets per image. Non-imaging techniques include Phase Doppler Interferometer (PDI), laser diffraction analyzers and optical probe arrays. In this phase of the design we would be best suited to use the imaging technique to quickly and cheaply confirm that we have either reached our target or not. This would allow us to develop procedures for producing suitable droplets consistently. However, to provide the means to baseline a CFD model, we would highly recommend a non imaging system. A PDPA system with the supporting software can gather the necessary data in real-time. This would greatly increase the number of validation tests that can be performed. The PDPA system is a flux measurement device that measures droplet size and velocity as well as distribution within the “probe volume”. The following description from Spraying Systems Co. describes the general PDPA system.

"Phase Doppler particle analyzers (PDPA)

PDPA are flux sampling instruments and fall into the non-imaging (single particle counter) category. These analyzers consist of a transmitter, receiver, signal processor and computer. The PDPA uses a low-power laser that is split into two beams or four beams for a 2-dimensional system. By utilizing a beam splitter and frequency module; these laser beams intersect again at a point referred to as the probe volume. When a drop passes through the probe volume, the scattered light forms an interference fringe pattern. The scattered interference sweeps past the receiver unit at the Doppler difference frequency, which is proportional to the drop velocity. The spatial frequency of the fringe pattern is inversely proportional to the drop diameter. A data analysis routine is used to convert the raw drop count into a meaningful drop size distribution.  The PDPA measures sizes in the 0.5 to 10,000 μm range using various optical configurations……."   (Spray Systems  Co.)

 
There are several manufacturers of Phase Doppler analyzers:

 • Artium Technologies, Inc., Sunnyvale, CA

(http://www.artium.com)

• Dantec Dynamics A/S, Skovlunde, Denmark

(http://www.dantecdynamics.com)

• TSI Incorporated, Shoreview, MN (http://www.tsi.com)

(Spray Analysis and Research Services (manual), pg. 11)

Phase Doppler Particle Analyzer: Figure courtesy of Spraying Systems Co.: “Understanding Drop Size”

In order to effectively relate the flow conditions to parameters readily usable by CFD modeling software a complete system offered by a laser imaging company should be used. These companies provide hardware and software that can rapidly evaluate the flow and provide data that can be more easily related to the CFD model, thus allowing verification of the model.

Suggested systems include the following:

“Artium Technologies Inc.’s PDI–X00MD is a modular system based on a laser Phase Doppler Interferometer. This system consists of an optical transmitter, receiver, signal processor and software. Most importantly the software which is called “Aims” can provide droplet size and velocity information. It is not indicated whether or not that this system provides correlations to volume flux and statistical analysis or is these values have to be calculated independently.

From TSI Incorporated the PDPA-System 1-component (velocity measurement) system is appropriate for the flow evaluation. It includes an emitter, receiver, signal processor and analysis & graphics software. This system provides data analysis on droplet size, velocity as well as statistical analysis with the provided “Flowsizer 2.0” software. This system also provides graphical representations of the flow characteristics and statistics in easily read charts.

Oxford laser systems incorporated has a laser based system which falls into the optical analysis category. This system utilizes software to count and measure droplets related lens and focal length. This system may be suitable for our needs. However, one must consider whether or not the spray pattern might obscure the inner areas of the flow.  The software in this system provides size, velocity and statistical data, in easily read graphs. It also provides a picture test area which is valuable in determining non-spherical droplet characteristics.

Of the three preceding systems the most desirable is the TSI PDPA system. This company offers the most accessories and provides a significant amount of detail in their methods. Also, their accompanying software provides more flow analysis than the other companies. 

Statistical Analysis

Volume Median Diameter (VMD) will be used to represent the spray droplet size due to its wide spread use by manufactures. This will give the ability select and test nozzles that will deliver the desired droplet size. The VMD is based on relating the droplet size to the total fluid volume. Specifically the VMD is where 50% of the fluid volume is accounted for in droplets smaller than the target value, and 50% of the fluid volume is accounted for in droplets larger than the target value. Also it may become necessary to find regions within the flow that have different characteristics. For certain tests areas with a large population of small droplets and few large droplets might be preferable. Locating these areas can be done by tabulating the droplet sizes (at varying pressures, distance…) into a histogram which would relate size to frequency of occurrence. In such a case the VMD would not be used, however it is an important tool in standardizing each test and a starting point in the classification of the nozzle characteristics.

NISTIR 6640:  Thermochemical and Thermophysical Properties of JP-10

Bruno, Huber, Laesecke, Lemmon, Perkins

National Institute of Standards and Technology, June 2006

Combustion Engineering

Borman, Ragland

McGraw Hill, 1998

Fluid Dynamics  

Moscow: Maik, Nauka.

Handbook of Mathematical Fluid Dynamics  

Friedlander, Serre

Fluid Power Dynamics 

 Mobley

Fluid Dynamics and the Transport of Droplets and Sprays 

 Sirignano

Fuels of Opportunity: Characteristics and Uses in Combustion Systems  

Tillman

Combustion Chemistry: Elementary Reactions to Macroscopic Processes 

 Faraday Division, Royal Society of Chemistry

Numerical Study On the Formation of Spray Cloud By Explosive Dispersal of Water Willed in V-shaped Trough

J.C. Liu, J. Ding, Y.T. Hu and L.F. Xie

Website: www.spray.com

An Engineer’s Practical Guide To Drop Size

By Rudolf J. Schick

Spraying Systems Co. 1997