FIRST

The main objectives of FIRST were to deliver a fuel spray atomisation prediction capability, which can represent the unsteadiness of the atomisation process, for gas turbine injectors and to deliver an improved soot / particulate modelling methodology for combustion Computational fluid dynamics (CFD).

SCITEK Consultants Experomental Fluid Mechanics for CFD Validation

Before the FIRST project there was very little information available to describe the properties of atomisation from Rolls-Royce aero engine rich and lean burn fuel injectors. Lefebvre correlations were typically used to estimate the fuel spray boundary conditions for CFD modelling of these injectors in combustor flows. This coarse method used for numerical modelling made accurate prediction of combustor flow fields, and consequently emissions, difficult. Actual measurements of the fuel particle sizes and velocities were required before further progress could be made in improving modelling techniques.

A new test facility was built in the combustion laboratories in Rolls-Royce Derby that was specifically designed to enable PDA measurements of fuel sprays from engine injectors. All of the PDA and flow visualisation measurements for this study were performed by SCITEK. Test geometries were designed and made that represented a single sector of an engine combustor so that the spray conditions were more closely representative of reality. Lean and rich burn fuel injectors were tested at a range of conditions relevant to their respective engine cycles and detailed measurements of droplet sizes and velocities were made at numerous locations. The work was extended to include sprays from alternative fuels as a comparison to a standard kerosene spray.

The measurements of the spray from the fuel injectors have now provided the boundary conditions required for CFD modelling of Rolls-Royce engine combustors and has described the progression of the atomisation process as the flow moves downstream away from the fuel injectors. This is vital information for the improvement of numerical models to predict engine reacting flows. The CFD models can now be validated against the spray measurements for a wide range of geometries and conditions and a step change in modelling accuracy will be the result. Future designs of aero engine combustors and fuel injectors will benefit from the improved modelling accuracy.

 

Pdf_icon_32x32 FIRST Overview Brochure

Wind Tunnel Seeders

Large Wind Tunnel Seeding Requirements

As part of any LDV and PIV measurements, seeding particles need to be introduced into the flow. For large size wind-tunnels the volume of air is large and “off the shelf” seeding generators cannot produce the density of seeding required. Smoke generators are generally considered to be the only devices that can generate large quantity of seeding and produce sub-micron particles. In LDV applications the small aperture of the receiving optics combined with the small size of the smoke particles produces weak signals and low data rates. In PIV applications the interrogation areas in large wind tunnels are required to be large and smoke particles are not ideal because they produce weak signals (or necessitate the use of high power lasers).

The SCITEK system solves these problems by making use of larger particles (1 to 6 microns) using high-pressure atomising nozzles. An additional advantage is that these nozzles deliver very large quantities of seeding particles (1010 particles per second). Also by using more than one nozzle the seeding can be spread over a large area and can be designed to produce high data rates, reducing experiment time and thus costs. For large wind tunnels the saving from reduced running time recovers the cost of the seeding system in just a few runs.

Applications

  • Approximately 1010 particles/second seed output per nozzle.
  • Small size nozzles present minimum disruption to the flow.
  • Each system is custom built to use as many nozzles as necessary.
  • High particle count ensures high data rates.
  • Stainless steel construction.
  • Can work at any back pressure (up to 1000psi).
  • Nozzles are mounted on rakes activated by valves remotely from the experiment.
  • Scalable to provide any quantity of seeding over large areas.
  • Can also be used in open-air experiments.

Particle Size Issues

The SCITEK seeder makes use of high-pressure (2000psi) impactor type atomising nozzles. The important characteristic of these nozzles is that they produce very small droplets (much smaller to those produced by swirl type atomisers). A typical distribution of particle counts across the full width of the spray cone of such a nozzle using water is shown in the figure. It can be seen that 72% of the droplets are below 6 microns which means that the majority of the droplets imaged by aPIV system will be below 6 microns. Although larger droplets are present their population, and consequently their influence in the measurements, is statistically very small. In cases where the larger droplets may be a problem SCITEK can provide aerodynamic filters which remove the larger droplets from the flow with almost no pressure loss to the flow.

Recommended Working Fluids

Both oil and water can be used with the SCITEK LS-HP seeders. For most applications oil is used to provide seeding particles into the flow because they do not evaporate as they travel through the tunnel. Where oil is not desirable for reasons of cost, availability or disposal difficulties, water can provide a suitable alternative. Obviously water is more easily available and can be disposed by evaporation or drainage at the end of the experiment. The disadvantage of using water as seeding material is that some evaporation may take place and reduce the quantity of small droplets, but on installations so far this has never proved to be a problem. One of our systems was installed on a high Mach number (0.8) wind tunnel where good PIV experiments were conducted with high data validation rates. From the images produced the size of the water droplets was estimated (from the scattered intensity) to be around 3 microns.

Powder Seeders

PS-10 & 20  Remote Operation Powder Seeder

Dantec Dynamics are acting as agents for our powder seeders.  For details and pricing please refer to the Dantec Dynamics web page, model number 10F01.

The PS-10 and PS-20 powder seeders are operationally equivalent, with the PS-10 seeder rated to a working pressure of 10 bar and the PS-20 seeder rated to 20 bar.

Applications

  • Powder Seeding for Particle Image Velocimetry
  • Powder Seeding for Laser Doppler Velocimetry
  • Flow visualisation
  • High temperature Combustion studies
  • High pressure flow applications (up to 20 bar [PS-20])
  • Seeding in Hostile Environments

PS-20 Powder seeder for high pressure applications

Specification

  • Up to 10 or 20 bar working back-pressure (PS-10 and PS20 respectively).
  • Controllable seeding density.
  • Dilution air available.
  • Stainless steel pressure vessel construction.
  • Remote Panel Controls – Electrically operated rate of powder dispersal and thus seed density. Pressure regulator to adjust air supply to seeder. Two pressure gauges to indicate air supply pressure and seeder back pressure. Compatible with solenoid valves for PLC or computer control.
  • Push-fit pipe connectors and colour coded air tubes for easy installation and adjustment of remote cable length.
  • Suitable for a variety of powders and particle sizes. Aluminium oxide or Titanium Dioxide powders are recommended. This seeder was specifically developed for LDA and PIV applications where small particle grain powders need to be used. For LDA applications it is recommended to use Aluminium Oxide powder of 0.3um size, available from us, or from metal polishing suppliers.
  • Maximum duration for continuous powder delivery is 30 minutes. A second powder container drum is provided for quick change-over.
  • Remote control panel enables the powder feed to be switched off when not needed thus extending the period between re-fills and minimising the quantity of powder used.
  • Quick re-fill operation without the need to stop experiment.
  • Controllability of seeder can be customised to suit other user requirements.

Performance

The biggest problem with other powder seeders is the break up of the powder to the smallest grain size. With grain sizes below 1um particles tend to agglomerate and unless are broken up by the seeder their ability to follow the flow is reduced. The Scitek PS-10 seeder employs sonic jets which create high shear flow fields which break up the powder while is being dispensed in the outlet chamber. The seeder was tested against an oil mist generator in a supersonic wind tunnel and measurements were made of the response of the particles along a normal shock wave using a Laser Anemometer operated in back-scatter. Aluminium Oxide powder with 0.3um size particles was used. As can be seen in figure 1 there is excellent agreement in the results obtained with the two types of seeding devices. The design of this seeder is such that clogging experienced by other powder seeders is inherently avoided. Our proprietary powder dispensing method also ensures uniform seeding density during operation.

Recommended Powders

Aluminium Oxide powders 0.3 microns in size or higher can be used and they are available from ourselves or from metal polishing suppliers. This unit was specifically designed for small size powders that do not flow readily. However, powders of larger grain size tend to flow more readily and can also be used.

Liquid Seeders

LS-10 Remote Operation Liquid Droplet Seeder

Dantec Dynamics are acting as agents for our liquid seeder.  For details and pricing please refer to the Dantec Dynamics web page, model number 10F02.

Applications

  • Droplet Seeding for Particle Image Velocimetry
  • Droplet Seeding for Laser Doppler Velocimetry
  • Oil mist for flow visualisation
  • Seeding for Mie Scattering combustion studies of flame shape
  • Aerosol research
  • Controlled oil mist for lubrication

Specification

  • Up to 10 bar working back-pressure.
  • Ten Laskin nozzles.
  • Approximate 108 particles/second. seed output per jet
  • Stainless steel pressure vessel construction.
  • Acrylic level indicator rated for 10bar max pressure.
  • Dilution air: Variable through central tube.
  • M6 tapped mounting holes compatible with NewportTM saddles.
  • Quick re-fill operation without the need to stop experiment.
  • Remote Panel: Pneumatically operate any number of nozzles (1-10) to control seed density remotely from the experiment.
  • Compatible with solenoid valves for PLC or computer control.
  • Push-fit pipe connectors and colour coded air tubes for easy installation and adjustment of remote cable length.
  • One way valves on all Laskin nozzles to avoid contamination of air supply pipes with seeding liquid.
  • Controllability of seeder can be customised to suit other user requirements.

Principle of Operation

The individual Laskin nozzles operate close to sonic conditions at typical air supply pressures. The high shear region between the emerging air and liquid atomises and entrains microscopic liquid droplets into bubbles which release the seed on bursting at the free surface. Bubble busting is a source of secondary atomisation that gives larger droplets, which are prevented from exiting the seeder by the horizontal baffle plate at the top of the device. In this device ten identical Laskin nozzles at the base of the device can be operated independently to give an easily controlled and repeatable seed density. Further dilution of the seed is made possible by means of an additional dilution jet that bypasses the liquid and mixes with the emerging mist.

Recommended Working Fluids

Typical seed materials include corn oil, olive oil, and silicone fluids (eg Dow Corning). Corn oil is frequently used in wind tunnel applications at atmospheric pressures and temperatures, as it is inexpensive and produces micron-sized droplets. Olive oil has a higher flash point or decomposition temperature, and is the liquid of choice for many internal combustion engine flows. Where operation at very high temperatures, and reduced pressures is required, silicone vacuum fluids are preferred. These liquids were developed for use in vacuum diffusion pumps and combine very low vapour pressure with high flash points and surface tension and kinematic viscosities similar to the vegetable oils. It is also possible to use this seeder to provide solid particle seeding by diluting powders with water or methanol.

Products

SCITEK Products

SCITEK has extensive experience of experimental fluid dynamics and of all the major laser diagnostic techniques, particularly in demanding and hostile environments. Where commercially available hardware was not found to be suitable for producing the best possible results, SCITEK developed its own. 

Our products have been developed from meany years of experience in flow measurements using optical techniques and are classed in three categories: Seeding Systems, Spray Cone Instrumentation and LDA and PDA Alignment tools. 

Seeding Systems

Our seeding systems are for use in flows where optical instrumentation (LDA and PIV) and disperse liquid or solid particles to be carried that are small enough to follow the flow so that fluid velocity can be measured. Our range of products covers small and large scale flow rigs. Our seeder designs have been developed to the point that we believe them to be superior to the alternative products in a wide range of applications and we can now offer them as high quality products. Please click on the links below to take you to the relevant product page. We are also happy to supply bespoke seeders engineered for individual requirements.

Pdf_icon_32x32 Read how NASA used SCITEK Seeder in CFD Validation Work.

 

Spay Cone Instrumentation

SCITEK offers instrumentation that can be used to measure the spray cone of spray nozzles. In its basic form the instruments uses a camera to image the spray and uses software developed by SCITEK to process the images to work out the angle of the spray. Illumination of the spray is not included in the basic product as most customers have their own illumination method. However, SCITEK can supply low cost illumination hardware to suit customer requirements.

  • Click here for more details.

LDA and PDA Alignment Tools

With regards to calibration tools SCITEK provides a system that can be used to carry out a black box calibration of the accuracy of frequency measurement of LDA and PDA processors. This product utilises a flashing LED that is driven by a calibrated frequency signal generator. A small 3D traverse is supplied with the LED source to allow accurate placement of the light source to the focusing point of receiving optics. The LED supplied is matched to the wavelength of your system. Visit the product page now.

For calibrating PDA systems SCITEK can provide a small water tank and calibrated latex particles that allow the user to evaluate the accuracy of their particle sizing system.