Analytical Technologies Singapore

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Laser Doppler Velocimetry (LDV)

Laser Doppler Velocimetry (LDV) is a technique which allows the measurement of velocity at a point in a flow field with a high temporal resolution. Whenever a micron-sized liquid or solid particle entrained in a fluid passes through the intersection of two laser beams, the scattered light received from the particles fluctuates in intensity.
 
Laser Doppler Velocimetry (LDV) makes use of the fact that the frequency of this fluctuation is equivalent to the Doppler shift between the incident and scattered light, and is thus proportional to the component of particle velocity which lies in the plane of two laser beams.

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Rayleigh scattering

Rayleigh scattering is the elastic scattering of light by particles much smaller than the wavelength of the light. That is the case for gas phase molecules and, therefore, this method is suited for laser imaging in gases. Rayleigh scattering of sunlight by atmospheric molecules is the reason for the observed blue colour of the sky, because the scattering efficiency varies inversely with the fourth power of the wavelength. 

For a single component gas with known scattering cross section the Rayleigh signal is directly proportional to the gas density. The scattered light is almost at the same wavelength as the incident light, i. e. Rayleigh scattering is not species selective. Rayleigh scattering requires either constant gas composition or known mole fractions of all major species for the density measurement of a gas mixture. In some cases Rayleigh scattering is stronger for one species than another, and it can be used to image mixing processes such as fuel – air mixing.

When gas composition and pressure are known Rayleigh imaging allows to measure planar temperature fields (Rayleigh Thermometry). Rayleigh scattering is much weaker than Mie scattering but more than two orders of magnitude stronger than Spontaneous Raman Scattering. Incandescence from soot and Mie scattering are processes that can totally obscure the Rayleigh signal.

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Interferometric Mie Imaging

IMI imaging of single droplets

Interferometric Mie Imaging (IMI) is a sizing technique for the evaluation of the diameter of spherical particles, droplets and bubbles similar to PDI. The working principle is based on the out-of-focus imaging of particles illuminated by a laser light sheet. The optical setup of a standard IMI system consists of a laser-light sheet and a digital camera with a high quality lens. Moving the camera chip to an out-of-focus position an interference fringe pattern becomes visible.

The visible fringe pattern corresponds exactly to the far field scattering which can be calculated by the Mie theory. The number of fringes within the aperture image depends on the diameter of the droplet and the aperture angle. With increasing particle size, the number of fringes increases. The exact size of the particles is determined by analysis of the fringe patterns. The size of the aperture image is a measure for the z-position of the particle along the line-of-sight.

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Mie scattering

Mie scattering is elastic scattered light of particles that have a diameter similar to or larger than the wavelength of the incident light. The Mie signal is proportional to the square of the particle diameter.
 
Mie scattering is much stronger than Rayleigh scattering and, therefore, a potential source of interference for this weaker light scattering process. There is a strong angular dependency of the scattered intensity especially for smaller particles which has to be considered for successful Mie imaging experiments. Mie scattering is often used to measure flow velocities applying Particle Image Velocimetry (PIV).

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Emission Spectroscopy: Spectral Imaging

Based on emission spectroscopy flame emission is analysed using spectral imaging. Flame emission imaging is a line-of-sight method like absorption and probes electronically excited species in contrast to LIF which probes ground state species. 2D flame species imaging is carried out with a set of filters mounted in front of the camera lens or 1D using an imaging spectrograph for correlated multi-species detection.


The flame emission (chemiluminescence) of flame radicals like OH*, CH*, C2* is used to e.g. visualize the reaction zone or to monitor flame stoichiometry via emission ratios of such radicals. Infrared emission visualises major flame components like unburnt fuel, CO2 or water.


2-color pyrometry (blackbody radiation) is applied to measure temperature fields in sooty flames. Temporal flame analysis (flame flicker) is measured using statistical information from a time series of flame images. High speed imaging is applied for flame propagation and extinction measurements.

 

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Particle Image Velocimetry (PIV)

What is PIV?

Particle image velocimetry is an optical method of flow visualization used in education and research. It is used to obtain instantaneous velocity measurements and related properties in fluids.

In Particle Image Velocimetry (PIV) light scattering particles are added to the flow. A laser beam is formed into a light sheet illuminating seeding particles twice with a short time interval Δt. In 2D-PIV the scattered light is recorded onto two consecutive frames of a high resolution digital camera. 

Stero Particle Image Velocity (PIV)

In Stereo-PIV two cameras at different observation angles are used to measure also the third (out-of-plane) component of the flow velocity in the light sheet.


For velocity calculation the particle image of each camera is subdivided into small interrogation windows. The average particle displacement within an interrogation window is determined by cross-correlation followed by the localisation of the correlation peak. From the known time difference Δt and the measured displacement in each direction the velocity components are calculated. Perspective correction, distortion compensation and image mapping of the two views are taken care of by Self-Calibration procedures. Advanced multi-pass image deformation techniques are used for higher accuracy and spatial resolution.

Spatial & Temporal Derivatives

From one velocity field a range of spatial derivatives can be calculated such as vorticity and shear stress. Ensemble statistics provide additional information like turbulent kinetic energy or Reynolds stresses. Time-resolved velocity fields recorded with high-frame-rate cameras and high frequency lasers allow for deeper dynamic insights about flow field evolution, fluid element trajectories, acceleration and turbulence statistics.

Tomographic PIV

From one velocity field a range of spatial derivatives can be calculated such as vorticity and shear stress. Ensemble statistics provide additional information like turbulent kinetic energy or Reynolds stresses. Time-resolved velocity fields recorded with high-frame-rate cameras and high frequency lasers allow for deeper dynamic insights about flow field evolution, fluid element trajectories, acceleration and turbulence statistics.

Contact us to find out how PIV can aid in your current & future researches.

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