Summary of "Non-intrusive spray measurements techniques"
Summary of Scientific Concepts and Techniques Presented
Non-Intrusive Spray Measurement Techniques Overview
The focus is on non-intrusive optical techniques used to measure spray characteristics such as velocity and droplet size. The main techniques discussed include:
- Particle Imaging Velocimetry (PIV)
- Particle Tracking Velocimetry (PTV)
- Phase Doppler Particle Analyzer (PDPA)
Particle Imaging Velocimetry (PIV) and Particle Sizing
PIV is primarily used to obtain velocity fields by tracking particles in motion. Particle sizing can be integrated into PIV by analyzing the intensity of scattered light.
Mie Scattering Theory
- Describes how light interacts with spherical droplets, involving reflection, refraction, and internal reflection.
- The scattered light intensity pattern (Mie scatter pattern) features lobes at different angles:
- Forward scatter lobes (main lobe, secondary lobe)
- Backscatter lobe
- The intensity of scattered light at a 90° scatter angle is proportional to the square of the droplet diameter: [ I \propto d^2 ]
Calibration
- A calibration constant ( k ) relates intensity to droplet diameter squared: [ I = k \times d^2 ]
- Calibration involves using droplets of known size to determine ( k ).
Challenges
- Variation in scatter angle across the field of view causes intensity changes, making ( k ) angle-dependent.
- Proper calibration across the entire image field is necessary to correct for this angular dependence.
Output
- Provides spatial distribution of total or mean surface area of droplets rather than individual droplet sizes.
- Useful for estimating surface area density in sprays.
Phase Doppler Particle Analyzer (PDPA)
PDPA measures both droplet size and velocity at a single point using laser optics.
Setup
- Two coherent laser beams intersect to create an interference fringe pattern within a measurement volume (~1 mm³).
- Transmitter: Splits laser into two beams with a phase shift introduced by a Bragg cell (phase shifter).
- Receiver: Consists of three spatially separated detectors collecting scattered light.
Measurement Principle
- Droplets passing through the fringes scatter light modulated by the fringe spacing.
- The scattered light intensity over time forms a “Doppler burst.”
Velocity Measurement
- Velocity is calculated from the frequency of intensity oscillations in the Doppler burst.
- Fringe spacing ( d ) depends on laser wavelength and beam intersection angle.
- Velocity formula: [ v = d \times f ] where ( f ) is the Doppler frequency.
- Fast Fourier Transform (FFT) is used to extract the peak frequency from the Doppler burst.
Size Measurement
- Droplet size affects the fringe spacing observed due to refraction inside the droplet.
- Multiple detectors measure phase differences in the Doppler burst signals.
- The phase difference between detectors correlates with droplet diameter (dependent on refractive index).
Third Detector
- Used for validation to ensure measurements are from refracted light rather than reflected light.
Limitations
- Single-particle measurement technique; high particle density near nozzles can cause overlapping signals.
- Typically used downstream where particle density is low (~1 particle/mm³).
Key Methodologies Outlined
Particle Sizing with PIV
- Capture grayscale intensity image of the spray.
- Use calibration with droplets of known size to find constant ( k ).
- Convert grayscale intensity to surface area distribution.
- Correct for scatter angle dependence through spatial calibration.
Velocity and Size Measurement with PDPA
- Generate two coherent laser beams with a phase shift.
- Focus beams to create a measurement volume with interference fringes.
- Detect scattered light intensity time series (Doppler bursts) with multiple detectors.
- Use FFT on intensity signals to find Doppler frequency → velocity.
- Use phase difference between detectors → droplet size.
- Use third detector for signal validation.
Researchers/Sources Featured
No specific researchers or external sources were explicitly named. The content appears to be from a lecture or tutorial by an expert in spray measurement techniques, possibly an academic or researcher in fluid mechanics or optical diagnostics.
This summary captures the core scientific principles and methodologies related to non-intrusive spray measurement techniques, focusing on PIV-based sizing and PDPA for simultaneous size and velocity measurement.
Category
Science and Nature
