Yingchun Wu

Coal powder measurement by digital holography with expanded measurement area

The field of view of digital in-line holography for flow field diagnostics is restricted to a small volume due to the finite size and the low spatial resolution of the available CCD. Expansion of the measurement cross section of digital holographic particle image velocimetry was investigated with a lens-based holography configuration. By sampling the chirp signal in the center lobe completely and undersampling the chirp signal in the second- and higher-order lobes by a magnified virtual recording plane produced by an imaging camera lens, the field of view is expanded. Simulation results show that the three-dimensional (3D) location and size of the relatively large particle can be reconstructed with good accuracy. A digital holographic particle image velocimetry system was established for coal particle flow field diagnostics. Compared with the lensless configuration, the field of view of the digital holography system was enlarged 1.9 times, up to 2.78 cm × 2.78 cm × 3 cm. The 3D location, size distribution, and the 3D vector field of coal powder were obtained. The results show that the application of digital in-line holography to measure large particle flow field is feasible.

Modified convolution method to reconstruct particle hologram with an elliptical Gaussian beam illumination

Application of the modified convolution method to reconstruct digital inline holography of particle illuminated by an elliptical Gaussian beam is investigated. Based on the analysis on the formation of particle hologram using the Collins formula, the convolution method is modified to compensate the astigmatism by adding two scaling factors. Both simulated and experimental holograms of transparent droplets and opaque particles are used to test the algorithm, and the reconstructed images are compared with that using FRFT reconstruction. Results show that the modified convolution method can accurately reconstruct the particle image. This method has an advantage that the reconstructed images in different depth positions have the same size and resolution with the hologram. This work shows that digital inline holography has great potential in particle diagnostics in curvature containers.

Concentration and composition measurement of sprays with a global rainbow technique

Applications of the global rainbow technique to measure the concentration of a sprayed bi-component solution or the composition and the relative proportions of two sprays of different solutions at a local point are investigated. For a dual spray, the global rainbow signal is processed by optimally fitting the initial global rainbow signal with two global rainbow signals. For each composition, the value of the refractive index and size distribution is measured. In the case of spray mixing, the relative proportion of each composition can be retrieved from this information. The algorithms are validated by the processing of simulated global rainbows. Experimental rainbows of water–ethanol solutions with volume concentration from 0% to 100% and two sprays of water and ethanol are measured. The limitations of the global rainbow technique for concentration and composition measurements are discussed.

Direct measurement of particle size and 3D velocity of a gas–solid pipe flow with digital holographic particle tracking velocimetry

The 3D measurement of the particles in a gas-solid pipe flow is of great interest, but remains challenging due to curved pipe walls in various engineering applications. Because of the astigmatism induced by the pipe, concentric ellipse fringes in the hologram of spherical particles are observed in the experiments. With a theoretical analysis of the particle holography by an ABCD matrix, the in-focus particle image can be reconstructed by the modified convolution method and fractional Fourier transform. Thereafter, the particle size, 3D position, and velocity are simultaneously measured by digital holographic particle tracking velocimetry (DHPTV). The successful application of DHPTV to the particle size and 3D velocity measurement in a glass pipes flow can facilitate its 3D diagnostics.

Phase interferometric particle imaging for simultaneous measurements of evaporating micron-sized droplet and nanoscale size changes

We have developed phase interferometric particle imaging (PHIPI) of Mie scattering to simultaneously measure the spherical and transparent droplet size at the micron scale and transient size changes at the nanoscale. The theoretical derivation of PHIPI reveals that the phase of interferometric fringes between direct transmission and reflection linearly shifts with the change in droplet size. After a proof-of-concept validation with simulation, a PHIPI system is then implemented with an elaborately designed Fourier imaging system and applied to measure single evaporating droplets of ethanol and n-Nonane. The results show that the PHIPI technique can determine the size change down to several nanometers, providing a powerful tool for accurate in-situ characterization of droplet dynamics, e.g., droplet evaporation and oscillation.

Intrinsic spatial shift of local focus metric curves in digital inline holography for accurate 3D morphology measurement of irregular micro-objects

A theoretical model of digital inline holography system reveals that the local focus metric curves (FMCs) of different parts of an irregular micro-object present spatial shift in the depth direction which is resulted from the depth shift. Thus, the 3D morphology of an irregular micro-object can be accurately measured using the cross correlation of the local FMCs. This method retrieves the 3D depth information directly, avoiding the uncertainty inherited from the depth position determination. Typical 3D morphology measurements, including the 3D boundary lines of tilted carbon fibers and irregular coal particles, and the 3D swimming gesture of a live Caenorhabdities elegans, are presented.

Characterizations of transparent particle holography in near-field using Debye series

The effects of the individual scattering process on the formations of both the particle hologram and its corresponding reconstructed three-dimensional particle image are investigated using the Debye series. A particle hologram model using the Debye series decomposes the object wave into different scattering modes and thus permits evaluating the effects of the individual scattering process [diffraction, reflection, transmission, refractions with (p-1) internal reflections] on the particle holography quantitatively. In the Gabor inline holography of a transparent droplet, the transmission light causes small discrepancies between the hologram fringes of an opaque particle (diffraction) and a transparent particle near the zero point of the Bessel-like modulation function, eventually giving rise to the glory spot in the center of the reconstructed dark particle image. For off-axis holography, this paper reveals the effects of reflection, particularly total reflection by bubbles, transmission, and refractions with (p-1) internal reflections of the scattered light on the formation and the reconstructed glory spot images of typical forward and backward off-axis holography.

Asymmetric wavelet reconstruction of particle hologram with an elliptical Gaussian beam illumination

We propose an asymmetric wavelet method to reconstruct a particle from a hologram illuminated by an elliptical, astigmatic Gaussian beam. The particle can be reconstructed by a convolution of the asymmetric wavelet and hologram. The reconstructed images have the same size and resolution as the recorded hologram; therefore, the reconstructed 3D field is convenient for automatic particle locating and sizing. The asymmetric wavelet method is validated by both simulated holograms of spherical particles and experimental holograms of opaque, nonspherical coal particles.

Measurement of microchannel flow with digital holographic microscopy by integrated nearest neighbor and cross-correlation particle pairing.

A micro digital in-line holographic particle tracking velocimetry (micro-DHPTV) system has been developed and applied to investigate the three-dimensional flow field in straight and Y-junction microchannels. The micro-DHPTV system comprises a cooled frame-transfer CCD camera and a double-pulsed laser. The processing algorithm introduced to evaluate the three-dimensional velocity is based on the combination of integrated cross-correlation and nearest neighbor matching algorithms, taking advantage of information from both the reconstructed particle field and the original holograms fringes patterns. Tests on simulated pairs of holograms show that the particles can be detected, located, and paired with high probability and accuracy. Results obtained in the straight and Y-junction microchannels show that the superimposed vector field is physically reasonable.

Coal Particle Measurement in a Pulverized Coal Flame with Digital Inline Holography

Digital inline holography with pulse illumination was applied to measure the 3D position and size of the burning coal particles in a laboratory-scale pulverized coal flame under strong temporal and spatial environment’s refractive index variations.