Investigation of the Flow Field in the Upper Part of a Cyclone with Laser and Phase Doppler Anemometry

The cyclone is a well known apparatus for separating particles out of a gas stream. With the modern laser diagnostic technologies of laser and phase Doppler anemometry (LDA and PDA), there is the potential to measure the flow and particle field inside the cyclone. The gas phase only measurements used micron‐sized oil seeding droplets, whereas the solid phase, chosen for the PDA particle size measurements, was limestone powder. To assess the possibility of measuring milled limestone particles with PDA, the measured size distribution was compared with those obtained by laser diffraction. The measurements inside the cyclone showed that the flow field in the upper part of the cyclone was different to that commonly thought. Therefore, the vertical height of the cyclone's vortex finder could be shortened without deterioration of the separation efficiency. The particles found in the hold‐up of the cyclone air flow were considerably larger than the average particle size in the feed pipe.     

Numerical study of glare spot phase Doppler anemometry

The phase Doppler anemometry has (PDA) been developed to measure simultaneously the velocity and the size of droplets. When the concentration of particles is high, tightly focused beams must be used, as in the dual burst PDA. The latter permits an access to the refractive index of the particle, but the effect of wave front curvature of the incident beams becomes evident. In this paper, we introduce a glare spot phase Doppler anemometry which uses two large beams. The images of the particle formed by the reflected and refracted light, known as glare spots, are separated in space. When a particle passes through the probe volume, the two parts in a signal obtained by a detector in forward direction are then separated in time. If two detectors are used the phase differences and the intensity ratios between two signals, the distance between the reflected and refracted spots can be obtained. These measured values provide information about the particle diameter and its refractive index, as well as its two velocity components. This paper is devoted to the numerical study of such a configuration with two theoretical models: geometrical optics and rigorous electromagnetism solution.     

Directional achromatic heterodyne fiber laser Doppler anemometer

The application of diode-pumped double-clad Nd:glass fiber lasers with high fundamental mode power for laser Doppler anemometry (LDA) will be presented. An acousto-optic modulator (AOM) is used as a diffractive beam splitter, so that the LDA calibration constant is wavelength independent, i.e., achromatic. Hence, broadband emitting powerful fiber lasers can also be employed. The directional discrimination of the LDA velocity measurement was achieved using the heterodyne technique with the frequency shift of the AOM element.     

Particle-Induced Limits of Accuracy in Laser Doppler Anemometry

Laser Doppler anemometry (LDA) is one of the most advanced optical measuring techniques for flow velocities and is widely used in industrial and academic laboratories. Based on numerous applications in the past, there is no doubt that LDA is one of the most accurate flow measuring techniques. However, recent investigations have shown that the period lengths of LDA signal bursts are not as constant as one might expect within the individual burst. This can induce an additional scatter in the signal frequency and in the determination of flow velocity from individual bursts. This paper describes experimental investigations which show qualitatively and quantitatively that the particle passage through the laser beams shortly before the point of intersection, i. e. the LDA measuring volume, yields a distorted LDA fringe pattern. The latter results in a scatter of the measured velocity data for those particles passing the center of the measuring volume at the same time.     

Planar dropsizing by elastic and fluorescence scattering in sprays too dense for phase Doppler measurement

An attempt was made to measure, non-intrusively, average droplet sizes in a dense cooling spray of water. The small droplet size and high number density presented severe problems to conventional nonintrusive measurement methodology with phase Doppler anemometry (PDA). A recently developed optical technique, with more promise for measurements in dense sprays, laser sheet dropsizing (LSD), was tried with more success. Sources of error were considered and the uncertainty of the drop sizes measured by LSD was estimated at ±7%, neglecting multiple scattering, dropsize distribution effects and the contributions of droplets at the edge of the laser beam. The greatest of the known contributions to uncertainty is the calibration of the technique against PDA. The greatest of the unknown contributions is likely to be multiple scattering in such dense sprays. Received: 1 March 2000 / Revised version: 25 May 2000 / Published online: 20 September 2000     

High-Resolution Phase Doppler Particle Sizing based on Hilbert transform signal processing

A new phase Doppler anemometry (PDA) signal processing method based on a Hilbert transform algorithm is introduced and analysed. By generating a 90° phase-shifted burst signal in the time domain, the envelope of the Doppler burst can be determined. In addition, this envelope is approximated by a Gaussian exponential function. The difference of the maxima of these Gaussian approximations for two related PDA bursts gives an estimate of the time difference between these time shifted signals. With the introduction of this estimation method, the restriction to the [0,360°] interval resulting from conventional signal analysis may be avoided in many cases. To investigate the dependence on SNR, burst position, burst frequency and sampling rate, results of computer simulations are presented. The feasibility of the method is demonstrated briefly by experimental results. Phase differences of more than 2000° arising from the measurement of monodisperse droplets by a conventional PDA setup could be determined.     

Laser Doppler anemometry

A brief survey is given of the principles and present state of the art in laser Doppler anemometry. Both the optical and signal-processing aspects are treated in some detail. The comparative features and special problems associated with the various methods of interpreting the Doppler signals are discussed, with particular reference to the more recently developed technique of photon correlation. A wide range of practical applications, including measurements on liquids and solids, is considered.     

LASER DOPPLER ANEMOMETRY MEASUREMENTS OF BUBBLE RISE VELOCITY AND DEPARTURE FREQUENCY

Previous experimental investigations have employed photographic techniques to quantify the rise velocity and departure frequency of bubbles departing from a boiling surface, a method that requires a significant time investment to provide accurate time-averaged results. The current study focuses on applying laser Doppler anemometry (LDA) techniques to quantify these parameters. Initially, the LDA system is employed to measure the rise velocity and departure frequency of air bubbles in FC-72, R-113, and water. These air bubble tests verify the ability of the LDA measurement technique to quantify the bubble parameters. The LDA technique is then used to quantify the bubble parameters from a wire during isolated nucleate boiling conditions in saturated FC-72. Highly time-dependent departure frequencies (7 to 242 Hz) are measured in the isolated nucleate boiling regime. LDA signals representing departure frequencies in the fully developed nucleate boiling regime are discussed.     

Experimental Analysis of the Response of a Laser/Phase Doppler Anemometer System to a Partially Atomized Spray

This work describes a systematic approach adopted to establish Laser and Phase Doppler Anemometry, LDA/PDA, experimental techniques that would allow velocity and dropsize measurements to be made over wide velocity and size ranges with confidence in partially atomized sprays. The analysis considers the sprays generated by different gasoline direct injection (GDI) systems injecting into air under atmospheric conditions. The upper limit to the dropsize range in the fuel sprays was confirmed using (a) an Oxford Lasers' VisiSizer and (b) droplets of a known size produced by a mono‐dispersed droplet generator. GDI fuel sprays are highly transient, optically dense and provide a high degree of penetration and atomization. The measurement problem is therefore one of the detection of small, high speed droplets inside a dense cloud of surrounding droplets. Furthermore, under the transients found at the start and end of injection and during high fuel loads, fuel elements in the form of sheets, ligaments and filaments are also injected. These liquid fuel elements subsequently break‐up, downstream from the nozzle, to form droplets of a much larger size class but with a much lower number density [1]. The co‐existence of these liquid fuel elements and the widely different size classes in the spray are considered to pose a problem for dropsize measurements by the PDA technique. In particular: the wide dynamic range of light intensities scattered by the fuel elements and droplets; the trajectory of large drops through the edges of the PDA measurement volume with its Gaussian intensity distribution [2] and the high probability of non spherical droplets. The work concludes that the LDA/PDA measurement technique, as applied here, is robust. It can discriminate between partially and fully atomized sprays, has a high probability of accurately measuring dropsizes larger than the measurement volume and give a realistic indication of ‘sizes’ for non spherical droplets. However, specification of the PDA system parameters must be strictly compatible with the measurement task to yield unambiguous results.     

Influence of the Measurement Volume on the Phase Error in Phase Doppler Anemometry. Part 1: Reflective mode operation

In phase Doppler anemometry (PDA), phase difference errors are caused by the Gaussian intensity distribution of the laser beam and the curvature of the phase fronts outside of the beam waist. This results in erroneous particle sizes. Based on a physical analysis by geometrical optics [1], this phase difference error is evaluated for reflected mode operation (Part I) and for refracted mode operation (Part II). By varying the particle trajectories statistically, the error in determining the particle size and the mass flow can be simulated. By the method described the error in the measured phase difference can easily be estimated.     

Review of LDA and PIV applied to the measurement of sound and acoustic streaming

A review of laser Doppler anemometry (LDA) and particle image velocimetry (PIV) with their application to the measurement of sound is presented. The fundamental principles behind LDA and PIV are discussed and extended to the application of sound measurement. Special attention is paid to analysis of LDA signals including the Hilbert transform, which enables amplitude information to be obtained about various frequency components of a signal and wavelet analysis, which allows non-stationary signals to be accurately analysed. The influence of the refractive index variations in a medium due to a sound wave on the laser beams of an LDA signal is discussed. Attention is also paid to acoustic streaming which arises due to high-intensity sound, and PIV results are presented to demonstrate the effect.     

Dynamic calibration of a laser diode LDA probe

An experimental method to quickly and precisely determine the calibration constant of a laser diode LDA probe with a standard LDA optics and a dual-channel Doppler signal processor operating in coincidence mode is described and the measurement results are presented.     

Coherent Scattering by Multiple Particles in Phase Doppler Interferometry

A theoretical model has been developed for studying the response of the phase Doppler interferometer when multiple particles are simultaneously present within the measurement probe volume. The developed model incorporates the geometrical optics theory for describing the coherent interaction between the scattered light signals of multiple particles, eachhaving different size, velocity, trajectory, and arrival time. The resulting Doppler signal is processed by a theoretical signal processor which can simulate the performance characteristic of different signal processing schemes that are widely used in phase Doppler interferometry, namely, zero-crossing counter, covariance, autocorrelation and DFT parocessors. The application of the developed model for studying the coherent scattering by two particles has been specifically addressed in this paper. It has been shown that a DFT processor can be used to simultaneously measure the size and velocity of the two particles in most instances. However, for more than two particles, the signal processing scheme becomes more complex because of a quadratic increase in the beat frequency components.     

Rapid communication Novel high-power laser Doppler anemometer using a diode-pumped fiber laser

We present for the first time the application of a diode-pumped double-clad fiber laser in laser Doppler anemometry. The use of fiber lasers with a fundamental transverse mode power of multiple Watts enables the realization of powerful laser Doppler anemometers with a high portability. Received: 22 October 1996     

A novel optical method in micro drop deformation measurements

A novel method was developed to determine the deformation of a micro drop on a substrate. The principle of the method is based on the scattering of parallel fringes pattern from the deformed droplet. Because only a very few components are needed together with a conventional laser Doppler anemometry (LDA) transmitting unit, the newly developed method is very easy to be implemented. Within the geometrical optics approach, it is possible to reconstruct the profile of a non-spherical micro drop from the scattered interference fringes. The spatial frequency of the fringes was analyzed using fast Fourier transform (FFT). The validation experiments for this method in the determination of micro drop deformation which is affected by gravity, capillary force and surface tension were performed and the results are agreed well with the theoretical analysis.     

Optical diagnostics and direct injection of liquid fuel sprays

The research described here addresses the problem of a paucity of high quality data on the full field structure of high pressure liquid fuel sprays for gasoline direct injection, GDI, engines. The paper describes the application of phase Doppler anemometry, PDA, and single-shot laser sheet Mie imaging to the study of GDI sprays and discusses the methodologies adopted for the experimental systems and the optimisation of the techniques. Experimental data is presented which defines the spray structure in terms of PDA vector and scalar fields and single-shot CCD digital images. The work demonstrates the essential complementary nature of the single point and planar optical diagnostics for spray studies.     

Velocity measurement inside a motored internal combustion engine using three-component laser Doppler anemometry

A three-component laser Doppler anemometry (LDA) system has been employed to investigate the structure of the flow inside the cylinder of a motored internal combustion engine. This model engine was reasonably representative of a typical, single cylinder, spark ignition engine although it did not permit firing. It was equipped with overhead valve gear and optical access was provided in the top and side walls of the cylinder. A principal objective was to study the influence of the inlet port design on the flow within the cylinder during the induction and compression strokes of the engine. Here, it can be noted that results obtained in an unfired engine are believed to be representative of the flow behaviour before combustion occurs in a fired engine (see P.O. Witze, Measurements of the spatial distribution and engine speed dependence of turbulent air motion in an i.c. engine, SAE Paper No. 770220, 1977; Witze, Sandia Laboratory Energy Report, SAND 79-8685, Sandia Laboratories, USA, 1979). Experimental data presented for an inclined inlet port configuration reveal the complex three-dimensional nature of the flow inside the model engine cylinder. Not surprisingly, the results also show that the inclined inlet port created flow conditions more favourable to mixing in the cylinder. Specifically, the inclined inlet flow was found to generate a region with a relatively high shear and strong recirculation zones in the cylinder. Inclining the inlet port also produced a more nearly homogeneous flow structure at top dead centre during the compression stroke. The paper identifies the special difficulties encountered in making the LDA measurements. The experimental findings are examined and the problems that arise in presenting time-varying three-dimensional data of this type are discussed. Finally, the future potential of this experimental approach is explored.     

A High Power,High Resolution LDA/PDA System Applied to Gasoline Direct Injection Sprays

A new generation LDA/PDA transmitter system has been designed and constructed. The heart of the optical system is a new type of Bragg cell. Advances in laser power handling and symmetrical splitting at high Bragg angles of the shifted and unshifted beams have made it possible to construct a simple yet elegant LDA/PDA transmitter. The optical system integrates the Bragg cell with a laser beam expander to offer variable beam separation and high beam expansion ratios to produce a measurement volume with a high spatial resolution and at high power levels. The transmitter is proving to be a significant contribution to LDA/PDA optical system design and, not only applicable as a research tool for use in flows of a demanding nature but, due to its simplicity, flexibility and cost, an asset to teaching. The LDA/PDA system is being applied to characterise the atomization of fuel by high pressure automotive injectors as found in Gasoline Direct Injection (GDI) engines. Although the PDA system was configured to measure two orthogonal velocity components (2D) and size, its operation was not restricted to this configuration. An analysis of the spray with the PDA system in 1D and size and the LDA system in 2D and 1D configurations indicated the complexity of the atomization and break-up processes occurring in the spray. Single-shot imaging was used to study the spatial structure of the spray as a function of time. Use of the light sheet imaging technique alone could lead to false impressions of the atomization process.     

Phase Doppler Anemometer for Measurements of Deterministic Spray Unsteadiness

A method and analysis was developed to quantify the amplitude of deterministic spray unsteadiness based on Phase Doppler Anemometry (PDA), which sampled time‐dependent droplet velocity and size measurements, in order to determine the fluctuations of droplet data rate and number density, which are quantities relevant to fluctuations of droplet concentration. The data processing method of the PDA measurements was assessed in a pulsed spray at a frequency of 20 Hz injected in a swirl‐stabilised burner. Comparisons between quantities relevant to droplet concentration fluctuations, measured by PDA and a light scattering technique, quantified the deterministic spray unsteadiness and agreed to within 15%. The developed PDA approach was applied in the swirl‐stabilised burner to measure the amplitude of deterministic spray unsteadiness of an otherwise steady spray, which was caused by the instability of the atomisation process. The intensity of deterministic fluctuations of droplet data rate and number density, occurring at a frequency range around 600 Hz due to the atomisation process, was quantified to 15% of the corresponding mean value and this spray unsteadiness generated fluctuations on the air and droplet velocity fields. The deterministic spray unsteadiness could survive up to the end of the recirculation zone of the air flow at the burner exit and, therefore, could influence flame stability.     

A fourier optics method for the simulation of measurement-volume-effect by the slit constraint

A new method is introduced to analyze the accuracy of phase-Doppler anemometry (PDA) for sizing large particles in two-phase flows. The method is based on Fourier optics theory (FOT) and geometrical optics theory (GOT) to calculate the intensity ratio of refractive and reflective light scattered by a sphere under a slit constraint. To examine the accuracy in calculating the light intensity, the results from the GOT method were compared with those of the direct simulation based on the generalized Lorenz-Mie theory (GLMT). This newly developed FOT/GOT method also predicts the results of phase jump due to the slit constraint. The performance of dual-mode phase-Doppler anemometry can also be simulated by this method.