A compact,laser diode based phase Doppler system

Recent advances in laser diode technology provide the opportunity to reduce the size and cost of laser Doppler and phase Doppler anemometry systems. The application of laser diodes to phase Doppler systems is discussed, and the construction of a compact, laser diode based phase Doppler system is described. Test measurements in a monosized drop stream, a water spray and a fluidized bed are reported, demonstrating the functionality of such a system.     

An LDA technique for in situ simultaneous velocity and size measurement of large spherical particles in a two-phase suspension flow

A relatively simple optical scheme using the reference-mode laser Doppler anemometry for the in situ measurement of flow properties of a dilute particle-fluid two-phase suspension having a predominant flow direction is hereby proposed. It is an extension of the established technique of optical gating for particle sizing which is fully integrated into the established technique of laser Doppler anemometry for velocity measurement. Particles that can be measured by this scheme are limited to those with sizes greater than the smaller dimension of the optical measuring volume. Inherent in the methodology is a procedure for providing information on the local particle number density and velocity distributions for each size range of the particles and the local velocity distribution of the continuous phase. The accompanying electronics and interfaces are also established for data processing and analysis in a mini computer. Validation of the scheme has been accomplished by controlled experiments using stainless steel balls and water droplets of 1 mm and greater in diameter.     

On the motion of particles in a fluid under the influence of a large velocity gradient

The motion of seeding particles as used in laser Doppler anemometry is investigated in the presence of a large velocity gradient across aerodynamic shocks under different flow conditions. Experimentally obtained results are presented and compared with theoretical predictions based upon the size distribution of the seeding particles used. It is found that the agreement of experimental and theoretical results depends on the flow conditions as well as on the particle material.     

Effect of particle polydispersity on particle concentration measurement by using laser Doppler anemometry

Measurement of particle concentration by laser Doppler anemometry (LDA) is studied on a vertical air jet seeded by a powder disperser with controlled particle and air flow rates. Particle arrival rate is utilized to retrieve particle number densities from conventional LDA operation. The effect of polydisperse nature of the particles is assessed. Comparisons between measured and estimated particle number densities suggest that only a certain portion of the particle population with a particle size to fringe spacing ratio around unity can be detected. Results indicate that reliable measurement of absolute particle concentration is possible for a particle population of narrow size distribution with an average diameter equivalent to fringe spacing. Present number density measurement technique which is useful for practical purposes with conventional LDA systems is found to yield physically reasonable profiles in both laminar and turbulent regimes.     

Experimental investigation of parameters effect on heat transfer of spray cooling

The effects of spray height, nozzle spray angle, inlet pressure and spray incident angle on heat transfer of spray cooling were studied by an experimental method. Multi-points thermocouples and infrared imaging device were used to measure temperature distribution on heating surface. A Doppler anemometry and a camera were applied to study the spray flow field. The mechanism of heat transfer of spray cooling was concluded on the basis of experimental data and spray characteristics. It is showed that parameters affect heat transfer by way of changing the flow field on the heating surface. Heat transfer performance can be optimized by a smaller spray angle nozzle, which sprays at a smaller spray height and a higher inlet pressure. The effect of incident angle on heat transfer depends on nozzle spray angle and the definition of distance of nozzle to surface.     

Bursting frequency in turbulent boundary layers

Employing laser Doppler anemometry and VITA techniques, the bursting frequency in turbulent boundary layers has been measured over the Reynolds-number range 320 to 1470. The result indicates that the mean and non-dimensional bursting frequency scaled with the variables appropriate for the wall region was constant and independent of Reynoids number. When the same data are plotted using the outer variables of boundary layer to normalize the bursting frequency, the non-dimensional frequency increases as the Reynolds number increases. This is in agreement with the results of Blackwelder et al. (1983) who used hot wire anemometry and VITA technique. The project is supported by the National Natural Science Foundation of China     

Secondary atomisation produced by single drop vertical impacts onto heated surfaces

The paper reports an experimental analysis of the secondary atomisation produced by the impact of a single drop on a solid heated surface. Different wall temperatures were used to study different boiling regimes. The size of secondary drops produced by the impact was measured by two techniques, namely the phase Doppler anemometry (PDA) and the image analysis technique (IAT); this allowed to extend the measurable size range from 5.5 μm up to few mm. Two impacting walls with different surface roughness were used to show the effect of this parameter on different atomisation regimes. The liquid viscosity was also varied in a limited range by using water–glycerol mixtures. Image analysis allowed also to define the details of the morphology of drop spreading and break-up.     

Distortion of LDA fringe pattern by tracer particles

For precise flow velocity measurements laser Doppler anemometry (LDA) is wide-spread in use in the laboratories of industry and universitarian research institutions. The LDA method has the advantage of being not intrusive and able to discriminate between forward and reverse velocities. So far, laser Doppler anemometry is believed to be one of the most accurate flow measuring techniques. However, recent investigations have shown that the period lengths of LDA signal bursts are not constant within an individual burst. This can induce an additional scatter in the signal frequency and in the determination of the flow velocity. Until now, the reason for the period variations has not been investigated in detail although the problem was observed before. This paper describes experimental investigations which show that the particle passage through the laser beams shortly before the point of superposition, i.e. the LDA measuring volume, yields a distorted LDA fringe pattern. Thus, the signal period length from an individual particle, passing the center of the measuring volume at the same time, varies according to the distortion of the fringe spacing.     

Effective particle size range in laser-Doppler anemometry

The present paper points out that all existing laser-Doppler anemometer systems do not only operate within a finite range of Doppler frequencies but also work within a relatively narrow range of signal amplitudes. It is shown that this corresponds to a finite, and usually to an extremely small, range of particle diameters which contributes to the final LDA measurements. Because of this, the particle size distribution has to be matched to the LDA-system used for measuring particle velocities. If this is not taken into account in particle seeding, low data rates will result in spite of very high particle passage rates through the measuring control volume. This is shown experimentally and is supported by theoretical considerations.The present investigation results in conclusions regarding optimum particle size distributions for laser-Doppler anemometry. If fluid velocity measurements are attempted rather than particle velocity measurements, the particles still have to satisfy well known size requirements that are flow, fluid and particle density dependent.The experimental study employs a combined optical system for simultaneous measurements of particle velocity, particle size and particle concentration. The system is used to measure those particles of a spectrum of oil droplets that contribute to the validated signal output of counter and transient recorder based LDA-electronic signal processing systems.     

Influence of precession on velocity measurements in a strong laboratory vortex

A strong laboratory vortex is generated in a cylindrical cell using a rotating disk and stretched by pumping the fluid out through a hole in the centre of the top of the cell. The velocity field is measured by means of laser Doppler anemometry and Doppler ultrasonic anemometry which are both non intrusive methods. The vortex exhibits a slight precession which induces temporal fluctuations of the velocity at the measurement point. Due to the centrifugal force, the tracers concentrate in a tubular region around the vortex, leading to spatial variations of the measurement counting rate. Under these two effects, the probability density function (PDF) of the one point velocity exhibits a strong non-Gaussian behaviour. In order to access the details of the velocity profile of the vortex in its own system of reference, the influence of the vortex precession, of the spatial variations of the concentration in tracers and of the intrinsic measurement dispersion is investigated and a model is proposed. It allows to recover statistically the characteristics of the vortex and to deduce the trajectory of its centre from the instantaneous velocity profiles. Received: 5 August 1998/Accepted: 20 February 1999     

The effects of flow split ratio and flow rate in manifolds

This paper reports a combined experimental and numerical investigation of three-dimensional steady turbulent flows in inlet manifolds of square cross-section. Predictions and measurements of the flows were carried out using computational fluid dynamics and laser Doppler anemometry techniques respectively. The flow structure was characterized in detail and the effects of flow split ratio and inlet flow rate were studied. These were found to cause significant variations in the size and shape of recirculation regions in the branches, and in the turbulence levels. It was then found that there is a significant difference between the flow rates through different branches. The performance of the code was assessed through a comparison between predictions and measurements. The comparison demonstrates that all important features of the flow are well represented by the predictions.     

光检测器参数对相位—多普勒粒子分析系统特性的影响

应用几何光学近似理论模型计算了激光相位多普勒系统中微粒的光散射特性,得到了散射光强与粒径关系、检测器表现的散射光强分布、相位差与粒径特性中的非线性跃变以及检测器形状对特性的影响。     

Étude expérimentale des courants de dérive induits par la houle au-dessus d'un fond ridé

Wave-induced current measurements have been carried out above rippled beds in a channel, using laser Doppler anemometry. Very simple formulas for the drift currents just outside the boundary layer above rippled and very rough beds in turbulent regime are proposed. The vertical profiles of drift currents in the bottom boundary layer are compared to the results of the recent model presented by Davies and Villaret.     

Comparative evaluation of some existing rate-type constitutive equations for a viscoelastic fluid undergoing wiggle flow

A comparative evaluation of existing rate-type constitutive equations is provided for a viscoelastic fluid undergoing accelerated flow. To this end, accurate point velocity and stress birefringence data previously obtained by laser Doppler anemometry and stress birefringence are utilized. For each constitutive equation, the numerical values of constants which yield the best fit with experimental data are determined via non-linear regression analysis. The best agreement between experimental and calculated normal stress differences is obtained with the White-Metzner equation. The success of this equation is attributed to the deformation rate dependence of its viscosity and time constant.     

The vortex structure behind an Ahmed reference model in the presence of a moving ground plane

Laser Doppler anemometry (LDA) data are presented for an Ahmed reference model employing various backlight angles. LDA velocity measurements are taken in a number of planes around and downstream of the model at a free-stream velocity of 25 m s⁻¹. During the testing a rolling road provided ground simulation and six-component force data were recorded. Data from these experiments are compared to previous experimental data. It is found that the inclusion of the ground simulation and the consequent supporting strut produces a reduction in the size and strength of the vortices shed from the back end of the Ahmed model when compared with previous analysis. It is further concluded this effect is primarily a result of the overhead strut, and that the rolling road has little effect on these upper vortices. In addition, vortices shed from the underside of the model, not reported in previous experimental work, are found and analysed.     

Measurement of multiple gas-bubble velocities in gas–liquid flows using hot-film anemometry

A dual-probe hot-film anemometry technique has been developed to measure multiple gas-bubble velocities corresponding to different gas-bubble size groups in air–water flows. A data reduction scheme using wavelet analysis combined with a phase-detection technique is used to discriminate the hot-film anemometer output signals into signals corresponding to different bubble size groups. The phase and bubble size discrimination is based on the magnitude and time derivative of the signal, and the streamwise length of the gas bubbles. A cross-correlation between the discriminated signals from the two probes yields an average time difference of arrival of the gas bubbles at the two sensor locations. The velocities are estimated from the distance between the sensors and the time difference of arrival. The mean bubble size is estimated from the chord length distribution. Measurements performed in vertical-up air–water slug flow show the technique to be a viable method for obtaining bubble velocity and size information. The velocity measurements from the hot-film anemometry are corroborated using a high-speed quantitative flow visualization system. Received: 22 December 1999/Accepted: 8 May 2001     

Time-resolved spectral-line-filtered velocimetry

Measurements were obtained with a velocimeter that simultaneously illuminates the particulate in a flow with argon and He–Ne lasers and the scattered light is passed through a carefully controlled iodine filter. The iodine cell absorbs the argon emission at a rate proportional to the velocity of the scattering particle, while the absorption of scattered He–Ne light is independent of velocity. A tunable acousto-optic filter is used to multiplex the argon and He–Ne signals collected from a single photodetector. The problem of detector alignment in conventional Doppler global velocimetry is thus avoided. Experimental results on a scattering disk and an axisymmetric jet are presented and compared with laser Doppler anemometry measurements. Received: 7 March 2000/Accepted: 5 January 2001     

A transient laser Doppler anemometry for measurement of rapidly changing velocities of solid bodies

A transient laser Doppler anemometry (LDA) is developed to extract the velocity history of a solid body that is transient in nature. A digital storage oscilloscope was used in place of the conventional counter to allow postprocessing of the recorded Doppler signals. By sliding a rectangular window with a length on the order of 1μs, frequencies of the rapidly changing Doppler signals were extracted using a refined signal processing method. This method, which incorporates the chirp-Z transform algorithm to the short-time Fourier transform routine for frequency enhancement, has proved its superiority over the conventional short-time Fourier transform and the zero-crossing methods; the latter was used in the authors' previously published works. The transient LDA was then applied to record transient responses of projectiles with different masses during impact on plates of various thicknesses, and interesting impact phenomena were discovered. From these applications, it is found that this method is robust and reliable for extracting the velocity history of a solid body during a transient event.