Performance tests of a new non-invasive sensor unit and ultrasound electronics

Industrial applications involving pulsed ultrasound instrumentation require complete non-invasive setups due to high temperatures, pressures and possible abrasive fluids. Recently, new pulser-receiver electronics and a new sensor unit were developed by Flow-Viz. The complete sensor unit setup enables non-invasive Doppler measurements through high grade stainless steel. In this work a non-invasive sensor unit developed for one inch pipes (22.5 mm ID) and two inch pipes (48.4 mm ID) were evaluated. Performance tests were conducted using a Doppler string phantom setup and the Doppler velocity results were compared to the moving string target velocities. Eight different positions along the pipe internal diameter (22.5 mm) were investigated and at each position six speeds (0.1–0.6 m/s) were tested. Error differences ranged from 0.18 to 7.8% for the tested velocity range. The average accuracy of Doppler measurements for the 22.5 mm sensor unit decreased slightly from 1.3 to 2.3% across the ultrasound beam axis. Eleven positions were tested along the diameter of the 48.4 mm pipe (eight positions covered the pipe radius) and five speeds were tested (0.2–0.6 m/s). The average accuracy of Doppler measurements for the 48.4 mm sensor unit was between 2.4 and 5.9%, with the lowest accuracy at the point furthest away from the sensor unit. Error differences varied between 0.07 and 11.85% for the tested velocity range, where mostly overestimated velocities were recorded. This systematic error explains the higher average error difference percentage when comparing the 48.4 mm (2.4–5.9%) and 22.5 mm (1.3–2.3%) sensor unit performance. The overall performance of the combined Flow-Viz system (electronics, software, sensor) was excellent as similar or higher errors were typically reported in the medical field. This study has for the first time validated non-invasive Doppler measurements through high grade stainless steel pipes by using an advanced string phantom setup.     

An unsteady flow structure on a heated rotating disk under mixed convection

A flow field under mixed convection on a heated rotating disk has been measured using an ultrasonic velocity profiler (UVP). The measured velocity field is a spatio‐temporal one as a function of radial coordinates and time. The objective of this paper is to clarify the vortex structure caused by the instability between buoyancy and centrifugal force. The vortex appears under typical conditions of Reynolds numbers and Grashof numbers and it moves toward the outside of the disk. This behavior can be classified into two patterns. The size of the vortex structure decreases with an increasing Reynolds number and increases with the Grashof number. The traveling velocity of the vortex increases with the Grashof number. Moreover, it decreases with an increasing Reynolds number in spite of increasing centrifugal force. According to these results, the region dominated by natural, forced, and mixed convection is classified in the relationship between Reynolds and Grashof numbers. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(6): 407–418, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20074     

Application of ultrasonic multi-wave method for two-phase bubbly and slug flows

This paper proposes a measurement technique for two-phase bubbly and slug flows using ultrasound. In order to obtain both liquid and gas velocity distributions simultaneously, a new technique for separating liquid and gas velocity data is developed. The technique employs a unique ultrasonic transducer referred to as multi-wave transducer (TDX). The multi-wave TDX consists of two kinds of ultrasonic piezoelectric elements which have different resonant frequencies. The central element of 3 mm diameter has a basic frequency of 8 MHz and the outer element has a basic frequency of 2 MHz. The multi-wave TDX can emit the two ultrasonic frequencies independently. In our previous investigations, both elements were connected with two ultrasonic velocity profile (UVP) monitors to measure liquid and bubble velocity distributions. However, the technique was limited to the measurement of bubbly flows at low void-fraction. Furthermore, it was impossible to synchronize the instantaneous velocities of liquid and bubbles because of the facility limitation. In order to overcome these disadvantages, cross-correlation method is employed for the measurements in this study. In order to apply the technique to flow measurements, ultrasound pressure fields are measured. As a result, it is found that the TDX must be set 20 mm away from the test section. The technique is applied to measuring bubbly and slug flows. By the combination of 2 and 8 MHz ultrasonic echo signals, the echo signals are distinguished between reflected from particles and bubbles. Compared with the results of obtaining with the multi-wave method and a high-speed camera, it is confirmed that the technique can separate the information of liquid and gas phases at a sampling rate of 1000 Hz.     

An experimental device for measuring radial flow velocity profiles of yield stress fluids

Measuring the radial flow velocity field of yield stress fluids (YSFs) between two parallel disks provides crucial data to understand the underlying flow phenomena. However, direct velocimetry of YSFs in the radial flow configuration remains a challenge, due to the complex fluid rheology and geometry constraints. In this paper, we present an experimental device for measuring YSF radial flow velocity profiles. Ultrasound Velocity Profiling (UVP) is used to non-intrusively measure the velocity profiles. The Tikhonov regularization method is implemented to obtain smooth velocity profiles, which are used to calculate the plug-flow region. Compared to our previous work on radial flow, the current contributions include: (i) additional structural frame members to maintain a constant aperture, (ii) wall slip reduction, and (iii) an improved velocity profile plug-detection algorithm. The results show that the experimental device and the measurement method are effective for further studying radial flow behavior of YSFs for industrial applications.     

Islanding detection of grid connected distributed generators using TLS-ESPRIT

A critical protection requirement for grid connected distributed generators (DG) is anti-islanding protection. In this paper, a new islanding detection method is proposed based on monitoring the generator's frequency. Two new features, the frequency of oscillation and the damping factor of the generator's frequency output waveform, are extracted using the total least square-estimation of signal parameters via rotational invariance techniques (TLS-ESPRIT) algorithm. The proposed method has been tested under various scenarios such as load change, short circuit, and capacitor switching.     

Effect of scale on entrainment in stirred tanks

In the present work, surface turbulence characteristics at the onset of entrainment in air water system have been investigated. For the present study, shear type entrainment in stirred tanks has been considered. Experiments have been performed in stirred tanks with different scales for different types and sizes of impellers. The results of the work reveal that radial RMS, axial RMS velocities and turbulent kinetic energy showed similar magnitudes at onset even at different scales. The RMS velocities as well as TKE magnitudes did not vary with type or size of impellers. Local energy dissipation rates have been estimated from autocorrelation function of fluctuating velocity. Very low magnitudes of local energy dissipation rates at onset have been observed.     

Effect of cooling temperature of electrodes on Joule-heating flow in cubic cavity

The effect of cooling condition of electrode plates on chaotic Joule-heating flow behavior in a cubic cavity has been experimentally investigated. Two electrode plates are immersed in the fluid body and placed on the opposing side wall. They are connected to an AC power source for internal volumetric ‘Joule-heating’ in the cavity. This chaotic flow which induced by Joule heating is observed within whole the cavity. The chaotic flow is investigated quantitatively by Ultrasonic Velocity Profiling (UVP) method. The chaotic flow behavior is also observed by using Particle Image Velocimetry (PIV) method to understand the effect of temperature distribution in this condition. The chaotic flow behavior generates fluctuations of temperature and velocity. As a result, cooling condition of electrode plates has a strong effect on the Joule-heating flow behavior that the vortex area occurred in the upper part of cavity. On the other hand, in the adiabatic condition, unstable flow appeared in whole cavity. In additional, velocity profiles are analyzed by Fast Fourier Transform (FFT) method about the frequencies of fluctuations. Furthermore, a numerical analysis using Finite Element Method, GSMAC-FEM, is also examined the Joule-heating flow behavior for cubic cavity.     

Flow field investigation in a rectangular shallow reservoir using UVP, LSPIV and numerical modelling

Low velocity and shallow-depth flow fields often are a challenge to most velocity measuring instruments. In the framework of a research project on reservoir sedimentation, the influence of the reservoir geometry on sediment transport and deposition was studied. An inexpensive and accurate technique for Large-Scale Particle Image Velocimetry (LSPIV) was developed to measure the surface velocity field in 2D. An Ultrasonic Doppler Velocity Profiler (UVP) and LSPIV techniques were used for verification and validation of the numerical simulations. The velocities measured by means of UVP allowed an instantaneous measurement of the 1D velocity profile over the whole flow depth. The turbulence large-scale structures and jet expansion in the basin have been determined based on UVP, LSPIV and numerical simulations. Vertical velocity distributions were defined to study the vertical velocity effect. UVP measurements confirm 2D flow map in shallow reservoir. LSPIV has potential to measure low velocities. The comparison between LSPIV, UVP and numerical simulation gives good agreements.     

Effects of the number of pulse repetitions and noise on the velocity data from the ultrasonic pulsed Doppler method with different algorithms

The ultrasonic pulsed Doppler technique known as the ultrasonic velocity profile (UVP) method has been widely used in many engineering fields. The analysis algorithms of the UVP, the number of pulse repetitions (N_pulse), noise and reflector conditions, etc. all affect the measurement accuracy. N_pulse is related to the temporal resolution, thus to improve this resolution it must be set as low as possible. However, it is known that the measurement accuracy of the instantaneous velocity becomes worse with decreasing values of N_pulse. In this study, UVP analysis algorithms including the fast Fourier transform (FFT), autocorrelation, and the wavelet transform (WT) were compared via simulations and experiments using varying values of N_pulse and the signal-to-noise ratio (SNR). We show that there is an appropriate N_pulse for each algorithm that depends on the SNR; specifically, the value of N_pulse increases with decreasing SNR. The difference between the algorithms for the velocity data was small under low noise conditions. However, a FFT with a Gaussian interpolation produced the best result under noisy conditions. In contrast the WT was relatively unaffected by noise. Therefore, a WT is the preferred choice for measuring velocity distributions if high sampling measurement is not required.