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Viscosity Measurements for Molten Glass |
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This model explores the generation and use of fundamental guided modes to measure the theological properties and discusses the sensitivity of those modes on the melt properties. The technique uses the effect of melt properties on ultrasonic guided wave reflections at the solid-fluid interface. An experimental procedure to study the variation of reflection factors with temperature using the three fundamental ultrasonic guided wave modes from the rods immersed in Newtonian molten glasses has been explored. These reflections factors can be used to measure the properties of the melts like viscosity, density and bulk modulus. Details of the experimental setup ( shown in Fig. 1) is as follows:
A box type resistive heating furnace with SiC heating elements with a rated power of 6 kW was used to melt the glass sample (10ml), which is inserted in an alumina crucible. Furnace has two opening for loading the sample and buffer rod. The water cooled buffer rod (alumina) used in these experiments serves the dual role of protecting the transducer from high temperatures because no piezoelectric transducers are available that can resist the elevated temperatures and the corrosive environment of a glass melt, and in providing an acoustic reflection interface with the melt for measurement of the reflected signal.
A 0.25 MHz frequency 1.0″ diameter piezoelectric shear transducer was used for flexural mode F(1,1) and torsional mode T(0,1) generation. A lithium niobate longitudinal bare crystal of 0.20 MHz frequency was used for longitudinal mode L(0,1) generation. The ultrasonic waves were transmitted to the glass melt through a ceramic buffer rod. The RF output of the pulser/receiver was connected to a digital storage oscilloscope to monitor and save the signal in the computer using a USB interface cable. The peak-to-peak amplitude and the time of flight of the saved signals were recorded.Experimental log viscosity values agree within » ± 5% error with NIST standard log viscosity values in most of the region across viscosity range of 18 - 6300 Pa-s and is shown in Fig. 2.
All the three modes were found to be sensitive to glass viscosity changes within the range of 101-104 Pa-s. While the torsional wave mode was determined to provide higher sensitivity, the flexural mode was found to be more applicable, for such measurements due to practical considerations. And also using the time of flight measurement in the buffer rod, temperature of the glass melt was measured in the range of 800-1300°C with the max error of 1% from thermocouple measurements. The low cost laboratory technique allows this sensor to function as a use-and-throw sensor with a small amount of glass sample. This prevents the need for tedious cleaning processes (for sensor reuse) that are required with current rotating high temperature viscometers. This experimental procedure is simple and can be used for in-line method of measuring properties and process monitoring.
All the three modes were found to be sensitive to glass viscosity changes within the range of 101-104 Pa-s. While the torsional wave mode was determined to provide higher sensitivity, the flexural mode was found to be more applicable, for such measurements due to practical considerations. And also using the time of flight measurement in the buffer rod, temperature of the glass melt was measured in the range of 800-1300°C with the max error of 1% from thermocouple measurements. The low cost laboratory technique allows this sensor to function as a use-and-throw sensor with a small amount of glass sample. This prevents the need for tedious cleaning processes (for sensor reuse) that are required with current rotating high temperature viscometers. This experimental procedure is simple and can be used for in-line method of measuring properties and process monitoring.
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Fig 1. Schematic of the experimental setup |
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Fig 2. Comparison of experimental
ultrasonic viscosity data with NIST data |
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References : 1. V. S. K. Prasad, K. Balasubramaniam, E. Kannan,and K. L Geisinger, “Viscosity Measurements Of Melts At High Temperatures Using
Ultrasonic Guided Waves” , Journal of Materials Processing Technology(Accepted) |
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Contaminant Detection during Cotton Processing |
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This is a research and development project undertaken with a private industry Sieger Spintech Equipments Pvt Ltd in Coimbatore, India. The sonic contaminant system was developed at the CNDE based on a unique and patent pending technology that allows for the detection of single fiber contaminants travel in cotton at speeds in excess of 10 m/s.
The contamination control system from operates based on the detection of contaminants using air coupled ultrasound. The technology uses Ultrasonic imaging to identify the contaminants based on density. The high accuracy and speed of the sensors enable identificaton of white polymers and hidden contaminants of different densities Signals from sensors are efficiently processed by unique embedded sonic signal processorsThe contaminants are collected in a bin or removed through the air duct. Used in spinning mills, these systems have a high level of customisation for different types of materials using the most flexible software with pre-defined size, colour and dimensional tolerance.
Technical specifications: Capacity: Up to 1000 kgs per hour. Control system: Industrial processor with latest imaging software. Special features: Defect free yarn Low power consumption No additional processing and handling required Remote operator panel with optional touch screen Separate enclosure for control and imaging system Pneumatics within the enclosure. |
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| Cotton and Contaminants. |
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| Fig 2 |
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The Sono-optic system in a cotton
processing plant. |
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Ultrasonic Burn Rate Measurements |
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The effort focused to develop and standardize an ultrasonic NDE method for the measurement of the burn rate of high rate combustion such as propellants. The basic principle is a pulse echo technique using an ultrasonic transducer coupled to the propellant. The transducer sends an ultrasonic beam into the propellant. This beam reflects off the burning surface of the propellant. With a sufficiently higher pulse repetition rate, a good estimate of the burn rate could be obtained.
The transducer is acoustically coupled to a piece of propellant (encased in an inhibitor material) kept in a chamber with/without a nozzle and a safety value. The chamber has pressure ports through which the chamber pressure is measured through a signal conditioner and the analog-digital converter. During the propellant burning, the ultrasonic transducer will send pulses through the coupler to the propellant, which will be reflected from the receding propellant burning surface. Knowing the velocity of sound in propellant (or by knowing the initial length of the propellant sample), the propellant length during the burning process is measured in real-time from the echo delay time. With sufficiently high pulse rate the instantaneous surface can be tracked to a good temporal accuracy. From this, the instantaneous burn rate is obtained and correlated to pressure. The variation of sound speed with pressure and distortion of propellant grain with pressure is accounted for by calibrating the speed of sound under pressure. With this setup, the errors due to ignition and extinction are removed. Also, since the instantaneous burn rate is measured, the variation of burn rate in a strand is also obtained. By using larger size strands, the effect of inhibition on burn rate is also reduced. Using the methodology used in this set up we would be able to measure the burning rate under actual rocket operating conditions, for eg. Erosive burning rate. Some of the advantages of the ultrasonic measurement of burn rate using the closed bomb approach ensured that the burning rate parameters were obtained in only one experiment. Some of the benefits include:
Steady and unsteady Flow Measurements.
One measurement for all pressures.
Provides the a and n values in one experiment.
In-situ measurement on real tests may be possible after confidence
levels have been established.
Data recorded and archived for further analysis.
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Online Monitoring of Welding - Guided Ultrasonic Waves |
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Virtually every industry uses welding process at some stage of manufacturing or in the repair and maintenance of process equipment. With increasing importance of structural integrity, the need to produce good quality weld has become inevitable. The use of Lamb waves is an attractive solution to this problem since Lamb waves can be excited on point of the structure and can be propagated over considerable distances. The guided wave advantage for weld inspection is the long-range of inspection the probes can be placed well away from the weld zone - the high temperature region and still monitor the changes in the weld.
The first order symmetric mode- S1 mode was chosen for carrying out the inspection. S1 mode was excited using wedges. The angle of the wedge was calculated using Snell’s law [4] and it was found for 2 mm thick Aluminium plate the angle was approximately 30° The mode that was excited was verified by using the group velocity data obtained using the experimental time of arrival of the signal.
It is evident from the results of the experiments conducted on thin welded aluminium samples that for the short range of distances involved, energy of the signal is much easier parameter to monitor the quality of the weld that is fabricated. It was also found that the dispersion characteristic of the higher order mode S1 did not affect the test results on a greater extent due the short range involved in testing. The sensitivity of S1 is greater and hence for short-range application such as that of the weld inspection we can use the higher order modes. Though this technique at its present state was able to detect most of the weld defects micro-porosity formed in aluminium welds could not be detected also classification of the defect type have not been explored. Even with these present limitations this technique if implemented online can serve as a rapid pre-inspection tool wherein the welder can avoid producing defective weld batches.
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Weld inspection setup |
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| Plot of the energy of the signal for Aluminium plate with notches |
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Comparison of energy of the signal received for various cases of welding |
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J. Ezhil, K. Balasubramaniam, and K. Prasad Rao, Inspection of Thin Welded Aluminium Plates using Ultrasonic Guided Waves,
Journal of Nondestructive Evaluation and Testing 4(2) 36-41 (2005). |
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High Viscosity Phase Transition Points Sensing |
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The work explores the generation and use of fundamental ultrasonic guided modes to measure the high viscosity glass transformation points of glasses during its heating process and thereby infer the approximate viscosity at these temperatures. The buffer rod based sensor uses the effect of the wetting of the melt glass, during the melting process, on the ultrasonic cylindrical guided wave reflection coefficient of both longitudinal (L(0,1)) and Flexural (F(1,1)) modes. It was observed that two transition points can be observed that signify the wetting phenomena of glass on to the buffer rod. The experimental procedure described in this paper has applications in laboratory tests as well as for process monitoring in industries. The results from the measurement of melt viscosity and identification of the transformation points using the proposed method, on three different glass samples were compared with the values provided by the National Institute of Standards Technology, USA and DSC measurements. This method also offers advantages in reduction in the amount of sample, cost, complexity in experiments and time required for conducting measurements when compared to mechanical high temperature viscometers.
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Variation of signal amplitude with temperature during first heating cycle. |
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| The first Transition point |
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The second transition point |
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Lamb Wave Flow & Cure Measurement for RTM |
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Resin Transfer Molding (RTM) process, especially the Vacuum Assisted Resin Transfer molding (VARTM), has received considerable attention in recent years, due to its increased utility in forming lightweight composite structural parts. However, the quality of the VARTM products depends highly on the complete filling and curing of the resin inside the mold. Hence, in recent years, different techniques for online and non intrusive monitoring of these process parameters have received great importance. In this paper, we discuss the capability of using the leaky plate waves for monitoring the flow and curing of resins. A 0.5 MHz shear transducer is used for generating plate waves in the base plates. Another shear transducer placed on the mold is used for receiving the waves leaking through the surface. Initially when there is no resin the transducer a very weak signal is picked up by the transducer as there is no coupling between the plate and the transducer. Once the resin fills the area below the transducer, it starts picking the leaky waves indicating that the resin has reached that portion. The curing of these resins is then monitored using the change in the amplitude as well as the time of flight of the leaky S0 wave mode. Experiments were carried on VARTM setup capable of making flat plates using poly-ester as the resin.
Wetting of the region by resin is determined by sudden increase in the amplitude of the leaky guided wave caused due to proper coupling between the fiber mats by the resin. The curing of resin is monitored by looking at time of arrival of the S0 mode. As the resin cures, the speed of the wave through the mat increases leading to decrease in time of flight. The amplitude was found to be affected by two factors. The first one is due to the overall decrease in the amplitude due to the leakage in the region before the transducer. Higher the viscosity of the loading fluid the signal was found to attenuate severely.
The main advantage is that a single transmitter is enough for generating the waves in all direction. Another added advantage is that, since the measurement is based on the time of flight data, the transducer orientation effects can be neglected. Even the effect of weight of the transducer can be avoided by using non-contact methods like laser vibrometer, air coupled ultrasound, or electro-magnetic pickup.
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Interferometric Sensor for Rheology Measurements |
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Fluids that are undergoing rheological changes are important for process control. Density, Adiabatic bulk modulus and Viscosity form a set of important mechanical properties of liquids. Their quantification provides information regarding the progress of many industrial processes and hence this method serves as a useful tool for on-line process control.
A double reflection ultrasonic interferometric technique based on the null method is proposed here for the measurement of rheological properties of fluids that addresses the first two issues stated above. Here, two tone burst (gated near-monochromatic wave) signals obtained from solid reflecting surfaces, one in contact with the liquid whose properties are being evaluated while the other that is in contact with a reference fluid, are allowed to interfere. The length of travel of the ultrasonic waves in the two solid delay lines was adjusted such that the interference causes a null minimum. Hence, any change in the conditions, either in the solid or the fluid media will cause phase difference between the two signals and the interfered signal will not satisfy the null condition. Hence, the interfered signal provides an extremely sensitive method for measuring small changes in the rheological properties of the liquids. This technique has potential for on-line monitoring of viscosity and/or density of liquids.
The longitudinal waves were used to measure the longitudinal impedance while shear waves are used to measure the shear impedance. Glycerin-water mixtures were used as the test material. A simple plane wave model was shown to provide satisfactory comparison with experiments. Potential advantages of the double delay line interferometric technique include:
- By appropriate choice of the reference fluid, this technique can measure (a) Subtle changes in fluid properties, (b)Viscosity in the low viscosity (cps) range may be measured.
- For low temperature applications (less than 200 C), it may be possible to auto-compensate for temperature variations by allowing the delay-lines to be thin and made of material with low coefficient of expansion.
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Interferometric rheology monitoring system using the double delay-line ultrasonic interferometry with typical transmitted and received L and S wave signals.
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| Simulation and experimental results for fixed lengths and varying frequencies of the Interferometer Sensor. |
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Ultrasonic measurement of viscosity and comparison
with rotating viscometer results. |
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K. Balasubramaniam, and S. Sethuraman “Ultrasonic Interferometeric Sensor for Rheological Changes of Fluids” Review of
Scientific Instruments 77, 084902 (2006) |
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Torsional guided wave sensor for flow front monitoring |
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Improper filling and under curing of viscous fluids such as polymeric resin have been a concern in many industries such as polymers, composites, etc., resulting in increased manufacturing costs and poor performance of the fabricated structures. Measuring the extent of flow of viscous fluids inside opaque molds has been a very important parameter in determining the quality of products in manufacturing process such as Injection molding and Resin Transfer Molding (RTM).
Different methods have been investigated to determine proper curing of the resin especially using guided waves in different waveguides. An ultrasonic torsional guided wave sensor has been discussed for monitoring the movement of flow front during filling of resins in opaque molds. A pair of piezoelectric normal shear transducers was used for generating and receiving the fundamental ultrasonic torsional guided wave mode in thin copper wires. The torsional mode was excited at one end of the wire, while the flowing viscous fluid progressively wet the other free end of the wire. The time-of-flight of the transient reflections of this fundamental mode from the air-fluid interface, where the wire enters the resin, was used to measure the position of the fluid flow front. Experiments were conducted on four fluids with different viscosity values.
For high viscous fluids, the transient reflected signals was very strong compared to other signals, such as reflections/scattering from wire end, and from small unavoidable “kinks” in the wire. As the viscosity of the fluid decreases, the strength of the reflected signals reduces as compared to the other reflected/scattered signals. Hence, two different post-processing algorithms (one based on time domain analysis and one based on frequency domain analysis) were developed for enhancing the weak transient reflected signal from the interface and suppressing the other stationary reflections and noises. Both methods showed an improvement in enhancing the contrast of the interface reflected signal. The algorithms were tested for cases where the reflected signals showed poor signal to noise ratio. The method 1, which uses time domain processing, was found to be better, since the frequency domain approach had residuals from stationary signals. However, the contrast of the moving signal to the background noise in image was found to be better in case of method 2. The post processed B-scan images were then used to plot the flow front profiles for different fluids. While, this sensor has been shown to work in certain fluids, this approach can be used in many other applications such as level sensing, and high temperature process monitoring and may be used for a wide range of fluids. However, this method was found to be relatively insensitive to fluids with viscosity less than 65 centipoises.
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Viswanathan, K., and K. Balasubramaniam, Review of Scientific Instruments 78 015110 (2007) |
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