J.A. Chavez

Methodology for extracting thermoelectric module parameters

The fundamental temperature-dependent parameters of a thermoelectric (TE) module: coefficient, electrical resistance, thermal conductance, and figure-of-merit are necessary, among other factors, for the conception of models for this type of devices. In this work we present a methodology for calculating these parameters from observable variables, which are obtained from temperature, voltage, and electric current measurements made on a working TE devices. The extracted parameters are plotted against the average temperature between the hot and cold side of the module. Results are verified by inserting average-value parameters into an electro-thermal SPICE model of a thermoelectric module and comparing simulation and measured (using a TE module characterization system) results for a given test configuration.

One-dimensional modeling of TE devices considering temperature-dependent parameters using SPICE

Based on simplified one-dimensional steady-state analysis of thermoelectric phenomena and on analogies between thermal and electrical domains, we propose both lumped and distributed parameter electrical models for thermoelectric devices. A couple of important advantages of the presented models are that the temperature dependence of material properties is considered and that they can be easily simulated using an electronic simulation tool such as SPICE. For a single free-standing pellet, comparisons are made between SPICE simulations using the proposed models and with numerical simulations carried out with Mathematica software. Results illustrate accuracy of the distributed parameter models and show how inappropriate it is to assume, in some cases, constant material parameters for an entire thermoelectric element.

Transient distributed parameter electrical analogous model of TE devices

This paper describes a one-dimensional distributed parameter transient model for thermoelectric devices implemented using analogies between the thermal and electrical domains, where thermal variables are described by their electrical analogues. The resulting electrical network can be tested by means of an electrical simulation tool such as SPICE. This approach facilitates simulation of a thermoelectric module and its interconnections with electronic control circuits and other thermal elements under varying boundary and initial conditions. Capabilities of the model are illustrated from simulations carried out for a free-standing thermoelectric element during the pulse cooling operation. Simulation results fit well with those obtained using other models reported in the literature as well as with numerical solutions.

High-power high-resolution pulser for air-coupled ultrasonic NDE applications

This paper describes the design and development of a high-power high-resolution pulser designed especially for air-coupled ultrasonic NDE applications requiring high material penetration capability. The pulser can deliver up to 800 V in a spike excitation type pulse to appropriate low/medium frequency transducers. Moreover, the pulser operates in three different modes: single pulse, double pulse (pulse cancellation technique) and external reference signal, where the last two operating modes are intended to enhance the resolution of ultrasonic inspections.

Ultrasonic monitoring of malolactic fermentation in red wines

The progress of malolactic fermentation in red wines has been monitored by using ultrasonic techniques. The evolution of ultrasonic velocity of a tone burst 1MHz longitudinal wave was measured, analyzed and compared to those parameters of oenological interest obtained simultaneously by analytical methods. Semi-industrial tanks were used during measurements pretending to be in real industrial conditions. Results showed that the ultrasonic velocity mainly changes as a result of the conversion by lactic acid bacteria of malic acid into lactic acid and CO2. Overall, the present study has demonstrated the potential of the ultrasonic technique in monitoring the malolactic fermentation process.

Ultrasound measurements for determining rheological properties of flour-water systems

Automation of the food industry requires fast and reliable measurements of the physical properties of materials during processing. The mixture of wheat flour, water, yeast and other ingredients produces a dough with specific viscoelastic characteristics capable of retaining gas and producing aerated goods. Within the baking industry, the control of dough properties is required to achieve final product quality and consistency. Traditional methods for dough testing are slow and off-line and do not provide fundamental rheological information. There is therefore a need for the development of fast and on-line instruments capable of providing relevant data for baking. Ultrasonics provides a nondestructive, rapid and low cost technique for the measurement of physical food characteristics. In this work, the rheological properties of dough are investigated using ultrasonic techniques. The measurements are correlated with dough quality tests. A common protocol for dough preparation is used for each type of measurement. Experimental results on different flour quality, water content and dough processing are presented. The relationship between measured ultrasonic parameters and final product quality is discussed.

Ultrasonic Lamb wave NDE system using an air coupled concave array transducer

Rapid and non contact ultrasonic NDE techniques are of great industrial interest. This paper describes an air-coupled ultrasonic inspection system based on concave linear arrays working on pitch-catch configuration, The system has been designed for real-time characterisation of sheet and plate manufactured materials such as paper and resin-fibre composites. The proposed system is based on air-coupled Lamb wave excitation and reception that performs a rapid measurement of the optimum input angle of the incident beam impinging the material surface. No mechanical parts are used for tuning the plate wave excitation with the angle, doing that electronically by steering the acoustic beam. This solution increases the exploration velocity and the measurement repeatability and system reliability. The main contributions are related to the utilization of a 0.8 MHz ultrasonic air-coupled concave array transducer. This transducer, using only 32 elements, is able to generate a 2.5 square cm size flat wavefront, steering up to /spl plusmn/26 degrees with 1.6 degree resolution, keeping the distance of wave flight and the impact point constant. The angular resolution can be improved up to 0.2 degrees using a novel micro-deflection technique, without any increment of the system complexity.

Wide dynamic range electronics for air-coupled Lamb-waves ultrasonic NDT using piezoelectric transducer arrays

The excitation and front-end electronics for an ultrasonic transducer array used in an air-coupled NDT system is presented. The main characteristic of the whole system is its wide dynamic range, making possible the inspection of thin and porous materials. The system is composed of two 32 elements concave array with a resonant frequency of 780 kHz. To excite the transducers, an array of pulsers has been designed. The transducers are excited by 500 ns and 500 V electrical pulses. The receivers are ultra low noise amplifiers with a 10 MHz bandwidth and 80 dB of gain.

Lumped and distributed parameter SPICE models of TE devices considering temperature dependent material properties

Based on simplified one-dimensional steady-state analysis of thermoelectric phenomena and on analogies between thermal and electrical domains, we propose both lumped and distributed parameter electrical models for thermoelectric devices. For lumped parameter models, constant values for material properties are extracted from polynomial fit curves evaluated at different module temperatures (hot side, cold side, average, and mean module temperature). For the case of distributed parameter models, material properties are calculated according to the mean temperature at each segment of a sectioned device. A couple of important advantages of the presented models are that temperature dependence of material properties is considered and that they can be easily simulated using an electronic simulation tool such as SPICE. Comparisons are made between SPICE simulations for a single-pellet module using the proposed models and with numerical simulations carried out with Mathematica software. Results illustrate accuracy of the distributed parameter models and show how inappropriate is to assume, in some cases, constant material parameters for an entire thermoelectric element.

Study of the time-dependence of the mechanical properties of doughs for flour strength evaluation

Both flour strength and dough processing affect the dough consistency that determines its potential for breadmaking purposes. Quick identification of poor dough quality would reduce problems with dough handling during further stages of the process and maximise productivity. Maintaining consistent production would contribute to better control of product quality and consequently lead to high levels of customer satisfaction. Ultrasonic measurements have already been carried out to characterise dough properties. The ultrasonic wave parameters generally measured include the velocity of propagation and the attenuation of the acoustic wave traveling through the sample. These measurements can be related to both viscoelastic and physical properties of the sample, providing the flour strength. However, due to the time-dependent nature of dough accurate measurements of the ultrasonic velocity and attenuation are sometimes difficult to attain especially in highly attenuating materials like dough. Furthermore, due to viscoelastic properties of dough when a sample is placed between both transducers it slowly flows away from the transducer surface producing changes in the values of the ultrasonic velocity and attenuation with time. The greater these changes are the softer the dough is. This makes necessary a settling time in order to get an accurate measurement. In this work, an alternative method for evaluating the flour strength using low intensity ultrasound is shown. The evolution with time of both velocity and attenuation is monitored and then related to the flour strength. Main advantage of this novel approach is that changes in time of ultrasonic velocity and attenuation are easy to monitor than carry out accurate measurements of them after a settling time. Experimental results on doughs with different flour strength are presented, compared and discussed. Automation of the food industry requires fast and reliable measurements of the physical properties of materials during processing. The mixture of wheat flour, water, yeast and other ingredients produces a dough with specific viscoelastic characteristics capable of retaining gas and producing aerated goods. Within the baking industry, the control of dough properties is required to achieve final product quality and consistency. Traditional methods for dough testing are slow and off-line and do not provide fundamental rheological information. There is therefore a need for the development of fast and on-line instruments capable of providing relevant data for baking. The ultrasonic wave parameters generally measured include the velocity of propagation and the attenuation of the acoustical wave traveling through the sample. These can be related to various of its physical properties. Sensors and instruments based on non-contact methods are especially attractive to the food industry to be employed in quality assurance, process control and non-destructive inspection (1-3), for being both hygienic and easy to maintain. However, there is still a need to develop new techniques that can perform precise evaluations of dough and flour quality. Currently, there are only a few studies using ultrasound for characterising flour-water systems (4-9). In this paper, the ultrasonic measurements of velocity and attenuation are used for the classification of flours intended for different purposes and are compared with conventional flour testing methods. The time-dependence of the mechanical properties of dough is also studied and the results are related to flour strength. The purpose of this study is to determine the potential of ultrasound for use to predict flour and dough quality by millers and bakers by means the determination of flour strength. Section II gives a short outline of the experimental procedure and set-up used during the experiments. In Section III the experimental results are explained and discussed in detail. Finally, conclusions are made in Section IV.