NIWeek 97 Annual Conference, National Instruments, Austin, TX, 1997. Published also in: "Virtual Instrumentation in Education: 1997 Conference Proceedings" Massachusetts Institute of Technology-June 12, 1997 and University of California at Berkeley, June 27, 1997, P/N 350357A-01, p.131-136, National Instruments Corporation, 1997.

DATA ACQUISITION AND CONTROL
USING LabVIEWâ VIRTUAL INSTRUMENT
FOR AN INNOVATIVE THERMAL CONDUCTIVITY APPARATUS

 M. Kostic
Associate Professor of Mechanical Engineering
Northern Illinois University

Products Used:
LabVIEW
DAQ
SCXI

Abstract
An innovative method and a novel research apparatus are being developed and tested to measure the thermal conductivity of non-Newtonian fluids while they are subjected to shearing flow. The emphasis here is given to the apparatus' instrumentation and computerized data acquisition and control, while a detailed description of the mechanical design and test results will be presented elsewhere. The measurement and control are accomplished and integrated by using a computerized data acquisition system and a comprehensive virtual instrument, developed using the LabVIEW application software. In addition, this system allows for easy modification and enhancement of virtual (software) instrument by modification of the software program.

The Challenge
Develop a computerized data acquisition system in order to integrate high accuracy measurements, data acquisition, interactive data processing and analysis for the feed-back control, and data and results display and storage.
 

The Solution
The measurement and control are accomplished and integrated by designing and implementing a computerized data acquisition system and a comprehensive virtual instrument, developed using the LabVIEW application software.
 

Introduction

Many unusual flow and heat transfer phenomena associated with high molecular polymeric solutions and other rheologically complex non-Newtonian fluids are being investigated and offer great application potentials. It is known that these fluids are affected by shearing flow: becomes fiber-like, non-uniform and non-isotropic. An innovative method and a novel research apparatus are being developed to measure the thermal conductivity of a fluid while it is subjected to shearing flow, thus measuring the thermal conductivity as a function of temperature and shearing parameters themselves [1-13]. This is contrary to the current state-of-the-art of measuring thermal conductivity under the condition of motionless fluid, to avoid convective heat transfer influence on the results. The emphasis here is given to the apparatus' instrumentation and computerized data acquisition design and its demonstration as a purposeful and typical application example, while a detailed description of the mechanical design and test results will be presented elsewhere [12,13].

Innovative Thermal Conductivity Apparatus And Instrumentation

The apparatus, see Figs. 1 and 2, consists of: (1) an innovative, concentric-cylinders thermal conductivity cell; (2) a high performance, variable controlled-voltage or -current, DC power supply for the main heater; (3) two common, variable-voltage, AC power supplies for the guard heaters; (4) a variable-speed DC motor with drive and controller; (5) a constant temperature bath, controlled by a high performance, digital immersion circulator; and (6) computerized data acquisition system with signal conditioning hardware. The measurement and control are accomplished and integrated by using a computerized data acquisition system and a comprehensive so called "virtual instrument," developed using the LabVIEW application software. The rotational speed is measured by a tachometer-sensor and controlled by a voltage-varying DC motor through a built in, solid-state, servo power-amplifier circuitry. The main heater is powered and controlled by a high-quality DC power supply, while two guard heaters are powered by common AC power supplies, and controlled, including over-heating protection, by the computerized system through solid-state relay switches. The computerized system hardware consists of a National Instruments' MIO plug-in data acquisition board, shielded cable assemblies, and a signal conditioning module with a cold-junction compensated terminal block for thermocouple signals. Brief descriptions of selected important components and functions are given below and in the next section.

Thermal Conductivity Cell

The actual geometry of an apparatus and test fluid sample consists of a circumferential narrow gap (see Fig.1), similar to the apparatus for viscosity measurements [3, 4]. In addition, the appropriate heat transfer flux in the transverse to test fluid flow direction is provided. The main test-section dimensions are: 2.598/ 2.488in , outer/inner cylinder diameters respectively, with the 0.055 in thick gap, filled with the test-fluid in-between. The inner-cylinder's in-the-test-fluid immersion length is 3.8 in. It is heated by three 1.3-in-diameter electrical-resistance heaters, the central main heater with height 1.44 in, and the two remaining guard heaters of 0.78 in high each. The inner cylinder with the heaters assembly is stationary, while the outer cylinder rotates (thus suppressing the Reynolds vortices) generating the Couette type-laminar flow of the test fluid. The two guard-heaters are controlled in such a way to maintain uniform axial temperature in the central, main-heater region, so that the latter heat flux is virtually in the radial direction only. Due to absence of the test-fluid's radial and axial velocities in the main-heater test-section region, the heat flux through the test fluid there is virtually transferred by conduction mode only. Thus, the measurement of the test-fluid's thermal conductivity, while undergoing shearing flow, is achieved. More detailed description of the apparatus is given in [11] and will be presented elsewhere [12,13].

Instrumentation and Measurement

The required variables for thermal conductivity measurement are heat flux and temperature gradient through the test fluid, as well as the shearing rate of the test fluid. The apparatus' instrumentation is described next:

• The thermal conductivity apparatus is instrumented and equipped with twelve thermocouples imbedded in the inner cylinder at two different radial and five different axial locations (see Fig. 2). In addition, a thermocouple is attached to the outside surface of the outer cylinder, at the center of the main-heater's axial location. This thermocouple rotates with the outer cylinder and is terminated at its top end with a "quick" connector. Due to this special circumstance, the outer cylinder temperature is measured after the steady state is achieved and after all other measurements are completed, by quick-stopping of the cylinder rotation and with the "quick" connector wired to the data acquisition system. Then, one more measurement of all other temperatures is performed to confirm the agreement with the corresponding measurements just before the quick stop. In addition, the outside-surface of outer-cylinder is measured during its rotation by a special thermocouple probe which rubs the surface under a slight pressure. The effect of rubbing is calibrated under different rotational and heating conditions. Due to the rotation of the outer cylinder, it is not known to the author if direct measurements of its temperature were done before. Two more thermocouples are used for measurement of constant temperature bath and room temperatures. All thermocouples are made from 30-gauge, T-type thermocouple wire and calibrated before and after assembly. These 16 thermocouples provide for the temperature gradient calculation needed for thermal conductivity measurement, and for confirming both, the unidirectional (radial only) heat flux through the test fluid and steady state thermal condition.
• The heat flux is measured through measurement of the DC voltage drop across the main heater and a precise current resistor (shunt), see Fig. 2.
• Finally, the fluid shear rate is calculated using the known test-section geometry and the measured rotational speed of the cylinder with a calibrated tachometer-sensor, see Fig. 2.

All measurements are repeated until the kinematics and thermal equilibrium is achieved. After that a number of final measurements are performed and results are obtained using statistical analysis, as described elsewhere [12,13].
 

Test Fluids and Results

Distilled water and the standard Newtonian fluids with known thermal conductivities are used for over-all calibration of the apparatus. Then, the thermal conductivity of the following non-Newtonian fluids, suspected to have shear-rate dependent thermal conductivity, will be measured as a function of shearing parameters: a) aqueous solutions of polyacrylic acid (Carbopol); b) aqueous solutions of polyacrylamide (Separan or Praestol); c) aqueous solutions of carboxymethyl cellulose (CMC); and d) aqueous solutions of polyethylene oxide (Poliox). At the time of writing, the apparatus, including the instrumentation and data acquisition as described next, has been completed, tested, and used for educational demonstration [11]. The project is in progress with the calibration of the apparatus being underway. However, the test results are not available yet, and will be presented after the completion of the project [12,13].
 

 Computerized Data Acquisition And LabVIEWÒ Virtual Instrument

Development and implementation of computerized data acquisition have the objectives of achieving more accurate measurement and interactive feed-back control. The computerized data acquisition and control system is schematically presented in Fig. 2. It consists of the following components made by National Instruments:

• AT-MIO-16DE-10 data acquisition (DAQ) board (E Series architecture, 100 kSamples/sec; 12-bit analog inputs, 16 single-ended/8 differential channels; two 12-bit analog outputs; two 24-bit, 20 MHz counter/timers; 32 digital I/O channels);
• SCXI-1000-slot signal conditioning chassis;
• SCXI-1122 16-channel isolated transducer multiplexer and signal conditioning module for thermocouple sensors;
• SCXI-1322 shielded terminal block;
• SCXI-1353 shielded cable assembly.
• Two CB-50 terminal blocks with short NB-1 ribbon cables attached to appropriate SCXI-1353 assembly adapters for analog and digital input/output and counter/timer connections.

A so called "virtual instrument" is developed, using the LabVIEW software application program. It integrates measurements, data acquisition, and interactive data processing and analysis for the feed-back control, and data and results display. The LabVIEW "virtual instrument's front-panel" interface is given in Fig. 3. It provides for accurate and interactive control and display of measured and analyzed variables for three major functions: (1) motor's rotation; (2) guard heaters' control and main heater power measurements; and (3) temperature measurements and overheating protection. The measured variables are displayed graphically, along with their average values, standard deviations and limits if appropriate, see Fig. 3. The control of all functions and data acquisition settings is conveniently provided through the virtual instrument's "front-panel" interface. The measured data are also stored in a file for future processing The algorithm of the LabVIEW "virtual instrument" software program is presented elsewhere [11]. The measurement and process control are enhanced by (see Figs. 2 and 3):

• implementing a feed-back control circuit via DAQ AO signal for DC motor-drive using a calibrated tachometer-sensor's DAQ AI signal;
• efficient and accurate feed-back control of guard-heaters' power by varying the "duty cycles" of DAQ Counter/Timer (CTR) pulse signals to control the corresponding solid-state relays (S/R);
• comprehensive over-heating protection control by switching off the solid-state relays if and when needed;
• interactive and comprehensive monitoring for the kinematics (via DAQ AI tachometer signal) and thermal steadiness (via SCXI-DAQ thermocouple signals) of all processes; and
• convenience of increasing the number of thermocouple sensors (by implementing the SCXI module and terminal) for more advanced measurements of temperature and heat fluxes.

 In addition to the research objectives, this apparatus is enhanced with appropriate documentation and labeling to be used as a typical and elaborate application for educational demonstration in Engineering Experimental Methods I and II courses (MEE 390 and 490) at the Mechanical Engineering Department of Northern Illinois University. The basics of

LabVIEW software are taught in these courses and students have used this apparatus as a purposeful application for demonstration of computerized instrumentation and data acquisition for interactive measurements and control.

 Conclusion

One of the objectives of this project is to utilize the latest powerful, yet inexpensive, technological developments: sensors and transducers, data acquisition and control integrated boards, computers and application software, for research and teaching by example. The designed, computerized measurement and data acquisition system, accomplishes the following objectives:

• acquire measured data with high speed and accuracy;
• interactively process and analyze measured data for immediate use or future post-processing;
• provide interactive and accurate feed-back process control - motor speed and guard-heating power, and
• interactively displays the raw/measured and processed/analyzed data in graphical and/or numerical forms.

 In addition, such a system allows for easy modification and enhancement of the so called "virtual (software) instrument" by modification of the software program. It is important to emphasize that functionality and quality of a virtual instrument is practically limited by our creativity.
 

Acknowledgment

The author acknowledges support by the Department of Mechanical Engineering and Graduate School of Northern Illinois University, and National Science Foundation support (Grant No. CTS-9523519). The author is also grateful for help in mechanical design and fabrication to Mr. Haibo Tong, graduate student and Mr. Al Metzger, instrument maker and technician supervisor, as well as for help in electronics design and fabrication to Mr. Bill Vickers, senior electronics technician.
 

References

1. Sengers, J.V., and M. Klein, Eds., "Technical Importance of Accurate Thermophysical Property Information," National Bureau of Standards Technical Note No. 590, 1980.from Incropera HT book
2. McLaughlin, E., "Theory of the thermal conductivity of Fluids," in R.P. Tye, Ed., Thermal Conductivity, Vol.2, Academic Press, London, 1969.
3. Jimenez, J. and M. Kostic, "A Novel Computerized Viscometer/Rheometer," Review of Scientific Instruments Journal, Vol.65(1), p. 229-241, American Institute of Physics (1994).
4. Kostic, M., "An Innovative Thermal Conductivity Measurement of Fluids with Changing and Anisotropic Structure While in Shearing Flow." National Science Foundation (NSF), Proposal # 92-288, Northern Illinois University, 1992.
5. Lee, D-L., "Thermal Conductivity Measurements of Non-Newtonian Fluids in a Shear Field," Ph.D. Thises, State University of New York at Stony Brook, 1995.
6. Hartnett, J.P. and M. Kostic, Heat transfer to Newtonian and non-Newtonian fluids in rectangular ducts. Advances in Heat Transfer, Vol.19, p. 247-356 (1989).
7. Bellet, M. Sengelin, and C. Thirriot. "Determination of Thermophysical Properties of Non-Newtonian Liquids Using a Coaxial Cylindrical Cell," Int. J. Heat Mass Transfer, Vol. 18, p. 1177, 1975.
8. National Instruments Web site: http://www.natinst.com .
9. National Instruments Catalogs and User's Manuals for the used hardware and software.
10. Wells, L and J. Travis, "LabVIEW for Everyone," Prentice Hall PTR, Upper Saddle River, NJ, 1997.
11. Kostic, M., "Instrumentation with Computerized Data Acquisition for an Innovative Thermal Conductivity Apparatus." ASEE 1997 Annual Conference, American Society for Engineering Education, 1997.
12. Tong, H., "A Novel Thermal Conductivity Measurement of Fluids with Changing and Anisotropic Structure Due to Shearing Flow." M.S. Thesis in progress, Northern Illinois University, DeKalb, IL, 1997.
13. Kostic, M. and H. Tong, "Innovative Thermal Conductivity Apparatus for Testing of Complex Fluids," Manuscript in progress, Northern Illinois University, DeKalb, IL, 1997.
     

Author Biography
Milivoje Kostic, Ph.D., P.E. is an Associate Professor in the Department of Mechanical Engineering at Northern Illinois University. He received his Ph.D. in 1984 from the University of Illinois. Professor Kostic's teaching and research interests are Thermodynamics, Fluid Mechanics, Heat Transfer and related Fluid/Thermal/Energy sciences; with emphases on new technologies, experimental methods, creativity, design, and computer applications.

Contact Information
M. Kostic, Ph.D., P.E.
Associate Professor of Mechanical Engineering
NORTHERN ILLINOIS UNIVERSITY
DeKalb, IL 60115-2854, USA
Phone: (815) 753-9975 or 753-9979
Fax: (815) 753-9975 or 753-0416
Web: www.kostic.niu.edu  or  http://www.ceet.niu.edu/faculty/kostic
E-mail: kostic@ceet.niu.edu

Professor M. Kostic's Web Site: www.kostic.niu.edu *Usage Policy & © Copyright by M. Kostic