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In-line viscosity measurement using ultrasonic Doppler velocimetry |
L. WANG1, K. L. McCarthy2, and M. J. McCarthy2. (1) Dept. of Biological & Agricultural Engineering, Univ. of California, Davis, 1 Shields Ave., 2030 Bainer Hall, Davis, CA 95616-5294, (2) Dept. of Food Science & Technology, Univ. of California, Davis, 1 Shields Ave., 222 Cruess Hall, Davis, CA 95616 Over the last ten years, there has been increased emphasis on the development of new techniques for the in-line measurement of the viscosity of fluid foods. The goal is to use viscosity as a key quality indicator and control variable, and/or to correlate viscosity to other key quality factors. The objective here is to develop an ultrasonic Doppler velocimetry (UDV) technique to provide viscosity characterization for in-line applications during steady pipe flow. This technique utilized a UDV velocity profile and independent pressure drop measurement to evaluate shear viscosity. The focus of this work is to provide a means to account and correct for fluid temperature variation at the measuring site. A linear temperature gradient is set up between the pipe wall and centerline. Experimentally, the velocity profile of the fluids is affected by the temperature gradient through the temperature dependence of both the viscosity and of the speed of sound. To correct for these factors, this work predicts the velocity profile under specified conditions and the effect on the measured viscosity. Two model fluids were evaluated, a 40% Newtonian sugar solution (5.2 mPa.s at 25C) and an 8.3 Brix tomato sauce as a power law fluid (n=0.18, K=12.4 Pa.sn at 25C). As an example, with a temperature gradient between 25C and 80C, the fluid velocity profile is significantly affected and is reflected in viscosities that vary from 5.2 to 1.3 mPa.s for this Newtonian fluid. Likewise, the apparent viscosity of the tomato sauce varies from 50 Pa.s to 5 Pa.s with the combined shear and temperature dependence. In contrast, correction for the temperature dependence of the speed of sound is at a level of 1.5% and 3.8% for the sugar solution and tomato sauce respectively. This characterization allows compensation for nonideal temperature conditions that may exist in factory applications of in-line viscosity measurement.
Session 43, Food Engineering: Rheology and texture
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