15D-5 |
Effect of curvature ratio on the degree of mixing during helical tube flow |
T. K. PALAZOGLU and K. P. Sandeep. Dept. of Food Science, North Carolina State Univ., Schaub Hall, Box 7624, Raleigh, NC 27695-7624 Mixing of fluid elements during continuous thermal processing of foods is desirable as it increases the rate of heat transfer and uniformity of the process. Secondary flow observed in helical heat exchangers and holding tubes is known to induce radial mixing, and its strength is dependent on the curvature ratio (ratio of tube diameter to coil diameter) of the helical tube. The objective of this study was to investigate the effect of curvature ratio on the degree of mixing during helical tube flow. Curvature ratios of 1/13 and 1/7 were tested. Cubic polystyrene (r=1013 kg/m³) and acrylic (r=1178 kg/m³) particles were covered with liquid crystal sheets that change color between 20 and 25°C. Particles were placed in an ice-water mixture to bring their temperature below 20°C before they were introduced into the stream of CMC solution (25°C). Color change on the particle surface was recorded by a camera at the inlet and outlet of the system. The video recordings were then analyzed using an image analysis software, and surface color maps were obtained. Injected dye streaks were also recorded and the length of complete dye swirls were compared. Flow in helical tube was simulated using a computational fluid dynamics software to explain the experimental results. Liquid crystal and dye injection experiments showed that a higher degree of radial mixing is attained when the tube is coiled to a tighter radius. Stronger secondary flow field is responsible for improved mixing and heat transfer characteristics. Simulation of the experiments by finite element flow modeling supports the experimental findings. The results of this study may be useful in designing new equipment and thermal processes for liquid and particulate foods that will yield improved mixing, and hence a product of better quality.
Session 15D, Food Engineering: Processing Technologies
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