14A-36

T. K. PALAZOGLU and K. P. Sandeep. Department of Food Science, North Carolina State University, Schaub Hall, Box 7624, Raleigh, NC 27695

Uniformity of the heating in a continuous thermal process increases with narrowing residence time distribution (RTD) of particles in the holding tube. In a double-helical holding tube (tube coiled in 8-shape), in addition to the secondary flow development, the axial maximum velocity region will shift from one side of the tube to the other as the flow proceeds from one loop to the next. This flow behavior in double-helical tube has the potential to further increase mixing, and hence narrow the RTD.

The objective of this study was to investigate the effect of curvature (ratio of tube to coil diameter) on RTD of multiple particles in a double-helical holding tube.

Double-helical holding tube of different curvature ratios (1/13, 1/10, and 1/7) was formed by coiling the tube in 8-shape between two cylindrical vertical stands. Spherical polystyrene (density=1010 kg/m³) and acrylic (density=1180 kg/m³) particles, each having a diameter of 0.014 m, were used as model food particles. The carrier fluid used was either water or carboxymethyl cellulose solution, which is a pseudoplastic fluid (K=0.2 Pa-sn, n=0.96), at different flow rates (0.76-1.51 kg/s). Digital image analysis was used to monitor particles at the entrance and exit of the holding tube (15.25 m long, 0.0508 m i.d.) as the particles at different concentrations (4-12% v/v) were recirculated continuously at room temperature.

Preliminary results indicate that RTD in the double-helical setup is narrower than that in a single-helical one only under certain combinations of process parameters. Flow rate was found to be the most significant factor affecting RTD. Data is being analyzed to quantify these effects.

Results of this study will be useful in designing heat exchangers and holding tubes for the food industry that will yield a uniformly processed product.