Residence time distribution and fluid-to-particle heat transfer coefficients in a holding tube having oval cross section

Finite element method with Galerkin's approach of weighted residuals was used to calculate point velocities over the cross section and the maximum to average velocity ratio in a holding tube with oval (OHT) or circular cross section (CHT). It was found that this ratio was 1.67 in OHT as compared to 2.04 in CHT. Fluid velocities on the cross section of OHT was entirely different from those in CHT. OHT was fabricated from an aluminum alloy and had cross sectional area equivalent to 2-inch sanitary tube. Residence time for fluid was measured experimentally using impulse technique while for solids a step method was used. Polystyrene spheres were injected into the holding tube to simulate food system with suspended particles. Fastest-particle velocities were measured by the use of white and black particles and counting the exiting particles at the exit end. For comparison similar experiments were conducted using a conventional holding tube. It was found that the residence time distribution was more uniform, had narrower spread and the maximum velocity was lower in OHT compared to CHT.;Fluid-particle heat transfer coefficients in the holding tubes were determined using a biological time temperature indicators. Vegetative cells of Bacillus stearothermophilus were immobilized in gellan cubes. Plastic spheres were injected at controlled rates along with test particles with different carrier fluid flow rates and temperatures to simulate conditions of multiple particles heat transfer in the stream. Levels of surviving microbes in the cubes were experimentally determined and compared with the number estimated by an explicit finite difference technique. It was found that the heat transfer coefficients were significantly higher in OHT as compared to CHT and this difference increased with flow rate and temperature. The heat transfer values in CHT were within the range reported by previous investigations.;These observations, show that the OHT not only had lower maximum-particle velocities but also a remarkably narrower residence time distribution. This implies that, to achieve a target sterility value a shorter holding tube would be required and reduced over processing of food could be expected compared to conventional holding tubes. OHT could be useful for aseptic processing of particulate foods because of uniform heating of food particles and lethality because of narrower residence time distribution. For processes where particle heating is controlled by surface convective heat transfer, faster particle heating will be obtained in OHT compared to CHT. Ohmic heating may also benefit from better residence time distribution in OHT.