Abstract:In order to improve the atomization characteristics of quick-frozen blueberries, a fan-shaped nozzle with multi-outlet was designed and optimized. It can spray a fan-shaped dry ice particle jet, which can ultimately freeze blueberries more evenly and quickly. A physical model of a multi-outlet fan-shaped nozzle and a computational model of the dry ice jet flow field for quick-frozen blueberries were established. Using Fluent software and the gas-solid two-phase dynamics model DPS, Realizable,k-ε turbulence model, a numerical simulation study was conducted on the process of quick-freezing blueberries by using a multi-outlet fan-shaped nozzle for dry ice particle spraying. The different V-shaped groove angles (60°, 70°, 80° and 90°) at the outlet of the fan-shaped nozzle were investigated, the effects of different angles on the flow field distribution of dry ice particles in quick-freezing chamber, as well as the freezing rate of blueberry and the freezing uniformity were studied under the same inlet flow rate and outlet aperture. The results showed that as the angle of the V-shaped groove was increased, the width of the fan-shaped impinging jet was decreased, and the flow velocity in the core region of the impinging jet was increased. When the V-shaped groove angle of the multi-outlet fan-shaped nozzle outlet was 70°, compared with 60°, 80°, and 90°, the freezing completion time distribution of the whole plate of blueberries in the quick-freezing chamber was the most concentrated, the overall freezing speed was fast, and the flow field was the most uniform. Therefore, it was the optimal outlet parameter for this nozzle model (inlet diameter of 30mm, inlet velocity of 0.25m/s, outlet was a combination of circularly arranged diameter of 5.2mm×6（there were six outlets with a hole diameter of 5.2mm）and centrally arranged diameter of 2mm×4). The optimal result of the simulation was then tested experimentally. The results showed that the whole plate of blueberries completed quick-freezing in 119s, with a freezing rate of 0.50cm/min. The error between the experimental and simulated cooling curves was 4.3%. Tests were conducted on anthocyanins content, soluble solids, mass fraction, and water loss rate of frozen blueberries after quick freezing. The results showed that the sensory quality of dry ice quick-frozen blueberries during storage was better than that of national standard quick-frozen blueberries.
