Research on Flow and Heat Transfer Characteristics of Multiple Impinging Jets on a Moving Conveyor Belt

     Air impingement cooling technology is one of the widely used methods to enhance the heat transfer in many engineering applications and manufacturing processes, such as cooling of gas turbines blades, annealing of metals and glass, textiles and paper drying, heating, cooling or drying of painted cylinders and foodstuffs, de-icing of aircraft wings and cooling of high power density electronic equipment. The process of the air jet impingement takes the compressed freezing air passing through the nozzles toward the product surface. Thereby provides an effective way to enhance heat and mass due to its thin hydrodynamic and thermal boundary layers in the stagnation region. What’s more, as a clean medium, the air is feasible to be massively applied without considering environment pollution and expensive maintenance. With these properties, The quick freezer is efficient equipment in the rapid-frozen industry.
     However, the airflow distribution of the impinging jet is affected by the conveyor belt motion, thereby affect the heat transfer performance of the food. In this research, impinging jets at three different jet nozzle forms were numerically analyzed to investigate the flow field and heat transfer characteristics on a moving target surface. To hunt for an optimal jet nozzle structure, we designed three different impinging jet exit nozzles, including slot, rectangular, and funnel-shaped nozzles. To simulate the cooling process of the impinging jets, three semi-confined domains with these jet nozzles are then established. Based on the principle of computational fluid dynamics (CFD), Fluent software is used to calculate the velocity, temperature, pressure, and mass flow rate distribution of multiple impinging jets. The Reynolds Averaged Navier Stokes (RANS) equations are used to model the jets domains using the k- epsilon turbulence model and the SIMPLE algorithm. The influence of jet nozzles on the velocity field, pressure field, and temperature field, and the effect of belt motions on the wind velocity, temperature, and mass flow rate are investigated according to the critical cross-sections of interest. Distributions of the temperature and wind velocity at four critical cross-sections of domains are compared using the conveyer belt speed varying from 0 to 0.02m/s). In all investigations, the nozzle to surface distance H/d  keeps as a constant. The results show that the freezing rate of foods mainly relates to temperature and wind velocity.  The jet impinging heat transfer is enhanced by using slot nozzles. For three impinging domains, the impinging jet with slot nozzles produces higher exit wind velocity, lower center temperature, and a better mass flow uniformity than others, which could better improve the heat transfer performance. It could increase the freezing rate of foods.