Effects of nozzle structures of air impinging freezer on heat transfer characteristics of steel strip surface

Abstract: In view of the low efficiency and high energy consumption of air impinging freezer, 2 kinds of nozzle structures were compared in this paper in order to find out the best structure and improve the Nusselt number and heat transfer uniformity. Based on the impinging freezing test bench, the performance of 2 kinds of nozzle was analyzed and compared by using computational fluid dynamics (CFD) Numerical simulation technology. The k-ε turbulence model was used. Since there was a temperature change during the jet impinging, the energy equation was selected. The cooling air inlet and outlet pressure were 250 Pa(Pin) and 0 Pa(Pout) respectively. For the frozen area, the cooling air inlet temperature was set to 230 K and outlet temperature was 235 K. The mass flow rate at the cooling air inlet is 0.064 4kg/s. The thermal conductivity of steel strip was 16.3 W/(m.°C). In order to verify the reliability of numerical simulation, experimental verification was carried out. Taking the circular orifice nozzle as an example, the outlet diameter of circular orifice nozzle was DE=10 mm, nozzle spacing was S=34 mm, nozzle number was 64(8 rows×8 ranks), and the ratio between nozzle-to-strip distances and outlet diameters was H/DE=2. The absolute velocity of steel strip surface in vertical direction at the outlet nozzle (Z direction) was measured. The error between simulation value and test value was 1.24%6.90%, thus it could be concluded that the numerical simulation of the impinging freezing test bench was reliable. Based on the Nusselt number distribution and heat transfer uniformity on steel strip, the heat transfer characteristics on steel strip surface under the circular orifice nozzles and circular funnel nozzles at the different ratio between nozzle-to-strip distances and outlet diameters were analyzed. The results showed that when the H/DE was in the range of 2-12, the average Nusselt number on steel strip surface under the circular funnel nozzle was about 5.41%-15.10% higher than that under the circular orifice nozzle. The change of the Nusselt number on steel strip surface under both 2 kinds of nozzle structures were greatly influenced by the H/DE and was less affected by the cross flow. The heat transfer uniformity η on steel strip surface under the circular funnel nozzle was about 7.06%-34.52% lower than that of the circular orifice nozzle. As the H/DE increasing, the η difference between the 2 kinds of nozzle structures was gradually decreased. This was because that for the circular funnel nozzle, the "convex" region which was formed between 2 kinds of nozzle structures could form a cross flow buffer zone. On the one hand, the flow direction of the cross flow was changed, so the cross flow velocity in the channel was decreased, and the adverse effect of cross flow was reduced. The average Nusselt number on steel strip surface under the circular funnel nozzle was higher, so that the heat transfer characteristics on steel strip surface was higher. On the other hand, the large vortex formed on the left side of the circular funnel nozzle enhanced the line B above the steel strip surface. The velocity in Z direction increased the Nusselt number on the line B, so the η value of the steel strip surface was decreased, so that the airflow in the air impinging freezer was relatively uniformity. By comparing the structures of the 2 nozzles, it is recommended to use a circular funnel nozzle in the case of the same air supply volume to reduce the freezing time, increase the output of the air impinging freezer, and improve the quality of the frozen food.