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In this paper, the fluid field in an air impinging freezing experimental table was studied by computational fluid dynamics (CFD) numerical simulation technology. The influence of the different diameter D and orifice spacing distance S on the heat transfer characteristics on the steel strip surface were analyzed, and the difference of the heat transfer effect between the orifice and slit nozzle were compared. The results show that when the orifice diameter is less, the heat transfer coefficient on the steel strip surface is less due to the effect of cross-flow. With the orifice diameter is increased, the effect of the resistance along the upstream is increased. The average Nusselt number on the steel strip surface is the highest at D=10mm. When the spacing distance is less, the distance between the first row of orifice nozzle in the upstream and the adhering wall is less and the velocity of flow is less. With the increased of spacing distance, the distance between the first row of orifice nozzle in the upstream and the last in the downstream is increased, and the effect of cross-flow is increased. The average Nusselt number on the steel strip surface is the highest at S=34mm.The effect of cross-flow under the slit nozzles is weak and the food can be better protected. The average Nusselt number on the steel strip surface under the orifice nozzles is 9.7% higher than that of the slit nozzle. Introduction Jet impingement is a commonly technique used in heating, cooling or drying the surface[1, 2]. The boundary layer on the food surface is destroyed by high-speed air and the heat transfer is greatly enhanced. It is one of the most effective and flexible local cooling technologies[3, 4]. Cross flow is defined as the fluid flow in the direction perpendicular to the impinging jet. It can be formed by external flow or accumulating spent flow. The cross-flow has an adverse effect on the heat transfer on the impinging surface. The cross-flow effect must be minimized in order to achieve effective heat transfer[5-7]. Nuntadusit C and Sarkar A et al[8, 9] used oil film technique to visualize the impinging surface flow. They found that the cross-flow effect became more notified along the axial direction. In the downstream area, the center of an impingement jet was shifted and the shapes of fluid flow on the impinging surface become irregular. Dano et al[10]determined the flow behavior of impinging surfaces by digital particle image velocimetry(DPIV) and flow visualization techniques. The results show that the heat transfer rate was maximum at the stagnation point, and the peak of the heat transfer coefficient was decayed along the cross-flow direction. According to the previously published works, the heat transfer characteristics on the impinging surface is significantly affected by the cross flow. In order to simplify the model of the air impinging freezer, the impinging freezing experiment table is as object of study in this paper. When the pressure difference between the inlet and the outlet of the cold air is constant, the influence of different orifice diameter and orifice spacing distance on the heat

Effects of Different Orifice Structure on Heat Transfer Characteristics of Steel Strip Surface in an Air Impinging Freezer