喷嘴结构对气流冲击式速冻机钢带表面换热特性的影响

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

为了改善现有速冻机存在的效率低、能耗大等问题,该文对比了2种速冻机的喷嘴结构,以期找到提高设备内部换热强度和均匀性的最佳结构。由于速冻机模型较为复杂,为了简化试验模型,本文以冲击式速冻试验台为依托,在验证所建立模型的可靠性的基础上,利用计算流体力学(computational fluid dynamics,CFD)数值模拟分析比较了气流冲击式速冻机2种喷嘴的换热性能。从钢带表面Nu分布和传热均匀性2个角度出发,分析了不同喷嘴到钢带表面距离与出口直径的比值(H/DE)下圆孔和圆漏斗喷嘴结构的钢带表面换热特性。结果表明,当H/DE值在2~12时,圆漏斗喷嘴结构的钢带表面Nu比圆孔喷嘴结构的Nu高5.41%~15.10%。2种喷嘴结构的钢带表面Nu变化受H/DE值影响较大,受横流影响较小。圆漏斗喷嘴结构的钢带表面传热均匀性指标η值比圆孔喷嘴结构的η值低7.06%~34.52%,圆漏斗喷嘴结构的钢带表面传热更均匀。对比2种喷嘴结构,在冷却空气入口处质量流量相同情况下,圆漏斗喷嘴结构的钢带表面能得到更高的Nu值和传热均匀度,更有利于减少食品的冻结时间,提高速冻机产能以及食品的冻结品质。

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-εturbulence 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 uniformityηon 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,theηdifference 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 theηvalue 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.