基于Minitab软件的速冻机内圆漏斗喷嘴结构优化

Optimization of circular funnel nozzle structure in air impinging freezer by Minitab

为了增强冲击式速冻机内的传热强度和均匀性,本文以冲击式速冻机试验台为依托,提出了一种新型喷嘴结构——圆漏斗喷嘴。在该试验台被试验验证可靠的基础上,利用数值模拟技术对该试验台流场进行了模拟计算,利用Minitab软件中的Plackett-Burman设计从圆漏斗喷嘴出口直径D_(E)、漏斗宽度L_(3)、漏斗高度L_(1)、射流高度L_(2)、喷嘴排数N、喷嘴间隙S和喷嘴到钢带的距离H等7个因素中得到漏斗宽度、射流高度和喷嘴排数等3个因素为影响钢带表面努塞尔数和传热均匀度的显著性因素,然后利用Box-Behnken设计,建立3个显著性因素与钢带表面平均努塞尔数Nu_(ave)和传热均匀指标η等2个响应值之间的数学模型,确定喷嘴最佳的结构参数。结果表明:最佳结果参数为漏斗宽度17 mm、射流高度50 mm、喷嘴排数为3;在此条件下钢带表面Nu_(ave)为448.68、传热均匀指标为0.232 9,通过数值模拟得到的值与方程预测值总体吻合,说明优化得到的圆漏斗喷嘴的新型结构参数能够使速冻机的速冻效率达到最佳。

In order to enhance the heat transfer intensity and uniformity in an air impinging freezer,an impinging freezing experimental table was designed as the research object,and a new nozzle structure—circular funnel nozzle was proposed.The numerical simulation technology was used to simulate the flow fields in the impinging freezing experimental table,which was testified by experiments.The flow medium was air,and the simulation process assumes:(1) The wall of static pressure chamber was adiabatic.(2) Air was an incompressible,homogeneous viscous fluid.(3) During the normal operation,the internal flow field of the model was regarded as steady state.The three-dimensional continuity equation,the momentum equation,the energy equation,the kinetic energy k equation and the turbulent dissipation ε equation were used.The cooling air inlet and outlet pressure were 250 Pa(P_(in)) and 0 Pa(P_(out)) respectively.For the frozen area,the cooling air inlet and outlet temperature was 230 K and 235 K.The mass flow rate at the cooling air inlet is0.064 4 kg/s.The thermal conductivity of steel strip was 16.3 W/(m·℃).Using the Plackett-Burman design,the significant factors for the average Nusselt number on the steel strip surface were obtained from the factors of outlet diameter D_E,funnel width L_3,funnel height L_1,jet height L_2,nozzle number N,nozzle spacing S and nozzle-to-surface distance H.These significant factors were funnel width L_3,jet height L_(2) and nozzle number N.The others had little effect on the average Nusselt number.So in the next study,these factors adopted the median values.Then,using the Box-Behnken design,a mathematical model between those three significant factors and the two response values which were average Nusselt number Nu_(ave) and the heat transfer uniformity index η on the steel strip surface was established to determine the optimal structural parameters.The F value in the regression equation can be used to determine the influence of the factors on the response value.Therefore,the order of the factors affecting the average Nusselt number and the heat transfer uniformity index on the steel strip surface was nozzle number>funnel width>jet height.The interaction between the funnel width and the number of nozzle rows,the jet height and the number of nozzle rows were the most significant in the response surface analysis figures,which were consistent with the results of the variance analysis.The results showed that the optimal structural parameters were funnel width L_3=17mm,jet height L_2=50 mm,nozzle number N=3.Substituting these parameters into the second-order polynomial equation,the average Nusselt number on the steel strip surface Nu_(ave)=448.68,heat transfer uniformity index η on the steel strip surface=0.232 9.The numerical simulation value was generally consistent with the predicted value,so it was reasonable to optimize the average Nusselt number and the heat transfer uniformity index on the steel strip surface by using the Box-Behnken design.