【作者】 刘斌；

【导师】 潘良明；

【作者基本信息】 重庆大学 ， 动力工程及工程热物理， 2012， 硕士

【摘要】 控制冷却技术是控制轧后钢板的冷却速度以细化铁素体晶粒，减少珠光体片层间距，阻止碳化物在高温下析出，以提高析出强化效果而采用的工艺，可作为在线进行淬火等热处理工艺的手段。喷淋冷却技术是重要的控制冷却技术之一，在冷却过程中，喷淋表面的换热系数是量化冷却介质冷却能力的重要参数，也是进行冷却控制设计的重要参数。本文针对圆柱孔形喷嘴、散射喷嘴以及缝隙式喷嘴的喷淋冷却进行了实验研究，并探讨了钢板厚度对喷淋冷却的影响；本文还实验研究了浸没冷却时冷却参数对浸没冷却速率的影响。研究发现，随着喷淋压力增加，冲击区中心冷却率变化速度加快，喷淋表面换热能力增大；对于圆柱孔型喷嘴，随着喷淋压力增加，喷淋距离对钢板表面冷却能力的影响有所减弱，而散射角喷嘴和缝隙式喷嘴的这种现象不明显。喷淋高度对换热的影响不明显，大多数情况下喷淋高度为100mm时冷却略快。而平流区换热情况较复杂，但散射角喷嘴则随着压力增加平流区中心冷却率变化速率增加；缝隙式喷嘴换热系数最大值出现在喷射压力为0.5MPa时，而圆孔柱型喷嘴的最大换热系数点则出现在0.8MPa。采用倾斜喷射时，冲击区中心冷却率变化速率减慢，换热系数减小，而平流区的换热系数较垂直喷淋时明显增大，冲击区和平流区的温差减小，采用倾斜喷淋时整个钢板的冷却均匀性有明显改善。垂直喷射时，散射角喷嘴同一截面平流区和冲击区温差最小，温度均匀性最好。当面雅克比数Ja>0.3时，表面热流密度随着Ja数的下降急剧变化，而后又平缓的上升或下降，冷却终了又急剧下降。换热系数h随Ja数的下降而上升，随后达到最大值。在冷却过程中喷淋表面的换热系数会随着表面温度降低先上升后下降，中间出现峰值，而峰值大概出现在Ja数在0.03-0.08之间。钢板厚度变化不会对钢板表面换热能力产生明显影响，而钢板的中心冷却速率变化率却随着厚度的增加明显下降。基于沸腾理论和喷淋冷却实验结果，建立了三种喷嘴喷淋冷却时的换热系数预测模型；模型预测结果与实验结果吻合较好。基于喷淋冷却过程中钢板表面温度的变化，针对工程中应用最为广泛的区间建立了平均换热系数的预测模型；模型预测结果与实验结果吻合很好。浸没冷却实验发现，液滴飞溅对高温钢板起到了预冷作用，在淹没液位上升过程中，高温钢板纵向导热越来越强，被淹没位置具有很高的中心冷却速率；研究还发现，高温钢板被淹没位置的中心冷却速率并不随浸没速率单调变化，而是在一定区内呈起伏变化，这说明在某个淹没速率下存在一个最小中心冷却速率的区间；高温情况下，初始温度对中心冷却速率影响不大。更多还原

【Abstract】 Controlled cooling is aimed at improving the structure and the properties of steelby controlling the cooling rate after rolling steel plate. It is also a kind of technologyused to enhance the precipitation effects by refining the ferrite grain, reducing thespacing between pearlite interlamellar and preventing the precipitation of carbides athigh temperatures and regarded as a method of heat treatment such as online cooling.Spray cooling is one of the most important controlled cooling technology. During theperiod of cooling, the heat transfer coefficient of spraying surface is an importantparameter to quantify the cooling capacity of the cooling medium and it is also animportant parameter in cooling control design. In this paper, we take experiments oncylindrical nozzle, scattering nozzle and crevice nozzle and explore the influence ofplate thickness on the spray cooling. Apart form the spraying experiment, the effect ofthe immersion cooling parameters has been studied in this paper as well.We found that with the increasing pressure of impact zone, the central cooling ratespeeds up and the capacity of heat transfer of spray surface increases. As for thecylindrical hole nozzle, the influence on cooling the surface of steel becomes weak. Asfor the scattering angle nozzle and slit injector, this phenomenon is not obvious.Spraying height does not have a big influence on heat transfer, in most cases, coolingbecomes slightly faster when the spray height is100mm. Heat transfer is very complexin advection zone while quite uniform in scattering angle nozzle, central cooling ratebecomes faster while the pressure speeds up. Capacity of heat transfer enhanced whilethe spray height increases.The gap type peaks at the circumstance of0.5MPa whilehole cylindrical peaks at0.8MPa.When we use the inclined spray, the cooling rate of impact zone decreases and theheat transfer coefficient reduced while the cooling rate of stratospheric acceleratesnoticeably along with the heat transfer coefficient (relative to the vertical spray). In theprocess of cooling, the temperature difference of impact zone and advection zonebecome smaller. The use of inclined spray improves all the uniformity significantly inthe whole steel plate. When we use vertical spray, the minimum temperature differencebetween impact zone and advection zone and the best temperature uniformity have beengot using the Scattering angle nozzle.According to the heat flux and heat transfer coefficient along with the Ja curve, when Ja is bigger than0.3, the heat flow of the upper and lower surface changedramatically with the change of Ja. As the Ja declines, the heat flux gradually rise ordecline and finally dropped sharply. Heat transfer coefficient h rises up with the declineof Ja and than arrive at the maximum. The heat transfer coefficient rises and thendeclines during the period of cooling and the maximum appears when Ja is between0.03and0.08.The research found that the thickness of steel plate doesn’t have any significantimpact on the capacity of the surface of steel plate while the cooling rate of the centralsteel plate decreases dramatically with the increases of the thickness of steel plate.In this paper, we establish three nozzle spray cooling heat transfer coefficientprediction model based on the boiling theory and cooling experiment which have beprovided by previous research.Model predictions and experimental results agree well with the hole cylindricalnozzle and the scattering angle nozzle error less than+-30%while the gap nozzle errorwithin+-40%. Surface temperature changes in the spray cooling process for the projectin the most widely used range from the early cold to a Jacques created than the numberof average hear transfer coefficient prediction model, model predictions andexperimental results are in good agreement.What’s more, under the immersion cooling condition, the liquid spatters canpre-cool the plate at high temperature; the heat transfer in the vertical direction has beenenhanced with the increasing liquid level and the center temperature cooling speed isvery high. The experiments reveal that the nonlinear relationship between the centertemperature dropping speed and the liquid level increasing speed, and it shows anU-shape trend which suggests that a minimum center temperature dropping speed existsduring rewetting with a certain liquid level increasing speed. With higher initialtemperature, the temperature dropping speed is affected by the initial temperaturemildly.更多还原