基于有限元的激光烧蚀棉花顶芽温度场模型与试验

    Finite element-based modeling and testing of temperature field in laser ablated cotton terminal buds

    • 摘要: 为探究激光烧蚀过程中温度对棉花顶芽的影响,该研究通过试验测定棉花的热物性参数,在传热理论模型基础上,借助有限元分析软件COMSOL Multiphysics建立随时间变化的激光烧蚀棉花顶芽温度场模型,并实现模型求解,根据激光烧蚀作用下的棉花顶芽温度分布,分析棉花顶芽在直径方向和深度方向的温度场变化,以及不同功率激光对棉花顶芽表面区域的烧蚀效果。研究结果表明:所构建的温度场数值模型可用于激光烧蚀棉花顶芽的模拟分析。与30 W功率激光相比,40和50 W功率激光的烧蚀时间分别缩短了45.55%和67.36%;棉花顶芽区域最高温度随激光光斑的增大而降低,2.5 mm光斑可在0.50 s内达到烧蚀温度,相比3.5 mm光斑,时间缩短了76.85%;含水率的升高会增加棉花顶芽的烧蚀时间,含水率为75%时,达到烧蚀温度的时间为0.52 s,相较含水率95%D 0.61 s 缩短了17.31%。在一定激光功率下,温度沿直径方向的扩散速度明显高于深度方向。激光功率50 W、烧蚀时间1 s未对棉花顶芽产生实质性的抑制作用,激光功率100 W、烧蚀时间1 s烧蚀顶芽,能够有效抑制棉花顶芽生长,且碳化烧蚀的程度越强,抑制效果越明显。研究结果可为棉花激光打顶理论研究和实际应用提供借鉴价值。

       

      Abstract: Cotton is one of the most widely cultivated cash crops in agriculture and textile industry worldwide. Innovative techniques have emerged to enhance the efficiency of cotton production in recent years. Among them, laser ablation can be expected to treat the cotton terminal buds in modern agriculture. The cotton terminal dominance can also be suppressed for the desired topping. Both the yield and quality of the cotton crop are improved to save the plant’s energy and resources towards the development of cotton bolls. Compared with traditional mechanical pruning, laser ablation can minimize the mechanical damage to the plant, which can often occur during physical pruning sensitive to pests and diseases. Moreover, the extensive use of chemical growth regulators can be avoided from agricultural practices to control plant growth. The environmental impact can be reduced to support healthy crop production in sustainable agriculture. Laser ablation can enhance the production potential after the treatment of cotton terminal bud. This study aims to examine the impact of the temperature field on the cotton terminal buds during laser ablation. A series of experiments were conducted to determine the thermal properties of cotton. A time-varying temperature field model was developed for the laser ablation using COMSOL Multiphysics finite element software. The heat transfer was then used to simulate the temperature distribution in the cotton shoot terminal meristems during laser ablation. There were great variations in the temperature both along the diameter and depth directions of the cotton shoot terminal meristem. A systematic investigation was also made to explore the effect of laser ablation on the surface erosion of the shoot terminal meristems. Finally, the numerical model was verified to be highly effective in the simulation. The results revealed that the ablation time was reduced by 45.55% and 67.36%, respectively, at the laser powers of 40 and 50 W, compared with the standard laser power of 30 W. Furthermore, the maximum temperature of the cotton shoot terminal meristems decreased with an increase in the laser spot size. Specifically, the ablation temperature reached in just 0.50 s, with a time reduction of 76.85% at a 2.5 mm spot size, compared with a 3.5 mm spot size. The higher water content further increased the ablation time. For instance, the reach time of the ablation temperature was 0.52 s at a water content of 75%, 17.31% faster than that at 95% water content. The speed of heat diffusion along the diameter direction was significantly higher than that along the depth direction under constant laser power. There was also the directional heat transfer of the laser ablation. Experimental results showed that a 50 W laser power with a 1 second ablation time significantly facilitated the growth of the shoot terminal meristems. However, the growth was effectively suppressed at a 100 W laser power with 1 second of ablation. Moreover, the greater the degree of carbonization and ablation was, the more pronounced the inhibitory effect on the shoot growth was. This finding can also provide valuable insights into the practical applications of laser ablation in cotton cultivation.

       

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