Application of Infrared Thermography for Characterizing Phase Change Process of Porous Media at Pore Scale
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摘要: 红外热成像技术是一种应用广泛、发展较快的新型数字化无损检测技术，但其目前仍存在测量精度较低、图像噪声影响较大、数据量巨大等缺点。本文利用红外热成像技术，在孔隙尺度下对多孔材料内石蜡和水的相变过程进行了表征，并对红外热成像测温精度和相界面的表征进行了优化。首先通过添加反射表面（褶皱的铝箔）获取测试过程中环境反射温度，以修正目标物体温度；同时通过引导滤波和主成分热像法（Principal component thermography, PCT）对红外热成像图片降噪，提高红外热像仪对温度场及相界面的检测精度。实验结果表明：环境反射温度修正法可以排除测试环境温度变化对红外测温结果的影响；引导滤波法选取合适的滤波半径和滤波参数后相界面轮廓更加清晰，并取得较好的降噪效果；主成分热像法除了降噪效果好，使相界面更清晰，并使处理数据量减少了4个量级。上述3种优化方法为红外热成像技术在孔隙尺度下多孔介质相变过程表征中的应用提供了技术支持。Abstract: Infrared thermography (IRT) is a new type of digital nondestructive testing technology that has developed rapidly and has found wide use. However, it still has some shortcomings, such as low accuracy, image noise, and requires a large amount of data. In this study, the phase change processes of paraffin and water in porous materials are characterized by IRT technology at pore scale, and the temperature measurement accuracy and phase interface characterization are optimized. First, the reflective surface (wrinkled aluminum foil) is added to obtain the environmental reflective temperature in the test process to correct the target object temperature; subsequently, guided filtering and principal component thermography (PCT) are used to reduce the noise of IRT images to improve the detection accuracy of the infrared thermal imager for temperature field and phase interface. The experimental results show that the temperature measured by the environmental reflection temperature correction method is closer to that measured by a thermocouple after eliminating the influence of changing environmental temperature; after selecting an appropriate filter radius and filter parameters, the contour of phase interface is clearer, and a better noise reduction effect is obtained. In addition to the good noise reduction effect, PCT makes the phase interface clearer and reduces the amount of processed data by four orders of magnitude. These three processing methods provide better optimization methods and theoretical support for infrared thermal imaging technology to characterize the phase transition process of porous media at the pore scale.
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图 2 将三维矩阵重构为二维矩阵
Figure 2. The three-dimensional matrix is reconstructed into a two-dimensional matrix
图 3 将二维矩阵重排为三维矩阵
Figure 3. The two-dimensional matrix is reconstructed into a three-dimensional matrix.
图 4 石蜡融化与凝固过程中吸热和放热曲线
Figure 4. Endothermic and exothermic curves of paraffin wax during melting and solidification.
图 5 复合相变材料(a)泡沫铜金属; (b)泡沫铜/石蜡复合式样
Figure 5. Phase transition material: (a) Copper foam metal; (b) Copper foam and paraffin composite pattern
图 6 多孔树脂模型(a)三维模型(b)孔间距
Figure 6. Model of porous resin: (a) three-D model; (b) pitch of holes
图 10 修正后温度、未修正温度与热电偶所测温度的对比
Figure 10. Comparison of corrected temperature, uncorrected temperature and temperature measured by thermocouple
图 11 泡沫铜/石蜡不同滤波窗口及参数下的对比
Figure 11. Comparison of copper foam/paraffin under different filtering windows and parameters
图 12 引导滤波处理前后泡沫铜/石蜡相界面对比
Figure 12. Comparison of copper foam/paraffin phase interface before and after guided filtering treatment:.
图 13 多孔树脂/水不同滤波窗口及参数下的对比
Figure 13. Comparison of porous resin/water under different filtering windows and parameters
图 14 引导滤波处理前后多孔树脂/水相界面对比
Figure 14. Comparison of porous resin/water interface before and after filtering treatment
图 15 泡沫铜/石蜡(a)奇异值(b)不同奇异值个数对应的Pj值
Figure 15. Copper foam/paraffin: (a)singular value; (b) Pj values corresponding to different singular values
图 16 多孔树脂/水(a)奇异值(b)不同奇异值个数对应的Pj值
Figure 16. Porous resin/water: (a)singular value; (b) Pj values corresponding to different singular values.
图 18 泡沫铜/石蜡不同截断值j对比图
Figure 18. Comparison charts of copper foam/paraffin with different cut-off values j
图 19 多孔树脂/水不同截断值j对比图
Figure 19. Comparison charst of porous resin/water with different cut-off values j
表 1 石蜡融化实验不同截断值与原图数据量对比
Table 1. Comparison between different cut-off values of paraffin melting experiment and data quantity of original image
Original drawing 22500×22500+38+38×38=506, 251, 482 j=1 22500×1+1+38×1=22, 539 j=2 22500×2+2+38×2=45, 078 j=3 22500×3+3+38×3=67, 617 j=4 22500×4+4+38×4=90, 156 
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表 2 水结冰实验不同截断值与原图数据量对比
Table 2. Comparison between different cut-off values of water icing experiment and data quantity of original image
Original drawing 22500×22500+94+94×94=506, 258, 930 j=1 22500×1+1+94×1=22, 595 j=2 22500×2+2+94×2=45, 388 j=3 22500×3+3+94×3=67, 785 j=4 22500×4+4+94×4=90, 380
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