【作者】 刘冬；

【导师】 严建华； 池涌； 岑可法；

【作者基本信息】 浙江大学 ， 工程热物理， 2010， 博士

【摘要】 随着我国社会经济的持续发展,工业和生活对于电力的需求持续增长。在我国电力工业中,燃煤电站仍占有主导地位,燃煤电站锅炉的安全,经济,稳定的运行是至关重要的。燃烧火焰温度场是燃烧诊断的核心,先进有效的火焰温度场测量技术对于研究电站锅炉煤粉燃烧具有重要的科学意义和实用价值。本文结合弥散介质的辐射特性和辐射传热的计算方法,以辐射反问题研究作为基础,配以辐射反问题求解方法,建立一套利用彩色CCD摄像机,适用于弥散介质的三维温度场重建模型及燃烧诊断方法。对于大空间弥散介质的温度场重建,其特点在于介质为吸收、发射和散射性介质,热辐射源很大,而接收器很小,因此需要建立一套介质适用性广,计算效率高的温度场重建模型,是本文研究的一个重点。从系统边界出射辐射信息反求系统内部的温度分布信息是典型的辐射反问题,反问题一般是严重病态问题,如何对大型病态重建矩阵方程进行有效快速合理的求解是研究的关键,是本文研究的另一个重点。首先给出了反问题不适定性的定义,研究了分析病态问题的有效方法(奇异值分解和Picard图),总结分析了各种反问题不适定方程的解法,分别为Tikhonov正则化方法,截断奇异值分解(TSVD)方法和最小二乘QR分解(LSQR)方法,并给出了正则化参数的选择方法—L曲线法。在此基础上,指出本文中使用的温度场重建辐射反问题病态方程的解法为LSQR算法,并给出讨论了求解炉膛三维温度场重建反问题重建方程的实际算例。详细给出了弥散介质辐射传递的Monte Carlo算法,并且采用文献上的算例进行了验证,计算结果与文献结果吻合的较好。在此基础上,讨论了利用Monte Carlo方法和透镜光学成像原理来计算弥散介质燃烧火焰的辐射光学成像。作为温度场重建的基础性研究,提出了在可见光波段,基于正向Monte Carlo方法的三维温度场重建模型,给出了具体的重建方程及求解方法,讨论了各种因素对温度场重建精度的影响。讨论分析了计算辐射传热的逆向Monte Carlo方法,在此基础上创新性地推导并提出了基于逆向Monte Carlo方法的三维温度场快速重建模型,该模型可用于吸收、发射和散射性介质,并且计算时间短,非常适合于大炉膛和小接收器的辐射传热计算,具有实时在线计算系数矩阵并进行求解的潜力。采用算例讨论了各种因素对三维温度场重建精度的影响。进行了正逆向Monte Carlo方法辐射传热计算及温度场重建的对比研究,发现逆向Monte Carlo方法的计算效率很高,可以达到实时在线建立重建方程并进行求解,两者重建误差相差并不是很大。还讨论了基于逆向Monte Carlo方法的二维均匀弥散介质温度场和辐射参数同时重建模型,以及危险废弃物焚烧回转窑二维截面温度场重建数值模拟研究。建立了利用CCD摄像机进行气体燃烧火焰三维烟黑的温度场和浓度场同时重建的模型,该模型考虑了火焰烟黑辐射能的实际三维容积发射与三维成像,是从三维重建区域上来进行重建,与区域内的火焰形式没有直接关系,因此可适用于对称火焰和非对称火焰,并通过数值模拟对该模型进行了数值验证,最后进行了实验应用研究,烟黑浓度分布与温度分布的重建结果较为合理,与文献上的一些温度分布情况具有一致性,为了验证重建温度分布值的准确性,使用热电偶对温度分布大致进行了测量,重建温度结果与测量温度结果吻合的较好。采用发展的基于逆向Monte Carlo方法的温度场快速重建模型对小型煤粉燃烧试验台火焰温度场进行了重建试验研究。重建温度场的高温区和低温区与文献重建结果对应较为一致,温度值大小也较为一致。重建结果温度较为细致清晰,重建出的回流区非常明显,并有偏斜,是由于火焰燃烧具有偏斜所致,把其中二维火焰图像转化为伪彩色温度图像,从伪彩色温度图像中可以清晰地看到煤粉回流区的情形,验证了本文的重建结果。进行了大型燃煤电站锅炉炉膛截面温度场和三维温度场重建试验研究,采用了Fluent数值模拟锅炉实际运行工况,进而得到介质的浓度分布,计算得到粒子云的辐射参数分布,比一般文献上直接假设辐射参数值来的更为准确。对于截面温度场重建,重建温度场中有个明显的高温区,高温区周围的温度较低,且温度大小范围在合理范围内,符合一般炉膛内的温度分布规律。重建结果与文献中计算的结果进行了对比,温度值大小较为一致,本文重建温度分布更为详细,显示出了更为细致的高温区分布情况,而且可以看出高温区有偏置。对于三维温度场的重建,重建温度场整体上呈现出中部温度高,四周温度低的趋势,温度范围也在合理范围内,与文献上得到的温度分布趋势相一致。计算了整体重建时间(包括建立重建矩阵方程的时间、读取系数矩阵的时间和求解时间)为1分钟左右,具有较好的在线重建能力。在线重建时,对于较为稳定的工况,可以使用同一系数矩阵,只需要求解方程即可,这时的计算时间只需要2-3s左右,具有更好的在线重建更新能力。最后,利用太赫兹时域光谱技术对0.2-1.6THz火焰碳黑的光学特性进行了初步研究,通过傅里叶变换得到了碳黑频域光谱,利用定点迭代法获得了太赫兹波段火焰碳黑的复折射率,并进行了对比。研究结果可为太赫兹波技术应用于光学燃烧诊断提供基础性数据,扩展了光学燃烧诊断应用的范围。更多还原

【Abstract】 The electricity requirement from industrial and daily life is increasing with the development of economy in our country. In our country electricity industrial, the pul-verized coal-fired power plant is dominant. It is very important for the safe, economic, and stable operation of power plant. Combustion flame temperature field is the center of the combustion diagnostics. The advanced and effective measurement technique of the combustion flame temperature field has the significant scientific meaning and practical value for the study on the combustion of furnace in power plant. Combined with radiative properties of participating medium and radiative heat transfer calcula-tion method, based on the study of inverse radiation problem, and with the solving method of the inverse radiation problem, three-dimensional (3-D) temperature recon-struction model and combustion diagnostics method are established by means of CCD camera in participating medium.The temperature field reconstruction in large participating medium has the char-acteristics as following, the medium is absorbing, emitting and scattering, and the ra-diative heat source is very large but the receiver is very small. Therefore, it is required to establish the reconstruction model for participating medium with high computing efficiency and this is the key point of this study. It is the typical inverse radiation problem that the exiting radiative information from the system boundary is used to retrieve the temperature distribution information in the system. Inverse radiation problem is normally the ill-posed problem. How to solve the large ill-posed recon-struction matrix equation efficiently, fast and reasonably is very important and this is another key point of this study.First, the ill-posed problem is defined and the effective methods for analyzing ill-posed problem (SVD method and Picard figure) are studied. Some kinds of solving methods (Tikhonov regularization method, TSVD method and LSQR method) are summarized and the L-curve method is used to give reasonable regularization pa-rameter. Based on the above, LSQR method is used in this study for solving the ill-posed temperature reconstruction equation and the simulation case is given.The Monte Carlo method is given in detail and the developed program is vali-dated by the case in the reference. The results are found to agree well with those in the reference. The combustion flame optical imaging is calculated using Monte Carlo method and lens optical imaging principle.Served as the fundamental study on the temperature field reconstruction,3-D temperature field reconstruction model based on the forward Monte Carlo method in the visible range is presented. The reconstruction equation and solving procedure are given in detail. The effects on the reconstruction accuracy are discussed.The backward Monte Carlo method is discussed.3-D temperature field recon-struction model based on the backward Monte Carlo method is deduced and presented innovatively. This model can be used for the absorbing, emitting and scattering me-dium, and the computing time is short and suitable for radiative heat transfer between the large furnace and small receiver. The model has the ability of the real-time coeffi-cient matrix calculation and the on-line matrix equation solving. The results using the forward and backward Monte Carlo methods for computing radiative heat transfer and reconstructing the temperature field are compared. It is found the backward Monte Carlo method has much higher efficiency but the reconstruction errors are almost the same. The simultaneous reconstruction model of temperature field and radiative pa-rameters based on the backward Monte Carlo method in the homogenous medium, and the simulation study of the section temperature field reconstruction in the rotary kiln for dangerous waste incineration are also discussed.Simultaneous measurement model of 3-D soot temperature and volume fraction fields in flame with CCD cameras is presented. This model considers the practical 3-D volume radiative energy emission and flame imaging. The consideration of this model is 3-D reconstruction zone, not the flame, so the reconstruction is not related with the flame form directly, which is suitable to the axisymmetric and asymmetric flames. The simulation is used to validate the model and the experiments are also car-ried out. The reconstructed temperature and volume fraction distributions are reason-able and agree with some reports in the reference. Also, the thermocouple is used to measure the flame temperature, and the reconstructed temperatures agree well with the measured ones.Temperature field of flame is reconstructed in small scale pulverized coal-fired test rig using temperature distribution fast reconstruction model based on the backward Monte Carlo method. Recontructed temperature field is in good agreement with the results from the literature. The reconstructed results are clear and the recirculating zone can be seen clearly and has some deflection. The two-dimensional flame image is converted into the pseudo temperature image, from which the recirculating zone can be seen more clearly. This validates the reconstruction results.The experimental studies on the section temperature field and 3-D temperature field reconstruction in the large pulverized coal-fired power plant furnace are performed. The Fluent simulation is used to obtain the medium radiative properties distribution according to the furnace operation parameters, which is more accurate than some ref-erences using assumed radiative parameters. For the section temperature field recon-struction, there is a obvious high temperature zone, rond which the temperatures are lower. The values of temperature are in a reasonable range. The reoncstructed results are compared with those in literature and it is found that the temperature range agrees well, but the reconstructed temperature in this paper has more details and presents more particulars. Moreover, it can be seen that the high temperature zone has deflec-tion. For the 3-D temperature field reconstruction, the temperatures in the middle part of the furnace are high and the surrounding part of the furnace is relatively low. The reconstructed temperature range is also reasonable. These characteristics of the recon-structed temperature field agree with the results in the references. The computing time is also reported. The whole computing time (include the reconstruction equation es-tablishing time, coefficient matrix reading time, and solving time of equation) is about one minute. In the on-line reconstruction, for the stable operation condition, the same coefficient matrix can be used and it is only needed to solve the equation. At this time, the reconstruction time is only 2-3s, which shows better ability for the on-line recon-struction.Finally, the terahertz time-domain spectroscopy technique was used to study the optical properties of the soot within 0.2-1.6THz and the frequency-domain spectra was obtained through the Fourier transform. The complex refractive index of the soot was deduced by the fixed-point iteration and comparisons are made. The deduced re-sults can provide the optical data of soot for the application of terahertz time-domain spectroscopy technique to the optical combustion diagnostics, and extend the optical combustion diagnostics application area.更多还原