基于阻尼LSQR-LMBC的火焰三维温度场重建

光场相机可以解决辐射测温多相机系统光路复杂、同步触发难等问题,在辐射成像三维温度重建时有其独特优势. LSQR是求解基于大型稀疏矩阵最小二乘问题的经典算法,该算法用于重建三维温度场时对温度初值依赖较大,在信噪比较低的情况下重建精度不理想.本文提出阻尼LSQR-LMBC重建算法,通过在LSQR方法中添加阻尼正则化项,提高火焰三维温度场重建的抗噪性能,并结合LMBC算法,实现吸收系数和三维温度场同时求解.在数值模拟部分,随着信噪比逐渐降低,阻尼LSQR的重建效果比LSQR更加稳定,在信噪比达到13.86 d B时,重建精度大约提高30%.阻尼LSQR-LMBC的平均重建误差为6.63%.用丁烷火焰进行了实验,重建的丁烷火焰三维温度场分布符合辐射火焰燃烧的特征,和热电偶的测温数据结果进行对比,相对误差在6.8%左右.     

Study on concurrent measurements of velocity and density distributions for high-speed CO2 flow

Velocity and density distributions of a high-speed and initial CO₂ jet flow have been analyzed concurrently by a developed three-dimensional digital speckle tomography and a particle image velocimetry (PIV). Two high-speed cameras have been used for the tomography and one for the PIV since a shape of a nozzle for the jet flow is bilaterally symmetric and the starting flow is fast and unsteady. The speckle movements between no flow and CO₂ jet flow have been obtained by a cross-correlation tracking method so that those distances can be transferred to deflection angles of laser rays for density gradients. The three-dimensional density fields for the high-speed CO₂ jet flow have been reconstructed from the deflection angles by the real-time tomography method, and the two-dimensional velocity fields have been calculated by the PIV method concurrently and instantaneously.     

彩色漫射物体的压缩全息层析成像

漫射物体的压缩全息利用其非相干散射密度函数在统计意义上满足稀疏先验这一假设,可以从多幅散斑图案实现漫射物体的层析重建,避免了散斑和不同平面的散焦图像之间的串扰。将单波长照明条件拓展到红、绿、蓝三色波长,提出一种新的适用于彩色漫射物体的压缩全息层析成像方法,搭建多波长照明条件下漫射物体的层析成像模型,并通过解压缩推理方法有效地分离不同平面的三维非相干密度函数。数值模拟实验表明,所提方法可以在多幅二维彩色散斑图案中成功进行彩色层析漫射物体的压缩重建,有效地抑制了散斑效应以及不同平面的散焦图像之间的相互串扰。     

Digital speckle photography and speckle tomography in heat transfer studies

The line-of-sight speckle photography of transparent media is used for quantitative measurements of the instantaneous temperature fields in 3D unsteady flows. Both electronic and photographic methods are employed for specklegram recording. The subsequent specklegram treatment uses the Young's fringes method as well as cross-correlation analysis of small interrogation areas of the recordings. Experimental data for three different heat transfer configurations are obtained and discussed. The first one is natural convection over extended vertical heated plates with forward facing steps, the second is unsteady 3D convective flow around a suddenly heated vertical thin wire, and the third one is a convective plume above a multi-jet flame. Both local and global Nusselt numbers are determined via measuring local surface temperature gradients for these convective flows. The results are compared with Ostrach's theory for a single vertical plate and with the data obtained by Mach–Zehnder interferometry. The 3D temperature fields are reconstructed for axisymmetric convective flows around a suddenly heated vertical wire using quasi-double projection measurement and the Radon inversion. 3D temperature distributions above the combustion zone are reconstructed using multi-projection speckle photography measurements and computerised tomography.     

Flame four-dimensional deflection tomography with compressed-sensing-revision reconstruction

Deflection tomography with limited angle projections was investigated to visualize a premixed flame. A projection sampling system for deflection tomography was used to obtain chronological deflectogram arrays at six view angles with only a pair of gratings. A new iterative reconstruction algorithm with deflection angle compressed-sensing revision was developed to improve reconstruction-distribution quality from incomplete projection data. Numerical simulation and error analysis provided a good indication of algorithm precision and convergence. In the experiment, 150 fringes were processed, and temperature distributions in 20 cross-sections were reconstructed from projection data in four instants. Four-dimensional flame structures and temperature distributions in the flame interior were visualized using the visualization toolkit. The experimental reconstruction was then compared with the result obtained from computational fluid dynamic analysis.     

Wavelength-scanning digital interference holography for optical section imaging

We propose and experimentally demonstrate a simple digital holographic method that allows reconstruction of three-dimensional object images with a narrow depth of focus or axial resolution. A number of holograms are optically generated by use of different wavelengths spaced at regular intervals. The holograms are recorded on a digital camera and reconstructed numerically. Multiwavelength interference of the holograms results in images of the contour plane whose thickness can be made arbitrarily narrow. Objects at different distances from the hologram plane are imaged clearly and independently, with complete suppression of the out-of-focus images. The technique is available only in digital holography and should have useful applications in holographic microscopy.     

Study on the reduction of speckle noise in the reconstructed image of digital hologram

As one of the most important 3-D display technique, reduction of speckle noise in the reconstructed image of digital holography should be grounded on the digital hologram itself. Based on the whole process of the recording and reconstruction of digital holography, the optical distributions of recorded object and reconstructed image of digital holography have been studied. It has been proposed that the root formation cause of speckle noise in its reconstructed image is the speckle noise formed on the recorded object surface when illuminated by coherent light because of its optical roughness. A novel approach has been presented to reduce speckle noise in digital holography by changing the interference structure of hologram itself. First, by reducing the speckle noise in the reconstructed image, the distribution of ideal reconstruction light with reduced speckle noise is acquired. Then in turn, taking the ideal reconstruction light with reduced speckle noise as ideal object light, a new hologram can be rebuilt, which can reconstruct the ideal object light. The experimental results are given to confirm the proposed method. Therefore, it offers a brand-new thought and practical way to reduce the speckle noise in the reconstructed image of digital holography.     

Increment of lateral resolution in digital holography by speckle noise removal

Experimental features such as wavelength, camera specifications and reconstruction distance determine the theoretical limit for lateral resolution in digital holography. However, the actual experimental resolution limit is about 50% below such theoretical limit due to the high-contrast speckle noise presented in the reconstructed holograms. Recently, a technique has been introduced to reduce the contrast of speckle noise that is based on the superposition of uncorrelated hologram reconstructions of the same static object [5] (Garcia-Sucerquia et al., 2006). By this approach of reducing the contrast of the speckle noise, it is experimentally shown that an improvement of the order of 50% can be reached when 100 reconstructed images are superimposed.     

虚拟和真实实验相融合的三维非完全数据层析

为了解决非完全数据条件下复杂流场的三维层析重建,结合计算流体力学(Computational fluid dynamics,CFD)、计算流动显示技术(Computational flo wimaging,CFI)和光学层析技术(Optical computerized tomography,OCT),提出了应用于非完全数据流场重建的虚拟实验和真实实验相融合(即CFD,CFI与OCT融合)的三维层析。非对称双峰温度场实验结果表明,与完全数据重建结果相比,虚拟实验与真实实验相融合的三维层析的重建双峰误差小于5％。因此能够更好地实现非完全数据条件下流场的三维重建,极大地提高非完全数据条件下流场重建的真实性,较好地与实际结果相吻合,从而为解决有限角条件下的严重非完全数据流场的重建开拓了一条新途径。     

Flow visualization and density measurement of compressible flows by a laser speckle method

Laser speckle method is a well known technique that is useful for both visualization and quantitative measurement. This technique was applied to the density measurement of Mach reflection of shock waves in the present experiment. The Object of the measurement is the density field of simple Mach reflection in relatively low shock Mach number. The non-uniform flow field is divided into three regions by incident, Mach and reflected shock waves. A shock tube was employed in the present experiment. Wedges of 20 degrees and 45 degrees were placed in the test section. YAG laser was employed as a light source. Speckle photograph was taken by a digital still camera. Simple subtraction between the reference and flow images shows a shock pattern and a degree of the correlation of speckle pattern in the flow field. Thus, we can obtain a visualized flow image showing a configuration of Mach reflection from speckle photograph. Speckle photographs which was obtained in the experiments were processed with cross-correlation method. A reconstructed density gradient vector map of Mach reflection was obtained. Comparing the experimental result with numerical one, the measured density gradient shows a good agreement with theoretical prediction.     

Three-dimensional tomography using a cubic-phase plate extended depth-of-field system

We use cubic-phase plate imaging to demonstrate an order-of-magnitude improvement in the transverse resolution of three-dimensional objects reconstructed by extended depth-of-field tomography. Our algorithm compensates for the range shear of the cubic-phase approach and uses camera rotation to center the reconstructed volume on a target object point.     

The influence of hologram aperture on speckle noise in the reconstructed image of digital holography and its reduction

Based on the whole process of the recording and reconstruction of digital holography, we study the formation cause of speckle noise in its reconstructed image and acquire the conclusion that the small size of hologram aperture diffraction aggravates the speckle noise of reconstructed image and the speckle noise has been one of primary noise sources in the reconstruction process. In order to reduce the speckle noise resulting from little hologram aperture diffraction, we set an appropriate aperture function matching the recording parameter and aperture size of hologram and deconvolve the reconstructed image with it. The validity has been proved in theory and experiment. Therefore, it offers a brand-new thought and practical way to reduce the speckle noise in the reconstructed image of digital holography.     

Simulation study on reconstruction model of three-dimensional temperature distribution within visible range in furnace

This paper presents a reconstruction model of three-dimensional temperature distribution in furnace based on radiative energy images captured by charge-coupled device （CCD） cameras within the visible wavelength range. Numerical simulation case was used in this study and a zigzag eccentric temperature distribution was assumed to verify the model. Least square QR-factorization （LSQR） method was introduced to deal with reconstruction equation. It is found that the reconstructed temperature distributions in low-temperature areas had some fluctuations and high-temperature areas were reconstructed well. The whole reconstruction relative error was mainly due to errors in low-temperature areas and the relative error for highest-temperature reconstruction was quite small.     

DME/Oxygen wall-stabilized premixed cool flame

Wall-stabilized cool flames have been studied through numerical analysis and a series of experiments. One- and two-dimensional numerical simulations were performed to estimate the characteristics of the wall-stabilized cool flames, such as flammability and temperature/species distributions. Based on the computational results, the ignition condition of the cool flame at a fixed wall temperature has been identified with the strain rate between the van't Hoff point and the cool flame extinction point. In experiments with an impinging jet burner and a heated plate, the spontaneous ignition of the cool flame on the heated wall has been successfully established under the conditions predicted by the present numerical simulations. Spatial distributions of the HCHO concentration and flame temperature were measured through formaldehyde Planar Laser-Induced Fluorescence (HCHO-PLIF) and thermocouple measurements, respectively. It is found that the measurement data show a reasonable accordance with the simulation results with reduced low-temperature reactivity.     

Numerical reconstruction and twin-image suppression using an off-axis Fresnel digital hologram

In this work we proposed a novel approach for the numerical reconstruction of digital holography that uses only an off-axis Fresnel digital hologram and can easily be carrying out in a personal computer. From the presented off-axis holographic principle, the reference wave accompanies to the numerically reconstructed image, also makes the separation to the zero-order and twin images. Therefore, the concept can be applied to suppress the blurring of numerical reconstruction by adding the extra reconstruction plane and the formularized numerically reference wave in the reconstruction procedure. As expected, the blurring images can be separated out and removed from the reconstructed image. Then, by performing the back propagating to the exact reconstruction plane, one can obtain the pure object image without the blurring. In contrast to the temporal phase-shifting digital holographic-based scheme that employs multiple exposures the technique can be more effectively and easily overcome the blurring problem, also provide the practical feasibility in the digital holographic application. PACS 42.40.-I; 42.30.Wb; 42.40.Ht     

基于多角度无透镜傅里叶变换数字全息的散斑噪声抑制成像研究

在数字全息成像中,散斑噪声严重影响了再现像的信噪比和成像分辨率,因此为了提高数字全息成像质量,迫切需要抑制再现像的散斑噪声.分析并给出了矩形散射光斑的强度协方差,定量计算了特定实验条件下产生退相关散斑图样的最小角度差.结合透镜的性质设计并搭建了多角度无透镜傅里叶变换数字全息成像系统,利用光纤端面在透镜焦平面的二维机械移动代替传统反射镜的旋转,使照明光束在不改变照明位置和大小的同时,可任意改变光束方向.移动光纤端面使多角度照明满足最小角度差,获取了81幅数字全息图.利用单次快速傅里叶逆变换实现数值再现,对多幅再现像的强度像求平均,实验结果表明散斑对比度降低为单幅再现像的14.6%,使图像信噪比提高了6.9倍.该抑制散斑的多角度数字全息成像方法有效的抑制了散斑噪声,且成像光路结构简单,可操作性强.     

Applications of laser speckle photography for thermal flow problems

Laser speckle photography measures projected beam deflection of a phase object. The beam deflection measured at the solid surface will directly determine the wall-temperature gradient. Thus, the laser-speckle photography allows the evaluation of the convection-heat-transfer coefficients without the need to correct for conduction and radiation heat losses, which otherwise would be necessary. Selected results and brief discussions are presented for several natural convection-heat-transfer problems. Second, high-temperature applications of laser speckle photography shows potential as a nonintrusive means of measuring flame temperature. A fairly good agreement is shown in comparison of the laser speckle data with intrusive thermocouple data. Suggestive discussions are made to further improve the accuracy and sensitivity of the laser speckle photography technique in high-temperature applications. Finally, tomographic reconstruction of laser speckle photography data is discussed using both actual test fields and computer-synthesized phantom fields. The conventional algebraic reconstruction technique (ART) has been modified to apply for the laser speckle tomography. The efficacy of ART is examined particularly under a limited number of projections.     

高温空气燃烧炉内耦合传热的数值模拟

用自行开发的三维湍流流动、燃烧、辐射传热和NO_x湍流生成的计算程序,用离散坐标方法(DOM)模拟炉内的辐射传热并与其它传热方式相耦合,与各个输运方程共同求解,对燃烧室内的温度场进行了数值模拟.通过改变高温空气的预热温度,数值分析了高温空气燃烧炉内采用燃气直接喷射技术(FDI)的温度分布特性,检验了所采用的耦合数值模拟技术的有效性.模拟计算结果表明,提高空气预热温度,炉内温度的峰值相应增大,温度梯度降低,温度分布更均匀,火焰更长.相关的实验结果与数值预报的结果对比表明了相一致的规律.     

炉膛三维温度场重建中Tikhonov正则化和截断奇异值分解算法比较

在煤粉锅炉诊断中火焰辐射能图像扮演着越来越重要的角色, 通过电荷耦合器件(CCD)获得的辐射能图像可以重建出炉内火焰三维温度场, CCD 用于获取视场角内的辐射能图像. 温度场重建的矩阵方程是一个严重病态的方程, 本文使用两种算法(Tikhonov正则化算法和截断奇异值分解(TSVD)算法)来重建温度场. 应用广义交叉检验算法来选取正确的正则化参数. 数值模拟的环境为一个10 m×10 m×10 m的三维炉膛, 系统被划分为10×10×10的1000个网格, 每个网格单元都是边长为1 m的立方体. 在正问题求解所得到的CCD接受信号基础上加上不同随机误差以模拟测量时的CCD接受信号. 研究两种算法重建后的温度重建误差、两者的重建时间, 以及最高温度的重建效果. 初步的研究结果显示, 一般情况下基于Tikhonov算法重建的温度场比基于TSVD算法重建的温度场误差要小, 计算所需时间短, 最高温度重建更准确.     

Flow and Heat Transfer Characteristics of in-Line Impinging Jets With Cross-Flow At Short Jet-To-Plate Distance

The aim of this research is to numerically and experimentally study the flow and heat transfer characteristics of in-line impinging jets in cross-flow. The jets from a row of round orifices are perpendicularly impinged on the inner surface of a rectangular wind tunnel at a short distance between the orifice plate and impinged surface (H) of 2D, where D is a diameter of the orifice. The jet velocity was fixed corresponding to Re = 13,400 for all experiments, and the cross-flow velocity was varied at three different velocity ratios (velocity ratio, jet velocity/cross-flow velocity) of 3, 5, and 7. The heat transfer characteristic was visualized using a thermochromic liquid crystal sheet, and the Nusselt number distribution was evaluated by an image processing technique. The flow pattern on the impinged surface was also visualized by an oil film technique. The numerical simulation was used to explore a flow interaction between the impinging jets and cross-flow. The results indicated that Nusselt number peak increased by the increasing cross-flow velocity for short jet-to-plate distance. For the range determined, the maximum local Nusselt number peak was obtained at VR = 3 as the consequence of high velocity and high turbulence kinetic energy of jet impingement.