Huai-Chun Zhou

Visualization of three-dimensional temperature distributions in a large-scale furnace via regularized reconstruction from radiative energy images: numerical studies

In this paper, the possibility of visualization of three-dimensional (3-D) temperature distributions in large-scale boiler furnaces from radiative energy images captured by multiple charge-coupled device (CCD) cameras mounted around the furnace is studied numerically. For the calculation of the radiative energy image formation, a fast algorithm proposed by the authors for pinhole imaging is used in this paper, which is based on the Monte Carlo method and combined with a concept of angular factor effective for image formation. This algorithm is applicable for the emitting, absorbing, and isotropic scattering medium. For the inversion of the 3-D temperature distributions which is an ill-posed problem, a modified Tikhonov regularization method is improved, where the finite difference regularizer is defined and can be used in 3-D cases, and the optimal regularization parameter is suggested to be selected by using a post-treatment method. For a 3-D unimodal temperature distribution, the numerical simulation results show that the reconstruction errors for the 3-D temperature distribution can be maintained at levels similar to the measurement error and the visualization quality of the temperature distribution is satisfactory. For a kind of bimodal temperature distribution, the reconstruction errors are higher than those for the unimodal distribution, but the bimodal feature of the temperature distribution can also be reproduced clearly.

The influences of mineral behaviour on blended coal ash fusion characteristics

Abstract The ash fusion characteristics and mineral behaviour of three kinds of individual coal ash and a series of two-component blended ashes were studied. The ash fusibility and chemical composition were measured and analysed. The samples were heated progressively from 800°C to the initial deformation temperature (IT) at 100°C intervals in reducing atmosphere. Mineral composition and type at each temperature interval were determined by X-ray diffraction (XRD) analysis. The results show that blended ash softening temperatures do not change linearly with blending ratios. It can lie between or lower or higher than that of the individual parent coals. Some combinations of component coal’s mineral produce low-melting eutectic minerals at high temperature and this is the main reason causing the non-arithmetic averaging of softening temperature of blended ash. It is consistent with the results from the ternary system phase diagram.

A Combustion-Monitoring System With 3-D Temperature Reconstruction Based on Flame-Image Processing Technique

Based on a flame-image processing technology, a real-time combustion-monitoring system with 3-D temperature reconstruction and visualization installed in a coal-fired furnace of a power plant was reported. A dozen of flame detectors with charge-coupled-device cameras were mounted along the height of the furnace to capture multiple digital flame images. A radiation energy signal (RES) was obtained from the flame images according to Wiens law of radiation. A series of in situ experiments have been done, and the results showed that the flame temperature distribution and the RES are sensitive to change in the combustion of the boiler and can be used to improve the combustion control in practical application.

A new way to calculate radiative intensity and solve radiative transfer equation through using the Monte Carlo method

Through using the Monte Carlo method to calculate the fraction distributions of the emission scattered within an emitting, absorbing and scattering gray medium with a vacuum or a cold black wall, a new solution for the angular distributions of radiative intensity in the medium is presented. The angular distributions of intensity in one or more points, or even everywhere, can be calculated from the given distributions of temperature and radiative properties in the medium without the requirement of radiative equilibrium. For the emitting, absorbing, isotropic scattering, parallel-plane, gray media, some important relationships between the new and the formal solutions of the radiative transfer equation are delivered. Validation results for the angular distributions of radiative intensity got for the medium with uniformly specified distributions of temperature and radiative property and optical thickness less than 2.0 demonstrate that the new solution agrees well with the formal solution in the same condition. The work presented in this paper extends the application of the common Monte Carlo methods. In the recent future, more complicated parallel-plane cases will be studied by the same idea, especially with different kinds of wall conditions and/or anisotropic scattering.

An inverse radiative transfer problem of simultaneously estimating profiles of temperature and radiative parameters from boundary intensity and temperature measurements

Abstract A parallel-plane space filled with absorbing, emitting, isotropically scattering, gray medium is studied in this paper. The boundary intensity and boundary temperature profiles are calculated for the inverse analysis. For the simultaneous estimation of temperature, absorption and scattering coefficient profiles in the medium, the sum of residuals of boundary intensity and temperature after being weighted by a balance factor is minimized through using a Newton-type iteration algorithm and the least-squares method. To avoid over-updating for the parameters, the relative updating magnitude during the iteration process is constrained not to be >0.5. It is shown that the boundary intensity measurement alone is not enough to estimate simultaneously the temperature (source) and the radiative properties (both absorption and scattering coefficients) when the measurement data contain sensitive random errors. The boundary temperature measurement can serve as a necessary supplementation to the boundary intensity to make this kind of inverse radiative transfer problem resolvable. It was shown that a compensation relationship between absorption and scattering coefficients makes it difficult to fix them accurately. Parabolic profiles for the three parameters are used to validate the estimation method. When the optical thickness approaches 4.0, the results for the radiative properties are not acceptable, although the result for temperature profile is reasonable. This means the method needs further improvements.

Simultaneous reconstruction of temperature distribution, absorptivity of wall surface and absorption coefficient of medium in a 2-D furnace system

For a 2-D furnace system filled with a gray medium, surrounded by gray emitting/absorpting and diffusely reflecting wall surfaces, the temperature distribution is reconstructed using an improved Tikhonov regularization method with radiative energy images detected from the boundary of the furnace, uniform absorptivity of both the wall surfaces and the medium being updated from the temperature images grasped from the boundary too. These steps are taken alternately till a convergence is reached. The measurement errors with normal distribution of standard square deviation of 0.01 are taken into consideration for the radiative energy image and temperature image data. The reconstruction errors for radiative properties vary from 1.45% to 10.75%, and for the highest temperature are within 2%. Comparatively, the reconstruction result for the sharper temperature distribution is not as good as that for the smoother temperature distribution. The applicability of the proposed method may be practically valuable.

The Solution of Transient Radiative Transfer With Collimated Incident Serial Pulse in a Plane-Parallel Medium by the DRESOR Method

A time-dependent distribution of ratios of energy scattered by the medium or reflected by the boundary surfaces (DRESOR) method was proposed to solve the transient radiative transfer in a one-dimensional slab. This slab is filled with an absorbing, scattering, and nonemitting medium and exposed to a collimated, incident serial pulse with different pulse shapes and pulse widths. The time-dependent DRESOR values, representing the temporal response of an instantaneous, incident pulse with unit energy and the same incident direction as that for the serial pulse, were proposed and calculated by the Monte Carlo method. The temporal radiative intensity inside the medium with high directional resolution can be obtained from the time-dependent DRESOR values. The transient incident radiation results obtained by the DRESOR method were compared to those obtained with the Monte Carlo method, and good agreements were achieved. influences of the pulse shape and width, reflectivity of the boundary, scattering albedo, optical thickness, and anisotropic scattering on the transient radiative transfer, especially the temporal response along different directions, were investigated.

A NEW MODEL OF RADIATIVE IMAGE FORMATION USED IN VISUALIZATION OF 3-D TEMPERATURE DISTRIBUTIONS IN LARGE-SCALE FURNACES

Since it is difficult to calibrate the absolute radiative energy information from any digital image data, a new model is proposed by which the temperature distributions in furnaces are no longer related with the absolute radiative energy images, but with a sort of temperature image. The temperature images are obtained through using the so-called monochromatic-reference method, described in detail. It is shown that the new model has its objectivity in reflecting the temperature distributions in furnaces, and does not change significantly with the aperture or shutter speed of the camera used. The easier calibration for the temperature than for absolute radiative energy makes this treatment more applicable.

Flexibility of a 300 MW Arch Firing Boiler Burning Low Quality Coals

Abstract Experimental investigations on the flexibility of a 300 MW Arch Firing (AF) coal-fired boiler when burning low quality coals is reported. Measurements of gas temperature and species concentration and char sampling using a water-cooled suction pyrometer were carried out along the furnace elevation. The carbon content and the size distributions of the char samples were obtained. The char morphology was examined using a field emission scanning electron microscope (FESEM). The char sampling was performed on this type of boiler for the first time. The results indicate that the flexibility of this boiler burning low quality coals under a moderate boiler load is better than its flexibility under a high boiler load. Because of the insufficient capacity of the coal pulverizers used, in case of low coal quality the pulverized coal fineness will drastically decrease under high boiler loads. This causes an increase in the loss due to incomplete mechanical and chemical combustion. This is the main cause of a low burnout degree of the pulverized coal and the decrease of the flexibility of this AF boiler under a high boiler load.

A FAST ALGORITHM FOR CALCULATION OF RADIATIVE ENERGY DISTRIBUTIONS RECEIVED BY PINHOLE IMAGE-FORMATION PROCESS FROM 2D RECTANGULAR ENCLOSURES

In the fast algorithm, the total energy received by pinhole image-formation elements is divided into two parts: the direct part and indirect part. The indirect energy was related to the direct part, and the latter was calculated through a concept of effective angle factor for image formation. For 2-D systems filled with emitting, absorbing, isotropically scattering medium and surrounded by diffusely reflecting wall surfaces, the results through using the fast algorithm approached the statistical average values of those calculated using the Monte Carlo method, and the computation time can be nearly 50 times faster.In the fast algorithm, the total energy received by pinhole image-formation elements is divided into two parts: the direct part and indirect part. The indirect energy was related to the direct part, and the latter was calculated through a concept of effective angle factor for image formation. For 2-D systems filled with emitting, absorbing, isotropically scattering medium and surrounded by diffusely reflecting wall surfaces, the results through using the fast algorithm approached the statistical average values of those calculated using the Monte Carlo method, and the computation time can be nearly 50 times faster.