Simultaneous Measurement of Three-Dimensional Temperature Distributions and Radiative Properties Based on Radiation Image Processing Technology in a Gas-Fired Pilot Tubular Furnace

An on-line three-dimensional temperature measurement experiment was carried out in a gas-fired pilot tubular furnace. Four flame image detectors were utilized to obtain two (red and green) monochromatic radiation intensity distributions, which can be calculated by the DRESOR method based on the radiation image processing technology. Then a revised Tikhonov regularization method was developed to reconstruct three-dimensional temperature distributions from the green monochromatic radiative intensity. Meanwhile, a Newton method combined with a least-squares method was used to simultaneously reconstruct radiative properties from the red one. The two calculation procedures were performed alternately, forming an iterative algorithm to a simultaneous reconstruction of temperature and radiative properties. The reconstructed temperatures agreed well with those measured by thermocouples for different cases with different calorific values and components of gas. The largest relative error was less than 3%, which validated the effectiveness and accuracy of this reconstruction algorithm. Moreover, the nonuniform radiative properties for the flame and nonflame regions were determined to improve the accuracy of temperature measurement by a rigorous comparison test. Finally a set of reasonable fixed radiative properties for the media and walls was chosen for the on-line detection of temperature. The visualized temperatures obtained by the present method agreed reasonably with those measured by thermocouples for all cases, with the largest relative error less than 5%. The present method based on radiation image processing technology is reliable for on-line temperature measurement and shows a good accuracy for its application in the combustion industry.     

Efficiency analysis of radiative slab heating in a walking-beam-type reheating furnace

The thermal efficiency of a reheating furnace was predicted by considering radiative heat transfer to the slabs and the furnace wall. The entire furnace was divided into fourteen sub-zones, and each sub-zone was assumed to be homogeneous in temperature distribution with one medium temperature and wall temperature, which were computed on the basis of the overall heat balance for all of the sub-zones. The thermal energy inflow, thermal energy outflow, heat generation by fuel combustion, heat loss by the skid system, and heat loss by radiation through the boundary of each sub-zone were considered to give the two temperatures of each sub-zone. The radiative heat transfer was solved by the FVM radiation method, and a blocked-off procedure was applied to the treatment of the slabs. The temperature field of a slab was calculated by solving the transient heat conduction equation with the boundary condition of impinging radiation heat flux from the hot combustion gas and furnace wall. Additionally, the slab heating characteristics and thermal behavior of the furnace were analyzed for various fuel feed conditions.     

Measurements of Some Characteristics of Thermal Radiation in a 400-kW Grate-Fired Furnace Combusting Biomass

A comprehensive experimental investigation relevant to thermal radiation has been performed in a grate-fired test furnace. Thermal radiation is the dominating mode of heat transfer in the grate-fired furnace and yet only a few studies have focused on thermal radiation. No previous works, to the authors' knowledge, have been carried out concerning measurements on radiative heat transfer in grate-fired furnaces. In this work measurements of temperature have been carried out for all boundaries and the flue gases at a large number of locations. The gas species volume fraction, particle mass–size distributions, and wall irradiation have also been measured at a number of spatial locations. These data are useful in a computational framework to describe the radiative heat transfer reaching the boundaries. Comparing modeled wall irradiation to the measured one makes it possible to obtain a deeper insight into the thermal radiative transport inside the grate-fired furnace.     

Radiative slab heating analysis for various fuel gas compositions in an axial-fired reheating furnace

A transient radiative slab heating analysis was performed to investigate the effect of various fuel mixtures on the performance of an axial-fired reheating furnace. The various fuel mixtures tested were assumed to be attained by mixing COG (coke oven gas) and BFG (blast furnace gas), which are the two main byproduct gases found in the integrated steel mill industry. The numerical prediction of radiative heat transfer was calculated using an FVM radiation solving method, which is a well-known and efficient method for curvilinear coordinates. The WSGGM (weighted sum of gray gas model) was also adopted to calculate the radiative heat transfer in composition dependent media. The entire furnace was divided into fourteen sub-zones to calculate the radiative thermal characteristics of the furnace without flow field calculations. Each sub-zone was assumed to have homogeneous media and wall temperatures. All of the medium and wall temperatures were computed by calculating the overall heat balance using some relevant assumptions. The overall heat balance was satisfied when the net heat input equaled the three sources of heat loss in each sub-zone, wall loss, skid loss, and slab heating loss.     

Transient radiative heating characteristics of slabs in a walking beam type reheating furnace

Transient radiative heating characteristics of slabs in a walking beam type reheating furnace is predicted by the finite-volume method (FVM) for radiation. The FVM can calculate the radiative intensity absorbed and emitted by hot gas as well as emitted by the wall with curvilinear geometry. The non-gray weighted sum of gray gas model (WSGGM) which is more realistic than the gray gas model is used for better accurate prediction of gas radiation. The block-off procedure is applied to the treatment of the slabs inside which intensity has no meaning. Entire domain is divided into eight sub-zones to specify temperature distribution, and each sub-zone has different temperatures and the same species composition. Temperature field of a slab is acquired by solving the transient 3D heat conduction equation. Incident radiation flux into a slab is used for the boundary condition of the heat conduction equation governing the slab temperature. The movement of the slabs is taken into account and calculation is performed during the residence time of a slab in the furnace. The slab heating characteristics is also investigated for the various slab residence times. Main interest of this study is the transient variation of the average temperature and temperature non-uniformity of the slabs.     

Effects of particulate concentrations on temperature and heat flux distributions in a pulverized-coal fired furnace

A simple methodology for numerical modelling of total heat transfer in an axisymmetric, cylindrical pulverized coal-fired furnace is introduced. The solution for the flow field and energy equations are coupled with the solution of the radiative transfer equation. The SIMPLER code is employed to solve all the equations numerically. The radiation part is modelled using the first-order spherical harmonics approximation. The radiative properties of the gases and particulates such as soot, coal/char and fly-ash are obtained locally to account for the temperature and concentration distribution effects. Using a k - ε model, the turbulence closure is obtained. Parametric studies are performed and are presented graphically to demonstrate the effects of particulate concentrations on the distributions of medium radiative and physical properties, temperature, and the wall total and radiative heat fluxes.     

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.     

Numerical investigation of combustion with non-gray thermal radiation and soot formation effect in a liquid rocket engine

A numerical analysis was carried out in order to investigate the combustion and heat transfer characteristics in a liquid rocket engine in terms of non-gray thermal radiation and soot formation. Governing gas and droplet phase equations with PSIC model, turbulent combustion model with liquid kerosene fuel, soot formation, and non-gray thermal radiative equations are introduced. A radiation model was implemented in a compressible flow solver in order to investigate the effects of thermal radiation. The finite-volume method (FVM) was employed to solve the radiative transfer equation, and the weighted-sum-of-gray-gases model (WSGGM) was applied to model the radiation effect by a mixture of non-gray gases and gray soot particulates. After confirming the two-phase combustion behavior with soot distribution, the effects of the O/F ratio, wall temperature, and wall emissivity on the wall heat flux were investigated. It was found that the effects of soot formation and radiation are significant; as the O/F ratio increases, the wall temperature decreases. In addition, as the wall emissivity increases, the radiative heat flux on the wall increases.     

A heat transfer model for the analysis of transient heating of the slab in a direct-fired walking beam type reheating furnace

A mathematical heat transfer model for the prediction of heat flux on the slab surface and temperature distribution in the slab has been developed by considering the thermal radiation in the furnace chamber and transient heat conduction governing equations in the slab, respectively. The furnace is modeled as radiating medium with spatially varying temperature and constant absorption coefficient. The steel slabs are moved on the next fixed beam by the walking beam after being heated up through the non-firing, charging, preheating, heating, and soaking zones in the furnace. Radiative heat flux calculated from the radiative heat exchange within the furnace modeled using the FVM by considering the effect of furnace wall, slab, and combustion gases is introduced as the boundary condition of the transient conduction equation of the slab. Heat transfer characteristics and temperature behavior of the slab is investigated by changing such parameters as absorption coefficient and emissivity of the slab. Comparison with the experimental work show that the present heat transfer model works well for the prediction of thermal behavior of the slab in the reheating furnace.     

A two-step discrete method for reconstruction of temperature distribution in a three-dimensional participating medium

A new two-step discrete method is proposed in this paper for reconstruction of three-dimensional temperature distribution in an absorbing, emitting and isotropically scattering medium. With this new method, the temperature of the wall is also considered. The local radiative source term is reconstructed in the first step through the discrete transfer method from the directional, exit radiation intensities measured by CCD cameras. Then, the temperature of each discrete element is calculated in the second step by subtracting the scattering contribution from the retrieved radiative source term through the discrete ordinate method. The least squares minimum residual algorithm is employed to solve the ill-posed reconstruction equations and the calculation is improved to reduce the computational cost. The performance of the proposed method is examined by numerical test problems with unimodal and bimodal temperature distributions. The ill-conditioning of the reconstruction problem is checked by the Picard condition. The effects of the measurement noise and the radiative properties on the reconstruction accuracy are discussed. The results show that the method proposed in this paper is capable of reconstructing the temperature distribution accurately in large, confined, participating media, even with noisy input data. The computation time reduction of this new method is significant when compared with other methods.     

Spectrally correlated radiation and laminar forced convection in the entrance region of a circular duct

The two-dimensional combined radiative and convective transfer in emitting and absorbing real gases in the entrance region of a duct with a jump of wall temperature is studied. The axial propagation of radiation is taken into account in the analysis. The flow field and the energy equations are solved simultaneously and the radiative properties of the flowing gases, CO₂ or H₂O, are modeled by using either the narrow-band correlated-k model or the global absorption distribution function (ADF) model. The results are presented in terms of temperature and radiative power fields, and of the evolution of bulk temperatures and of heat transfer coefficients. Due to the axial component of the radiative flux, the gas is preheated or precooled before the change in wall temperature and this induces a persistent difference between the results of 1-D and 2-D radiation analyses. Some differences between CO₂ and H₂O temperature and radiative power profiles, due to the different structures of their spectra, are put in evidence. The ADF model, only suitable for gray walls, is shown to be less accurate when the gas is heated than when it is cooled.     

Heating Process Simulation of Steel Coil in Bell‐Type Annealing Furnace

This study examines two important parameters: the convective heat‐transfer coefficient and radiative heat‐transfer coefficient, which have a significant impact on coil temperature in a furnace. A new three‐dimensional model is proposed for convective heat transfer, and the factors affecting the Nusselt number (Nu) are studied using the orthogonal test method. Finally, the relationship between the Nu number and flow rate is determined. Considering the complex geometric structure of a furnace, this study uses the Monte Carlo method to calculate the angle factor and obtains the radiant heat flux using a radiation network diagram. The calculated values are applied to steel coil temperatures for accurate boundary conditions. The results show that the temperature simulated by using the mathematical model is in good agreement with the experimental data obtained with the thermocouple insert experiment.     

The influence of furnace design on the NO formation in high temperature processes

High temperature processes produce high NO_x emissions due to their elevated working temperatures. Strong regulations for emissions of pollutants [1] from industrial plants lead the operators to optimize their furnaces. In this paper a three-dimensional mathematical model for turbulent flow and combustion on the basis of turbulence-chemistry interactions and radiative heat transfer taking into account spectral effects of surrounding walls and combustion gases is described. The transport equation for radiative intensity was split into different wavelength ranges. A block-structured finite volume grid with local refinements was used to solve the governing equations. The calculation domain is subdivided into a number of subdomains which are linked within the solver based on the message passing interface (MPI) library. Computed distributions of velocity, temperature, species distribution and heat fluxes are given. Results of a parametric study in a producing horseshoe furnace by increasing the height of the furnace with regard to NO_x concentration distributions are presented.     

CFD simulations on a coal–coal reburning test facility

Fuel staging or fuel reburning is a possible primary measure for the reduction of NO_x emissions from fossil-fired steam generators. It is intended to investigate if this primary measure can be applied efficiently to coal-fired combustion systems where pulverized coal is also used as a reburning fuel: coal reburning over a coal fire. This technology has to be tested accordingly. The company ENEL SpA is at present evaluating, together with ANSALDO Energia SpA, the option of demonstrating and extending the use of this technology in its coal-fired utility steam generators. Both companies have decided to erect a boiler simulation facility (5 MWth) at ENEL's experimental area Santa Gilla (Cagliari). This facility is intended to replicate the time/temperature profiles of real boilers. One tool to support the design of firing systems is CFD simulation. Consequently, CFD simulation was applied to perform a ‘proof of concept’. Most important for the design of firing systems for the furnaces of utility steam generators or test facilities is the distribution of temperature inside the combustion chamber and the furnace exit temperature—especially the radiation transport responsible for the amount of energy transferred from the gaseous environment of the hot furnace to the furnace walls. Subsequently, this determines the mean furnace exit temperature. This is why the accuracy of temperature calculations depends directly on the radiation transport model, the gas emissivity approximations, the particle emissivity and the radiative properties of the furnace enclosure. The results of several combustion simulations of the coal reburning test facility are shown and discussed. Estimates of the possible effect of different wall temperatures on the combustion process itself and on the gas temperature inside the furnace are presented. © 1997 by John Wiley & Sons, Ltd.     

燃烧室内三维温度场的辐射反问题

本文提出了一种在介质辐射特性已知的条件下,由壁面入射辐射热流的测量值反演燃烧室内三维温度场的方法。该方法是在辐射传递方程离散坐标近似的基础上,用求目标函数极小值的共轭梯度法进行反演计算。通过对吸收系数、散射不对称因子、反照率、壁面黑度和燃烧室大小尺寸等参数对反演精度影响的分析,结果表明,即使存在随机测量误差,这些参数对温度场反演精度的影响也不大,本文所提出的方法可较精确地反演燃烧室内三维温度场。     

燃煤锅炉炉膛断面温度场可视化实验研究

在一台300MW燃煤锅炉上进行了炉膛断面温度场可视化实验研究，采用炉膛燃烧数值模拟程序计算沿炉膛高度平均颗粒容积份额的变化，计算出温度场重建所需的辐射参数，由火焰图像重建的炉膛温度场结果表明，燃烧器区域之上某一个断面的温度场呈现典型的单峰分布特征，且其偏料情况与电厂运行经验基本一致；250MW负荷水平下的最高温度比200MW负荷水平下的最高温度高约80K；断面温度场可视化结果刷新一次的时间不超过5s，满足在线监测的要求。     

环形加热炉热工过程CFD数值模拟及其应用

借助CFD商业软件CFX，考虑流动、辐射、燃烧等，对宝钢环形加热炉的热工过程进行了数值模拟，得到了炉膛内的温度、速度矢量等热工参数的分布图。通过简化处理，对环形加热炉各控制段内的加热过程进行了模拟分析，并根据所得到的管坯表面辐射热流、热电偶温度及管坯表面温度研究了各个段总括热吸收率的分布情况，主要分析了总括热吸收率在管坯圆周方向、长度方向以及各个段之间的变化规律。     

优化壁温计算模型及其在电站锅炉壁温在线监测中的应用

针对电站锅炉壁温在线监测以及炉内汽温和壁温计算的要求,对电站锅炉过热器和再热器炉内汽温和壁温的计算方法进行了优化,提出了炉内汽温和壁温的分段计算模型,对该计算模型涉及的辐射因数、角系数、辐射穿透率、沿炉膛宽度的偏差系数和沿屏高度的偏差系数进行了详细研究,并通过数值模拟对沿炉膛宽度的偏差系数和沿屏高度的偏差系数进行了优化和修正.结果表明:该模型可对锅炉过热器和再热器受热面各点温度进行实时计算,能满足电站锅炉壁温在线监测的要求.     

Experimental investigations on overall cooling effect of ribbed channel with air bleeds

An experimental investigation on overall heat transfer performance of a rectangular channel, in which one wall has periodically placed oblique ribs to enhance heat exchange and cylindrical film holes to bleed cooling air, has been carried out in a hot wind tunnel at different mainstream temperatures, hot mainstream Reynolds numbers, coolant Reynolds numbers and blowing ratios. To describe the cooling effect of combined external coolant film with the internal heat convection enhanced by the ribs, the overall cooling effectiveness at the surface exposed in the mainstream with high temperature was calculated by the surface temperatures measured with an infrared thermal imaging system. The total mass flow rate of cooling air through the coolant channel was regulated by a digital mass flow rate controller, and the blowing ratio passing through the total film holes was calculated based on the measurements of another digital-type mass flow meter. The detailed distributions of overall cooling effectiveness show distinctive peaks in heat transfer levels near the film holes, remarkable inner convective heat transfer effect over entire channel surface, and visible conductive heat transfer effect through the channel wall; but only when the coolant Reynolds number is large enough, the oblique rib effect can be detected from the overall cooling effectiveness; and the oblique bleeding hole effect shows the more obvious trend with increasing blowing ratios. Based on the experimental data, the overall cooling effectiveness is correlated as the functions of Re_m (Reynolds number of hot mainstream) and Re_c (Reynolds number of internal coolant flow at entrance) for the parametric conditions examined.