锅炉声波除灰的声学分析

声波除灰技术是提高锅炉换热效率、保证锅炉安全运行的重要手段之一,早在９０年代初就已经在国内外工业锅炉上推广应用,但是与之相关的多数基本问题目前还没有得到很好的解决,成为制约技术发展和应用的重要障碍［１］。本文采用Ｈｅｌｍｈｏｌｔｚ积分方程数值计算了锅炉换热器管排的声场散射问题,得到了管束表面及管排周围的声场分布特性,针对除灰要求进行了声学分析,对当前声波除灰技术的发展及应用均具有实际意义。     

声波除灰技术的理论及应用

声波除灰技术是近年来发展起来的一项新技术，已广泛用于电厂、石化及一般工业锅炉。该技术有很多优于其他除灰技术的优点，但有些作用机理尚需进一步研究。本文重点介绍声波除灰技术的物理声学基础、除灰机理、技术特点及应用情况，并提出具体的结论和建议。     

锅炉受热面积灰在线监测的研究

本文主要进行了电站锅炉受热面积灰在线监测的研究工作,在选择监测参数的过程中,放弃了传统的热有效系数和灰污系数,而选择了相对较易监测的灰污特征参数,并运用人工神经网络之BP网络预测各种工况下受热面清洁时的吸热量,最终推算出灰污特征参数;基于以上理论,充分利用电厂DAS数据资源,在不增加额外测点的条件下,开发和实现了对流受热面积灰的计算机在线监测以及优化吹灰指导。     

交变流动回热器的热声功能和回热功能

本文从解析的角度论证了交变流动回热器不仅具有经典热力学理论所认为的回热功能,而且具有热声理论所论述的热声功能,并指出交变流动斯特林热机进行的不是传统热力分析认为的两个等温过程和两个等容回热过程组成的斯特林循环。     

混合堆FEB—E粒子抽除和氘氚回收系统的概要设计

实验混合堆FEB依靠偏滤器排出粒子及其携带的能量。排出的粒子包括聚变反应产物α粒子、等离子体表面相互作用产物杂质以及没能产生聚变反应的氘氚燃料粒子等。FEB－E粒子抽除和燃料回收循环系统的任务是抽除上述氦灰、杂质以及大量的没能产生聚变反应的氘氚燃料粒子等燃烧废气,以能实现有效的堆芯等离子体纯度控制和密度控制;同时将排出废气中没能产生聚变反应的氘氚燃料粒子分离、纯化和回收,即实现氘氚工艺处理。     

Effects of gas and soot radiation on soot formation in counterflow ethylene diffusion flames

Numerical study of soot formation in counterflow ethylene diffusion flames at atmospheric pressure was conducted using detailed chemistry and complex thermal and transport properties. Soot kinetics was modelled using a semi-empirical two-equation model. Radiation heat transfer was calculated using the discrete-ordinates method coupled with an accurate band model. The calculated soot volume fractions are in reasonably good agreement with the experimental results in the literature. The individual effects of gas and soot radiation on soot formation were also investigated.     

团簇状碳黑颗粒在丙烯臭氧氧化和光氧化体系中的重构

大气中的碳黑颗粒具有不规则的分形团簇结构. 以电火花放电产生的团簇碳黑颗粒为研究对象,发现颗粒的团簇结构在丙烯臭氧氧化和光氧化体系中均发生重构,形成更为紧实的结构. 研究表明这种重构是由丙烯氧化产生的过氧自由基和羟基自由基引起的. 研究结果有助于对大气中团簇状碳黑颗粒老化过程的进一步了解. 由于重构直接改变了颗粒的粒径分布,这一过程可能导致碳黑颗粒在大气中物理化学特性的重大改变. 关键词： 团簇碳黑颗粒 重构 丙烯 烟雾箱     

Scattering and propagation of terahertz pulses in random soot aggregate systems

Scattering and propagation of terahertz pulses in random soot aggregate systems are studied by using the generalized multi-particle Mie-solution(GMM) and the pulse propagation theory. Soot aggregates are obtained by the diffusion-limited aggregation(DLA) model. For a soot aggregate in soot aggregate systems, scattering characteristics are analyzed by using the GMM. Scattering intensities versus scattering angles are given. The effects of different positions of the aggregate on the scattering intensities, scattering cross sections, extinction cross sections, and absorption cross sections are computed and compared. Based on pulse propagation in random media, the transmission of terahertz pulses in random soot aggregate systems is determined by the two-frequency mutual coherence function. Numerical simulations and analysis are given for terahertz pulses(0.7956 THz).     

Investigation of mass and energy coupling between soot particles and gas species in modelling ethylene counterflow diffusion flames

A numerical model is developed aiming at investigating soot formation in ethylene counterflow diffusion flames. The mass and energy coupling between soot solid particles and gas-phase species is investigated in detail. A semi-empirical two-equation model is chosen for predicting soot mass fraction and number density. The model describes particle nucleation, surface growth, and oxidation. A detailed kinetic mechanism is considered for the gas phase and the effect of considering radiation heat losses is also evaluated. Simulations were done for a range of conditions that produce low-to-significant amounts of soot using three strategies: first by changing the strain rate imposed on the flow field, second, by changing the oxygen content in the oxidant stream, and third, by changing the pressure. Additionally, the effect of the transport model chosen was analysed. The results showed that, for the flames studied and within the limits of the present work, the soot and gas radiation terms are of primary importance for numerical simulations. Additionally, it was shown that the soot mass and thermodynamic properties coupling terms are, in general, a second-order effect, with an importance that increases as soot amount increases. As a general recommendation, the radiation terms have always to be considered, whereas full coupling has to be employed only when the soot mass fraction, Y_S, is equal to or larger than 0.008. If a higher precision is required, with errors less than 1%, full coupling should be taken into account for Y_S ≥ 0.002. For lower soot amounts, the coupling through soot mass and thermodynamic properties may be neglected as a first approximation, but an error on the total mass conservation will be present. Additionally, discrepancies from considering different transport models (detailed or simplified) are larger than those found from not fully coupling the phases.     

Effects of benzene and naphthalene addition on soot inception in a well-stirred reactor/plug flow reactor

A well-stirred reactor (WSR) followed by a plug flow reactor (PFR) is being used to study soot inception. Soot size distributions were measured using two different dilution probes followed by a nano-differential mobility analyzer (nano-DMA). One of the dilution probes was developed for the PFR section, while the second probe was specifically developed for use in the WSR section. Results are presented on the effect of residence time on the soot size distributions obtained for fixed dilution ratio and equivalence ratio. In addition, a technique to inject aromatics and PAH species in the transition region between the WSR and PFR was developed. Results are presented on the effect of benzene and naphthalene on the soot size distributions obtained for differing seeding concentrations and residence times. The results demonstrate for the first time the sensitivity of the soot particle size distribution to the seeding of aromatic species in a WSR/PFR.     

Mathematical modeling of laser induced heating and melting in solids

An analytical method for treating the problem of laser heating and melting is developed in this paper. The analytical method has been applied to aluminum, titanium, copper, silver and fused quartz and the time needed to melt and vaporize and the effects of laser power density on the melt depth for four metals are also obtained. In addition, the depth profile and time evolution of the temperature of aluminum before melting and after melting are given, in which a discontinuity in the temperature gradient is obviously observed due to the latent heat of fusion and the increment in thermal conductivity in solid phase. Additionally, the calculated melt depth evolution of fused quartz induced by 10.6 μm laser irradiation is in good agreement with the experimental results.     

Influence of pressure on near nozzle flow field and soot formation in laminar co-flow diffusion flames

Soot formation from combustion devices, which tend to operate at high pressure, is a health and environmental concern, thus investigating the effect of pressure on soot formation is important. While most fundamental studies have utilised the co-flow laminar diffusion flame configuration to study the effect of pressure on soot, there is a lack of investigations into the effect of pressure on the flow field of diffusion flames and the resultant influence on soot formation. A recent work has displayed that recirculation zones can form along the centreline of atmospheric pressure diffusion flames. This present work seeks to investigate whether these zones can form due to higher pressure as well, which has never been explored experimentally or numerically. The CoFlame code, which models co-flow laminar, sooting, diffusion flames, is validated for the prediction of recirculation zones using experimental flow field data for a set of atmospheric pressure flames. The code is subsequently utilised to model ethane-air diffusion flames from 2 to 33 atm. Above 10 atm, recirculation zones are predicted to form. The reason for the formation of the zones is determined to be due to increasing shear between the air and fuel steams, with the air stream having higher velocities in the vicinity of the fuel tube tip than the fuel stream. This increase in shear is shown to be the cause of the recirculation zones formed in previously investigated atmospheric flames as well. Finally, the recirculation zone is determined as a probable cause of the experimentally observed formation of a large mass of soot covering the entire fuel tube exit for an ethane diffusion flame at 36.5 atm. Previously, no adequate explanation for the formation of the large mass of soot existed.     

Catalytic soot oxidation in microscale experiments: Simulation of interactions between co-deposited graphitic nanoparticle agglomerates and platinum nanoparticles

Continuously regenerating catalytic soot traps are under development to reduce particulate emissions from diesel exhaust. A good understanding of the processes that take place during soot oxidation is needed to optimize diesel soot trap performance. To gain insight into these processes from the perspective of nanoparticle technology, the effects of catalyst particle size and the interparticle distance between soot and catalyst particles were measured. A model catalyst was prepared by depositing Pt nanoparticles on a SiO/SiO₂-coated transmission electron microscope (TEM) grid. A soot surrogate composed of graphitic nanoparticle agglomerates generated by laser ablation was deposited on the same surface. This system simulates, morphologically, catalytic soot traps used in practice. The reaction was carried out in a tubular flow reactor in which the gas phase simulated diesel exhaust gas, composed of a mixture of 10% O₂ and 1000 ppm NO with the remainder N₂. The progress of the carbon nanoparticle oxidation was monitored off-line by analysis of electron microscopy images of the agglomerates before and after reaction. This experimental method permitted the correlation of reaction rate with particle sizes and separation distances as well as catalyst surface area in the direct environs of the soot particles. The experimental results revealed no effect of Pt catalyst particle size in the range 7–31 nm on the rate of reaction. Also observed were a decrease in the rate of reaction with increasing distance between carbon agglomerates and catalyst particles and a linear dependence of the reaction rate on the fractional catalyst surface area coverage.     

Effect of driving frequency on non-linear coupling between ultrasound transducer and target under inspection in Sonic Infrared Imaging

Sonic Infrared (IR) Imaging, also referred as vibrothermography, is a novel Nondestructive Evaluation (NDE) technology to find cracks through infrared imaging of vibration-induced crack heating. The vibration source plays an important role in the detection of cracks. In this paper, the effect of driving frequency on the ultrasound vibration to the thermal imaging is presented. The research is organized by using different frequency system and coupling materials on the same aluminum bar sample. The analysis is conducted by combination of the vibration waveforms with the IR images and signals. Correlation analysis between the acoustic energy and the thermal energy in the crack is discussed as well.     

Prediction of soot and thermal radiation in a model gas turbine combustor burning kerosene fuel spray at different swirl levels

Combustion of kerosene fuel spray has been numerically simulated in a laboratory scale combustor geometry to predict soot and the effects of thermal radiation at different swirl levels of primary air flow. The two-phase motion in the combustor is simulated using an Eulerian–Lagragian formulation considering the stochastic separated flow model. The Favre-averaged governing equations are solved for the gas phase with the turbulent quantities simulated by realisable k–? model. The injection of the fuel is considered through a pressure swirl atomiser and the combustion is simulated by a laminar flamelet model with detailed kinetics of kerosene combustion. Soot formation in the flame is predicted using an empirical model with the model parameters adjusted for kerosene fuel. Contributions of gas phase and soot towards thermal radiation have been considered to predict the incident heat flux on the combustor wall and fuel injector. Swirl in the primary flow significantly influences the flow and flame structures in the combustor. The stronger recirculation at high swirl draws more air into the flame region, reduces the flame length and peak flame temperature and also brings the soot laden zone closer to the inlet plane. As a result, the radiative heat flux on the peripheral wall decreases at high swirl and also shifts closer to the inlet plane. However, increased swirl increases the combustor wall temperature due to radial spreading of the flame. The high incident radiative heat flux and the high surface temperature make the fuel injector a critical item in the combustor. The injector peak temperature increases with the increase in swirl flow mainly because the flame is located closer to the inlet plane. On the other hand, a more uniform temperature distribution in the exhaust gas can be attained at the combustor exit at high swirl condition.