Electrochemical mass transfer modeling of a complex two phase heat transfer problem: Case of a prototype slagging gasifier

The local and averaged forced-convective heat transfer coefficients were estimated from measured local and averaged mass transfer coefficients in a model slagging-gasifier hearth pool using the Chilton-Colburn analogy. A solution of ferri/ferrocyanide and buffer with addition of CMC (carboxymethylcellulose) was used for the electrochemical mass transfer measurements. This solution had similar properties to those of the slag in the real gasifier. The influence of natural convection due to the differences in temperatures in the hearth was also estimated. Values of heat transfer coefficient similar to those estimated by British Gas for the prototype Westfield gasifier were found using the mass transfer modelling method. Published in Russian in Elektrokhimiya, 2008, Vol. 44, No. 4, pp. 447–458. The text was submitted by the authors in English.     

Can extensional viscosity be measured with opposed-nozzle devices?

Opposed-nozzle devices are widely used to try to measure the extensional viscosity of low-viscosity liquids. A thorough literature survey shows that there are still several unanswered questions on the relationship between the quantity measured in opposed-nozzle devices and the true extensional viscosity of the liquids. In addition to extensional stresses, opposed nozzle measurements are influenced by dynamic pressure, shear on the nozzles, and liquid inertia. Therefore the ratio of the apparent extensional viscosity that is measured to the shear viscosity that is independently measured is greater than three even for Newtonian liquids. The effect of inertia on the extensional measurements is analyzed by computer-aided solution of the Navier-Stokes system, and by experiments on low-viscosity Newtonian liquids(1 mPa sS 800 mPa s). The effect of nozzle separation-to-diameter ratio on the average residence time of the liquid is analyzed under the assumption of simple extensional flow kinematics. The average residence time of the liquid is independent of this ratio unless the radial inflow section of the extensional flow volume is related to the nozzle separation. Experiments indicate that in some cases widening the gap lowers the apparent extensional viscosity that is measured, whereas in other cases the opposite is true. In the light of these theoretical considerations and experimental observations, the use of systematic corrections to extensional viscosity measurements on non-Newtonian liquids is not recommended. Thus opposed nozzle devices should be considered as useful indexers rather than rheometers. Finally, measurements on a series of semi-dilute solutions of high molecular weight poly(ethylene oxide) in. water are also reported.Dedicated to the memory of Anastasios C. Papanastasiou     

粉碎电磁波的性质和应用

粉碎电磁波是一种新的电磁波,是由无穷个源以无穷个相位在一个局部范围内发射的波,它与普通电磁波有完全不同的性质,波动性几乎已消失,而以粒子性为主,这样它穿透导体的能力几乎大了一百倍.同时它的传播是以粒子扩散方式进行的,具有一种保持在地球原来位置的特性,因此它具有与运动载体反方向运动的趋势. 关键词： 粉碎电磁波 谐振子 反向运动     

气流床气化炉壁面熔渣行为模拟

本文对气流床煤气化炉内高温合成气和壁面渣层的流动与传热传质过程进行了分析,建立了渣层流动、传热传质和相变数理模型,采用VOF方法对渣层和气体之间自由界面进行追踪,采用射线追踪方法(DTRM)计算辐射换热,采用焓法计算渣层相变,熔融态渣滴在渣层表面的沉积通过源项引入.应用所建立的数学模型对实验室规模的Texaco煤气化炉进行了模拟.结果表明:气化炉壁面换热系数从上到下先增加再减少最后再增加;炉膛温度升高,固态渣层厚度减少;壁面温度升高,固态渣层厚度减少.     

The impact of dilatation,scrambling, and pressure transport in turbulent premixed flames

Premixed turbulent flames feature strong interactions between chemical reactions and turbulence that affect scalar and turbulence statistics. The focus of the present work is on clarifying the impact of pressure dilatation/flamelet scrambling effects with a comprehensive second-moment closure used for evaluation purposes. Model extensions that take into account flamelet orientation and molecular diffusion are derived. Isothermal pressure transport is included with an additional variable density contribution derived for the flamelet regime of combustion. Full closure is assessed by comparisons with Direct Numerical Simulations (DNSs) of statistically ‘steady’ fully developed premixed turbulent planar flames at different expansion ratios. Subsequently, the prediction of lean premixed turbulent methane–air flames featuring fractal grid generated turbulence in an opposed jet geometry is considered. The overall agreement shows that ‘dilatation’ effects contribute to counter-gradient transport and can also increase the turbulent kinetic energy significantly. Levels of anisotropy are broadly consistent with the DNS data and key aspects of opposed jet flames are well predicted. However, it is also shown that complications arise due to interactions between the imposed pressure gradient and combustion and that redistribution is affected along with the scalar flux at the leading edge. The latter is strongly affected by the reaction rate closure and, potentially, by pressure transport. Overall, the derived models offer significant improvements and can readily be applied to the modelling of premixed turbulent flames at practical rates of heat release.     

Opposed round jets issuing into a small aspect ratio channel cross flow

Round jets (diameter D) discharging into a confined cross flow (dimension 3.16D × 21.05D) are investigated experimentally. Two configurations are considered: (1) a single jet (momentum flux ratio, J = 155) and (2) two opposed jets with two different momentum flux ratios (J = 60, and 155). A two-component laser-Doppler anemometer is used to make a detailed map of the normal stresses and mean velocities in the symmetry plane of the jets. In addition, smoke-wire and laser-sheet visualization are used to study the flow.

The rate of bending of the single confined jet is found to be higher than the rate of bending of an unconfined jet with the same momentum flux ratio. In the far field, the jet centerline velocity is observed to decay more slowly than the unconfined jet, indicating poor turbulent diffusion of linear momentum. Annular shear layer vortices are visualized on the upstream edge of the jet in the near field. In the far field, the flow visualization suggests that the jet loses its integrity and fragments into independent regions that are convected by the cross flow.

In the opposed jet configuration at the high momentum flux ratio (J = 155), the jets impinge in the center of the duct, and a pair of vortices is observed upstream of the impingement region. The flow visualization implies that the impingement vortices form quasi periodically and have a finite life span. In the impingement region, the jets are observed to penetrate alternately beyond the symmetry plane of the duct. In the two-jet configuration with J = 60, the jets do not impinge on each other owing to the higher rate of bending. Instead, the flow visualization indicates that the shear layers of the jets penetrate to the central region and periodically pinch off regions of the potential-like cross-flow fluid where they meet. The pinch-off regions of cross-flow fluid are convected by the turbulent flow for large distances, yet remain essentially unmixed.     

Vaporisation characteristics of methanol,ethanol and heptane droplets in opposed stagnation flow at low temperature and pressure

A computational model is developed and applied to study the vaporisation behaviour of three liquid fuels. This fundamental study is motivated by a need to understand how the performance of direct-injection-spark-ignition (DISI) engines may be affected by changes in fuel composition, especially alcohols. Currently, most DISI engines are designed for homogeneous-charge combustion, where the in-cylinder fuel injection, vaporisation and mixing is accomplished during the intake and early in the compression process. Thus the temperature and pressure are low, compared to post-compression conditions. The two-phase axisymmetric model is based upon an ideal opposed stagnation flow field. Liquid droplets are carried in one air stream that is met by an opposed air flow. Because of stagnation-flow similarity, the mathematical model can be represented as a one-dimensional boundary-value problem. Results show significant differences between methanol, ethanol and heptane fuels, which have potentially important impacts on the design and modification of fuel-injection systems for direct-injection engines with alternative fuels.     

双高煤干法气化炉的气化试验研究

以高灰熔点高灰分煤的高效气化为背景,提出了一种新型干法煤粉气化炉结构模型,并开展了双高煤在该结构试验炉上的试验研究。结果表明,为了获得较好的气化指标,在试验工况下,合适的氧碳原子比和蒸汽煤比取值区间分别为0.95～1.05和0.05～0.12 kg/kg;典型优化工况下的气化结果表明了该气化炉能够适用于双高煤的高效气化。     

逆向复式射流预燃室燃烧器

预燃室是一种高效、低ＮＯｘ排放很有发展前途的洁净煤燃烧技术。但是，目前由于其体积过大，使它在实际应用中受到限制。本文叙述的逆向复式射流预燃室燃烧器结构简单、体积小、燃烧效率高、煤种适应性强，可望成为符合洁净煤技术要求的高效、低ＮＯｘ主燃烧器。     

流化床气化炉半焦细粉的燃烧特性及其动力学研究

利用热天平研究了三种不同变质程度煤流化床气化半焦细粉的燃烧特性,考察了灰分对燃烧行为的影响,同时采用三种不同的动力学计算方法对其燃烧动力学进行了分析。研究结果表明,挥发分的存在能显著降低低阶褐煤半焦细粉的着火点;高温下半焦细粉中矿物质的分解导致其DTG曲线出现较为明显的二次峰。细粉中的灰分对其燃尽性能和综合燃烧反应性都具有一定的抑制作用。动力学分析结果表明,燃烧活化能随转化率的增加而增加;三种常用动力学分析方法中,Vyazovkin对半焦细粉的燃烧拟合效果最好。