A numerical study of convective-scale downdrafts in the outer core of tropical cyclones in vertically varying environmental flows

The characteristics of convective-scale downdrafts in the outer core of tropical cyclones in the lower-and upper-layer vertical wind shear(VWS)are investigated based on two high-resolution idealized numerical experiments.Four types of outer-core downdrafts,originating from the lower troposphere,the midtroposphere,the upper level,and the tropopause,respectively,are found.The downdrafts originating from the lower and mid troposphere can penetrate down near the surface,and those originating from the tropopause in upper-layer VWS tend to penetrate more downward than in lower-layer VWS.Downdrafts tend to be located in the more upwind portion of the downshear-right quadrant in lower-layer VWS than in upper-layer VWS.The frequency of downdrafts outside and upwind of the parent updraft increases with the increasing downdraft top height.Vertical momentum budgets indicate that downward-oriented buoyancy due to the evaporational cooling of rainwater and precipitation drag mainly contribute to the occurrence of low-level downdrafts,and the midlevel and upper-level downdrafts originate due to precipitation drag and are strengthened by the downward-oriented,buoyancy-induced perturbation pressure gradient.The processes governing the downdrafts from the tropopause are different between the two experiments.More icy-type particles are produced and transported outward at upper levels in the lower-layer shear experiment,resulting in larger downward-oriented buoyancy due to the sublimational cooling of icy-type particles and contributing to the development/maintenance of the downdraft from the tropopause in that experiment.However,the downwardoriented perturbation pressure gradient leads to the development/maintenance of the downdraft from the tropopause in the upper-layer shear experiment.

Insights into Convective-scale Predictability in East China: Error Growth Dynamics and Associated Impact on Precipitation of Warm-Season Convective Events

This study investigated the regime-dependent predictability using convective-scale ensemble forecasts initialized with different initial condition perturbations in the Yangtze and Huai River basin(YHRB)of East China.The scale-dependent error growth(ensemble variability)and associated impact on precipitation forecasts(precipitation uncertainties)were quantitatively explored for 13 warm-season convective events that were categorized in terms of strong forcing and weak forcing.The forecast error growth in the strong-forcing regime shows a stepwise increase with increasing spatial scale,while the error growth shows a larger temporal variability with an afternoon peak appearing at smaller scales under weak forcing.This leads to the dissimilarity of precipitation uncertainty and shows a strong correlation between error growth and precipitation across spatial scales.The lateral boundary condition errors exert a quasi-linear increase on error growth with time at the larger scale,suggesting that the large-scale flow could govern the magnitude of error growth and associated precipitation uncertainties,especially for the strong-forcing regime.Further comparisons between scale-based initial error sensitivity experiments show evident scale interaction including upscale transfer of small-scale errors and downscale cascade of larger-scale errors.Specifically,small-scale errors are found to be more sensitive in the weak-forcing regime than those under strong forcing.Meanwhile,larger-scale initial errors are responsible for the error growth after 4 h and produce the precipitation uncertainties at the meso-β-scale.Consequently,these results can be used to explain underdispersion issues in convective-scale ensemble forecasts and provide feedback for ensemble design over the YHRB.

Syngas cleaning with nano-structured micro-porous ion exchange polymers in biomass gasification using a novel downdraft gasifier

Sulphonated nano-structured micro-porous ion exchange polymers, known as sulphonated PolyHIPE Polymers （s-PHPs） were used in syngas cleaning to investigate their impact on tar composition, concentration and dew point depression during the gasification of fuel cane bagasse as a model biomass. The results showed that the s-PHPs used as a secondary syngas treatment system, was highly effective at adsorbing and reducing the concentration of all class of tars in syngas by 95%-80% which resulted in tar dew point depression from 90 ~C to 73 ~C. It was shown that tars underwent chemical reactions within s-PHPs, indicating that tar diffusion from syngas was driven by chemical potential. It was also observed that s-PHPs also captured ash forming elements from syngas. The use of s-PHPs in gasification as well as in an integrated thermochemical biorefinery technology is discussed since the tar loaded s-PHPs can be used as natural herbicides in the form of soil additives to enhance the biomass growth and crop yield.

Pre-Operational Analysis of a Prototype Downdraft Gasifier Fueled by Bamboo

Biomass gasification is a thermochemical conversion process that dates back to the 19th century. Nevertheless, designing and operating a gasifier system is not an easy task. Every biomass feedstock has different characteristics and the gasifier needs to be designed according to those qualities. Hence, many laboratory analyses on bamboo were carried out for this study. This study also concentrates on finding the best possible process variables for a bamboo fueled downdraft gasifier through a sensitivity analysis. A software program called Thermoflex was used for this purpose and the effect of gasifier temperature, air-fuel-ratio, moisture content of the fuel and temperature of pre-heated air on the syngas composition were simulated. The results show that bamboo is a decent gasification feedstock because of its low ash and sulfur content and satisfactory energy value. The simulations reveal that the best gas quality is obtained with the gasifier temperature between 700℃ and 800℃, A/F-ratio of 1.25 - 1.75 and dry basis moisture content between 10% and 15%.

Pyrolysis Model of Single Biomass Pellet in Downdraft Gasifier

By coupling the heat transfer equation with semi-global chemical reaction kinetic equations, a onedimensional, unsteady mathematical model is developed to describe the pyrolysis of single biomass pellet in the pyrolysis zone of downdraft gasifier. The simulation results in inert atmosphere and pyrolysis zone agree well with the published experimental results. The pyrolysis of biomass pellets in pyrolysis zone is investigated, and the results show that the estimated convective heat transfer coefficient and emissivity coefficient are suitable. The mean pyrolysis time is 15.22%, shorter than that in inert atmosphere, and the pellet pyrolysis process in pyrolysis zone belongs to fast pyrolysis. Among the pyrolysis products, tar yield is the most, gas the second, and char the least. During pyrolysis, the temperature change near the center is contrary to that near the surface. Pyrolysis gradually moves inwards layer by layer. With the increase of pyrolysis temperature and pellet diameter, the total pyrolysis time, tar yield, char yield and gas yield change in different ways. The height of pyrolysis zone is calculated to be 1.51—3.51 times of the characteristic pellet diameter.

Two-dimensional CFD simulation and pilot-scale experimental verification of a downdraft gasifier: effect of reactor aspect ratios on temperature and syngas composition during gasification

This paper focuses on a two-dimensional CFD simulation of a downdraft gasifier and a pilot-scale experiment for verification using wood pellet fuel.The simulation work was carried out via the ANSYS-Fluent CFD software package with in-house coding via User Defined Function.Three gasification parameters were taken into account in the simulation and validation to achieve highly accurate results;namely,fuel consumption,temperature profile,and syngas composition.After verification of the developed model,the effects of aspect ratios on temperature and syngas composition were investigated.Results from simulation and experimental work indicated that the fuel consumption rate during the steady state gasification experiment was 1.750±0.048 g/s.The average steady state temperature of the experiment was 1240.32±14.20 K.In sum,the fuel consumption and temperature profile during gasification from modeling and experimentation show an error lower than 1.3%.Concentrations of CO,CO2,H2,and CH4 were 20.42 vol%,15.09 vol%,8.02 vol%,and 2.6 vol%,respectively,which are comparable to those of the experiment:20.00 vol%,15.48 vol%,8.00 vol%,and 2.65 vol%.A high concentration of syngas is observed in the outer radial part of the reactor because of the resistive flow of the air inlet and the synthesis gas produced.The average temperatures during the steady state of the gasifier with aspect ratios(H/D)of 1.00,1.38(experiment),and 1.82 were 978.77±11.60,1256.46±9.90,and 1368.94±9.20 K,respectively.The 1.82 aspect ratio reactor has the smallest diameter,therefore the radiative heat transferred from the reactor wall affects the temperature in the reactor.Syngas compositions are comparable.Inverse relationships between the aspect ratios and the syngas LHV,(4.29–4.49 MJ/N m3),cold gas efficiency(29.66%to 31.00%),and carbon conversion(79.59%to 80.87%)are observed.

Performance and Economic Analysis of Mulberry Paper Handmade Dryer by Using Downdraft Gasifier

The mulberry paper handmade dryer uses downdraft gasifier, which is a continuous hot air dryer. The downdraft gasifier uses charcoal or wood chip as fuel to produce the producer gas for a dryer heat source. Two steps operation of a dryer as follows： The frst was to reduce mulberry paper pulp moisture by an air vacuum pump; the second was a continuous hot air drying process. The optimum condition drying, the capacity of dryer, the fuel consumption, the drying constant （k） and economics analysis were investigated. It was found that the first step could be to reduce mulberry paper pulp moisture content about 25% and the suitable condition drying was 80 ~C drying temperature, 0.04 kg/s air mass flow rate and 0.29 m/min chain conveyor speed, respectively. The capacity of this dryer was 20 sheets per hour. The quality of mulberry paper product was very good （based on the standard of mulberry paper community 41/2546） and the fuel consumption rate was 5 kg/h （charcoal）. The drying constant was about 0.532933-0.541367 min~ and the drying constant was a function of drying temperature （T）, air mass flow rate （F） and conveyor speed （10 as equation of k（T, F, V） = 0.567494 ＋ 0.000422T- 1.40588F- 0.000205 V （R2 = 0.9254） and the breakeven point of dryer was 0.79 years.

Performance Analysis of a Woodchip Downdraft Gasifier： Numerical Prediction and Experimental Validation

The study deals with a multi-faceted theoretical approach, symbolic, analytical and numerical, based on the chemical equilibrium assumption, addressed at predicting the performance trends of downdrafi wood-gasification processes so to assess the optimal ranges of input parameters, in particular the equivalence ratios, suitable to achieving the highest cold gas efficiencies whilst keeping the more the possible tar-free the produced bio-syngas. The time-steady, zero-dimensional model has been developed within MATLAB （the computing language and interactive environment from Matrix Laboratory） and solved by enforcing the constraints posed by the equilibrium constants in relation to two reactions, gas-water shift and methanation. Particular care is devoted toward verifying the real attainment of the equilibrium condition, as attested by an actual presence of products from the equilibrium reactions together with a zero difference AE between the energy flows entering and exiting the system, an issue often overlooked. With respect to other similar theoretical approaches, the numerical model, assisted by the symbolic counterpart for better interpretation and intrinsic validation of results, shows a distinct advantage in predicting rather accurately the syngas composition for varying gasification temperatures, as attested by cross comparisons with experimental data directly taken on an instrumented, dedicated, small-scale downdraft gasifier operational at DIME/SCL （the Savona Combustion Laboratory of DIME, the Dept. of Mechanical, Energy, Management and Transportation Engineering of Genova University）. The behavior of cold gas efficiency clearly points out that, from an energy conversion point of view, the optimal gasification temperatures turn out comprised between 900 ℃ and 1,000 ℃： this range is indeed characterized by the highest concentrations in the energy-rich syngas components CO and H2. For higher temperatures, as induced by higher air-to-fuel ratios, the progressive oxidation of above components, together with increasing nitrogen levels, would decrease the bio-syngas heat values.

兼吸式移动床生物质热解炭化装置设计与试验

针对目前内热式移动床生物质热解炭化试验研究平台条件不足的问题,开发了一种内加热兼吸式移动床热解炭化装置,模拟连续式热解炭化工艺,设计了供气系统、出炭系统、产物收集系统、气体净化系统、温度监控系统,以稻壳作为原料,开展了上吸和下吸式热解炭化试验,研究了停留时间和吸气方式对内热式热解炭化产物的影响。结果表明,随着停留时间的增加,上吸和下吸内热式热解炭化变化趋势基本相同,挥发分和固定碳含量均呈下降趋势,灰分含量增加,磷及氮元素含量均下降,氧元素含量上升,氢元素变化趋势不明显,热值分别从20.7MJ/kg和22MJ/kg下降到14.6MJ/kg和15.2MJ/kg;比表面积分别从0.73m^(2)/g和0.78m^(2)/g上升到3.84m^(2)/g和3.95m^(2)/g,生物炭孔隙结构得到了发展。该系统结构合理,运行稳定可靠,密封效果良好,可有效控制进气方式、进气量、保温炭化时间等试验因素。为内加热炭化工艺参数试验研究提供了重要支撑。

两次强下击暴流致灾大风过程对比

利用多普勒天气雷达、常规探空和地面观测数据、1 min降水量和5 min间隔加密自动气象站观测数据,对比分析了2017年6月2日和8月6日山东两次强下击暴流风暴(简称6·2超级单体和8·6强单体)雷达特征及地面致灾大风的成因。研究表明:两次致灾大风过程在强天气尺度和有利中尺度环境下分别形成超级单体和强单体风暴并触发系列下击暴流,最强下击暴流发生时垂直积分液态水含量先跃增后骤降,6·2超级单体伴随中气旋顶和底高度的剧烈下沉。两次强下击暴流触地前均出现强反射率因子核的快速下降、底层高径向速度和强辐散、中层径向辐合和高空强辐散特征。6·2超级单体旋转特性强、中气旋深厚,低层伴随弧形入流缺口和勾状回波。8·6强单体中低层辐合特征显著,风暴前端低层伴有由雷暴出流和前侧入流形成的辐合带。两次强下击暴流引起地面致灾大风的过程中负浮力效应基本相当,6·2超级单体冷池密度流效应更明显,8·6强单体动量下传效应更显著。潍坊南孙站位于风暴移动方向正前侧,前侧辐散气流与同向快速移动的风暴叠加,是导致37 m·s^(-1)极端大风的重要原因。

既有地铁车站纵向通风烟气控制模式试验研究

在北京某地铁车站开展现场试验,对不同烟气控制条件下烟气蔓延规律及楼梯口向下气流速度进行分析研究,同时利用FDS建立对应实际尺寸的数值模拟模型,增加烟气控制模式改变的工况,基于判断能见度危险来临时间对站台排烟能力展开分析研究。研究表明:针对不同烟气控制模式,烟气蔓延规律不尽一致,对于该既有车站而言开启全部风机及仅开启1号风机会造成楼梯口负压情形,即站台发生火灾烟气不会蔓延至站厅;分析楼梯口断面向下气流速度,可知开启全部风机时,楼梯口风速满足规范的楼梯口向下气流速度1.5 m/s的要求,而仅开启1号风机,一侧站台无法满足规范要求;在烟气蔓延规律分析及断面风速分析的基础上,可知1号风机对车站安全起主要作用;结合楼梯口断面风速试验数据与能见度危险来临时间模拟数据,可知开启全部风机相较仅开启1号风机大幅度提升了站厅安全性,但对站台排烟能力提升作用较小。

基于氢能催化生物质富氧气化预条件研究

探究催化剂和气化剂的使用条件,有利于生物质气化生成高品质燃气,这对生物质气化技术的进一步发展具有推动作用。因此,通过分析生物质气化研究的现状,对比空气当量比、燃气组分、燃气热值、气化效率和燃气焦油含量等参数,了解布朗气(HHO)催化对相应反应的影响规律,最后确定了采用下吸式固定床对生物质进行富氧气化的预条件:布朗气的流量为气化剂流量的1%,富氧浓度为90%±5%,炉内温度为1100℃。

兰州城区TSP高浓度污染与自然降尘的关系

从宁夏到甘肃中部有一个自北向南的少雨“干舌”,在垂直速度场上形成一个从河西东部向南伸下来的下沉运动区,冬季下沉运动区大而强烈,有利于空气中的沙尘下沉堆积。经分析,兰州城区自然降尘量占年降尘量的54%。随着沙尘暴、扬沙和浮尘天气的出现,自然降尘量由1月份的32%增大到5月份的60%;按春、夏、秋、冬四季划分,城区TSP浓度中自然降尘量各占58%、33%、28%和32%。全年中11月城郊浓度差值最大0.59mg.m-3,其主要原因是11月刚进入采暖期,除原有工业源外,又增加了采暖锅炉和生活小炉灶,TSP排放量增大,加之该月风速最小、天气过程少,是兰州市出现严重污染的主要原因。

三次雷暴导致的阵风锋过程分析

利用多普勒天气雷达资料和自动站资料对2012年5月16日江苏、6月9日京津冀地区以及7月10日河北的三次阵风锋过程进行综合分析。结果表明:持续下沉的冷空气形成雷暴高压是阵风锋产生的直接原因。雷暴高压形成过程中,一方面,下沉气流在较小区域内迅速堆积形成雷暴高压,另一方面,新旧单体不断更替,风暴内稳定的下沉气流使雷暴高压发展。雷暴高压内强辐散气流与环境空气形成阵风锋。随着雷暴高压的移动和增强,阵风锋向前移动和增强,当雷暴高压减弱,阵风锋也逐渐消亡。温度梯度与气压梯度越大,瞬时大风越强,阵风锋也越强。阵风锋产生的瞬时大风与窄带回波的强度值不一定成正比。中层径向辐合对阵风锋产生有提前预示作用,提前量为半小时左右,辐合持续时间越长,阵风锋生命史越长。

下吸式生物质固定床气化炉研究进展

随着全球能源需求的不断增加和化石能源的日趋枯竭,生物质固定床气化技术作为一种清洁的可再生能源利用技术,引起许多研究者的关注。与上吸式固定床气化炉相比,下吸式具有可燃气焦油含量低、炭转化率高、可燃气热值高、可燃气产品用途广的优点。笔者详细介绍了下吸式生物质固定床气化炉的原理、分类、单段下吸式固定床以及两段下吸式固定床的研究现状,并且在综述的基础上,对下吸式固定床气化炉的应用研究提出了展望。

下击暴流形成机理及监测预警研究进展

下击暴流指对流单体强下沉气流引发的地面或地面附近的爆发性辐散出流,单个下击暴流会导致千米尺度地面强阵风,而下击暴流簇可导致较大范围间断性地面灾害性强阵风,其形成机制亦不限于强下沉气流辐散。文章回顾了下击暴流的界定,然后分为孤立风暴产生的下击暴流和中尺度对流系统内嵌的下击暴流两种情况进行讨论,内容包括对流大风和下击暴流产生的物理机理、风暴结构特征以及基于多普勒天气雷达的预警技术。在上述回顾基础上,对下击暴流形成机理及监测预警难点进行了讨论,提出了与下击暴流相关的亟需研究的问题。

生物质气化技术的再认识

近现代,生物技术在工业、农业和能源领域得到广泛应用,对世界科技和经济发展起到重大的变革和促进作用。由于化石燃料资源性枯竭问题和环境污染问题,寻找一种清洁、可再生的替代燃料和燃料生产技术已迫在眉睫。生物质气化技术作为一种清洁的可再生能源利用技术得到了快速发展,然而由于气化设备自身不够成熟以及未对气化副产物(生物质炭和生物质提取液)加以有效利用等问题,严重阻碍了生物质气化技术的商业化推广和运行。生物质气化多联产技术是指基于生物质下吸式固定床气化的气、固、液三相产品多联产及其产品分相回收、利用技术。该技术的提出,以及相关核心设备的开发成功与应用,为生物质气化技术的进一步发展提供了新的思路。笔者详细介绍了气化技术发展的历史和困境、生物质气化多联产技术的路线和核心设备以及多联产技术产品的开发和应用情况。

往复柱塞泵转套式配流系统泵腔流场仿真研究

往复柱塞泵转套式配流系统是一种结构紧凑、密封可靠的新型配流系统。针对其流量倒灌和压力超调问题,利用软件Fluent,采用UDF(User-Defined-Function)功能和滑移网格与动网格技术,对往复柱塞泵转套式配流系统泵内的非定常流动进行了仿真研究。仿真结果表明,配流系统只在排油向进油过渡的瞬间出现压力超调,并产生短时间压力震荡;往复柱塞泵进油阶段,进油腔内液压油流速较慢,流动范围较大,排油阶段,液压油流速较大,流动范围很小;整个工作周期内进油口与泵腔之间无流量倒灌现象,但出油口与泵腔在每个过渡瞬间都出现倒灌,倒灌流量较小、时间较短。

下吸式气化炉木屑高温蒸汽气化制取富H_2实验

设计了生物质高温蒸汽气化实验平台,主反应器为高温蒸汽发生系统和带有喉口的下吸式气化炉。利用该实验平台对木屑进行高温蒸汽气化研究,气化过程通入的蒸汽温度控制在600～1 000℃。实验结果表明：高温蒸汽既是气化过程的气化剂又是部分热载体,能有效提高气化效率,并维持炉内温度场的稳定。实验条件下,气化气可燃组分体积分数达到77%以上,当蒸汽温度为（948±4）℃时,气化气中H2体积分数达到（51.83±0.12）%,气体热值为9.81 MJ/m3,H2/CO组分比达到2.17,气化气可持续稳定燃烧,气化性能较为理想。

基于Fluent的固定床生物质气化炉冷态压力场研究

以下吸式固定床生物质气化炉物理模型为研究对象,应用流体仿真软件Fluent,对冷态气化炉在单、双层气化剂配风工况下的床层压力场进行仿真研究。通过气化炉多点测压实验,对仿真结果进行验证。利用欧拉-拉格朗日方法分析气化炉冷态流场分布特性,根据伯努利方程说明气化炉床层压力场的变化原因。结果表明,仿真与实验结果的误差值在2.5%以内,气化剂配风工况的变化改变了炉内流场,双层气化剂配风使氧化层压力场平均值为14.98 kPa,高于单层配风工况,且轴向压力分布均匀。