太极式鱼道水力特性试验研究及数值模拟

针对传统鱼道一经建成,其固体边界条件已经固定,池室内水流流速分布变化不大而难以适应多种鱼类通过的缺点,该文研究一种通过太极圆盘和八卦爻条消减水流能量,形成多态流速场以适应多种鱼类洄游的太极式新型鱼道,并对其进行了水力模型试验和数值分析。首先通过模型试验得到了鱼道在不同工况下的流态和沿程水深变化,然后通过数值计算得到了与模型试验相近的结果,并进一步分析了太极式鱼道的表面流速、近底流速及关键横断面流速分布。结果表明:太极式新型鱼道具有显著的消能减速效果,最浅处水深达到无太极圆盘时相应最浅水深的2倍左右,断面平均最大流速为0.95 m/s,相对于无太极圆盘的情况降低50%左右,池室内水流呈现多态化,另外随太极圆盘方位不同流速场亦有明显变化,该种鱼道可为鱼类提供更多适宜的洄游条件。     

文丘里洗涤器净化生物质燃气流场数值模拟及试验研究

针对文丘里洗涤器净化生物质气化燃气中的焦油液滴问题,借助CFD软件Fluent对文丘里管内部流场进行数值模拟,得出其压力与速度分布特性,从能量转化角度分析了文丘里除焦过程流场变化规律,结果表明文丘里喉管段压力大幅减小,其压力能转化为燃气流体动能,形成了较大的湍流提高净化效率。通过试验和数值模拟研究文丘里内部流场总体压力损耗,将液气比3L/m3、喉口气速70～120m/s条件下的文丘里管总体压力损失试验与模拟数据对比表明二者结果基本吻合。对文丘里净化生物质燃气除焦效率的研究表明喉口气速大于90m/s时,利用经验公式根据文丘里洗涤器实际总体压力损失可以有效估算除焦效率,误差范围在7%以内;喉口气速大于90m/s、液气比高于2L/m3时除焦效率可以达到90%以上。研究结果可以为文丘里洗涤器净化生物质燃气的工程应用提供参考。     

生物多孔介质热风干燥数学模型及数值模拟

为了研究生物多孔介质在热风干燥过程中的热质传递机理以及其内部应力应变分布规律,根据生物多孔介质中温度、水分及应力之间复杂的耦合关系,基于菲克扩散定律、傅立叶导热定律和热弹性力学理论,建立了对流干燥条件下,含湿多孔介质内部传热传质过程热-湿-力双向耦合的数学模型。采用有限差分法编制相应的计算程序,对其进行数值计算,数值结果与马铃薯和胡萝卜对流干燥试验结果之间的相对误差均小于5%;进一步分析了干燥特性曲线,以及温度、干基含水率和应力应变的时空分布;最后分析了风温、风速等干燥条件以及多孔介质厚度对干燥过程的影响,结果表明:在一定试验条件下,风温越高,风速越大,切片厚度越薄,干燥时间越短。研究为改善生物多孔介质热质传递现象物理机理的理解提供参考。     

果蔬类多孔介质干燥传质过程的分子动力学模拟

果蔬类多孔介质内部水中溶解有大量的营养物质（溶质）,在干燥过程中溶质的迁移与湿分的传递同时进行,其内部微孔内的干燥传质机理尚不明确。为了揭示果蔬类多孔介质干燥过程中内部溶液的迁移机理,确定果蔬微孔结构特性对干燥传质过程的影响规律。该研究采用分子动力学方法模拟研究了果蔬类多孔介质微孔道中的干燥传质过程,构建了光滑壁面溶液扩散过程模型与粗糙壁面溶液扩散过程模型。模拟过程采用SPC/E水分子模型,选取OPLS-AA全原子力场和正则系综,溶液势函数选用静电库伦相互作用与Lennard-Jones相互作用,中心水分子的初始速度由高斯分布给出,采用Velocity-Verlet算法,用SHAKE算法固定水分子,x、y方向施加周期性边界条件,z方向上施加固定壁面边界条件。从分子水平模拟分析了果蔬类多孔介质内部溶液的扩散过程,并以马铃薯的热风干燥试验结果进行模型的验证。得出试验值与KCl溶液粗糙壁面模型的模拟值最为接近,其最大相对误差为17.39%;与纯水模型的模拟值相差最大,说明溶质的存在对水分扩散系数的影响不可忽略,且粗糙壁面模型更接近于真实孔道结构。从径向分布函数分布可以看出K+、Cl-对水分子的氢键结构有破坏作用。K+、Cl-均存在两层水化层,H2O分子以O原子靠近K+,以H原子靠近Cl-。溶质浓度、孔道直径、壁面粗糙度因子和相面积分数均对孔隙中的水分扩散系数有重要影响。随着孔道内KCl溶液质量分数的增大,其水分扩散系数逐渐减小。孔道直径变大、粗糙壁面粗糙度因子减小和粗糙壁面相面积分数增大,均会导致干燥过程中水分扩散系数增大。研究结果为果蔬干燥品质及工艺优化分析提供了理论依据。     

密度驱动条件下饱和介质盐分迁移规律

为探究一维饱和介质中考虑密度驱动条件下的盐分迁移规律,在室内开展了从土柱顶端释放NaCl盐溶液的密度驱动下盐分迁移试验,结果表明：浓度为100 g/L的盐溶液从顶部释放后仅720 min土柱底部浓度即达到最大值;土柱顶部释放的盐分浓度越高密度驱动效果越明显,迁移到土柱底部（深度70～100 cm）的盐分含量占比越高;释放盐分浓度为6、12、40、60、100、150 g/L时底部盐分含量占比分别为33.3%、36.7%、38.0%、40.0%、40.2%、47.1%。为对比密度驱动和浓度弥散作用下的盐分迁移规律差异,开展了从土柱底部释放高浓度盐溶液的反向弥散试验,结果表明：试验期内密度驱动对盐分迁移的影响效果远大于浓度弥散作用,试验进行30 d后弥散作用仅对土柱底部20 cm范围内的浓度分布产生影响;底部释放盐分浓度越高,向上的弥散通量越大,但对浓度的重分布影响不明显,浓度为6、40、60、100 g/L的NaCl溶液试验进行15 d后土柱底部20 cm范围内含盐量占比依次增大,分别为37.5%、62.4%、72.7%、82.7%。该研究量化分析了在密度驱动和弥散作用下盐分的迁移规律,可为干旱灌区盐分平衡规律的研究提供一定的理论依据。     

橡塑渗灌管渗水性能实验研究

对一种橡塑渗灌管产品,应用蒸馏水进行自由渗流实验。结果表明：渗水速率随渗水时间的延长逐渐降低,最终趋于相对稳定。在0～7 mH₂O压力范围内,渗水速率与水头压力之间呈幂函数关系,渗水均匀性随水头压力发生变化;同应用蒸馏水相比,应用自来水实验导致渗水速率和渗水均匀性降低,增大水头压力有助于提高渗灌管的抗堵塞性能。研究表明,供试橡塑渗灌管的渗水性能受水头压力影响较大,渗水均匀性有待提高。     

砂质多孔介质中土壤颗粒的迁移

通过室内模拟试验,研究了不同土壤悬液浓度和不同多孔介质组成条件下土壤颗粒在砂质多孔介质中的截留和迁出特征,以揭示土壤颗粒在砂质多孔介质中的迁移特征。结果表明,土壤颗粒在砂质多孔介质中迁移时会受到土壤颗粒粒级、浓度和介质组成的影响。截留作用在土壤颗粒通过砂质多孔介质时起主导作用,土壤颗粒截留比例随着土壤颗粒浓度的增大逐渐增加;而随着多孔介质中粗砂比例的增大,其逐渐降低。土壤总颗粒、50μm和1~50μm土壤颗粒迁出量分别与该粒级相应投入量呈幂函数增加趋势,而1μm土壤颗粒的迁出量与其投入量呈线性增加趋势。土壤总颗粒迁出量与介质中粗砂比例之间存在线性增加关系。因此,多孔介质组成、颗粒粒径和浓度在土壤颗粒迁移过程中起着重要作用,在多孔介质中土壤颗粒和污染物迁移研究方面需要充分考虑这些因素。     

Modeling finger development and persistence in initially dry porous media

The mechanism for the growth and persistence of gravity-driven fingered flow of water in initially dry porous media is described. A Galerkin finite element solution of the two-dimensional Richards equation with the associated parameter equations for capillary hysteresis in the water retention function is presented. A scheme for upstream weighting of internodal unsaturated hydraulic conductivities is applied to limit smearing of steep wetting fronts. The growth and persistence of a single finger in an initially dry porous media is simulated using this numerical solution scheme. To adequately simulate fingered flow, it was found that the upstream weighting factor had to be negative, meaning that the internodal unsaturated hydraulic conductivities were weighted more by the downstream node. It is shown that the growth and persistence of a finger is sensitive to the character of the porous media water retention functions. For porous media where the water-entry capillary pressure on the main wetting function is less than the air-entry capillary pressure on the main drainage function, a small perturbation will grow into a finger, and during sequential drainage and wetting the finger will persist. In contrast, for porous media where the water-entry capillary pressure on the main wetting function is greater than the air-entry capillary pressure on the main drainage function, the same small perturbation will dissipate by capillary diffusion. The finger widths derived from the numerical simulation are similar to those predicted by analytical theory.     

Mineralization and denitrification in upland, nearly saturated and flooded subtropical soil I. Effect of nitrate and ammoniacal nitrogen

 The influence of fertilizer N applied through nitrate and ammoniacal sources on the availability of nitrate, supply of C, and gaseous N losses via denitrification (using acetylene inhibition technique) in a semiarid subtropical soil (Typic Ustochrepts) was investigated in a growth chamber simulating upland [60% water-filled pore space (WFPS)], nearly saturated (90% WFPS), and flooded (120% WFPS) conditions. The rate of denitrification was very low in the upland soil conditions, irrespective of fertilizer N treatments. Increasing water content to nearly saturated and flooded conditions resulted in four- to sixfold higher rates of denitrification within 2 days, suggesting that the denitrifying activity commences quickly. Results of this study reveal that (1) under restricted aeration, these soils could support high rates of denitrification (∼6 mg N kg^–1 day^–1) for short periods when nitrate is present; (2) application of fertilizer N as nitrate enhances N losses via denitrification (∼10 mg N kg^–1 day^–1) – however, the supply of available C determines the intensity and duration of denitrification; (3) when fertilizer N is applied as an ammoniacal form, nitrification proceeds slowly and nitrate availability limits denitrification in flooded soil; (4) the nearly saturated soil, being partially aerobic, supported greater nitrification of applied ammoniacal fertilizer N than flooded soil resulting in higher relative rates of denitrification; and (5) under aerobic soil conditions, 26 mg mineral N kg^–1 accumulated in control soil over a 16-day period, demonstrating a modest capacity of such semiarid subtropical soils, low in organic matter, to supply N to growing plants. Received: 7 June 1999     

磁力泵冷却循环回路的设计及数值模拟

为避免磁力泵温升过高导致永磁体退磁及隔离套损坏,该文对磁力泵冷却循环回路的设计方法进行了探讨,采用ANSYS-APDL软件计算出了隔离套的涡流发热,根据热平衡确定冷却循环流量并设计了冷却循环回路。基于SIMPLEC算法和标准k-ε湍流模型,通过求解三维N-S方程及能量方程,对冷却循环回路内部流场及温度场进行了数值分析。从数值模拟可以看出,冷却循环回路内部流动为圆周运动和直线运动合成的螺旋运动。对比内循环、外循环2种方式表明,内循环方式隔离套底部温升最高、压力较低;外循环方式温度场分布较均匀,最高温升小于10 K,满足设计要求。在冷却循环流量相同的情况下,轴孔孔径在设计尺寸一定范围内波动对外循环方式的冷却效果影响不大,轴孔分别为3、4、5 mm,其最高温升分别为9.2、9.3、9.4 K并且分布基本相同。通过分析不同转速下冷却循环回路的流场、温度场,发现当内磁转子不转动时,流场最高温度达到了386 K,而随着转速的增加最高温度逐步降低,表明增加泵的转速能够促进不同流体层间的热量交换,改善冷却循环回路的冷却效果。该研究可为磁力泵冷却循环回路的设计提供参考。     

Pore network and water retention characteristics of volcanic porous media

Aggregate media are often characterized by multi‐porous systems, which have structural and water retention characteristics that depend on the complex interaction between intra‐ and inter‐aggregate pores. Here we investigate the structure and water retention dynamics of rigid aggregate volcanic materials. In particular, we focus on commercially used pumices, lapilli and zeolites. The aim was to estimate the air and water content through complex dual‐porous systems, and thus to evaluate their suitability for vegetation growth. Both inter‐ and intra‐aggregate characteristics were determined by means of mercury intrusion porosimetry, X‐ray microtomography and water retention curves. The wilting point was determined with pressure plates, a dew point hygrometer and the sunflower method to assess their reliability at small matric potentials. Results indicate that aggregate porous media were bimodal and their heterogeneous pore network affected the water retention dynamics because (i) the large inter‐aggregate pores allowed a rapid drainage near saturation and (ii) the intra‐aggregate porosity held water available for root uptake and plant growth. In contrast, volcanic powders were less affected by the inter‐ and intra‐aggregate dual‐porosity. The use of a dew point hygrometer instead of pressure plates for determining small matric potentials is also suggested because pressure plates might over‐estimate the water content because of poor plate and soil conductance. However, the reference potential at wilting point should be set at values greater than ?1471.5 kPa (?784.8 kPa) to consider the interaction between plant roots and porous media with small hydraulic conductivity. Results from this work indicate that aggregate multi‐porous media allow the simultaneous supply of oxygen and available water for plants, although the heterogeneous nature of the pore network involves uncertainties regarding water balance and root–matrix interactions.     

多孔介质干燥过程传热传质研究

该文根据非平衡态热力学和相平衡理论，建立了多孔介质对流干燥内部传热传质过程的二维数学模型，该模型充分考虑了传热与传质之间的相互耦合，用控制容积积分法采用全隐格式对该模型进行分析求解，并与玉米干燥实验数据进行对比。结果表明，表面对流换热系数、干燥介质温度和湿度的变化对热、湿迁移过程均有较大影响，而表面对流传质系数对湿迁移过程影响不大，温度和湿度梯度的耦合产生“局部增湿”现象。     

氧化石墨烯和五价砷在改性多孔介质中的共迁移特征

砷是农田土壤重金属污染的主要元素之一,在砷污染农田土壤的修复过程中往往忽视纳米颗粒能够使结合态的砷重新释放,导致有效态砷浓度升高,探究土壤中黏土矿物对氧化石墨烯(Graphene Oxide,GO)和五价砷(As(V))在多孔介质中迁移行为的影响,对进一步完善农田土壤砷修复理论以及提高农作物产量、保护人体健康具有重要意义。该研究利用蒙脱石和高岭石改性石英砂,通过砂柱迁移试验系统地研究了GO、As(V)和GO-As(V)在填加0%、10%、30%和50%的蒙脱石和高岭石改性石英砂柱中的迁移行为。研究结果表明,随着高岭石和蒙脱石改性石英砂填加比例的增加,GO和As(V)的迁移能力均呈降低趋势,且GO和As(V)在不同条件下的迁移曲线均存在显著差异(P<0.05);GO在50%高岭石和蒙脱石改性石英砂柱中的回收率相对于石英砂柱分别下降了14%和17%,As(V)分别下降了15%和12%;在共迁移试验中,GO和As(V)在石英砂柱中回收率分别上升至99%和100%。分析表明,As(V)在蒙脱石改性石英砂柱中的迁移能力大于高岭石改性石英砂,而GO与之相反;当GO和与As(V)共迁移时,二者...     

胶体作用下饱和土壤中Cd运移的数值模拟

通过室内土柱出流实验探讨了饱和壤砂土中Cd在SiO2胶体作用下的运移行为,并用对流-弥散方程(Convection-Dispersion Equation,CDE)对实验结果进行了数值模拟。结果表明,土壤中SiO2胶体的存在抑制了Cd的运移,土壤对Cd的吸附以动力学反应为主。耦合Freundlich等温吸附的单点非平衡模型(One Site Nonequilibrium Model,OSM)能较好地模拟Cd在壤砂土中的运移,模拟得到的吸附特性参数β较小,表明壤砂土吸附表面的不均一性较强。SiO2胶体的穿透时间较早,耦合沉淀-释放反应的CDE模型可以很好地描述SiO2胶体在壤砂土中的运移。胶体作用下的CDE模型成功地模拟了SiO2胶体存在时Cd的运移;同时,参数敏感性分析结果表明,胶体对Cd的吸附速率系数kamc、kaic越小,解吸速率系数kdmc、kdic越大,越有利于Cd的运移。     

自控温室番茄生产的光合数值模拟

使用CIRAS-1便携式光合仪测定了荷兰大型玻璃自控温室内番茄光合能力对环境条件的响应,由实测值和参考文献确定了有关参数,运用非直角双曲线叶面积积分模型模拟温室不同气象条件下番茄净光合量的增减,探讨了提高光合产量的调控措施。     

胶体作用下饱和土壤中Cd运移的数值模拟

通过室内土柱出流实验探讨了饱和壤砂土中Cd在SiO2胶体作用下的运移行为,并用对流-弥散方程(Convection-Dispersion Equation,CDE)对实验结果进行了数值模拟.结果表明,土壤中SiO2胶体的存在抑制了Cd的运移,使Cd穿透时间较晚,出流浓度较低,土壤对Cd的吸附以动力学反应为主.耦合Freun-dlich等温吸附的单点非平衡模型(One Site Nonequilibrium Model,OSM)能较好地模拟Cd在壤砂土中的运移,模拟得到的吸附特性参数β较小,表明壤砂土吸附表面的不均一性较强.SiO2胶体的穿透时间较早,耦合沉淀-释放反应的CDE模型可以很好地描述SiO2胶体在壤砂土中的运移.胶体作用下的CDE模型成功地模拟了SiO2胶体存在时Cd的运移;同时,参数敏感性分析结果表明,胶体对Cd的吸附速率系数κamc、κaic越小,解吸速率系数κdmc、κdic越大,越有利于Cd的运移.