Simulation of light-limited algae growth in homogeneous turbulence

A study is performed of the effect of turbulent mixing on algae growth rate under light-limiting conditions. In order to isolate the effect of light fluctuations and to eliminate possible effects of mass transfer and other side effects of mixing, a numerical model of algae growth is employed. The study examines three aspects of turbulent mixing using models of increasing complexity. First, mixing increases the depth of the “mixed layer”—the layer of approximately uniform algae concentration—spreading the algae produced near the surface over a thicker fluid region. We demonstrate that mixed layer depth has no effect on the net algae production rate. Second, mixing causes motion of algae cells across the optical gradient. A single-frequency harmonic model is used to demonstrate the influence of periodic motion on algae production rate for different frequencies and concentration values. The model results explain previous apparently contradictory experimental observations. Significant enhancement of algae production rate with mixing is observed for small values of the ratio of illuminated layer depth to total fluid depth; however, growth rate enhancement saturates beyond a certain mixing frequency. Third, turbulent mixing involves a broad range of fluid time and length scales. A direct numerical simulation of homogeneous turbulence coupled to the algae growth model is used to show that the main conclusions of the single-frequency model can be carried over to a more realistic turbulent flow by appropriate choice of the mixing time scale.     

A stochastic Lagrangian approach for simulating the effect of turbulent mixing on algae growth rate in a photobioreactor

It is well known that mixing caused by fluid turbulence can cause significantly increased growth rate of algae in photobioreactor flows under certain conditions. In general, flows in which the light penetrates into only a small fraction of the reactor flow field have the largest growth rate enhancement in the presence of fluid mixing. The current paper seeks to develop a computationally efficient prediction method for algae growth rate in practical photobioreactors using a combination of commercially available RANS turbulence models and a stochastic Lagrangian model for the turbulence fluctuations. The stochastic Lagrangian algae growth rate model is first validated by comparison with simulations of algae growth rate obtained by direct numerical simulation of homogeneous turbulence. We then demonstrate the stochastic Lagrangian model approach for prediction of algae growth rate in turbulent pipe flow, which is representative of the primary photoreaction component in many tubular algae production facilities. The results illustrate how algae growth rate increases as the pipe flow Reynolds number is increased.     

紫外线对给水除藻作用的研究

以武汉东湖水为原水,采用不同的光强和照射时间组合,对原水进行紫外光预处理来加强除藻效果,后续工艺为栅条絮凝、气浮。试验结果表明:2.8mW/cm2照射1.5min可达到83.5%的除藻率,2.5mW/cm2照射6min除藻率为87.6%。光强和时间对藻的抑制存在交互作用,增大光强可提高除藻率,但当光强小幅增大时,随着照射时间的延长会出现小剂量效应。针对试验期间的原水,2.5mW/cm2照射6min的效果最好。经过紫外线预处理后,水的浊度略有升高,溶解氧减少,pH值降低,CODMn降低。     

聚丙烯/纳米CaCO3复合材料的结晶行为研究

用超重力法制备的纳米CaCO3和PP熔融共混制备了PP/CaCO3复合材料,并对PP/CaCO3复合材料的结晶行为进行了详细研究。差示扫描量热分析表明,纳米CaCO3粒子的加入加快了PP的结晶速率,缩短了半结晶时间,130℃时含15份纳米CaCO3的PP/CaCO3复合材料的半结晶时间比纯PP的减少了8.92 min;结晶度有轻微下降,结晶温度为126.5 ℃时纯PP的结晶度为44.33 %,含15份纳米CaCO3的PP/CaCO3复合材料的结晶度为35.9 %。动力学研究数据表明,等温结晶过程符合Avrami方程,PP/CaCO3的n和k值都大于纯PP的;利用偏光显微镜观察了PP/CaCO3复合材料的结晶形貌及结晶生长过程,纳米CaCO3粒子的加入使球晶数量明显增多,意味着CaCO3起到了结晶成核剂的作用。     

Process Parameter Screening for the Microalga Chlamydomonas asymmetrica in Batch and Turbidostat Cultivations

Algae offer tremendous potential for the production of various high-value products (HVPs). Besides the light quantity and quality, factors such as temperature, medium composition, pH, and aeration are important algae-specific process parameters that influence growth behavior and their chemical composition. Screenings for algae and their HVPs are ongoing, but the available data for their cultivability and suitability as production organisms are limited. Turbidostat processes enable a simple and fast screening for light-specific influences, as constant biomass concentrations will result in constant self-shading. Here, the microalgae Chlamydomonas asymmetrica was cultivated in cylindrical photobioreactor screening modules. A detailed characterization the light and temperature dependency of the organisms' HVPs is provided.     

蓝藻生长时变系统非线性动力学分析及水华预测方法

为解决现有蓝藻生长动力学模型难以有效描述实际水体中蓝藻生长时变系统的非线性动力学特性,导致水华预测准确性不高的问题,构建蓝藻摄食和营养盐循环模型,并考虑水温、光照等主要影响因素随时间变化对蓝藻生长的影响,进一步建立蓝藻生长时变系统非线性动力学模型,对其常值参数采用遗传算法与数值算法结合的方法进行优化率定,对其时变参数采用多元时序方法进行建模预测,根据分岔理论及时变系统理论分析水华暴发行为的非线性动力学机理,实现对蓝藻生长时变系统的水华预测。通过太湖流域监测实例表明,与现有研究相比,引入时变参数的蓝藻生长动力学模型更能反映蓝藻生长时变系统下水华暴发行为的非线性动力学特性,其水华预测结果更为准确。     

基于优化理论的藻类生长模型及在水华预测中的应用

在对水华产生机理分析和研究的基础上,建立了用于藻类生长模型,研究了有约束条件的非线性极小值问题求取最优参数估计值的方法。结合实验数据对模型中涉及的参数,运用多参数同时估计法进行估计并验证。仿真与实验结果表明了该模型能够较好地描述水华从产生到爆发过程的变化规律,为水华的预警及治理提供参考依据。     

景观水体藻类水华影响因素研究进展

随着人民生活水平的日益提高,人们对周围环境的要求也越来越高,而目前由于藻类大量生长引发的水华现象越来越频繁,危害也越趋严重,甚至连一些景观城市河道水体也频有发生,给周围的人民群众的日常生活和身心健康带来了较大的困扰。水华究竟是怎么发生的,哪些因素对它影响比较大,又是如何影响的,是文章所要讲诉的内容。文章分别从物理因素(温度、光照、pH、水文)、化学因素(营养盐)和其它因素(微量元素、生物因素)3个方面系统的对水华的影响因素进行了总结,详细的阐述了每种因素的作用机理以及部分适合藻类生长的最佳条件,同时对一些未来要重点进一步研究的方向进行了探讨与展望。     

Overall crystallization behavior of polypropylene–clay nanocomposites; Effect of clay content and polymer/clay compatibility on the bulk crystallization and spherulitic growth

The effect of clay nanoparticles on the overall crystallization (isothermal crystallization, spherulitic growth, and nonisothermal crystallization) behavior of polypropylene (PP) was studied by means of differential scanning calorimetry and polarized light optical microscopy. In addition, the changes produced by the compatibility between the filler and the matrix were analyzed by using more hydrophobic clays or incorporating PP grafted with maleic anhydride (PP‐g‐MA). Different models were used to predict the relative degree of crystallinity and several parameters were analyzed. A clear nucleating effect of clay nanoparticles was found on the experimental behavior (induction time, half‐crystallization time, and overall crystallization time) and also deducted from the models parameters (Avrami exponent, rate constant, nucleation activity, activation energy). The effect was also related with the matrix/clay compatibility. In addition, the polarized light optical microscopy showed that the number of spherulites increased and their size decreased when clay was incorporated, which is also an indication of the heterogeneous nucleating behavior of such particles. We also noted faster spherulitic growth and increasing K_g (the model parameter). © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

A new hydration kinetics model of composite cementitious materials,part 1: Hydration kinetic model of Portland cement

This work is the first part of an overall research project that aims to model the hydration kinetics of modern composite cementitious materials. The aim of this part is to build a hydration kinetic model to characterize the hydration behavior of cement at all ages. In this paper, the nucleation and growth of CSH and the diffusion of water are described by a modified BNG model and a modified Jander's model. The kinetic parameters are obtained by simulative fitting of the model to the experimental data. The linear functions between the nucleation and growth rates and the W/C ratios are given in this paper. The apparent activation energy of the nucleation rate constant, growth rate constant, and generalized diffusion constant are approximately 36.0, 37.5, and 42.0 kJ/mol, respectively. Finally, the phase compositions of hardened cement paste are calculated according to the kinetic model and the reaction formula of cement.     

A mathematical model of microalgal growth in light-limited chemostat culture

A mathematical model for light-limited growth of a continuous microalgal culture is proposed. Average light intensity inside the photobioreactor was calculated, taking light attenuation by algae into account. The biomass absorption coefficient was determined by means of two new parameters, the pigment-free biomass, Y_B, and the total pigment absorption, Y_p (absorption coefficient normalized to total pigment content). The model reproduced the steady states reached and the dynamic behaviour of the system when the dilution rate was changed.     

光照强度及温度对青草沙水库蓝藻暴发风险的模拟试验

青草沙水源地正成为上海市新兴的重要水源地,其水源安全直接影响地区安全和发展,蓝藻暴发[1-3]是危害其水源安全的重要方面。以长江中下游地区常见的铜绿微囊藻、四尾栅藻作为试验藻种,选择光照强度、温度为影响因子,设计不同因素水平进行室内单因素模拟试验和原水稳态模拟试验,并结合青草沙水库实际水质水文条件进行分析,以寻找该地区蓝藻生长的优势条件。结果表明铜绿微囊藻比四尾栅藻对温度变化更敏感;铜绿微囊藻在低光照下有较高的适应性和较高的比增殖速率,四尾栅藻对强光照更为敏感;在3 300~6 600 lux、24℃的青草沙原水中,铜绿微囊藻倍增周期为20 d左右,对温度变化较光照变化更敏感。应加强青草沙水库库首、滞水区等局部水域水质监测,并据此及时提出预警和防范措施,同时应在夏季预期有持续高温晴天出现时提高预警级别,防范持续日照增长后蓝藻暴发。     

Static and dynamic light scattering studies of the crystallization of poly(ethylene oxide) from dilute solutions

We have monitored in situ the self-seeded crystallization of poly(ethylene oxide) from dilute toluene solutions by dynamic light scattering. In supercooled dilute solution, the radii (R) of the crystals grow linearly with time. the rate constant obtained from the slope of the plot R versus time depends on the temperature and the molecular weight of the polymer, both of which determine the degree of the supercooling. The maximum crystal size obtained from solutions of fixed concentration also depends on the temperature and the polymer molecular weight. It appears that crystal growth is limited because of molecular weight fractionation. Static light scattering from suspensions of stable crystals provides information on the crystal morphology. A comparison of experimental and theoretical particle scattering functions suggests that the crystals form short cylinders and that the crystal growth from the seeds is primarily two-dimensional. Some comparisons to melt crystallization are possible. The preparation and use of tiny seed crystals is critical to the success of these studies.     

Nutrients fixation by algae and limiting factors of algal growth in flooded rice fields under semi-arid Mediterranean conditions: case study in Thessaloniki plain in Greece

The aim of this study is to assess the fixation of the major nutrients C, N, P, Ca, Mg, K, Na by algal biomass produced in the rice fields of Thessaloniki plain in Greece under semi-arid Mediterranean conditions and to evaluate the limiting factors for their growth. Measurements were performed in experimental rice-field following the regional conventional practices (C–H treatment): (a) direct sowing, (b) continuous flooding with few intermissions, (c) use of nitrogen fertilizers at 176 kg N ha^?1, and (d) application of herbicides (active ingredients benzofenap and clomazone). Herbicides were not applied in a small part of the field isolated by bunds (C–NH treatment) in order to assess possible limiting effects of herbicides on algae growth. Climatic data, measurements of rice crop characteristics and water quality of the ponded water in the C–H part of the experimental field were also obtained in order to assess the limiting effects of light, temperature and nutrients based on a modelling approach. Green algae were found to be dominant in the specific system. Considering the two treatments, the results showed that herbicides did not affect algae growth probably due to the short period of exposure followed by the continuous flooding. Nutrients fixation by algae for C–H followed the order C (52.1 %) > Ca (5.6 %) > K (3.5 %) > N (2.4 %) > Mg (0.3 %) ≈ Na (0.3 %) > P (0.24 %) with a final dry biomass production at 1,118 kg ha^?1. Based on the measurements and model simulations the most limiting factors under the regional conventional practices of rice cultivation were the temperature at the initial and final stage of rice growing season, the light when the leaf area index of rice was >2 and phosphorus concentration in the ponded water. The mean algae growth rate during the flooding period was estimated at 8.2 kg ha^?1 day^?1, while the maximum rate was estimated at 15.9 kg ha^?1 day^?1 at the initial growth stages of rice before the beginning of intense light limitations from rice crop coverage.     

Growth of zinc dendrites in acidic zinc chloride solutions

The heights and widths of zinc dendrites were measured in pure zinc chloride solutions in the concentration and pH range prevailing in zinc-halogen load leveling batteries.At constant overpotential the height increases linearly with time, indicating a constant growth rate. The higher the concentration and the overpotential, the faster is the growth. The critical overpotential for initiation of dendrite growth is 14 mV independent on the concentration. The growth rate increases when the pH decreases. The influence of hydrogen gas evolved at low pH on the growth rate is discussed. Propagation rate is discussed in terms of an extended Barton-Bockris model. When the height of the dendrite is smaller than the diffusion layer thickness, mass transport to the tip is linear and the dendrite propagates in an exponential manner. When the height is larger than the diffusion layer, mass transport to the tip is spherical and the propagation is linear with time. At constant overpotential the widht is a linear function of the square root of the height, indicating that the dendrite has a shape which is preserved during the growth and the tip radius is maintained constant as predicted by the theory.     

Relaxation of stresses in crazes at crack tips and rate of craze extension

The Barenblatt theory of cohesive stresses at crack tips is used to investigate the effect of the relaxation of craze stresses at crack tips on the rate of craze extension. The craze stresses are equated to the cohesive stresses of the Barenblatt theory. The cancellation by the cohesive/craze stress of the singularity that would exist at the crack tip in their absence is assumed to hold for an extending craze. With this assumption, relaxation of the craze stresses produces craze extension, an effect which has been called ‘relaxation controlled growth’ by Williams and Marshall. A general equation relating the rate of change of craze length to the rate of change of stress intensity factor (K₁) and the rate of change of the craze stress is derived. It is argued from this equation that uniform crack growth with a constant craze length can occur only at constant K₁. Using plausibility arguments for the behaviour of the craze stress with time and position in the craze, and assuming a generalized Dugdale model, differential equations for the rate of craze extension with no crack growth are derived for the constant load and constant K₁ cases. These equations relate the rate of change of craze length to the craze stress at the tip of the crack. Assuming a specific form for the time dependence of this stress, the equation for the constant K₁ case is solved to yield an expression for the craze length as a function of time.     

硫酸铜灭藻影响因素的试验研究

目的通过对硫酸铜灭藻影响因素的试验研究,确定出硫酸铜灭藻的最佳工况条件.方法以叶绿素a法作为检测方法,通过烧杯试验,确定处理含藻水的最佳硫酸铜投加量、接触反应时间、pH等参数.结果硫酸铜最佳投加量为1.5 mg/L;硫酸铜适宜的接触反应时间为8～12 h;pH值6.6左右时,硫酸铜藻类去除率较高,pH值＞9时,藻类去除率显著降低.结论 pH值对硫酸铜灭藻效果的影响较大;温度对硫酸铜灭藻有一定的影响;水中还原物质浓度对硫酸铜灭藻影响较小;藻类浓度不同对硫酸铜灭藻效果有一定的影响.     

Photoelectrochemical kinetics of Eosin Y-sensitized zinc oxide films investigated by scanning electrochemical microscopy under illumination with different LED

The overall efficiency of the light-induced charge separation in dye-sensitized solar cells depends on the kinetic competition between back electron transfer and dye regeneration processes by a redox electrolyte. In a previous study, the reduction of the intermittently formed photo-oxidized dye molecules by iodide ions in the electrolyte phase was investigated using the feedback mode of a scanning electrochemical microscope (SECM) and a quantitative model had been derived. Here we provide a more thorough experimental verification of this model by variation of the excitation wavelength, light intensities and mediator concentrations. Nanoporous ZnO/Eosin Y films prepared by self-assembly were used as model electrodes and were used with an iodide/triiodide electrolyte. The experimentally found effective rate constants could be related to the rate constant for the reaction of the dissolved donor with photo-oxidized Eosin Y bound to ZnO and the absorption spectrum of the dye and confirmed the assumption made in the derivation of the model. For the regeneration process of Eosin Y, a rate constant of k_ox with different light emitting diodes and light intensities is determined.     

温度、营养和光强对发状念珠藻生长的影响

研究了温度、营养和光强对发状念珠藻(Nostoc flagelliforme Born. et Flah.)生长的影响. 结果表明,温度、营养和光强的变化影响其生长速率和细胞的生化组成. 藻细胞在25℃温度与45 μmol/(m2×s)光强下培养分别得到最大比生长速率;而N, P和Ca2+缺乏显著降低细胞的比生长速率,微量元素As和K缺乏对细胞生长速率没有明显影响. 随着培养温度的增加,胞外多糖含量逐渐下降;而不同光强对胞外多糖含量也具有显著影响,高光强和低光强下的多糖含量均高,而15~45 mmol/(m2×s)光强下的胞外多糖含量仅是高光强和低光强实验组的10%~20%左右;营养成份的缺失不利于多糖的合成,缺Ca等对胞外多糖的分泌影响较大. 适宜的生长温度(25℃)下蛋白质含量高于其它温度;高光强[90~180 μmol/(m2×s)]下的蛋白质含量显著高于其它光强;营养成份缺失的实验组蛋白质含量均比未缺失的对照组(BG11)低.     

Mixed-mode fatigue behavior of degraded toughened epoxy adhesive joints

Open-faced asymmetric double cantilever beam (ADCB) specimens of toughened epoxy-aluminum adhesive joints were aged either in a constant humidity environment or a cyclically changing environment to study the mixed-mode fatigue behavior. Under constant humidity environments, the fatigue threshold strain energy release rate initially decreased with aging time until it reached a constant minimum value for long times. In contrast, the crack growth rates continued to increase with aging time. It is hypothesized that at crack growth rates close to threshold the fatigue behavior is governed by the epoxy matrix, whereas at relatively high crack growth rates the fatigue behavior is governed by the loss of the rubber toughening mechanism. Increasing the aging temperature accelerated the degradation of the joints leading to a reduction in the time to reach the constant minimum value and increased the crack growth rates.Under a cyclic aging environment with intermittent salt spray, neither the threshold strain energy release rate nor the crack growth rates degraded until four weeks of aging. The superior fatigue performance of these joints compared to joints aged in constant humidity environments was due to the lower water concentrations in the adhesive while aging. This conclusion was supported by moisture uptake measurements of the adhesive in deionised and salt water environments that showed simple Fickian behavior at room temperature and dual-Fickian behavior at higher temperature. The salt water environment produced osmotic pressure that decreased the moisture concentration in the second stage of diffusion.