Predicting diffused-bubble oxygen transfer rate using the discrete-bubble model

A discrete-bubble model that predicts the rate of oxygen transfer in diffused-bubble systems is evaluated. Key inputs are the applied gas flow rate and the initial bubble size distribution. The model accounts for changes in the volume of individual bubbles due to transfer of oxygen and nitrogen (and hence changing partial pressure), variation in hydrostatic pressure, and changes in temperature. The bubble-rise velocity and mass-transfer coefficient, both known functions of the bubble diameter, are continually adjusted. The model is applied to predict the results of diffused-bubble oxygen transfer tests conducted in a 14-m deep tank at three air flow rates. All of the test data are predicted to within 15%. The range of bubble diameters (0.2-2 mm) spans the region of greatest variation in rise velocity and mass-transfer coefficient. For simplicity, the Sauter-mean diameter is used rather than the full bubble size distribution without loss of accuracy. The model should prove useful in the design and optimization of hypolimnetic oxygenation systems, as well as other diffused-bubble applications.     

Predicting the initial rate of water absorption in clay bricks

The effect of product characteristics and processing conditions on the initial rate of water absorption of 15 clay bricks was investigated and the influence of porosimetric parameters (amount, size and tortuosity of pores) as well as of phase composition (amount of calcium-silicates and amorphous phase) was established. The suction behaviour of bricks, which may be brought back to the models of [Gummerson RJ, Hall C, Hoff WD. The suction rate and the sorptivity of bricks. Brit Ceram Trans J 1981;80:150–2.] and [Hoffmann D, Niesel K. Quantifying capillary rise in columns of porous material. Am Ceram Soc Bull 1988;67(8):1418.], was also evaluated on the basis of both the product microstructure and the liquid physical properties. According to the model of Beltran et al. (1988), which reveled to be sufficiently reliable, the values of the capillary coefficient K_s were calculated and their correlation with the experimental ones has been provided. For a given liquid and in the same experimental conditions, the results indicate that varying in a controlled way the product microstructure (i.e. decreasing the pore size, increasing the pore tortuosity and/or controlling the amorphous/new formed phases ratio) should allow to design materials having a most suitable behaviour.     

Predicting oxygen transfer of fine bubble diffused aeration systems--model issued from dimensional analysis

The standard oxygenation performances of fine bubble diffused aeration systems in clean water, measured in 12 cylindrical tanks (water depth from 2.4 to 6.1m), were analysed using dimensional analysis. A relationship was established to estimate the scale-up factor for oxygen transfer, the transfer number (N(T)) The transfer number, which is written as a function of the oxygen transfer coefficient (k(L)a(20)), the gas superficial velocity (U(G)), the kinematic viscosity of water (nu) and the acceleration due to gravity (g), has the same physical meaning as the specific oxygen transfer efficiency. N(T) only depends on the geometry of the tank/aeration system [the total surface of the perforated membrane (S(p)), the surface of the tank (S) or its diameter (D), the total surface of the zones covered by the diffusers ("aerated area", S(a)) and the submergence of the diffusers (h)]. This analysis allowed to better describe the mass transfer in cylindrical tanks. Within the range of the parameters considered, the oxygen transfer coefficient (k(L)a(20)) is an increasing linear function of the air flow rate. For a given air flow rate and a given tank surface area, k(L)a(20) decreases with the water depth (submergence of the diffusers). For a given water depth, k(L)a(20) increases with the number of diffusers, and, for an equal number of diffusers, with the total area of the zones covered by the diffusers. The latter result evidences the superiority of the total floor coverage over an arrangement whereby the diffusers are placed on separate grids. The specific standard oxygen transfer efficiency is independent of the air flow rate and the water depth, the drop in the k(L)a(20) being offset by the increase of the saturation concentration. For a given tank area, the impact of the total surface of the perforated membrane (S(p)) and of the aerated area (S(a)) is the same as on the oxygen transfer coefficient.     

Oxygen transfer rate estimation in oxidation ditches from clean water measurements

Standard methods for the determination of oxygen transfer rate are based on assumptions that are not valid for oxidation ditches. This paper presents a realistic and simple new method to be used in the estimation of oxygen transfer rate in oxidation ditches from clean water measurements. The new method uses a loop-of-CSTRs model, which can be easily incorporated within control algorithms, for modelling oxidation ditches. Further, this method assumes zero oxygen transfer rates (KLa) in the unaerated CSTRs. Application of a formal estimation procedure to real data revealed that the aeration constant (k = KLaVA, where VA is the volume of the aerated CSTR) can be determined significantly more accurately than KLa and VA. Therefore, the new method estimates k instead of KLa. From application to real data, this method proved to be more accurate than the commonly used Dutch standard method (STORA, 1980).     

李家河水库重力流输水工程的水锤防护研究

关阀水锤的防护是长距离重力流输水工程安全的关键问题。根据管线特性,通过水锤计算的特征线法及计算机编程,对李家河水库重力流输水管线初始设计工况及进一步采取水锤防护措施工况进行了水力过渡计算与分析,并提出了最佳水锤防护方案,以期为工程设计提供参考。     

居住小区给水管道设计流量计算探讨

从理论的角度分析了现行设计秒流量公式和最大小时平均秒流量公式，针对在居住小区设计流量计算中存在的不足，提出了具体的解决方法和计算步骤。     

一种低压中速离心式水雾喷头的开发和实验研究

根据对喷头结构参数的理论计算，设计了一种低压中速离心式水雾喷头，并与目前常见的低压高速水雾喷头进行了灭火实验效果比较，结果表明新设计的水雾喷头能更好抑制和扑灭A类火、B类火及AB类混合火，并节省用水量。     

阶段式变流量系统循环水泵扬程计算探讨

①1　传统计算方法在阶段式变流量系统的设计中 ,各种技术资料对水泵的计算提出了要求 ,当系统出于节电考虑而采用阶段式变流量质调节时 ,循环水泵按以下原则进行选择 :对中小型系统 ,多采用二阶段式变流量 ,2台循环水泵的流量分别为计算值的 10 0 % ,75 % ;扬程分别为计算值的     

基于冷水量变化的冷水机组性能测试与故障诊断

研究了定流量/变温差和变流量/定温差条件下冷水量变化对机组性能的影响。结果表明:大型冷水机组蒸发器侧冷水大范围变流量不会影响系统的稳定性;在一定范围内冷水机组制冷量与冷水流量呈线性变化,系统的性能系数COP基本保持稳定,这为冷水泵的节能运行提供了良好的依据;蒸发器盘管中冷水流量较大时,允许的冷水流速变化范围可以适当增加;冷水量调节的极限速度为10.1%/min,否则会造成冷水机组运行不安全;蒸发器冷水流量变化引起的冷水机组性能变化可作为故障诊断的判断依据。     

变流量水系统调速水泵运行测试分析

调速水泵效率变化的主要原因是管网阻抗变化.对于管网阻抗变化较大的变流量水系统,为了保证各个工况下并联调速水泵的高效运行以及水泵的相互备用,水泵应同型号多台布置.在运行中应采取水泵调速控制和台数控制相结合的方法,且各水泵的转速应相同.     

Application of the gas tracer method for measuring oxygen transfer rates in subsurface flow constructed wetlands

The oxygen transfer rate (OTR) has a significant impact on the design, optimal operation and modelling of constructed wetlands treating wastewater. Oxygen consumption is very fast in wetlands and the OTR cannot be determined using an oxygen mass balance. This problem is circumvented in this study by applying the gas tracer method. Experiments were conducted in an unplanted gravel bed (dimensions L × W × d 125 × 50 × 35 cm filled with a 30-cm layer of 10-11-mm gravel) and a planted horizontal subsurface flow constructed wetland (HSSFCW) (L × W × d 110 × 70 × 38 cm filled with a 30-cm layer of 3.5-mm gravel with Phragmites australis). Tap water saturated with propane as gas tracer (pure or commercial cooking gas, depending on the test) was used. The mass transfer ratio between oxygen and commercial propane gas was quite constant and averaged R = 1.03, which is slightly lower than the value of R = 1.39 that is usually reported for pure propane. The OTR ranged from 0.31 to 5.04 g O₂ m⁻² d⁻¹ in the unplanted gravel bed and from 0.3 to 3.2 g O₂ m⁻² d⁻¹ in the HSSFCW, depending on the hydraulic retention time (HRT). The results of this study suggest that the OTR in HSSFCW is very low for the oxygen demand of standard wastewater and the OTR calculations based on mass balances and theoretical stoichiometric considerations overestimate OTR values by a factor that ranges from 10 to 100. The gas tracer method is a promising tool for determining OTR in constructed wetlands, with commercial gas proving to be a viable low-cost alternative for determining OTR.     

Predicting disinfection by-product formation potential in water

Formation of regulated and non-regulated disinfection by-products (DBPs) is an issue at both potable water and wastewater treatment plants (W/WWTPs). Water samples from W/WWTPs across the USA were collected and DBP formation potentials (DBPFPs) in the presence of free chlorine and chloramine were obtained for trihalomethane (THM), haloacetic acid (HAA), haloacetonitrile (HAN), and N-nitrosodimethylamine (NDMA). With nearly 200 samples covering a range of dissolved organic carbon (0.6-23 mg/L), ultraviolet absorbance (0.01-0.48 cm⁻¹ at 254 nm wavelength), and bromide (0-1.0 mg/L) levels, power function models were developed to predict the carbonaceous DBP (C-DBP) and nitrogenous DBP (N-DBP) precursors spanning 3 orders of magnitudes. The predicted THM and HAA formation potentials fitted well with the measured data (analytical variance of less than 22%). Inclusion of dissolved organic nitrogen (DON) into the HANFP model improved the predictions. NDMAFP was the most difficult one to predict based upon the selected water quality parameters, perhaps suggesting that bulk measurements such as DOC or UVA₂₅₄ were not appropriate for tracking NDMAFP. These are the first such DBPFP models for wastewater systems, and among the few models that consider both C-DBPs and N-DBPs formation potentials from the same water sources.     

水源热泵机组地下水变流量运行特性研究

通过研究满液式水源热泵机组在不同负荷率时的运行情况和地下水水泵变频耗功,得到了地下水地源热泵系统变流量运行特性,并确定了机组部分负荷下,地下水地源热泵系统性能最佳时地下水需求量百分数;提出了具有较高系统综合性能的运行方案.采用地下水变流量技术可节约运行费用和地下水需求量,增加的初投资一年内可回收.     

变流量水系统压差旁通控制系统设计

介绍了压差旁通控制系统的作用、系统组成、作用原理、安装位置以及相关设备的选择,分析了系统存在的问题及其原因,并介绍了确保该控制系统长期良好运转的主要措施。     

胶河调水工程输水隧道地质勘察研究

对胶河调水工程输水隧道的地质条件进行了勘察研究,着重对工程概况、地基承载力、地下水文特征、场地地震效应、隧洞围岩稳定性分析,并针对分析结果提出合理化建议,解决了水资源空间分布不均的现状。     

溶液-湿空气热质交换过程的匹配研究

吸湿性溶液与空气的热质交换过程既有热量的交换又有质量的交换,提高该过程的可逆程度是提高溶液除湿空调系统性能的重要手段。在对溶液的性质进行合理简化的基础上,分析了溶液与空气的热质交换过程的流量比和过程需要吸收或释放的热量,得到在不同工况下,可逆过程所要求的流量比和应该补充或带走的热量。认为这一结论有助于溶液除湿空调流程的设计和相关参数的选取。     

居住小区室内外给水流量衔接点计算人数及其给水保证率的探讨

本文通过推算2009版《建筑给水排水设计规范》(以下简称2009版规范)住宅小区室内外流量衔接点生活给水秒流量的给水保证率,提出简化住宅小区室内外秒流量衔接点分类、统一居住小区室内外给水流量衔接点计算人数的技术方案。     

小流量大温差LiBr溶液换热器传热性能的实验研究

搭建实验台,对小流量大温差条件下溴化锂水溶液在多套管式换热器和钎焊板式换热器中的传热性能进行了研究,并测试了溴化锂水溶液回收的饱和制冷剂水余热.基于实验数据拟合出溴化锂溶液钎焊板式换热器对流换热特征数方程,并与文献中的相应方程进行比较,以验证拟合方程的可靠性.实验结果表明,该方程可用于指导溴化锂溶液在钎焊板式换热器中流动的热力水力特性的理论分析,为换热器优化设计提供依据.     

Model for oxygen transfer in rotating biological contactor

Rotating biological contactor is being widely used for wastewater treatment but there is an apparent lack of knowledge about the rate at which oxygen transfer occurs, in physical and biological system. In this study the transfer of oxygen from air to water by a rotating disc air-liquid contactor in physical system is investigated. The oxygen transfer model suggested by Kim and Molof, Water Sci. Technol. 14 (1982) 569, was modified and the developed model is termed as modified Kim and Molof model. The model was calibrated by using available data in literature and validated by experiments conducted in this study. The effect of significant physical parameters was integrated into a single term and is termed as volume renewal number. The modified Kim and Molof model was compared with the other available models. The coefficient of determination (R(2)) for the modified Kim and Molof model obtained is 0.95 which is much higher than in the other available models. Thereby the model is expected to estimate oxygen transfer more accurately. Further, a simplified linear model between K(L)a and the volume renewal number is proposed. Both modified Kim and Molof and linear model estimate the overall oxygen transfer coefficient (K(L)a) accurately.