Impact of bubble and free surface oxygen transfer on diffused aeration systems

The primary location of oxygen transfer in a diffused aeration system is examined by separately determining the surface air-water and bubble-water mass transfer coefficients. The mass transfer model developed to determine the mass transfer coefficients advances the McWhirter and Hutter (A.I.Ch.E. J. 35(9) (1989) 1527) model by tracking oxygen and nitrogen transfer into and out of the bubbles as they rise to the water surface. The resulting vertical profiles of the liquid-phase equilibrium concentration inside the bubble and the gas-phase oxygen composition give insight into how the bubble-water concentration gradient changes over depth. The surface mass transfer coefficient, k(Ls)a(s), is 59-85% of the bubble mass transfer coefficient, k(L)a(b), and the driving concentration difference is smaller for surface transfer. Surface transfer and bubble transfer both contribute significantly to oxygen transfer; however, bubble transfer is the primary mode of oxygen transfer for this system at the air flow rates used. Further experiments demonstrate that most of the surface transfer occurs above the bubble plume.     

Membrane properties change in fine-pore aeration diffusers: full-scale variations of transfer efficiency and headloss

Fine-pore diffusers are the most common aeration system in municipal wastewater treatment. Punched polymeric membranes are often used in fine-pore aeration due to their advantageous initial performance. These membranes are subject to fouling and scaling, resulting in increased headloss and reduced oxygen transfer efficiency, both contributing to increased plant energy costs. This paper describes and discusses the change in material properties for polymeric fine-pore diffusers, comparing new and used membranes. Three different diffuser technologies were tested and sample diffusers from two wastewater treatment facilities were analysed. The polymeric membranes analysed in this paper were composed of ethylene-propylene-diene monomer (EPDM), polyurethane, and silicon. Transfer efficiency is usually lower with longer times in operation, as older, dilated orifices produce larger bubbles, which are unfavourable to mass transfer. At the same time, headloss increases with time in operation, since membranes increase in rigidity and hardness, and fouling and scaling phenomena occur at the orifice opening. Change in polymer properties and laboratory test results correlate with the decrease in oxygen transfer efficiency.     

Effects of plume spacing and flowrate on destratification efficiency of air diffusers

This study adopts techniques of computational fluid dynamics (CFD) to analyze the combined effect of adjacent plumes of an air-diffuser system on its destratification efficiency. Lab experiments were carried out to calibrate and verify the CFD models in thermally stratified freshwater. The CFD simulation and lab experiment results were analyzed to relate destratification efficiency with four non-dimensional variables. The results indicate that destratification number, D(N), has the best relationship that includes air flowrate, stratification frequency, water depth, and bubble slip velocity. Since plume spacing and air flowrate are the major control variables of the system, especially in the field, two charts showing the relationships between destratification efficiency, plume spacing, and destratification number are developed for guiding their control in its design and operation.     

Biomass effects on oxygen transfer in membrane bioreactors

Ten biomass samples from both municipal and industrial pilot and full scale submerged membrane bioreactors (MBRs) with mixed liquor suspended solids concentrations (MLSS) ranging from 7.2 to 30.2g L(-1) were studied at six air-flow rates (0.7, 1.3, 2.3, 3, 4.4 and 6m(3)m(-3)h(-1)). Statistical analyses were applied to identify the relative impacts of the various bulk biomass characteristics on oxygen transfer. Of the biomass characteristics studied, only solids concentration (correlated with viscosity), the carbohydrate fraction of the EPS (EPS(c)) and the chemical oxygen demand (COD) concentration of the SMP (SMP(COD)) were found to affect the oxygen transfer parameters k(L)a(20) (the oxygen transfer coefficient) and alpha-factor. The relative influence on k(L)a(20) was MLSS>aeration>EPS(c)>SMP(COD) and on alpha-factor was MLSS>SMP(COD)>EPS(c)>aeration. Both k(L)a(20) and alpha-factor increased with increasing aeration and EPS(c) and decreased with increasing MLSS and SMP(COD). MLSS was found to be the main parameter controlling the oxygen transfer.     

水冷式表冷器传热研究（4）：表冷器总传热系数的一个新实验公式

认为目前使用的表冷器总体传热系数实验公式中未考虑管壁及助片的导热,故利用接触因数和表冷器外表面换热系数的关系,提出了一个新的实验公式,对该公司进行了实验验证,拟合精度较高。     

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.     

采用吸湿剂处理湿空气的流程优化分析

分析了吸湿剂与空气直接接触的热湿传递过程,得到了溶液与空气热湿传递过程中传热、传湿阻力表达式,比较了溶液-空气不同进口状态的传热、传湿不匹配系数.在溶液进口状态等浓度线上进行的热湿传递过程传热、传湿不匹配系数最小,而等焓线上发生的过程的传热、传湿不匹配系数较大.在构建溶液与空气的热湿处理流程中,应尽可能使除湿与再生过程贴近溶液等浓度线进行.固体吸湿剂与溶液之间的性质差异使得固体吸湿剂与空气间的热湿传递过程一般沿着等焓线发生.     

低气压条件下电加热器自然对流换热性能测试

从理论上分析了表面传热系数与气压的关系,实验测试了电加热器在低压与常压下的换热性能,得出以下结论:低压与常压下表面传热系数的比值与气压比值的0.2次幂成正比;电加热器在低压与常压下散热功率基本相等;表面温度随气压的降低而升高。     

Water surface contacting cover system--the basic study for improving the oxygen transfer coefficient and the BOD removal capacity

In order to construct an environmental aeration basin in a sewage treatment plant, the suggested novel aeration basin was examined for the oxygen transfer coefficient by using tap water on a lab-scale and pilot-scale. We called it water surface contacting cover system (WSCCS) that was installed with a cover plate on the wafer surface and a slant plate in aeration basin, compared to a standard aeration basin (SRAS, spiral roll aeration system). Finally, the novel basin could be compared by adopting the slant and cover plates into the actual sewage treatment plant (W x L x D (m)=5.0 x 2.65 x 4.25, treatment capacity: 600 m3/d). As a result, only by adding a slant plate, the oxygen transfer coefficient (K(L)a) increased approximately by 15% more than that with a SRAS basin. And the slope of slant plate was optimized at 10 degrees . Additionally, in the WSCCS, the increasing efficiency of the oxygen transfer coefficient, affected only by the cover, was proved at about 25% on the lab-scale aeration basin and about 20% on the pilot plant. After all, the increasing effect of the oxygen transfer coefficient on WSCCS that was installed with the water contacting cover and the slant plate on standard aeration basin was proved to be above 35%. The distance between the air diffuser and the slant plate was 20-30 cm in the pilot-scale plant. As an effective factor for increasing the oxygen transfer rate, the width/depth ratio of aeration basin was proved to be 2. Through the short-term results of actual sewage treatment plant, it was known that BOD removal efficiency could be improved by about 3% by adopting tile WSCCS not but SRAS.     

湿工况下表冷器总传热系数实验公式的改进

指出了湿工况下水冷式表冷器总传热系数的常用实验公式中因析湿系数未考虑风速的影响且其指数形式不完备而造成的较大误差。使析湿系数的指数形式完备,误差可明显减小。风速对析湿系数的影响可以直接加到总传热系数的公式中,当风速变化较大时,总传热系数的误差仍能保持在很小的范围内。     

Experimental investigation on heat transfer characteristics of heat exchanger with bubble fin assistance

ABSTRACT

Heat transfer area per unit volume (m²/m³) of heat exchangers decides the size of the heat exchangers, over the period of years heat exchangers have undergone numerous development in this aspect. One such attractive design is plate heat exchanger which is very compact and has high thermal effectiveness, whereas the flow nature of fluid through this type of heat exchangers is complex. The most common type of plate heat exchanger is chevron type which suffers from large pressure drop (Δp) at higher heat transfer rates, to overcome this problem bubble finned heat exchangers are designed. In this paper the performance of bubble finned heat exchanger is studied with single phase fluid flow condition. The comparative study of finned and no fin configuration shows that the former has 1.8 times higher rate of heat transfer at lower value of Reynold’s number, further the flow remains laminar hence the Δp is minimum.     

三维高效强化传热管传热系数的试验研究(1):蒸发强化传热

试验测试了一种可用于满液式壳管换热器的管外侧强化沸腾和强化冷凝的双效强化传热管.用热阻分离法计算了使用R134a制冷剂时制冷剂侧和水侧的表面传热系数.试验结果表明,在较大的热流密度(大于20 kW/m2)下该管较传统的强化沸腾传热管具有较大的池沸腾表面传热系数;在较大的热流密度(大于15 kW/m2)下较传统的强化冷凝管具有较大的管外冷凝表面传热系数.     

General expressions for the calculation of air flow and heat transfer rates in tall ventilation cavities

Solar heated tall ventilation cavities including solar chimneys are used to enhance natural ventilation of buildings. A validated CFD model was used to predict the buoyancy-driven air flow and heat transfer rates in vertical ventilation cavities with various combinations of heat distribution on two vertical walls ranging from symmetrical to fully asymmetrical heating. The natural ventilation rate and heat transfer rate have been found to vary with the total heat input, heat distribution on the cavity walls, cavity width and height and inlet opening position. General expressions for these variables have been obtained and presented in non-dimensional terms, Nusselt number, Reynolds number, Rayleigh number and aspect ratio (H/b), as Nu = f(Ra, H/b) and Nu = f(Ra, Re) or Re = f(Ra, Nu), for natural ventilation design.     

光伏构件的传热系数检测

本文首先阐述了建筑围护结构对光伏构件传热性能的要求,然后根据相关标准测试了三种光伏构件,分析影响传热系数的原因,最后分析了三种光伏构件在全国各气候分区的适应性。     

Convection heat transfer in the cone calorimeter

Estimates of the convection heat transfer coefficient (HTC) in the cone calorimeter are obtained from a near-steady-state analysis of the temperature histories of horizontal coated steel plates. Corrections are applied to the derived HTC values to account for changes of the surface emissivity as a function of temperature. Furthermore, estimates are obtained of the additional heat losses through the sides of the sample holder and the insulation layer on which the test sample rests. The derived HTC values are analysed and correlations presented in the form of Nu=f(Ra) as well as h=f(T), where Nu is the plate Nusselt number, Ra is the plate Rayleigh number, h is the HTC and T is the plate temperature.     

Radiative and convective heat transfer coefficients of the human body in natural convection

The purpose of this study was to investigate the convective and radiative heat transfer coefficients of the human body, while focusing on the convective heat transfer area of the human body. Thermal sensors directly measuring the total heat flux and radiative heat flux were employed. The mannequin was placed in seven postures as follows: standing (exposed to the atmosphere, floor contact); chair sitting (exposed to the atmosphere, contact with seat, chair back, and floor); cross-legged sitting (floor contact); legs-out sitting (floor contact); and supine (floor contact). The radiative heat transfer coefficient was determined for each posture, and empirical formulas were proposed for the convective heat transfer coefficient of the entire human body under natural convection, driven by the difference between the air temperature and mean skin temperature corrected using the convective heat transfer area.     

基于传递系数法的滑坡稳定性分析和评价

以甘肃省临夏市某大型滑坡为例,通过分析滑坡的基本特征和形成机制,采用传递系数法对不同工况下滑坡的稳定性进行分析与评价,确定了滑坡在天然状况下处于基本稳定状态,暴雨及地震条件下处于欠稳定～不稳定状态。     

水侧污垢对空调系统的影响

介绍了空调水系统中污垢形成的原因及其对系统的影响,提出了预防污垢的办法。     

浅析PVC塑料型材内腔结构对传热系数的影响

本文通过CAE软件简要分析介绍了PVC型材内腔结构对型材传热系数Uf的影响。     

水膜作用下导热塑料盘管换热性能实验研究

水膜作用下换热盘管的换热性能主要受盘管总传热系数Ktotal的影响。本文将通过理论分析和实验研究比较盘管材料导热系数λ对Ktotal的影响以及管内流体强迫对流Re数、管外喷淋密度Г对三种管材的换热盘管Ktotal的影响,以此分析导热塑料盘管在水膜作用下的换热性能。