On the reoxygenation efficiency of diffused air aeration

One technique used to increase the dissolved oxygen concentration of polluted waters is the bubbling of air through a diffuser pipe located at depth, thereby producing a bubble curtain from which oxygen transfer to the water occurs.

The results of laboratory studies on the aeration efficiency of a diffuser placed along the entire width of a flume, perpendicular to a cross flow are presented (two dimensional aeration). Parameters investigated include (1) diffuser type-porous materials with mean pore sizes of 40, 90 and 180 μm and perforated pipes with 0.4, 0.6 and 1.0 mm diameter ports (2) air flow rate per unit width 3–53 m³ (m h)⁻¹ and (3) cross flow velocity (2.5–15 cm s⁻¹. The effect of variation from the two dimensional situation is also discussed as well as the consequence of using oxygen instead of air, and the sensitivity to discharge angle, port spacing and the free surface. The measured efficiencies are compared with theory as well as available laboratory and field data.

The major conclusions are (1) aeration efficiencies using diffused air aeration are on the order of 2–13%m⁻¹ (2) the aeration efficiency increases with increasing cross flow velocity and decreasing air flow rate per unit width (3) aeration efficiencies using porous filters, for air flow rates less than 40 m³ (m h)⁻¹, are significantly higher (a factor of 2–3) than those achieved using perforated pipes (4) changing the pore size from 40 to 180 μm, the port size from 0.4 to 1.0 mm or the port spacing does not significantly effect the aeration efficiency (5) aeration using oxygen is somewhat less efficient than that using air. However, since equivalent oxygen bubbles contain approximately five times more oxygen than air bubbles, more oxygen is transferred on an absolute basis at the same gas flow rate using compressed oxygen as opposed to air (6) aeration efficiency resulting from aeration over a portion of the entire width can be reasonably predicted using the results of the two dimensional studies and (7) the available laboratory and field data compare well with the results of these laboratory studies.     

Investigation of air quality,comfort parameters and effectiveness for two floor-level air supply systems in classrooms

The method of distributing the outdoor air in classrooms has a major impact on indoor air quality and thermal comfort of pupils. In a previous study, ([11] Karimipanah T, Sandberg M, Awbi HB. A comparative study of different air distribution systems in a classroom. In: Proceedings of Roomvent 2000, vol. II, Reading, UK, 2000. p. 1013–18; [13] Karimipanah T, Sandberg M, Awbi HB, Blomqvist C. Effectiveness of confluent jets ventilation system for classrooms. In: Idoor Air 2005, Beijing, China, 2005 (to be presented).) presented results for four and two types of air distribution systems tested in a purpose built classroom with simulated occupancy as well as computational fluid dynamics (CFD) modelling.     

Effects of impurities on oxygen transfer rates in diffused aeration systems

A series of unsteady-state reaeration tests were performed in a 500-L tank at 0.81-4.58 m3/h diffused-air flow rate and 288-302 K water temperature. Three different types of impurities: soybean oil, surfactant, and diatomaceous earth were doped to simulate the impurities in wastewaters and the effects of the impurities on the oxygen transfer rate were investigated. The ASCE and the two-zone oxygen mass-transfer models were used to analyze the unsteady-state reaeration data and the volumetric mass-transfer coefficients determined from the unsteady-state reaeration data were correlated as a function of the diffused-air flow rate, water temperature, and impurity concentration. The results showed that the alpha factors based on the ASCE model are less sensitive to the impurity concentration while the presence of the impurities significantly reduces the alpha factors in the gas bubble zone. The saturation DO concentration and volumetric oxygen mass-transfer rate can be predicted by the two-zone model along with the correlation obtained in this study.     

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.     

Influence of air supply parameters on indoor air diffusion

This paper presents the field distributions of air velocity, temperature, contaminant concentration, and thermal comfort in an office with displacement ventilation for different air supply parameters such as the effective area, shape, and dimension of the diffuser and the turbulence intensity, flow rate, and temperature of the air supplied. The research is conducted numerically by using an airflow computer program based on a low-Reynolds-number k-ε model of turbulence. It can be concluded that the effective area, shape, and dimension of the diffuser and the turbulence intensity of the air supplied have little effect on the room air diffusion except at floor level. The influence of the flow rate and temperature of the air supplied is very significant on the air diffusion as well as on the thermal comfort and indoor air quality.     

VRV空调新风系统设计

介绍了VRV空调常见的新风处理方式。结合h-d图,分析了不同的新风处理方式对应的气流组织形式,指出选择新风处理设备时应使新风处理过程的热湿比与实际要求一致。     

工位空调送风气流微环境评价

模拟了三种送风速度下送风气流在人体活动区形成的速度场、温度场及PMV的分布，结果表明人员活动区存在送风不均匀和温度分层现象，有助于解决吹风感；PMV值在 1和-1之间，可以满足人体热舒适要求。     

变风量空调系统的建模与控制

介绍了变风量空调系统的基本原理。结合变风量空调系统的特点,在基于被控房间数学模型的基础上,将模糊控制和常规PID控制相结合,提出了一种模糊PID控制方法并将其应用于变风量空调室温控制中。     

变风量空调系统的新风

介绍了ASHARE标准关于变风量空调系统中新风量确定的方法，常用的新风量测量方法和控制方法以及新风的利用，探讨了测试中发现的新风量确定和新风品质等问题。     

家用空调系统概念

阐述了住宅环境的空气调节要求，以及房间空调器、单元式空调机和户式集中空调机组的局限性，指出这些设备的基本功能均不能够全面满足空气调节的要求，必须以系统方式解决。提出了家用空调系统概念，在GB50019基础上，以GB50096特殊要求等予以限定，从而形成家用空调系统的要求。     

空调系统中制冷剂泄漏浅析

通过对不同制冷剂充灌量下单元式空调机运行情况的分析表明,制冷剂泄漏对机组运行具有危害性,需要加低压保护装置.     

变风量空调系统应用探讨

本文主要介绍ＶＡＶ系统在香港地区的应用，并谈及该系统在内地的使用情况，认为只要技术水平进一步提高，ＶＡＶ系统生产成本进一步降低，ＶＡＶ系统会在国内得到普遍应用。     

基于全球典型城市全年气象参数的数据中心空调系统节能性分析

数据中心空调系统能耗与室外气象参数紧密相关,全球不同区域的数据中心空调系统能耗有很大差异.结合全球四大典型城市(纽约、伦敦、北京、上海)的气候特点,总结了数据中心常用水冷式冷水系统的优化措施,分析了优化后国内外数据中心空调系统的节能性.     

空调系统空气冷却器和流量调节阀的选配策略

分析了空气冷却器和三通调节阀的阻力特性,阐述了温度精确控制工程中的空调设备选用原则,实验验证了空调系统动态调节时三通调节阀的调节稳定性和系统稳定的负荷输出特性。     

下送风空调气流组织设计方法

介绍了下送风气流组织中影响人体热舒适的四个主要因素：送风口型式，送风口到人体的距离，送风速度，送风温度。按因素重要性和最佳舒适性要求，提出了两种气流组织设计方法。     

变风量空调系统发展状况

介绍了国外变风量(VAV)系统的研究和应用现状。分析了国内VAV系统研究和应用中存在的问题,指出了其发展方向。总结了变风量空调系统的发展趋势和技术关键。     

多区域变风量空调系统的新风控制策略

研究了多区域变风量空调系统的实时新风控制。概括了5种控制策略,分述了各自的实施方法,给出了各种控制方法下的新风比计算式,并分析比较了它们的控制效果。     

冬季地板送风参数的研究

结合某工程,采用送风区域内的竖向温度分布代替工作区的温度分布,根据PMV-PPD指标和PD指标要求,得到满足人体热舒适性要求的冬季地板送风速度和送风温差;并计算了该系统的通风效率,结果显示优于传统的混合通风方式。