Modeling of a cryogenic liquid pool boiling by CFD simulation

A boiling model is developed by Computational Fluid Dynamics (CFD) code to calculate the source term of a cryogenic liquid spill. The model includes the effect of the changing ground temperature on the vaporization rate of the cryogenic liquid. Simulations are performed for liquid nitrogen. The model can describe different boiling regimes (film, transition and nucleate). The heat flux calculated for each boiling regimes are compared to the experimental data from literature. The developed numerical model seems to have a good ability to predict the heat flux for the film boiling stage. Model development is still necessary to improve the prediction of the nucleate boiling regime. Overall, the approach shows very promising results to model the complex physical phenomena involved in in the vaporization of cryogenic liquid pool spilled on ground.     

基于CFD的两种紫外消毒模型的比较研究

为了预测紫外反应器的消毒效率,提高模拟的准确度,对两种紫外光强模型进行了比较分析。利用CFD技术,采用数值模拟的方法,对线源和柱源两种光强模型进行了光强分布以及消毒效率的模拟,并采用生物实验进行了验证。结果表明:与柱源模型相比,线源模型在近灯管处强度高42.3%,在1.2 cm处二者趋于一致;针对同一管式紫外反应器,透光率为95%时,线源模型的模拟结果偏大,柱源结果偏小;透光率为80%时,线源及柱源模型的模拟结果都偏大,流量及模型影响较小,透光率成为主要影响因素。综合比较,柱源模型适用于小型管式紫外灯消毒数值模型的开发。     

旋风分离器内高速旋转流场的数值计算方法选择

数值计算方法已成为研究旋风分离器内部流场的重要途径。利用计算流体力学方法对旋风分离器内部气相流场进行了数值模拟研究。分析了不同湍流模型、离散化方式和压力插值方式对旋风分离器内部计算流场的影响,并将所得数值结果与已知的实验数据进行了对比,以期得到最适合的数值模型。结果表明:对于旋风分离器内部复杂流场的流动,运用雷诺应力模型(Low-Re Stress-Omega)能够较好地预测出强制涡中心涡流的运动情况,同时采用SIMPLEC算法、二阶迎风离散化方式和PRESTO压力梯度插补格式能够获得最好的预报结果。     

高速土工离心机温升的数值模拟

目的 数值计算高速土工离心机机室温度分布,研究不同真空度和侧壁温度对机室温升的影响,并为高速土工离心机提出温控方案。方法 针对一款在建的高速土工离心机,采用SRF方法和RNG k-ε湍流模型对其进行数值计算,对比不同转速下离心机的温升。针对加速度为1 500g的运行工况,对比不同机室压力和侧壁温度对机室温升的影响。结果 高速土工离心机以1 500g加速度运行时,机室最高温度可达83 ℃。运行压力从100 kPa降至3 kPa,机室最高温度下降约15 ℃,侧壁温度每降低10 ℃,机室最高温度降低约5 ℃。另外,真空度配合侧壁冷却无法满足散热要求时,可考虑在机室顶部靠侧壁布置面积不小于顶部面积1/4的冷却环。结论 利用CFD数值计算方法,定量得到了高速土工离心机机室温度随转速、真空度和侧壁温度的变化,为其冷却方案的设计提供了参考。     

基于WRF-CFD耦合的复杂城市形态街道通风性能模拟研究

目的研究静稳天气下复杂城市形态街道的通风性能。方法用WRF中尺度气象模式耦合CFD流体模型,通过选取冬季一次静稳天气过程,对南京市不同城市形态的街区通风性能进行分析,评价不同形态街区的通风性能,给出现有城市通风性能的状态,并对城市规划提供合理的意见。结果静稳天气条件下,人口密集居住的住宅区,通风性能严重不足,大部分街区风速只有0.3m/s,不利于居民的健康生活。局部区域的通风性能主要由街道的交叉口位置和街道与主导风向的位置关系决定,此次静稳天气综合型切片的中心区域通风性能明显优于中心区和住宅区。在迎风向的街谷或道路的通风性能明显,在静稳天气下能够达到2～3m/s,很好地起到了城市通风走廊的作用。结论合理的城市设计和建筑布局能够有效地增加局部区域,尤其是高密度地区的通风性能,有利于污染物等的扩散以及居民的健康。     

Numerical simulation study on air quality in aircraft cabins

Air pollution is one of the main factors that affect the air quality in aircraft cabins, and the use of different air supply modes could influence the distribution of air pollutants in cabins. Based on the traditional ceiling air supply mode used on the B737NG, this study investigated another 3 different kinds of air supply modes for comparison: luggage rack air supply mode, joint mode combining ceiling and luggage rack air supply, and joint mode combining ceiling and individual air supply. Under the above 4 air supply modes, the air velocity, temperature and distribution of air pollutants in a cabin full of passengers were studied using computational fluid dynamics (CFD), and carbon dioxide (CO₂) and formaldehyde were selected as 2 kinds of representative air pollutants. The simulation results show that the joint mode combining ceiling and individual air supply can create a more uniform distribution of air velocity and temperature, has a better effect on the removal of CO₂ and formaldehyde, and can provide better air quality in cabins than the other 3 modes.     

CFD-based simulation of dense gas dispersion in presence of obstacles

Quantification of spatial and temporal concentration profiles of vapor clouds resulting from accidental loss of containment of toxic and/or flammable substances is of great importance as correct prediction of spatial and temporal profiles can not only help in designing mitigation/prevention equipment such as gas detection alarms and shutdown procedures but also help decide on modifications that may help prevent any escalation of the event.The most commonly used models - SLAB (Ermak, 1990), HEGADAS (Colenbrander, 1980), DEGADIS (Spicer & Havens, 1989), HGSYSTEM (Witlox & McFarlane, 1994), PHAST (DNV, 2007), ALOHA (EPA & NOAA, 2007), SCIPUFF (Sykes, Parker, Henn, & Chowdhury, 2007), TRACE (SAFER Systems, 2009), etc. - for simulation of dense gas dispersion consider the dispersion over a flat featureless plain and are unable to consider the effect of presence of obstacles in the path of dispersing medium. In this context, computational fluid dynamics (CFD) has been recognized as a potent tool for realistic estimation of consequence of accidental loss of containment because of its ability to take into account the effect of complex terrain and obstacles present in the path of dispersing fluid.The key to a successful application of CFD in dispersion simulation lies in the accuracy with which the effect of turbulence generated due to the presence of obstacles is assessed. Hence a correct choice of the most appropriate turbulence model is crucial to a successful implementation of CFD in the modeling and simulation of dispersion of toxic and/or flammable substances.In this paper an attempt has been made to employ CFD in the assessment of heavy gas dispersion in presence of obstacles. For this purpose several turbulence models were studied for simulating the experiments conducted earlier by Health and Safety Executive, (HSE) U.K. at Thorney Island, USA (Lees, 2005). From the various experiments done at that time, the findings of Trial 26 have been used by us to see which turbulence model enables the best fit of the CFD simulation with the actual findings. It is found that the realizable k-? model was the most apt and enabled the closest prediction of the actual findings in terms of spatial and temporal concentration profiles. It was also able to capture the phenomenon of gravity slumping associated with dense gas dispersion.     

Evaluating the potential for overpressures from the ignition of an LNG vapor cloud during offloading

Ignition of natural gas (composed primarily of methane) is generally not considered to pose explosion hazards when in unconfined and low- or medium-congested areas, as most of the areas within LNG regasification facilities can typically be classified. However, as the degrees of confinement and/or congestion increase, the potential exists for the ignition of a methane cloud to result in damaging overpressures (as demonstrated by the recurring residential explosions due to natural gas leaks). Therefore, it is prudent to examine a proposed facility’s design to identify areas where vapor cloud explosions (VCEs) may cause damage, particularly if the damage may extend off site.An area of potential interest for VCEs is the dock, while an LNG carrier is being offloaded: the vessel hull provides one degree of confinement and the shoreline may provide another; some degree of congestion is provided by the dock and associated equipment.In this paper, the computational fluid dynamics (CFD) software FLACS is used to evaluate the consequences of the ignition of a flammable vapor cloud from an LNG spill during the LNG carrier offloading process. The simulations will demonstrate different approaches that can be taken to evaluate a vapor cloud explosion scenario in a partially confined and partially congested geometry.     

NO70燃烧系统在燃煤锅炉脱硝中的应用

介绍了NO70燃烧系统的特点以及计算机流体力学模拟原理。分析了使用NO70燃烧系统技术对旺隆电厂的2×100MW四角切圆燃煤锅炉进行改造,有效降低氮氧化物排放量并提高机组效率的案例。     

3D model for a secondary facultative pond

This paper describes a comprehensive model of wastewater treatment in secondary facultative ponds, which combines 3D hydrodynamics with a mechanistic water quality model. The hydrodynamics are based on the Navier-Stokes equation for incompressible fluids under shallow water and Boussinesq assumptions capturing the flow dynamics along length, breadth and depth of the pond. The water quality sub model is based on the Activated Sludge Model (ASM) concept, describing COD and nutrient removal as function of bacterial growth following Monod kinetics, except for Escherichia coli removal, which was modelled as first order decay. The model was implemented in the Delft3D software and was used to evaluate the effect of wind and the addition of baffles on the water flow pattern, temperature profiles in the pond and treatment efficiency. In contrast to earlier models reported in the literature, our simulation results did not show any significant improvement in COD removal (based on the ASM concept) with addition of baffles or under intermittent wind-induced mixing. However, E. coli removal efficiency, based on a first order decay approach, showed a fair improvement in the presence of baffles or intermittent wind-induced mixing. Furthermore, simulations with continuous wind effect showed a decrease in removal efficiency for COD but a further increase in E. coli removal efficiency. Such contrasting results for two different approaches in modelling could indicate that the first order decay concept might not be appropriate to describe all the interactions between biochemical processes in a pond. However, these interpretations remain theoretical, as the model needs validation with field data.