The mathematical model for steady magnetohydrodynamic flow between parallel porous plates with an angular velocity

ABSTRACT

The model is developed as the steady magnetohydrodynamic (MHD) flows with an angular velocity between parallel porous plates are considered. The problem is solved analytically by using similarity transformation, whose solution deals with increasing fluid flow with an angular velocity. The applications in MHD are power generators, polymer technology, cooling system, petroleum industry, aerodynamics heating used. The objective of this paper is to analyse the steady MHD flow of viscous fluid with an angular velocity between parallel porous plates when the fluid is being withdrawn through both the walls of the channel at the same rate. The problem is reduced to a third-order linear differential equation which depends on a Suction Reynolds number R and M ₁ for which an exact solution is obtained.     

喷雾降温效果影响因素的数值模拟分析

采用计算流体力学软件(Fluent)对室外空间喷雾降温过程进行了数值模拟,在不同环境风速、喷雾压力、喷嘴流量工况下,模拟模型区域的温度场、速度场、相对湿度分布。环境风速较小时,局部区域降温幅度大,范围窄,温度梯度大;环境风速增大使液滴飘散效果更好,虽然降温幅度减小,但降温区域分布更广,温度分布更均匀。环境风速较小的地区,可以减小单个喷嘴流量,缩短喷嘴布置间距,使喷嘴作用范围内温度分布更加均匀。环境风速较大的地区,宜适当增加单个喷嘴流量并增大下游喷嘴间距,利用环境风速使液滴飘散。此外,在环境风速较大的情况下还可以适当降低喷嘴高度使人员可感知区域的温降增大。在相同环境风速下,喷雾压力对降温范围与降温幅度的影响较小,可适当降低喷雾压力,以提高喷雾系统运行的经济性。     

脉冲袋式除尘器喷吹管内流场的模拟研究

脉冲袋式除尘器清灰时,传统等孔径喷吹管各喷口喷气量的不均匀,将导致沿喷吹管方向滤袋清灰效果不同,清灰效果差。本文针对等孔径喷吹管喷吹过程,利用CFD软件进行非稳态模拟,得到了喷吹充分发展时刻的管内气流速度、静压分布,以及喷吹管各喷口的气体质量流量。最后用修正方法对喷吹管喷口尺寸进行优化设计,提高了喷吹气流均匀性,为脉冲袋式除尘器喷吹系统的改进提供了理论依据。     

考虑分界面的黏性泥石流下游演进数学模型研究

基于分界面理论,以泥石流屈服深度对应的面为分界面,将黏性泥石流分为理想流体和Bingham流体,建立了模拟黏性泥石流演进过程的数学模型和相应计算方法。该模型基于泥石流的运动特征,注重不同层间泥石流流速的差异,并可合理反映泥石流运动速度对其演进形态的影响机制;同时,全面考虑了黏性泥石流在演进过程中所呈现的"舌状体"和"龙头"由上向下翻落现象,因此可更准确描述黏性泥石流在演进过程中的运动状态。采用3组模型试验结果对建立的数学模型与计算方法进行了验证,结果表明:模型计算得出的泥石流泛滥范围和最大堆积厚度与模型试验结果误差在±5%以内,验证了模型的合理性。     

格子Boltzmann方法在斜坡非饱和带土体大孔隙流研究中的应用

大孔隙结构的建立和大孔隙流的渗流模拟能定性和定量的解释大孔隙流的产生机理及其动态变化过程。采用CT扫描获取斜坡非饱和带大孔隙域,基于不考虑外力项的格子Boltzmann方法,通过确定离散速度模型、平衡态分布函数和分布函数的演化方程三大组成部分,其中边界条件与计算假设依据染色示踪试验设定,定性定量化研究二维大孔隙流的渗流过程,研究结果表明:该方法能清晰定量化描述大孔隙流湿润前锋的推进过程,大孔隙效应明显,渗流场流速随着时间推移逐渐趋于稳定,大孔隙效应逐渐减弱;在连通性较好、孔径较大的区域流体流速较快,且在大孔隙中心流速最大,往两侧逐级递减;下层深度处的流体流量和流速往往受控于上一层大孔隙的孔径尺度。格子Boltzmann方法的应用能为大孔隙流特别是三维渗流过程的研究提供新手段。     

空调喷口内部流道型线对射流流场的影响

利用FLUENT 6.1模拟软件对4种不同内部流道型线的喷口的流速分布、喷口轴线流速、喷口出口断面流速、喷口出口断面的总压及静压进行了数值模拟,对喷口内部流道型线对射流流场的影响进行了研究。     

Entrainment and its implications on microclimate ventilation systems: Scaling the velocity and temperature field of a round free jet

Research on microclimate ventilation systems, which mostly involve free jets, points to delivery of better ventilation in breathing zones. While the literature is comprehensive, the influence of contaminant entrainment in jet flows and its implications on the delivery of supplied air is not fully addressed. This paper presents and discusses entrainment characteristics of a jet issued from a round nozzle (0.05 m diameter), in relation to ventilation, by exploring the velocity and temperature fields of the jet flow. The results show a trend suggesting that increasing the Reynold number (Re) reduces ambient entrainment. As shown herein, about 30% concentration of ambient air entrained into the bulk jet flow at Re 2541 while Re 9233 had about 13% and 19% for Re = 6537/12 026 at downstream distance of 8 diameters (40 cm). The study discusses that “moderate to high” Re may be ideal to reduce contaminant entrainment, but this is limited by delivery distance and possibly the risk of occupant discomfort. Incorporating the entrainment mixing factor (the ratio of room contaminants entrained into a jet flow) in performance measurements is proposed, and further studies are recommended to verify results herein and test whether this is general to other nozzle configurations.     

氧-丙烷气割割嘴流动和燃烧的数值模拟

运用CFD模拟软件,就一种适用于丙烷燃烧的新型割嘴,对氧-丙烷割嘴内外氧气与丙烷的流动燃烧进行了冷态模拟分析,结果显示割嘴出口速度场分布均匀。采用概率密度函数燃烧模型进行氧-丙烷燃烧模拟,研究流场和温度场的分布,结果表明此新型割嘴能够达到良好的燃烧效果,能够满足气割要求。     

Computational analysis of reduced-mixing personal ventilation jets

In this paper we develop a detailed computational fluid dynamics (CFD) model of a personal ventilation (PV) setup comprising a PV nozzle, seated thermal manikin and floor diffuser, then use experimental velocity and tracer gas concentration data for the same setup to validate the CFD model. Specifically, we compare CFD results with the experimental results obtained with both a conventional round nozzle and a novel low-mixing co-flow nozzle directing a PV fresh air jet toward the breathing zone (BZ) of a seated thermal manikin in a thermally controlled chamber ventilated also by a floor diffuser behind the manikin. The CFD model shows excellent agreement with the experimental data. We then exercise the CFD model to study the effect of nozzle exit boundary conditions such as turbulence intensity and length scale, flow rate and temperature, and manikin temperature on the air quality in the BZ of the heated manikin. It is shown that the air quality of the novel PV system is sensitive to the nozzle exit turbulence intensity and flow rate, and insensitive to jet temperature within the 20–26 °C range, and to body temperature within a clo range of 0–1. A companion paper presents in detail the experimental set up and results used to validate the CFD model discussed in this paper.     

特大断面黄土隧道静力稳定性分析

用六节点曲边节理单元模拟复合式衬砌中间防水层,采用二维弹塑性静力有限元模拟某黄土隧道的开挖修建过程,分析了该隧道的静力稳定性。计算中用释放荷载法模拟开挖效应,采用Mohr-Coulomb屈服准则及关联流动法则,采用初始刚度迭代法求解平衡方程,对超过屈服点的应力进行调整。文中最后提出了抑制隧道围岩变形的技术措施,给出了隧道衬砌支护参数的建议。     

旋芯雾化喷嘴仿真与实验研究

为研究旋芯雾化喷嘴的喷放特性,通过建模仿真了单孔旋芯雾化喷嘴内流场及外流场,研究分析了内流场的雾化机理及外流场的雾滴分布特性,并结合实际喷放验证了仿真的准确性。研究表明：旋芯喷嘴喷放过程中能大幅度提高液体喷放速度,并在喷口处形成空穴,显著提高雾化效率;该喷嘴在低压下即可实现不大于80 μm 粒径的雾化,且粒径分布均匀。     

Modeling of window-sized solar chimneys for ventilation

The mathematical model for predicting airflow velocity in a solar chimney has been developed through predicting temperature of the absorber, air in flow channel and glass cover. Experimental validation of the model has been done using a solar chimney having less than a 1-m-high absorber. Investigations have been carried out with three different combinations of air gap and size of the inlet opening for entry of air in the chimney. Good agreement between observed and calculated results has been obtained. The small size of the analyzed solar chimney has opened possibilities of utilizing windows as solar chimneys since the flow velocity upto 0.24 m/s has been experimentally recorded.     

Analysis of FDS 6 Simulation Results for Planar Air Curtain Related Flows from Straight Rectangular Ducts

CFD results are discussed for jet flows through a straight square duct, which is an interesting configuration in the context of air curtain flows for smoke and heat control in buildings in case of fire. The CFD package Fire Dynamics Simulator, Version 6.0.1, is used. Special focus is given to the impact of the inlet boundary condition on the flow field in the near-field region. Investigation of different orifice configurations (W = 2 cm width, variable span-wise length), including calculations inside a straight square duct (2 cm × 2 cm, with variable length) ahead of the air orifice, reveals a small vena contracta effect when the orifice is flush with a solid boundary, leading to an acceleration of the flow in the symmetry plane in the near-field region. The vena contracta effect disappears if the co-flow at the nozzle exit is aligned with the jet. More important is the effect of the duct length (precursor domain length, serving as method to generate inflow turbulent conditions for the main computation): imposing a top hat velocity profile, a sufficiently long duct (i.e., L = 20W) is required for the flow to become fully developed at the orifice. The CFD results confirm an analytical correlation for the ratio of the entrance length to the hydraulic diameter of the duct as function of the Reynolds number, provided the duct width is used as characteristic length scale. Using a sufficiently fine mesh, i.e., 10 cells across the characteristic dimension of the nozzle, the evolution of the mean and RMS stream-wise velocity along the centerline, as well as their profiles across the nozzle width, are shown to be captured accurately in the CFD results.     

Design and analysis of Coanda effect nozzle with two independent streams

ABSTRACT

This project aims at presenting a method of making use of the ACHEON Coanda effect based on velocity. The main component of the model is HOMER nozzle. It consists of two inlets, in which streams of different velocity can be made to flow. It is made possible by using the PEACE concept. This project is an analysis work of six different 2D models varying in their geometry. Deflection at various angles is produced. From the base design, five designs are made by varying their domain and they are analysed at different conditions. Higher deflection angle is produced in either direction. The graphs are plotted for pressure and also in terms of velocity. The concept of thrust vectoring is used here. This project helps in creating more flexibility of thrust vectoring nozzle. It overcomes the limitations of traditional systems related to jet deflection exhausts.     

Firefighter Nozzle Reaction

Nozzle reaction and hose tension are analyzed using conservation of fluid momentum and assuming steady, inviscid flow and a flexible hose in frictionless contact with the ground. An expression that is independent of the bend angle is derived for the hose tension. If this tension is exceeded owing to anchor forces, the hose becomes straight. The nozzle reaction is found to equal the jet momentum flow rate, and it does not change when an elbow connects the hose to the nozzle. A forward force must be exerted by a firefighter or another anchor that matches the forward force that the jet would exert on a perpendicular wall. Three reaction expressions are derived, allowing it to be determined in terms of hose diameter, jet diameter, flow rate, and static pressure upstream of the nozzle. The nozzle reaction predictions used by the fire service are 56% to 90% of those obtained here for typical firefighting hand lines. Sharing these findings with the fire protection community can improve the safety of firefighters.     

Hydraulic design aspects for supercritical flow in vortex drop shafts

ABSTRACT

Vortex drop shafts, as special sewer manholes, operate optimally if an adequate energy dissipation is guaranteed and the integrity of the structural components is safeguarded. The results of an experimental study on a vortex drop shaft with supercritical inflow are discussed herein. The hydraulic behaviour of the spiral inlet, the vertical shaft and the dissipation chamber is described. Based on detailed flow observations, useful recommendations for designing these structures are provided. It is demonstrated that a relation adopted for supercritical bend flows provides a reliable estimation of the maximum wave height along the inlet. A procedure for predicting the rotational flow angles and the velocity distribution along vertical shafts with swirling flows is developed. Water levels and pressure measurements in the dissipation chamber are further analysed to identify maximum forces acting on the chamber invert and to derive preliminary design equations.     

地铁用间接蒸发冷却器换热性能影响因素

为解决地铁站冷却塔设置难题,提出了一种采用低速电机驱动旋转布水装置的间接蒸发冷却器,在两种布置方式下,对其换热性能进行了单因素实验,并运用正交实验法对较优布置方式下影响换热器换热的因素进行了分析。结果表明:两种布置方式下,喷嘴与蒸发冷却器的间距、两组换热管束间距均存在最佳值,喷嘴双侧旋转布水优于单侧旋转布水;换热器平行气流布置且喷嘴双侧旋转布水为较优布置方式,此时,换热器换热量随喷水量、转速、空气速度、冷却水进口温度的增加以及喷水温度、空气温度的降低而增大,其中,冷却水进口温度对换热器换热影响最为显著,其他因素对其换热的影响从主到次顺序为:喷水量、空气温度、空气速度、喷水温度、转速、冷却水流量。     

深埋隧洞涌水预测非达西流模型及数值模拟

 由于深部岩体具有高温、高地应力、高地下水压等特点,深埋隧洞开挖必然会带来高水力梯度引起的控水断裂高流速非达西流问题。基于单裂隙高流速非达西流试验结果,建立二次型的非达西流本构模型,及非达西流影响系数与渗透率的关系式。通过构建统一的达西–非达西流本构方程,建立达西–非达西流有限元分析方法,采用共轭梯度迭代法,进行非达西流有限元方程的迭代求解。最后将理论与方法应用于深埋隧洞的涌水预测中。通过对比达西与非达西流结果表明,对于裂隙宽度较大情况,采用非达西流理论计算是十分必要的,且断层(发育溶蚀裂隙)的宽度及多条断层的交叉位置等对隧道涌水量具有较大的影响。     

Backflow air and pressure analysis in emptying a pipeline containing an entrapped air pocket

ABSTRACT

The prediction of the pressure inside the air pocket in water pipelines has been the topic for a lot of research works. Several aspects in this field have been discussed, such as the filling and the emptying procedures. The emptying process can affect the safety and the efficiency of water systems. Current research presents an analysis of the emptying process using experimental and computational results. The phenomenon is simulated using the two-dimensional computational fluid dynamics (2D CFD) and the one-dimensional mathematical (1D) models. A backflow air analysis is also provided based on CFD simulations. The developed models show good ability in the prediction of the sub-atmospheric pressure and the flow velocity in the system. In most of the cases, the 1D and 2D CFD models show similar performance in the prediction of the pressure and the velocity results. The backflow air development can be accurately explained using the CFD model.     

Experimental analysis of the air velocity and contaminant dispersion of human exhalation flows

Human exhalation flow is a potential source of pathogens that can constitute a cross‐infection risk to people in indoor environments. Thus, it is important to investigate the characteristics of this flow, its development, area of influence, and the diffusion of the exhaled contaminants. This paper uses phase‐averaged particle image velocimetry together with a tracer gas (CO₂) to study two different exhalation flows over time: the exhalation of an average male (test M) and an average female (test F), using a life‐sized thermal manikin in a supine position. The exhalation jets generated for both tests are similar in terms of symmetrical geometry, vorticity values, jet opening angles, and velocity and concentration decays. However, there is a difference in the penetration length of the two flows throughout the whole exhalation process. There is also a time difference in reaching maximum velocity between the two tests. It is also possible to see that the tracer gas dispersion depends on the momentum of the jet so the test with the highest velocity decay shows the lowest concentration decay. All these results are of interest to better understand cross‐infection risk.