Numerical study on natural convection in three-dimensional rectangular enclosures

Natural convection in three-dimensional rectangular enclosures has been analyzed numerically using a strongly stabilized QUICK (Quadratic Upstream Interpolation for Convective Kinematics) scheme. Computations are performed for the different cases of temperature boundary conditions to see the effect of temperature disturbance on three-dimensional motion. The effect of the Rayleigh number is mainly investigated and two dimensional approximation limit is examined as well. The results show that the temperature disturbance imposed on the end wall reinforces the axial flow and magnifies the three-dimensional effect.     

Numerical study of natural convection and entropy generation of Cu-water nanofluid around an obstacle in a cavity

In this work, natural convection and entropy generation in a square cavity with an obstacle filled with Cu-water nanofluid is numerically studied. Horizontal walls of the cavity are adiabatic and vertical walls are maintained at a different constant temperature. The study has been done for the Rayleigh numbers between 10³ and 10⁶, the obstacle dimensions (W/L) of 0.1?C0.5 and for base fluid as well as nanofluid. It is found that, using the nanofluid overall leads to increase the flow strength, Nusselt number and entropy generation and decrease the Bejan number especially at high Rayleigh numbers. It is observed that by increasing the obstacle dimensions, the entropy generation increases and the Bejan number decreases, but the effect of the obstacle dimensions on Nusselt number depends on Rayleigh number. For the present thermal system, the increasing Nusselt number compared to increasing entropy generation due to increase obstacle dimensions is significant at low Rayleigh numbers.     

大型液化天然气储罐的发展研究

世界能源日益紧张,天然气在世界能源结构中的比重将赶超石油成为世界第一大能源.到目前为止我国尚无大型液化天然气贮罐国家标准,也无自行设计、建造的大型液化天然气储罐,也未掌握液化天然气核心技术,因此加强对液化天然气储罐的研究对推动我国液化天然气工业的发展,解决日益恶化的环境问题和能源危机都具有十分重要的现实意义.简述了液化天然气的优点,按照不同的分类依据对液化天然气储罐进行了分类,结合国内外液化天然气储罐的建造状况对其进行了分析,并提出了液化天然气储罐大型化的发展趋势,对于液化天然气储罐的研究和工程应用具有一定的参考价值.     

纸带法测定天然气、煤气和液化石油气中的硫化氢

本文描述用GD-101硫化氢自动分析仪测定天然气、煤气和液化石油气中的硫化氢。纸带法操作简单、方法自动、可取代碘量法、层析法和比色法,具有推广价值。     

天然气储罐的损伤机理及修复研究

压力容器在焊接、安装和检验、使用过程中经常产生裂纹。以某油田天然气公司的Ⅲ类储罐为例,其技术参数为：公称容积为1500m^3;工作压力为3.0MPa;工作温度为-20-50℃;直径为1.16×10^4mm;壁厚为35mm;介质为天然气（C1、C2、C3、C4、C5^＋及微量杂质如H2S和H2O）;壳体材料为N—TuF50钢;罐体焊接安装后未经回火处理。对该储罐壳体开罐检测时发现其内表面有裂纹,且每次检查裂纹的数量均有所增加。裂纹的形成严重地威胁储罐的安全运行,为了防止裂纹损伤加剧,需找出裂纹产生的机理,进而修复已发生的损伤。     

Numerical simulation of natural convection in annuli with internal fins

The solution for the natural convection in internally finned horizontal annuli is obtained by using a numerical simulation of time-dependent and two-dimensional governing equations. The fins existing in annuli influence the flow pattern, temperature distribution and heat transfer rate. The variations of the fin configuration suppress or accelerate the free convective effects compared to those of the smooth tubes. The effects of fin configuration, number of fins and ratio of annulus gap width to the inner cylinder radius on the fluid flow and heat transfer in annuli are demonstrated by the distribution of the velocity vector, isotherms and streamlines. The governing equations are solved efficiently by using a parallel implementation. The technique is adopted for reduction of the computation cost. The parallelization is performed with the domain decomposition technique and message passing between sub-domains on the basis of the MPI library. The results from parallel computation reveal in consistency with those of the sequential program. Moreover, the speed-up ratio shows linearity with the number of processor.     

A study on natural convection from two cylinders in a cavity

Steady-state natural convection heat transfer characteristics from cylinders in a multiply-connected bounded region are clarified. A spectral finite difference scheme (spectral decomposition of the system of partial differential equations, semi-implicit time integration) is applied in numerical analysis, with a boundary-fitted conformai coordinate system through a Jacobian elliptic function with a successive transformation to formulate a system of governing equations in terms of a stream function, vorticity and temperature. Multiplicity of the domain is expressed explicitly.     

液化气中微量含氧化合物的测定

采用中心切割技术,将二甲醚从烃类中切割出来,用一台色谱仪同时对液化气中微量甲醇、甲基叔丁基醚和二甲醚进行测定,结果表明测定精度能够满足分析要求,准确度较高,测定方法操作简便,分析速度快。     

伺服液位计在液化气球罐区中的应用

伺服液位计不仅可以测量液位,还可以测量不同液位的分界面、液体的单点温度、平均温度、分段密度、平均密度。比起差压式液位计、雷达液位计来,它的检测精度高,不受介质的温度、压力、积液变化的影响,可以通过Modbus通讯的方式,把测量的液位、温度信号送到控制系统,节约投资,减少维护工作量。在目前的液化气球罐液位测量管理技术中,它比较先进,应用良好。     

A numerical study on the conjugate natural convection in a circular pipe containing water

In this paper, the effect of material property of pipe on the conjugate natural convection in a circular pipe containing water was investigated by solving the unsteady incompressible Navier-Stokes equations coupled with energy equations of the water and pipe. Natural convection and conduction of water inside the pipe was coupled with the conduction of the pipe whose bottom was subject to uniform heat source. From the present grid resolution and time-step independent solutions, it has been confirmed that the water temperature inside a PVC pipe was higher than that inside a steel pipe due to the smaller heat capacity of PVC and that the streamline patterns of the two cases were found to be opposite because the thermal diffusivity of steel (PVC) is larger (smaller) than that of water such that steel (PVC) pipe is heated faster (slower) than water. Furthermore, a quantitative comparison of heat flux to water was performed by examining the distributions of the heat flux along the inside walls of steel/PVC. The average temperature of water inside steel was found to be higher than that inside PVC at the initial stage of heating. On the other hand, PVC provided a larger heat flux to water when it reached a steady value.     

Experimental study on the gas jet characteristics of a diesel-piloted direct-injection natural gas engine

Journal of Mechanical Science and Technology - The natural gas (NG) jet characteristics of NG/diesel dual-fuel injection under different NG injection pressures and dual-fuel injection intervals...     

Study on natural convection in a rectangular enclosure with a heating source

The natural convective heat transfer in a rectangular enclosure with a heating source has been studied by experiment and numerical analysis. The governing equations were solved by a finite volume method, a SIMPLE algorithm was adopted to solve a pressure term. The parameters for the numerical study are positions and surface temperatures of a heating source i.e., Y/H=0.25, 0.5, 0.75 and 11°C≦ΔT≦59°C. The results of isotherms and velocity vectors have been represented, and the numerical results showed a good agreement with experimental values. Based on the numerical results, the mean Nusselt number of the rectangular enclosure wall could be expressed as a function of Grashof number.     

Numerical study of droplet dynamics in a PEMFC gas channel with multiple pores

The water droplet motion in a PEMFC gas channel with multiple pores, through which water emerges, is studied numerically by solving the equations governing the conservation of mass and momentum. The liquid-gas interface is tracked by a level set method which is based on a sharp-interface representation for accurately imposing the matching conditions at the interface. The method is modified to implement the contact angle conditions on the walls and pores. The dynamic interaction between the droplets growing on multiple pores is investigated by conducting the computations until the droplet growth and sliding motion exhibits a periodic pattern. The numerical results show that the configuration subject to droplet merging is not effective for water removal and that the wettability of channel wall strongly affects water management in the PEMFC gas channel. This paper was presented at the 7th JSME-KSME Thermal and Fluids Engineering Conference, Sapporo, Japan, October 2008. Gihun Son received his B.S. and M.S. degrees in Mechanical Engineering from Seoul National University in 1986 and 1988, respectively, and Ph.D. degree in Mechanical Engineering from UCLA in 1996. Dr. Son is currently a professor of Mechanical Engineering at Sogang University in Seoul, Korea. His research interests are in the area of multiphase dynamics, heat transfer, and power system simulation. Jiyoung Choi received a B.S. degree in Mechanical Engineering from Sogang University in 2005. He is a graduate student of Mechanical Engineering at Sogang University in Seoul, Korea. Choi’s research interests are in the area of PEM fuel cell and microfluidics.     

Numerical analysis of the combustion process in a four-stroke compressed natural gas engine with direct injection system

In this paper, a numerical study to simulate and analyze the combustion process occurred in a compressed natural gas direct injection (CNG-DI) engine by using a multi-dimensional computational fluid dynamics (CFD) code was presented. The investigation was performed on a single cylinder of the 1.6-liter engine running at wide open throttle at a fixed speed of 2000 rpm. The mesh generation was established via an embedded algorithm for moving meshes and boundaries for providing a more accurate transient condition of the operating engine. The combustion process was characterized with the eddy-break-up model of Magnussen for unpremixed or diffusion reaction. The modeling of gaseous fuel injection was described to define the start and end of injection timing. The utilized ignition strategy into the computational mesh was also explained to obtain the real spark ignition timing. The natural gas employed is considered to be 100% methane (CH₄) with three global step reaction scheme. The CFD simulation was started from the intake valves opening until the time before exhaust valves opening. The results of CFD simulation were then compared with the data obtained from the single-cylinder engine experiment and showed a close agreement. For verification purpose, comparison between numerical and experimental work are in the form of average in-cylinder pressure, engine power as well as emission level of CO and NO. This paper was presented at the 9^th Asian International Conference on Fluid Machinery (AICFM9), Jeju, Korea, October 16–19, 2007.     

Experimental study of natural convection heat transfer from a nonuniformly heated flat plate simulating PV panel

Journal of Mechanical Science and Technology - In view of the characteristic of Photovoltaic (PV) conversion, an experimental study has been conducted to investigate the natural convection heat...     

Numerical study on the vertical bridgman crystal growth with thermosolutal convection

A numerical analysis has been carried out to investigate the influences of thermosolutal convection on the heat and mass transfer and solute segregation in crystals grown by the vertical Bridgman technique. The governing equations are solved by a finite-volume method using the power law scheme and the SIMPLE algorithm in which body-fitted coordinate system has been used. A primary convective cell driven by thermal gradients forms in the bulk of the domain, while a secondary convective cell driven by solutal gradients forms near interface. As the solutal Rayleigh number increases, secondary cell becomes to be stronger and has a great influence on the radial concentration along the interface.     

Numerical study on the low density gas flows in a plasma etch reactor

The direct simulation Monte Carlo method is employed to predict the etch rate distribution on Al wafer for a chlorine feed gas flow. The etching process of an Al wafer in a plasma etch reactor is examined by simulating molecular collisions of reactant and product. The surface reaction on the Al wafer is simply modelled by one-step reaction: 3Cl₂+2Al → 2AlCl₃. The gas flow inside the reactor is compared for six different nozzle locations. The present numerical results show that the etch rate increases with the mass flow rate of source gas Cl₂. It is also shown that the flow field inside the reactor is significantly affected by the nozzle locations.     

An experimental study on the transition criteria of open channel natural convection flows

Natural convection experiments were carried out for a wide range of the Grashof number from 10⁴ to 5 × 10⁹ or for the Rayleigh number from 10⁷ to 10¹³ in order to seek the proper transition criteria from laminar to turbulent. Using the analogy concept, heat transfer systems were simulated by corresponding mass transfer systems. The copper sulfate electroplating system was chosen as the mass transfer system. The experimental results closely reproduced the McAdams’s correlation for laminar and Fouad’s for turbulent. The Sherwood numbers obtained from the experiment were proportional to the 1/4 power of Rayleigh number or Grashof number at laminar region and the near 1/3 power at turbulent region as the well known theory. This paper concludes that the proper transition criteria of the natural convection should be the Grashof number of 10⁹. The originality of this paper comes from the fact that the study deals with very large value of Schmidt number and that by using the analogy experiment methodology, high values of Rayleigh number and Grashof number were achieved with a relatively short test facility.

     

Resonant enhancement of natural convection in a cubic enclosure under time-periodic magnetizing force

A numerical investigation was carried out for natural convection of air (paramagnetic fluid) in a cubic enclosure subjected to constant gravity and time-periodic magnetizing force. Since the magnetizing force expressed as the gradient of magnetic induction squared in each direction, has three-dimensional characteristics, three-dimensional modeling of system would be required to investigate the effect of magnetizing force more accurately. In this study as a sequent research, the basic analysis of Kang and Hyun [16] was extended to the case of three-dimensional and to cases when the magnitude of magnetizing force is varied. Two geometrical configurations for the location of electrical coils, were considered. The amplification characteristics of heat transfer rate in the cube according to the change of the magnitude of magnetizing force, were investigated especially when the resonance occurs. The numerical results showed that the heat transfer is enhanced appreciably by application of large magnitude of magnetizing force. In addition, the results for two- and three-dimensional cases were also compared.

     

Effect of circular cylinder location on three-dimensional natural convection in a cubical enclosure

This paper presents the results of immersed boundary method-based three-dimension numerical simulations of natural convection in a cubical enclosure with an inner circular cylinder at a Prandtl number of 0.7. This simulation spans three decades of Rayleigh number, Ra, from 10³ to 10⁶. The location of the inner circular cylinder is changed vertically along the centerline of the cubical enclosure. This study primarily focuses on the effects of both buoyancy-induced convection and the location of the inner circular cylinder on heat transfer and fluid flow in the cubical enclosure. In the range of Rayleigh numbers considered in this study, the thermal and flow fields eventually reach steady state, regardless of the location of the inner cylinder. When Ra is 10³, the end wall of the cubical enclosure has a negligible effect on the thermal and flow fields in the enclosure. However, in the range of 10⁴ ≤ Ra ≤ 10⁶, the effect of the end wall on heat transfer and fluid flow in the enclosure depends on both the location of the inner cylinder and the Rayleigh number. Detailed analysis results for the distribution of streamlines, isotherms, and Nusselt numbers are presented in this paper.