Natural convection heat transfer estimation from a longitudinally finned vertical pipe using CFD

In this study, CFD analysis of air-heating vaporizers was conducted. A longitudinally finned vertical pipe was used to represent the air-heating vaporizer in the CFD model. Nitrogen gas was used as the working fluid inside the vertical pipe, and it was made to flow upward. Ambient air, which was the heat source, was assumed to contain no water vapor. To validate the CFD results, the convective heat transfer coefficients inside the pipe, h_i-c, derived from the CFD results were first compared with the heat transfer coefficients inside the pipe, h_i-p, which were derived from the Perkins correlation. Second, the convection heat transfer coefficients outside the pipe, h_o-c, derived from the CFD results were compared with the convection heat transfer coefficients, h_o-a, which were derived from an analytical solution of the energy equation. Third, the CFD results of both the ambient-air flow pattern and temperature were observed to determine whether they were their reasonability. It was found that all validations showed good results. Subsequently, the heat transfer coefficients for natural convection outside the pipe, h_o-c, were used to determine the Nusselt number outside the pipe, Nu_o.. This was then correlated with the Rayleigh number, R_a. The results show that R_a and Nu_o have a proportional relationship in the range of 2.7414×10¹² ≤ Ra ≤ 2.8263×10¹³. Based on this result, a relation for the Nusselt number outside the pipe, Nu_o, was proposed. This paper was recommended for publication in revised form by Associate Editor Man Yeong Ha Hyomin Jeong is currently a professor of Mechanical and Precision Engineering at Gyeongsang Nation University. He received his ph.D. in mechanical engineering from the University of Tokyo in 1992 and he joined Arizona State University as a visiting professor from 2008 to 2009. His research interests are in fluid engineering, CFD, cryogenic system, cascade refrigeration system and ejector system, mechanical vapor compression Hanshik Chung is a professor of Mechanical and Precision Engineering at Gyeongsang National University. He obtianed his Ph.D. in Mechanical Engineering from Donga University. He joined Changwon Master’s College and Tongyeong Fisher National College as an assistant Professor in 1988 and 1993, respectively. His research fields extend into the thermal engineering, heat transfer, solar heating & cooling system, LNG vaporizer optimum, solar cell, hydrogen compressor for fuel cell and making fresh water system from sea water     

颗粒物料传质过程的数值仿真与实验研究

利用质量平衡方程、能量平衡方程、传热方程和传质方程，建立了颗粒物料在转筒干燥器干燥过程中的传质数学模型，并使用有限元法进行了仿真研究，该模型能够较好地预测干燥过程中物料颗粒含湿量的变化，仿真结果与实验结果吻合良好。该仿真程序对此类问题的研究具有参考价值。     

电解加工间隙中的传质过程及其对电解加工的影响

这里应用流体力学、传质学和电化学理论,分析了电解加工间隙中处于紊流状态的电解质溶液内的传质过程。在此基础上建立了阳极电流密度的计算方法,并讨论了外加电压、电解质浓度、流速等参数对电解加工的影响。     

基于CFD的风冷摩托车发动机气缸体的传热仿真研究

针对单缸风冷摩托车发动机气缸体的传热问题,基于热力学第一定律建立了进气过程中气缸壁传热量的数学模型,分析了气缸周壁的传热方式,通过CAD三维软件建立了发动机气缸体的三维模型,并进行计算网格划分,给定边界条件,应用计算流体力学( CFD)软件对发动机气缸体传热过程进行数值模拟计算,得出气缸体的温度分布图,从而获得气缸壁传给进气气体的热量值,为进一步研究摩托车发动机气缸体的传热问题提供参考.     

基于参数化建模和CFD数值模拟的船舶球鼻艏优化设计

针对传统船舶球鼻艏形状设计存在的问题,对基于NURBS方法的船体完全参数化建模和基于Rankine源的面元法的船舶阻力数值计算进行了研究,对一类组合型优化策略结合设计变量、约束条件和优化目标进行了分析,通过建立一类自动优化平台对船舶球鼻艏型线进行了优化设计。通过优化平台对船体球鼻艏各项参数进行了自动优化调整直至满足减小船舶兴波阻力的优化目标。应用所构建的优化框架,以兴波阻力最小化为目标对某一滚装船球鼻艏型线进行了优化设计。研究结果表明,型线优化后的船体兴波阻力得到下降,船艏周围兴波得到了改善,验证了所提出型线优化设计方案的有效性。     

基于CFD技术优化50 L发酵罐内气体分布器

通过CFD模拟技术对50L通气搅拌发酵罐内不同气体分布器的传质混合效果进行气液双相流模拟。首先对发酵罐自带气体分布器进行流场模拟,针对模拟结果提出两种改进气体分布器。结果表明:双层环形气体分布器表现出一种较理想的混合传质效果,为此类发酵罐的优化和放大提供一种有效的技术手段。     

Measurement of film fluctuations and aerated characteristics during gas-liquid slug-churn flow transition by wire-mesh sensor

Vertical upward gas-liquid slug flows are frequently encountered in chemical processes and petroleum industries. The measurement of the film fluctuations and the aerated characteristics is of great significance for uncovering the mechanism of slug-churn flow pattern transitions. In this study, a conductance wire-mesh sensor (WMS) measurement system is designed based on a Field Programmable Gate Array (FPGA) to visualize the structures of vertical gas-liquid flows. Liquid film flooding is a significant factor prompting the transition from slug to churn flow. Based on the WMS data, the 3D film structures are derived to indicate film instability during the flow pattern transition. Three types of film fluctuations in stable slug flow, unstable slug flow, and churn flow are presented. Liquid slug aeration is another important factor contributing to the slug-churn flow transition. The spatial distribution and the diameters of the gas bubbles in the liquid slug are detected by the WMS. The coalescence behavior of the bubbles is uncovered. Finally, mechanistic models based on the film flooding and slug aeration are constructed to predict the boundary of the flow pattern transition. The performance of the film flooding model and slug aeration model in predicting the onset of churn flow is evaluated.     

A study of signal noise reduction of the mass air flow sensor using the flow conditioner on the air induction system of heavy-duty truck

The mass air flow meter is a critical sensor that works based on thermal hot wire technology, used to determine the fuel to be injected into the cylinder and calculate the fuel-air ratio. In order to measure the airflow rate accurately, the flow should be uniform and smooth upstream of the sensor. The flow disturbance with a short straight length upstream of the flow meter results in the noise of the sensor signal. This noise causes unstable mass flow measurement on the system. Flow conditioners can be used to smooth the velocity profile of the flow. In this study, experimental and numerical methods were used to characterize the performance and operating accuracy of the mass flow meter used in heavy-duty truck applications. The flow conditioners were implemented to smooth the velocity profile around the mass flow meter that was disrupted by bends. The flow structures with and without flow conditioner were examined using Particle Image Velocimetry (PIV) to measure the time-averaged velocity. As well as the validated computational fluid dynamics (CFD) model provides data to understand the flow uniformity effect of the conditioner on the mass airflow (MAF) sensor. The optimization study was performed using a full factorial design of experiment (DOE) for flow conditioner design. A robust methodology was developed for the flow conditioner characteristics and mass airflow sensor implementation on the air induction system.     

Transient pressure characteristics in a pressure regulating system by using 1-D analytic valve modeling

Typically, a Pressure Control Valve (PCV) system is constructed with 3 to 4 PCV lines in order to prepare for both a future demand increase and a continuous supply of power after an emergency shutdown of the operating line. However, some operation failure cases that do not follow the original design concepts of a PCV system have been reported in the field. In this study, an accurate 1-D PCV system numerical model was built and a 1-D compressible flow theory was introduced for analytic valve modeling to find solutions for this problem. Several numerical analyses were successfully performed to examine the generation and propagation characteristics of the transient pressure and to clarify the relationships between the transient pressure or surge wave and each factor or parameter relating to fluid dynamics behavior in a PCV system. The relationship between the transient pressure and other factors, such as the size of pipe and header linked after a PCV, the pipe size of the added components linked after the header, and the generation time of the transient pressure and influence of the Slam Shut Valve were investigated in detail. Finally, in order to reduce the strength of the transient pressure and to delay its propagation, this study demonstrates the benefit both of enlargement of the pipe linked to the component added after the PCV system, and the installation of a dissipation component between the operation line and the other lines.     

Wet gas overreading characteristics of a long-throat Venturi at high pressure based on CFD

Based on the operational conditions of the PetroChina Southwest Oil & Gas well field, this study aims to explore the wet gas flow overreading (OR) characteristics of a nonstandard long-throat Venturi by the means of computational fluid dynamics (CFD) technique. The studied prototype structure size is an inner diameter of 50 mm, a diameter ratio of 0.4 and a throat length of 50 mm. According to the field experiment, the simulation pressure is 3 MPa gauge. Through a comparative study of the multiphase flow models and turbulence models, combined with the analysis of the Baker׳s flow regime and interparticle space under the field conditions, this paper eventually employed DPM model and Eulerian model for wet gas simulation, respectively, and RSM for turbulence model. An equivalent droplet diameter adjustment method was implemented to improve the precision of prediction. During post-processing, the liquid phase distributions and the wall pressure profiles were investigated. The numerical results indicate that the differential pressure in convergent section of long-throat Venturi by using DPM model is less than that by using Eulerian model, and the differential pressures in the divergent section by using the two models are analogous. Afterwards, the OR prediction correlations based on the differential pressure ratio method were proposed, and then compared and validated by the industrial field tests. The root mean square errors (RMSE) and the average relative errors predicted by Eulerian model were 4.24% and 3.78%, 5.69% and 5.01% by using DPM model, respectively. In conclusion, Eulerian model is more suitable for wet gas flow prediction. And some advice on the improvement of the multiphase flow simulation is provided to get a more preferable performance in wet gas flow prediction.     

CFD investigation of the influence of volute geometrical variations on hydrodynamic characteristics of circulator pump

Improper design of volute geometry can be the main cause that leads to unsteady pressure pulsation and radial force in pumps. Therefore, it is important to understand the influence of volute geometrical parameters on hydrodynamic characteristics of pump and the mechanism. However, the existing studies are limited to investigate the influence of only one or two volute geometrical parameters each time, and a systematic study of the influence of the combinations of different volute geometrical parameters on the pump’s hydrodynamic characteristics is missing. In this paper, a study on the understanding of the influence of volute geometrical variations on hydrodynamic characteristics of a high speed circulator pump by using computational fluid dynamics(CFD) technology is presented. Five main volute geometrical parametersD3,A8,a0,j0 andRt are selected and 25 different volute configurations are generated by using design of experiments(DOE) method. The 3D unsteady flow numerical simulations, which are based on the SSTk-w turbulence model and sliding mesh technique provided by CFX, are executed on the 25 different volute configurations. The hydraulic performance, pressure pulsation and unsteady radial force inside the pump at design condition are obtained and analyzed. It has been found that volute geometrical parametersD3 andA8 are major influence factors on hydrodynamic characteristics of the pump, whilea0,j0 andRt are minor influence factors. The minimum contribution from bothD3 andA8 is 58% on head, and maximum contribution from bothD3 and A8 is 90% on pressure pulsation. Regarding the pressure pulsation intensity, two peaks can be found. One is in the tongue area and the other is in the diffusor area. The contributions are around 60% from tongue and 25% from diffusor, respectively. The amplitude of pressure pulsation has a quadratic polynomial functional relationship with respect toD3/D2 andA8/A10, and fluctuating level of radial force has a quadratic polynomial functional relationship with respect toD3/D2. While for the other volute parametersa0,j0 andRt, no special function has been found related to pressure pulsation and radial force. The presented work could be a useful guideline in engineering practice when designing a circulator pump with low hydrodynamic force.     

Experimental investigation on the void fraction characteristics of a sudden expansion tube using wire-mesh sensors

Void fraction is an essential parameter of gas-liquid two-phase flow and experiments were executed to investigate the void fraction fluctuation characteristics of gas-liquid two phase flow through a sudden expansion tube. Two 16 × 16 wires mesh sensors were applied to measure the phase distribution of upstream pipe(pipe-32) and downstream pipe(pipe-50). The superficial gas velocity is in the range of 3.46 m/s - 22.46 m/s and the superficial liquid velocity ranges from 0.034 m/s to 0.414 m/s. Flow pattern evolution of upstream and downstream pipes was reconstructed and compared. The experiment results show that, in contrast to pipe-32, the void fraction of pipe-50 shows different trends with the increase of liquid and gas velocity. Liquid-carrying capacity is essential in the relationship between the void fraction of pipe-32 and pipe-50. The critical superficial liquid and gas velocities are proposed to characterize the liquid-carrying capacity. The maximum critical superficial gas and liquid velocity is 15.56 m/s and 0.207 m/s, respectively. Besides, a model is proposed to describe the relationship of void fraction between pipe-32 and pipe-50. It is found that the prediction error is less than ±10% in the case of annular flow.     

A study on the modeling of transitional lateral force acting on the berthing ship by CFD

To evaluate the unsteady motion in laterally berthing maneuver, it is necessary to estimate clearly the magnitudes and properties of hydrodynamic forces acting on ship hull in shallow water. A numerical simulation has been performed to investigate quantitatively the hydrodynamic force according to water depth for Wigley model using the CFD (Computational Fluid Dynamics) technique. By comparing the computational results with the experimental ones, the validity of the CFD method was verified. The numerical solutions successfully captured some features of transient flow around the berthing ship. The transitional lateral force in a state ranging from the rest to the uniform motion is modeled by using the concept of circulation.     

基于CFD技术的发动机水套选型及优化设计

水套是发动机冷却系统的重要组成部分,良好的水套设计能够使得各缸冷却液分布合理,换热均匀。在水套设计过程中工程师常常面临多种水套方案的选型工作,因此明确各方案的流动特点是非常关键的工作。介绍了一起在发动机的正向开发中利用CFD手段确定水套流场及在此基础上进行水套优化选型的实例。     

铝表面结霜的试验研究及CFD模拟

结霜是制约热泵在低温工况下运行的主要因素之一。针对蒸发器结霜问题,通过试验及数值模拟的方法研究了水平铝表面结霜特性。试验观察了铝表面霜层生长的形貌特征,对比了不同冷表面温度及来流空气特性对结霜的影响。结果表明:铝表面温度越低,霜层生长初在始阶段时间越长,风速大于1.5 m/s之后,风速的变化主要影响霜层生长成熟阶段,并通过计算流体力学(CFD)模拟分析了霜层密度及导热系数的变化,验证结果表明,该模型能够较好的预测铝表面结霜。     

利用APS实现大气气溶胶数密度和质量浓度的测量

利用自行研制的空气动力学粒谱仪(APS:aerodynam ic particle sizer)测量北京地区大气气溶胶不同粒径段的粒子数密度与质量浓度。对二者进行相关性分析的基础上,运用多元回归方法,得出利用粒子数密度谱获取颗粒物(PM10)质量浓度谱的经验公式,并对公式进行实验验证。同时分析北京市初春大气气溶胶粒子不同空气动力学直径的质量浓度谱特征,以期对北京市大气污染总量控制方案及对策提供科学依据。     

基于ANSYS ICEM CFD和CFX数值仿真技术

为了准确方便地计算和分析流体的传热和流动情况,文中以分析某浸入式水口浇钢温度场和流场为例介绍了一种基于ANSYS ICEM CFD和CFX数值仿真技术在连续铸钢过程中的应用.该方法是利用ANSYS ICEM CFD中进行流体数值仿真的前处理操作,然后将生成的网格导入到CFX中施加边界条件进行求解计算,可以大大提高数值仿真效率,对CAE工程分析人员具有很大的借鉴意义.     

A naphthalene sublimation study on heat/mass transfer for flow over a flat plate

It is important to completely understand heat/mass transfer from a flat plate because it is a basic element of heat/mass transfer. In the present study, local heat/mass transfer coefficient is obtained for two flow conditions to investigate the effect of boundary layer using the naphthalene sublimation technique. Obtained local heat/mass transfer coefficient is converted to dimensionless parameters such as Sherwood number, Stanton number and Colburnj-factor. These also are compared with correlations of laminar and turbulent heat/mass transfer from a flat plate. According to experimental results, local Sherwood number and local Stanton number are in much better agreement with the correlation of turbulent region rather than laminar region, which means analogy between heat/mass transfer and momentum transfer is more suitable for turbulent boundary layer. But average Sherwood number and average Colburnj-factor representing analogy between heat/mass transfer and momentum transfer are consistent with the correlation of laminar boundary layer as well as turbulent boundary layer.     

红外探测中潜艇冷热尾流的传热传质特性

本文采用有限体积法建立了1/72龙鲨Ⅱ号核潜艇的三维计算模型,结合动参考系、用户自定义函数和物性多项式函数等实现了高速旋转螺旋桨和海水温度密度分层的仿真。基于该模型,探讨了螺旋桨高速旋转、海水温度密度分层和高温热尾流喷射等因素对潜艇冷热尾流传热传质特性的影响,所得结论如下:高速旋转螺旋桨促使热尾流后向延迟距离增大、海表温差减小,忽略旋转时海表温差的绝对误差和相对误差分别为3.23mK和52.7%;水下航行潜艇扰动温度密度分层海水浮升形成冷尾流温差信号,与温度密度均匀海水相比,海表温变区域显著增大、尾流温差由6.13mK增大到84mK;通过海表上游冷尾流特征判断是否存在水下航行潜艇,若存在,再结合海表下游热尾流特征实现潜艇位置的精确反演。上述结论可为优化潜艇冷热尾流的数值仿真精度提供参考与借鉴。