Flow visualization of Rayleigh–Bénard convection for cubical cavity

Natural convective flow of air inside the cubical cavity is investigated numerically. The temperature of the bottom wall is kept higher than that of top cold wall, and other four walls are assumed to be adiabatic. Attention has been paid to the convective discretization schemes, like upwind, QUICK, total variation diminishing, normalized variable diagram (NVD) schemes that are compared with respect to accuracy. The output is validated with respect to the results available in the literature. A parallel computing message passing interface code is adapted to run the simulations. From the results, it is observed that the NVD scheme gives better results among all the employed convective discretization schemes irrespective of the mesh structure. Thus, in this article, self filtered central differencing which is a family of NVD, is used. From the enormous output data, along with the streamlines, contours of isotherms, the new technique of energy pathlines, and field synergy are used to visualize the fluid flow and heat transfer mechanism arising in the system in the range of Ra from 10³ to 10⁶. Free energy streamlines are observed with small Ra, whereas trapped energy streamlines are observed with high Ra. When Ra increases, synergy angle increases and implies that the synergy between the velocity vector and temperature gradient gets reduced and leads to increasing values of average Nusselt number (Nu).     

Mixed convective flow in a bottom heated lid‐driven cubical cavity: Energy streamlines and field synergy

Analysis of three dimensional natural convective lid‐driven cavity flow is carried out numerically. The top wall is assumed to slide in its own plane at a constant speed. Isothermal temperature is maintained at horizontal walls in which the bottom wall is assumed to be at a higher temperature than the top wall. Governing equations of this problem, expressed in dimensionless form are solved by using the finite volume method. Numerical results are computed for the control parameters arising in the system, namely, the Reynolds number (Re) and Richardson number (Ri) in the range of 100 ≤ Re ≤ 1000 and 0.001 ≤ Ri ≤ 10. The contours of isotherms, streamlines, Vortex corelines, energy pathlines, and field synergy are used to visualize the flow and thermal characteristics. The simulated results are corroborated with those available in the literature. When Re = 100 and 400 with growth of Ri there are "free" energy streamlines and they exhibited symmetric nature near the boundaries. The participation of convective thermal energy and kinetic energy is insignificant compared to conductive thermal energy, where the velocity components are modest. When Re = 1000 with increase of Ri, "trapped" energy streamlines are detected. Energy streamlines occupy substantial part. This is due to the result of high Re, with increasing Ri, kinetic energy and convective thermal energy get dominated and hence "trapped" streamlines formed. As Re increases, synergy angle increases for distinct Ri values. So the synergy between temperature and velocity gets worse. The synergy angle of buoyant‐aiding flow is high while the buoyant‐opposing flow is significantly less than that of forced convection flow when Ri = 1. This gives the relation between temperature field and velocity at buoyant‐aiding flow, which is at the worst situation leading to increasing average Nusselt number.     

Analysis of visualization techniques of bottom heated lid–driven square cavity

In the present study, an attempt is made to explore the flow visualization techniques inside the bottom heated lid–driven square cavity. The governing equations along with boundary conditions are solved numerically. The convection differencing schemes namely, upwind difference, quadratic upstream interpolation for convective kinetics, Superbee, and self‐filtered central differencing schemes are discussed and are used to simulate the flow using message passing interface (MPI) code. An attempt has been made to analyze the flow behavior inside the cavity using streamlines, isotherms energy streamlines, and field synergy by varying the Reynolds number (Re) and Richardson number (Ri). The simulated results (100≤ Re ≤ 1000 and 0.001≤ Ri ≤ 10) are validated with previous results in literature. It is observed that the computational cost for all the differencing schemes gets reduced tremendously when the MPI code is implemented. Flow becomes quasi‐two‐dimensional for Ri < 1. Overall, Nusselt number increases mildly with cavity inclination for the forced convection–dominated case (Ri = 0.1) while it increases much more rapidly with inclination for natural convection–dominated case (Ri = 10).     

Analysis of new visualization techniques of left heated lid‐driven square cavity

In the present study, an attempt is made to explore the flow visualization techniques inside the left‐heated lid‐driven square cavity. The governing equations along with boundary conditions are solved numerically. The simulated results (100 ≤ Re ≤ 1000 and 0.001 ≤ Ri ≤ 10) are validated with previous results in the literature. The convection differencing schemes, namely, UPWIND, QUICK, SUPERBEE, and self‐filtered central differencing are discussed and are used to simulate the flow using MPI code. It is observed that the computational cost for all the differencing schemes get reduced tremendously when the MPI code is implemented. Plots demonstrating the influences of Re and Ri in terms of the contours of the fluid streamlines, isotherms, energy streamlines, and field synergy principle are presented.     

湍流场协同模型构建及减阻应用

在稳态、无体积力流动场协同模型的基础上,将场协同原理与湍流流动有效结合,引入Favre平均和RANS,根据Navier-Stokes方程建立了不可压湍流场协同模型。基于最小机械能耗散原理,在约束条件下推导使黏性耗散函数满足极小值的数值解,将不可压缩湍流模型与流动减阻相关联,构建了湍流场协同减阻模型。以后台阶流动验证模型的有效性,结果显示:改变结构后,黏性耗散值从0.633 0 W减小到0.245 0 W,优化了61.3%。     

含加热圆管方腔内自然对流的数值研究

采用数值计算方法对含不同直径圆管以及相同直径圆管位置不同方腔内的层流自然对流进行了研究。以冷热壁面温度差为基准的瑞利数Rn为10^6,以圆管壁面热流密度为基准的Ra为10^8。计算结果表明,当圆管处于方腔中间位置时,随着圆管直径的增大,圆管表面局部努塞尔数呈减小趋势。当圆管直径不变时,由于在不同位置处浮力作用的强弱不同,随着圆管在方腔内位置的改变,方腔内流场结构和温度场分布也会发生变化。整个计算结果可为工程设计提供参考。     

热辐射能(波)选择性强化吸收的场协同机理

从场协同原理的角度出发，分析了材料对热辐射能(波)选择性吸收过程，提出了材料对入射辐射能(波)的作用实质是材料内阻尼振子组成的力场和入射辐射场之间的相互作用；调整这两个场之间的协同关系，可以改变和改善材料的选择性热辐射性能。基于此得到了强化材料对入射热辐射能(波)选择性吸收的机理。研究表明：减弱入射辐射场与材料内阻尼振子组成的力场之间的协同关系，可以提高材料的吸收率和发射率；相反，强化这两个场之间的协同关系，可使材料表现出较高的反射率。     

Numerical analysis on heat transfer enhancement by longitudinal vortex based on field synergy principle

Three-dimensional numerical simulation results are presented for a fin-and-tube heat transfer surface with vortex generators. The effects of the Reynolds number (from 800 to 2 000) and the attack angle (30° and 45°) of a delta winglet vortex generator are examined. The numerical results are analyzed on the basis of the field synergy principle to explain the inherent mechanism of heat transfer enhancement by longitudinal vortex. The secondary flow generated by the vortex generators causes the reduction of the intersection angle between the velocity and fluid temperature gradients. In addition, the computational evaluations indicate that the heat transfer enhancement of delta winglet pairs for an aligned tube bank fin-and-tube surface is more significant than that for a staggered tube bank fin-and-tube surface. The heat transfer enhancement of the delta winglet pairs with an attack angle of 45° is larger than that with an angle of 30°. The delta winglet pair with an attack angle of 45° leads to an increase in pressure drop, while the delta winglet pair with the 30° angle results in a slight decrease. The heat transfer enhancement under identical pumping power condition for the attack angle of 30° is larger than that for the attack angle of 45° either for staggered or for aligned tube bank arrangement. Translated from Journal of Xi’an Jiao Tong University, 2006, 40(7): 757–761 [译自: 西安交通大学学报]     

Numerical investigation and analysis of heat transfer enhancement in channel by longitudinal vortex based on field synergy principle

3-D numerical simulations were presented for laminar flow and heat transfer characteristics in a rectangular channel with vortex generators. The effects of Reynolds number (from 800 to 3 000), the attack angle of vortex generator (from 15° to 90°) and the shape of vortex generator were examined. The numerical results were analyzed based on the field synergy principle. It is found that the inherent mechanism of the heat transfer enhancement by longitudinal vortex can be explained by the field synergy principle, that is, the second flow generated by vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient. The longitudinal vortex improves the field synergy of the large downstream region of longitudinal vortex generator (LVG) and the region near (LVG); however, transverse vortex only improves the synergy of the region near vortex generator. Thus, longitudinal vortex can enhance the integral heat transfer of the flow field, while transverse vortex can only enhance the local heat transfer. The synergy angle decreases with the increase of Reynolds number for the channel with LVG to differ from the result obtained from the plain channel, and the triangle winglet performs better than the rectanglar one under the same surface area condition. __________ Translated from Journal of Xi’an Jiaotong University, 2006, 40(9): 996–1000 [译自: 西安交通大学学报]     

Discussion on the convective heat transfer and field synergy principle

The convective “heat” transfer is actually mainly carried out by the motion of hotter or colder particles from one system into another system. Therefore, the best convective “heat” (strictly speaking, internal energy) transfer is the case where velocity vectors are always perpendicular to the isothermal surfaces (or isotherm in 2D cases). This conclusion has been named “field synergy principle”. In this paper, some field synergy exact solutions are presented to further develop the principle. The concrete physical meanings of the derived analytical solutions are analyzed. The method of separating variables with addition and other extraordinary approaches are adopted in the derivation.     

Numerical study and field synergy analysis on CO selective methanation packed-bed reactor

Carbon monoxide selective methanation (CO-SMET) is one of the most efficient technologies for hydrogen purification and CO deep removal. This paper applies the field synergy principle for a deep understanding on the chemically reactive flow in a CO-SMET tubular reactor. The variation of CO conversion rate under different operating conditions is interpreted, at the first time, as relevant to the variation of the synergy angle between temperature and gas concentration fields. Sensitive analyses of the bed pressure, CO/CO₂ ratio, heat exchange modes, etc., are studied to obtain the profile of field synergy angle in the inlet gas temperature range of 373 K – 873 K. It is found that the region with synergy angles between 0° and 70° enhances the heat transfer between mass transfer and contributes the main output of CO conversion. This work provides a fundamental basis on the future optimal design of CO–SMET reactors.     

基于多场协同理论的炉膛燃烧传热分析

为了探究炉膛传热性能,利用场协同理论对不同条件下的炉膛燃烧流场进行了分析。结果表明：在炉膛燃烧区壁面附近及中心部分区域,协同角α较小,而介于这两部分的大块区域,协同角α较大。协同角β大的区域占比相对较大。在整个燃烧区域,尤其是喷口附近,协同角γ较大,且分布比较均匀。协同角φ分布也比较均匀,大角度φ主要集中在壁面附近。在炉膛中心大部分区域以及炉膛顶部及折焰角处协同角θ较大,但是在喷口附近,协同角θ较小。     

CFD study on local fluid-to-wall heat transfer in packed beds and field synergy analysis

To reach the target of smaller pressure drop and better heat transfer performance, packed beds with small tube-to-particle diameter ratio (D/d _p<10) have now been considered in many areas. Fluid-to-wall heat transfer coefficient is an important factor determining the performance of this type of beds. In this work, local fluid- to-wall heat transfer characteristic in packed beds was studied by Computational Fluid Dynamics (CFD) at different Reynolds number for D/d _p=1.5, 3.0 and 5.6. The results show that the fluid-to-wall heat transfer coefficient is oscillating along the bed with small tube-to-particle diameter ratio. Moreover, this phenomenon was explained by field synergy principle in detail. Two arrangement structures of particles in packed beds were recommended based on the synergy characteristic between flow and temperature fields. This study provides a new local understanding of fluid-to-wall heat transfer in packed beds with small tube-to-particle diameter ratio.     

自然对流温度场节能改造

本文通过热流理论分析和传热计算,指出了某一运行多年的自然对流温度场的不足,为了节能增效,采取了快捷的改造措施。工程竣工后试运转表明节能率可达50%,对所有自然对流温度场的改造具有普遍指导意义和重要参考借鉴作用。     

Three-field synergy of solar energy for induced the enhancement of the oxidation of acrylonitrile in coordination with the production of hydrogen

In the review of the successful solar thermal electrochemical process (STEP) of acrylonitrile oxidation for the effective wastewater treatment, the process was actually driven by solar two fields - thermofield and electrofield, essentially activated and motivated for both thermochemistry and electrochemistry. In this paper, the synergistic system of solar three fields, induced by the primary photofield, and sub-thermofield and sub-electrofield, was designed and employed firstly for promoted the efficiency of the solar utilization and pollutant oxidation plus hydrogen production. With the correlative action, the three sub-chemical processes were induced by the solar three fields. The action actually conducted a three-field synergy of solar energy with a combination of the thermo-activation, photocatalysis and electrochemistry of the pollutant oxidation. Exemplified by acrylonitrile, the solar oxidation plus hydrogen production was theoretically and experimentally investigated by the single-field, coupled two-fields and coupled three-fields patterns. The results indicated that the coupled three-field pattern achieved high efficiencies in the solar utilization and oxidative reaction plus the hydrogen production, which was superior to ones of the single or two fields. The solar thermofield enables that the activated acrylonitrile was apt to be thermally decomposed, greatly in favor of the subsequent photo- and electrooxidation. The photocatalytic efficiency driven by the single photofield was reached at a rate of 31.01%. The electrolysis efficiency powered by single electrofield gained a rate of 24.56%. For the combination of the solar three-field pattern, the oxidation efficiencies run up to a rates of 32.74%, 38.06%, 55.01% and 76.01% during 60 min at the 25 °C, 40 °C, 60 °C, 75 °C, respectively. Especially, a joint of the coupled field realized the 6.38 times of the COD removal rate of acrylonitrile in comparison with the single field pattern. Due to the easy anodic oxidation of acrylonitrile and operation under the high temperature, the cathodic reduction of water was enhanced for the production of hydrogen in the electrolysis of the less potential plus an addition of photocatalysis. The experimental data and mechanistic analysis significantly revealed that the system achieved such a synergetic action. The full mineralization plus the hydrogen production was attributed to a coupling and matching integration of the solar three fields and subchemistries.     

电弧炉中电流场的分析与节能研究

本文选择电炉实用结构作为物理模型,对电弧炉中的电流场作了一些必要的假设,用分布电荷法来求三相电炉中电流场中的边值问题的近似解,推导有关场的分布及工作电导的公式,在此基础上按照电炉的等效电路图,在一定条件下求得提高电炉效率的最佳极距,以达到节能的目的。     

太阳能热水系统在寒冷地区油气田倒班点的应用

介绍位于内蒙古地区乌审旗境内油气田某倒班点太阳能热水系统的应用实例,分析了太阳能热水系统在寒冷地区应用的实用性、经济性和优势,并针对该类地区的区域特点说明了系统设计应着重考虑的问题及应对措施。运行结果证明太阳能热水系统完全能够在寒冷地区推广应用．节能效果明显。     

Numerical study of fluid flow and heat transfer in a flat-plate channel with longitudinal vortex generators by applying field synergy principle analysis

Three dimensional numerical simulations are performed on laminar heat transfer and fluid flow characteristics of a flat-plate channel with longitudinal vortex generators (LVGs). The effects of two different shaped LVGs, rectangular winglet pair (RWP) and delta winglet pair (DWP) with two different configurations, common-flow-down (CFD) and common-flow-up (CFU), are studied. The numerical results indicate that the application of LVGs effectively enhances heat transfer of the channel. According to the performance evaluation parameter, (Nu/Nu₀)/(f/f₀), the channel with DWP has better overall performance than RWP; the CFD and CFU configurations of DWP have almost the same overall performance; the CFD configuration has a better overall performance than the CFU configuration for RWP. The basic mechanism of heat transfer enhancement by LVGs can be well described by the field synergy principle.     

Experimental study on heat and fluid flow in LNG tank heated from the bottom and the sidewalls

Heat and fluid flow in a layer heated from the bottom and the sidewalls simulating an underground LNG tank is experimentally studied under high Rayleigh number (7.5×10¹⁰<Ra<1.5×10¹³) conditions by electrochemical mass transfer technique. The experiment yielded the following results. When sidewalls are heated, the heat transfer along the bottom surface is reduced. Heat transfer along sidewalls is independent of bottom heating, and is modeled by an equation for laminar natural convection flow even for Ra>10⁹. Convective flow pattern in the tank is visualized by the Schlieren technique. The results, combined with local mass transfer measurement, show that Sh of the bottom surface is reduced in the area where impinging downward flow exists. It is caused by the suppression of thermal plume formation by the downward flow. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(7): 417–430, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20031     

Thermodynamic models and energy distribution of single-phase heated surface in a boiler under unsteady conditions

A coal-fired power unit frequently operates under unsteady conditions; thus, in order to acquire scientific energy analysis of the unit, thermodynamic analysis of a single-phase heated surface in a boiler under such conditions requires investigation. Processes are analyzed, and distributions of energy and exergy are qualitatively revealed. Models for energy analysis, entropy analysis, and exergy analysis of control volumes and irreversible heat transfer processes are established. Taking the low-temperature superheater of a 610 t/h-boiler as an example, the distribution of energy, entropy production, and exergy is depicted quantitatively, and the results are analyzed.