Computer Simulation of Turbulent Flow through a Hydraulic Turbine Draft Tube

Based on the Naviev-Stokes equations and the standard κ-ε turbulence model, this paper presents the derivation of the governing equations for the turbulent flow field in a draft tube. The mathematical model for the turbulent flow through a draft tube is set up when the boundary conditions, including the inlet boundary conditions, the outlet boundary conditions and the wall boundary conditions, have been implemented. The governing equations are formulated in a discrete form on a staggered grid system by the finite volume method. The second-order central difference approximation and hybrid scheme are used for discretization. The computation and analysis on internal flow through a draft tube have been carried out by using the simplee algorithm and cfx-tasc flow software so as to obtain the simulated flow fields. The calculation results at the design operating condition for the draft tube are presented in this paper. Thereby, an effective method for simulating the internal flow field in a draft tube has been explored.

Experimental study on pressure fluctuation characteristics of slug flow in horizontal curved tubes

In order to study the pressure characteristics of slug flow in horizontal curved tubes,two kinds of curved tubes with central angle 45° and 90° respectively are studied,of which are with 0. 5m circumference and 26 mm inner diameter are used. When the superficial liquid velocity or the superficial gas velocity is constant,the pressure fluctuations and the probability distribution of the average velocity of slug flow are clear for all of the five experimental conditions. The results of experiment show that the pressure characteristics of slug flow in curved tubes have periodic fluctuations. With the rise of central angle,the period of pressure fluctuation is more obvious. The system pressure of the slug flow increases with the increasing of superficial liquid/gas velocity. Meanwhile,the probability distribution of pressure signal shows regularity,such as unimodal,bimodal or multimodal.

Numerical investigation on flow and heat transfer characteristics of corrugated tubes with non-uniform corrugation in turbulent flow

Based on finite volume method, the pressure drop and heat transfer characteristics of one smooth tube and ten different axisymmetric corrugated tubes, including two with uniform corrugation and eight with non-uniform corrugation, have been studied. A physical model of the corrugated tube was built, then the numerical simulation of the model was carried out and the numerical simulation results were compared with the empirical formula.The results show that: the friction factor decreases with the increase of Reynolds number ranging from 6000 to 57000, the value of which in the corrugated tubes with non-uniform corrugation(tube 03–10) are smaller than those with uniform corrugation(tube 01–02). The geometry parameters of tube(01) have advantages on the heat transfer enhancement in low Reynolds number flow region(from 6000 to 13000) and tube(07–08)have advantages on the heat transfer enhancement in high Reynolds number flow region(from 13000 to 57000). The vortex, existed in each area between two adjacent corrugations called second flow region, is the root of the enhancement on heat transfer in the corrugated tubes. The effectiveness factor decreases with the increasing of Reynolds number and the performances of the corrugated tubes with pitch of 12.5 mm have advantages than these of 10 mm under the same corrugation geometric parameter.

THEORETICAL STUDY ON THE BIFURCATION OF VORTEXES STRUCTURE FOR FLOW IN CURVED TUBE

The number and distribution of the singular points of streamlines in the cross-section of steady flow through a curved tube ate discussed by using the method of topological structure analysis. And a theoretical criterion is obtained for the bifurcation of flow vortexes for the secondary flow turning from two-vortex structure into four-vortex structure. Furthermore, the critical Dean number for bifurcation and the semi-analytical expressions of stream function and axial velocity are given by using Galerkin technique. The result of calculation is consistent with the theoretical criterion.

NUMERICAL INVESTIGATION OF THREE-DIMENSIONAL VISCOUS INCOMPRESSIBLE FLOWS IN DIVERGENT CURVED CHANNELS AND TURBULENT MODEL STUDY

In order to make the numerical calculation of viscous flows more convenient for the flows in channel with complicated profile governing equations expressed in the arbitrary curvilinear coordinates were derived by means of Favre density-weighted averaged method, and a turbulent model with effect of curvature modification was also derived. The numerical calculation of laminar and turbulent flown in divergent curved channels was carried out by means of parabolizeil computation method. The calculating results were used to analyze and investigate the aerodynamic performance of talor cascades in compressors preliminarily.

An advanced turbulator with blades and semi-conical section for heat transfer improvement in a helical double tube heat exchanger

In present work,a helical double tube heat exchanger is proposed in which an advanced turbulator with blades,semi-conical part,and two holes is inserted in inner section.Two geometrical parameters,including angle of turbulator’s blades(θ) and number of turbulator’s blades(N),are considered.Results indicated that firstly,the best thermal stratification is achieved at θ=180°.Furthermore,at the lowest studied mass flow rate(m = 8 × 10^(-3) kg/s),heat transfer coefficient of turbulator with blade angle of 180° is 130.77%,25%,and 36.36% higher than cases including without turbulator,with turbulator with blade angle of θ =240°,and θ =360°,respectively.Moreover,case with N=12 showed the highest overall performance.At the highest studied mass flow rate(m = 5.842 × 10^(-2) kg/s),heat transfer coefficient for case with N=12 is up to 54.76%,27.45%,and 6.56% higher than cases including without turbulator,with turbulator with N=6,and with turbulator with N=9,respectively.

An application of space-filling curves to improve results of turbulent aerodynamics modeling with convolutional neural networks

When carrying out calculations for turbulent flow simulation,one inevitably has to face the choice between accuracy and speed of calculations.In order to simultaneously obtain both a computationally efficient and more accurate model,a surrogate model can be built on the basis of some fast special model and knowledge of previous calculations obtained by more accurate base models from various test bases or some results of serial calculations.The objective of this work is to construct a surrogate model which allows to improve the accuracy of turbulent calculations obtained by a special model on unstructured meshes.For this purpose,we use 1D Convolutional Neural Network(CNN)of the encoder-decoder architecture and reduce the problem to a single dimension by applying space-filling curves.Such an approach would have the benefit of being applicable to solutions obtained on unstructured meshes.In this work,a non-local approach is applied where entire flow fields obtained by the special and base models are used as input and ground truth output respectively.Spalart-Allmaras(SA)model and Near-wall Domain Decomposition(NDD)method for SA are taken as the base and special models respectively.The efficiency and accuracy of the obtained surrogate model are demonstrated in a case of supersonic flow over a compression corner with different values for angleαand Reynolds number Re.We conducted an investigation into interpolation and extrapolation by Re and also into interpolation byα.

Experimental research on stabilities, thermophysical properties and heat transfer enhancement of nanofluids in heat exchanger systems

Stable TiO_2–water nanofluids are prepared by a two-step method, stabilities of nanofluids are investigated by precipitation method and transmittance method respectively, and thermal conductivities and viscosities are also measured. An experimental system for studying the heat transfer enhancement of nanofluids is established,and heat transfer and flow characteristics of TiO_2–water nanofluids in heat exchanger systems with a triangular tube and circular tube are experimentally studied. The effects of nanoparticle mass fractions(ω = 0.1 wt%–0.5 wt%) and Reynolds numbers(Re = 800–10000) on the heat transfer and flow performances of nanofluids are analyzed. Fitting formulas for Nusselt number and resistance coefficient of nanofluids in a triangular tube are put forward based on the experimental data. The comprehensive performances of nanofluids in a triangular tube are investigated. It is found that nanofluids in a triangular tube can significantly improve the heat transfer performance at the cost of a small increase in resistance coefficient compared with that in a circular tube, especially the resistance coefficients are almost the same between different nanoparticle mass fractions at turbulent flow. It is also found that the comprehensive evaluation index η decreases with Reynolds number at laminar flow but a critical maximum value appears at turbulent flow.

变截面半圆通道内超临界二氧化碳热工水力性能研究

建立了不同半圆通道的数学和物理模型,并通过与实验数据的比较,验证了所建立模型的准确性。研究了超临界二氧化碳(S-CO_(2))在等截面半圆通道、渐扩型和渐缩型通道内的热工水力换热性能,并分析了不同通道和压力对S-CO_(2)在变截面半圆通道内的热工水力性能的影响。结果表明:相较于均匀截面半圆通道,渐扩型通道恶化传热,渐缩型通道强化传热;入口和出口半径比为1.0:0.5的渐缩型半圆通道的总体换热系数最大提升了39.93%,流动传热综合性能评价因子PEC最大为1.346;当压力越靠近临界压力或热通量较低时,传热性能越高。最后,从场协同性和湍动能分布的角度解释了渐缩型变截面通道强化传热的原因。研究结果可为S-CO_(2)循环系统冷却器的设计与优化提供新的思路和理论指导。

Simple Power-type Heat Transfer Correlations for Turbulent Pipe Flow in Tubes

The paper presents three power-type correlations of a simple form, which are valid for Reynolds numbers range from 3.10^3 ≤ Re ≤ 10^6, and for three different ranges of Prandtl number： 0.1 ≤ Pr ≤ 1.0, 1.0≤ Pr ≤ 3.0, and 3.0 ≤Pr ≤ 10^3. Heat transfer correlations developed in the paper were compared with experimental results available in the literature. The comparisons performed in the paper confLrm the good accuracy of the proposed correlations. They are also much simpler compared with the relationship of Gnielinski, which is also widely used in the heat transfer calculations.

Numerical simulation of turbulent flow in helically coiled open-channels with compound cross-sections

Turbulence structure in a helically coiled open channel flow is numerically simulated using three different turbulence models--the Launder and Ying model, the Naot and Rodi model, and the nonlinear k-ε Model (SY model). Simulation results were compared with observation of (i) turbulent flows in alternating point-bar type channel bends with rectangular sections, and (ii) straight open channel flows with compound cross-sections. Based on calculations of the impact of various channel curvatures on turbulence characteristics, accuracy of the three turbulence models was analyzed with observed data as a qualitative reference. It has been found out that the Launder and Ying model and the nonlinear k-ε Model are able to predict the same general trend as measured data, and the simulation of the effect of the centrifugal force on the formation of secondary currents produces a correct pattern.

Particle Image Velocimetry Measurement of Flow Across Tube Bundle in Waste Heat Boiler

In the present paper the experimental investigations of flow across staggered tube bundles in waste heat boiler are conducted by mans of PIV (Particle Image Velocimetry) system. Flow visualization and velocity distribution of the wake between different cylinders are measured in detail. It is concluded that there are still Von Karman vortices in the wake and the phenomena of vortex shedding, pairing, merging are observed in the flow. The Von Karman vortices can’t fully developed because of the existence of the downstream cylinder There is interaction between main streams and vortices, and the development of the vortex is enhanced by this interaction. Meanwhile some statistical results are performed. The distribution of correlated variables of the velocity fluctuations u’ u’, u’ v’, v’ v’ and the space correlation coefficients are obtained.

AN INVESTIGATION OF FLOW CHARACTERISTIC OF AERATED DRAG REDUCTION IN TUBE

Under the aerated conditions of wall and top in tube, the turbulent flow in the tube was measured by using LDA. The turbulent structure of the flow field and the mechanism of aerating drag reduction in the tube were discussed. It is shown that the energy dissipations of turbulence flow and mean flow will reduce and the flow velocity (or flow rate) will increase by injecting mini bubbles to the wall or top of tube, namely the effect of aerating drag reduction is attained.

Numerical Investigation of Developing Turbulent Flow in a Helical Square Ductwith Large Curvature

A fully elliptic numerical study has been carried out to investigate the three-dimensional turbulent developing flow in a helical square duct with large curvature. A two-layer zonal model is proposed and used, in which the whole region is divided into a viscosity-affected near wall layer and a fully turbulent region. A DSM closure is applied in the former, and a one-equation model is solved in the latter. The results presented in this paper cover a Reynolds number range of (l- 10) x 104. The development of flow is found to be dominated by radial pressure gradient and Dean-type secondary motion. The distribution of Reynolds stresses in fully developed flow exhibit a complex pattern of turbulence anisotropy The development of peripherally averaged friction factor and the distribution of local friction factor in fully developed flow are given and discussed.

Which Friction Factor Model Is the Best？ A Comparative Analysis of Model Selection Criteria

The ongoing research for model choice and selection has generated a plethora of approaches. With such a wealth of methods, it can be difficult for a researcher to know what model selection approach is the proper way to proceed to select the appropriate model for prediction. The authors present an evaluation of various model selection criteria from decision-theoretic perspective using experimental data to define and recommend a criterion to select the best model. In this analysis, six of the most common selection criteria, nineteen friction factor correlations, and eight sets of experimental data are employed. The results show that while the use of the traditional correlation coefficient, R2 is inappropriate, root mean square error, RMSE can be used to rank models, but does not give much insight on their accuracy. Other criteria such as correlation ratio, mean absolute error, and standard deviation are also evaluated. The AIC （Akaike Information Criterion） has shown its superiority to other selection criteria. The authors propose AIC as an alternative to use when fitting experimental data or evaluating existing correlations. Indeed, the AIC method is an information theory based, theoretically sound and stable. The paper presents a detailed discussion of the model selection criteria, their pros and cons, and how they can be utilized to allow proper comparison of different models for the best model to be inferred based on sound mathematical theory. In conclusion, model selection is an interesting problem and an innovative strategy to help alleviate similar challenges faced by the professionals in the oil and gas industry is introduced.

刚性植被群对连续弯道水力特性的影响研究

为了探究植被对天然弯曲河流水力特性的影响,通过连续弯曲水槽试验,设计均匀、簇状两种布设形式分别模拟人工规律种植以及自然丛生的植被群,对比分析不同的布设形式对弯道水流流场、水动力轴线、紊动能分布的影响。结果表明:①连续弯槽中水流流场出现明显的流速分区:高流速区靠近凸岸,低流速区则分布在凹岸附近;底层水流与表层水流相比,高流速区分布范围扩大,低流速区减小;布设植被后高流速区分布范围增加,低流速区则剧减,且簇状布设的影响幅度明显大于均匀布设。②连续弯槽水流的水动力轴线偏离正常位置,其沿程表现出先贴近凸岸后脱离凸岸的动态变化过程;植被布设大大延缓了该进程,且簇状布设的延缓程度大于均匀布设。③连续弯槽水流的紊动能沿程分布呈现出不均匀性,最大值出现在弯顶附近区域;从紊动能数值变化可知,植被布设使得水流的紊动能普遍增加,且簇状布设的影响程度大于均匀布设。

液态金属绕流管束流动传热进展

基于液态金属的螺旋管式换热器具有紧凑、换热能力强的特点,在热化学制氢、第四代核能、太阳能高温热发电、余热回收等能源化工系统极具价值,液态金属绕流管束流动传热问题越来越受到重视。然而,绕流管束湍流传热较复杂,实验和数值模拟难度较大,目前尚未有相关可靠文献综述,阻碍了该类换热器设计与技术进步。本文回顾了液态金属绕流管束相关研究,首先指出了液态金属流动传热特性与其他流体的异同,然后简述并比较了液态金属流动传热经验关系式,推荐了该类型换热器设计的流动传热经验公式,紧接着应用经验关系式分别对比了不同工质绕流管束、液态金属流经不同流道的流动传热性能。指出液态金属湍流传热具有一定强化潜力,且绕流管束带来形阻较大,建议采取减阻措施。本文为后续涉及液态金属绕流管束的换热器设计提供了参考。

大型中心型换热器内流量分配特性数值分析

为了解非能动安全壳热量导出系统大型中心型集管式换热器内部流量分配特性,并为其后续设计提供相应参考依据。本文对大型中心型集管式换热器内部流量分配特性开展了数值计算研究。结果表明:增大传热管内流动阻力、增大联箱直径、弧形三通的应用、采用等径分配联箱与汇流联箱等方式,均有利于换热器内流量的均匀分配。而中心管间距过小则会对流量分配带来明显不利影响。传热管管束顺排或叉排布置方式对流量分配不均匀性及换热器内流动阻力基本无影响。

冷却条件下渐扩/渐缩管内超临界压力二氧化碳的传热特性

采用SSTk-ω湍流模型,数值计算了冷却条件下超临界压力二氧化碳(SCO_(2))在三种水平管(等截面管、渐扩管和渐缩管)内的传热特性,研究了不同运行参数(压力、质量流量及热通量)对传热性能的影响。结果表明,与等截面管相比,渐扩管有效地强化了传热,采用渐扩管时,SCO_(2)的总传热系数最大提高了47.98%。然而,相比等截面管,渐缩管却削弱了传热。最后,从类冷凝和湍流场分布的角度阐明了传热强化的物理机理。为SCO_(2)冷却器的优化设计提供了新的思路和理论指导。

在线样品制备技术耦合液相色谱-质谱系统在食品危害物检测中的应用进展

食品安全检测具有重要意义,但食品样品基质复杂,测定其中危害物时,通常需要以下几个步骤:样品制备,即采用适合的样品前处理方法,在不同基质中将目标物分离出来;分离纯化,利用色谱系统进一步分离纯化目标物;定性定量分析,基于目标化合物的性质选择合适的检测器进行分析。其中样品制备是关键步骤之一,将样品制备过程与液相色谱系统耦合可以实现样品的在线自动化分析。与传统人工处理过程相比,在线分析不仅能够减少人工操作误差,保证良好的精密度和重复性,而且可以降低溶剂消耗,避免样品间交叉污染,同时节约分析时间,提高检测效率。本文主要介绍目前常用的在线样品制备技术,包括在线固相萃取(on-line SPE)、管内固相微萃取(in-tube SPME)、湍流色谱法(TFC),详细阐述了其基本原理和耦合设备。在线样品制备技术耦合液相色谱-质谱系统分为两个维度,主要依赖于阀切换技术,将样品制备(第一维度)和液相色谱系统(第二维度)之间建立物理连接,随后采用相应的检测器进行分析。第一维度的作用主要是去除样品杂质,净化分离目标物,为第二维度对目标物进行定性定量检测做准备。此外,文章对3种在线净化系统适用的不同净化填料进行了总结讨论。最后综述了近10年来3类在线系统在食品兽药残留、农药残留、毒素污染以及其他危害物检测中的应用和研究进展,并对该领域存在的问题和发展趋势进行了探讨和展望,以促进在线样品制备技术进一步应用于食品安全检测。