惰性颗粒抑制CH4/O2/N2爆炸过程的二维数值模拟与实验验证

使用带基元化学反应的两相Euler方程,对惰性颗粒抑制高温火团诱导的CH4/O2/N2爆炸过程进行了二维数值模拟.其中,采用全耦合二阶精度的TVD格式求解气相方程,采用MacCormack格式求解惰性颗粒相方程,同时用分步方法处理方程组的耦合刚性,用Gear隐式方法处理化学反应刚性.计算并讨论了不同颗粒相浓度条件下,气相压力场、气相密度和颗粒相浓度的分布、爆炸衰减的气相结构以及两相间的能量传递.结果表明,惰性颗粒相的堆积可阻碍气相爆炸波的传播,使其衰减为引导激波,引导激波继续向颗粒相传递能量,而进一步衰减最终使得爆炸波被抑制.随着颗粒相浓度的增大,爆炸波的抑制更明显.计算结果与大型水平管的实验结果进行了比较,定性证明了计算的合理性.     

砂石颗粒堆积床内流动特性研究

为了降低核电站严重事故中碎片床冷却性分析的不确定性,采用2个尺寸范围的砂石颗粒模拟构建碎片床,并进行了单相与两相流动实验。基于测量的单相流动阻力压降和Ergun方程计算出砂石颗粒的有效直径,在此基础上进行气-水两相流动实验,测量并获得了颗粒堆积床内的两相流动阻力压降,验证碎片床内两相流动阻力模型。结果表明:对于小尺寸砂石颗粒堆积床,其两相流动阻力压降随气相雷诺数的增大呈现上升趋势,在气相雷诺数较低时,Lipinski模型计算值与实验值吻合较好,随着气相雷诺数增大,实验值逐渐接近Reed模型计算值;对于大尺寸颗粒堆积床,相间摩擦力对两相流动阻力有重要影响,其两相流动阻力压降随气相雷诺数的增大呈先下降后上升的趋势。     

AP1000核电汽轮机中调门内两相流动特性数值模拟

利用FLUENT对某AP1000核电汽轮机中压阀内的两相流动进行数值模拟。分别对调节阀在90°、40°、10°3种开度下的两相流与单相流流场比较分析,并对阀门压损以及影响压损的因素进行研究。结果表明:两相流在阀内的最大流速低于同等开度下单相流的最大流速,湿蒸汽两相流随开度的压损变化规律与单相流相似,但两相流的压损将近为单相流的两倍;蒸汽中水滴粒径大小对压力损失的影响较小,而蒸汽含水率对阀门压损影响非常大。     

流化床反应器内气固两相流动特性的研究

基于颗粒动力学理论模拟颗粒相流动,应用流体与颗粒两相流理论考虑两相相间作用,建立了流化床核反应器内多相流流动的计算流体动力学模型,数值模拟研究了流化床核反应器内的流体动力行为。计算结果表明,应用Gi-daspow曳力模型得到的沿截面颗粒浓度分布与已有实验结果的分布趋势比较接近。在中心喷射区的中心处颗粒浓度较高。随着径向距离的增大,逐渐降低到局部最小值后颗粒浓度逐渐上升。在环隙区域内颗粒浓度基本保持不变。分析了流体与颗粒间作用力、颗粒弹性恢复系数等对流化床核反应器内流体动力特性的影响。研究表明,颗粒碰撞恢复系数越大,流场内沿截面颗粒浓度分布变得越均匀。     

微颗粒涡流管惯性分离特性数值模拟

微颗粒摄入对燃气涡轮关键部件产生侵蚀、沉积等问题,严重危害其可靠性与安全性。提出了一种阵列涡流管分离装置,建立了气-粒两相流耦合计算模型,考虑颗粒碰撞和流体相对颗粒作用力,通过分析涡流管内部流动特性、颗粒运动轨迹等,研究了涡流管分离器分离机理,并对比了两种排尘结构对颗粒分离效率的影响。结果表明:涡流管能够有效实现沙尘分离,在两种分离结构颗粒清除口截面积相等的条件下,单清除口分离效率为99.96%,流量损失为6.96%;对称双清除口分离效率为99.90%,流量损失为7.08%,单清除口结构能更快达到颗粒大量分离,减少颗粒在涡流管中的堆积,具有更好的颗粒分离能力。     

高水头电站水轮机沙水流动特性的数值研究

研究泥沙磨损有助于降低水轮机磨损,延长水轮机使用寿命。运用Partical两相流模型对高水头多泥沙电站的水轮机全流道内部沙水两相流动进行数值计算,并分别分析转轮进口沿叶高20%、50%、80%流面的流动特性。计算结果表明,压力从固定导叶进口到转轮出口圆周方向的周向性较好,分布合理,最低压力高于空化压力。活动导叶上泥沙相体积分数最高在导叶头部位置,尾部泥沙相体积分数也较高,导叶靠近头部处和导叶尾部泥沙速度最大。转轮叶片各叶高流面泥沙相体积分数均在叶片工作面尾部达到最大,叶片头部和尾部位置泥沙速度均较高。研究结果对水轮机选材、关键磨损部位的预测及防护方案具有指导意义。     

燃气轮机回流式燃烧室内两相流动数值模拟及分析

利用流体分析软件STAR—CD对某回流式燃气轮机燃烧室的内部流场进行了三维冷态数值模拟及两相流反应数值模拟。建立了燃烧室的三维计算几何模型及计算网格,计算了燃烧室的单相流场及喷雾两相流场。在计算中气相采用N-S方程求解,采用高雷诺数κ-ε湍流模型及SIMPISO算法;液相采用Lagrange法处理,采用颗粒群轨道模型。根据计算结果进行流动分析,为进一步进行燃烧室内部燃烧过程的数值计算分析及改善燃烧室的结构设计、降低排放奠定了基础。     

利用T型三通测量气液两相流体的流量和干度

利用T型三通的相分离特性 ,从被测气液两相流体中分流分离出一部分单相气体 ,通过测量这部分单相气体的流量确定被测气液两相流体的流量或干度。这种方法把两相流体的流量测量转化成了单相流的测量 ,测量仪表的稳定性和可靠性能得到显著改善 ,测量精度得以大幅度提高。试验结果证实 ,分流系数与被测两相流的干度成正比。在本文实验范围内 ,流量和干度的平均测量误差小于± 5 %。     

两相工频感应电炉

两相工频感应电炉是采用T/L接线的变压器取代具有两只感应线圈的单相工频感应电炉的电源装置,使单相结构两相运转,以保持三相电网的平衡。介绍了两相工频感应电炉的原理、电压和容量计算,三相电流的不平衡度及经济效益。     

振荡条件下金属颗粒对气液两相流动与混合特性影响的试验研究

为进一步提升柴油机活塞腔的换热性能,通过加入金属颗粒来提升振荡状态下的两相混合程度。采用振荡流动试验方法,结合数字图像处理技术,研究金属颗粒在振荡状态下的运动学特性和两相振荡流动特性,进一步讨论转速、充液率对金属颗粒运动和最大气泡直径和混合率的影响。结果表明：转速是影响气液两相流振荡流动效果的主要因素。随着转速的增加,金属颗粒到达方腔上壁面时间缩短,瞬时速度增大,相对于270 r/min,330 r/min冲击上壁面的平均速度变化率增加133%,冲击壁面的强度增加。相对于25%充液率,75%充液率的金属颗粒冲击上壁面的最大瞬时速度降低68%,充液率过高,降低颗粒冲击上壁面强度。50%充液率左右和高转速条件下,颗粒冲击壁面强度降低,但气液两相流混合程度最好。     

Entropy Generation in a Circular Tube Heat Exchanger Using Nanofluids: Effects of Different Modeling Approaches

The main goal of this paper is to compare single- and two-phase modeling approaches for forced convection flow of water/TiO₂ nanofluid. The considered geometry is a horizontal tube with constant wall heat flux boundary condition where flow regime is turbulent. A computational fluid dynamics (CFD) approach is utilized for heat transfer and flow field estimation of the single-phase and three different two-phase approaches, namely, volume of fluid, mixture, and Eulerian models. Results are presented for Reynolds numbers ranging from 9000 to 21,000, for different nanoparticle diameters ranging from 20 to 40 nm, and for values of volume fractions ranging from 0 to 4%. The obtained results show that the values of entropy generation for thermal and turbulent dissipation are very close for the single-phase and mixture models. Numerical investigation showed that the values of entropy production for pure water are identical regardless of the CFD approach; however, when the volume fraction of nanoparticles increases, differences between the models appear.     

Dimensional analysis on the evaporation and condensation of refrigerant R-134a in minichannel plate heat exchangers

Two-phase flow analysis for the evaporation and condensation of refrigerants within the minichannel plate heat exchangers is an area of ongoing research, as reported in the literatures reviewed in this article. The previous studies mostly correlated the two-phase heat transfer and pressure drop in these minichannel heat exchangers using theories and empirical correlations that had previously been established for two-phase flows in conventional macrochannels. However, the two-phase flow characteristics within micro/minichannels may be more sophisticated than conventional macrochannels, and the empirical correlations for one scale may not work for the other one. The objective of this study is to investigate the parameters that affect the two-phase heat transfer within the minichannel plate heat exchangers, and to utilize the dimensional analysis technique to develop appropriate correlations. For this purpose, thermo-hydrodynamic performance of three minichannel brazed-type plate heat exchangers was analyzed experimentally in this study. These heat exchangers were used as the evaporator and condenser of an automotive refrigeration system where the refrigerant R-134a flowed on one side and a 50% glycol–water mixture on the other side in a counter-flow configuration. The heat transfer coefficient for the single-phase flow of the glycol–water mixture was first obtained using a modified Wilson plot technique. The results from the single-phase flow analysis were then used in the two-phase flow analysis, and correlations for the refrigerant evaporation and condensation heat transfer were developed. Correlations for the single-phase and two-phase Fanning friction factors were also obtained based on a homogenous model. The results of this study showed that the two-phase theories and correlations that were established for conventional macrochannel heat exchangers may not hold for the minichannel heat exchangers used in this study.     

A comprehensive comparison of various CFD models for convective heat transfer of Al2O3 nanofluid inside a heated tube

This study presents a comprehensive comparison between various models in numerical/CFD approaches to investigate a case study of the laminar forced convection flow of Al₂O₃/water nanofluid with 1.6% volume fraction and Re = 1600 in a heated tube. The quantitative deviation in Nusselt number for the case study is reported using (i) four types of single-phase models, including Newtonian and non-Newtonian single-phase models with assessing the effect of two different thermal dispersion models based on velocity and temperature gradient (ii) four types of two-phase models, including Eulerian, mixture (types 1 and 2) and discrete phase models. According to the results, non-Newtonian single-phase model predicts more accurate Nusselt number than Newtonian single-phase model, with average errors of 5.98% and 4.84% respectively. Incorporating the dispersion models in non-Newtonian single-phase approach, the average error decreases to 2.07% for dispersion models type 1 and 3.33%, for dispersion models type 2. Regarding two-phase models, Eulerian, mixture type 1, mixture type 2, and discrete phase model show the average error of 2.79%, 17.57%, 5.87% and 2.73% respectively. The repeatability and the consistency of the findings of some of most accurate models was checked for 0–2% nanoparticle volume fraction and also for Re ranging from 745 to 1600. This study benefits when comes to selecting a suitable model for a similar type case study.     

A Demonstrative Study on the Two-phase vs. Single-phase Modeling of Buoyancy-driven Flows of Enclosed Nanofluids

A demonstrative numerical study on natural convection of water-based nanofluids in square enclosures with different boundary conditions imposed at the walls, and different orientations with respect to the gravity vector, is performed using both the single-phase and the two-phase approaches, with the main scope to evaluate in what measure the single-phase approach fails in describing the basic heat and fluid flow features, as well as in determining the thermal performance of nanofluids. The system of the mass, momentum and energy transfer governing equations is solved by way of a computational code based on the SIMPLE-C algorithm. Empirical correlations are used for the calculation of the effective thermal conductivity, the effective dynamic viscosity, and the thermophoretic diffusion coefficient. The following configurations are investigated: a tilted cavity differentially-heated at two opposite walls; a vertical cavity partially-heated at the bottom wall and cooled at both sides; and a vertical cavity differentially-heated at the vertical and horizontal walls. It is found that the non-uniform distribution of the suspended solid phase throughout the enclosure gives rise to a solutal buoyancy force, whose competition with the thermal buoyancy force results in a periodic flow detectable only if the two-phase approach is applied. Moreover, the impact of the dispersion of the nanoparticles into the base liquid, which turns out to be notably higher at higher average temperatures, is found to be systematically underestimated by the single-phase approach.     

Numerical study of convective heat transfer of nanofluids in a circular tube two-phase model versus single-phase model

Laminar convective heat transfer of nanofluids in a circular tube under constant wall temperature condition is studied numerically using a CFD¹ approach. Single-phase and two-phase models have been used for prediction of temperature, flow field, and calculation of heat transfer coefficient. Effects of some important parameters such as nanoparticle sources, nanoparticle volume fraction and nanofluid Peclet number on heat transfer rate have been investigated. The results of CFD simulation based on two-phase model were used for comparison with single-phase model, theoretical models and experimental data. Results have shown that heat transfer coefficient clearly increases with an increase in particle concentration. Also the heat transfer enhancement increases with Peclet number. Two-phase model shows better agreement with experimental measurements. For Cu/Water nanofluid with 0.2% concentration, the average relative error between experimental data and CFD results based on single-phase model was 16% while for two-phase model was 8%. Based on the results of the simulation it was concluded that the two-phase approach gives better predictions for heat transfer rate compared to the single-phase model.     

基于FLO-2D与冲量模型的泥石流危险度分区方法及应用

鉴于正确的泥石流危险度分区对合理规划布局泥石流堆积区经济活动的重要作用,采用FLO-2D模型对泥石流的运动堆积过程进行数值模拟,求得泥石流堆积扇上流速、流深分布及以格网为单位的泥石流体流速和流深的时间变化过程,结合GIS技术对计算所得数据进行空间分析,以泥石流体冲量为分区指标,建立泥石流堆积扇危险度分区模型,并以四川省石棉县马颈子沟泥石流为例,模拟该沟百年一遇泥石流运动堆积过程。结果表明,模拟堆积范围与实际堆积面积吻合度达86.16%;根据格网内泥石流体冲量模长,可将泥石流堆积扇危险度分为极度、重度、中度、轻度危险区四个等级。     

Phase change characteristics and their effect on the performance of hydrogen recirculation ejectors for PEMFC systems

The working fluid of the hydrogen recirculation ejector in proton exchange membrane fuel cell (PEMFC) systems is humid hydrogen containing water vapour. However, previous studies on the hydrogen recirculation ejector using computational fluid dynamics (CFD) were based on the single-phase flow model without considering the phase change of water vapour. In this study, the characteristics of the phase change and its effect on the ejector performance are analysed according to a two-phase CFD model. The model is established based on a non-equilibrium condensation phase change. The results show that the average deviation of the entrainment ratio predicted by a single-phase flow model is 25.8% compared with experiments involving a hydrogen recirculation ejector, which is higher than the 15.1% predicted by the two-phase flow model. It can be determined that droplet nucleation occurs at the junction of the primary and secondary flow, with the maximum nucleation rate reaching 4.0 × 10²⁰ m^?3s^?1 at a primary flow pressure of 5.0 bar. The higher temperature, lower velocity, and higher pressure of the gas phase can be found in the mixing region due to condensation, resulting in a lower entrainment performance. The nucleation rate, droplet number, and liquid mass fraction increase remarkably with an increasing primary flow pressure. This study provides a meaningful reference for understanding phase change characteristics and two-phase flow behaviour in hydrogen recirculation ejectors for PEMFC systems.     

Two-Phase Flow Modeling in Microchannels and Minichannels

In this article, three different methods for two-phase flow modeling in microchannels and minichannels are presented. They are effective property models for homogeneous two-phase flows, an asymptotic modeling approach for separated two-phase flow, and bounds on two-phase frictional pressure gradient. In the first method, new definitions for two-phase viscosity are proposed using a one-dimensional transport analogy between thermal conductivity of porous media and viscosity in two-phase flow. These new definitions can be used to compute the two-phase frictional pressure gradient using the homogeneous modeling approach. In the second method, a simple semitheoretical method for calculating two-phase frictional pressure gradient using asymptotic analysis is presented. Two-phase frictional pressure gradient is expressed in terms of the asymptotic single-phase frictional pressure gradients for liquid and gas flowing alone. In the final method, simple rules are developed for obtaining rational bounds for two-phase frictional pressure gradient in minichannels and microchannels. In all cases, the proposed modeling approaches are validated using the published experimental data.     

NUMERICAL INVESTIGATION OF HEAT TRANSFER IN INTENSIVE COOLING TUBES WITH TURBULENCE BUSHES

The heat transfer in intensive cooling tubes with turbulence bushes is studied numerically. Based on experimental data, a two-dimensional, two-phase heat transfer model was developed, which describes the coupled temperature fields in the wire and the annulus. Due to the very high wall temperature of the wire and the special flow conditions, convection-controlled film boiling occurs, in which the heat transfer depends mainly on the parameters of single-phase convection. The asymmetric turbulent flow is described by an appropriate one-dimensional model. In order to investigate shape variations of the bushes, additional experiments or two-dimensional flow simulations are required. Therefore, further modeling is focused on the single-phase heat transfer considering the two-dimensional flow. This numerical model is also validated by experimental results. The numerical experiments show the mechanism of the shape influence of the turbulence bushes on the pressure drop and the heat transfer coefficient. They permit optimization of newly designed turbulence bushes and deliver the characteristic bush parameters for the two-phase model.     

Radial mass-transfer enhancement in bubble-train flow

Compared with single-phase laminar pipe flow, two-phase bubble-train flow shows a significantly increased rate of mass transfer between liquid and wall. The present work is a study of this effect over a wide range of the governing variables. The problem has been numerically simulated and experimentally examined using the copper dissolution method. Furthermore, a mathematical correlation for the Sherwood-number of the system is presented and the mechanism of transport enhancement is elucidated.