A Direct Pressure-Linking Method for Turbulent Flow Computations

A new numerical method based on the direct approach is presented for turbulent flow computations. The velocity–pressure coupling is done by solving locally, in each control volume and in all possible ways, a linear system for velocity components and pressure. This method uses less memory than a standard SIMPLER implementation, and it shows good numerical stability. The problem of turbulent flow in a straight duct is used to test the ability of this procedure with three different Reynolds numbers. All examples are computed with moderate grids, finding good agreement and reproducing the strong pressure variation in the viscous sublayer.     

电场作用下的液体乙醇微尺度层流扩散燃烧

采用实验和数值模拟方法研究了电场作用下液体乙醇层流扩散微燃烧的火焰结构、火焰无量纲高度和宽度以及Re之间的关系.结果表明,实验拍摄火焰结构图像与数值模拟高温区结构相似,正向电场火焰结构尺寸较反向电场高;H/d与Re呈线性关系,正向电场的H/d随Re增加幅度较反向电场快;W/d随Re先是线性增加,后期变化幅度较小;火焰中...     

Flow Patterns and Heat Transfer for Flow Boiling in Small to Micro Diameter Tubes

An overview of the recent developments in the study of flow patterns and boiling heat transfer in small to micro diameter tubes is presented. The latest results of a long-term study of flow boiling of R134a in five vertical stainless-steel tubes of internal diameter 4.26, 2.88, 2.01, 1.1, and 0.52 mm are then discussed. During these experiments, the mass flux was varied from 100 to 700 kg/m²s and the heat flux from as low as 1.6 to 135 kW/m². Five different pressures were studied, namely, 6, 8, 10, 12, and 14 bar. The flow regimes were observed at a glass section located directly at the exit of the heated test section. The range of diameters was chosen to investigate thresholds for macro, small, or micro tube characteristics. The heat transfer coefficients in tubes ranging from 4.26 mm down to 1.1 mm increased with heat flux and system pressure, but did not change with vapor quality for low quality values. At higher quality, the heat transfer coefficients decreased with increasing quality, indicating local transient dry-out, instead of increasing as expected in macro tubes. There was no significant difference between the characteristics and magnitude of the heat transfer coefficients in the 4.26 mm and 2.88 mm tubes but the coefficients in the 2.01 and 1.1 mm tubes were higher. Confined bubble flow was first observed in the 2.01 mm tube, which suggests that this size might be considered as a critical diameter to distinguish small from macro tubes. Further differences have now been observed in the 0.52 mm tube: A transitional wavy flow appeared over a significant range of quality/heat flux and dispersed flow was not observed. The heat transfer characteristics were also different from those in the larger tubes. The data fell into two groups that exhibited different influences of heat flux below and above a heat flux threshold. These differences, in both flow patterns and heat transfer, indicate a possible second change from small to micro behavior at diameters less than 1 mm for R134a.     

Heat Transfer Correlations for Elongated Bubbly Flow in Flow Boiling Micro/Minichannels

An improved conventional-to-micro/minichannel criterion was proposed by using the Bond number and the liquid Reynolds number. In micro/minichannels, bubbles tend to be confined and elongated in the channel and the conventional two-phase flow theory loses its applicability. As significant disagreement in experimental trends and heat transfer mechanisms was reported for flow boiling in micro/minichannels in the literature, it is not possible to explain the discrepancy and predict all data points by a single correlation without considering the different flow patterns. In this study, heat transfer correlations for elongated bubbly flow in flow boiling micro/minichannels were developed based on a collected micro/minichannel heat transfer database. The newly developed correlations not only can present a decent overall accuracy, but also estimate the parametric trends correctly. More than 97% of the data points can be predicted by the proposed correlations within a ±50% error band for elongated bubbly flow. Also, a flow-pattern-based model can be developed by combining the developed elongated bubbly flow correlations with previous annular flow correlations for predicting flow boiling heat transfer in micro/minichannels.     

大型循环流化床锅炉固体颗粒流动及分布的数值模拟

对引进的４１０ｔ／ｈＰｙｒｏｆｌｏｗ商用循环流化床锅炉燃烧系统固体的流动过程及其分布特性进行了数字模拟,建立的流动模型采用小室模型模拟气固两相沿轴向的流动,采用“核心－环”流动结构模拟气固两相在径向的不均匀性,模拟时考虑了给料,固体颗粒磨损以及床料的宽筛分等特性,为引进大型商用循环流化床锅炉技术的消化吸收进行了初步的基础研究,并为进一步分析研究该循环流化床锅炉的整体特性奠定了基础。     

Reynolds-Averaged Navier-Stokes Equations Describing Turbulent Flow and Heat Transfer Behavior for Supercritical Fluid

Supercritical fluid has been widely applied in many industrial applications.The traditional Reynolds-averaged Navier-Stokes(RANS)equations are directly applied for turbulent flow and heat transfer of the supercritical fluid,ignoring turbulent effect of the thermal physical properties due to the intense nonlinearity.This paper deduces a set of Reynolds-averaged Navier-Stokes equations for supercritical fluid(SCF-RANS equations)to depict turbulent flow and heat transfer of the supercritical fluid taking all the physical parameters as variables.The SCF-RANS equations include many new correlation terms due to fluctuation of the thermal physical properties.Model methods for the new correlation term have been discussed for closing the SCF-RANS equations.Some of them have relatively mature models,while others are completely new and need profound physical theoretical analysis for proposing reasonable models.This paper provides referable information for these new correlations as far as authors know.The SCF-RANS equations not only provide the formulation special for flow and heat transfer of the supercritical fluid,but also represent the most sophisticate form of the RANS equations,for every involved physical property has been considered as variable without any simplification.     

One-Dimensional Semimechanistic Model for Flow Boiling Pressure Drop in Small to Micro Passages

Accurate predictions of two-phase pressure drop in small to micro-diameter passages are necessary for the design of compact and ultra-compact heat exchangers, which find wide application in process and refrigeration industries and in the cooling of electronics. A semimechanistic model of boiling two-phase pressure drop in the confined bubble regime is formulated, following the three-zone approach for heat transfer. The total pressure drop is calculated by time-averaging the pressure drops for single-phase liquid, elongated bubble with a thin liquid film, and single-phase vapor. The model results were compared with experimental data collected for a wide range of tube diameters (4.26, 2.88, 2.02, 1.1, and 0.52 mm) for R134a at pressures of 6–12 bar. In this model's present form, its predictions are close to those of the homogeneous flow model but it provides a platform for further development.     

Review on Heat and Fluid Flow in Micro Pin Fin Heat Sinks under Single-phase and Two-phase Flow Conditions

This article reviews recent studies on the hydrodynamic and thermal characteristics of micro pin fin heat sink (MPFHS). In the studies reviewed in this article, liquid coolants such as water, HFE-7000, HFE-7200, R-123 were tested under both single-phase and two-phase flow conditions. Analytical, computational and experimental research studies were covered with a focus on configurations with traditional arrangements of micro pin fins (MPF) as well as original designs such as oblique finned MPFs, variable density MPF, vortex generators and herringbone structures. Single-phase flow results highlighted pressure drop penalty with achieved heat transfer enhancement. Many studies revealed the inability of conventional correlations to predict the hydrodynamic and thermal characteristics and proposed new correlations for different operating conditions and geometrical specifications. Regarding the studies on two-phase flows the number of performed studies is less than the ones in single-phase flow regime although the diversity of utilized coolants is more. Under flow boiling conditions, the focus was on determining flow patterns among MPFs for different arrangements and under different operating conditions. Unlike the studies on single-phase flows, the data could be relatively well predicted using the earlier suggested model by Lockhart and Martinelli with appropriate coefficients for different arrangements of MPFs.     

Computations with the multiple reference frame technique: Flow and temperature fields downstream of an axial fan

In numerical computations, axial fans are typically abstracted as two-dimensional surfaces, and this forms the basis for the “Lumped Fan” (LF) model. The LF-model relies on experimentally derived P–Q (fan) curves which should conform to the published test codes. Despite its simplicity, the LF-model’s accuracy depends on the application and acceptable error margin. Therefore, with decreasing error margins in thermal engineering, there has been an interest in accurate fan modeling techniques such as “Multiple Reference Frame” (MRF) model. The current effort provides a two-part validation of the MRF model results for an axial fan against the relevant experiments: comparison of the (i) P–Q curves, and (ii) temperature distribution in an electronic enclosure. In the former, both fan models exhibit good agreement with the experiments, however the flow structures determined by the two models exhibit substantial differences. Given these flow structures’ strong influence on the temperature field, the MRF model demonstrates good correlation and significantly better agreement with the measured temperature distribution, compared to those of the LF-model. In light of these findings, benefits, limitations, and possible applications of both models are also discussed.     

微型燃气轮机燃烧室性能的数值研究

针对Capstone公司的C30微型燃气轮机的燃烧室，采用κ—ε湍流模型、EBU—Ambenius湍流燃烧模型描述其燃烧流动，采用扩展Zeldivch机制描述HOx生成；应用分区结构化网格和SIMPLE算法求解控制方程，进行了三维燃烧流动的数值模拟研究，同时对燃烧室的整体性能进行了分析。通过数值计算及结果分析，着重研究了环型贫燃预混燃烧室的燃烧组织形式对燃烧室性能的影响，并探讨了流动控制板对燃烧室内燃烧流动和燃烧室出口HOx分布的影响。数值研究的主要目的是配合新型微型燃气轮机的研制，获得微型燃气轮机燃烧室的设计经验，为研制既有高燃烧效率和燃烧稳定性，又有低HOx排放特性的燃烧室奠定基础。     

Modeling and Coupling Particle Scale Heat Transfer with DEM through Heat Transfer Mechanisms

The detailed heat transfer mechanisms particle interior, gas film around particles, gas gap between contact surfaces, and rough surface are considered to model heat transfer between particles. The validation of the heat transfer model is accomplished and the predicted results show good agreement with other experiments. From the quantitative comparison of four heat transfer paths, it is revealed that the heat transfer through gas gap and rough surface could be neglected for a particle diameter larger than 2 mm. Furthermore, the detailed heat transfer model is coupled with the discrete element method (DEM) to calculate macro effective thermal conductivity (ETC) of fixed beds, and the accuracy and applicability is verified by comparing with other estimated and experimental results. The influence of particle diameter, density, specific thermal capacity, and thermal conductivity on ETC is investigated. Results show that the proposed heat transfer model provides an effective and accurate way to couple with DEM in the particle system.     

微型能源动力装置及微尺度燃烧研究

基于燃料燃烧的微型能源动力装置具有高能量密度特性,可提供瓦到百瓦级的能量输出,因此在过去的20年间受到广泛关注。国内外学者研制了微型的燃气轮机、内燃机、推进装置、燃烧器、热电转换装置及热光电转换系统等不同类型的能源动力装置。然而,由于微尺度条件下燃烧环境和常规尺度存在差异,材料、密封及润滑等方面的技术瓶颈,目前大部分微型能源动力装置的性能未能到达预期的目标。由于微尺度燃烧基础理论有别于传统的常规尺度燃烧理论,随着其重要性的凸显,国内外学者对其进行了广泛深入的研究,更加清晰地揭示了微尺度火焰及燃烧的基本特性。本文首先介绍了国内外微型能源动力装置及系统的研究进展,然后对微尺度条件下预混及非预混火焰的研究现状进行了总结,在本文的最后部分提出了微燃烧相关亟待解决的科学及工程问题。     

微型燃气轮机低热值燃料燃烧室喷嘴流动实验研究

为适应生物质气低热值、组分变化大的特点,达到燃烧室燃烧稳定、低排放要求,在某60 kW级微型燃气轮机环形燃烧室结构的基础上,新设计了一种具有不同预混孔结构的新喷嘴,并在上海交通大学微型燃气轮机单喷嘴燃烧室实验台上对原喷嘴及新设计喷嘴进行冷态流动实验,对比分析不同稀释孔直径及工质参数条件下原喷嘴及新喷嘴对燃烧室空气流量分配比及过量空气系数的影响。研究表明:燃料流量、空气流量变化会影响燃烧室空气流量分配,空气温度变化对燃烧室空气流量分配无影响;燃料热值降低会导致燃烧室过量空气系数增大,需要匹配直径更大的燃烧室稀释孔;相比于原喷嘴新喷嘴的流量分配比较大,为使其适应低热值燃料,需要匹配的燃烧室稀释孔直径为11.0 mm,该条件下新喷嘴可适应CH_4摩尔分数为50%~90%的燃料。     

The Flow Resistance and Heat Transfer Characteristics of Micro Pin-Fins with Different Cross-Sectional Shapes

The flow resistance and heat transfer characteristics of deionized water flowing through a rectangular channel (60 mm × 5.2 mm × 0.5 mm) with staggered array micro pin-fin circular, diamond, and elliptical groups are experimentally investigated over Reynolds numbers ranging from 8 to 1,000, and the investigation shows that the flow resistance increases due to the endwall effect and large pin-fins density at low Re. With the increase in Re, the endwall effect is weakened, but the flow resistance still increases due to the appearance of vortex resistance, and the heat transfer is enhanced due to the flow disturbance or transition from laminar flow to turbulent flow. The experimental results are also compared with predictions of the theoretical correlations for the staggered array micro pin-fin groups, and the comparisons indicate that only the correlation related to the diamond shaped micro pin-fin groups approximately agrees with experimental data, and the other correlations do not describe well the flow and heat transfer characteristics covering laminar, transitional, and turbulent states in circular and elliptical test sections.     

柴油机Urea-SCR系统内流场与化学反应耦合研究

提出三维流场与一维化学反应相耦合的方法对柴油机尿素(urea)-选择性催化还原(SCR)系统进行全尺寸模拟,真实反映SCR系统内部气流运动状态对化学反应的影响。选择柴油机典型工况的排气温度、流量及压力等物理信息作为边界条件,对SCR系统进行流场分析,截取催化器前端截面的速度切片,建立流速均匀性系数与工况的函数模型,研究各流速分布下运行参数对NO_x转化效率及副产物N_2O生成的影响,为优化SCR系统控制策略提供指导。结果表明,不同工况下柴油机SCR催化器前端各区域流速差异明显,且流速均匀性系数在0.6～0.8区间内,氨氮比为1.1且NO_2、NO_x之比在0.0～0.5区间内时,NO_x转化效率最高,副产物N_2O的生成最少。     

供热汽轮机抽汽供热循环耦合汽水流量分配计算的研究

以常规热平衡法为基础,推导出了考虑加热器散热损失的供热机组T级抽汽供热循环系统热平衡矩阵方程,并将等效热降法应用于供热机组T级抽汽供热循环耦合汽水流量的分配计算,计算结果与常规热平衡法完全一致.本方法简单明晰,通用性好,为供热机组热力系统发电与供热的整体耦合分析计算奠定了基础,并可简捷快速地计算出热力系统连接方式以及局部参数变化对抽汽供热循环耦合汽水流量的影响.     

Eigenfunction Expansion Solution for Boundary-Layer Equations in Cylindrical Coordinates: Simultaneously Developing Flow in Circular Tubes

The Generalized Integral Transform Technique (GITT) is employed, via a novel eigenfunction expansion, in the solution of the steady-state continuity, momentum, and energy equations under the boundary-layer formulation and cylindrical coordinates, and applied to the solution of simultaneously developing laminar flow inside circular ducts. The streamfunction formulation is adopted to automatically satisfy the continuity equation and to eliminate the pressure field. A fourth-order eigenvalue problem is thus considered for the velocity field, eliminating the difficulties associated with the singularity at the channel centerline through this recently introduced expansion basis. A thorough analysis of convergence behavior is undertaken for both the velocity and temperature proposed eigenfunction representations, and here illustrated for representative values of governing parameters and positions along the channel. Results for quantities associated with applications, such as the product of the friction factor–Reynolds number and Nusselt numbers, are also computed along the entrance region for different values of the governing parameters, and tabulated for reference purposes. Critical comparisons with previous results in the literature are also performed, in order to validate the numerical code developed and to inspect the adequacy of previously proposed approximate solutions.     

水平微细管内CO2流动沸腾流态研究

针对CO_2水平微细管内流动沸腾换热流态及流态转变特性进行实验研究。实验工况:热流密度(5~35 k W/m~2)、质量流率(50~600 kg/(m~2·s))、饱和温度(-40~0℃)、管径(0.5~1.5 mm)。实验表明:CO_2在微细管内实际流态分别是泡状流、弹状流、间歇流、层流、波状流、混状流、环状流和雾状流;干涸过程中的流态主要为环状流-雾状流、波状流-雾状流的过程及不稳定的环状流;通过理论计算获得CO_2微细管内流动沸腾换热流态图,流态图显示热流密度对高干度区域流态转变有显著影响,质量流率大小直接决定了换热过程所经历的流态;不同饱和温度工质热物性不同改变了流型;理论分析所采用的流态形式与实际CO2在微细管内所具有的流态类型基本一致。     

Optimal Homotopy Asymptotic Solutions for Nonlinear Ordinary Differential Equations Arising in Flow and Heat Transfer due to Nonlinear Stretching Sheet

Optimal homotopy asymptotic method (OHAM) is used to obtain solutions for nonlinear ordinary differential equations (ODEs) arising in fluid flow and heat transfer at a nonlinear stretching sheet. The solutions for skin friction and temperature gradient for some special cases are tabulated and compared with the available numerical results in the literature. Moreover, OHAM is found to be very easy to use and the technique could be used for solving coupled nonlinear systems of ordinary differential equations arising in science and engineering.     

叶顶微射流对小型高负荷离心压气机性能及流场结构的影响

以某小型高速离心压气机为研究对象,采用数值方法研究了微射流对压气机性能和叶轮叶顶流场结构的影响。研究结果表明：射流为1%设计流量时,失速裕度能够提高3.12%,稳定工作范围拓宽28.17%;在设计点,原型离心压气机叶顶来流马赫数达1.8以上,叶顶存在复杂激波/间隙泄漏流干扰,工作稳定性较差,微射流改变了“λ”状的激波结构,使前缘激波的强度减弱,后掠角度减小,并且降低了叶顶的负荷水平;微射流能够抑制间隙泄漏流的周向运动,并削弱激波/间隙泄漏流之间的相互作用,间隙泄漏涡不易发生破裂、溃散,极大增强了压气机工作的稳定性。