用分形理论处理煤飞灰微粒在声场中的团聚现象

通过对煤飞灰微粒在声场中团聚实验研究的观测发现,团聚后的煤飞灰粒子并非为球形或近似为球形,因其特殊的形状,将其简化为当量球形来处理,模型预测结果与实际情况偏差较大。运用分形和分维理论对前人的团聚模型作了修正,用修正后的模型对煤飞灰微粒团聚作预测,并与前人的计算结果进行比较     

三角形阻流件几何参数对传热系数影响的数值研究

对设置三角形阻流件的水平矩形通道内的换热问题进行了二维数值模拟,在Re=1500～35000范围内,研究了阻流件顶角大小和高度对恒热流加热壁面的水平通道内的对流传热系数的影响状况.结果表明:当Re≤8000时,阻流件顶角的变化对Nu数影响不明显;当Re＞8000后,随着顶角α的增加,Nu数先上升后下降;当顶角在30°时,换热效果最好.阻流件高度对Nu数的影响很大,在相同的顶角下,高度越高,Nu数越大.     

Improvements in FFD Modeling by Using Different Numerical Schemes

Indoor environment design and air management in buildings requires fast simulation of air distribution. A fast fluid dynamics (FFD) model seems very promising. This work was to develop the FFD by improving its speed and accuracy. Enhancement of computing speed can be realized by modifying the time-splitting method. Improvements in accuracy were achieved by replacing the finite-difference scheme by the finite-volume method and by proposing a correction function for mass conservation. Using the new FFD model for several indoor air flows, the results show significant reduction in computing time and great improvements on accuracy.     

Numerical Modeling of Combined Radiation and Conduction Heat Transfer in Mineral Wool Insulations

This article addresses numerical modeling of coupled heat conduction and radiation in mineral wools under steady-state condition for prediction of its effective thermal conductivity. The radiative heat transfer is modeled using the Monte Carlo Ray-Trace Method. The radiation model is based on a random distribution of fibers in the media. The radiation distribution factor is employed in order to compute the fraction of the total radiation emitted from one fiber that is absorbed by another, due to both direct radiation and to all possible reflections within the enclosure. The radiation model is coupled with the nonlinear heat conduction equation. The results obtained by the proposed model compare well with experimental measurements of the heat flow meter apparatus. The method is easy to code, and the number of calculations during each iteration is considerably reduced.     

Weighted Sum of Gray Gas Modeling for Nongray Radiation in Combusting Environment Using the Hybrid Solution Methodology

A hybrid solution approach is proposed to solve the radiation transfer equation (RTE) in combusting environment using the weighted sum of gray gases model for modeling the absorption coefficient. The hybrid approach uses a selective combination of the finite-volume method (FVM) and P1 approximation to solve the RTE for different gray gases. A comprehensive study with a wide range of test cases with homogenous and nonhomogenous media is carried out to study the accuracy, speed, and convergence rate of the hybrid solution approach. The differences between hybrid and FVM results are described, and the sources of errors are discussed.     

Effects of Thermal Radiation for Electronic Cooling on Modified PCB Geometry under Natural Convection

In this study, the discrete ordinates method (DOM) model is employed to estimate the effect of thermal radiation from multiple heat sources in a natural-convection flow field. It is found that the flow field around the chips can be altered by natural convection as induced by radiative heat transfer. The influence of thermal radiation is higher than 65% when the chipboard is in a vertical orientation. Furthermore, even if the chip surface temperature is only 317 K, the influence of radiative heat transfer is still up to 18%. Therefore, radiative heat transfer cannot be ignored for electronic component computational fluid dynamics simulation under natural convection.     

平板V型小翼各参数对风力机功率系数的影响

为提高风力机的功率系数Cp,对平板V型小翼的参数进行了优化试验.采用正交试验法选取5个基本参数,分析了各个参数对风力机功率的影响,找出了对其影响明显的参数以及小翼尺寸与叶片尺寸的相对关系.     

余热锅炉辐射受热面辐射系数的影响因素

论述了余热锅炉内辐射传热的影响因素,计算得到了烟气入口温度、三原子气体含量、含尘量、饱和蒸汽压力、有效辐射层厚度、积灰厚度等因素对辐射系数C′的影响关系,以此指导余热锅炉的设计。     

不同结构燃烧室内流场的三维模拟

利用AVL Fire软件对105柴油机进气、压缩过程进行三维瞬态数值模拟,模拟结果给出了两种形状燃烧室详细的流场结构.在压缩上止点之前,A型燃烧室(直口ω形)内挤流速度明显低于B型燃烧室(缩口ω形),压缩上止点后B型燃烧室逆挤流强度大,持续时间长,说明缩小燃烧室开口直径能有效增强挤流和逆挤流;B型燃烧室的湍流强度明显高于A型燃烧室,B型燃烧室最大湍动能约为A型燃烧室的两倍.     

分形几何及其在汽轮机组振动故障征兆提取中的应用

包含丰富汽轮机转子振动和故障信息的一维时间振动波形和二维轴心轨迹图具有分形的特性。研究它们的分形特性,计算不同振动故障类型的分形维数。结果表明不同的故障类型的分形维数不同。分形维数与故障类型之间存在某种对应关系。分形维数的计算的确可以作为故障征兆撮的一种方法加以应用。     

A Fast Numerical Simulation for Modeling Simultaneous Metal Flow and Solidification in Thin Cavities Using the Lubrication Approximation

A numerical algorithm for modelling steady flow of liquid metal accompanied by solidification in a thin cavity is presented. The problem is closely related to a die cast process and in particular to the metal flow phenomenon observed in thin ventilation channels. Using the fact that the rate of metal flow in the channel is much higher than the rate of solidification, a numerical algorithm is developed by treating the metal flow as steady in a given time-step while treating the heat transfer in the thickness direction as transient. The flow in the thin cavity is treated as two dimensional after integrating the momentum and continuity equations over the thickness of the channel, while the heat transfer is modelled as a one-dimensional phenomenon in the thickness direction. The presence of a moving solid-liquid interface introduces non-linearity in the resulting set of equations, and which are solved iteratively. The location and shape of the solid-liquid interface are found as a part of the solution. The staggered grid arrangement is used to discretize the flow governing equations and the resulting set of partial differential equations is solved using the SIMPLE algorithm. The thickness direction heat-transfer problem accompanied by phase change is solved using a control volume formulation. The results are compared with the predictions of the commercial software FLOW3D® which solves the full three-dimensional set of flow and heat transfer equations accompanied with solidification. The Reynolds's lubrication equations accompanied by the through-the-thickness heat loss and solidification model can be successfully implemented to analyze flow and solidification of liquid metals in thin channel during the die cast process. The results were obtained with significant savings in CPU time.     

江苏省太阳总辐射月经验系数计算方法的对比研究

基于江苏省淮安、吕泗、南京及周边省份莒县、郑州、固始、合肥、宝山、杭州、屯溪共计10个辐射代表站建站以来历年逐月太阳总辐射和日照百分率数据,拟合各辐射代表站月经验系数a、b值作为样本,采用反距离权重插值法及REOF分区法两种气候学方法计算江苏省各无辐射观测气象站的月经验系数a、b,并分析不同气候学方法计算的月经验系数估算太阳总辐射量与实测值的差异。结果表明,采用气候学计算方法估算太阳总辐射量实际可行;以两种不同气候学方法计算的月经验系数估算太阳总辐射量效果均较好,但采用REOF分区法所得的经验系数更适用于江苏省太阳总辐射量的气候学估算。     

增压锅炉烟气发生器燃烧流场数值模拟与结构优化

本文采用数值模拟方法,对增压锅炉烟气发生器设计结构方案的燃烧流场进行数值分析,分析内部的流动规律以及原始设计中存在的问题。通过改进烟气发生器的几何结构,得出更加合理的流场分布,从而指导它的结构优化设计。数值计算结果表明,所采取的改进措施的效果明显,达到了优化增压锅炉烟气发生器燃烧段设计方案的主要目的。     

Field Method Research for Leak-off Coefficient Analysis Using Instantaneous Shut-in Pressure

The accurate monitor and prediction of fracturing pressure for formation is very important to hydraulic fracturing treatment operation, but whether hydraulic fracturing is successful or not, the fracturing fluid plays a very important role, leak-off coefficient is the most leading parameters of fracturing fluids. Mini-frac test was the most commonly used tools for leak-off coefficient analysis, but it has the shortcoming of time-consuming and costly that can not meet the requirement of the production. The main purpose of this paper is to introduce a simple and convenient leak off coefficient analysis method with more inexpensive and time-saving than former methods. Based on ISIP （instantaneous shut-in pressure） method, a new field method of leak off coefficient field analysis model was presented. According to twice ISIP of the fracturing treatment in field operation, therefore, fluid leak off coefficient and formation characteristic can be studied quickly and reliably. More than 40 wells were fractured using this field method. The results show that average liquid rates of post-fracturing was 20 m3/d which double improvement compared with the past treatment wells. It had an important role for fracturing treatments in low permeability used in field application. reservoirs, the new model for real time analysis and adjust is successful     

Numerical Modeling for Multiphase Incompressible Flow with Phase Change

ABSTRACT

A general formula for the second-order projection method combined with the level set method is developed to simulate unsteady, incompressible multifluid flow with phase change. A subcell conception is introduced in a modified mass transfer model to accurately calculate the mass transfer across the interface. The third-order essentially nonoscillatory (ENO) scheme and second-order semi-implicit Crank-Nicholson scheme is employed to update the convective and diffusion terms, respectively. The projection method has second-order temporal accuracy for variable-density unsteady incompressible flows as well. The level set approach is employed to implicitly capture the interface for multiphase flows. A continuum surface force (CSF) tension model is used in the present cases. Phase change and dynamics associated with single bubble and multibubbles in two and three dimensions during nucleate boiling are studied numerically via the present modeling. The numerical results show that this method can handle complex deformation of the interface and account for the effect of liquid–vapor phase change.     

Modeling for Shell-Side Heat Transfer Coefficient and Pressure Drop of Helical Baffle Heat Exchangers

This study has suitably modified the existing heat transfer and pressure drop correction factors of the modified Bell–Delaware method used for heat exchangers with segmental baffles, taking into consideration the helical baffle geometry. These correction factors are presented in parametric formulas based on the Taborek presented procedure for segmental baffles. These formulas are functions of the geometrical and physical parameters of discontinuous helical baffles. In addition, the parametric formulas are presented graphically based on the Stehlik method. The correction design method proposed by Stehlik for the helical baffle is presented in detail and a theoretical model for shell-side heat transfer and pressure drop is developed. In general, the results show that the present model matches more closely with the graphically proposed correction factors of Stehlik. In order to calculate the shell-side heat transfer coefficient and pressure drop using the present method, a computational code has been developed by the authors. In addition, in order to examine the validity, the results of the code for a case study are compared with the results obtained from EXPRESS software and experimental formulas presented by Zhang. The results of comparison show that the proposed method is accurate and can be used by designers confidently.     

A Numerical Model for Freeze Drying Processes with Infrared Radiation Heating

A numerical model has been developed for accurate prediction of freeze drying processes subjected to infrared radiation heating. A spectral two-flux radiation model was incorporated into freeze drying analysis to consider the spectral absorption characteristics of moisture-rich products. The temperature of the infrared heater was found to significantly influence the freeze drying behaviors. At a fixed intensity level at the product surface, a higher heater temperature increased the drying time while temperature in the product was significantly reduced. This result was discussed in relation with the spectral composition of light emitting from infrared heaters.     

地板辐射采暖系统的数值模拟

采用标准K-ε模型,对低温地板辐射采暖系统进行数值模拟,研究采暖房间的流场与温度的分布状况,得出辐射采暖的一些突出优点;具体分析了埋管不同保温层和地表装饰材料对房间的流场分布及能量的有效利用率的影响。     

抽水蓄能电站调压室阻抗系数的三维数值模拟

选用适合调压室流动的三维可视化k-ε双方程湍流模型以及考虑边壁粗糙度的壁函数,采用一阶精度及S IM PLE算法,以FLUENT流体计算软件为计算平台,对抽水蓄能电站调压室流场进行了数值模拟。通过对比和分析表明,在分流、合流两种流态下利用计算流体动力学CFD方法计算得到的水头损失系数值与常规物理模型试验得到的结果,在规律和数值上基本吻合,表明计算流体动力学可以应用于抽水蓄能电站调压室的优化设计中。     

碳粒燃烧能量分配系数的数值计算

应用改进的移动火焰锋面(MFF)模型对碳粒燃烧的能量分配系数进行了研究,得到了考虑CO在颗粒边界层内燃烧及CO2发生表面还原反应的能量分配系数表达式．在一实际煤粉火焰条件下,计算了可在单膜(SF)模型中直接应用的可比能量分配系数,并与传统SF模型的取用值进行了比较．计算结果表明,碳粒直径越小,平均能量分配系数越大;随着碳粒温度的升高,能量分配系数下降．在本文的计算条件下,平均能量分配系数约为0．7,平均可比能量分配系数值高于0．5,远大于传统单膜模型(SF-CO)计算的可比能量分配系数0．3．