Conjugate Mixed Convection Heat Transfer in Plane Laminar Wall Jet Flow

A numerical investigation of heat transfer from a uniformly heated slab of finite thickness by plane laminar wall jet flow under combined forced and natural convection, i.e., mixed convection, is presented. The problem has been solved for two classical cases such as Pr ? 1 and Pr ? 1. The effects of the Grashof number (Gr), Reynolds number (Re), Prandtl number (Pr), and thermal conductivity ratio (R_k) between the slab and fluid medium are investigated on the heat transfer characteristics, i.e., local Nusselt number, interface temperature, and average Nusselt number.     

Numerical Investigation of Double Diffusive Mixed Convection Laminar Flow in Two Sided Lid Driven Porous Cavity

This paper presents the numerical study of mixed convection in a two‐sided lid driven porous cavity due to temperature and concentration gradients. The top and bottom walls are stationary and insulated. The left and right walls are moving at an equal velocity (V_o) in the same direction. The temperature and concentration are kept high at the right wall and low at the left wall. The governing equations are discretized using finite volume method. The pressure–velocity coupling is performed by the SIMPLE algorithm. A third order differed QUICK scheme is applied at the inner nodes and a second order central difference scheme is used at the boundary nodes. The flow behavior and heat transfer are analyzed for different nondimensional numbers, such as, 1 × 10^?4 ≤ Ri ≤ 10, 1 × 10^?4 ≤ Da ≤ 0.1 and 0.7 < Pr < 10. The present numerical results are compared with the literature and are in good agreement. For the above selected nondimensional numbers, the heat and fluid flow behavior is investigated using local and average Nusselt (Nu) and Sherwood (Sh) numbers. Results show that the convection flow is significant up to Da = 0.1, beyond that the effect of porosity is negligible. The effect of Prandtl number (Pr) on average Nu is found to increase significantly.     

弱剪切平面湍流射流的直接数值模拟

采用直接数值模拟方法研究了弱剪切平面湍流射流的涡量场。其中“弱剪切”是指射流出口速度与伴随流速度的大小在同一数量级(文中为2：1)。计算结果精确捕捉到了大涡卷起、两涡配对及特殊的三涡合并现象。纵向速度U的平均分布与实验值符合得很好。由于涡量场关于射流轴线的完全对称性，雷诺应力的分布较为特殊。图6参8     

大涡模拟二维气固两相平面射流

首先用大涡模拟对二维平面射流进行了数值模拟，并就断面流向速度分布、速度脉动量分布以及雷诺切应力分布与实验值进行对比分析，得出大涡模拟方法可以较好地模拟二雏平面射流。在此基础上对二维气固两相平面射流中直径为30μm(St=2．5)的颗粒扩散特性进行数值模拟。发现由于大尺度涡团的作用，颗粒分布于大尺度涡团的外缘而不是集中于中线附近。这与过去基于时均雷诺方程的数值模拟结果得到的颗粒主要集中于中线的结论有所不同，但与直接模拟二维平面混合层中颗粒运动扩散情况相吻合。     

Forced Convection of Metallic Foam Heat Sink under Laminar Slot Jet Confined by Parallel Wall

This investigation numerically explores the fluid flow and heat transfer characteristics of the metallic foam heat sink under the laminar slot jet confined by a parallel wall. The Prandtl number is 0.7, and the range of Reynolds numbers is 100–500. The parameters of interest in this work are the porosity (?), pore density (PPI), effective solid conductivity (k _s *), jet nozzle width (W), ratio of the porous sink length to the jet nozzle width (L/W), ratio of the jet-to-sink distance to the jet nozzle width (C/W), and ratio of the porous sink height to the jet nozzle width (H/W). The simulation data reveal that the Nusselt number of the system with a metallic porous heat sink was much better than that of the system without a porous sink, for a given volumetric flow rate and value of (C+H)/W. The porous properties (such as ? and k _s *) and the system configurations (such as L/W and H/W) strongly influenced the cooling performance. The effect of the PPI, W, and C/W values on the heat transfer characteristics of the system was negligible. The effect of the fluid flow on the thermal results was examined. Finally, the correlations of the stagnation Nusselt number and the average Nusselt number were also determined using the numerical data for a system with the size of a common multi-chips module.     

Mean Flow and Thermal Characteristics of a Turbulent Dual Jet Consisting of a Plane Wall Jet and a Parallel Offset Jet

The standard high-Reynolds number two-equation k ? ? model is used to study the flow and thermal characteristics of a dual jet consisting of a plain wall turbulent jet and a parallel turbulent offset jet (hereafter, dual jet). The flow and thermal characteristics are presented in the form of streamlines, mean velocity vector, turbulent kinetic energy, dissipation of turbulent energy, Reynolds stresses, and isothermal contour plots. The variation in local heat flux and local Nusselt number on the bottom wall is also presented. The finite-volume-method-based SIMPLE algorithm is utilized for understanding the complex nature of flow arising due to a dual jet. The convective flux is discretized using the power-law upwind scheme, while the diffusive term is discretized using the central difference scheme. To study the effect of offset ratio, which is defined as the ratio of height of the jet from the horizontal wall to the width of the jet (nozzle), it is varied between 3 and 15 at an interval of 2. It is noted that the presence of a wall jet in addition to the parallel offset jet has a significant effect on flow and thermal characteristics.     

Developing Flow of Mixed Convection in a Vertical Rectangular Duct with One Heating Wall

Heat transfer and flow patterns of laminar mixed confection in the developing region of a vertical rectangular duct with one heating wall have been investigated numerically in this study. The parabolic boundary layer model is adopted to predict the three-dimensional buoyancy-assisted flow field. Governing equations art solved by using the SIMPI.KC method coupled with a forward marching, implicit finite difference scheme. The velocity and temperature distributions as well as the stream wise variations of local heat flux and fluid pressure are calculated under various geometric parameters. Results show that the flow characteristics are significantly dependent on the buoyancy effect (Gr / Re), the aspect ratio of cross section of duct (H). and the Prandtl number (Pr). The numerical solutions obtained have been compared and verified with existing studies. Meanwhile, the analytical solutions of fully developed flaw recently reported 181 are found to exactly portray the behavior of flow far downstream.     

Numerical Investigation of Laminar Mixed Convection in a Cubic Cavity by MRT-LBM: Effects of the Sliding Direction

Laminar mixed convection in a cubic cavity is studied numerically using the multi-relaxation-time (MRT) lattice Boltzmann method (LBM). The sidewalls of the cavity are thermally adiabatic, while the fixed bottom and top moving walls are maintained at uniform temperatures. Simulations are performed for various values of Richardson number (Ri), sliding angle (θ), and Reynolds number (Re). It is revealed that the Nusselt number increases for different values of Reynolds number as the moving lid angle enhances. In addition, the distribution of the local Nusselt number on the bottom wall follows the direction of the moving lid.     

Numerical Experiments on the Study of Mixed Convection Flow in Cylindrical Geometry

Steady, laminar flow of an incompressible fluid with mixed convection over an isothermal circular cylinder is considered. The governing full nonlinear Navier–Stokes equations with energy equation are solved by using a high-accuracy finite difference scheme in cylindrical polar coordinates. The results are discussed in detail for 5 ≤ Re ≤ 40, 0.7 ≤ Pr ≤ 7, and 0 ≤ Ri ≤ 4 and the results are in agreement with the experimental and other computed data. An increase in the mixed convection parameter leads to reduction of the drag coefficient. The fluid with higher Pr generates vortices that destabilize the flow. The local Nu for the mixed convection cases are higher than the pure forced convection counterparts. The normalized mean Nusselt number (N_m) does not significantly change for flows with very small Richardson numbers. However, for larger Ri, it increases linearly with Ri.     

竖壁降膜波动特性数值模拟研究

建立了竖直壁面降膜流动的二维几何模型,运用VOF方法对雷诺数200～1 000的降膜的波动特性进行了数值模拟。研究了液膜的波动特征、液膜流动方向的速度变化以及液体雷诺数对液膜波动的影响规律。结果表明:根据液膜的形态可以将流动区域分为入口区、发展区和稳定区三部分。入口区的液膜相对比较平滑,发展区的液膜表现为频率较高的小幅波动,稳定区的液膜波动幅度增加而频率减小;入口区的最大流速大于初始流速,发展区的最大流速在初始流速上下波动,而稳定区流速小于初始流速;随着液体雷诺数的增加,液膜厚度增加而波幅降低。     

Regularized Lattice Boltzmann Simulation of Laminar Mixed Convection in the Entrance Region of 2-D Channels

A numerical study of incompressible laminar mixed convection in the entrance region of 2-D vertical and inclined channels using the regularized lattice Boltzmann method is presented. Individual distribution functions with lattice types D2Q9 and D2Q5 are considered to solve fluid flow and thermal fields, respectively. Reynolds number is held constant at 100 and Grashof number is varied from 10³ to 10⁶. The channel inclination angle is varied from 0 to 60°. The aspect ratio of channel is equal to 5. Predicted velocity and temperature fields are in good agreement with velocity and temperature fields found from the finite volume code Fluent.     

Asymmetrical Non-Uniform Heat Flux Distributions For Laminar Flow Heat Transfer With Mixed Convection In a Horizontal Circular Tube

Non-symmetric heat flux distributions in terms of gravity in solar collector tubes influence buoyancy-driven secondary flow which has an impact on the associated heat transfer and pressure drop performance. In this study the influence of the asymmetry angle (0°, 20°, 30° and 40°) with regard to gravity for non-uniform heat flux boundaries in a horizontal circular tube was investigated numerically. A stainless steel tube with an inner diameter of 62.68 mm, a wall thickness of 5.16 mm, and a length of 10 m was considered for water inlet temperatures ranging from 290 K to 360 K and inlet Reynolds numbers ranging from 130 to 2000. Conjugate heat transfer was modelled for different sinusoidal type outer surface heat flux distributions with a base-level incident heat flux intensity of 7.1 kW/m². It was found that average internal heat transfer coefficients increased with the circumferential span of the heat flux distribution. Average internal and axial local heat transfer coefficients and overall friction factors were at their highest for symmetrical heat flux cases (gravity at 0º) and lower for asymmetric cases. The internal heat transfer coefficients also increased with the inlet fluid temperature and decreased with an increase in the external heat loss transfer coefficient. Friction factors decreased with an increase in fluid inlet temperature or an increase in the external heat loss transfer coefficients of the tube model.     

Numerical Simulation of Steady Mixed Convection Around Two Heated Circular Cylinders in a Square Enclosure

In this paper, a numerical study has been carried out to investigate the steady-state mixed convection around two heated horizontal cylinders in a square two-dimensional enclosure. The cylinders are located at the middle of the enclosure height and the walls of the cavity are adiabatic. Streamlines and isotherms are produced and the effects of cylinder diameter, Reynolds number, and Richardson number on the heat transfer characteristics are numerically analyzed. The average Nusselt number over the surface of cylinders and average nondimensional temperature in the enclosure are also presented. The results show that both heat transfer rates from the heated cylinders and the dimensionless fluid temperature in the enclosure increase with increasing Richardson number and cylinder diameter. However, the trend of average Nusselt number and nondimensional temperature variation is completely opposite when Reynolds number increases. In addition, by increasing the cylinders diameter and Richardson number, the left cylinder is less affected by the inlet flow than right one.     

Numerical Simulation of Double Diffusive Mixed Convection in a Lid-Driven Square Cavity Using Velocity-Vorticity Formulation

In this article, convection driven by combined thermal and solutal concentration buoyancy effects in a lid-driven square cavity is examined using velocity-vorticity form of Navier-Stokes equations. The governing equations consist of vorticity transport equation, velocity Poisson equations, energy equation, and concentration equation. Validation results are discussed for convection due to heat and mass transfer in a lid-driven square cavity at Re = 500, Le = 2, and G_RT  = G_RS  = 100. These results indicate that the present velocity-vorticity formulation could predict the characteristic parameters of flow, temperature, and solutal concentration fields using a much coarser mesh compared to the mesh used in a stream function-vorticity formulation. The capability of the proposed algorithm to handle complex geometry is demonstrated by application to mixed convection in a lid-driven square cavity with a square blockage. The effect of buoyancy ratio on the convection phenomenon is discussed for buoyancy ratio varying from ? 100 to 100 at Re = 100. Under opposing temperature and concentration gradients along the vertical direction, the negative buoyancy ratios give rise to aiding flows.     

Unsteady Mixed Convection of a Confined Jet in a Fluid-Superposed High Porosity Medium

Numerical study of a confined jet impingement cooling of a fluid-superposed porous medium heated from below is conducted to investigate the oscillatory mixed convection. The effects of the Rayleigh number (2 × 10⁵ ≤ Ra ≤ 1 × 10⁶) and the Darcy number (1 × 10^?5 ≤ Da ≤ 5 × 10^?4) on the heat transfer are investigated for different Péclet numbers. It is found that, the average Nusselt number increases with the increase in Darcy number or Rayleigh number. The values of average Nusselt number are found to oscillate with time for some combination of Rayleigh numbers (Ra ≥ 4 × 10⁵) and Péclet numbers (200 ≤ Pe ≤ 1000), at which the oscillatory convection occurs. The oscillation of average Nusselt number is investigated for different porous medium height and porous medium-to-fluid heat capacity ratio.     

贫化电炉喷射流场气泡羽流数值模拟

针对现有贫化电炉所使用的还原喷枪存在还原率低等问题,采用水力学物理模拟和数值模拟相结合的研究方法,对喷射搅拌过程进行了计算流体力学仿真和水力实验,研究结果为优化A公司贫化电炉的还原工艺过程提供了一定的理论依据,并根据试验需求提出黄金分割边缘检测算法以观察气泡羽流形态,模拟和试验结果符合Freymuth(1966)所观察的"旋涡成对"现象.     

聚合射流氧枪射流特性模拟研究

建立了聚合射流氧枪数学模型,基于CFD计算软件对有无伴随流的超音速氧气射流特征进行了模拟,以射流温度和密度计算云图阐明了火焰伴随聚合射流的基本原理,分析了不同伴随流条件下火焰伴随聚合射流核心区长度的变化规律。模拟结果表明:火焰伴随流氧枪射流核心区长度优于纯氧聚合射流,是传统超音速氧枪核心区长度的4倍左右,主氧周围甲烷和副氧反应所产生的高温低密度火焰能够保护主氧射流,使主氧射流形成高度聚合状态;随燃气伴随流热值的提高,聚合射流核心区长度增加;随甲烷燃气压力以及环境温度的升高,聚合射流核心区长度变长。     

流线形喷嘴时射流泵流场的数值模拟

为分析流线形喷嘴时射流泵的水力特性,采用紊流数值模拟方法,对流线形喷嘴时射流泵流场进行了三维计算。结果表明,随着流量比的增大,工作液流核区衰减得越慢,在喉管入口段工作液提升被吸液的区域有所减小;流量比越大喉管内紊动能最大值出现的位置会靠后且其数值会降低,两股液体混掺作用会变弱;喉管内压力随流量比的增大而逐渐降低且负压区范围有所扩大,最低负压发生在喉管壁处。同一流量比下,流线形喷嘴时射流泵的压力比略微高于圆锥形喷嘴时,但差别很小。流线形喷嘴时射流泵流场的计算成果,可为研究射流泵水力特性提供参考。     

Numerical Analysis of Laminar Forced Convection in Corrugated-Plate Channels with Sinusoidal,Ellipse, and Rounded-vee Wall Shapes

Steady, two-dimensional, developing laminar forced convection is computed in channels of ten modules of three out-of-phase (symmetric) wall corrugations: sinusoidal-wavy-shaped (SWS), rounded-ellipse-shaped (RES), and rounded-vee-shaped (RVS). Different geometric configurations of the three shapes are studied. Fluid flow and heat transfer are examined in a typical module in the fully-developed region for Reynolds numbers in the range of 25 to 300 for a Prandtl number of 2.29. Over the ranges of the geometric parameters studied, the SWS corrugation has, in general, the lowest friction factor and highest average Nusselt number values. The RES corrugation has the highest heat transfer per unit pumping power at an inlet Reynolds number of 300.     

Numerical Simulation of Natural Convection and Radiation Heat Transfer in Two-Dimensional Enclosure on Hybrid Grids

Natural convection and radiation heat transfer interaction commonly exists in engineering problems, and a numerical method for combined natural convection and radiation heat transfer is very important in practical engineering applications. In this article, the finite-volume method (FVM) for radiation is formulated and implemented in the fluid flow solver GTEA on hybrid grids. For comparison and validation, three test cases, an equilateral triangular enclosure and a square enclosure with/without baffles, are chosen. Then, natural convections in a cavity with/without baffles are simulated with the present FVM to take into account the radiation heat transfer effects. All the results obtained by the presented FVM agree very well with the exact solutions as well as results obtained by the zone method. Natural convection under low gravity is researched with Gr = 0.7 and 700, and the radiation effects on the temperature distribution are also studied with variation conduction-radiation numbers, Nr = 0.06, 0.1, and 0.15. It is found that the solutions are sensitive to the conduction-radiation number but do not change very much with the Grashof number.