FLOW BOILING OF WATER AND n -HEPTANE IN MICRO CHANNELS

Heat transfer characteristics during flow boiling in micro channels with rectangular cross section were studied using a thermographic measuring method. The characteristic length of the channels investigated was varied in a region of 300 w m up to 700 w m. The channels were designed as Joule heating pipes. Thus, the evaporation was achieved under conditions of nearly constant heat fluxes at the heating wall of the channel. The thermographic measuring method was used to examine the wall temperature. The high spatial and temporal resolution of this thermographic measuring method makes it possible to detect the axial position of the different boiling regions. Furthermore, it allows conclusions to be made on which flow conditions occur in the different sections of the channel. Experimental results are shown with water and n -heptane as the fluid to be vaporized. The results of measurements are discussed and a correlation is given of the location where the dryout starts with the characteristic parameters. The dependence of the pressure drop in such channels on the technological parameters has also been presented.     

NUMERICAL STUDY OF TURBULENT FLOW AND HEAT TRANSFER IN A CONVEX CHANNEL OF A CALORIMETRIC ROCKET CHAMBER

A numerical study on flow and heat transfer in double-wave cross-corrugated passages with different structure parameters was conducted. The three-dimensional governing equations for mass, momentum, and heat transfer were solved using a control volume finite difference method and a validated low-Reynolds number k-? model. The effects of Reynolds number and structure parameters, including pitch ratio (P₁/P₂) and height ratio (H₁/H₂), were studied. It was found that with a decrease in height ratio, the mainstream flow changed from a pattern dominated by L-shaped flow to one dominated by Z-shaped flow, whereas pitch ratio had almost no influence on the flow pattern. The average Nusselt number Nu_av first increased and then decreased gradually with either an increase in the pitch ratio or a decrease in the height ratio. Pressure drop showed the same trend as heat transfer performance. The best performance evaluation criterion number (g) of double-wave passage was nearly 20% higher than that of the corresponding single-wave passage, whereas the worst was nearly 40% lower. On the whole, the double-wave plate with H₁/H₂ = 5 showed better overall performance. The double-wave plate with P₁/P₂ = 1 had better overall performance for Re < 5,000, whereas that with P₁/P₂ = 3 was better for Re > 7,500.     

ANALYSIS OF TWO-PHASE RADIATION IN THERMALLY DEVELOPING POISEUILLE FLOW

Combined conductive and radiative heat transfer in a thermally developing two-phase Poiseuille flow in a cylindrical duct is studied here. A two-phase radiative transfer equation (RTE) considering radiation by both gas and particles is taken into account. A complexform of nonlinear integrodifferential RTE is solved by the discrete ordinates method (DOM, or so called SN method) in axisymmetric geometry. After such validation, namely, the solution in a two-dimensional channel flow between two flat plates is compared with that solved by the zone method, the program is then applied to fully developed gas-particle two-phase flow in a cylindrical duct. A parametric study is performed for gas and particle absorption coefficients, particle number density, particle emissivity, and wall emissivity. The results show a significant effect of two-phase radiation on the thermal characteristics. However, in all cases, it was found that conduction is predominant near the wall.     

CHOKED FLOW CALCULATIONS OF TWO-PHASE BUBBLY FLOW

A new choked flow criterion based on the system eigenvalues derived by the characteristic analysis of a hyperbolic nonequilibrium two-fluid model is employed in the RELAP5/MOD3 code. A modification of the choked flow model produced by the earlier works of Trapp and Ransom is elaborated so that better predictions of choked flow rate can be made in the two-phase bubbly flow. Marviken choked flow tests on the large-scale break of pipes are assessed by using the present criterion. The assessment results demonstrate more accurate predictions of choked flow rate in the bubbly flow regime without any adjustment than those of the earlier calculations by using the equilibrium choked flow criterion.     

PEMFC翅脉流道传质及输出性能分析

为研究流道结构对质子交换膜燃料电池（PEMFC）反应气体质量传输及输出性能的影响,建立翅脉流道、叶脉流道及蛇形流道的三维PEMFC几何模型,并对比3种流道的反应气体浓度分布、压力分布及电流密度分布,最后对翅脉流道结构参数进行优化。结果表明,与蛇形流道、叶脉流道相比,翅脉流道能明显改善流道和扩散层内反应气体浓度分布的均匀性,有利于强化反应气体向催化层的质量传递;翅脉流道能减小气体压力分布梯度,使反应气体扩散更加充分;翅脉流道的平均膜电流密度更大,有利于促进电化学反应稳定进行;翅脉流道能改善PEMFC的输出性能,翅脉流道峰值功率密度比蛇形流道、叶脉流道分别提高7.72%和6.25%;减小翅脉流道的直流道长度或圆弧流道圆心角,可提升翅脉流道输出性能。     

NUMERICAL INVESTIGATION OF TWO-DIMENSIONAL LAMINAR FLOW AND SOLUTE TRANSPORT IN A CHANNEL WITH SOME SYMMETRIC EXPANSIONS AND CONTRACTIONS

Numerical investigation of two-dimensional (2D) laminar flow and solute transport in a channel with some sudden symmetric expansions and contractions has been performed using the fictitious regions method. This method allows us, instead of solving Navier-Stokes equations in a complex domain, to solve equations with suitably continued coefficients in a rectangle. Stream function-vorticity variables are used in the present paper. Dependence of the flow and solute transport from the dimensions of the channel expansions and contractions is numerically investigated for different values of Reynolds and Péclet numbers using a finite differences method on a relatively fine grid.     

NUMERICAL PREDICTION OF BUOYANCY-INDUCED PERIODIC FLOW PATTERN AND HEAT TRANSFER IN A LID-DRIVEN ARC-SHAPE CAVITY

Numerical simulation has been carried out to study the unsteady phenomenon of a buoyancy-induced periodic flow and convection heat transfer in a lid-driven arc-shape cavity. The governing equations in terms of the stream function-vorticity formulation are solved by the finite-volume method coupled with a body-fitted coordinate transformation scheme. In the range of the Reynolds number (Re) from 100 to 2,000 and Grashof number (Gr) up to 5 2 10 7 , the heat transfer characteristics and flow pattern have been predicted. Attention has been focused on the combined effects of the inertial and buoyant forces exerted on the fluid. Results show that only when the inertial and buoyant forces are of approximately equal strength that can the periodic flow pattern be observed. For an inertia-dominant or buoyancy-dominant situation, the periodic flow pattern is not visible.     

THERMOPHORESIS OF PARTICLES IN GAS-PARTICLE TWO-PHASE FLOW WITH RADIATION EFFECT

The radiation effect on the thermophoresis of particles is analyzed for a gas-particle twophase laminar flow. Two-phase radiation by both gas and particles is considered; in addition, the thermal nonequilibrium between gas and particle is taken into account. It is concluded that the particle diffusion velocity as well as particle concentration depends strongly on the optical radius of gas or particle. In general, the radiation was found to decrease the particle diffusion. In case that gas as well as particle radiation exists, the deposition of particle is mainly influenced by the gas. The effects of parameters such as the optical radius, conduction to radiation parameter, thermal loading ratio, and wall emissivity on the cumulative collection efficiency E ( x ) are also considered. As the optical radius and tube wall emissivity increase, E ( x ) decreases. The increase in conduction to radiation parameter N and thermal loading ratio CL leads to an increase in E ( x ).     

ENHANCEMENT OF HEAT TRANSFER IN TURBULENT SEPARATED AND REATTACHING FLOW BY LOCAL FORCING

This article presents numerical solutions for solving the problem of a mixed convective micropolar fluid flow and heat transfer along a vertical wavy surface with a discontinuous temperature profile. The overall surface is equally divided into a heated section succeeded by an unheated section alternately. The problems in the present study have been formulated by using a simple transposition theorem and the cubic spline collocation method. Eringen has applied the spline alternating direction implicit (SADI) procedure to solve the governing momentum, angular momentum, and energy equations those formulated. Along the wavy surface, the velocity, temperature, and microrotation profiles are presented. The influences of micropolar parameters R, u , geometry, and Gr/Re 2 number on the skin friction coefficient and Nusselt number have been studied in this work. The results demonstrate that the skin friction coefficient consists of a mixture of two harmonics in micropolar fluids and in Newtonian fluids. As the vortex viscosity parameter (R) increases, the heat transfer rate decreases, but the skin friction increases. In addition, when the spin gradient viscosity parameter ( u ) increases, the skin friction decreases. Comparisons between a Newtonian fluid and a micropolar fluid are also discussed.     

NUMERICAL PREDICTIONS OF HEAT TRANSFER AND FLUID FLOW CHARACTERISTICS FOR SEVEN DIFFERENT DIMPLED SURFACES IN A CHANNEL

Heat transfer and fluid flow characteristics for seven different dimpled surfaces on one surface of a channel are predicted numerically using version 6.1.18 of FLUENT. The turbulent model employed is a realizable κ–? model without a wall function. The different dimples investigated are spherical dimples, tilted cylinder dimples, cylinder dimples, in-line triangular dimples, reverse in-line triangular dimples, staggered triangular dimples, and reverse staggered triangular dimples. Results show the existence of a centrally located vortex pair and vortex pairs near the spanwise edges of each dimple for the three circular dimple types, which augment local magnitudes of eddy diffusivity for momentum and eddy diffusivity for heat. Advection of reattaching and recirculating flows from locations within the spherical-type dimple cavities, as well as strong instantaneous secondary flows and mixing within the vortex pairs, are especially apparent. For the four triangular types of dimples, only one primary flow circulation zone generally is present within individual dimples. In all cases, regions of augmented streamwise vorticity show approximate correspondence to locations where eddy diffusivities for momentum and heat are increased. Overall, the highest heat transfer augmentations, and the most significant local and overall increases to eddy diffusivity for momentum and eddy diffusivity for heat, are produced by the spherical dimples and the tilted cylinder dimples.     

NUMERICAL SIMULATION OF THREE-DIMENSIONAL SEPARATED FLOW AND HEAT TRANSFER AROUND STAGGERED SURFACE-MOUNTED RECTANGULAR BLOCKS IN A CHANNEL

ABSTRACT

Numerical results simulating a three-dimensional laminar separated flow and heat transfer around staggered surface-mounted rectangular blocks in a plane channel are presented. Treated in the present study is a case of staggered three-row blocks. The finite-difference method is employed to solve the Navier-Stokes and energy Equations directly, and the resulting finite-difference Equations are solved with the SMAC method for Re = 100–500 and Pr = 0.7. The present numerical results are found to simulate well the visualization results such as horseshoe vortices and recirculating flow. The heat transfer coefficient greatly varies on the different side surfaces of blocks and also with Reynolds number.     

A NUMERICAL INVESTIGATION OF THE CONVECTIVE HEAT TRANSFER IN UNSTEADY LAMINAR FLOW PAST A SINGLE AND TANDEM PAIR OF SQUARE CYLINDERS IN A CHANNEL

The unsteady laminar flow and heat transfer characteristics from square cylinders located in a channel with a fully developed inlet velocity profile were studied numerically. The time-averaged Nusselt number for each face and the time-averaged cylinder Nusselt number (Nu) were determined, as well as aerodynamic characteristics such as cylinder lift, drag, and eddy-shedding Strouhal number (St) . The results show that the cylinder Nu decreases for both the single and the tandem pair of cylinders as they approach the channel wall. The upstream eddy-promoting cylinder significantly reduces the drag of the downstream cylinder as compared with that of the single cylinder. The St decreases as the wall is approached and is larger for the tandem pair than for the single cylinder for all positions.     

矩形管道多声路超声波测流的数学模型及计算方法

本文从分析压力管道水流流速分布规律出发,分别应用高斯和马尔柯夫数值计算方法,得出了矩形管道多声路超声波测流流量计算、声道分布坐标及权重系数的数学模型,并用现场实测资料进行了验证。     

特隆勃墙通道内自然对流边界层流动

利用标度分析得到表征特隆勃墙通道内空气自然对流的宏观特征参数与系统的运行控制参数和结构参数之间的内在相关关系式,并利用直接数值模拟计算结果对其进行验证和定量化.     

TURBULENT FLOW AND HEAT TRANSFER IN A CHANNEL WITH A SQUARE BAR DETACHED FROM THE WALL

Computations are conducted to study the heat transfer and friction in a channel with a mounted square bar of different sizes detached from the channel wall. The Reynolds number (Re) based on channel height ranges from 10 4 to 105, whereas the bar height to channel height (d H) varies from 0.15 to 0.35. The channel walls are subjected to a constant wall temperature. The standard k- epsilon turbulence model and a modified version were used in conjunction with the Reynolds-averaged momentum and energy equations for the simulations and were compared thereafter. A finite volume technique with staggered grids combined with the SIMPLEC algorithm is applied. Results show that the local and global Nusselt numbers on the channel walls are strongly modified by the unsteady vortex shedding induced by the bar. The displacement of the bar from the channel axis toward the wall did not cause an increase in the global heat transfer coefficient on the channel walls compared with the bar centered in the channel.     

A NUMERICAL STUDY OF TWO-PHASE FLOW USING A TWO-DIMENSIONAL TWO-FLUID MODEL

Two-phase flow is studied numerically in two dimensions using a two-fluid equation system. The development is based on the surface-tension force terms incorporated in the momentum equations. The governing equations become hyperbolic type for which an upwind method such as flux vector splitting (FVS) avails. As a benchmark problem, a two-phase shock tube is first used to study the wave propagation characteristics and interaction between the gas and liquid phases. Fluid sedimentation due to the density difference and cavity growth in a duct with a bend are showed next. Advantages and capabilities of the present formulation are discussed in some detail.     

BOUNDARY ELEMENT ANALYSIS OF VISCOUS CHANNEL FLOW

The boundary element method (BEM) is used to investigate laminar viscous flows in internal channels. A direct iterative scheme is used to cope with the nonlinear character of the integral equations. To achieve convergence, an underrelaxation technique is employed at relatively high Reynolds numbers. Numerical examples of Poisseuille and backward-facing step flow problems are considered. It is found that BEM gives accurate solutions up to a certain Reynolds number for each case.