生物质超临界水气化制氢反应建模及数值模拟

建立了管式反应器中生物质超临界水气化制氢反应的数学模型,同时提出了以葡萄糖做为生物质模型化合物的全局气化反应动力学模型。模型计算结果与实验值的比较表明该模型能较好的预测反应器出口温度与气体产物组份分布。利用该模型数值模拟计算得到了反应器中温度场、速度场基本情况以及化学反应速率分布的基本规律。该文通过计算还讨论了反应器入口水温、反应器壁温以及物料和预热水之比对反应器内气化反应的影响,得出一系列重要结论。该模型对生物质超临界水反应器系统的优化设计与化学反应最佳工况的选择有一定的实用价值。     

NUMERICAL ERRORS OF THE GALERKIN FINITE-ELEMENT METHOD FOR NATURAL CONVECTION OF A FLUID LAYER OR A FLUID-SATURATED POROUS LAYER

This paper describes the effects of element size and formula used for the calculation of temperature gradients on the local and average Nusselt numbers of natural convection for the widely used Galerlan finite-element method. Two cases of laminar two-dimensional natural convection are examined, namely, a fluid layer and a porous layer. The numerical error in the Nusselt numbers decreases with decreasing element size. The maximum error occurs at the position of a maximum of the local Nusselt number. In addition to the effect of element size, the Nusselt numbers are shown to vary with the formula used for calculating temperature gradients. The Nusselt numbers extrapolated to zero element size for different formulas are found, in both cases, to be virtually identical and also to agree well with the experimental data and the results computed by finite-difference methods     

近海区域风速数值模拟试验分析

在可持续能源发展战略的实施中，风能等可再生能源的开发利用是重要的战略选择。其中风能资源的评估是其开发利用的关键，如何利用有限的观测资料进行资源评估和分析就成为迫切需要解决的问题。该文利用中尺度气象模式MM5对海上风场进行模拟分析，模拟结果在风的分布趋势很好与多年平均实况吻合，为我国海上风资源的开发利用提供有力的科学依据。     

NUMERICAL MODELING OF A NUCLEATE BOILING SURFACE

ABSTRACT

A computer program developed to analyze nucleate boiling over a heated surface is described. The model solves the three-dimensional transient conduction equation within the heater. The conduction solution is coupled with closure relationships to mimic the bubble dynamics and the associated heat transfer coefficients. Sample problems are run using a copper surface subject to partial nucleate boiling in saturated water at atmospheric pressure. The results are shown to be in good qualitative agreement with the pertinent experimental observations.     

考虑转捩的风力机翼型动态失速数值模拟

以风力机专用翼型的动态失速为对象,采用一种基于流场当地变量的Gamma-Theta转捩模型配合SSTk-ω湍流模型进行数值模拟,研究转捩对动态失速性能的影响和动态失速下的转捩规律。结果表明,使用考虑转捩效应,能够使动态失速过程中上仰段大迎角状态下失速和下俯段气流再附的模拟得到改善。在动态失速上仰段,上表面转捩由后缘分离泡向前缘分离泡的转变过程较快,导致转捩点迅速前移;而在下俯段,前缘分离泡向后缘分离泡的转变过程中经过了自然转捩和再层流化的过渡,因此转捩点的移动较上仰段平滑。     

NUMERICAL MODELING OF INTERIOR BOUNDARIES

The numerical modeling of interior boundaries of finite and infinitesimal volume (area) is described for finite volume numerical methods. The treatment of passive structures such as solid obstacles and infinitesimally thin porous and nonporous walls, as well as hydro-dynamically active structures such as pumps and fans, are discussed. Properties peculiar to the pressure-velocity coupling are stressed, while more generally applicable techniques for other dependencies are shown. Heal transfer and turbulence effects complementary to the hydrodynamics are discussed. An example is presented showing application of the techniques to the flow of air about a very large directly air-cooled heat exchanger.     

NUMERICAL SHAPE OPTIMIZATION FOR HIGH PERFORMANCE OF A HEAT SINK WITH PIN-FINS

The design optimization of a 7 × 7 pin-fin heat sink is performed numerically. To achieve higher thermal performance of the heat sink, the thermal resistance at the junction of the chip and the heat sink and the overall pressure drop in the heat sink have to be minimized simultaneously. The fin height (h), fin width (w), and fan-to-heat sink distance (c) are chosen as the design variables, and the pressure drop (ΔP) and thermal resistance (θ _ja ) are adopted as the objective functions. To obtain the optimum design values, we used the finite-volume method for calculating the objective functions, the Broydon-Fletcher-Goldfarb-Shanno method for solving the unconstrained nonlinear optimization problem, and the weighting method for predicting the multiobjective problem. The results show that the optimum design variables for the weighting coefficient of 0.5 are as follows: w = 4.653 mm, h = 59.215 mm, and c = 2.669 mm. The objective functions corresponding to the optimal design are calculated as ΔP = 6.82 Pa and θ _ja  = 0.56 K/W. The Pareto solutions are also presented for various weighting coefficients, and they offer very useful data for designing a pin-fin heat sink.     

NUMERICAL MODELING OF BUOYANCY-DRIVEN FLOWS IN A ROTATING CYLINDRICAL CAVITY: COMPARISON OF A FINITE ELEMENT MODEL WITH A SPECTRAL MODEL

Abstract

A finite element model is developed for the prediction of the motion of rotating Boussinesq fluid driven by buoyancy. The computations art performed for the axisymmetric regime in an annular cavity for Reynolds number varying from 0 to 2500. The results are compared with those of an earlier study of this problem using a spectral Tau-Chebyshev method. The good agreement found assesses the finite element model. Finally, a complementary convergence analysis gives the sensitivity of the model to mesh refinement.     

DIRECT NUMERICAL SIMULATION OF LAYER MERGING IN A SALT-STRATIFIED SYSTEM

Time-dependent double-diffusive convection was studied numerically to clarify the mechanism of layer merging in a salt-stratified system. Using the Chebyshev collocation method, a typical example of stably stratified salt fluid subject to a lateral temperature difference in a rectangular enclosure (Ar = 1.25) is considered for realistic values of parameters (Ra_T = 2.7 X 10⁷, N_i = 0.882, Pr = 7.15, and Sc = 685). Two cases that differ by the initial salt concentration profile, i.e., linear and steplike profiles, are examined. Although globally, in both cases, the layer merging process is characterized by the mass transfer across the interface separating two convection layers, the two instances are quite different with respect to the interface structure. For the linear profile, vertical motion due to salt fingers is dominant, whereas for the steplike profile horizontal motion due to strong shear flows prevails. In particular, in the latter case, unlike for the linear profile case, traveling plumes perpendicular to shear flows that lead to the time variations in temperature and concentration are periodically generated within the interface. Predictions obtained with the simulations are in good agreement with experimental data.     

NUMERICAL STUDY ON COOLING PHENOMENON OF WATER WITH SUPERCOOLED REGION IN A HORIZONTAL CIRCULAR CYLINDER

Ice formation in a horizontal circular has been studied numerically. From the numerical analysis results, it was found that there were three types of freezing patterns and that the freezing phenomenon was affected largely by density inversion and cooling rate. The type of freezing pattern largely depends on the secondary flow, which is generated by density inversion. When supercooling energy is released before the development of the secondary flow, an annular ice layer grows. If the energy is released when the secondary flow is considerably developed and the supercooled region is removed to the upper half part of the cylinder, an asymmetric ice layer grows. If the energy is released after perfect development of the secondary flow, instantaneous dendritic ice formation over the full region occurs. Furthermore, the secondary flow was found to have an effect on heat transfer characteristics. The heat transfer rate becomes small at the instant when the secondary flow is generated, but it becomes large with the development of the secondary flow. It is concluded that for the facilitation of heat transfer it is desirable to keep water, in its liquid phase until the secondary flow is perfectly developed. This study gives an instruction on the performance improvement of a capsule-type ice storage tank.     

NUMERICAL MODELING OF HELICAL FLOW OF PSEUDOPLASTIC FLUIDS

The laminar helical flow of non-Newtonian pseudoplastic fluids in concentric and eccentric annuli with a rotating inner cylinder has been investigated numerically. A finite volume algorithm with a nonstaggered grid system is used to analyze the problem. A nonorthogonal curvilinear coordinate system is employed to handle the irregular geometry of aneccentric annulus. The power-law constitutive equation is used to model the shear rate dependent viscosity of a pseudoplastic fluid. The computer code is validated against an available analytical solution for helical flow in a concentric annulus. It is observed that for a certain axial pressure gradient the axial flow rate increases within creasing rotational speed of the inner cylinder. The torque needed to rotate the inner cylinder decreases with increasing axial pressure gradient. These are explained in terms of the shear-thinning effect of a pseudoplastic fluid. The discharge as well as torque are found to increase with increasing eccentricity. The flow field in an eccentric annulus is complex in nature since vigorous secondary flow is produced in addition to the primary axial helical flow. The location and extent of the secondary flow is studied and theresults are presented for various eccentricities. The results will be useful in planning oil and gas well drilling operations.     

NUMERICAL SIMULATION OF FLOW PATTERN FORMATION IN A BOTTOM HEATED CYLINDRICAL FLUID LAYER

Abstract

The temporal formation of the buoyancy-driven flow structures in a bottom heated, shallow, cylindrical fluid layer was numerically studied. The unsteady three-dimensional Navier-Stokes and energy equations were discretized by the power law scheme and solved by the fully implicit Marker-and-Cell method. Computations were carried out for the pressurized argon (Pr=0·69) and water (Pr=6·1) layers for various Rayleigh numbers and heating rates of the layer. In the pressurized argon layer at a slightly supercritical Rayleigh number with Ra_f;=1·05Ra_c a steady straight roll pattern was formed when the heating rate was very slow (a=0·001) after a long transient stage. When the heating rate was raised to a=0·01, a very different structure tike U-rolls was formed at steady state. In the water layer with Ra_f=l·05Ra_c, a straight roll pattern was again formed, but at a equals;0·07. At Ra_f;=1·13Ra_c, curved rolls with the three foci at the sidewall were formed for a=0·01. A pattern in the form of U-rolls appears at a=0·01. Regular concentric circular rolls prevail at a=1·0. When the Rayleigh number is further raised to 1·23Ra_c, the resulting steady flow is dominated by incomplete circular rolls with open ends near θ=0°     

NUMERICAL INVESTIGATION OF AN INTERNAL LAYER IN TURBULENT FLOW OVER A CURVED HILL

The development of an internal layer in a turbulent boundary layer flow over a curved hill is investigated numerically. The turbulent flow equations are solved by a control volume based, finite-difference method. The turbulence is described by a multiple-time-scale turbulence model. Computational results show that the internal layer is a strong turbulence field that develops beneath the external boundary layer and is located very close to the wall. The turbulence field of the boundary layer flow over the curved kill is compared with that of a turbulent flow over a symmetric airfoil (which has the same geometry as the curved hill except that the leading and trailing edge plates were removed) to study the influence of a strongly curved surface on the turbulence field. The turbulence structure in the near-wall region of the curved hill is almost the same as that of the airfoil in most of the curved region even though the approaching external flows are quite different. Results show that the development of the wall shearing stress and separation of the boundary layer at the rear of the curved hill depend mostly on the streamline curvature and are only slightly influenced by the external boundary layer flow.     

A NUMERICAL TECHNIQUE FOR DYNAMIC COUPLED THERMOELASTICITY PROBLEMS WITH RELAXATION TIMES

A numerical method is proposed for Green and Lindsay' s dynamic thermoelasticity problems. The semidiscrete equations resulting from standard finite element formulations are mathematically manipulated so that the-resulting coupled heat equation and elastic equations both become symmetric. Such process allows for taking advantage of the traditional uncoupled thermal-structural solution procedures. Unconditionally stable time integration schemes are recommended to solve these matrix equations. The proposed technique is very effective and versatile in solving both finite speed thermal wave and stress wave propagation problems     

NUMERICAL STUDY OF CONVECTION HEAT TRANSFER DURING THE MELTING OF ICE IN A POROUS LAYER

A numerical study is made of the melting of ice in a rectangular porous cavity heated from above. The Landau transformation is used to immobilize the ice-water interface, and the Darcy-Boussinesq equations are solved by a finite-difference technique. Results are analyzed in terms of the heating temperature and the aspect ratio of the cavity. A comparison is made with the case of melting from below. It was found that melting from above is more effective than melting from below when the heating temperature is between 0 and 8°C: convection arises earlier, the melting process is faster, and the total melt at steady state is thicker. The critical time for onset of convection is minimum when the upper boundary is heated at 6°C. At this heating temperature, one also obtains a maximum heat transfer rate (Nusselt number).     

A NUMERICAL STUDY OF THE EFFECT OF WINDOW CONFIGURATION ON THE EXTERNAL HEAT AND SMOKE SPREAD IN BUILDING FIRE

This article presents a numerical study of external heat and smoke spread caused by compartment fires in multistory buildings. Usually, a building fire causes not only hazard to the occupants in that compartment, but also poses a possible threat to the occupants in other compartments, in particular to those located at upper stories. In this study, three scenarios with different window configurations are performed on the basis of the Navier–Stokes equations and the standard k– k model. Analysis of the computational results found that the window configurations and area as well as its location affect the heat and smoke pattern and in turn the safety of the occupants at upper stories. This implies that window design for multistory buildings, in terms of fire safety, should be carefully considered.     

风-水电联合优化运行分析

以风-水电联合运行后的风电场效益最大为目标,提出了将概率遍历搜索和局部弹性搜索相结合的混合遗传算法来进行风-水电联合优化运行的仿真分析.仿真结果表明风-水电联合供电不但提高了风电场的收益,同时也平滑了风电场的功率输出,这将有利于提高风电在电力系统中的份额,促进我国风电产业的发展.     

IMPORTANCE OF GRID REFINEMENT IN NUMERICAL MODELING OF CHEMICAL VAPOR DEPOSITION PROCESSES

The importance of grid resolution near the substrate surface for accurate prediction of the deposition rates in chemical vapor deposition modeling has been demonstrated. The exercise is conducted through numerical modeling of the chemical vapor deposition of silicon in an atmospheric-pressure, circular, impinging-jet reactor. Silicon is deposited from gaseous silane (SiH 4 ) supplied in a dilute condition premixed in a hydrogen carrier gas. The substrate temperature is kept fixed at 1,333 K. The model includes variable fluid properties and buoyancy forces in the hydrodynamic model. The Bousinesq approximation is not used because the temperature gradient is large. In addition to the hydrodynamic and thermal solution, both gas-phase reactions in the bulk gas and surface reactions on the susceptor are included in the model. The mesh-independent solution and the deposition rate of silicon on the wafer surface are presented. It is observed that a very fine mesh near the substrate surface, within the concentration boundary layer for the intermediate species such as silylene (SiH 2 ), is required to establish grid independency and accurate prediction of the deposition rate. For the specific deposition process modeled in this study, about 7 control volumes had to be placed within the SiH 2 concentration boundary layer at the substrate surface.     

带Gurney襟翼翼型改型的气动性能的数值研究

对NACA4424翼型及其对应的带有Gurney襟翼翼型修改后的翼型进行了数值计算,针对风力机应用,来流风速为2～16m/s,迎角为0°～20°;计算结果表明,在研究的所有条件下,修改后的钝尾缘翼型对流场有强烈的下洗作用,明显修改了翼型压力面和吸力面的压力分布,压力面压力更高,吸力面压力更低,因此翼型升力及升阻比均比翼型原型及单纯的Gurney襟翼翼型得到显著增加。     

NUMERICAL MODELING OF CONJUGATE HEAT TRANSFER DURING IMPINGEMENT OF FREE LIQUID JET ISSUING FROM A SLOT NOZZLE

The conjugate heat transfer from discrete heat sources to a two-dimensional jet of a high Prandtl number fluid discharging from a slot nozzle is considered. The variation of solid and fluid properties with temperature was taken into account in the numerical simulation. The geometry of the free surface was determined iteratively. The influence of different operating parameters such as jet velocity, heat flux, plate thickness, plate material, and the location of the heat generating electronics were investigated. It was found that in addition to jet Reynolds number (Re) plate thickness and its thermal conductivity have significant influence on temperature distribution and average Nusselt number (Nu).