非正南方向太阳集热器面积补偿的理论研究

该文以昆明和北京为例,对我国南北地区分别选用平板型管翼式集热器和圆柱吸热体真空管集热器进行了模拟计算。结果显示,水平面上日均太阳辐射、环境温度、屋顶方位及倾角对集热面积补偿有不同程度的影响;其中,屋顶方位角和倾角是影响集热器处于非正南和正南方向且最佳倾角时采光面积之比A/A0的主要因素。本文的研究成果可为太阳能建筑一体化设计提供部分有价值的参考数据。     

Numerical Simulation of Bubble Dynamics in a Uniform Electric Field by the Adaptive 3D-VOSET Method

This article presents a three-dimensional numerical simulation of the effect of a uniform electric field on the dynamics of bubbles in a viscous fluid. The two-phase interface is captured utilizing a coupled volume-of-fluid and level set (VOSET) method by solving the full Navier–Stokes equations coupled with electric field equations. To track the interface more accurately, the dynamically adaptive octree grids are used to refine the grids around the interface. The effects of different parameters such as the electric Bond number, the ratio of electrical permittivity, the gravitational Bond number, and the Reynolds number on the motion and deformation of the bubble are investigated. According to the computational results, it is found that the electric field has a significant influence on the bubble dynamic behavior. Increase of the electric Bond number or the ratio of electrical permittivity results in the larger deformation and rising velocity of the bubble. For a higher electric Bond number and the Reynolds number, separations of the tail of the bubble are observed. In this case, the jet above the bubble is strong enough to turn the spherical bubble to a toroidal shape.     

光伏阵列倾角对性能影响实验研究

对8块不同朝向和倾角的太阳电池组件输出情况进行了为期一年多的测试,并对实验结果进行了详细的分析和研究.得出:水平布置以及小倾角布置时,灰尘、雨水等因素对光伏阵列产能有较大影响;全年以朝向正南220倾角太阳电池组件产出的电能最多.     

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).     

Wind Effect on Combined Convection and Surface Radiation Heat Losses of a Fully Open Cylindrical Cavity With Insulation

Wind effect, of both the wind incidence angle and the wind speed, on convection and surface radiation heat losses of a fully open cylindrical cavity with constant bottom wall temperature was numerically investigated. The impacts of cavity tilt angle and wall temperature were also considered. Temperature contours, velocity contours, and vectors inside and around the cavity were presented. The variations of average convection and radiation heat loss Nusselt numbers Nu_c and Nu_r and percentages of heat losses with related parameters (wind speed, wind incidence angle, tilt angle, and bottom wall temperature) were also shown. In the end, correlations about Nu_c and Nu_r for practical applications were proposed. Results show that compared with no-wind condition, Nu_c under a wind condition is almost always higher except for head-on wind with velocity of 1.5 m/s, while Nu_r is always lower. Nu_c varies slightly, while Nu_r increases rapidly as the bottom wall temperature increases. With the existence of wind, the effect of tilt angle on heat transfer becomes more complex. A critical wind direction close to 30° is detected, which maximizes Nu_c and percentage of convective heat loss. The results also demonstrate that wind speed, wind incidence angle, and tilt angle should be considered simultaneously when analyzing heat transfer inside the cavity under a wind condition.     

Droplet retention on an incline

The present study seeks to understand and predict droplet retention on smooth hydrophobic surfaces. The droplet shape and the advancing and receding contact angles are experimentally measured as a function of droplet size under the action of a gravitational force at different inclination angles. The advancing and receding contact angles are correlated with static contact angle and Bond number. A Volume of Fluid – Continuous Surface Force model with varying contact angles along the triple contact line is developed to predict droplet shape. The model is first verified against a two dimensional analytical solution. It is then used to simulate the shape of a sessile droplet on an incline at various angles of inclination and to determine the critical angle of inclination as a function of droplet size. Good agreement is found between experimental measurements and predictions. The contact line profile and contact area are also predicted. The contact area predictions based on a spherical-cap assumption are compared to the numerical predictions and are found to underpredict the droplet contact area.     

HEAT TRANSFER FROM THE COLD WALL OF A SQUARE CAVITY TO THE HOT ONE BY OSCILLATORY NATURAL CONVECTION

ABSTRACT

The periodic natural convection in an inclined square enclosure is studied numerically. One of the cavity sides is maintained at a constant temperature, and the temperature of the opposite wall is varied by the sine law. The mean value of this temperature in a period was lower than the given constant temperature. The other two walls of the enclosure were adiabatic. The system of Navier–Stokes Equations in Boussinesq approximation is solved numerically by the control-volume method with the SIMPLER algorithm. Solutions are obtained for Grashof number equal to 5 × 10⁵ and Prandtl number equal to unity in a wide range of oscillation frequency variation. The heat transfer dependency on oscillation frequency is studied for different values of inclination angle. It is found that there is a possibility of heat transferring from the colder wall to the hotter one in the whole range of problem parameters. The possible causes of such a phenomenon are analyzed.     

NUMERICAL ANALYSES OF SALT-FINGER PHENOMENA NEAR THE INTERFACE BETWEEN TWO LIQUID LAYERS IN A CUBIC ENCLOSURE

The salt-finger phenomena in a two-layer system that consists of hot solution (upper layer) and cold water (lower layer) was studied by a finite difference method at the Prandtl number Pr = 6; the aspect ratio A = 1; the Lewis number Le = 100; the buoyancy ratio N = 0.5, 1, or 2; and the Rayleigh number Ra = 10 5 , 10 6 , or 10 7 . Salt finger occurred earlier for large N, but the shape of the finger was almost identical to those for other N's. At high Ra the temperature gradient was steep and then a number of fingers were formed. In the three-dimensional (3D) analyses the salt-finger shape was similar to those of twodimensional (2D) analyses. The cylindrical Benard-type roll cell convection occurred near the interface with the diffusion of heat, and many salt fingers grow up from the Benardtype convection. The fingers locate in order up and down with the increase in distance from midheight.     

A NUMERICAL STUDY OF ANISOTROPY AND CONVECTION DURING SOLIDIFICATION

A fixed-grid enthalpy formulation is developed to model convection and anisotropic conduction during solidification from the side or top wall of a rectangular cavity containing pure gallium. The solid-liquid interface is tracked with the use of an interface energy-balance equation. An improved discretization method is developed to prevent nonphysical oscillations due to discontinuous temperature and velocity gradients in the vicinity of the solid-liquid interface. Numerical results are compared to experimental data from the literature on the morphology and positions of the interface. The effect of anisotropic conduction in the solid interacting with thermal convection in the liquid has been studied.     

Heat transfer measurements for smooth and rough tilted semi-cylindrical cavities

This paper presents the effect of roughness on heat transfer for semi-cylindrical cavity. Heat transfer measurements are performed on smooth and rough surfaces for different tilt angles. The results are compared with a rectangular cavity of the same surface area to study the effect of cavity shape on heat transfer. The roughness composed of 0.002 m diameter rods installed along the length of the cavity at equal spacing. Roughness shows a large effect on heat transfer for the semi-cylindrical cavities. Two competing effects are present with the existence of roughness. Roughness may increase the blockage effect on the flow that can cause the buoyancy force to decrease, but on the other hand it increases the turbulence intensity resulting in a higher heat transfer. Both effects are function of tilt angles. Heat transfer for the cylindrical cavity is higher than the rectangular cavity for all tilt angles. This increase in heat transfer for the cylindrical cavity is due to the absence of sharp corners that can slow the buoyancy driven convection mechanism.     

基于参数化建模方法的双级旋流器流场研究

首先在软件UG和ANSYS Workbench的基础上提出了参数化建模方法。然后采用该方法研究了某双级旋流器(一级采用倾斜孔,二级为径向旋流器)的四个几何参数对其冷态流场特性的影响,主要结论有:(1)一级旋流数随斜切孔周向倾斜角的增大先增大后减小,随斜切孔直径的增大稍有增大,受二级旋流器的叶片安装角和出口外半径的影响很小;(2)二级旋流数随二级旋流器的叶片安装角和出口外半径的增大而大幅增大,受斜切孔的周向倾斜角和直径的影响可以忽略;(3)总旋流数随斜切孔周向倾斜角的增大先增大后减小,随斜切孔直径的增大而减小,随二级旋流器的叶片安装角和出口外半径的增大而大幅增大;(4)一二级旋流器流量之比受斜切孔周向倾斜角的影响很小,随叶片安装角的增大而大幅增大,仅改变斜切孔直径时流量之比和面积之比成正比,仅改变二级旋流器出口外半径时流量之比随面积之比的增大而增大;(5)相同条件下,双级旋流器旋向相反时小于旋向相同时的总旋流数。对于该旋流器的数值模拟研究过程证明,采用参数化建模方法是一种有效的快速设计手段,可以在较短时间内完成大量的改变几何参数的方案比较过程,快速地实现方案筛选,大大加快研究进度。     

太阳能集热系统过热影响因素分析

通过分析太阳能供热系统集热、蓄热和用热各个子系统的动态热量热平衡关系,建立太阳能集热系统热量传递数学模型,并结合集热系统内部流体过热汽化原理,提出集热系统过热度评价指标,并对集热系统在不同流量、水箱容积以及不同连接方式等条件下的过热程度进行分析计算。结果表明,集热系统过热度随热媒流量、集热器倾角和水箱容积取值的增大而减小,并通过对上述3种因素进行敏感性分析得到集热器倾角对系统过热度的影响最大。     

Effects of diameter ratio of adiabatic circular cylinder and tilt angle on natural convection from a square open tilted cavity

A numerical analysis is carried out to study the performance of natural convection inside a square open tilted cavity filled with air. An adiabatic circular solid cylinder is placed at the center of the cavity and the sidewall in front of the breathing space is heated by a constant heat flux. The top and bottom walls are kept at the ambient constant temperature. Two‐dimensional forms of Navier–Stokes equations along with the energy equations are solved using the Galerkin finite element method. Results are obtained for a range of Grashof numbers from 10³ to 10⁶ at Pr = 0.71 while the tilt angle varies from 0 to 45° and the diameter ratio of the cylinder is considered to be 0.2, 0.3, and 0.4 with constant physical properties. The parametric studies for a wide range of cylinder diameter ratios and cavity tilt angles show significant features of the present problem in terms of stream functions and temperature profiles. The computational results indicate that the heat transfer coefficient is strongly influenced by the above governing parameters. It is also found that the average Nusselt number decreases when the diameter ratio increases. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21001     

A new methodology to optimise solar energy extraction under cloudy conditions

The orientation and tilt position of the solar panel affect the amount of solar radiation that falls on the panel surface over the course of the day and indeed the year. The choice of tilt angle for a solar panel is fundamental to its efficient operation because incorrectly positioning the solar panel leads to an unnecessary loss in potential power. In the past, much work has been done by authors to determine the optimum tilt angle by applying existing models to their locations. This approach has been successful in climates with the most favourable solar potential, where greater than 90 percent of the solar radiation arrives as direct beam radiation. The accuracy of these models in these locations has been attributed to the low presence of cloud cover and the consequential dominance of the beam radiation portion of the global radiation. Countries located above 45°N however, (Northern Europe), require a different approach to optimising the tilt angle as they receive the least amount of direct radiation with approximately half arriving as diffuse radiation, due to frequent, heavy cloud cover. This paper reviews existing methods and describes a means of predicting the solar radiation in a frequently overcast climate and proposes a method for choosing the optimum tilt angle in such a climate. The effect of different load profiles on the optimum tilt angle is also investigated. The solar radiation model is then used to predict the solar radiation for Cairo, Egypt to show that the model has a global application and is not limited to frequently overcast climates.     

The effect of off-south orientation on the performance of flat-plate solar collectors

There are many instances in which an off-south installation of a flat-plate solar collector is more compatible with a building's orientation than a due-south installation. In these cases it is important to determine the magnitude of the performance sacrificed by conforming to the building architecture. This study investigates the collector performance and optimum tilt as functions of the off-south angle, collection temperature, number of glass covers and the relative amounts of direct and diffuse radiation. It was found that the yearly energy collection for a given collector tilt is insensitive to the off-south angle and that in some cases it actually improves with increasing azimuthal angle. It was also found that for a given azimuthal angle an optimum collector tilt exists which is between 3 and 10° less than the latitude. Calculations were based on New York City weather.     

OPTIMUM TILT ANGLE FOR SOLAR COLLECTION SYSTEMS

For non-tracking solar collection systems, the tilt angle has a predominant effect on the quantity of energy that the system can intercept. In the present work, a computational algorithm is developed for the calculation of the optimum tilt angle that would orient a non-tracking solar collection system (concentrating or non-concentrating) in its best position for the maximum average daily, monthly seasonal or yearly intercepted radiation. The optimum tilt angles were obtained for latitudes ranging from l0–50oN on monthly, seasonal and yearly bases. A case study is applied on Riyadh City (latitude 24.9°N) to investigate the sensitivity of intercepted radiation when the tilt angle varies from that of the optimum value. The results show that, on a monthly basis when the collector is mounted at the yearly optimum tilt angle, the loss of radiation intercepted is less than 10% as compared to the monthly optimum tilt angle. The optimum seasonal tilt angle reduces the incident radiation by less than 2% from that of the monthly optimum tilt angle.     

Bubble Dynamics and Heat Transfer during Pool and Flow Boiling

A large number of studies of bubble growth rate and departure diameter have been reported in the literature. Because of uncertainty in defining the shape of an evolving interface, empirical constants are invariably used to match the model predictions with data. This is especially true when force balance is made on a vapor bubble to determine the departure diameter. In this paper, the results of an alternate approach based on a complete numerical simulation of the process are given. Single and multiple bubbles are considered for both pool and flow boiling. The simulations are based on the solution of the conservation equations of mass, momentum, and energy for both phases. Interface shape is captured through a level set function. A comparison of bubble shape during evolution, bubble diameter at departure, and bubble growth period is made with data from well-controlled experiments. Among other variables, the effect of magnitude of gravity and contact angle is explicitly investigated.     

Optimum tilt angle for a flat plate solar collector

One of the important parameters that affects the performance of a flat plate solar collector is its angle of tilt with the horizontal. This is due to the fact that the variation of tilt angle changes the top loss coefficient and the amount of solar radiation reaching the absorber plate. A mathematical model has been developed for calculating the useful energy gained by a flat plate collector under various operating conditions. The model is then used to determine the optimum tilt angles for a typical collector in Basrah on a daily basis, as well as on the basis of a specified period. The optimum tilt angle is found by searching its value for which the useful energy gained by the collector is a maximum for a particular day or a specified period.From the results, it is observed that changing the tilt angle eight times in a year determines the useful energy in Basrah near its value which is found by changing the tilt angle daily to its optimum value. Furthermore, the influence of various operating parameters is studied on the daily optimum tilt angle.     

Free Convective Heat Transfer in Tilted Longitudinal Open Cavity

Heat transfer experiments were performed to investigate the effects of inclination and channel height-to-gap ratio on free convection in a simulated fin-passage with a strategic aim of devising a criterion for selecting the optimal fin length that could provide the maximum free convective capability. The ranges of parameters investigated include the Grashof number, up to 500,000; channel height-to-gap ratios of 1, 2, and 3; and tilt angles of 0°, 30°, 60°, 90°, 120°, 150°, and 180°. Selections of local and spatially averaged Nusselt number results demonstrate the manner by which the Grashof number, tilt angle, and channel height-to-gap ratio interactively affect the heat transfer. In conformity with the experimentally revealed heat transfer physics, the correlation of a spatially-averaged Nusselt number over two parallel walls and the bottom surface of an open-ended channel is derived that permits the individual and interactive effects of the Grashof number, tilt angle, and channel height-to-gap ratio on heat transfers to be evaluated. A criterion for selecting the optimal height-to-gap ratio of the fin channel is subsequently formulated as a design tool for maximizing the convective capability of a free convective fin assembly.     

负载缺电率用于独立光伏系统的最优化设计

在确定光伏方阵最佳倾角时,应综合考虑方阵面上太阳辐射量的连续性、均匀性和极大性。“夏半年”和“冬半年”周期内入射到倾斜面上的平均日辐射量Ｈ１和Ｈ２相等或在盯等之前Ｈ２有极大值,则其对应角度即为最佳倾角。通过改变相应放电深度的简便计算方法,即可得到应于不同负载缺电率的一系列光伏方阵与蓄电池容量组合,从而确定最方阵和蓄电池容量。