A numerical simulation of pool boiling using CAS model

This paper presents a new numerical model, called the CAS model, for boiling heat transfer. The CAS model is based on the cellular automata (CA) technique that is integrated into the popular SIMPLER algorithm for CFD problems. In the model, the CA technique deals with the microscopic nonlinear dynamic interactions of bubbles while the traditional CFD algorithm is used to determine macroscopic system parameters such as pressure and temperature. The popular SIMPLER algorithm is employed for the CFD treatment. The model is then employed to simulate a pool boiling process. The computational results show that the CAS model can reproduce most of the basic features of boiling and capture the fundamental characteristics of boiling phenomena. The heat transfer coefficient predicted by the CAS model is in excellent agreement with the experimental data and existing empirical correlations.     

Some numerical considerations in the simulation of low-Ma number hydrogen/air mixing layers

In this work numerical simulations of hydrogen/air mixing are conducted which demonstrate the generation of spatial pressure gradients arising from the diffusion of species (hydrogen versus air) of high molecular weight disparity. Convective velocities arising as a result of the spatial gradients have been evaluated with respect to molecular weight ratio between stratified layers in coflow and stationary diffusion layers. The implications of results for the spatially-constant thermodynamic pressure assumption for low-Mach-number flows are discussed. Additionally, the impact of the choice of diffusion model on the results has been assessed.     

Numerical simulation of stacked microchannel heat sink with mixing-enhanced passive structure

The paper is focused on the investigation of numerical simulation of stacked two-layer microchannel heat sink with enhanced mixing passive microstructure. In contrast to the smooth microchannel studies in the literature, the microchannel with embedded passive microstructure is chosen. The computational fluid dynamics (CFD) will be used to simulate the flow and heat transfer in a stacked two-layer microchannels with multiple MEMS easy-processing passive microstructures. To simulate the conjugated heat transfer among the heatsink and fluid, the three-dimensional conjugated model is used to solve this problem. The important parameters (e.g. the ratio of embedded structure height to microchannel height and fluid property) are investigated. The ratio of embedded structure height to microchannel height is changed from 0.13 to 0.26. The microchannel Reynolds number is fixed at 14.8. The stacked microchannel with passive structures has better performance than the smooth microchannels.     

Direct numerical simulation of gas-solid two-phase mixing layer

In this paper, the spatially evolving of the higher Reynolds numbers gas-solid mixing layer under compressible conditions was investigated by a new direct numerical simulation technology. A high-resolution solver was performed for the gas-phase flow-field, particles with different Stokes numbers were traced by the Lagrangian approach based on one-way coupling. The processes of the vortex rolling up and pairing in the two-dimensional mixing layer were captured precisely. The large-scale structures developed from the initial inflow are characterized by the counter-rotating vortices. The mean velocity and the fluctuation intensities profiles agree well with the experimental data. Particles with smaller Stokes numbers accumulate at the vortex centers due to the smaller aerodynamic response time; particles with moderate Stokes numbers tend to orbit around individual streamwise vortices and in the periphery of paring vortices; particles with larger Stokes numbers disperse less evenly, showing a concentration distribution in the flow field.     

Calculation of the polycrystalline PV module temperature using a simple method of energy balance

The performance of a photovoltaic module is studied versus environmental variables such as solar irradiance, ambient temperature and wind speed. Two types of simplified models are studied in this paper: a PV module temperature model and a PV module electrical efficiency model. These models have been validated utilizing experimental data from two experiments: a 850 Wp grid connected photovoltaic system and a p-Si module with eight temperature sensors integrated into the module. Both models have been coupled to determine the PV array output power versus the three meteorological parameters. This simple model using a simple energy balance and neglecting the radiation effects is in good agreement with the experimental data.     

A new method for numerical simulation of thermal contact resistance in cylindrical coordinates

In this paper, a random numbers model using an innovative equiperipheral grid in cylindrical coordinates has been proposed to predict the contact spot distribution of two rough surfaces at various loads. The ability of this method to predict the contact spot distribution has been proven through comparison with results using a conventional equiangular grid. Further, a network method using such an equiperipheral grid has been developed in order to solve a three-dimensional heat conduction problem where two cylindrical specimens were connected to each other along the longitudinal direction. A uniform heat flux is given at the bottom surface of specimen I, a uniform temperature is maintained at the top surface of specimen II, and thermal insulation is assumed at the outer radius of the two specimens. The present numerical results have been compared to calculations using conventional equiangular grids and to experimental results obtained for cylindrical brass specimens. The present results are shown to compare much more closely with experimental measurements than previous calculations using conventional numerical models.     

Design optimization of insulation usage and space conditioning load using energy simulation and genetic algorithm

Architectural design is a process to find the best solution to satisfy various design criteria. To achieve sustainable and green design, performance simulations are often used to verify these criteria and modify the design. The conventional approach of manual trial-and-error is too time-consuming to be practical. Introducing optimization technique can greatly improve the design efficiency and help designers find the optimal design. In this paper, modeFRONTIER was used as the design optimization environment to find the best insulation strategy to minimize the space conditioning load of an office building located in Nanjing, China while keeping the insulation usage at minimum. EnergyPlus was integrated into the optimization tool by writing a DOS batch file to automate the work flow. The search engine was the genetic algorithm and it proved to be able to generate a well-defined Pareto frontier in a reasonable number of runs. Based on the Pareto frontier, the designer can specify his preferences and select the final design. The case study shows that an energy simulation program can be effectively integrated into a design optimization environment to find the optimal design. The technique presented has a broad application in architectural design, especially when the design considerations are multi-objective.     

Effects of turbine damping on performance of an impulse turbine for wave energy conversion under different sea conditions using numerical simulation techniques

This paper presents the work carried out to predict the behavior of a 0.6 m impulse turbine with fixed guide vanes with 0.6 hub to tip (H/T) ratio under real sea conditions.This enhances the earlier work done by authors on the subject by including the effects of damping applied by the turbine. Real wave data for different wave sites were used as the input data. A typical oscillating water column (OWC) geometry has been used for this simulation. Standard numerical techniques were employed to solve the non-linear behavior of the sea waves. Considering the quasi-steady assumption, uni-directional steady flow experimental data were used to simulate the turbine characteristics under irregular unsteady flow conditions. The test rotor used for this simulation consisted of 30 blades with elliptical profile with a set of symmetric, fixed guide vanes on both up-stream and down-stream sections of the rotor, with 26 vanes each. The results show that the performance of this type of turbine depends on the level of damping applied by the turbine and the prevailing wave site conditions. The objective of this paper is to predict the effects of applied damping on the behavior of impulse turbine under irregular, unsteady conditions for wave power conversion using numerical simulation.     

Experimental and numerical investigations of mixing in raceway ponds for algae cultivation

The current high interest in the algae sector is leading to the development of several demo/commercial scale projects, either for the food market or bioenergy production. Raceway Ponds (RWPs) are a widely used technology for algae mass cultivation. RWPs were developed long time ago, and thus capital and operating costs are well assessed. Nevertheless, room still exists to further reduce operational costs. A possible route towards energy optimization and therefore operational cost reduction can be identified through a better understanding of the mixing phenomena.The focus of the present work is that vertical mixing, defined as the cyclical movement of the algal cells between surface and bottom layers of the culture, cannot be completely determined by considering only turbulence, and therefore it is not represented by the Re number.A 3D Computational Fluid Dynamic (CFD) analysis of a conventional RWP was carried out based on a multi-phase “Volume of Fluid” model, in order to investigate the flow field of the culture in the pond. The CFD results were compared with experimental measures on a 20 m² pilot RWP. Once agreement among CFD and experimental results was shown, a statistical evaluation of the trajectories calculated for algae particles in the flow was carried out. The aim of this statistical evaluation was to define the level of vertical mixing in different sections of the pond.The model proposed was then used to scale-up the results to a demo/pre-commercial size RWP (500 m²). The standard deviation of the actual trajectory was calculated with respect to the undisturbed trajectory for each section modeled.The results of the simulation showed that a limited mixing is to be expected in RWPs. In the long straight parts of the pond vertical mixing is poor and algae tend to settle to the bottom. Only in the bends the vortexes produced by flow separation move part of the culture from the bottom to the top and vice-versa. This result does not fit with the practice, typically observed in large scale ponds, of reducing vortexes around the bends by placing baffles. The method described can be applied to different pond designs operated at different culture velocities.     

Three-dimensional coupling numerical simulation of two-phase flow and electrochemical phenomena in alkaline water electrolysis

Alkaline water electrolysis has the advantage of scalability for industrial-scale mass production of hydrogen; however, it is operated under a lower current density than other methods of water electrolysis because a high overpotential resulting from ion transport limitations will occur at high current density. Bubble dynamics can both prevent ion transport by its existence and accelerate it by bubble-induced flow. In this study, we conduct three-dimensional coupling numerical simulations of two-phase flow and electrochemical phenomena to elucidate the mechanisms by which microscale bubble dynamics influence ion transport and the cell overpotential. We find that the flow induced by rising microbubbles enhances ion transport to the anode and suppresses the cell overpotential. Moreover, bubble atomization further suppresses the overpotential because smaller bubbles approach the anode more closely than larger ones and accelerate ion transport to the anode surface.     

Modeling and numerical simulation of solar chimney power plants

The solar chimney power plant is a simple solar thermal power plant that is capable of converting solar energy into thermal energy in the solar collector. In the second stage, the generated thermal energy is converted into kinetic energy in the chimney and ultimately into electric energy using a combination of a wind turbine and a generator. The purpose of this study is to conduct a more detailed numerical analysis of a solar chimney power plant. A mathematical model based on the Navier-Stokes, continuity and energy equations was developed to describe the solar chimney power plant mechanism in detail. Two different numerical simulations were performed for the geometry of the prototype in Manzanares, Spain. First, the governing equations were solved numerically using an iterative technique. Then, the numerical simulation was performed using the CFD software FLUENT that can simulate a two-dimensional axisymmetric model of a solar chimney power plant with the standard k-epsilon turbulence model. Both the predictions were compared with the available experimental data to assess the validity of the model. The temperature, velocity and pressure distributions in the solar collector are illustrated for three different solar radiations. Reasonably good quantitative agreement was obtained between the experimental data of the Manzanares prototype and both the numerical results.     

Three-dimensional numerical simulation of heat and fluid flow in noncircular microchannel heat sinks

A three-dimensional model of heat transfer and fluid flow in noncircular microchannel heat sinks is developed and analyzed numerically. It is found that Nusselt number has a much higher value at the inlet region, but quickly approaches the constant fully developed value. The temperature in both solid and fluid increases along the flow direction. In addition, the comparison of thermal efficiencies is conducted among triangular, rectangular and trapezoidal microchannels. The result indicates that the triangular microchannel has the highest thermal efficiency.     

Analysis of thermal systems using the entropy balance method

This study investigates the applicability of the second law of thermodynamics using an entropy balance method to analyse and design thermal systems. As examples, the entropy balance method is used to analyse a single stage chiller system and a single stage heat transformer, both with lithium-bromide/water as the working fluid. The entropy method yields not only the same information as is conveyed by the methods of energy and exergy analysis, but it also predicts clearly the influence of irreversibilities of individual components on the coefficient of performance and its effectiveness, based on the process properties, rather than on ambient conditions. Furthermore, this method is capable of presenting the overall distribution of the heat input by displaying the additional heat required to overcome irreversibility of each component without ambiguity.     

Numerical simulation of high-power synthetic jet actuator flowfield and its influence on mixing control

Detailed two-dimensional unsteady numerical simulation is carried out to investigate a high-power synthetic jet actuator flow field and its design characteristic. Simultaneously, mixing control mechanism of coaxial jets with actuators is also studied. Firstly, excitation frequency （rotating speed）, piston displacement and its exit slot width have effect on the controlling ability and controlling efficiency of actuator. With the invariable model and con- cerned parameters, the actuator becomes more desirable as the rotating speed increases. Average velocity and maximal velocity at the actuator exit section increase as the piston displacement enlarges or the exit slot width decreases. But the actuator does not always exhibit good performance with the narrower exit. Secondly, the synthetic jets also have the ＂push＂ effect on the coaxial jets, which results in the fluctuation of vorticity and temperature distribution of mixing flowfield. Finally, the employment of synthetic jet actuator can achieve mixing enhancement significantly.     

基于CFD数值模拟下的蒸汽喷射器性能预测

建立了蒸汽喷射器热力学模型,并验证了该模型性能及结构计算的可靠性。并针对实际情况中,对该模型进行了动态模拟。结果显示：当工作流体的温度升高时,工质流体的质量流量都会增加,引射系数存在峰值,峰值所对应温度为热力学模型设定温度值;当引射流体的温度升高时,其质量流量也会随之增大,而工作流体的质量流量则较稳定,因此引射流体温度与压力的升高可以改善喷射器的性能;当背压升高时,在一定压力范围内,工质流体的质量流量都趋于稳定,而当背压超过热力学模型设定背压值时,引射流体的质量流量便随背压的升高而急剧下降,喷射器性能严重恶化,故认为该压力值为喷射器的临界背压。本文研究结果对喷射器的设计计算具有一定的指导作用。 关键词：蒸汽喷射器;热力学模型;数值模拟;引射系数     

Towards a sustainable energy balance: progressive efficiency and the return of energy conservation

We argue that a primary focus on energy efficiency may not be sufficient to slow (and ultimately reverse) the growth in total energy consumption and carbon emissions. Instead, policy makers need to return to an earlier emphasis on “conservation,” with energy efficiency seen as a means rather than an end in itself. We briefly review the concept of “intensive” versus “extensive” variables (i.e., energy efficiency versus energy consumption) and why attention to both consumption and efficiency is essential for effective policy in a carbon- and oil-constrained world with increasingly brittle energy markets. To start, energy indicators and policy evaluation metrics need to reflect energy consumption, as well as efficiency. We introduce the concept of “progressive efficiency,” with the expected or required level of efficiency varying as a function of house size, appliance capacity, or more generally, the scale of energy services. We propose introducing progressive efficiency criteria first in consumer information programs (including appliance labeling categories) and then in voluntary rating and recognition programs such as ENERGY STAR. As acceptance grows, the concept could be extended to utility rebates, tax incentives, and ultimately to mandatory codes and standards. For these and other programs, incorporating criteria for consumption, as well as efficiency, offers a path for energy experts, policymakers, and the public to begin building consensus on energy policies that recognize the limits of resources and global carrying capacity. Ultimately, it is both necessary and, we believe, possible to manage energy consumption, not just efficiency, in order to achieve a sustainable energy balance. Along the way, we may find it possible to shift expectations away from perpetual growth and toward satisfaction with sufficiency.

Application of Numerical Simulation Method to Predict the Performance of Wave Energy Device with Impulse Turbine

This paper presents the work carried out to predict the behavior of a 0.6 m Impulse turbine with fixed guide vanes with 0.6 hub-to-tip (H/T) ratio under real sea conditions. In order to predict the true performance of the actual Oscillating Water Column (OWC), the numerical technique has been fine tuned by incorporating the compressibility effect. Water surface elevation verses time history based on Pierson Moskowitz Spectra was used as the input data. Standard numerical techniques were employed to solve the non-linear behavior of the sea waves. The effect due to compressibility inside the air chamber and turbine performance under unsteady and irregular flow condition has been analyzed numerically. Considering the quasi-steady assumptions, unidirectional steady flow experimental data was used to simulate the turbine characteristics under irregular unsteady flow conditions. The results show that the performance of this type of turbine is quite stable and efficiency of air chamber and the mean conversion     

Comparison of seven numerical methods for determining Weibull parameters for wind energy generation in the northeast region of Brazil

This paper deals with the analysis and comparison of 7 (seven) numerical methods for the assessment of effectiveness in determining the parameters for the Weibull distribution, using wind speed data collected in Camocim and Paracuru cities, State of Ceará, in the northeast region of Brazil, in the period from August 2004 to April 2006, obtained by the Department of Infrastructure of the State of Ceará. One method is not well known, namely the equivalent energy method, and its performance is compared to the others. By using the methods of analysis of variance, RMSE (root mean square error), and chi-square tests to compare the proposed methods, this study aims to determine which ones are effective in determining the parameters of the Weibull distribution for the available data, in an attempt to establish acceptable criteria to a better utilization of wind power in the State of Ceará, which is a national prominence in the use of renewable sources for electricity generation in Brazil.     

CH4/air预混气体双层多孔介质燃烧特性数值模拟研究

基于局部热平衡假设,定义了无量纲参数——火焰宽度比,在过量空气系数为1.2条件下,研究了CH_4/air预混气体在双层多孔介质中浸没燃烧和表面燃烧的燃烧特性。结果表明:表面燃烧具有更高的烟气出口温度以及更高的火焰宽度比。浸没燃烧火焰轮廓类似抛物线,而表面燃烧的火焰宽度比则基本不变;入口速度不同时,距离着火面同一位置浸没燃烧火焰宽度基本保持不变,火焰轮廓仍类似于抛物线;而对于表面燃烧,相同位置的火焰宽度比在很小范围内(约为0.03)呈现先增大后减小的变化规律。同时研究也表明表面燃烧具有更高的NO_x排放,且随着速度的增加两种燃烧方式NO_x排放变化规律一致,均呈现先增大后减小再增加的变化规律。在速度为1.1～1.2μm/s左右时,两种燃烧方式NO_x排放大致相当,这表明在不扩大污染的条件下,可以使用表面燃烧获得更多的对外辐射以节约能源。     

Three-dimensional numerical simulation on plastic damage in small punch specimen of Zirconium

Small punch test (SPT) technique was used to evaluate the mechanical properties of Zirconium in this paper. The dimension of the disc specimen is φ 10 × 0.5 mm. Plastic damage in small punch specimen of Zirconium was investigated both experimentally and numerically, because it has great influence on small punch specimen. In order to simulate the plastic damage in the small punch specimen of Zirconium, the 3D finite element model incorporated with Gurson–Tvergaard–Needleman (GTN) plastic damage constitutive equation was established. Void growth and initiation of ductile crack of the small punch specimen were predicted. Results show that damage occurs on the bottom side of the specimen and grows across the specimen until complete failure, which has good agreement with the observation in the experiment.