Effect of Nanoparticle Type and Surfactant on Heat Transfer Enhancement in Spray Cooling

In this study,the heat transfer characteristics of nanofluids used in spray cooling systems were examined.Three nanofluids,i.e.,Cu,CuO,and Al2 O3,respectively,with volume fractions ranging from 0.1%to0.5%,as well as different volume fractions of a surfactant Tween 20,were used.In addition,their contact angles were measured to examine the heat-transfer characteristics.Under the same experimental conditions,with the increase in the volume fraction of the Cu nanoparticles from 0.1%to 0.5%,the maximum heat flux qmax increased from 3.36 MW/m2 to 3.48 MW/m2 from the impinging central point to r=30 mm(r is the distance from the impingement point),and the corresponding temperature of qmax increased from 400℃to 420℃.Results revealed that with increasing Tween 20 concentrations,the contact angle decreased because of the decrease in the surface tension of nanofluids and improvement of the wetting ability,and the corresponding qmax increased from 3.48 MW/m2 to 3.94 MW/m2 at the impact central point.     

Characteristics of the Mach Disk in the Underexpanded Jet in which the Back Pressure Continuously Changes with Time

When the high-pressure gas is exhausted to the vacuum chamber from the nozzle, the underexpanded supersonic jet contained with the Mach disk is generally formed. The eventual purpose of this study is to clarify the unsteady phenomenon of the underexpanded free jet when the back pressure continuously changes with time. The characteristic of the Mach disk has been clarified in consideration of the diameter and position of it by the numerical analysis in this paper. The sonic jet of the exit Mach number Me=1 is assumed and the axisymmetric conservational equation is solved by the TVD method in the numerical calculation. The diameter and position of the Mach disk differs with the results of a steady jet and the influence on the continuously changing of the back pressure is evidenced from the comparison with the case of steady supersonic jet.     

An Experimental Study of Carbon Dioxide Condensation in Mini Channels

In this study, the local characteristics of pressure drop and heat transfer were investigated experimentally for carbon dioxide condensation in a multi-port extruded aluminum test section, which had 10 circular channels each with 1.31 mm inner diameter. The CO2 was cooled with cooling water flow inside the copper blocks that were attached at both sides of the test section. The temperatures at the outer surface of the test section were measured with 24 K-type thermocouples embedded in the upper and lower surfaces along the length. Local heat fluxes were measured with 12 heat flux sensors to estimate the local enthalpies, temperatures and heat transfer coefficients. Bulk mean temperatures of CO2 at the inlet and outlet of the test section were measured with 2 K-type thermocouples. The measurements were performed for the pressure ranged from 6.48 to 7.3 MPa, inlet temperature of CO2 from 21.63 to 31.33℃, heat flux from 1.10 to 8.12 kW/m2, mass velocity from 123.2 to 315.2 kg/m2s, and vapor quality from 0 to 1. The results indicate that pressure drop is very small along the test section, heat transfer coefficient in the two-phase region is higher than that in the single-phase, and mass velocity has important effect on condensation heat transfer characteristics. In addition, experimental data were compared with previous correlations and large discrepancies were observed.     

An Experimental Study on Aerodynamic Sound Generated from Wake Interference of Circular Cylinder and Airfoil Vane in Tandem

The purpose of this study is to investigate the characteristics of aerodynamic sound generated from wake inter-ference of circular cylinder and airfoil vane located in tandem and to clarify the generation mechanism of thesound source with discrete frequency.The effects of the interval between the cylinder and the airfoil on the char-acteristics of aerodynamic sound are investigated by acoustic measurement,flow visualization and explorationtest of sound source.The relation between the flow field and the sound field with discrete frequency noise(DFN)is shown,and then it is found that the downstream airfoil works as the sound source of DFN,which has the fre-quency of vortex shedding from the upstream cylinder,when the interval of two bodies is longer than a criticaldistance.     

Forced Convective Heat Transfer and Pressure Drop of Air Flowing in a Rectangle Duct with Cross-Ribs on the Opposite Walls

Experiments were conducted to investigate the forced convective heat transfer and flow friction of turbulent airflow in a rectangular duct with cross-ribs attached at the two principal walls in the Reynolds number range from 5000 to 40000. The effect of the rib cross angle (45° 60° 75° and the height (4 mm, 5 mm) of the cross-ribs on the forced convection and flow friction were tested. Non-dimensional correlations for the duct average Nusselt number and friction factor of cross-ribs duct were developed from the test data. Experiments were also conducted for the corresponding parallel ribs to compare their relative performance. The experimental results show that both of the convective heat transfer coefficient and friction factor were increased with cross-ribs, with 45°cross-ribs being the best. Compared with parallel ribs normal to the flow direction under identical flow rate and identical pumping power constraints, the cross-ribs can enhance heat transfer in the lower Reynolds number region, while i     

Configuration of Shock Waves in Two-Dimensional Overexpanded Jets

An experimental and analytical study has been carried out to obtain the clear understanding of a shock wave transition associated with a steady two-dimensional overexpanded flow. Two-dimensional inviscid theory with respect to a shock wave reflection is used in the present study on the characteristic of shock waves. The results obtained from the flow analysis are compared with those obtained from flow visualizations. It is shown that in the region of regular reflection, the angle of an incident shock wave becomes lower than that calculated by two shock theory with an increment in the ratio pe/pb of the nozzle exit pressure pe to the back pressure pb. It is indicated that the configuration of shock waves in overexpanded jets is influenced by the divergent angle at the nozzle exit. Also it is shown from the flow visualization that a series of shock waves move into the nozzle inside with a decrease in pressure ratio pe/pb, even if the pe/pb is under overexpanded conditions.     

Experimental Study on Coal Multi-generation in Dual Fluidized Beds

An atmospheric test system of dual fluidized beds for coal multi-generation was built.One bubbling fluidized bedis for gasification and a circulating fluidized bed for combustion.The two beds are combined with two valves:one valve to send high temperature ash from combustion bed to the gasification bed and another valve to sendchar and ash from gasification bed to combustion bed.Experiments on Shenhua coal multi-generation were madeat temperatures from 1112 K to 1191 K in the dual fluidized beds.The temperatures of the combustor are stableand the char combustion efficiency is about 98%.Increasing air/coal ratio to the fluidized bed leads to theincrease of temperature and gasification efficiency.The maximum gasification efficiency is 36.7% and thecalorific value of fuel gas is 10.7 MJ/Nm3.The tar yield in this work is 1.5%,much lower than that of pyrolysis.Carbon conversion efficiency to fuel gas and flue gas is about 90%.     

Experimental Investigation of Self—sustained Oscillation in a Traveling Wave Thermoacoustic System

A traveling wave thermoacoustic engine consisting of a loop tube with a resonator has been tested. The onset characteristic together with the transition of oscillation mode from traveling wave to standing wave and the periodic shifting between modes in this system are investigated experimentally. The process of self-sustained thermoacoustic oscillation in this heat engine is described and analyzed through phase space distribution reconstructed from the time series of acoustic signal.     

Laser Measurements in High Speed Compressors for Rotor-Stator Interaction Analysis

This paper deals with the experimental quantification of the unsteady effects of the interactions between rotor and stator rows in high speed compressors. Due to the fact that the levels of the periodic fluctuations arising from the unsteady interaction may be low compared with the random fluctuations arising from the measurement uncertainties, it is crucial to minimize the errors inherent to the used technique. The first part of the paper concentrates on technical details relative to the experimental process. The second part is devoted to the data postprocessing. Two tools for analysing the rotor-stator interactions are presented. The first tool is based on a decomposition of the flow field which was initially introduced to solve numerical problems when attempting to calculate the flow field in a multi-row configuration. The second tool is based on a spectral analysis of the signal, that qualifies the interaction in a sense of circumferential spinning lobes. Experimental results obtained within both an     

CFD Simulation of Reactor Internal Flow in the Scaled APR＋

A series of 1/5 scale reactor flow model tests have been conducted in order to determine the hydraulic characteristics of the APR＋ （advanced power reactor plus）. The objective of test was to determine the core inlet flow field of the model reactor in order to provide input information required by the open core thermal margin analysis code such as TORC. In this study, in order to examine the validity of the results of reactor flow model tests and the applicability of CFD （computational fluid dynamics） in the simulation of reactor internal flow, CFD simulation was conducted with the commercial multi-purpose CFD software, ANSYSCFX V. 14. It was found that the velocity field in the downcomer had the inhomogeneous feature. Relative high velocity region was located in the core region. This result was different from measurement and this difference may result from the fact that some internal structures were not modeled with the real geometry but treated as the porous domain.     

下击暴流作用下定日镜表面风压数值模拟研究

为了研究下击暴流作用下塔式太阳能定日镜表面的风压分布特征,文章采用计算流体动力学方法对下击暴流作用下,不同径向位置和不同工作俯仰角的定日镜表面风压进行了数值模拟,并将模拟结果与大气边界层近地风作用下定日镜表面的风压特性进行了比较分析。分析结果表明:当定日镜正常工作时,下击暴流作用下,迎风面风压呈现出中间高两边低的分布趋势,风压峰值位于定日镜中部,背风面风压中间低两边高;随着俯仰角逐渐增大,下击暴流作用下,定日镜迎风面压力峰值中心从定日镜下边缘逐渐上移,最大压力值和高压区范围也逐渐增大,背风面负压值逐渐减小且谷值中心逐渐下移;与常规风相比,下击暴流作用下,定日镜表面风压受径向距离影响明显,当镜面垂直于地面时,定日镜迎风面和背风面表面风压随着定日镜与下击暴流风暴中心之间径向距离的增大而减小;在定日镜抗雷暴下击暴流强风的设计过程中,须要考虑下击暴流和常规风的速度场、气压场的不同,及其所导致的定日镜表面风压分布特征的变化。     

大型风光互补并网发电系统研究

以甘肃酒泉地区瓜州某风场为例,介绍了该地区的风能、太阳能资源情况。揭示了该地区的风能和太阳能资源具有很好的风光互补关系。阐述了风光互补容量的匹配计算方法。仿真试验分析表明：对于瓜州48MWN．电场,配置10MW的光伏电站．互补效果最好。     

Development of a simulation model for a three-dimensional wind velocity field using Ténès Algeria as a case study

A mathematical simulation model was developed that can determine the three-dimensional wind velocity field over a complex terrain. The Ténès area in the Valley of Cheliff in Algeria was used as a case study. This region is exposed to south-west circulation that makes it favorable to the use of wind energy. Knowledge of wind fields is crucial for predicting the dispersion of pollutants, for forecasting meteorological weather, for fire spread prediction and in the design and implementation of wind turbines. By means of a mass consistent model, an in-house program was developed to calculate the three-dimensional wind velocity field in the study region. The model was supported by a numerical box in which flow through is allowed for in the upper and lateral boundaries. The bottom boundary through which no flow through occurs was determined by the topographic relief at the surface. From measured wind velocities, observed values were calculated by interpolation-extrapolation. Using an optimization method, the adjusted velocities were obtained from constraints, observed velocities and the continuity equation. The model was verified with wind point data, the relative error did not exceed 6%.     

Assessing the determinants of local acceptability of wind-farm investment: A choice experiment in the Greek Aegean Islands

This paper aims at analysing the factors, which motivate communities to resist the installation of wind farms in their vicinity. To this end, the choice experiment methodology was employed in communities in two Greek Aegean Islands to assess the determinants of preferences towards different wind-farm projects. Unlike other studies, the willingness to accept welfare measure was adopted. The results of our analysis show that the conservation status of the area where the wind farms are to be installed, along with the governance characteristics of the planning procedure are the most important determinants of local community welfare in relation to wind farms. In contrast to other studies, we find that the physical attributes of wind farms appear to be of less relative importance from a local community welfare point of view. Implications for the EU's future energy policy are drawn.     

风场和吞吐流对狭长型雁鸣湖流场的影响

基于二维非结构网格水动力模型,对实测风场和吞吐流作用下的雁鸣湖进行流场数值模拟,水位模拟结果与实测结果拟合较好,说明该模型能够反映雁鸣湖水动力变化过程。在此基础上定量研究不同风速、风向、吞吐流对雁鸣湖水动力的影响。研究表明,风对湖泊流场的变化起主导作用,其中风速是影响湖泊流速的主要因素;风向对湖泊环流的方向起主导作用;吞吐流是调节水环境的主要手段,对整体湖流影响较小,但对于局部水域有一定的影响。因此,在常年主导风向下,适当地增加吞吐流有利于改善湖泊流场。     

基于PIV测试的风轮主要声源区域雷诺应力特征分析

为了获悉风轮主要声源区域的流动机理,在风洞开口段,在不同风速和尖速比下,文章对风轮展向X/C=0.5区域不同相对弦长处的流场进行了PIV测试。测试结果表明:由于翼型表面发生流动分离导致流体速度脉动,雷诺应力迅速增加,使叶片与来流相互作用产生压力脉冲;随着相对弦长的增加,雷诺应力均有不同程度的增大,当X/C为0.4~0.8时,雷诺应力的变化最为明显;对比不同风速、尖速比、相对弦长处的雷诺应力数据发现,随着尖速比的增加,雷诺应力增大最明显的区域向前缘移动,流动分离位置提前,而风速变化对流动分离位置没有影响;发生流动分离后的主要声源区域的雷诺应力呈现单峰值,流体脉动程度较剧烈;对比不同工况下主要声源最大声压级和X/C=1处中心最大雷诺应力值发现,两者变化趋势一致且易受尖速比变化的影响。文章以实验测试的方法揭示了风轮主要声源区域雷诺应力表现的流动特征,研究成果对于叶片的优化设计和降噪方法的改进提供了可行的解决思路。     

三种地形数据下WRF模式风场模拟对比试验研究

为提高近地面风场模拟精度,将SRTM3和ASTER高精度地形数据引入WRF中尺度模式中,并结合模式原有地形数据GTOPO30,对香港地区2013年11月风场进行模拟,同时选取模拟区域内29个地面气象监测站实测数据对不同地形数据下的10m高处风速模拟结果进行检验。结果表明,WRF模式能够较好地模拟近地面风场的日变化趋势,尤其在海拔较高的区域,模拟结果与实际情况非常吻合;相比于GTOPO30,SRTM3和ASTER数据能够更加准确描述研究区域的地形特征,两者对原地形高程的修正幅度均比较有限,用于近地面风速模拟时均能小幅改善模拟效果。     

On the dependence of an empty flanged diffuser performance on flange height: Numerical simulations and PIV visualizations

Flanged diffuser shrouding small wind turbine, is among the most tested devices for increasing wind energy. The height of the flange is between the geometric futures of the diffuser that contributes efficiently in improving diffuser performances. Results obtained from numerical simulations and PIV visualizations show that when a flange is mounted at the outlet area of the diffuser, two contra-rotating vortices were created at this location. These two vortices move away from each other in the flow direction as the flange height increases and they seem to lengthen in the streamwise direction and to extend in the two directions when the flange height becomes taller. A critical ratio (Flange height/Inlet section diffuser diameter = 0.1) has been found. Beyond this value, due to the remoteness of vortices from the flange, the flange height seems to be without significant effect on increasing wind velocity.     

Experimental validation of a computational fluid dynamics code to predict the wind speed in street canyons for passive cooling purposes

Natural ventilation is one of the most efficient passive cooling techniques for buildings. Knowledge of the wind speed in street canyons is the necessary condition for the application of such a technique in dense urban configurations. Thus, prediction techniques to evaluate the microclimate and dispersion parameters in street canyons, has become a subject of intense scientific research in recent years. In most of cases, wind flow and pollutant dispersion characteristics have been studied numerically and experimentally with a view to provide an insight in urban dispersion. The microscale model MIMO was employed in order to perform a three-dimensional modelling of the wind field within three typical deep street canyons, located in the centre of Athens. Computational results were compared to field data collected during consecutive three-day experimental campaigns that took place in the summer period. Results from the computations have shown that the wind field in urban areas is quite complex, presenting areas of very low wind speeds and convergence of vortices. The model underestimated the measured wind speed intensities, which may be partly explained by the uncertainty of specific input parameters, the necessary simplifications for the application of such models and finally the geometrical complexity of the area modelled. Finally, computations were performed for a reference velocity of 2 m/s, which is considered to be a threshold value. The wind field developed departed from the one observed in the previous cases.     

Numerical simulation and parametric optimization on the container type energy storage system

本文以储能集装箱为研究对象,根据锂电池发热特性,应用标准k-湍流模型、D-O辐射模型,实现了集装箱内流场、温度场的数值模拟。对基准工况与优化工况进行了相关数值模拟,基准工况模拟结果表明：距离空调近端与远端出风均匀性存在较大差别,存在电池模块间温差较大情况。通过在风道内加设导流板,并进行结构调整,对原有工况进行优化,优化工况模拟结果表明：优化后流场与温度场分布更加均匀,能够有效降低电池模块间温差。