会议室空调气流组织形式的数值模拟研究

办公区里的会议室为重要工作场所,其环境质量的优劣直接影响工作的效率。以计算流体力学和传热学为基础,利用CFD软件对室内混合通风与置换通风两种气流组织进行数值模拟。结果表明:在温度场、速度场、温度效率及人体热舒适性等方面,置换通风均优于混合通风方式。     

A comparison of CFD‐simulations and measurements of the temperature stratification in a mixing box of an air‐handling unit

This paper presents a comparison between CFD‐simulations and measurements of the temperature stratification in a mixing box of an air‐handling unit. We have used data from field measurements during a period of over a year for different outside temperatures. We performed two‐dimensional CFD‐simulations for four different outside temperatures with commercially available software. The measurements as well as the simulations show that the temperature difference between the upper part and the lower part of the duct downstream of the mixing box is considerable. It increases, as the outside temperature decreases. However, the discrepancies between the measurements and the simulations are large. The reasons for this are uncertain boundary conditions and modelling errors leading to an inaccurate simulation result. The stratification downstream of the mixing box implies large sensor errors and the use of the mixed air temperature for control and fault detection must therefore be questioned. Averaging sensors, which take a mean value over the duct section, can be used but do not consider differences in velocities and are therefore not accurate either. In order to, for example, use CFD as a tool to decide the optimal sensor location a more accurate model and more information regarding the boundary conditions is needed. Copyright © 2001 John Wiley & Sons, Ltd.     

飞机座舱应用置换通风系统的节能分析

通过置换通风系统和混合通风系统的比较,阐述了置换通风系统的特点和优势.运用暖通空调领域设计置换通风系统的方法,并结合ISO7730 - 2005、ASHRAE62 - 1989和民航飞机相关标准对座舱通风的规定,以节能分析的方式得出了置换通风在飞机座舱通风系统应用上的节能优势.     

Study on nonpremixed methane/air combustion from flame structure and NOX emission aspect for different burner head structures

In the present study, the air turbulator, which is a part of a nonpremixed burner, is investigated numerically in terms of its effects on the diffusion methane flame structure and NO_X emissions. A computational fluid dynamics (CFD) code was used for the numerical analysis. At first, four experiments were conducted using natural gas fuel. In the experimental studies, the excess air ratio was taken constant as 1.2, while the fuel consumption rate was changed between 22 and 51 Nm³/h. After the experimental studies, the CFD studies were carried out. Pure methane was taken as fuel for the simulations. The nonpremixed combustion model with the steady laminar flamelet model (SFM) approach was used in the combustion analyses. Methane‐air extinction mechanism with 17 species and 58 reactions was used for the simulations. The results obtained from the CFD studies were confronted with the measurements of the flue gas emissions in the experimental studies. Then, a modified burner head was analysed numerically for the different air turbulator blade numbers and angles. The CFD results show that increasing the air turbulator blade number and angle causes the thermal NO emissions to be reduced in the flue gas by making the flame in the combustion chamber more uniform than the original case. This new flame structure provides better mixing of the fuel and combustion air. Thus, the diffusion flame structure in the combustion chamber takes the form of the partially premixed flame structure. The maximum reduction in the thermal NO emissions in the flue gas is achieved at 38% according to the original case.     

置换通风的研究与应用现状

简述了置换通风的原理,回顾了置换通风应用的发展过程,总结了置换通风系统的优缺点,介绍了置换通风的设计方法和设计关键参数的确定,分析了置换通风的工作机理,介绍了置换通风的CFD研究,并指出置换通风是一种值得推荐的暖通空调方式。     

"Bump燃烧室"内新概念稀扩散燃烧混合气形成机理的研究

基于自行研制的实验装置,用片状激光诱导荧光法(PLIF)对普通商用柴油喷雾的撞壁混合过程进行了实验研究,并用CFD数值分析软件对其进行了模拟计算,二者结果基本吻合．平板和实际燃烧室的实验及计算结果均表明,撞壁射流在遇到限流沿(Bump)后会剥离壁面,形成二次空间射流,扩大撞壁射流与空气的空间混合体积及混合速率,出现与周围空气迅速混合的“闪混”现象,减少壁面燃油堆积量．计算结果还表明,Bump的存在改变了缸内气流运动的流场结构,Bump附近旋向相反的“双涡结构”极大地增强了二次空间射流对周围空气的卷吸,促进了燃油与空气的混合,是Bump燃烧室内稀混合气形成及稀扩散燃烧的关键所在．     

EnergyPlus软件在置换通风设计中的应用研究

介绍了EnergyPlus软件中的置换通风模型,采用EnergyPlus辅助某办公室置换通风系统设计,并采用CFD软件模拟了室内温度分布,结果表明,EnergyPlus可以精确地预测置换通风系统负荷,系统设计能满足热舒适要求。     

CFD modelling and experimental investigation of an ejector refrigeration system using methanol as the working fluid

This paper presents results of computational fluid dynamic (CFD) analysis and experimental investigation of an ejector refrigeration system using methanol as the working fluid. The CFD modelling was used to investigate the effect of the relative position of the primary nozzle exit within the mixing chamber on the performance of the ejector. The results of the CFD were used to obtain the optimum geometry of the ejector, which was then used to design, construct and test a small‐scale experimental ejector refrigeration system. Methanol was used as the working fluid, as it has the advantage of being an ‘environmentally friendly’ refrigerant that does not contribute to global warming and ozone layer depletion. In addition, use of methanol allows the ejector refrigeration system to produce cooling at temperatures below the freezing point of the water, which of course would not be possible with a water ejector refrigeration system. CFD results showed that positioning the nozzle exit at least 0.21 length of the mixing chamber throat's diameter upstream of the entrance of the mixing chamber gave better performance than pushing it into the mixing chamber. Experimental values of coefficient of performance (COP) between 0.2 and 0.4 were obtained at operating conditions achievable using low‐grade heat such as solar energy and waste heat. Copyright © 2001 John Wiley & Sons, Ltd.     

多污染热源置换通风的应用与节能

简述了多污染热源置换通风的原理、特性及其与混合通风的比较 ,列举了工程应用实例 ,并对其节能效果进行了分析讨论 ,指出多污染热源置换通风是一种值得推广的通风方式     

Air flow and thermal comfort simulation studies of wind ventilated classrooms in Malaysia

A CFD software called VORTEX is used as a tool to simulate air flow and thermal comfort in naturally wind ventilated classrooms of an educational institution, which are at different locations, have different configurations and slightly differing outdoor environmental conditions. Simulations of the various classrooms are compared to get the most thermally comfortable and uncomfortable naturally ventilated classroom. An analysis of the simulations will be done, taking into consideration, among others, location of inlets and outlets and the sheltering effect of the surrounding built-up environment. Recommendations will then be made on how to improve the ventilation of the least comfortable room, based on hypothetical simulation results.     

Simulation of Velocity and Temperature Distributions of Displacement Ventilation System with Single or Double Heat Sources

In order to obtain a better understanding of flow characteristics of displacement ventilation,thethree-dimensional numerical models are developed using the CFD technology.The numericalsimulation results are verified by experiments,based on this,the velocity and temperature distributionof three-dimensional displacement ventilation system with single and double heat sources are studied.Velocity and temperature fields under two different cases of heat source are analyzed and compared.The numerical results show that there are three layers in vertical temperature fields of displacementventilation system with single or double heat sources,and the vertical temperature distribution ofsingle heat source is different from that of double heat sources.When indoor load is large,the comfortrequirement of people indoor can't be satisfied with displacement ventilation system only,thus anadditional refrigeration system is necessary.Furthermore,under the condition of two heat sources,thedisplacement ventilation parameters can't be computed simply according to single heat source inletparameters,therefore the interaction between heat sources should be considered.     

Experimental study for comparison of thermal comfort and air age between two combined ventilation systems with chilled ceiling considering occupant density

Due to the global spread of diseases and epidemics, the need to maintain a clean indoor atmosphere has received increasing attention in recent years. Therefore, there will be a need to clearly estimate and define the areas that affect human exposure to pollutants, taking into account the occupied density, which is the primary importance of this research. The capacity of the chilled ceiling combined with mixing ventilation and personal ventilation systems has been studied and compared to the chilled ceiling with mixing ventilation in terms of mean air age, temperature distribution, CO₂ concentration, and thermal efficiency, with the best flow rate of the proposed system considering the occupied density in a thermally insulated office room experimentally in the climate of Iraq (Hilla, a hot and dry climate). Twelve tests were performed for four different cooling loads with cooled ceilings (0%, 0.25%, 50%, and 80%), at a constant supply air flow rate with two PV airflow modifiers for three cases. As the cooling load treated by a chilled ceiling increased, the average air temperature increased with height in all cases. The lowest values of average air age appeared in the occupied area in the case of a chilled ceiling with mixing ventilation. This study shows that the chilled ceiling combined with a mixing ventilation and personal ventilation system with a flow rate of 7.5 L/s provides thermal comfort and higher air quality in the occupied area. based on the values of air exchange efficiency and occupants’ air exchange efficiency. As a result, a flow rate of 7.5 L/s is the best option for protecting occupants from direct pollution in the breathing zone and in the surrounding microclimate, because the lower the ventilation rate, the less air is changed for occupants.     

基于三维数值模拟的蒸汽喷射器结构优化

蒸汽喷射器可以有效利用余热资源,是一种节能环保的流体机械。采用计算流体力学(CFD)软件对蒸汽喷射器进行三维数值模拟,研究了喷嘴出口马赫数、混合室长度、等截面积混合段长度和扩压室长度对喷射器引射比和临界压力的影响。模拟结果表明:混合室长度存在一个最佳值,且最佳混合室长度与喷嘴出口马赫数有关;等截面积混合段长度和扩压室长度对引射比的影响不大,但是会影响喷射器的临界压力。研究结果对于蒸汽喷射器的设计和结构优化有一定的意义。     

Dispersion modeling and assessment of natural gas containing hydrogen released from a damaged gas transmission pipeline

This paper performs a simulation and assessment of dispersion of natural gas containing hydrogen released from transmission pipeline using a Computational Fluid Dynamics (CFD) approach. A 3D CFD model is established to evaluate the dispersion behavior of hydrogen-enriched natural gas in the hydrogen-natural gas mixing station. The simulations include a matrix of scenarios for hydrogen doping ratios, gas release rates, wind speeds and wind directions. The development process of flammable gas cloud is predicted, and the dangerous area generated in the hydrogen-natural gas mixing station is assessed. Additionally, the effects of some critical factors on flammable gas dispersion behavior are analyzed. The simulations produce some useful outcomes including the parameters of flammable gas cloud and the dangerous area in the station, which are useful for conducting a prior risk assessment and contingency planning.     

Effect of fuel jet arrangement on the mixing rate inside trapezoidal cavity flame holder at supersonic flow

Fuel mixing inside the supersonic combustion chamber is a significant process for development of modern scramjets. In this article, computational fluid dynamic (CFD) approach is applied to investigate the effect of various fuel injections on the mixing rate inside the supersonic combustion chamber. The mixing of hydrogen jets with four different arrangements inside the cavity flame holder is comprehensively studied. In order to examine the effect of multi jets within a cavity flameholder, a three-dimensional model is established and Navier-stocks equations are solved to simulate the flow and mixing zone inside a cavity region. Obtained results show that the injection of hydrogen jet from the bottom of cavity flame holder considerable enhances the ignition zone within the cavity. Moreover, the backward fuel injection is more superior to forward fuel injection since low-pressure vortex could significantly distribute the fuel and enlarge the mixing zone inside the cavity flame holder.     

Rapid prediction of exergy destruction in data centers due to airflow mixing

ABSTRACT

A novel lumped parameter approach is introduced to predict data center exergy destruction due to airflow mixing, resulting in a speedup of several orders of magnitude compared to detailed computational fluid dynamics (CFD) simulations. Both lumped parameter and detailed CFD methods agree within 8.5% for 11 test cases on an example data center. A significant time-saving design strategy is also introduced using two detailed CFD simulations to predict bulk flow parameters, and then applying lumped parameter analysis on flow-independent design parameters. The strategy shows agreement within 0.39% when computer room air conditioner (CRAC) supply temperature is varied from 12–20°C.     

地板送风与置换通风特性比较

介绍了置换通风与地板送风的原理。从原理出发,对地板送风与置换通风在气流组织、通风效率、 空气品质和热舒适等方面的特性进行了比较。     

Hydrogen risk analysis for a generic nuclear containment ventilation system

Hydrogen safety issue in a ventilation system of a generic nuclear containment is studied. In accidental scenarios, a large amount of burnable gas mixture of hydrogen with certain amount of oxygen is released into the containment. In case of high containment pressure, the combustible mixture is further ventilated into the chambers and the piping of the containment ventilation system. The burnable even potentially detonable gas mixture could pose a risk to the structures of the system once being ignited unexpectedly. Therefore the main goal of the study is to apply the computational fluid dynamics (CFD) computer code – GASFLOW, to analyze the distribution of the hydrogen in the ventilation system, and to find how sensitive the mixture is to detonation in different scenarios. The CFD simulations manifest that a ventilation fan with sustained power supply can extinguish the hydrogen risk effectively. However in case of station blackout with loss of power supply to the fan, hydrogen/oxygen mixture could be accumulated in the ventilation system. A further study proves that steam injection could degrade the sensitivity of the hydrogen mixture significantly.     

Scale-up methodology for automatic biomass furnaces

This work presents a methodology to perform the scale-up of a solid fuel furnace to a higher heat output with maintaining or improving the burn-out quality. As basis to derive the scale-up concept, an example of a 35 kW screw burner for biomass fuels is investigated. Based on the Pi-theorem, the relevant dimensionless parameters are derived and similarity rules for the scale-up are proposed as follows: As initial conditions, the height to diameter ratio of the combustion chamber, the mean Reynolds number in the combustion chamber and the mean square velocity through the combustion chamber shall be kept constant or in the case of the Reynolds number may also increase. Additionally the effective momentum flux ratio between the secondary air injected in the combustion chamber and the gases from the pyrolysis and gasification section also shall be kept constant to maintain the mixing conditions between combustible gases and secondary air. Finally the thermal surface load on the screw also shall be kept constant. The influence of different scale-up approaches on thermal surface load, gas velocity, pressure losses, Reynolds number and height-to-diameter ratio are compared and discussed and a scaling approach to increase the heat output from 35 kW to 150 kW is described. For a theoretical validation of the scale-up, CFD simulations are performed to investigate the predicted pollutant emissions and the pressure loss for the scaled 150 kW furnace.     

直喷式柴油机进气涡流比与喷油系统参数优化匹配的多维仿真研究

基于多维数值模拟方法,研究了直喷式柴油机进气涡流比与喷油系统参数之间的匹配关系,并分析了进气涡流对柴油机油束撞壁的影响。研究结果表明:对于较小口径比的柴油机燃烧室,进气涡流对燃油混合的促进作用主要发生在油束撞壁以后,并影响燃油蒸气在燃烧室壁面附近的轴向分布情况;为了获得理想的燃油蒸气空间分布,涡流比的匹配值应随着喷孔数的增加而逐渐减小;通过改变喷油器的油束夹角,可以改善燃油蒸气在燃烧室壁面处的轴向分布情况,改善发动机的燃烧效果。