Integration of three-dimensional CFD results into energy simulations utilizing an Advection-Diffusion Response Factor

Indoor climates have a three-dimensional spatial distribution caused by three-dimensional airflow. To understand the building performance, we must integrate these spatial distributions into building simulations. However, conventional energy simulations are based on the assumptions that perfect mixing of air streams with different temperatures occurs. Therefore, it has been difficult to evaluate the effectiveness of energy conservation methods that utilize a thermal distribution mechanism within a room. Taking into account the above conditions, we have developed a calculation method that can achieve more accurate time-series analysis. This is accomplished by combining the newly developed method with the conventional energy simulation method. In the new method, we calculate, in advance, the heat response in a static flow field using computational fluid dynamics (CFD) analysis. Then we calculate Advection-Diffusion Response Factors and integrate them into the energy simulation as a factor in the three-dimensional thermal distribution within a room. In this paper, we show a calculation example using the model for high ceilings with high-temperature exhaust. As a result, we conclude that our new calculation method, in combination with a dynamic heat load calculation, will offer possibilities for a long-term, non-steady-state energy simulation, even on personal computers, based on the room temperature distribution data obtained using steady-state calculations with CFD analysis.     

Thermal simulation: Response factor analysis using three-dimensional CFD in the simulation of air conditioning control

The heat generated from an air-conditioning equipment or other thermal loads is distributed throughout a room by a three-dimensional airflow. This three-dimensional airflow creates a three-dimensional heat distribution in a room. To better understand building performance, we must integrate this spatial distribution into building simulations. Thus, three-dimensional computational fluid dynamics (CFD) analysis is necessary in design process because most conventional building energy simulations still employ a temperature that is averaged across the space of a room. However, usually only a few cases of CFD analyses are executable in real design process because of the large computational load they require. This paper presents a new, simplified method to calculate heat transport phenomena in rooms, based on a few cases of CFD analysis, and to integrate data into a nodal analysis. This method can be used to calculate an indoor environment, including the spatial distribution of temperature, with a computational load that is much lighter than it is in a simulation using CFD alone. Furthermore, in terms of precision, it is a far more reliable method than the conventional simulation, which assumes the perfect mixing of heat in a room. In the paper, we apply this method to simulate the control of air conditioning. Ordinarily, the reproduction of the phenomena shown in the calculation examples requires substantial manpower and costly computing resources for experimentation or CFD analysis. With our calculation method, it is possible to reproduce the same calculation results in a very short time with a PC. And we checked the potential to the practical use through a verification calculation with CFD analysis.     

带内热源试验厂房温度和气流场模拟研究

机械通风是带内热源厂房常用的主动式设计策略,对于提高建筑环境舒适度和改善室内空气品质等方面具有至关重要的作用,结合工程实际,利用计算流体动力学(简称CFD)方法对机车试验厂房温度和气流场进行了模拟研究,基于SST湍流模型,采用CFX软件进行了计算,提出了通过边界条件的等效处理综合多个计算域为一个计算域的流场分析方法,得到了2种通风方式下厂房内部的温度、速度和矢量分布图,对比分析发现,侧排风机设计方案可以有效抽吸热流,使工作环境满足通风散热要求。     

Coupled simulations for naturally ventilated rooms between building simulation (BS) and computational fluid dynamics (CFD) for better prediction of indoor thermal environment

The coupling strategies for natural ventilation between building simulation (BS) and computational fluid dynamics (CFD) are discussed and coupling methodology for natural ventilation is highlighted. Two single-zone cases have been used to validate coupled simulations with full CFD simulations. The main discrepancy factors have also been analyzed. The comparison results suggest that for coupled simulations taking pressure from BS as inlet boundary conditions can provide more accurate results for indoor CFD simulation than taking velocity from BS as boundary conditions. The validation results indicate that coupled simulations can improve indoor thermal environment prediction for natural ventilation taking wind as the major force. With the aids of developed coupling program, coupled simulations between BS and CFD can effectively improve the speed and accuracy in predicting indoor thermal environment for natural ventilation studies.     

Comparison of STAR-CCM+ and ANSYS Fluent for simulating indoor airflows

Suitable air distributions are essential for creating thermally comfortable and healthy conditions in indoor spaces. Computational fluid dynamics (CFD) is widely used to predict air distributions. This study systematically assessed the performance of the two most popular CFD programs, STAR-CCM+ and ANSYS Fluent, in predicting air distributions. The assessment used the same meshes and thermo-fluid boundary conditions for several types of airflow found in indoor spaces, and experimental data from the literature. The programs were compared in terms of grid-independent solutions; turbulent viscosity calculations; heat transfer coefficients as determined by wall functions; and complex flow with complicated boundary conditions. The two programs produced almost the same results with similar computing effort, although ANSYS Fluent seemed slightly better in some aspects.     

计算机模拟在大空间建筑幕墙结露分析中应用

应用IES软件建立大空间三维几何模型,分层模拟冬季及夏季的室内热环境,选用相关参数作为CFD分析的边界条件.根据该边界条件,利用STAR-CD软件对大空间内各点的空气温度进行CFD模拟分析,通过幕墙最不利点表面温度同露点温度的比较,进行结露分析.     

航站楼使用阶段钢结构温度取值研究

为了准确确定大跨度钢结构在使用阶段的温度作用和方便钢结构温度变化模拟分析,对厦门高崎机场T4航站楼屋盖钢结构夏季的温度进行了全面测试,提出太阳辐射与室外气象温度的日变化计算模型。根据气象条件、室内分层温度控制方法以及屋面构造做法,通过CFD模拟技术研究高大空间温度场沿高度方向的变化规律,并在天窗部位考虑了太阳辐射的影响。结果表明:金属屋面具有良好的保温隔热性能,室内与室外钢结构的最高温度均与最高气温非常接近,钢结构昼夜温差主要受室外气温变化的影响;受到太阳辐射的影响,天窗部位钢结构的温度明显高于最高气温;CFD模拟计算得到高大空间空气温度沿竖向分布的结果与实测值的变化趋势相一致,顶部热滞留区的温度明显高于等温空调区,计算温度与实测温度相对误差为5%~10%;通过实测与模拟分析,可以较为合理地确定在使用阶段大跨度钢结构的温度,该方法可供类似大型公共建筑设计时参考。     

Quality control of computational fluid dynamics in indoor environments

Computational fluid dynamics (CFD) is used routinely to predict air movement and distributions of temperature and concentrations in indoor environments. Modelling and numerical errors are inherent in such studies and must be considered when the results are presented. Here, we discuss modelling aspects of turbulence and boundary conditions, as well as aspects related to numerical errors, with emphasis on choice of differencing scheme and computational grid. Illustrative examples are given to stress the main points related to numerical errors. Finally, recommendations are given for improving the quality of CFD calculations, as well as guidelines for the minimum information that should accompany all CFD-related publications to enable a scientific judgment of the quality of the study.     

Computational simulations and optimization of flow and temperature distributions in a large-scale power plant building

For reduced carbon dioxide and pollutant emission, it is often as effective, if not more, to minimize energy use on the consumption side, as to maximize the efficiency on the power supply side. In this study, we seek to fully characterize and optimize the heating, ventilation, and air conditioning (HVAC) electrical energy use in a large-scale structure: a power-plant building that houses boilers, turbines and other operating equipment. We use a fully three-dimensional computational fluid dynamics (CFD) model of this building, measuring 80 m in width, 120 m in length and 60 m in height, replicating the complex internal and external geometries, in order to simulate the flow and temperature distributions under a wide range of ambient and HVAC operating conditions. The flow patterns and temperature distributions in this building structure are computationally simulated in detail, wherein the computed temperatures are validated through spot measurements. The detailed understanding of the flow patterns and temperature distributions then allows for optimization of the HVAC configuration. Identification of the problematic flow patterns and temperature mis-distributions, leads to some corrective measures, for optimization of the temperature distributions. The basic principles of fluid mechanics and heat transfer, applied in conjunction with CFD simulation results, can result in substantial improvements under both hot- and cold-weather conditions, in most cases with relatively simple, implementable modifications.     

The 24-h unsteady analysis of air flow and temperature in a real city by high-speed radiation calculation method

Heat island phenomenon is an important issue in environmental studies. Many studies involving observations and simulations have been performed. Computational Fluid Dynamics (CFD) analysis including the effects of solar radiation and longwave radiation heating/cooling are limited in the extreme conditions at midday, when solar radiation intensity are at maximum; and the 24-h unsteady analyses are not done due to the difficulties of the boundary conditions. Authors developed Computer Graphics (CG) method for calculating solar radiation and longwave radiation with high speed, and developed the 24-h unsteady analytical method from the data calculated by Weather Research and Forecasting (WRF). The integrated CFD was applied to the real city. The results showed that the integrated CFD was the useful tool to analyze the heat island phenomena.     

不同保温形式下典型热桥的热湿传递研究

为了研究建筑热桥在室内外热湿条件综合作用下墙体内部温湿度分布,本文以Henry提出的热湿传递模型为基础,建立了建筑热桥二维热湿传递数学模型,并以上海地区冬季室外典型年气象参数作为计算条件,计算了L形建筑热桥在该条件周期作用下墙体内部温湿度分布和热流密度变化,对热桥局部保温提出了建议.     

Development of new indices to assess the contribution of moisture sources to indoor humidity and application to optimization design: Proposal of CRI(H) and a transient simulation for the prediction of indoor humidity

Many indoor and outdoor factors (e.g., the presence of occupants, hot-water supply equipment, the use of hygroscopic materials, and ventilation) contribute to indoor humidity. It is important to investigate and understand the contribution of each factor to indoor humidity and to establish an effective method for the design and control of indoor humidity. In this study, indoor humidity was treated as a linear summation of the contribution of various factors, all of which can cause an increase or decrease in indoor humidity. New indices for assessing the contribution of factors to the humidity distribution in a room are proposed as Contribution Ratios of Humidity (CRI_(H)) 1, 2, and 3 which can be calculated based on Computational Fluid Dynamics (CFD) simulations. Furthermore, a transient simulation based on CRI_(H)1 and the Contribution Ratio of Indoor Climate (CRI_(C)) was developed to predict the indoor humidity distribution. A 100-day transient analysis was performed in a living room in which moisture-buffering materials were used. The simulation results were compared with those from a well-mixed zonal model and a CFD transient analysis to confirm the effectiveness of the approach. The analysis provided the three-dimensional spatial distribution of indoor humidity and temperature with good prediction accuracy. The calculation time was approximately equal to that of the well-mixed zonal model and much faster than that of the CFD transient analysis.     

Detailing the effects of geometry approximation and grid simplification on the capability of a CFD model to address the benchmark test case for flow around a computer simulated person

This paper details the use of a simplified CFD model to predict the flow patterns around a computer simulated person in a displacement ventilated room. The use of CFD is a valuable tool for indoor airflow analysis and the level of complexity of the model being investigated is often critical to the accuracy of predictions. The closer the computational geometry is to the real geometry of interest, the more accurate the corresponding results are expected to be. High complexity meshes enable elaborated geometries to be resolved. The drawback is, however, their increased computational cost. The Fire Dynamics Simulator (FDS) model (Version 5) enabled to investigate the effects of geometry and computational grid simplification on the accuracy of numerical predictions. The FDS model is based on a three-dimensional Cartesian coordinate system and all solid obstructions are forced to conform to the underlying numerical grid which is a potential limitation when dealing with complex geometries such as those of a human body. Nevertheless, the developed computational model was based exclusively on a three-dimensional rectangular geometry. At the same time, in order to limit the total number of grid cells, a relatively coarser grid than those used for similar simulations was adopted in the investigation. The developed model was then assessed in terms of its capability of reproducing benchmark temperature and air velocity distributions. The extent to which numerical results depend on different simulation settings was detailed and different boundary conditions are discussed in order to provide some guidance on the parameters that resulted to affect the accuracy of the predicted results. The comparison between numerical results and measurements showed that a simplified CFD model can be used to capture the airflow characteristics of the investigated scenario with predictions showing a favourable agreement with experimental data at least in the qualitative features of the flow (the detailed investigation of the local airflow field near the occupant can not be probably conducted apart from considering the real human geometry). Significant influence of simulator geometry and of boundary conditions was found.     

Airborne spread of expiratory droplet nuclei between the occupants of indoor environments: A review

This article reviews past studies of airborne transmission between occupants in indoor environments, focusing on the spread of expiratory droplet nuclei from mouth/nose to mouth/nose for non‐specific diseases. Special attention is paid to summarizing what is known about the influential factors, the inappropriate simplifications of the thermofluid boundary conditions of thermal manikins, the challenges facing the available experimental techniques, and the limitations of available evaluation methods. Secondary issues are highlighted, and some new ways to improve our understanding of airborne transmission indoors are provided. The characteristics of airborne spread of expiratory droplet nuclei between occupants, which are influenced correlatively by both environmental and personal factors, were widely revealed under steady‐state conditions. Owing to the different boundary conditions used, some inconsistent findings on specific influential factors have been published. The available instrumentation was too slow to provide accurate concentration profiles for time‐dependent evaluations of events with obvious time characteristics, while computational fluid dynamics (CFD) studies were mainly performed in the framework of inherently steady Reynolds‐averaged Navier‐Stokes modeling. Future research needs in 3 areas are identified: the importance of the direction of indoor airflow patterns, the dynamics of airborne transmission, and the application of CFD simulations.     

Inverse modeling methods for indoor airborne pollutant tracking: literature review and fundamentals

Reduction in indoor environment quality calls for effective control and improvement measures. Accurate and prompt identification of contaminant sources ensures that they can be quickly removed and contaminated spaces isolated and cleaned. This paper discusses the use of inverse modeling to identify potential indoor pollutant sources with limited pollutant sensor data. The study reviews various inverse modeling methods for advection-dispersion problems and summarizes the methods into three major categories: forward, backward, and probability inverse modeling methods. The adjoint probability inverse modeling method is indicated as an appropriate model for indoor air pollutant tracking because it can quickly find source location, strength and release time without prior information. The paper introduces the principles of the adjoint probability method and establishes the corresponding adjoint equations for both multi-zone airflow models and computational fluid dynamics (CFD) models. The study proposes a two-stage inverse modeling approach integrating both multi-zone and CFD models, which can provide a rapid estimate of indoor pollution status and history for a whole building. Preliminary case study results indicate that the adjoint probability method is feasible for indoor pollutant inverse modeling. PRACTICAL IMPLICATIONS: The proposed method can help identify contaminant source characteristics (location and release time) with limited sensor outputs. This will ensure an effective and prompt execution of building management strategies and thus achieve a healthy and safe indoor environment. The method can also help design optimal sensor networks.     

CFD boundary conditions for contaminant dispersion,heat transfer and airflow simulations around human occupants in indoor environments

Indoor computational fluid dynamics (CFD) simulations can predict contaminant dispersion around human occupants and provide valuable information in resolving indoor air quality or homeland security problems. The accuracy of CFD simulations strongly depends on the appropriate setting of boundary conditions and numerical simulation parameters. The present study explores influence of the following three key boundary condition settings on the simulation accuracy: (1) contaminant source area size, (2) convective/radiative heat fluxes, and (3) shape/size of human simulators. For each of the boundary conditions, numerical simulations were validated with experimental data obtained in two different environmental chambers. In CFD simulations, a small release area of a contaminant point source causes locally high concentration gradients that require a very fine local grid system. This fine grid system can slow down the simulations substantially. The convergence speed of calculation is greatly increased by the source area enlargement. This method will not influence the simulation accuracy of passive point source within well-predicted airflow field. However, for active point source located within complicated airflow filed, such an enlargement should be carried out cautiously because simulation inaccuracy might be introduced. For setting thermal boundary conditions, convection to radiation heat flux ratio is critical for accurate CFD computations of temperature profiles around human simulators. The recommended convection to radiation (C:R) ratio is 30:70 for human simulators. Finally, simplified human simulators can provide accurate temperature profiles within the whole domain of interest. However, velocity and contaminant concentration simulations require further work in establishing the influence of simplifications on the simulation accuracy in the vicinity of the human simulator.     

Principles and applications of probability-based inverse modeling method for finding indoor airborne contaminant sources

Building indoor air quality (IAQ) has received growing attentions lately because of the extended time people spend indoors and the increasing reports of health problems related to poor indoor environments. Recent alarms to potential terrorist attacks with airborne chemical and biological agents (CBA) have further highlighted the research needs on building vulnerability and protection. To maintain a healthful and safe indoor environment, it is crucial to identify contaminant source locations, strengths, and release histories. Accurate and prompt identification of contaminant sources can ensure that the contaminant sources can be quickly removed and contaminated spaces can be effectively isolated and cleaned. This paper introduces a probability concept based prediction method—the adjoint probability method-that can track potential indoor airborne contaminant sources with limited sensor outputs. The paper describes the principles of the method and presents the general modeling algorithm and procedure that can be implemented with current computational fluid dynamics (CFD) or multi-zone airflow models. The study demonstrates the application of the method for identifying airborne pollutant source locations in two realistic indoor environments with few sensor measurement outputs. The numerical simulations verify the feasibility and accuracy of the method for indoor pollutant tracking applications, which forms a good foundation for developing an intelligent and integrated indoor environment management system that can promptly respond to indoor pollution episodes with effective detection, analysis, and control.     

Investigating the potential use of natural ventilation in new building designs in Turkey

The most economic air conditioning of living and working places can be achieved by natural ventilation if sufficient. This provides not only the circulation of clear air, but also the decrease of indoor temperature, especially, during hot summer days, provided that the temperature of clear air is lower than that of indoor. From the geometric optimization point of view, both size and position of windows in buildings are important parameters to obtain a uniform indoor air velocity distribution.In this study, the potential use of natural ventilation as a passive cooling system in new building designs in Kayseri, a midsize city in Turkey located at 38.44°N and 35.29°W, was investigated by computational fluid dynamics (CFD). Using the FLUENT 6.2 program, which employs finite element methods, indoor air velocity distributions with respect to changing wind direction and magnitude were obtained in living places of different dimensions. The simulation results suggest that natural ventilation can be used to provide a thermally comfortable indoor environment during the summer season in the study area. The study presents useful design guidelines for natural ventilation at both site planning and individual building levels.     

黑瞎子岛植物园建筑风环境分析

BIM技术近年来发展迅速,黑瞎子岛植物园在建筑设计初期就成功地引入了BIM技术。建筑风环境设计中,CFD技术应用广泛,本文着重介绍了CFD技术结合BIM技术的切入和应用,并对植物园室外风环境、室内自然通风、空调流场及热舒适性等进行了模拟计算。根据模拟的建筑风环境动态调整和优化建筑布局和朝向,优化建筑布局。同时,根据CFD的室内温湿度场计算,校核植物生长条件需求,采用局部空气调节以实现节能。     

Fast and informative flow simulations in a building by using fast fluid dynamics model on graphics processing unit

Fast indoor airflow simulations are necessary for building emergency management, preliminary design of sustainable buildings, and real-time indoor environment control. The simulation should also be informative since the airflow motion, temperature distribution, and contaminant concentration are important. Unfortunately, none of the current indoor airflow simulation techniques can satisfy both requirements at the same time. Our previous study proposed a Fast Fluid Dynamics (FFD) model for indoor flow simulation. The FFD is an intermediate method between the Computational Fluid Dynamics (CFD) and multizone/zonal models. It can efficiently solve Navier–Stokes equations and other transportation equations for energy and species at a speed of 50 times faster than the CFD. However, this speed is still not fast enough to do real-time simulation for a whole building. This paper reports our efforts on further accelerating FFD simulation by running it in parallel on a Graphics Processing Unit (GPU). This study validated the FFD on the GPU by simulating the flow in a lid-driven cavity, channel flow, forced convective flow, and natural convective flow. The results show that the FFD on the GPU can produce reasonable results for those indoor flows. In addition, the FFD on the GPU is 10–30 times faster than that on a Central Processing Unit (CPU). As a whole, the FFD on a GPU can be 500–1500 times faster than the CFD on a CPU. By applying the FFD to the GPU, it is possible to do real-time informative airflow simulation for a small building.