Dynamisch‐thermisches CFD‐Verfahren mit angepaßter Regelungsmethode

Dynamic thermal CFD approach using an adaptive control method. Previously published studies have presented a novel freeze‐flow method for reducing CPU requirements of long‐term dynamic thermal simulations using CFD programs. This works by intermittently switching between solution of the full dynamic equations and solution of the enthalpy equation only (frozen flow). This paper describes a new automated control method for this switching strategy and shows an additional decrease in simulation time. In tests with models for mechanical and natural ventilation and for free convection in a sealed room with constant and time varying thermal boundary conditions, a simulation time reduction of up to 93% was achieved when compared to a continuous fully dynamic simulation.     

Natural ventilation in high-rise buildings with double facades,saving or waste of energy

Double facades are built to allow natural ventilation in high-rise buildings and buildings with high outside noise levels. To evaluate the energetic performance, three buildings with double facades were monitored for at least 1 year (Siemens building in Dortmund, Victoria Insurance Company in Düsseldorf and RWE Tower in Essen). In one building all the air conditioning facilities had been removed and the facade replaced by a double façade, one building had cooling equipment without mechanical ventilation (concrete core tempered ceiling) and one building had cooling equipment combined with mechanical ventilation. The results document the indoor climate, the boundary conditions for further planning and the possibilities for high-rise buildings with no or only limited cooling facilities. The research was carried out at the University of Dortmund and founded by the state of Nordrhein–Westfahlen (AG-Solar).     

Energy efficiency optimization of combined ventilation systems in livestock buildings

Good ventilation system in livestock buildings is necessary for removing excess moisture and heat and for improving building environment in general. Natural ventilation does not require energy consumption and on the other hand, animals would not be affected by electrical power failures. Because natural ventilation depends largely on temperature difference between inside and outside air and wind velocity and direction it is very important in early stages of building design to provide orientation and accurate opening areas. Numerical simulation of natural ventilation and computation of fluid dynamics in livestock buildings can be usefully integrated in whole ventilation system optimization and related energy consumption decrease. Even in mechanical system ventilation, from flow field obtained in numerical simulation it is possible to optimize these systems. CFD analysis is generally restricted to the study of buildings’ environment flows and space study, and the designer must supply boundary conditions in the form of external and internal buildings’ envelope/wall surface conditions. Finally, the needs for further research and engineering development are outlined.     

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.     

采用"二区+CFD"模型研究点源自然通风及其羽流

讨论了几个典型的羽流流量计算公式,结合一种二区自然通风模型,建立了热分层高度、通风量和中和界高度的计算式。基于单步静态耦合(或预定边界条件)的思想,采用“二区 CFD”模型的方法研究了自然通风及其羽流。二区模型提供CFD模拟边界条件,CFD模拟反向检验二区模型,同时检验各羽流公式的适用性。     

The effect of ceiling configurations on indoor air motion and ventilation flow rates

The purpose of this paper is to evaluate the effects of a building parameter, namely ceiling configuration, on indoor natural ventilation. The computational fluid dynamics (CFD) code Phoenics was used with the RNG k–? turbulence model to study wind motion and ventilation flow rates inside the building. All the CFD boundary conditions were described. The simulation results were first validated by wind tunnel experiment results in detail, and then used to compare rooms with various ceiling configurations in different cases. The simulation results generated matched the experimental results confirming the accuracy of the RNG k–? turbulence model to successfully predict indoor wind motion for this study. Our main results reveal that ceiling configurations have certain effects on indoor airflow and ventilation flow rates although these effects are fairly minor.     

CFD modelling of natural displacement ventilation in an enclosure connected to an atrium

Computational fluid dynamics (CFD) is used to investigate buoyancy-driven natural ventilation flows in a single-storey space connected to an atrium. The atrium is taller than the ventilated space and is warmed by heat gains inside the single-storey space which produce a column of warm air in the atrium and drive a ventilation flow. CFD simulations were carried out with and without ventilation openings at the bottom of the atrium, and results were compared with predictions of analytical models and small-scale experiments. The influence of key CFD modelling issues, such as boundary conditions, solution controls, and mesh dependency were investigated. The airflow patterns, temperature distribution and ventilation flow rates predicted by the CFD model agreed favourably with the analytical models and the experiments. The work demonstrates the capability of CFD for predicting buoyancy-driven displacement natural ventilation flows in simple connected spaces.     

基于室外风环境CFD模拟的建筑规划设计

立足于优化建筑小区规划设计的理念,介绍了如何建立适合建筑小区模拟的模型与相应的边界条件.通过建筑小区自然通风的CFD模拟实例,说明了评价室外风环境的方法,以及如何在建筑规划设计前期与建筑方案结合,及时评价小区风环境设计的合理性.建筑小区风环境CFD模拟在规划前期的介入,对建筑规划设计的优化具有指导建议.     

CFD assessment of the performance of lateral ventilation in Double Glazed Façades in Mediterranean climates

Implementation of Double Glazed Façades (DGF) in buildings has been an object of broad study and application in recent years, both in new and existing constructions. However, there is little experience in predicting the operating behavior of a DGF. Sometimes the results obtained are not satisfactory and an extra energetic cost is necessary to obtain suitable comfort conditions in the inner space of the building, especially in Mediterranean climates, where large solar gains are a constant condition along throughout the year, and such large semi-transparent areas can produce significant over-heating in buildings. Computational Fluid Dynamics (CFD) has proven to be a useful tool for modeling air flow and heat transfer in DGF including conduction, convection and radiation heat transfer phenomena. The aim of this work was to evaluate, by means of CFD, the influence of several lateralventilation systems including the variation of the flow inlet (e.g., uniform or swirl flow inlet) and analyzing different inlet flow configurations (e.g., uniform, inclined, upper or lowercrossed inlet flow). From these CFD simulations, a comparative efficiency study, in terms of reduction of solar load gains and energetic requirements, was performed comparing the obtained results for horizontal ventilation against vertical ventilation, which was previously studied by this research group. It was found that using horizontal ventilation schemes would reduce the required air volumetric flow rate within the DGF for obtaining similar reductions in solar load gain to those obtained with vertical ventilation, when the construction and operation parameters of the DGF are equal for both cases.     

Wind tunnel simulation studies on dispersion at urban street canyons and intersections—a review

Increased traffic emissions and reduced natural ventilation cause build up of high pollution levels in urban street canyons/intersections. Natural ventilation in urban streets canyons/intersections is restricted because the bulk of flow does not enter inside and pollutants are trapped in the lower region. Wind vortices, low-pressure zones and channeling effects may cause build up of pollutants under adverse meteorological conditions within urban street canyons. The review provides a comprehensive literature on wind tunnel simulation studies in urban street canyons/intersections including the effects of building configurations, canyon geometries, traffic induced turbulence and variable approaching wind directions on flow fields and exhaust dispersion.     

Application of integrating multi-zone model with CFD simulation to natural ventilation prediction

Predicting the performance of natural ventilation is difficult, especially for large scale naturally ventilated buildings, due to the lack of accurate and efficient prediction tools. This paper presents a strategy, integrating a multi-zone model and computational fluid dynamics (CFD), to improve natural ventilation prediction and design methods. Large openings and atrium configurations are broadly used in naturally ventilated buildings to promote buoyancy force and optimize air movement. How to properly deal with this typical configuration for a multi-zone model and integrated simulation is discussed and compared in this paper. In order to validate a newly developed multi-zone model program, MMPN, this paper investigated both buoyancy ventilation and wind-buoyancy combined ventilation. Integration strategies, transferring data (velocity or pressure) from a multi-zone model program to CFD as boundary conditions, are also studied.     

基于风速补偿的自然通风房间热舒适性模拟研究

为获得自然通风条件下室内的热舒适评价方法,利用Fluent软件,根据杭州市典型室外气象参数确定边界条件,模拟居住建筑室外绕流场、室内通风场和温度场的分布规律;运用基于风速补偿的热适应性评价模型分析室内热舒适状况,并将此法与PMV模型结果对比,表明前者定义舒适的阈值较小,且正向偏离较大,后者则能较好反映自然通风房间的热环境。     

Untersuchungen zum Raumklima und zur Fensterlüftung in Schulen

Investigations on indoor environmental conditions and natural ventilation in school buildings. There are more than 40,000 school buildings in Germany. Most of them are awaiting retrofitting. Retrofitting is not only intended to improve the energetic standards, but first of all to improve indoor environmental conditions for pupils and teachers. As most of German schools are not equipped with mechanical ventilation systems, natural ventilation controlled by the occupants opening the windows is the main way to maintain healthy and comfortable conditions in the classrooms. For that purpose the influence of window opening behaviour of the occupants on the indoor environment was measured in two German schools. Temperature and carbon dioxide concentration of the indoor air as well as the outdoor climate conditions were measured. In one school the frequency of window opening was recorded. Besides high CO₂‐concentrations especially during winter, some of the investigated classrooms are additionally to cold in winter or to hot in summer. Some classrooms do not have a shading device or the shading device is insufficient or it constricts the ventilation of the room. Windows are used as controls in some degree during lessons and during the breaks. But windows get closed after lessons and stay closed until next morning. There is no night or early morning ventilation in summer. A significant weak to moderate positive correlation between total open window ratio and indoor temperature has been found. Correlation between total open window ratio and outdoor temperature is not significant in most cases or show a weak correlation coefficient.     

Predicting integrated thermal and acoustic performance in naturally ventilated high-rise buildings using CFD and FEM simulation

The study of ventilation windows for both natural ventilation and noise mitigation has drawn significant attention recently. This paper presents the numerical approaches to analyse the integrated thermal and acoustical performance of ventilation windows, for a residential building in tropical climate which employs double-layer noise mitigation window for natural ventilation. Given a set of outdoor wind conditions, the distributions of indoor flow and temperature fields are simulated using Computational Fluid Dynamics (CFD) model. The thermal comfort is evaluated using statistical Predicted Mean Vote (PMV) method. For the acoustic performance, noise radiation from road traffic is assumed as the noise source, and the sound insulation of building façade is simulated using Finite Element Method (FEM). From the simulation results, it is found that the thermal satisfaction response is closely related to the inlet wind temperature and speed, and the window opening size greatly affects the ventilation performance. From the case study in Singapore, during certain season, day/night time and with sufficient wind flow, the ventilation window can provide enough fresh air, maintain adequate thermal comfort and quiet acoustic environment for the occupants. The numerical approaches presented in this paper are applicable to general window design studies, and the simulation findings can be incorporated into green building planning. The advantages of using simulation approaches are highlighted and their limitations are discussed.     

Predicting thermal and energy performance of mixed-mode ventilation using an integrated simulation approach

Mixed-mode ventilation can effectively reduce energy consumption in buildings, as well as improve thermal comfort and productivity of occupants. This study predicts thermal and energy performance of mixed-mode ventilation by integrating computational fluid dynamics (CFD) with energy simulation. In the simulation of change-over mixed-mode ventilation, it is critical to determine whether outdoor conditions are suitable for natural ventilation at each time step. This study uses CFD simulations to search for the outdoor temperature thresholds when natural ventilation alone is adequate for thermal comfort. The temperature thresholds for wind-driven natural ventilation are identified by a heat balance model, in which air change rate (ACH) is explicitly computed by CFD considering the influence of the surrounding buildings. In buoyancy-driven natural ventilation, the outdoor temperature thresholds are obtained directly from CFD-based parametric analysis. The integrated approach takes advantage of both the CFD algorithm and energy simulation while maintaining low levels of complexity, enabling building designers to utilize this method for early-stage decisionmaking. This paper first describes the workflow of the proposed integrated approach, followed by two case studies, which are presented using a three-floor office building in an urban context. The results are compared with those using an energy simulation program with built-in multizone modules for natural ventilation. Additionally, adaptive thermal comfort models are applied in these case studies, which shows the possibility of further reducing the electricity used for cooling.     

Simulation of buoyancy-driven natural ventilation of buildings—Impact of computational domain

Two computational domains have been used for simulation of buoyancy-driven natural ventilation in vertical cavities for different total heat fluxes and wall heat distributions. Results were compared between cavities with horizontal and vertical inlets. The predicted ventilation rate and heat transfer coefficient have been found to depend on the domain size and inlet position as well as the cavity size and heat distribution ratio. The difference in the predicted ventilation rate or heat transfer coefficient using two domains is generally larger for wider cavities with asymmetrical heating and is also larger for ventilation cavities with a horizontal inlet than those with a vertical inlet. The difference in the heat transfer coefficient is generally less than that in the ventilation rate. In addition, a ventilation cavity with symmetrical heating has a higher ventilation rate but generally lower heat transfer coefficient than does an asymmetrically heated cavity. A computational domain larger than the physical size should be used for accurate prediction of the flow rate and heat transfer in ventilation cavities or naturally ventilated buildings with large openings, particularly with multiple inlets and outlets. This is demonstrated with two examples for natural ventilation of buildings.     

Iterative Anwendung zweier Rechenmodelle zur Beurteilung der Lüftungserfordernisse für eine Vermeidung von Schimmelpilzwachstum

Iterative application of two simulation tools to enable the determination of the required airing to preclude mould growth in corners. A newly developed whole building model is able to determine the effect of different ventilation schemes on indoor climate. Feeding the resulting surface temperatures and humidities to a biohygrothermal model allows to assess the risk of mould growth. For given ambient conditions it is thus possible to determine the necessary minimum ventilation rate by iteratively applying the two models. As an example, different building constructions are investigated, showing the effect of different natural ventilation schemes (continuous ventilation versus pulse ventilation by opening the window) on the minimum mean air change rate required for preventing mould growth.     

Wind tunnel investigation on influence of fluctuating wind direction on cross natural ventilation

The performance of wind driven natural ventilation is influenced significantly by the boundary conditions set for the wind. In real conditions the wind direction is fluctuating constantly so it is important to consider this fluctuation in experiments and simulations. This paper investigates the influence of fluctuating wind direction on cross-ventilation using wind tunnel experiments with the aim of improving the evaluation accuracy for natural ventilation. A periodically fluctuating wind direction was designed and reproduced in the experiment. Rapid Response FIDs (Flame Ionization Detector) were used to monitor the concentration of tracer gas. An index named diluting flow rate (DFR) is introduced to evaluate the ventilation performance of this kind of experiment. The results indicate that the DFRs of fluctuating cases are approximately 65–100% of the maximum airflow rate and DFR is influenced by the wind speed, the opening size and the wind direction fluctuation. Informed by the experimental data the mechanism of this combined influence is discussed in this paper.     

Natural ventilation potential of high-rise residential buildings in northern China using coupling thermal and airflow simulations

Natural ventilation is regarded as one of the most energy-efficient ways of ventilating a building. Suitable methods for predicting ventilation performance are essential for regulating indoor air parameters in buildings. This study establishes a method to predict the natural ventilation potential for residential buildings. The average annual ventilation rate (N) and annual cooling load saving ratio (ACSR) for the top six types of residential buildings were measured and analyzed under different conditions. The N calculation formula was summarized to calculate the natural ventilation air change rate for each of the designated buildings. In addition, the logarithmic regression curves of the ACSR (with N) were also obtained and then used to predict the natural ventilation potential for specific climatic conditions. The simulation results could be used to guide engineers in deciding when and where natural ventilation can be incorporated as an energy-efficient feature without affecting indoor comfort. Moreover, accurate strategic analysis could also be used to assist architects evaluate the potential of natural ventilation at the architectural pre-design stage.     

地铁隧道活塞风井通风性能的数值模拟研究

本文采用CFD方法对某设有站台屏蔽门和活塞风井的地铁站中的自然通风系统进行了研究。文中建立了全尺寸几何模型,采用动网格方法模拟列车运行时引起的流域边界变化,通过求解三维非稳态雷诺时均N-S方程来获得地铁隧道、站台区域以及通风竖井中的流动特点。本文定义了2个参数,有效排风量Ne和有效进风量Ns,用来评价活塞风井的通风效率,主要分析了实际列车运营状况下,地铁站不同的风井数量、风井位置对活塞风井通风性能的影响。研究结果对于拟建地铁站的风井结构设计具有参考意义。