Difficulties and limitations in performance simulation of a double skin façade with EnergyPlus

Currently, several whole-building simulation tools (e.g., esp-r, EnergyPlus, TRNSYS, TAS, IES VE, IDA ICE, VA114, BSim, etc.) are used to assess the energy performance of double-skin façade (DSF) buildings. The aforementioned tools are well suited to assess energy performance of conventional building systems or whole buildings; however, it is questionable whether such tools can accurately describe the transient heat and mass transfer phenomena that occur in the complex three-dimensional geometry of DSFs. This paper describes an empirical validation of the EnergyPlus simulation tool for performance simulation of a DSF. A series of experiments were conducted for cavity airflow and thermal behavior of the DSF and then compared with simulation outputs. In this paper, it is shown that there are significant differences in both thermal and airflow behavior of DSFs between the measurements and simulation predictions by EnergyPlus. This study investigates three cases causing the differences and elucidates what should be considered when modeling DSFs using EnergyPlus.     

On CFD simulation of wind-induced airflow in narrow ventilated facade cavities: Coupled and decoupled simulations and modelling limitations

Heat and mass transfer modelling in building facades with ventilated cavities requires information on the cavity air change rates, which can be a complex function of the building and cavity geometry and the meteorological conditions. This paper applies Reynolds-averaged Navier–Stokes (RANS) CFD to study wind-induced airflow in the narrow (23 mm) ventilated facade cavities of an isolated low-rise building. Both coupled and decoupled simulations are performed. In the coupled simulations, the atmospheric boundary layer wind-flow pattern around the building and the resulting airflow in the cavities are calculated simultaneously and within the same computational domain. In the decoupled simulations, two separate CFD simulations are conducted: a simulation of the outdoor wind flow around the building (with closed cavities) to determine the surface pressures at the position of the cavity inlet and outlet openings, and a simulation of the cavity airflow, driven by these surface pressures. CFD validation is performed for the external and internal (cavity) flows. It indicates an important modelling limitation: while both laminar and turbulent cavity airflow can be accurately reproduced with low-Reynolds number modelling, this method fails in the transitional regime. The valid CFD results (outside the transitional regime) are analysed in terms of cavity airflow patterns and cavity air change rates per hour (ACH) for different cavity positions, wind speeds and wind directions. The CFD results of cavity air speed and ACH compare favourably with values from previous experimental studies. The coupled and decoupled simulation results are compared to provide an indication of the local losses. It is concluded that future work should focus on adapting RANS CFD low-Reynolds number models to accurately model cavity flow in the transitional regime.     

基于CFD模拟的双层玻璃幕墙通风性能研究

双层玻璃幕墙作为新型的建筑外围护结构,对于建筑内部的采光、通风、隔热起着非常重要的作用。衡量双层玻璃幕墙通风性能最重要的两个指标是内部空气流动速度及热传导,利用CFD软件模拟得出与实际数据最吻合的电脑模型,利用这个标准电脑模型的参数,模拟得出不同几何尺寸的双层玻璃幕墙的这两项指标的图像及数据。进一步定量研究影响双层玻璃通风性能的关键几何因素,通过比较得出双层玻璃幕墙通风口及空腔大小对其通风性能的影响的一般规律,并以此来指导商业生产。     

Airflow and heat transfer in double skin facades

Airflow and heat transfer simulation was conducted for a DSF system equipped with a venetian blind, using computational fluid dynamics (CFD) with RNG turbulence model, for a three-level combination of slat tilt angle and blind position. The CFD prediction was validated using experimental data collected for a mechanically ventilated DSF equipped with venetian blinds. The predicted trends in glass and blind surface temperatures of the CFD model are compared well with the experimental measurements. The present study indicates that the presence of venetian blinds influences the surface heat transfer coefficients (SHTCs), the temperature and the air distribution in the DSF system. For the cases considered, the changes in the position of the blinds (outer, middle, and inner) have more effect on the distribution of temperature, velocity, and SHTCs compared to the changes in the slat angles (θ = 0°, 45°, 90°).     

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.     

Energy performance assessment of double-skin façade with thermal mass

To mitigate the overheating problem in the warmer seasons, and thereby improve thermal performance and energy efficiency of the double-skin façade (DSF) system, this study introduced an innovative design approach involving the integration of passive thermal mass technique with the air channel of the conventional DSF. To assess the contribution of this integration to energy efficiency of the system, a numerical model was developed, capable of determining the thermal performance of the conventional DSF. The numerical model is composed of airflow and thermal models. This paper briefly describes the development of the models as well as the models’ verifications. Models were then used to carry out a series of parametric studies to investigate the effect of thermal mass on the energy performance of the integrated system.The simulation results revealed that mechanically ventilated DSF can save energy based on configuration from 21% to 26% in summer and from 41% to 59% in winter as compared to conventional DSFs without thermal mass. The results also showed the total saving for a naturally ventilated DSF is negligible year-round.     

CFD modelling and thermal performance analysis of a wooden ventilated roof structure

Thermal comfort and energy saving are objectives of key significance that building design must meet. Since a low energy building can be obtained as a result of the good realization of all its components, roofs call for particular attention as they represent a large part of a building’s total surface area. In this paper the benefit of using ventilated roofs for reducing summer cooling load is investigated. The investigation has been conducted comparing a ventilated roof assembly with different channel heights (3 cm, 5 cm, and 10 cm) to the same non ventilated structure, assuming buoyancy-driven airflow. Direct comparison between the open and the closed roof structures as a function of different cavity heights and outside environmental conditions is presented. To provide fundamental information about the thermal performance of these building envelope components, the computational fluid dynamics (CFD) model has been used to develop correlations for the characterization of the airflow and heat transfer phenomena in the ventilation cavity which have been implemented in a whole year energy simulation software. The present analysis shows a conflicting discrepancy among the indexes of performance describing the actual energy saving potential of a ventilated roof.     

CFD modelling of naturally ventilated double-skin facades with Venetian blinds

This study describes computational fluid dynamics (CFD) modelling of naturally ventilated double-skin facades (DSFs) with Venetian blinds inside the facade cavity. The 2D modelling work investigates the coupled convective, conductive and radiative heat transfer through the DSF system. The angles of the Venetian blind can be adjusted and a series of angles (0°, 30°, 45°, 60° and 80°) have been modelled. The modelling results are compared with the measurements from a section of a prototype-facade testing facility and with predictions from a component-based nodal model. Agreement between the three methods is generally good. It is thought that discrepancies in the results are caused by the simplification of the CFD model resulting in less turbulence mixing within the facade cavity. The CFD simulation output suggests that the presence of the Venetian blinds is able to enhance the natural ventilation flow within the facade cavity and significantly reduce the heat gains to the internal environment. It was also found that the convective heat transfer coefficients on the glazing surfaces are insensitive to the blind angles. The work demonstrated the capability of CFD for modelling complicated heat transfer processes through the DSF system and offered some guidance for CFD practitioners who wish to model similar type of flow.     

CFD modelling of an industrial air diffuser—predicting velocity and temperature in the near zone

This article describes experimental and modelling results from CFD simulation of an air diffuser for industrial spaces. The main objective of this paper is to validate a manufacturer model of the diffuser. In the air diffuser, the low velocity part is placed on top of a multi-cone diffuser in order to increase airflow rates and maximize the cooling capacity of a single diffuser unit. This kind of configuration should ensure appropriate performance of industrial air diffusers, which is discussed briefly at the end of the article. The paper illustrates the importance of a simulation model jointly with the manufacturer's product model and the grid layout near the ventilation device to achieve accurate results. Parameters for diffuser modelling were adapted from literature and manufacturer's product data. Correct specification of diffuser geometry and numerical boundary conditions for CFD simulations are critical for prediction. The standard k–ε model was chosen to model turbulence because it represents the best-known model utilized and validated for air diffuser performance. CFD simulations were compared systematically with data from laboratory measurements; air velocity was measured by ultrasonic sensors. Results show that CFD simulation with a standard k–ε model accurately predicts non-isothermal airflow around the diffuser. Additionally, smoke tests revealed that the flow around the diffuser is not completely symmetrical as predicted by CFD. The cause of the observed asymmetry was not identified. This was the main reason why some simulation results deviate from the measured values.     

Convective heat transfer prediction in large rectangular cross-sectional area Earth-to-Air Heat Exchangers

An Earth-to-Air Heat Exchanger (ETAHE) uses the ground's thermal storage capacity to dampen ambient air temperature oscillations by delivering the outdoor air to the indoors through a horizontally buried duct. With their lower airflow resistance, large cross-sectional area ETAHEs have been found to be more energy efficient than the conventional small ones, especially when integrated in hybrid ventilation systems. However, the lack of available methods for determining the heat convection at the duct surfaces has made accurate energy simulation and proper system design overly difficult. In this study, numerical experiments using computational fluid dynamics (CFD) were conducted to investigate the airflow and thermal behavior in the large ducts. A two-layer turbulence model was used to ensure accuracy in resolving the flow information in the near-wall region, which is critical for predicting accurate heat convection. The modeling method was verified by comparing its results with measurements from literature. Parametric studies were conducted to identify the influential design and operation variables for the heat convection. Thirty numerical experimental setups designed with the Latin Hypercube Sampling method were simulated to prepare a database with six design parameters as the simulation inputs and average Nusselt numbers over the duct ceiling, wall, and floor as the outputs. Based on the database an artificial neural network (ANN) model was trained to build a mathematical relation between the design variables and the Nusselt numbers. The developed ANN model showed very accurate prediction when compared with test data.     

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.     

Fast simulation of temperature distribution in air conditioned rooms by using proper orthogonal decomposition

Real-time control of air conditioning needs dynamic models of temperature distribution and air velocity field in rooms. Complete models, based on computational fluid dynamics (CFD), give this information, but they are too time-consuming for real-time applications. Therefore, a reduced model is needed. In mixed convection, when the air temperature has negligible variations, the velocity field may be considered fixed. In this case, the size of a CFD model may be reduced by solving only the energy balance equation, then putting this equation in the form of state-space and finally by reducing its order by proper orthogonal decomposition (POD). This algorithm was applied to a room equipped with a fan coil. Four fixed airflow fields, corresponding to negligible air temperature variation, were considered, resulting in four airflow patterns: the air jet sticks to the ceiling and reaches the opposite wall, the air jet falls before reaching the opposite wall, the air jet reaches the ceiling without extending along it, and the air jet falls before reaching the ceiling. The reduced model obtained from these airflow patterns was validated by comparing with CFD results for step variation of inlet temperature. As the order of the reduced model is always lower than seven, it may be used for real-time control applications.     

Modeling ventilated double skin façade—A zonal approach

This article presents the application of the zonal approach for modeling airflow and temperature in a ventilated double skin facades (DSF). The zonal airflow equation, power-law, was employed to calculate the airflow through the shading device and cavities. The zonal energy equation was used to evaluate the temperature distribution in the DSF system. The predicted temperature distributions were verified using measured values and parametric studies were conducted to identify the influence of height, flow rate and presence of venetian blinds on the inlet–outlet temperature difference. The influence of changing the values of each parameter was found to be more apparent during the day than during the night. The inlet–outlet temperature difference increased as height of the DSF increased and when venetian blinds were installed but it was found to decrease as the airflow rate increased. The results had revealed that the zonal approach can be employed to provide information on the performance of DSF faster and at very low computational resource.     

Site-specific prediction for energy simulation by integrating computational fluid dynamics

Currently, energy simulations (ES) utilize various outdoor variables such as outside surface, ground surface, sky, and air temperature, using coefficients or simplified equations. These variablesuse empirical correlations that are sometimes insufficient in certain location. Variables such as air temperature at the base of the layer are informed from weather data that may not accurately represent the physical microclimate of the site, and may therefore reduce the accuracy of simulation results. This research investigates utilizing computational fluid dynamics (CFD) with Monte Carlo stochastic model to predict site specific temperature parameters for energy simulation. This will allow more realistic and robust energy simulation results for specific site conditions.     

Comparison of different decontaminant delivery methods for sterilizing unoccupied commercial airliner cabins

Effective decontamination is crucial if an airliner cabin is contaminated by biological contaminants, such as infectious disease viruses or intentionally released biological agents. This study used computational fluid dynamics (CFD) method as a tool and vaporized hydrogen peroxide (VHP) as an exemplary decontaminant and Geobacillus stearothermophilus spores as a simulant contaminant to investigate three VHP delivery methods for sterilizing two different airliner cabins. The CFD first determined the airflow and the transient distributions of the contaminant and decontaminant in cabins. Auxiliary equations were implemented into the CFD model for evaluating efficacy of the sterilization process. The improved CFD model was validated by the measured airflow and simulated contaminant distributions obtained from a cabin mockup and the measured efficacy data from the literature. The three decontaminant delivery methods were (1) to supply the mixed VHP and air through the environmental control system of a cabin, (2) to send mixed VHP and air through a front door and to extract them from a back door of a cabin, and (3) to send directly VHP to a cabin and enhance the mixing with air in the cabin by fans. The two air cabins studied were a single-aisle and a twin-aisle airliner one. The results show that the second decontaminant delivery method (displacement method) was the best because the VHP distributions in the cabins were most uniform, the sterilization time was moderate, and the corrosion risk was low. The method displaced the existing air by the air/disinfectant solution, rather than dispersive mixing as the other two methods.     

Development of a fast fluid dynamics-based adjoint method for the inverse design of indoor environments

The computational fluid dynamics (CFD)-based adjoint method may be appropriate for the inverse design of indoor environments, considering both accuracy and efficiency, but a single design still requires tens of hours with the use of a personal computer. To speed up the inverse design process, this study evaluated four fast fluid dynamics (FFD) models in terms of solving the Navier–Stokes equations, integration with turbulence models, and solving the adjoint equations. This study implemented the FFD solvers in OpenFOAM and validated them for predicting steady-state and transient indoor airflow. This study then validated the FFD solvers for solving the adjoint equations and the FFD-based adjoint method for inverse identification problems and inverse designs in indoor environments. The results showed that FFD was 20 times faster than CFD in predicting transient indoor airflow, and similar computational accuracy could be maintained; the FFD-based adjoint method was 4–16 times faster than the CFD-based adjoint method in the inverse design process.     

Development of new and validation of existing convection correlations for rooms with displacement ventilation systems

Building airflow, thermal, and contaminant simulation programs need accurate models for the surface convective boundary conditions. This is, especially, the case for displacement ventilation (DV) systems, where convective buoyancy forces at room surfaces significantly affect the airflow pattern and temperature and contaminant distributions. Nevertheless, for DV, as a relatively new ventilation system, the convective correlations are adopted from more traditional mixing ventilation correlations, or non-existent. In this study, the existing recommended correlations are validated in a full-scale experimental facility representing an office space. In addition, new correlations are developed for floor surfaces because the current literature does not provide necessary correlations, even though, the floor surface is responsible for >50% of the total convective heat transfer at the envelope. The convective correlations are typically functions of a surface-air temperature difference, airflow parameters, and characteristic room dimensions. Validation results show that the floor convection correlations expressed as a function of volume flow rate are much stronger than the correlations expressed as a function of a temperature difference between the surface and local air. Consequently, the new convection correlation for floor surfaces is a function of the number of hourly room air changes (ACH). This correlation also takes into account buoyant effects from local floor heat patches. Experimental data show that the existing correlation can be successfully applied to vertical and ceiling surfaces in spaces with DV diffuser(s). Overall, the new and the existing convection correlations are tabulated for use in building simulation programs, such as annual energy analyses or computational fluid dynamics.     

Assessing the effects of sliding doors on an operating theatre climate

Surgical site infections (SSIs) are the second to third most common site of health care associated infections (HAIs). It is very important to maintain good indoor air quality (IAQ) and the best ventilation system in the operating theatre (OT) to ensure health and safety for the patient and surgical team, also to reduce the risk of post-operative wound infection. Then a significant consideration in OTs is the control of aerosols, anesthesia gases and smoke. The present paper uses the airflow modeling based on computational fluid dynamics (CFD) to study the indoor climate of a standard ISO5 class OT with an ultraclean air filter system and a total ceiling unidirectional diffuser. The OT has a heating, ventilation, and air conditioning (HVAC) plant with a laminar airflow (LAF) or ultraclean ventilation. A simple method to analyze the effects of different sliding door conditions combined with crossing persons and persons with a stretcher crossing, on the OT climate, airflow patterns and the indoor pressures scheme, is provided. The proposed simulation method, that belongs to the indirect approach for a numerical simulation of solid object movements in a fluid, provides important knowledge on complex flow phenomena combined with multi-physical conditions. Results obtained by transient simulation show disruptions of the airflow inside the OT and different airflow displacement and distribution caused by surgery staff movements and sliding door opening and closing, but in particular static pressure changes in the HVAC plant system with important effects on ventilation system working conditions and its energy performances.     

Modelling and simulation of the near-wall velocity of a turbulent ceiling attached plane jet after its impingement with the corner

At present, ceiling-mounted diffusers are very popular for indoor air distribution, particularly in offices, owing to greater efficiency in the distribution of the air supply and a more comfortable indoor environment. The objective of this study is to construct an effective model to design the indoor airflow of an attached plane jet after its impingement with the corner in a room. In this study, a full-scale test facility was set up to obtain detailed experimental data. One commercial CFD tool, CFX 11.0, was used to simulate the air velocity distribution of an attached plane air jet bounded by the ceiling and an insulated wall. One semi-empirical model was also constructed to predict the impingement jet velocity. The results show that bout the semi-empirical model and CFX 11.0 were able to predict the maximum velocity of an impinging jet at low Reynolds numbers, 1000 and 2000, with an inaccuracy of ±11%. However, the semi-empirical model could be more conveniently used to predict the maximum jet velocity decay after its impingement the corner in a room than CFD simulation in terms of accuracy and the time required to design the indoor airflow pattern.     

Experimental and numerical analysis of heat transfer and airflow on an interactive building facade

The envelope of a building is mainly responsible for its energy demand. Different kinds of double skin facades (DSFs) are nowadays used as a building envelope to reduce the energy demand and improve aesthetical view of buildings. Although DSF are already extensively used, their thermal performance is not well understood. This study presents a decoupling method capable to evaluate thermal performances and analyze fluid phenomena in a DSF. The solar radiation effects were evaluated with an analytical model and computational fluid dynamics (CFD) simulations were used to evaluate complex flow and thermal effect on a commercial DSF. With the decoupling approach to account for the effects of solar radiation and flow, the numerical results obtained by the CFD approach agree well with the experimental data collected on a full scale test room with a ventilated DSF. The method can be used to establish a database to develop a tool for DSF design.