Airflow Network Model Research Articles

A two-level optimisation approach for underground natural ventilation based on CFD and building energy simulations

Natural ventilation is a historical and promising passive design for conserving building energy. However, a noticeable scarcity exists in design strategies and academic studies on underground natural ventilation. This study introduces a two-level optimisation approach for underground natural ventilation, considering form and environmental characteristics. At the initial level, computational fluid dynamics simulations were employed to quantify and visualise the momentary ventilation performances of various passive strategies. These simulations also furnished precise wind pressure coefficients. Subsequently, at the second level, this study predicted the annual energy savings of the proposed prototype by integrating an airflow network model with building energy simulations. To adapt design parameters, sensitivity analysis and multi-objective optimisation techniques were applied. These numerical models underwent validation against measured data from analogous studies. The results reveal that the ‘inlet buried pipe + outlet shaft’ led to a 6.9 % reduction in energy consumption for heating, ventilation, and air-conditioning in underground hybrid ventilation compared to underground mechanical ventilation. Moreover, it achieved a 28.8 % decrease compared to aboveground mechanical ventilation. Optimising air temperatures following climate, soil conditions, and building characteristics is imperative, aligning with design standards. This study contributes a quantitative and visual approach to support underground natural ventilation and addresses the limitations of empirical designs, providing a foundation for future research and practical applications.

On the effect of pressure coefficient source on the energy demand of an isolated cross-ventilated building

Natural ventilation is a simple and effective measure to both reduce the cooling demand of buildings and improve the indoor air quality. In the prediction of heating and cooling demands by means of building energy simulations (BES), the use of pressure coefficients (Cp) from databases as input for the airflow network model is the common approach. Cp values for the same building typology may differ according to the adopted database and are generally unavailable for buildings with complex geometry. Employed Cp values may lead to differences in BES results. This manuscript presents a comparison, for different wind directions, between the Cp distributions and mean values on the facades of a detached building obtained with full-scale CFD – Reynolds-averaged Navier-Stokes (RANS) and large eddy simulation (LES) – simulations, from a database and from wind-tunnel experiments. The obtained pressure coefficients are used in the BES of a naturally ventilated building and the energy demand difference between the four approaches is quantified. Four climate zones (tropical, dry/desertic, temperate, continental) are considered. Although, in terms of accuracy of Cp prediction, LES outperforms RANS for all the wind directions considered, annual cooling energy demand is found to be relatively insensitive to the source of Cp for the current case study, while predicted peak cooling values differ up to 10.8%. On the other hand, the prediction of annual heating energy demand in cold climates varies up to 3% depending on the Cp source employed for BES simulations.

Optimum window-opening control for naturally ventilated buildings with phase change materials in the hot and humid climate of Indonesia

This study aims to determine the optimum window-opening control that maximizes annual thermal comfort using ventilative cooling, i.e., night and comfort ventilation, in naturally ventilated buildings where phase change materials (PCMs) are used. A thermal energy simulation (TES) coupled with an airflow network model was constructed to evaluate the comfort ventilation and thermal storage effects of the PCMs by controlling the window-opening patterns. The EnergyPlus-based coupled TES model was validated by comparing with the field measurement results of a full-scale experimental building in Indonesia. The root-mean-square errors of the temperature and air speeds at the center of the target building ranged from 0.2 to 0.5 °C and 0.1–0.2 m/s, respectively. The accuracy of the coupled TES was similar to that of computational fluid dynamics coupled with the heat balance analysis, which can simulate the spatial distributions of temperature and air speeds. The annual simulation results showed that prioritizing the thermal storage effect of the PCMs in the morning and comfort ventilation in the afternoon increases the thermal comfort period. For the optimum window-opening control to maximize thermal comfort in the hot and humid climate, an indoor temperature of 27 °C can be considered a criterion to open the windows for comfort ventilation in addition to night ventilation for the thermal storage effect. When this criterion was applied, the thermal comfort period in the room with PCMs increased to 83.3 %. Accordingly, the reduction in electricity consumption for space cooling was 41 % less than that in a control room without PCMs.

Assessment of year-round wind-driven ventilation by an integrated ventilation model

This study integrates a physical model for cross ventilation and empirical formulae for single-sided ventilation to predict wind-driven ventilation of buildings. The prediction model uses reduction factors to account for the different effects of wind direction, window type, nearby buildings, and building length. The predicted ventilation rates are validated by the results of wind tunnel experiments, a CFD model, and the airflow network model in the EnergyPlus model. The integrated model is then utilized to realistically assess the natural ventilation of a residential building under varying wind speeds and directions throughout a year. The simulation results demonstrate that double-sided cross-ventilation can achieve a ventilation rate roughly 20 times larger than single-sided ventilation under the same wind speed and opening area. In addition, the EnergyPlus model underestimates the annual average ventilation rate by neglecting the shear-induced ventilation when the prevailing wind direction is parallel to the building openings. The change of prevailing wind direction in different seasons can have a substantial effect on the ventilation rate, especially when the external openings of the building are limited to certain sides. Thus, the assessment of natural ventilation of a building must take into account the building orientation and seasonal variation of natural wind conditions.

Urban microclimate and climate change impact on the thermal performance and ventilation of multi-family residential buildings

Urban settings and climate change both impact on energy use and thermal comfort inside buildings. This paper first presents a study of changes in energy demand in residential buildings considering the overlapping effect of climate change and urban heat island intensity in two European locations; Cadiz (Spain) and London (United Kingdom), representing temperate and hot European climates and moderate and dense urban settings. Future-urban weather files were generated and simulations were run considering energy demand and indoor thermal comfort. In hot climate regions such as the one of Cadiz, future climate will increase the cooling demand and the additional impact of the UHI leads to a further increase of up to +28% of total energy demand compared to the current climate without considering urban effects. Future-urban weather conditions will be detrimental also for buildings in London, where the annual energy demand is predicted to increase by up to the 16% if future climate and urban effects are included. This is due to a higher increase in cooling demand compared to the reduction for the heating need. The paper also presents a method to take into account microclimatic conditions in naturally ventilated buildings, especially the effect of wind variations around the building which impacts natural ventilation rates. Air and surface temperature and wind speeds were studied using ENVImet and the resulting microclimatic conditions were used as inputs to the EnergyPlus Airflow Network model for the calculation of the building ventilation rates. It was found that ventilation rates are reduced (in comparison to meteorological weather files) and this reduction impacts negatively on internal operative temperatures. A thermal comfort analysis was carried out indicating that the selection of a suitable weather file and microclimatic conditions is essential for more accurate predictions of internal thermal comfort and will assist in the sizing of passive and active systems to avoid overheating.

A mathematical model for assessing transient airborne infection risks in a multi-zone hospital ward

Quantification of the parameters influencing airborne infection risks associated with indoor activities in different settings can help in understanding outbreak dynamics and the effective implementation of mitigation strategies. This is particularly important in hospitals, where the consequence of infections amongst vulnerable individuals can be significant. Despite the transient occupancy and inter-connected nature of hospital wards, the majority of airborne risk models assume steady-state conditions and well-mixed air in a single zone. We propose a multi-zone model with both a fixed and time-varying infectious person. We use an adapted version of the Wells–Riley model to estimate transient airborne virus concentration, and account for time-varying behaviour in the indoor setting. Through a coupling with a CONTAM airflow network model, we incorporate the effects of ventilation patterns on inter-zonal flow rates to represent likely airflow in a realistic hospital ward. We use this approach to explore the difference in predictions from transient versus steady-state models across several scenarios. Results suggest that a steady-state approach could lead to an overestimation of infection risk or underestimation of quanta emission, especially when the infectious person is only present for a short period of time. The difference between models is most apparent in poorly ventilated spaces, illustrating how risks can build over time when infectious occupant movement is more frequent than the removal rate due to ventilation. The model highlights the importance of considering transient factors when assessing infection risks to ensure that the most effective mitigation strategies are implemented to address long and short timescale risks.

A multi-factor optimization method based on thermal comfort for building energy performance with natural ventilation

Natural ventilation is one of the most potential passive measures to improve the building energy performance. This study introduces a multi-factor optimization method for energy performance of buildings with natural ventilation. To maximize the utilization of natural ventilation, this method uses natural-ventilation strategy (Strategy NV) as the air conditioning system operation strategy. In Strategy NV, the air conditioning system is only on when the natural ventilation cannot fulfill the occupants’ thermal needs. Two thermal comfort models and airflow network model are combined to obtain Strategy NV. Support vector regression is applied to train a surrogate model which is used to predict the annual energy consumption in Strategy NV. Introducing the surrogate model as objective function, particle swarm optimization is used to find the optimal solution. A case study in hot summer and cold winter climate zone is given to verify the feasibility and correctness of this method. The optimal solution has 656 GJ annual energy consumption in Strategy NV, including 254 GJ annual thermal energy consumption. The optimal solution saves annual thermal energy consumption by 21% in Strategy NV comparing with Strategy FT (full time operation strategy within occupied period). The annual thermal energy consumption of the optimal solution in Strategy NV is at least 26% smaller than that of four commonly used building designs in Strategy NV; and it is at least 43% smaller than that of the four commonly used building designs in Strategy FT. The optimal solution has the least total air conditioning hours comparing with the four commonly used building designs.

Simulation case study of stack pressure impact on thermal load in high-rise buildings

The reference building models in the EnergyPlus tool do not consider the stack effect. This is not an issue for small and low-rise buildings. However, this is significant in evaluating the heating and cooling load of high-rise buildings. This study evaluates and quantifies this stack effect in building energy simulation, EnergyPlus. The high-rise commercial building model in the EnergyPlus reference building set was used as a simulation test bed. The vertical multi-stories shaft zones were added inside the core zone in the existing building model. Then, the airflow network model was set in EnergyPlus so the internal airflow including the vertical and horizontal directions through the shaft zone as well as the core and perimeter zones were configured. The simulation model was calibrated against the actual pressure distribution between the core and perimeter zones in all floors of high-rise buildings from the literature. This model calibration was conducted manually by changing the discharge coefficient of the openings between thermal zones. The heating and cooling load of the proposed model were compared to the existing model which does not consider the vertical airflow through the shaft zones. The quantification with relative load mismatch was analyzed by floors and environment including the temperature difference between indoor and outdoor, and wind speed. Moreover, the impact of the air mixing and infiltration on the heating and cooling load was quantified. The result of this study reveals the significance of the stack effect in calculating the heating and cooling load.

Air infiltration rates in residential units of a public housing estate in Hong Kong

The air infiltration rate in a high-rise residential building can influence indoor air quality and building energy consumption. This study was carried out to estimate the air infiltration rates of all the residential units in Cheung Sha Wan Estate, a public housing estate in Hong Kong. Computational fluid dynamic (CFD) simulations were conducted for the neighborhood to obtain the wind pressures on the building envelopes through the use of annual weather data from the Hong Kong Observatory. Various cases with different wind speeds and directions were conducted to cover the majority of the possible scenarios. The resulting wind pressures were used as input data in the multizone airflow network model to calculate the infiltration rates of all the residential units. On-site measurements of infiltration rate were carried out in four residential units in Cheung Sha Wan Estate for estimation of the effective leakage areas for the multizone airflow calculations. Based on the calculated results and the probability of occurrence for each scenario, statistical measures of the estimated air infiltration rates were determined. The results show that the mean annual-average infiltration rate among the 596 residential units of Cheung Tai House was 0.26 h−1 with a standard deviation of 0.11 h−1. Furthermore, the infiltration rate was positively associated with the wind speed, negatively associated with the volume of the residential unit, and insensitive to the floor level. When the arrangement of the surrounding buildings provided a clear path for the wind, the infiltration rate was higher.

Thermal analysis of the cooling system with the circulation between rotor holes of enclosed PMSMs based on modified models

• Novel cooling structure of circulation between rotor holes is designed. • The deflector pressure is calculated based on the element-momentum theory. • The air friction pressure drop is modified by introducing a spiral correction coefficient. • The convective heat transfer coefficient of each heat transfer surface is refined. • Adopting the designed cooling structure, the temperature rise is greatly reduced. Accurate temperature predictions and cooling structure design for permanent magnet machines are essential to ensure safe and stable operation. In this paper, a cooling system with circulation between rotor holes for enclosed permanent magnet synchronous machines is proposed. The modified lumped parameter models of airflow network and thermal network are established, respectively, so as to predict the air velocity and temperature distribution quickly and accurately. In the airflow network model, the calculation method of deflector pressure is proposed based on blade element theory. The equation for the pressure drops in the rotor holes is modified, and the influence of rotor rotation on air velocity is better considered. To improve the accuracy of the thermal network model, each part of the motor is equivalent to three typical heat transfer surfaces, and the corresponding calculation methods of the convective heat transfer coefficient are listed. The parameters of the cooling structure are analyzed and designed based on the lumped parameter models. Compared with the original motor without the deflector, the maximum temperature-rise of the rotor is decreased by 16%. The accuracy of the calculation models is verified by experiment and simulation.

Overheating assessment in Passivhaus dwellings: the influence of prediction tools

Thermal comfort during the summer in the Passivhaus concept relies mainly on natural ventilation to provide indoor cooling. Do airflow modeling tools accurately predict overheating in summer and for anticipated warmer climates? What effect do simplifications of airflow modeling techniques have on the overheating assessment of Passivhaus dwellings? Measured data and a calibrated thermal model are employed in the present study to address this question. The calibrated model is then used to create a standalone building energy model (BEM), a BEM coupled with an airflow network model (AFN), and a BEM coupled with an AFN supported by the wind pressure coefficients obtained from computational fluid dynamics (CFD) simulation. The outcome of each modeling approach was then compared against each other within three different European climates. Results showed that the overheating frequency prediction found with the default design infiltration and natural ventilation inputs commonly used in the literature agreed fairly with those obtained from the AFN + CFD in temperate or colder climates (&lt; 2% difference), but were significantly underestimating overheating in Passivhaus buildings located at warmer climates (9.4% difference). For Passivhaus dwellings in warmer climates, the airflow modeling approach is unlikely to provide for an accurate estimation of the overheating incidence. <em><strong>Practice relevance</strong></em> With overheating becoming a major issue in Europe, the prediction of the indoor thermal performance by using dynamic building simulations became common practice by researchers and practitioners. This study scrutinized the three most common methods of modeling airflow phenomena (BEM, AFN and CFD) when performing building energy simulations. For Passivhaus buildings located in warmer climates, the choice of airflow modeling approach is important to obtain an accurate prediction of overheating. The study found that BEM is unlikely to be suitable for predicting the extent and frequency of overheating in home and warm climates. The AFN + CFD approach provides a more reliable approach for predicting overheating in hot and warm summer climates. For Passivhaus homes in locations with cooler summers, the difference between modeling approaches became mostly insignificant, so the simpler BEM models may be sufficient when performing an overheating analysis in that context.

The Performance Evaluation of Ventilated Windows in the Simultaneous Improvement of Energy Efficiency and Indoor Air Quality in Office Buildings: A Case Study

Energy efficiency and indoor air quality (IAQ) are two crucial required features in a building. Simultaneous improvement of energy efficiency and IAQ in a building can pave the way for obtaining a green building certification. This paper examined the performance of the airflow windows’ supply and exhaust operating modes in energy-saving and providing IAQ criteria. The analytical zonal model coupled with the airflow network model was used to evaluate the system’s thermal performance and the induced airflow. The simulation was done for an office building located in Shiraz, Iran. The results showed that the energy performance of ventilated windows is positive in nine months of the year. Compared to a conventional double-glazed window, the maximum energy savings is about 10%, which occurs in August. It is predicted that using ventilated windows in office buildings in Shiraz can improve the window’s thermal performance by an average of about 5%. The results also showed that ventilated windows could provide the fresh air needed for the building in 250 days of the year to achieve the desired IAQ index (based on ASHRAE 62.1 standard). Furthermore, the effects of glass aspect ratio, airflow channel thickness, and the size of inlet/outlet openings on energy efficiency and IAQ of the suggested window were studied. Results indicated that in the climatic conditions of Shiraz, the exhaust operating mode is much more efficient than the supply mode.

The role of complex airflow simulation tools for overheatingassessment of passive houses

Passive Houses are characterized mainly by construction concepts that greatly reduce energy usage during the winter, but that can lead to significant overheating during the hotter summer days. Since in the Passive House concept thermal comfort during the summer mainly relies on natural ventilation to provide indoor cooling, the importance of airflow modeling tools for overheating prediction needs to be investigated. This research analyzes the effect of simplifications commonly made in airflow modeling techniques on the overheating assessment of Passive Houses by collecting measured data and calibrating a thermal model with a Passive House case study. Utilizing the calibrated model, a standalone Building Energy Model (BEM), BEM coupled with an Airflow Network Model (AFN), and BEM coupled with an AFN supported by the wind pressure coefficient values obtained from Computational Fluid Dynamics (CFD) simulation were created. The outcome of each modeling approach was then compared against each other. Results showed that the default infiltration and natural ventilation input values commonly utilized in literature, when compared to those obtained from either the AFN or AFN+CFD, are significantly overestimating the natural ventilation potential of Passive House buildings, resulting in a lower number of overheating hours (39.9% decrease) and inaccurate overheating evaluation outcomes. Therefore, the paper concludes that the use of at least an AFN is necessary when estimating the overheating hours of Passive Houses.

Validation of thermal simulations of a non-air-conditioned office building in different seasonal, occupancy and ventilation conditions

In this work, a methodology for the validation of non-air-conditioned building thermal simulations is proposed. Having certainty in these simulations can give confidence to building designers on the possibility to avoid the use of mechanical air-conditioned systems or to reduce the period of their use, thus increasing the building's energy efficiency. The main features of the proposed methodology that differentiate it from the previous ones are: i) the separation of data inputs which values are known with uncertainty into those that have more influence on indoor air temperature and those with more impact on surface temperatures; ii) to carry out the calibration process in two stages having as comparison variables indoor air temperature in the first stage and adding surface temperatures in the second stage; and iii) to carry out the validation process in different seasonal, occupancy and ventilation conditions. The case study is an office building, simulations are performed in EnergyPlus employing the Airflow Network model for infiltration and ventilation. Quantitative comparisons are made using eight metrics. The results show the advantages of carrying out the second stage of validation proposed in this work. The validation results show that the building model obtained from the calibration process is suitable to simulate the building in different seasonal, occupancy and ventilation conditions, and can be used with certainty to test strategies to improve thermal comfort in the building. For the case study, two strategies are tested showing important reductions in thermal discomfort on occupancy hours during the critical hot season.

Simulations of Airflow in the Roof Space of a Gothic Sanctuary Using CFD Models

For a deep understanding of the airflow in an environment of historic wooden trusses, it is necessary to analyze the object using simulation methods. To calculate the amount of air passing through the structural openings (components) using dynamic simulation, multi-zone network models based on the simplicity of modeling the individual zones are suitable. For a more detailed analysis of airflow and temperature distribution within one space, a computational fluid dynamics (CFD) simulation model was performed. The air volume through openings and surface temperatures was adopted from the multi-zone airflow network model. By using this simulation technique during a sunny summer day four characteristic states of air movement were simulated in the attic: more intense flow at noon and at midnight caused by a large temperature difference between air and surrounding surfaces and, subsequently, less intense flow when the air was mixed up effectively. The temperature distribution in the cross-sections did not only indicate an increase in temperature with increasing height (up to 50 °C at noon) but also a temperature increase near the southern roof. The surface temperature of the masonry walls was stable (19–33 °C), while the air temperature fluctuated. The image of the flow was completed by ventilation through the tower, which acted as a solar chimney. The airflow through the door to the tower was almost 0.5 m3 s−1 at summer midnight.

Scrutinizing modeling and analysis methods for evaluating overheating risks in passive houses

In response to the increasing demand for energy efficient houses, passive houses are quickly becoming a required standard for new buildings. Despite their energy efficiency, the occupants have reported thermal discomfort during the summer. This study investigates the effects of choices made in the process of evaluating overheating risks in passive houses. Key questions raised in this study relate to the effects of different thermal comfort criteria, the relevance of the building’s energy model to represent airflow phenomena, and the role of uncertainty associated with simulation models utilized for the analysis. The overheating assessment results were observed to be highly dependent on thermal comfort criterion choices, and the energy model was found to undervalue the overheating hours by 33.33% due to design infiltration and ventilation rate overestimation. This result highlighted the necessity of properly modeling airflow phenomena by coupling the thermal and airflow network models for overheating analysis. The sensitivity analysis results indicated that although most of the dominant uncertain parameters are included in the thermal model, when analyzing the output variance caused by the uncertainty related to each modeling method, both the thermal (82.5%) and airflow network (17.5%) models showed a significant variance in the number of overheating hours.

On-site measurement and numerical investigation of the airflow characteristics in an aquatics center

• Air infiltration and indoor thermal environment of an aquatics center is measured. • A multi-zone airflow network model is established using CONTAM. • Effects of outdoor parameters, air tightness on air infiltration are analyzed. • Retrofit suggestion are provided based on measurement and simulation results. • Air infiltration after retrofit is reduced to 4400–9500 m³/h, about 96% lower.

寒冷気候に適した超高断熱・高気密住宅の熱・湿気性能に関する数値シミュレーション （その3）換気経路を考慮した多数室計算による室内環境の検討

寒冷気候に適した超高断熱・高気密住宅の熱・湿気性能に関する数値シミュレーション （その3）換気経路を考慮した多数室計算による室内環境の検討

Natural ventilation potential from weather analyses and building simulation

Naturally ventilated buildings' performance relies on their design and climatic conditions. Thus, when investigating the natural ventilation potential (NVP), the meteorological data plays a crucial role. This study discusses NVP assessment under two approaches: a general evaluation, using only meteorological data; and a specific investigation through building simulation. Both analyses included in-situ measured data and open-access weather file. When compared, the climatic data presented some differences in the frequency of wind direction/speed values. Still, they showed similarities in the hourly data, which were perceived in the NVP general analysis. A multi-zone Airflow Network model was employed for specific NVP examinations. Following a structured calibration process, using measured data from an experimental campaign, the validated model presented a final total Goodness-of-fit (GOF) of 2.98%. Subsequently, an annual building simulation was carried out with a hybrid operation mode. The number of occupied hours with natural ventilation and heating loads presented the most significant difference to the detriment of the weather file used, depending on the zone. A greater discrepancy in thermal comfort evaluation was perceived according to the adaptive standard employed. The paper discusses some challenges faced at the NVP analyses and the calibration process, ending with some suggestions on how to address these issues.

Modelling building infiltration using the airflow network model approach calibrated by air-tightness test results and leak detection

This paper presents a computational approach to air infiltration modelling and simulation validated by the blower door test results. In order to evaluate the potential of the airflow network method, three simulation models of the infiltration test were developed and calibrated by field measurements of leaked air change rate per hour at 50 Pa. Models were developed for existing building designs and constructed in low-energy standards differing in construction type and tightness. All leaks were precisely measured during field tests, defined as openings or cracks, numerically described and included in the model. The simulation results of calibrated models for other pressure differences revealed that the models’ accuracy is satisfactory. The differences between field tests and simulation results do not exceed 2.5%. Additionally, the calibrated models were used to estimate the infiltration heat losses of buildings in three different locations under continental climatic conditions. The results were compared with the steady-state method calculations made for the same building models and climatic conditions. It was proved that the steady-state method gives higher results of heat demand to cover infiltration losses than the simulation method. The final results depend on building location and vary between four and nine times. Practical application: The computational modelling and building performance simulations are increasingly commonly used in engineering design. The proposed method of air-tightness modelling and calibration can be used at any phase of a building’s lifecycle, from design and construction to exploitation and maintenance. Using the proposed techniques, it is possible to estimate more realistic processes of air infiltration and its effect on a building’s energy consumption in comparison with the steady-state method. Moreover, the analysis includes the dynamic effect of boundary conditions (external air temperature, wind speed and direction), as well as the effects of the building site and the surroundings.