Combination of natural ventilation approaches is a new trend for free space cooling/heating in buildings. A critical review was then undertaken to provide an overview of the combined technologies ...that hope to initialize new ideas and promote future endeavors. The advantages of the integrated natural ventilation systems can be summarized into several principles, including achieving beyond the existing performance by single system, maintaining indoor temperature stability, realizing heat energy recovery, overcoming the inadequacy of a single system, and providing a more comprehensive and useful energy-saving scheme. Most of the existing studies on combined systems are found based on thermal buoyancy, while only a small amount dealt with the combination of wind-driven and buoyancy-induce due to the complexity. Parametric studies in most previous studies focused on several major ones, so a systematic analysis is critically needed to address the performance of the overall combination to achieve stable and durable performance. A thoughtful investigation is also required to avert unpredictable delivery of air flow, such as through the manipulation of external wind forces. The related research focuses should also be shifted following the trend of multi-storey buildings under the rapidly growing population. No guideline was found that arranges these natural ventilation systems in terms of performance and applicability for their practical selections and usages. Also, the thermal bridge breaking in cold winter and condensation in summer may compromise the natural ventilation performance and durability, and longevity of buildings. The studies on the coupling between different natural ventilation systems are still insufficient, requiring quite a bit of effort in future works.
•Combined natural ventilation systems in single building were reviewed.•Advantages of combined natural ventilation systems were summarized.•Existing systems much reply on thermal buoyancy but not combined buoyancy and wind.•Research focuses should be shifted following the trend of multi-storey buildings.•Studies on coupling different natural ventilation systems are still insufficient.
Phase Change Materials have been acknowledged for their potential to be used as passive strategy for improving energy efficiency and occupants’ thermal comfort in buildings. However, their ...performance still needs to be enhanced to have them effectively used. In this view, this study investigates the potential improvement of PCMs performance for passive cooling application by efficient natural ventilation in residential building stock. Therefore, coupled dynamic simulation and optimization analysis is performed to explore the optimum melting temperature of PCM integrated in the external building envelope to minimize cooling loads in different Italian climate zones. Moreover, various natural ventilation control strategies are implemented to assess their influence on the process of PCM charge-discharge cycle. Results show that PCM inclusion in the building envelope provides significant cooling savings, up to about 300 kWh/year in mild climates. Furthermore, both night and temperature controlled natural ventilation are able to enhance the efficiency of PCMs thermal energy storage charge-discharge cycle. However, the optimum performance is obtained by coupling PCMs with natural ventilation controlled by indoor/outdoor temperature difference in all considered climate contexts. Accordingly, considerable building cooling energy need reduction is achievable through the optimum combination of PCMs and natural ventilation control, especially in milder climates.
•PCM passive cooling performance coupled with natural ventilation control is studied.•Summer optimum PCM melting temperature is explored via optimized dynamic simulation.•PCM is included in the building envelope in various Italian climate zones.•PCMs provide significant passive cooling contribution, especially in mild climates.•PCM coupled to T-controlled natural ventilation further improves energy saving.
Natural ventilation is crucial for reducing the energy consumption and carbon emissions of buildings. However, it is often impossible to meet indoor thermal requirements in hot climates. In this ...study, an integrated natural ventilation system with an underground pipe gallery for pre-cooling and a built-in chimney to enhance the thermal pressure was proposed to cool a computer room. A full-scale experimental platform was constructed to confirm the feasibility of the system. The system performance was investigated by comparing two experimental scenarios, thermal pressure-driven natural ventilation and fan-assisted ventilation, during the transition season in a hot outdoor environment. The underground pipe gallery pre-cooled chimney ventilation system effectively provided a suitable indoor environment for the occupied area, with a mean temperature of 29.1 °C and relative humidity of 57.6 %. The proportion of hours in which the hourly average temperature around the server rack meets the Class C level for computer rooms specified in the national standard is 66.7 %. The maximum pre-cooling capacity of the underground pipe gallery was 3.5 kW. When the assisted fan was operated, the environmental stability of the equipment area further improved, and the system had the potential to operate continuously for 24h to provide a suitable indoor environment.This study provides a strategic solution to passive building design by demonstrating the viability of using an underground pipe gallery for pre-cooling in natural ventilation systems.
This paper has analysed the influence of cross-sectional area and aspect ratio of shaft on natural ventilation in urban road tunnel fires by Large Eddy Simulation (LES). Results have shown that the ...performance of smoke exhausting process displays different smoke extraction modes with the increasing of the cross-sectional aspect ratio. When the cross-sectional aspect ratio increases to a certain value, the hot smoke will be adhered to the upstream wall of the shaft and be exhausted through the upstream region. In this case, the cold air from tunnel bottom penetrates the vertical shaft leading to the smoke exhausting process inefficiently. The 2-D line plume in near region could be used to predict the process of hot smoke exhausting by shaft upstream after penetration and the predicted values give a good agreement with the simulated values. It comes to conclusion that the shaft with larger cross-sectional aspect ratio should be divided into several smaller shafts in order to exhaust as much smoke as possible and avoid overmuch entrainment of fresh air.
•The accuracy of natural ventilation analysis relies largely on how the Influence Region is chosen.•Only including the adjacent layer of surrounding buildings is not sufficient.•Three layers of ...surrounding buildings are typically required for modeling low-rise neighborhoods.•Fewer surrounding buildings are required for wide canyons and high-rise landscapes.•Downstream buildings can be moderately excluded in the Influence Region.
Natural ventilation is one of the most important design options for green buildings, which reduces energy use and improves thermal comfort. Computational Fluid Dynamics (CFD) simulations have been used increasingly for natural ventilation design in urban neighborhoods. The accuracy of such simulations relies largely on how the CFD domain is chosen. In the domain, we define the Influence Region as the area where the surrounding buildings must be modeled explicitly to predict the ventilation flow rate accurately. This study presents the early efforts to determine the adequate size of the Influence Region in the CFD domain using a coupled indoor-outdoor CFD simulation, in which the air change rate (ACH) no longer varies noticeably with increasing number of surrounding obstacles. Convergence charts of ACH as a function of an increasing number of surrounding building layers are generated using various urban parameters (e.g., wind condition, aspect ratio, building height relative to surroundings, downstream obstacles, and non-idealized surroundings). Our analysis demonstrated that only including the adjacent layer of surrounding obstacles is not sufficient for predicting correctly the ACH because of the artificial channeling effect between buildings. For both normal and oblique wind directions, three layers of surroundings are required for regular street canyons with an aspect ratio H/W=1. In the case of wide canyons (H/W=1/3), two layers of surroundings are needed because there is less flow interference between upstream and downstream obstacles. For the urban configuration, where the target building is significantly taller than nearby structures, the ACH on higher floors does not vary much with increasing amount of surroundings, which significantly reduces the required number of buildings in the Influence Region. In addition, buildings at the side and downstream of the target building can be moderately excluded in the Influence Region as long as the most adjacent downstream layer of obstacles is modeled. A real urban configuration with non-uniform spacing among buildings is evaluated. We showed that the required size of the Influence Region that is derived from uniform building arrays still generally applies to non-idealized landscapes. This study demonstrates the importance of assessing the sensitivity of the selected Influence Region in CFD simulations to reduce unintended modeling errors and computing expense.
Outdoor ventilation is very important for a healthy and livable urban environment. It is strongly influenced by wind speed and direction, which in turn are affected by urban morphology. This paper ...first provides a detailed review of the literature for CFD studies of outdoor ventilation for generic urban configurations. The review indicates that there is a clear lack of studies for urban configurations where not all parallel streets have equal street widths. Next, the paper presents Computational Fluid Dynamics (CFD) simulations of outdoor ventilation for generic configurations with parallel streets of equal and unequal street widths. The 3D steady RANS equations with the standard k- model and the passive scalar transport equation are used to calculate the effective local mean age of air at pedestrian level as an indicator of pollutant removal efficiency. The study is based on grid-convergence analysis and on validation with previously published wind-tunnel measurements. The influence of a central and wider main street on the wind-velocity pattern and on the effective local mean age of air of the surrounding area is analyzed for different wind directions. For wind directions oblique or perpendicular to the main street, the presence of this main street generally improves the ventilation efficiency because the main street acts as a sink of clean air. However, this is generally not the case for the parallel wind direction, where the higher flow rate through the main street reduces the flow rates through the parallel narrower streets, negatively affecting their ventilation efficiency.
•Natural ventilation is dominant passive design strategy in tropical climate.•Maximum proficiency of natural ventilation depends on heat avoidance techniques.•Ventilation shaft especially active ...stack increases indoor air velocity.•Aperture size and building orientation elevate the efficiency of ventilation.
Energy consumption in the building sector is a major concern, especially in tropical climates where high temperatures and humidity force occupants to use electro-mechanical ventilation. Passive design strategies, in particular the application of natural ventilation, are one of the main techniques to moderate temperatures in buildings. Furthermore, many studies have shown reduced operating costs, better thermal comfort and indoor air quality, to be some of the advantages of the application of natural ventilation in buildings. Although existing studies support the efficiency of natural ventilation, the efficiency and practicability of architectural elements to maximise ventilation in buildings remains problematic. This study reviews studies on natural ventilation with other passive design strategies in tropical climates in order to support the argument for the application of natural ventilation in tropical climates. Through a review of studies on the operation of natural ventilation in buildings, it also identifies the most effective architectural elements and techniques in building façades and ventilation openings. The results indicate that ventilation shafts, window-to-wall ratio and building orientation should be applied in future construction. This study also identifies some further specific elements that are worth further investigation, including the shape of louvered windows, different forms of apertures and vernacular elements.
By necessity hydrogen-powered vehicles will be parked in covered and underground car parks. This has implications for the safety of life and property, and the development of regulations, codes and ...standards governing passenger vehicles and car parks. This study utilises Computational Fluid Dynamics (CFD) to investigate unignited hydrogen release and dispersion from 700 bar onboard storage in a naturally ventilated covered car park. The impact of leak diameter and angle of leak direction on the formation of the flammable cloud and the implications for vehicle passengers, first responders and car park ventilation are discussed. A typical car park with dimensions LxWxH = 30 × 28.6 × 2.6 m with two opposing vents based on the British Standard (BS 7346–7:2013) was considered. Releases through three different Thermally Activated Pressure Relief Devices (TPRD) with diameters of 3.34, 2.00 and 0.50 mm were compared, to understand the gas dispersion, specifically the dynamics of envelope formation for 1%, 2% and 4% vol of hydrogen. Concentrations in the vicinity of the vehicle and of the vents were of particular interest. It was shown how blowdown through a TPRD diameter of 3.34 mm leads to the formation of a flammable cloud throughout the majority of the car park space in less than 20 s. However, such a flammable envelope was not observed to the same extent for a TPRD diameter of 2 mm and the flammable envelope is negligible for a 0.5 mm diameter TPRD. A release through a 2 mm TPRD diameter resulted in concentrations of 1% hydrogen along the length of the car park ceiling within 20 s, which should activate hydrogen sensors, in contrast an upward release through a 0.5 mm diameter led to concentrations of 1% reaching a very limited area of the ceiling. Downward TPRD release angles of 0°, 30° and 45° were considered, and while an angle of 30° and 45° directed the hydrogen away from the car body, a downward release at 0° briefly surrounded the car doors and passenger escape routes with a flammable cloud. The study highlights the importance of release angle and demonstrates that a TPRD diameter of 0.5 mm is safer for the particular scenario considered. Larger diameter TPRDs have previously been shown to be unacceptable from a pressure peaking perspective and this study questions their use safety in a naturally ventilated covered car park.
•Numerical investigation of release from onboard storage in a covered car park.•Release and dispersions from a range of TPRD diameters is simulated.•A 0.5 mm diameter TPRD was found to be inherently safer for 700 bar storage.•The angle of TPRD release was shown to have implications for passenger egress.•Results support the inherently safe design of hydrogen fuel cell vehicles.
Reduced-scale experiments and simulations are important approaches in natural ventilation research, and the similarity requirement is fundamental for generalising the flow characteristics obtained ...from reduced-to full-scale conditions. However, the similarity requirement of a nonisothermal natural ventilation flow in a reduced-scale model poses additional challenges because of the reduced approaching flow, which can potentially result in Reynolds dependence issues. This study investigated the Reynolds number (Re) independence of indoor airflow in natural ventilation under isothermal and nonisothermal conditions using computational fluid dynamics (CFD) with Reynolds-averaged Navier–Stokes. A wind tunnel experiment was first conducted to validate the accuracy of the CFD using a reduced-scale model. Indoor airflow fields characterised by the same Archimedes number (Ar) but with varying approaching wind velocities and temperatures were compared between the full-scale and 1/10 reduced-scale simulations. The dimensionless ventilation rate showed the least dependence on the Re number, while the temperature field was very sensitive to the Re number, especially in the near-wall region. However, the temperature field on the ventilation pathway is much less dependent on the Re number, the deviation of which is less than 10 % compared to the full-scale simulation. The temperature distribution in the reduced-scale simulation exhibits a thermal stratification pattern similar to that in the full-scale simulation.
•The Re-independence of non-isothermal indoor airflow are investigated by CFD.•The indoor airflow between 1:10 reduced-scale model and full-scale were compared.•Low Re number effect should be considered in predicting the ventilation rate.•The temperature near the wall region is most sensitive to the Reynolds number.
•Parameters influencing the prediction accuracy for indoor air temperatures in nearly zero energy buildings are analysed.•A model was calibrated and validated for a nearly zero energy building in ...naturally ventilated mode only.•Many physical parameters and parameters related to occupant behaviour were identified as being influential.•The choice of occupancy schedules had a major impact on the accuracy of results.•Recommendations for predicting air temperatures in nearly zero energy buildings are presented.
As the cooling energy demand in buildings is set to increase dramatically in the future, the exploitation of passive solutions like natural ventilation could prove vital in reducing the reliance on mechanical systems. Models that can predict air temperature accurately in naturally ventilated mode are key to understanding the potential of natural ventilation now and in the future. This article presents a simulation based case study of a retrofitted nearly zero energy test-bed university building, in naturally ventilated mode only. The study had three aims: (1) calibration and validation of a whole building energy model, (2) a comparative analysis of occupancy schedules and opening control strategies, and (3) a comparison of researcher and practitioner approaches. Results showed the detailed building model was capable of predicting room level air temperature with a low level of error (0.27 °C ≤ RMSE ≤ 1.50 °C) that was well within the limits of existing calibration standards (MBE ±10%, CVRMSE <20%). The comparative analysis highlighted the need to consider occupancy schedules that have a wide range of diversity, and opening control strategies that reflect the manual and automated relationship in natural ventilation systems. The approach comparison highlighted that both practitioner and researcher approaches to simulating both occupancy schedules and opening control strategies showed similar levels of performance for the application considered. The paper also provides recommendations for those modelling air temperatures and thermal comfort in nearly zero energy buildings.
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