Buoyancy-driven Natural Ventilation Research Articles

Buoyancy-driven natural ventilation performance of a multi-story building with vertical shaft designed by using dimensionless design approach

Components with a tall vertical space attached to multi-story buildings are used to assist natural ventilation, which however unavoidably results in different buoyant forces on different stories. To achieve equal flow rates on all stories, a dimensionless design approach was developed in literature. This study aims to examine the reliability of the dimensionless approach for designing the buoyancy-driven ventilation of multi-story buildings. Moreover, it validates the application of the approach to scenarios where the heat source strengths on stories vary. The experimental and numerical results confirm the reliability of the dimensionless approach for designing naturally ventilated buildings with uniform airflow rates and air temperatures on each story. In the case of a story with flexible heat flux, changing the heat flux affects the ventilation rates on all the stories. In particular, when the heat flux on this story is zero, buoyant ventilation continues and may even be observed on the third story. Furthermore, an increase in the heat flux on the first or second story enhances stack ventilation, whereas increasing the heat flux on the third story has a damping effect on ventilation in the other stories. Additionally, stories with identical heat fluxes had similar ventilation rates and indoor temperatures.

Experimental investigation of buoyancy-driven natural ventilation in a building with an atrium using particle image velocimetry (PIV) method

ABSTRACT Natural ventilation is one of the main passive ways to reduce energy needs by delivering fresh air into the building without the help of mechanical systems. In this perspective, using the water bath model and particle image velocimetry, an experimental study of the relationship between fluid velocity caused by buoyancy-driven natural ventilation in a building with single-sided and cross ventilation has been conducted. A case study building with a jointed atrium has been considered for the six initial conditions tests. In the test model which has two low-level inlet openings, fluid velocity in single-sided ventilation mode was meaningfully higher than in cross-ventilation mode, ranging from 18% to 32%. In contrast, by changing the position of the windows to higher levels, cross ventilation creates a flow with 28% higher velocity. Analyzing velocity data demonstrates that, in all tests, the air change rate caused by buoyancy force in cross ventilation mode is more than in single-sided one. Furthermore, in the tests with two high-level openings acting as outlet openings, by opening both sides’ windows in the building, the air change rate increases significantly by 71.75%.

Buoyancy-driven natural ventilation: The role of thermal stratification and its impact on model accuracy

Since the invention of mechanical ventilation systems, natural ventilation has been deemed inferior compared to active systems for ventilation of buildings. The recent COVID-19 pandemic and raising awareness of climate change issues have rekindled the interests in natural ventilation as a sustainable method for ventilation and pollutant removal. Modelling natural ventilation is challenging due to uncontrollable outdoor conditions. Simple models such as the well-mixed air model assume uniform indoor air temperature. However, thermal stratification can induce significant temperature differences in the vertical direction, thereby violating the well-mixed assumption. This study evaluates the performance of the well-mixed model, the two-layer stratification model, and computational fluid dynamics (CFD) models in predicting the indoor air temperature under buoyancy-driven displacement ventilation. Compared to experimental measurements, the well-mixed model significantly overpredicts the indoor air temperature without thermal stratification since it assumes a uniform indoor air temperature. The two-layer stratification model overpredicts the upper layer air temperature and underpredicts the lower layer air temperature. The CFD models can capture the trend of the thermal stratification of a gradual increase in temperature with height. However, the CFD models underpredict the indoor air temperature, possibly due to errors introduced by the assumption of adiabatic indoor surfaces. Since simplified models cannot resolve thermal stratification, high-fidelity models, such as CFD models, should be used to model natural ventilation. Experimental studies of natural ventilation should include measurements of the thermal stratification, as well as the temperatures or heat fluxes on the indoor surfaces so the results can be used to develop and evaluate numerical models.

Impacts of storey number of buildings on solar chimney performance: A theoretical and numerical approach

Under the fact that solar chimney was less investigated in multi-storey buildings, a theoretical model was then developed for solar chimney-induced buoyancy-driven natural ventilation. This is the first study addressing the influences of the storey number on ventilation rates for multi-storey solar chimney (SC) buildings. A storey correction coefficient was proposed to predict the SC-induced ventilation at various floors with identical air inlet areas. The theoretical model was established to elucidate the relationship among ventilation flow rates, solar radiation intensity, vent sizes and storey number (f), where the numerical results have also been validated. Although the total SC ventilation performance is enhanced, its enhancement with a higher chimney cavity was less effective when compared to those solar chimneys in single-storey buildings. This is due to the higher chimney cavity hindering the ventilation performance of the lower floors. The volume flow rate decreased exponentially for the top floors of each building when the two-storey building increased to a seven-storey building. For buildings with more than three storeys, the overall volume flow rate was more sensitive to the cavity gap than the solar radiation intensity, with an improvement in ventilation by 45.6% compared to 26.0% under the same conditions, respectively. To maximize the total flow rate, the optimal cavity gap should increase gradually from 0.2 m to 1.5 m for single-to seven-storey buildings. The findings of this study contribute to a further application of solar chimneys in multi-storey buildings.

Full-scale validation of CFD simulations of buoyancy-driven ventilation in a three-story office building

Computational fluid dynamics (CFD) is frequently used to support the design of naturally ventilated buildings; however, the model accuracy should be thoroughly assessed, ideally through validation with full-scale measurements. The present study aims to (1) validate transient CFD simulations with uncertainty quantification (UQ) for buoyancy-driven natural ventilation against full-scale experiments in an operational atrium building, and (2) quantify the sensitivity of the CFD results to the thermal boundary conditions. The UQ and sensitivity analysis consider uncertainties in the initial and boundary conditions for the temperatures. Considering the volume-averaged air temperature on each floor, the predictions and measurements agree well with discrepancies less than 0.3 °C. When considering the temperature averaged over smaller zones on each floor, two trends can be observed. First, in zones not adjacent to windows, the discrepancies between the CFD and measurements can be explained by uncertainty in the boundary conditions and the measurements. Second, in zones adjacent to windows, higher discrepancies are observed due to oscillations in the inflow jets just downstream of the windows, and due to geometrical simplifications in the CFD model. The sensitivity analysis demonstrates that the boundary conditions for the thermal mass surface temperature and the outdoor temperature have a dominant effect on the indoor air temperature predictions, with their relative importance varying as a function of proximity to the windows.

Improving thermal model predictions for naturally ventilated buildings using large eddy simulations

Natural ventilation involves complex flow and heat transfer processes that are challenging to model accurately and efficiently. Computational fluid dynamics (CFD) simulations can accurately represent the flow and heat transfer physics but are computationally expensive; while building thermal models are computationally efficient but they use reduced-order models to represent complex flow phenomena. In this study, we aim to use CFD to develop building-specific, accurate correlations for the flow and heat transfer rates in buoyancy-driven natural ventilation flow, such that these correlations can be used to improve the accuracy of a dynamic thermal model. First, a CFD model of an operational building is validated with full-scale experiments. Next, the validated CFD model is used to run nine large eddy simulations, from which correlations for the convective heat transfer and the natural ventilation flow rate and heat transfer are derived. The results suggest that standard correlations for these processes can differ significantly from the building-specific values. The CFD-based correlations are then used in a dynamic thermal model to improve its accuracy. The resulting CFD-informed dynamic thermal model accurately predicts indoor air temperature, surface temperature and heat transfer, while the model employing the standard correlations (without inputs from CFD) only predicts the indoor air temperature accurately, not the surface temperature and heat transfer. The fast yet accurate CFD-informed dynamic thermal model can be used to run many simulations in a short time, supporting uncertainty quantification to quantify the effect of variability in the operating conditions and real-time prediction of the natural ventilation process.

A field study on effects of openings on thermal performance of natural cooling efficiency for atrium buildings

Introduction. In this paper, we investigate the temperature stratification of buoyancy-driven natural ventilation of the atrium of building at ULK-MGSU through field experiments. The process of ventilation with different openings ratios in the translucent roofing and ground floor entrance doors are analyzed to reveal the physical insights. With this aim, the main focus of the study is to consider the temperature fields during cooling the atrium premises that increase the thermal performance of the administrative building at ULK in the summer. An expensive ventilation solution by the optimum design of the inlet-to-outlet opening area ratio in the translucent roofing covering is utilized to improve thermal comfort without reducing the level of illumination. Materials and methods. In this study, field measurements were applied to investigate and compares temperature stratification by floors of naturally ventilated ULK atrium building with different outlet sizes and locations under hot period conditions. The results of field measurement was utilized to develop the baseline model for the computational fluid dynamics (CFD) simulation in future work. Results. These results reveal that the sizes and locations of openings in the atrium building affect on modification of the indoor thermal condition. Moreover, energy efficiency is improved thanks to buoyancy-driven changes in air flow rate in an atrium with multiple openings. Conclusions. This study shows that it can be possible reduce indoor air temperatures by 5 °C during the summer period. In addition to the large inlet openings at different atrium levels, a high ratio of the outlet opening area (>10 %) is recommended. The existing atrium of the building was opened 5 % of the total top-glass roof area, which helps to improve the performance of buoyancy-driven ventilation in order to achieve better atrium cooling performance and prevent the detrimental reverse air movement.

Buoyancy-driven natural ventilation characteristics of thermal corridors in industrial buildings

The continuous setup of heating plants such as kilns and boilers may form thermal corridors in industrial buildings resulting in high air temperature, which could be harmful to workers. To solve these problems, this study presents an investigation of the buoyancy-driven natural ventilation of a high-temperature corridor via computational fluid dynamics (CFD) simulation. First, a field measurement was conducted in a typical thermal corridor of a ceramic factory to define the boundary conditions for the simulation. Then, a CFD model was established and validated using reduced-scaled model experimental data. Next, the flow and temperature fields of the thermal corridor under buoyancy-driven natural ventilation were examined using the verified model. It was shown that the natural ventilation in the side corridor is generated by an asymmetric heat channel, and the flow in the main corridor belongs to the natural convection of a semi-closed square heating cavity. Finally, the single-story industrial building aspect ratio was fixed at 3.3, and five different parameters of building and heating plant setups on the natural ventilation in corridors were analysed. For example, the heat source and air opening aspect ratios varied from 12.5 to 50 and from 25 to 100, respectively. The results indicated that the height of the heat source bottom has a more significant effect on the indoor thermal environment than other situations. The maximum ventilation flow rate was observed at the heat source position zs/H = 0.04 and opening position H0/H = 0.05. When the overhead height zs/H reached 0.08, the flow in the main corridor turned to a symmetric heat channel, and the temperature in this region decreased sharply. This research can provide a reference for the natural ventilation design of high-temperature corridors in factories.

Review on buoyancy-driven natural ventilation in an enclosure with various types of heat sources

This paper reviews indoor heat convection and buoyancy-driven natural ventilation in enclosed space with heat sources in different forms such as point, line, plane, volume or combination of them. The indoor thermal flow is driven by these heat sources and accumulated in the enclosure. Thermal plume evolves based on its dynamic law above heat sources and is conversely affected by the thermal environment and geometric structure. Therefore, the dynamic of thermal-driven flows and the restriction by the thermal environment and geometric framework are both of interest in the field of indoor heat convection and buoyancy-driven natural ventilation. Based on this fact, the indoor thermal convection can be divided into two basic components which are buoyant plume above the heat source and indoor thermal stratification flow. Research and analysis on these laws and restriction are of significance in not only the advances in building thermal environment technology but also further cognition and effective solutions for current engineering practice.

Simulation study on indoor thermal environment improvement in multi-story buildings assisted by natural ventilation

Buoyancy-driven natural ventilation has the potential to improve the indoor thermal environment without any energy consumption. In this study, the applicability of the dimensionless area design method was studied, and the airflow rate and excess temperature deviation of rooms on different floors were verified. The results demonstrate that the airflow rates on the first, second and third floors are 887.68, 861.84 and 820.93 m3/h, respectively, and the corresponding airflow rates per capita are 88.77, 86.18 and 82.09 m3/h, respectively. In addition, the average indoor temperatures on the first, second and third floors are 298.78, 299.05 and 299.58 K, respectively, with corresponding excess temperatures of 5.72, 5.99 and 6.52 K. The deviation of airflow rate in the room is within the range of 0.59%-4.2%, indicating that the dimensionless area design method is suitable for designing naturally ventilated buildings.

Optimal temperature sensor placement in buildings with buoyancy-driven natural ventilation using computational fluid dynamics and uncertainty quantification

Natural ventilation and cooling can significantly reduce building energy consumption, but inherent variability in the boundary and operating conditions makes robust design and operation a challenging task. Computational models can be leveraged to evaluate the effects of these uncertainties and support robust design; however, the model accuracy should be thoroughly assessed, ideally through validation with carefully designed full-scale measurements. This study presents a novel approach for using computational fluid dynamics (CFD) simulations with uncertainty quantification to optimize temperature sensor placement in buildings with buoyancy-driven natural ventilation, such that the measurements are representative of the volume-averaged temperature, while also supporting analysis of the spatial variability in the temperature field. The approach uses a polynomial chaos expansion method to quantify the effect of the variability in the boundary and initial conditions on the predicted buoyancy-driven flow and indoor temperature field. Results for an operational building that employs night-time cooling are analyzed to identify sensor locations that will support robust observation of the time series of the volume-averaged temperature and the temperature range. The approach is shown to significantly reduce the potential error between the temperature recorded at a random location and the true volume-average temperature, while also providing a representative measure of the temperature range in the building. The results indicate that this approach can also support identifying optimal locations for temperature sensors used for operational control.

Distribution characteristics of indoor oxygen concentration under natural ventilation in oxygen-enriched buildings at high altitudes

Diffuse oxygen supply is an important means to improve the indoor oxygen environment of buildings and ensure physiological and psychological health of immigrants in plateau areas. Existing research on oxygen enrichment strategies at high altitudes has mainly focused on confined spaces under mechanical ventilation, with few studies on the distribution of indoor oxygen concentration under natural ventilation in actual buildings. This study used a verified computational fluid dynamics (CFD) method to investigate the indoor oxygen distribution with practical consideration of natural ventilation at high altitudes. The results showed that the oxygen distribution under wind-driven natural ventilation was more nonuniform than that under buoyancy-driven natural ventilation, with the ratio of local oxygen concentration to overall-mean oxygen concentration, the k value, between 0.8 and 1.3 under wind-driven natural ventilation and between 0.9 and 1.1 under buoyancy-driven natural ventilation. The effects of meteorological condition and oxygen source position on indoor spatial oxygen distribution characteristics were explored with careful examination in human occupied zone under lying, sitting and standing postures. The results can provide implications for effective and energy saving design of indoor oxygen supply system in plateau buildings.

Wind-driven ventilation of Double Skin Façades with vertical openings: Effects of opening configurations

Insufficient airflow in the cavity of Double Skin façade (DSF) systems has resulted in the overheating risk of the cavity and the resulting reduction in thermal performance in cooling dominant climates. Previous research has focused more on buoyancy-driven natural ventilation, and there is a lack of research on utilising wind-driven ventilation in the cavity as a dominant natural force for the heat dispersion from the cavity. Additionally, geometrical features of the cavity and wind direction as influential factors on wind-induced airflow have not been addressed in DSF studies. This paper presents a detailed evaluation of the impact of opening configurations on the ventilation performance of an integrated tall building with DSF with respect to four wind orientations (0°, 30°, 60°, and 90°) to the facade. The evaluation is based on three ventilation performance indicators: (i) induced airflow rate, (ii) wind speed ratio, and (iii) airflow distribution across the cavity length. High-resolution 3D steady RANS CFD simulations of cavity ventilation were performed for a range of sizes of the DSF cavity. The CFD simulations were validated against wind-tunnel measurements. The results show that the ratio between the areas of the front and lateral openings is a critical factor for improving the ventilation in the cavity at the wind incident angle 0° < ϴ < 30°. As wind direction increases to 60° and 90°, cavity size tends to be of the highest importance. Employing three front openings on the external skin of DSF (M3) can increase the efficacy of the system in a broader range of wind direction and cavity size.

Influences of radiation on the prediction of thermal environment in a transient buoyancy-driven natural ventilation classroom

Computational fluid dynamics (CFD) simulations are performed to reveal the significant effects of radiation on predictive accuracy of thermal environment in a transient buoyancy-driven natural ventilation classroom. The present numerical method is verified by the experimental results. The time evolutions of airflow field, radiation heat flux, buoyancy force, and airflow rate are analyzed in detail. In addition, the effects of radiation on thermal comfort are evaluated by three different indices. It is found that radiation plays a critical role in the establishment of the indoor thermal environment. The proportion of radiation heat flux increases with the effective radiation area but decreases with the power of the occupants. Finally, the indoor space can be divided into five different occupied zones depending on how much radiated heat the occupants emit. Radiation can change the indoor heat flux distribution, directly leading to a smaller buoyancy force and ventilation rate. As a result, the thermal comfort except for any draft is linearly affected by both the power and the effective radiation area, and the greater the proportion of radiation heat flux is, the better the indoor thermal comfort is. These conclusions demonstrate that radiation has a noticeable impact on the dynamic evaluations of indoor thermal environment.

Experimental study on the performance of hybrid buoyancy-driven natural ventilation with a mechanical exhaust system in an industrial building

Hybrid ventilation is an effective means of minimizing ventilation energy and improving indoor environment. A scale experimental model with a heat source was created for hybrid buoyancy-driven natural ventilation with a mechanical exhaust system. The aim of this study is to examine the performance of hybrid ventilation in an industrial building. The temperature distributions and hybrid ventilation efficiencies with different mechanical exhaust velocities were analyzed. Results showed that the hybrid ventilation efficiency first increased and then decreased with the mechanical exhaust velocity. A critical mechanical exhaust velocity was identified and the hybrid ventilation efficiency reached maximum at the critical mechanical exhaust velocity. The critical mechanical exhaust velocity was 1.4 m/s at the heat flux of the heat source q = 200 W and 1.0 m/s at q = 500 W, and the corresponding ventilation efficiencies were 24.4 and 6.69, respectively. Four modes of hybrid ventilation were investigated, and ventilation strategies of different modes of hybrid ventilation were given. An excessive mechanical ventilation rate will cause consumption of ventilation energy to increase and may lead to short circuiting of airflow and a bad thermal environment. These results should prove helpful in designing of hybrid ventilation systems for industrial buildings.

Efficiency of Different Roof Vent Designs on Natural Ventilation of Single-Span Plastic Greenhouse

In the summer season, natural ventilation is commonly used to reduce the inside air temperature of greenhouse when it rises above the optimal level. The greenhouse shape, vent design, and position play a critical role in the effectiveness of natural ventilation. In this study, computational fluid dynamics (CFD) was employed to investigate the effect of different roof vent designs along with side vents on the buoyancy-driven natural ventilation. The boussinesq hypothesis was used to simulate the buoyancy effect to the whole computational domain. RNG K-epsilon turbulence model was utilized, and a discrete originates (DO) radiation model was used with solar ray tracing to simulate the effect of solar radiation. The CFD model was validated using the experimentally obtained greenhouse internal temperature, and the experimental and computed results agreed well. Furthermore, this model was adopted to compare the internal greenhouse air temperature and ventilation rate for seven different roof vent designs. The results revealed that the inside-to-outside air temperature differences of the greenhouse varied from 3.2 to 9.6oC depending on the different studied roof vent types. Moreover, the ventilation rate was within the range from 0.33 to 0.49 min-1. Our findings show that the conical type roof ventilation has minimum inside-to-outside air temperature difference of 3.2oC and a maximum ventilation rate of 0.49 min-1.

Thermo-solutal buoyancy driven air flow through thermally decomposed thin porous media in a U-shaped channel: Towards understanding persistent underground coal fires

Natural ventilation for underground coal fires (UCF) is characterized by thermosolutal buoyancy driven air flow through thermally decomposed porous coalbed in an analogous U-shaped channel. Conventional models in terms of natural ventilation fail to include effects of solutal buoyancy and decomposed porous media. This paper aims to improve models for better prediction of thermosolutal buoyancy driven air flow, and further to facilitate the understanding of persistent burning of UCF. An experimental research framework was proposed to quantify buoyancy-driven natural ventilation for UCF. High-volatile bituminous coal was sampled for UCF experiments. Three fire depths of −1.6, −2.6, and −3.6 m were considered. Four models were developed and testified by experimental data. Results showed that proposed two models had good performances in prediction of natural ventilation for UCF. Validated models indicated that air velocity induced by solutal buoyancy is linearly proportional to molar mass difference between air and smoke, and effect of decomposed porous media can be characterized by temperature-dependent discharge coefficients and power exponent of natural ventilation model.

A Grey Box Modeling Method for Fast Predicting Buoyancy-Driven Natural Ventilation Rates through Multi-Opening Atriums

The utilization of buoyancy-driven natural ventilation in atrium buildings during transitional seasons helps create a healthy and comfortable indoor environment by bringing fresh air indoors. Among other factors, the air flow rate is a key parameter determining the ventilation performance of an atrium. In this study, a grey box modeling method is proposed and a prediction model is built for calculating the buoyancy-driven ventilation rate using three openings. This model developed from Bruce’s neutral height-based formulation and conservation laws is supported with a theoretical structure and determined with 7 independent variables and 4 integrated parameters. The integrated parameters could be estimated from a set of simulated data and in the results, the error of the semi-empirical predictive equation derived from CFD (computational fluid dynamics) simulated data is controlled within 10%, which indicates that a reliable predictive equation could be established with a rather small dataset. This modeling method has been validated with CFD simulated data, and it can be applied extensively to similar buildings for designing an expected ventilation rate. The simplicity of this grey box modeling should save the evaluation time for new cases and help designers to estimate the ventilation performance and choose building optimal opening designs.

Effect of Solar Chimney and PCM cooling ceiling to the air flow inside a naturally-ventilated building

Computational fluid dynamics (CFD) gives significant contributions to building design fields. Building ventilation is the most common problem to simulate in CFD software. 2D and 3D simulation can be used to simulate cross ventilation within the building. ANSYS Fluent is commonly used CFD software, and it has been used in this study to assess the environment of an atrium building under buoyancy-driven natural ventilation and wind-driven natural ventilation. Buoyancy is one of the main forces driving flows. Buoyancy effect happened when the moving fluid is lighter or heavier than surrounding fluid. This paper will elaborate the use of ANSYS Fluent in assessment the environmental performance of a building. Some cases on a building were set in this study. The aim is to simulate accurately buoyancy-driven air flow inside a building for natural ventilation, visualize and analyse simulation results, provide architectural/engineering solution to natural ventilation of buildings. Cooler air appears from ceilings and creates cooling to the room down. Meanwhile, solar chimney generates wind flow across the building due to temperature differences within the chimney.

A holistic framework to utilize natural ventilation to optimize energy performance of residential high-rise buildings

A novel holistic framework was established using Building Information Modelling (BIM) to estimate accurately the potential of natural ventilation of residential high-rise buildings. This framework integrates Computational Fluid Dynamics (CFD) simulation, multi-zone-air-flow modelling, and Building Energy Simulation (BES) to calculate ventilation rates under the mechanisms of wind-, buoyancy- and wind and buoyancy-driven ventilation. The framework was applied to a 40-storey residential building in Hong Kong for estimating the potential of natural ventilation in residential high-rise buildings. The results show that the building can save up to 25% of the electricity consumption if the building employs wind-driven natural ventilation instead of mechanical ventilation. The electricity consumption can be further reduced up to 45% by facilitating the buoyancy-driven natural ventilation. However, natural ventilation is found to be effective only if the temperature difference between indoor and outdoor is less than 2 °C. The study suggests to orienting residential high-rise buildings at an oblique angle with the prevalent wind direction than positioning perpendicular to the prevalent wind direction. Furthermore, the framework recommends promoting the wind-driven natural ventilation at top floors of residential high-rise buildings and to facilitate wind and buoyancy-driven natural ventilation at middle and lower floors of the buildings.