Buoyancy-driven Ventilation Research Articles

Experimental study of buoyancy-driven ventilation with a single opening between two adjacent building zones

In adjacent building zones connected by a single opening, a thermal plume produces a buoyant layer accumulating in the upper space of the experimental zone. The pressure difference between inside and outside the chamber will lead to air flowing in from outside and buoyant fluid flowing out from inside. This study aims to examine the effect of the vertical opening combination on flow regimes in adjacent building zones. The findings indicate that there may be three different flow regimes depending on the relative positions of the opening in the shared wall and the neutral level. The steady flow in two regimes with double stratification formed in the two zones is modeled separately and the theoretical prediction is compared with the experimental results. The flow regime of natural ventilation in the adjacent zones is only related to the height of the outlet, heat source height and effective opening area of the ventilation zones, but not to the buoyancy flux. The interface changes linearly with the height of the heat source at first until the nozzle is immersed in the buoyancy layer, and then the interface height remains constant. The relative size of the openings will also affect the volume flow rate through them, and keeping the areas of the openings along the flow passage equal will maximize the ventilation rate. These research results can provide layout guidelines for building doors and windows and will help designer improve the natural ventilation effect of adjacent building zones.

Buoyancy-Driven Ventilation in the Interior-Most Flats of an Apartment Building with Different Corridor Vent Opening Condition

In the present work, computational simulations are conducted for an apartment building to investigate the bouncy-driven airflow in the corridor and inside the rooms. Two types of corridor vent openings are provided through which the hot light air moves to the ambient. Two floors are considered for the study. The simulations are conducted with three different temperature difference conditions between walls and room inside air such as; ∆T=10OC, ∆T=15OC, ∆T=20OC for two different corridor vent opening configurations. When the corridor vent openings are kept only in front of the doors and windows, recirculation zones below the roof near the air entry region are observed and part of the air escaped through the inlet. In the case of a wide corridor vent opening, air does not escape through the inlet. With increase in ΔT, the airflow increases irrespective of vent positions. Mainly the area of the rooms is well ventilated closer to the door position. It is also observed that the lower floor is relatively well-ventilated as compared to the upper floor. The ACH increases with increase in vent opening and it also increases with increase in ∆T.

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.

Weather-based operation strategy for a dynamically compartmentalized double-skin façade system

This study proposed a weather-based operation strategy for a dynamically compartmentalized double-skin façade (DSF) system. The system features two separate air cavities, each of which serves as an inlet or an outlet. The cavities are placed at different levels to facilitate the buoyancy-driven ventilation and the preheating effect. To enhance the performance, the compartmentalized DSF needs to be operated in active cooling (AC), passive cooling (PC), and hybrid heating (HH) modes based on the outdoor condition. Therefore, this study proposed the appropriate operation strategy for the system and evaluated the influence of the strategy on building's thermal performance. Various mode-control thresholds for switching between AC–PC and between PC–HH modes were examined. A validated building simulation model was used to evaluate the operation strategy through a case study, and the results were discussed on the basis of the two types of performance indicators (PIs): the cooling and heating energy use intensity (for AC and HH modes) and hot and cold discomfort hours (for PC and HH modes). A lower AC–PC threshold increases the cooling energy while decreasing the hot discomfort. The influence of a PC–HH threshold on discomfort hours is not linear and vary by modes; therefore, PIs in each mode should be prioritized to determine the optimal threshold. The case study indicated that the DSF with the proposed operation strategy saved 28% cooling energy and 98% heating energy in comparison to a single-skin façade system. Significant hot discomfort may occur in the HH mode, emphasizing the importance of adaptive operations.

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.

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.

Effects of the Design Parameters of Ridge Vents on Induced Buoyancy-Driven Ventilation

With ridge vents that are commonly used in building ventilation applications as the research object, this study analyzed how design parameters affect the efficiency of thermal buoyancy-driven ventilation induced by ridge vents through computational fluid dynamics (CFD). The design parameters of ridge vents include the width S, height H, and eave overhang E. In consideration of engineering practices, the parameter ranges were set as follows: S = 1.2, 1.8, 2.4, and 3 m; H = 0.3, 0.6, 0.9, and 1.2 m; and E = 0, 0.3, and 0.6 m. The results show that when a ridge vent is under buoyancy-driven ventilation, the height H serves as the dominant design parameter. Correlation equations of the induced ventilation rates with the relevant ridge vent design parameters are provided.

Buoyancy-driven ventilation of an enclosure containing a convective area heat source

Buoyancy-driven ventilation of an enclosure containing a convective area heat source with low surface emissivity is studied in this paper. Based on the dynamics of turbulent plumes generated from an area heat source, the mathematical models for dimensionless thermal stratification height are deduced and integrated into a unified formula in this study. And a mathematical model governing the transition zone between these two zones is derived and developed. In order to test the validity of the theoretical model, a series of simulated models were conducted to compare theoretical results with simulated results. It was found that the transition zone will disappear directly when virtual origin distance zv/Ds≥−1.14, which results in that the thermal stratification height can only be obtained in the zones close to the area source and far away from the area source. And the thermal stratification height varies widely when the A∗/H2 values are from 0 to 0.1. In addition, thermal stratification height is negatively correlated with the dimensionless source diameter ξs. This study can be used as a basis for follow-up study of buoyancy-driven flows by area heat source in naturally ventilated building.

Exergetic review on passive and active systems for ventilation

The conceptualization of warm/cool and wet/dry exergies for a better understanding of various heating/cooling systems has been found very useful for describing the whole thermodynamic process of various built-environmental systems, from building envelopes to mechanical heating and cooling systems. In parallel to these sub-concepts of exergy, the conceptualization of “high-pressure/low-pressure” exergies, which is associated with mechanical non-equilibria, is also necessary for full exergetic description of flow and circulation within the built-environmental systems, from natural and mechanical ventilation to the circulation of hot or cold water between heat exchangers. This paper describes the essence of “high-pressure/low-pressure” exergies and also demonstrates a couple of example calculations, which clarify how natural ventilation systems function from the exergetic viewpoint. The conclusion reached by the present analyses is that the exergy input and consumption for natural ventilation is much smaller than those for mechanical ventilation: e.g., for the number of air change at the rate of 2 h-1 in a room whose floor area is 36 m2, the exergy inputs for wind-driven and buoyancy-driven ventilation are approximately 0.1% of the electricity input for the same air change rate with mechanical ventilation.

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.

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.

Experimental Comparison Between Three Types of Opaque Ventilated Facades

Background:The growing interest for the energy efficiency of building technologies has led the construction sector towards the adoption of Opaque Ventilated Facades (OVFs) as high-performance solutions for building systems.Objective:The aim of this study is to determine the optimal thermal inertia of the outer surface of ventilated facades with respect to the indoor comfort and the reduction of the outdoor overheating.Method:An experimental study was carried out in Central Italy (Mediterranean climate), by comparing the thermo-physical performance of three opaque ventilated façades, characterized by different positions of the mass (hollow bricks) within the air cavity. One has no mass and is enclosed by a Lightweight (L) cladding; one has an Internal Mass (IM) right adjacent to the insulation layer and an external lightweight cladding; the last one has an External Massive cladding (EM). The three prototypes (L, IM and EM walls), were installed on a test room and simultaneously monitored in the summer season.Result:The experimental outcomes demonstrate that the EM wall outperforms the others in terms of cooling efficiency, as the incoming heat fluxes towards the indoors are considerably reduced. Moreover, such a configuration led to the lowest surface temperatures on the outer slab, thus contributing to the mitigation of the external environmental overheating.Conclusion:Overall the External Mass (EM) solution was found to be the best choice, being beneficial for mitigating the outdoor surrounding temperatures and enhancing the buoyancy-driven ventilation.

Reduced-scale experimental model and numerical investigations to buoyance-driven natural ventilation in a large space building

This research establishes a reliable and effective methodology for evaluating the buoyancy-driven ventilation performance in a large space building through a reduced-scale experimental model and full-scale prototype numerical simulation. The measured values are modeled back to the corresponding environmental parameters for the prototype building by using similarity analysis. The experimental data are compared with numerical simulation results, and good agreements with relative errors no more than 1% in temperatures and airflow rates are achieved. The thermal comfort conditions in the building are numerically investigated in terms of well-known thermal comfort index. Results indicate that pure buoyancy-driven ventilation cannot keep people thermally comfortable when the ambient temperature is high, and mechanical ventilation is needed for such a building with a large glass ceiling. The proposed methodology provides a useful procedure to quantify the thermal conditions in such a large space building with a semi-transparent glazed ceiling, and the results of this study can be considered at the initial design stage of the building to obtain a comfortable indoor thermal environment.

Gaseous pollutant transmission through windows between vertical floors in a multistory building with natural ventilation

Natural ventilation is an effective strategy to control thermal comfort in buildings, and can be enhanced depending on the window style. The combination of natural ventilation and window can also facilitate the removal or dilution of gaseous pollutants from indoor sources in newly decorated buildings. However, the windows on the same facade may cause gaseous pollutant cross-transmission during single-sided natural ventilation between households on different floors close to the source. Although some research has focused on the pollutant cross-transmission in buildings, the simplification of windows into rectangular openings often affects accurate knowledge of pollutant transmission characteristics. Therefore, this investigation explored gaseous pollutant cross-transmission through real windows during single-sided, buoyancy-driven ventilation in a multistory building. Six types of windows were modeled for the indoor pollutant of gaseous formaldehyde (HCHO). Computational fluid dynamics (CFD) was utilized to solve characteristics of pollutant transmission inside and outside the multistory building. The results indicated that the ventilation rates, thermal profiles and pollutant transmission inside and outside the building varied for each window type, although the open window areas were identical. The re-entry ratio of exhausted air entering upper floors and the infection risk of epidemic viruses caused by airborne cross-transmission was sensitive to ventilation rates and window configurations, while the sensitivities for window configurations varied case by case. The comparisons also revealed that the specification of ambient temperature and pollutant release rate ultimately did not affect the evaluation of pollutant cross-transmission using CFD.

Adjoint-based optimization of displacement ventilation flow

We demonstrate the use of the ‘Direct-Adjoint-Looping method’ for the identification of optimal buoyancy-driven ventilation flows governed by Boussinesq equations. We use the incompressible Reynolds-averaged Navier-Stokes (RANS) equations, derive the corresponding adjoint equations and solve the resulting sensitivity equations with respect to inlet conditions. We focus on a displacement ventilation scenario with a steady plume due to a line source. Subject to an enthalpy flux constraint on the incoming flow, we identify boundary conditions leading to ‘optimal’ temperature distributions in the occupied zone. Our results show that depending on the scaled volume and momentum flux of the inlet flow, qualitatively different flow regimes may be obtained. The numerical optimal results agree with analytically obtained optimal inlet conditions available from classical plume theory in an ‘intermediate’ regime of strong stratification and two-layer flow.

Numerical analysis of wind microclimatic effects on a plastic greenhouse protected by a windbreak in different seasonal periods

Windbreaks are frequently used to protect greenhouses. Agricultural plastic nets are widely used for the construction of such windbreaks replacing the traditional natural windbreaks formed by trees. When installed against the dominant wind direction at an optimized distance from the greenhouse side walls, windbreaks may reduce wind loads. An additional effect of the provided protection against strong cold winds is the reduction of heat losses during the winter period. However, the effect of windbreaks in situations when cooling is necessary, particularly in cases of low wind speeds during summer period, has not been investigated so far. The present work aims to analyse the energy efficiency and the wind flow around and through an arched-roof greenhouse with two side and one roof opening (windows) in two extreme cases, namely a) a high velocity cold wind causing important heat losses on the greenhouse (closed windows), and b) a low velocity wind speed resulting to buoyancy-driven ventilation in the greenhouse (open windows). In the first case the reduction of heat losses due to the windbreak are explored. In the second case, the possible dissipation of the ventilation airflow and the corresponding temperature variations by the windbreak is investigated. Computational Fluid Dynamics (CFD) numerical simulations are used in the present work.

Buoyancy-Driven Ventilation Generated by the Double-Skin Façade of a High-Rise Building in Tropical Climate: Case Study Bandung, Indonesia

High-rise buildings in tropical region is identical to the use of mechanical Air Conditioning in massive scale. Nevertheless, there is an encouragement to high-rise buildings to reduce its energy consumptions, since they consume quite large amount of energy. This challenge can be overcome with various of strategies, one of them, by means of reducing the cooling load of mechanical Air Conditioning in high-rise building. Prospects come from the modern tall building design strategies, for example the use of double-skin facade to give addition of building skin which could provide indoor temperature protection from outside. Double-skin facade system has continued to increase in buildings in a tropical region such as in Indonesia. However, there is another potential of double skin facade, which is the possibility to increase the buoyancy effect in the air gap between the skin and building envelope. The possibility needs to be studied in order to give a proper way in designing double-skin facade of a high-rise building, especially on Bandung-Indonesia tropical climate. This paper explores the potential of double-skin facade in driving the air inside the facade to generate natural ventilation for a high-rise building in Bandung climate condition. Two parameters are used in exploring the buoyancy force, the width of double-skin facade and the temperature of the skin facade. In general, double-skin facade of a high-rise building in tropical climate can generate buoyancy driven ventilation for the building, it relates strongly to the distance between of the double-skin facade and the building envelope.

SIMULATION OF FREE CURRENT FLOWS IN BUOYANCY-DRIVEN VENTILATION SYSTEMS

Objectives . The aim of the study is to analyse the effect of the design and methods for heating the ventilation duct of a buoyancy- driven system on the formation of free convective air currents in it. Methods. The study of free convection under the conditions of interior problem was carried out using the CFD software, based on the finite volume method with unstructured grid. Ansys Fluent software was used as a calculation tool in the study, due to its having a high convergence of numerical solutions offering full-scale measurements of convective currents.To evaluate the reliability of the results obtained, a validation procedure was carried out, allowing us to determine how accurately the selected conceptual model describes the investigated flow through a comparison of experimental and numerical data. Results . The results of numerical modelling of free convective currents occurring in the heated channel of the ventilation system of the top floor of a multi-storey residential building are presented in the article. In the course of the study, the air velocity at the entrance to the ventilation duct was found to depend on the calculated temperature difference θ ˚C for various heating methods. A gradual increase in the discrepancy between the numerical simulation and experimental results is observed if the calculated temperature difference > 20 ° C. This phenomenon is due to the fact that with increased duct temperature, it is quite difficult to achieve even heating under actual conditions; this is especially noticeable when considering the variant when the vertical part of the vent duct and the take-off are both heated. The maximum deviation of the results is 4.4%. The obtained velocity profiles in the calculated sections indicate the impact of the ventilation take-off on the nature of the air flow motion. Conclusion . One of the drawbacks of the existing systems of natural ventilation of residential buildings is the low efficiency of work in the warm and transitional periods of the year, especially in bathrooms. The use of buoyancy-driven ventilation with vertical heating of the vent duct combined with the take-off allows a stable air exchange in the rooms to be provided.

Energy Aspects of Gravitational Ventilation in the Heating Season

Natural ventilation is currently widely used in existing buildings in order to assure the fresh air needed by occupants. The low investment and operational costs are the most important advantages of this type of ventilation. However, the dependency on the meteorological parameters has to be considered as disadvantage of the natural ventilation. In case of buoyancy-driven ventilation, the variation of the outdoor temperature results in the variation of the infiltrated air flow, CO2 concentration in the indoor air and energy demand of the ventilation. The air inlet and outlet orifices have to be properly chosen in order to meet the indoor air quality and energy requirements at the same time. In this paper the CO2 concentration and energy aspects of gravitational ventilation are discussed in case of a typical block of flats.

Optimal design and control of buoyancy-driven ventilation

ABSTRACTRelationships between airflow rates and opening areas of importance for design and control are analysed for buoyancy-driven ventilation in a room with two openings and uniform temperature. The optimal ratio between the inlet and outlet areas is found. The consequences of deviations from the optimum are discussed. Also, discussed are measures to be taken in order to avoid bidirectional flow in any of the openings, and it is discussed how to ensure constant airflow rate, when structural restrictions imply reduction of inlet or outlet. The analyses are carried out with the temperature difference and the net heat input, respectively, as independent, thermal variable.