Natural Displacement Ventilation Research Articles

Night cooling by hybrid supply ventilation – analytical predictions of airflow rates and the ‘hybrid ventilation triangles’

Overheating in buildings can be mitigated during periods of excessively warm weather through the application of a night purge ventilation strategy. Building on the mathematical model of Waterson and Hunt (2024), we investigate the emerging field of hybrid ventilation – specifically the rates of airflow achieved by the simultaneous combination of ‘stack-driven’ natural displacement ventilation and a mechanical supply. Whilst the focus of Waterson and Hunt (2024) was specifically on predicting the time taken to complete a purge, herein we focus on airflow rates and in doing so elucidate the somewhat counter intuitive role that the mechanical supply has on the natural ventilation flow rates. Our results challenge the perception that a hybrid approach must be superior to both solely natural and mechanical strategies. Focusing on the variation in airflow rates during a hybrid purge, our model reveals: (i) a hybrid purge offers an improvement over a solely mechanical purge only during one of the two distinct flow regimes identified; and (ii) the manner in which the stack-driven and mechanical components combine is highly nuanced and cannot be predicted by the linear superposition of the constituent stack-driven and mechanical contributions.

Night cooling by hybrid ventilation – analytical predictions of the purge time

Many low-energy buildings rely on a night purge strategy during periods of excessively warm weather in order to flush out excess heat and thereby help prevent overheating on the following day. A typical purge utilises either a mechanical or a natural displacement ventilation strategy. However, we investigate purging by the simultaneous application of natural and mechanical ventilation, i.e. using a hybrid strategy. Hybrid ventilation is widely promoted as a means of overcoming the inherent limitations of both natural and mechanical ventilation, although relatively little is understood about its operation on a fundamental level. Such was the lack of knowledge on hybrid ventilation that even the time taken to complete a purge was not known at the outset of this work. Through the development of a mathematical model, supported by a series of laboratory experiments, we show that the time taken to purge a room using hybrid ventilation can be established analytically, and is controlled by two ratios: (i) the flow rate of the mechanical supply (the ‘forcing’) relative to the initial flow rate through the vents in the absence of mechanical forcing, and (ii) the ratio of the floor-level and ceiling-level vent areas. The analytical predictions show close agreement with our measurements. Supplementing a natural displacement ventilation strategy with even a modest mechanical supply can reduce significantly the time taken to purge warm air from a room. Some of the implications of our findings for preventing overheating in buildings by increasing the efficacy of a night cooling strategy are discussed. Practical Application This research is directly applicable to practitioners involved in the first-order design of hybrid ventilation systems – particularly designs which incorporate a ‘night purge’ regime. Governing equations are derived and the key parameters controlling the rate of purging of warm air are revealed, allowing informed first-order design decisions to be made without the need for expensive and time-consuming computational simulations.

Theoretical analysis of natural displacement ventilation in a warm space

The transient development of natural displacement ventilation in a warm space with a localized source of heat is investigated theoretically to understand the fluid mechanics in detail. One of four flow modes, A, B, C and D, may develop during the transient evolution. The flow pattern of mode B, C or D can be further subdivided into two cases respectively according to a critical value of the reduced gravity of indoor original air. When the plume rises to an interface between two air layers, it may evolve to be less buoyant than that of the above layer. The subsequent flow pattern would develop in three ways, determined by whether the plume momentum and the penetrative entrainment are considered. Although the differences among the predictions corresponding to the three flow regimes are slight, the simplified model of penetration without entrainment is recommended, because it is the most reasonable and can be solved easily.

Experimental Research on the Temperature Distribution of Natural Displacement Ventilation in the Room under the Influence of Wind Pressure and Air Supply

Air pollution and building energy efficiency are two hot research contents at present. Based on the comfort, energy-saving and environmental protection characteristics of natural displacement ventilation, an experiment was been conducted to explore the influence of outdoor air pressure and different parameters of air supply outlet on indoor temperature distribution. The results showed that with the outside wind pressure increasing, thermal stratification height increases. When the outdoor air pressure exceeds 0.05625Pa, the indoor temperature fluctuations caused by side air supply was greater than the bottom, therefore, natural displacement ventilation with bottom air supply is better than with side air supply in the stability of temperature. When the outdoor air pressure exceeds 0.1Pa, the stratification disorder and even doesn’t exist. From the view of comfort and energy savings, the side air supply is better than the bottom.

Comparison of measured and simulated performance of natural displacement ventilation systems for classrooms

Children spend the majority of their weekdays in classrooms that often have low indoor air quality and limited financial resources for the initial and running costs of mechanical ventilation systems. Designing effective natural ventilation (NV) systems in schools is difficult due to the intense use of the classroom spaces and the dependence of NV on building geometry and outdoor conditions. Building thermal and airflow simulation tools are fundamental to predict NV system performance in the design phase. These predictions of these tools must be validated (preferably with data from real buildings). This paper presents a set of detailed measurements of buoyancy driven natural DV systems of three classrooms located in two buildings in Lisbon (Portugal). The rooms are located in two educational buildings, a kindergarten and a university, and have different buoyancy driven natural DV systems (with and without chimneys). The experimental measurements are used to validate a three-node DV model implemented on the open-source thermal building simulation software EnergyPlus. The validation results show that the building thermal simulation model tested is able to predict bulk airflow rate with an average error of 16%. In addition, a good agreement is also obtained for the vertical temperature prediction: an average error of 4% corresponding (average deviation of 0.7°C). Analysis of the kindergarten rooms results revealed, that as expected, increasing chimney height from 1 to 4m has a significant positive impact in NV system performance. The performance of natural DV systems depends on the number of thermal plumes in the room. For the same sensible heat load, increasing the number of plumes lowers the average occupied zone air temperature and increases the bulk airflow rate. In light of the complexity of the cases tested, NV with uncontrolled boundary conditions, the results of the comparisons performed between measurements and simulations should contribute to increase confidence in the use of EnergyPlus to simulate buoyancy driven natural DV systems.

A study on flow stratification in a space using displacement ventilation

This paper presents a theoretical analysis approach and experimental results on the stratified flow in a reduced-scale model using mechanical and natural displacement ventilation. Theoretical analysis is based on fundamental conservation equations and turbulent plume theory. The salt-bath technique is employed to conduct the analogous experiments to building ventilation problems and the reduced-scale acrylic model is used to observe the flow patterns in the laboratory. The light-attenuation method is used to analyze flow stratification in the analogous experiments. The model is divided into two rooms, which have the same cross-section area and volume, by an interior divider. The room having a buoyancy source is denoted as the ‘forced room’, and the other room is denoted as the ‘supply room’, which provides a constant flow rate into the space for mechanical displacement ventilation cases. This research focuses on analyzing convection flow properties and stratification distribution in the forced room. The research results show that the stratified flow in the forced room is controlled by the supply flow rate and slightly by the buoyancy source strength for mechanical displacement ventilation. The flow properties are normalized to be dimensionless parameters under the condition of a fixed buoyancy flux, and the dimensionless interface level and the dimensionless reduced gravity of the buoyant layer change with the dimensionless flow rate. As the supply flow rate increases, the stability of stratification becomes weak and there is an intermediate stratified layer formed between the fresh ambient and polluted buoyant layers. This study shows that the stability of stratification and the thickness of the intermediate stratified layer are dependent on the ratio of buoyancy force to inertia force in the room using displacement ventilation.

Theoretical predictions of transient natural displacement ventilation

The transient natural displacement ventilation driven by a localized heat source at the floor of a room is examined to understand the airflow behavior and the relevant thermal stratification characteristic. Two modified theoretical models are developed on the basis of classical plume theory and Kaye and Hunt’s model. The buoyant layer is modeled as composed of a near-ceiling layer and a warm layer rather than being well-mixed. Some assumptions on the buoyancies of the near-ceiling layer and the outflow through the upper opening are made in the two modified models for simplicity. Comparisons are made between the predictions of Kaye and Hunt’s model and the two modified models and experimental data reported in the literature. Two modified models are shown to perform better than Kaye and Hunt’s model. Meanwhile, the predictions of the modified model I seem to agree slightly better with the experimental data than those of the modified model II. Typically, the average relative deviation between the three models’ predictions of the lowest interface height and the experimental data are about 4.2% (modified model I), 4.6% (modified model II), and 4.9% (Kaye and Hunt’s model).

Quasi-steady states in natural displacement ventilation driven by periodic gusting of wind

AbstractWe investigate the natural displacement ventilation of a space connected to a body of warm fluid through high- and low-level vents. The space is subject to discrete periodic gusts of wind entering at high level from a cold exterior. The cold exterior air entering the space produces buoyancy differences between the space and the body of warm fluid, driving a ventilation flow. Initially we examine the case of a series of identical gusts of wind modelled as turbulent buoyant thermals. New laboratory experiments show that an approximately two-layer stratification is established and the height of the interface is quasi-steady if the period between thermals is much less than the draining time of the space but longer than the fall time of individual thermals. Experiments also show that the interface height depends on the average buoyancy flux associated with the wind gusts, the time between thermals as well as the geometric properties of the vents. This contrasts with the case of a continuous source of buoyancy where the interface height depends only on the geometric properties of the vents and is independent of the buoyancy flux. We develop a quasi-steady two-layer model of the flow based on the classical theory of turbulent thermals and show that it is consistent with our new experimental data. We generalize the model to explore the sensitivity of the results to temporal variations in the size of thermals. We then extend the model to explore the effects of longer interval times between successive thermals and find a two-layer stratification still develops but that the interface height now varies cyclically in time. We then discuss the implications of these results for the ventilation of a shopping mall subject to gusts of wind.

The effect of floor heat source area on the induced airflow in a room

We examine the role of heat source geometry in determining rates of airflow and thermal stratification in natural displacement ventilation flows. We modify existing models to account for heat sources of finite (non-zero) area, such as formed by a sun patch warming the floor of a room. Our model allows for predictions of the steady stratification and ventilation flow rates that develop in a room due to a circular heat source at floor level. We compare our theoretical predictions with predictions for the limiting cases of a point source of heat (yielding a stratified interior), and a uniformly heated floor (yielding a mixed interior). Our theory shows a smooth transition between these two limits, which themselves result in extremes of ventilation, as the ratio of the heat source radius to the room height increases. Our model for the transition from displacement to mixing ventilation is compared to previous work and demonstrates that the transition can occur for smaller sources than previously thought, particularly for rooms with large floor area compared to ceiling height.

The response of natural displacement ventilation to time-varying heat sources

We examine transients caused by sudden changes in heat load in a naturally ventilated chamber. The space we consider has an isolated heat source, modeled as an ideal plume, and is connected to the exterior via openings at the top and bottom. Pressure differences between the exterior and interior that arise due to the buoyancy in the space drive a natural ventilation flow through the space that generates a two-layer system with buoyant (warm) fluid in the upper layer and ambient fluid in the lower layer. We develop two mathematical models, one assuming perfect mixing of each layer, the other accounting for stratification. We compare both models to small scale laboratory experiments. Neither model is significantly better than the other, and both give good agreement with the experiments. Using these models we identify many appealing features of the type of natural ventilation studied here. These include the fact that a well designed system is self-controlling. The manner in which it controls itself is very robust, because the larger the change in heat load the faster the system will readjust. Also there is a resistance against the interface penetrating deeply into the occupied zone, even for very large changes in heat load.

CFD modelling of natural displacement ventilation in an enclosure connected to an atrium

Computational fluid dynamics (CFD) is used to investigate buoyancy-driven natural ventilation flows in a single-storey space connected to an atrium. The atrium is taller than the ventilated space and is warmed by heat gains inside the single-storey space which produce a column of warm air in the atrium and drive a ventilation flow. CFD simulations were carried out with and without ventilation openings at the bottom of the atrium, and results were compared with predictions of analytical models and small-scale experiments. The influence of key CFD modelling issues, such as boundary conditions, solution controls, and mesh dependency were investigated. The airflow patterns, temperature distribution and ventilation flow rates predicted by the CFD model agreed favourably with the analytical models and the experiments. The work demonstrates the capability of CFD for predicting buoyancy-driven displacement natural ventilation flows in simple connected spaces.

On the natural displacement flow through a full-scale enclosure, and the importance of the radiative participation of the water vapour content of the ambient air

This paper presents experimental data for the temperature stratification established within a full-scale enclosure, for the natural displacement ventilation flow driven by a source of buoyancy at floor level within the space with air as the working fluid. A range of predictive techniques is also investigated for the flow, and for each technique a critical comparison with the experimental data is presented. It is confirmed that the salt-bath modelling technique and related mathematical model of Linden et al. (J. Fluid Mech. 212 (1990) 309) is not appropriate for the prediction of this type of ventilation flow in full-scale buildings. Computational fluid dynamics is a powerful technique that can yield realistic predictions for this type of ventilation flow. Even this approach, however, requires further investigation before it can be used routinely. In particular, this paper illustrates the effect of applying different models of turbulence, and highlights the requirement to employ a complete thermal radiation model when using the CFD technique to predict this class of flow.