A laboratory experiment of shear-induced natural ventilation

When the wind direction is parallel to the opening façade, the wind shear near the building opening generates turbulence and entrains air across the opening. This kind of shear-induced ventilation cannot be predicted by the orifice equation because the time-averaged pressure difference across the opening is close to zero. This study uses wind tunnel experiments and the tracer gas decay method to investigate the ventilation rate of shear-induced ventilation. The influences of opening area A, external wind speed U and wind direction on the ventilation rates Q, of single-sided and two-sided openings are systemically examined. The experimental results indicate that the dimensionless ventilation rate, Q^* = Q/UA, of shear-induced ventilation is independent of the wind speed and opening area, and the value of Q^* of two-sided openings is larger than that of a single-sided opening. In addition, a cosine law was used to predict the ventilation rate of building with two-sided openings under various wind directions, and the results are compared with the prediction of the multizone ventilation model COMIS.     

Numerical simulation of natural ventilation of a factory roof cavity

The study targets the reduction of roof solar heat gain through the use of natural ventilation in a cavity of a factory roof. In the laboratory experiment [1], the average air velocity reached 0.25 m/s. A simulation program was developed to calculate the heat and air flow attained in the experiment. An airflow passage was divided into sections to trace the pattern of the air temperature rise. When the cavity was divided into 20 sections, it was enough to trace the temperature rise pattern, and hence to calculate buoyancy for natural ventilation. Then the simulated air velocities, temperatures and heat transportations were compared with the experimental results. The molecular viscosity and thermal conductivity of the air were modified to adjust the simulation results to the experimental results in a wide range of experimental conditions. When they were multiplied with a magnitude of 30 equally, the least root mean square of the ratio of deviations of the heat transportation was obtained. This simulation could predict the heat transportation as a result of natural ventilation with a root mean square of the deviation of 0.25 in a short calculation time.     

A naturally ventilated cavity roof as potential benefits for improving thermal environment and cooling load of a factory building

The impact of natural ventilation of a roof cavity on improvement of the thermal environment and reduction of cooling load of a factory building is discussed. A computer program was developed with the logic in a companion paper [1] to observe the effect of cavity ventilation on the operative temperature of the occupied zone in the factory. Comparisons were made between factories with a cavity roof and a single roof in the Japanese climate. Results showed that the cavity roof was superior to the single roof in lowering the operative temperature by about 4.4 °C. When the factory was air conditioned, the cooling load reduction reached approximately 50% during the summer to maintain an operative temperature of 26 °C. Results showed that a naturally ventilated cavity roof has excellent potential for improving the indoor thermal environment and energy savings of factory buildings without complicated cooling installations and life time power consumption.     

Enhancement of stack ventilation in hot and humid climate using a combination of roof solar collector and vertical stack

In the hot and humid climate, stack ventilation is inefficient due to small temperature difference between the inside and outside of naturally ventilated buildings. Hence, solar induced ventilation is a feasible alternative in enhancing the stack ventilation. This paper aims to investigate the effectiveness of a proposed solar induced ventilation strategy, which combines a roof solar collector and a vertical stack, in enhancing the stack ventilation performance in the hot and humid climate. The methodology selected for the investigation is physical experimental modelling which was carried out in the actual environment. The results are presented and discussed in terms of two performance variables: air temperature and air velocity. The findings indicate that the proposed strategy is able to enhance the stack ventilation, both in semi-clear sky and overcast sky conditions. The highest air temperature difference between the air inside the stack and the ambient air (T_i−T_o) is achieved in the semi-clear sky condition, which is about 9.9 °C (45.8 °C–35.9 °C). Meanwhile, in the overcast sky condition, the highest air temperature difference (T_i−T_o) is 6.2 °C (39.3 °C–33.1 °C). The experimental results also indicate good agreement with the theoretical results for the glass temperature, the air temperature in the roof solar collector’s channel and the absorber temperature. The findings also show that wind has significant effect to the induced air velocity by the proposed strategy.     

The effect of internal heat source and opening locations on environmental natural ventilation

Two-dimensional computational simulations are performed to examine the effect of open location on natural ventilation for room with an internal heat source. The room is symmetrically cooled from the sides and insulated from lower and upper walls. The analysis of the fluid flow and heat transfer characteristics of the natural ventilation was carried out by the method of streamline and temperature contours. Three different cases of local open length on lower and upper walls are studied for Rayleigh numbers ranging from 10³ to 10⁵ and open aspect ratio varied from 0.0 to 0.8. The results show that the enhancement of the heat transfer is the greatest at high Rayleigh numbers and open aspect ratios. The best position for the open locations was found out to be on the upper wall as shown in case 3.     

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.     

A qualitative ventilation model for prototype cavity structure

Creating a physical model structure that is able to simulate different ventilation scenarios is essential for improving the understanding of passive designs that are both sustainable and environmentally acceptable. The purpose of this investigation was to build a physical prototype model that could be heated from both the outside and inside to duplicate an occupied structure during the morning hours. This model was then used to provide information for two Computational Fluid Dynamic (CFD) models to firstly compare and then validate results obtained from experimental data. A sample of computational findings was initially presented in Chawynski et al. (2003). This paper presents the experimental sections used to validate numerical simulation in Chawynski et al. (2003) and incorporates findings from Chawynski (2004). The experimental component consisted of approximately 80% of the overall investigation. The findings enabled a better scientific understanding of how a structure’s thermal performance can be improved by mechanically forcing air (to simulate outside wind) inside to both lower and create an overall uniform internal air temperature distribution in a cavity enclosure. A chart-based model is proposed to qualitatively gauge the temperature inside a cavity enclosure for different ventilation scenarios.     

Possibility of using roof openings for natural ventilation in a shallow urban road tunnel

Naturally ventilated urban vehicular tunnels with multiple roof openings have increased in China. Unnecessary gas (polluted air or fire smoke) are expected to be exhausted out through openings. Whether its safety standards can be satisfied or not still needs to be verified. In this paper, a safe CO concentration was firstly discussed, and a heat risk level of very high to extreme up to 46 °C was given. Secondly, a real 1410 m tunnel was proposed, and a 1/10 scale model tunnel was reproduced. Ambient winds of 0.95 m/s in prototype and 0.3 m/s in model were considered. Under normal traffic test, a track circuit was constructed with model vehicles moving on it to form traffic wind, and once the air velocity was larger than 0.31 m/s, the airflows were found to be not relevant to the Reynolds number. The traffic winds were weakened by openings. For three of all tested traffic, the actual air velocities were larger than the required ones, so its air qualities were satisfied. In firing test, two sets of burning experiments were conducted with which the heat release rates (HRR) were 8.35 kW and 13.7 kW. Large amounts of smoke were exhausted out of openings, and the high-temperature was not significant. Full-scale numerical simulations were carried out to verify the experimental results respectively using Fluent 6.0 for normal traffic and FDS 4.07 for firing. The simulations were compared well with the experiments. Further FDS simulations show that the openings’ mass flow rates are influenced little by ambient temperature; with the increasing length of the buried section, much smoke accumulate inside leading to a high temperature; having 4–5 openings in one shaft group is oversize in the actual engineering design.     

Experimental study of natural roof ventilation in full-scale enclosure fire tests in a small compartment

An analysis of full-scale fire test experimental data is presented for a small compartment (3×3.6×2.3 m). A square steady fire source is placed in the center of the compartment. There is an open door and a horizontal opening in the roof, so that natural ventilation is established for the well-ventilated fire. A parameter study is performed, covering a range of total fire heat release rates (330, 440 and 550 kW), fire source areas (0.3×0.3 m and 0.6×0.6 m) and roof ventilation opening areas (1.45×1 m, 0.75×1 m and 0.5×1 m). The impact of the different parameters is examined on the smoke layer depth and the temperature variations in vertical direction in the compartment. Both mean temperatures and temperature fluctuations are reported. The total fire heat release rate value has the strongest influence on the hot smoke layer average temperature rise, while the influence of the fire source area and the roof opening is smaller. The hot smoke layer depth, determined from the measured temperature profiles, is primarily influenced by the fire source area, while the total fire heat release rate and the roof opening only have a small impact. Correlations are given for the hot smoke layer average temperature rise, the buoyancy reference velocity and the total smoke mass flow rate out of the compartment, as a function of the different parameters mentioned. Based on the experimental findings, it is discussed that different manual calculation methods, widely used for natural ventilation design of compartments in the case of fire, under-predict the hot layer thickness and total smoke mass flow rate, while the hot layer average temperature is over-estimated.     

An analytical and numerical study of solar chimney use for room natural ventilation

The solar chimney concept used for improving room natural ventilation was analytically and numerically studied. The study considered some geometrical parameters such as chimney inlet size and width, which are believed to have a significant effect on space ventilation. The numerical analysis was intended to predict the flow pattern in the room as well as in the chimney. This would help optimizing design parameters. The results were compared with available published experimental and theoretical data. There was an acceptable trend match between the present analytical results and the published data for the room air change per hour, ACH. Further, it was noticed that the chimney width has a more significant effect on ACH compared to the chimney inlet size. The results showed that the absorber average temperature could be correlated to the intensity as: (T_w = 3.51I^0.461) with an accepted range of approximation error. In addition the average air exit velocity was found to vary with the intensity as (ν_ex = 0.013I^0.4).     

Effect of volumetric heat sources on hysteresis phenomena in natural and mixed-mode ventilation

The summer-time cooling efficiency of hybrid buildings depends critically upon exploiting multiple environmental resources to dispose of waste heat. To this end, many previous studies have explored the role of wind, which exerts different static pressures on a building's windward and leeward facades. Here, we consider how this methodology may be extended to the converse problem of winter-time heating wherein hot, buoyant air is purposefully supplied to the interior space using a coupled ventilation scheme. A “blocked” flow regime is desired such that cold air inflow is impeded; to avoid interstitial condensation, the pressure distribution within the building must favor outflow through designated extraction vents. For the idealized geometry considered here, blocked conditions represent a unique solution to the flow equations in well-defined regions of parameter space. The likelihood of blocking may be increased through prudent choice of extraction vent size/orientation depending on the external forcing conditions. A discussion of the inherent tradeoffs associated with multi-season design of hybrid buildings is also presented.     

A study of natural ventilation in a refuge floor

Under the Building Codes of Hong Kong Special Administration Region, the provision of refuge floors has been an indispensable element in high-rise building design since 1996. Wind-induced cross natural ventilation is an important design criterion of a refuge floor since it helps to prevent any smoke entering to become persistent state remained (logging) on the refuge floor. This paper reports a study of refuge floor natural ventilation induced by wind flow around a high-rise building with a refuge floor arriving from different wind incidence angles. The study is based on CFD simulations which are validated by wind tunnel measurements. The refuge floor under investigation has a main services core at the centre and support walls flush with the building walls along two opposite sides. The results reveal that at all wind angles, wind is able to enter the refuge space from the windward side and escape from the leeward side. At some wind angles, wind is found to re-enter the refuge space from the leeward opening and accumulate behind the main services core. Based on the results, we suggest that stairs connecting to the refuge space should be located inside the side corridors formed by the internal and external side walls.     

Solar chimney—A passive strategy for natural ventilation

Passive ventilation systems are being increasingly proposed as an alternate to mechanical ventilation systems because of their potential benefits in terms of operational cost, energy requirement and carbon dioxide emission. Solar chimney is an excellent passive ventilation system which relies on natural driving force, that is, the energy from the sun. A significant amount of research work has been done on solar chimney since the 1990s. This article presents an overview of solar chimney research that has taken place in the last two decades. The review focuses on two main areas of research - the effects of geometry and inclination angle on the ventilation performance of a solar chimney. The experimental investigations of solar chimney have dominated the existing literature. However, numerical modelling of solar chimney using computational fluid dynamics (CFD) technique has attracted increasing attention. Moreover, this review found that solar chimney as a passive ventilation strategy has not been fully understood.     

Contribution of natural ventilation in a double skin envelope to heating load reduction in winter

This study examined the contribution of a double skin envelope (DSE) to the heating energy savings brought about by natural ventilation in office buildings. A DSE was applied to the east- and west-facing walls on an actual three-floor building. Field measurements and computer simulations were performed in winter.     

Full-scale experiment research and theoretical study for fires in tunnels with roof openings

In order to assess the possibility of exhausting smoke through passive roof openings and the influence of smoke on personnel in the tunnels, full-scale fire experiments in tunnels with roof openings are carried out, which were rarely reported in the previous references. The data of smoke propagation, smoke sedimentation, velocity field and temperature field are measured. On the basis of the smoke longitudinal propagation laws, the prediction model of calculating backlayering distance is built. The Kurioka model and the built mathematical models are validated by those experiments. All the experimental data presented in this paper can be further applied for verification of numerical models, and bench-scale experimental results. Those full-scale experimental results and theoretical analysis can also be used for directing tunnel fire research, which afforded scientific gist for fire protection and construction of road tunnel with roof openings.     

A venturi-shaped roof for wind-induced natural ventilation of buildings: Wind tunnel and CFD evaluation of different design configurations

Wind tunnel experiments and Computational Fluid Dynamics (CFD) are used to analyse the flow conditions in a venturi-shaped roof, with focus on the underpressure in the narrowest roof section (contraction). This underpressure can be used to partly or completely drive the natural ventilation of the building zones. The wind tunnel experiments are performed in an atmospheric boundary layer wind tunnel at scale 1:100. The 3D CFD simulations are performed with steady RANS and the RNG k-? model. The purpose of this study is twofold: (1) to evaluate the accuracy of steady RANS and the RNG k-? model for this application and (2) to assess the magnitude of the underpressures generated with different design configurations of the venturi-shaped roof. The CFD simulations of mean wind speed and surface pressures inside the roof are generally in good agreement (10–20%) with the wind tunnel measurements. The study shows that for the configuration without guiding vanes, large negative pressure coefficients are obtained, down to −1.35, with reference to the free-stream wind speed at roof height. The comparison of design configurations with and without guiding vanes shows an – at least at first sight – counter-intuitive result: adding guiding vanes strongly decreases the absolute value of the underpressure. The reason is that the presence of the guiding vanes increases the flow resistance inside the roof and causes more wind to flow over and around the roof, and less wind through it (wind-blocking). As a result, the optimum configuration is the one without guiding vanes.     

Transient natural ventilation of a space with localised heating

The natural ventilation of a well mixed, pre-heated room with a point source of heating, and openings at the base and roof is investigated. The transient draining associated with the room being warmer than the exterior combined with the convective flow produced by the point source of heat leads to a fascinating series of transient flow regimes as the system evolves to the two-layer steady-state regime described by Linden, Lane-Serff and Smeed [1]. As the room begins to ventilate, a turbulent plume rises from the point source of heat to the ceiling, and typically forms a deepening layer of hot air. However, with a weak heat source, then at some point the ascending plume will intrude beneath the layer of original fluid. Otherwise, the ascending plume always reaches the top of the room as the system evolves to a steady state. We develop a simplified model of the transient evolution and test this with some new laboratory experiments. We conclude with a discussion of the implications of our results for real buildings.     

The effect of gradual changes in wind speed or heat load on natural ventilation in a thermally massive building

We examine the transitions in flow regime which can occur in naturally ventilated thermally massive buildings subject to changes in the wind and buoyancy forcing. For a range of heat loads there are both wind-dominated and buoyancy-dominated flow regimes. However, outside this range, only the steady state wind-dominated or buoyancy-dominated flow can develop. As a result of this non-linearity, and the different timescales for the evolution of the air and of the thermal mass, the transient evolution of the system caused by changes in either the heat load or the wind forcing can be complex. We develop a simplified model to identify the influence of the thermal mass on transitions in flow regime caused by changes in heat load or wind forcing. We show that the interior air responds rapidly to changes in the forcing, and as a result, the thermal mass can then act as a slowly evolving heat source or heat sink. In some situations this can lead to temporary buffering of the interior temperature, followed by a second, rapid transition in the interior temperature and ventilation regime as the system adjusts to the new steady state.     

Coupling of thermal mass and natural ventilation in buildings

The coupling of thermal mass and natural ventilation is important to passive building design. Thermal mass can be classified as external thermal mass and internal thermal mass. Due to great diurnal variation of ambient air temperature and solar radiation intensity, heat transfer through building envelopes, which is called external thermal mass, is a complex and unsteady process. Indoor furniture are internal thermal mass, affecting the indoor air temperature through the process of absorbing and releasing heat. In this paper, a heat balance model coupling the external and internal thermal mass, natural ventilation rate and indoor air temperature for naturally ventilated building is developed. In this model, the inner surface temperature of building envelopes is obtained based on the harmonic response method. The effect of external and internal thermal mass on indoor air temperature for six external walls is discussed of different configurations including lightweight and heavy structures with and without external/internal insulation. Based on this model, a simple tool is developed to estimate the indoor air temperature for certain external and internal thermal mass and to determine the internal thermal mass needed to maintain required indoor air temperature for certain external wall for naturally ventilated building.