Stratification and oscillations produced by pre-cooling during transient natural ventilation

This paper investigates the transient natural ventilation of a warm room which vents to a cool exterior through a stack connected to the roof of the room, and into which air is drawn through a lower vent connected to a cooling unit. The temperature differences between the interior and exterior create positive buoyancy driving the ventilation. This results in fresh exterior air being drawn into the room through the cooler connected to the lower vent. After being cooled, this fresh air enters the room and displaces the existing warm air upwards and out through the stack. A sharp interface then develops between a lower layer of cooled invading air and an upper layer of warm original air. This interface ascends as the room continues to drain. However, the pre-cooled air is negatively buoyant, and so the flow rate and the rate of ascent of the interface gradually fall as the cold lower layer deepens. As a result, the temperature of the pre-cooled air progressively decreases towards that of the chiller, and the lower layer becomes stably stratified. Eventually, the ventilation ceases and the interface stops ascending when the positive buoyancy associated with warm air in the upper layer and stack and the negative buoyancy associated with cooled air in the lower layer are in balance. However, this equilibrium is unstable to downward motion. Colder, less buoyant air from the exterior in time displaces warm buoyant air in the stack, reducing the positive buoyancy of the upper layer and causing it to be outweighed by the negative buoyancy of the lower layer. A downward draining flow then commences. As the exterior air descends through the stack, it mixes with warm air in the upper layer, leading to the temperature of the upper layer evolving towards that of the ambient. As a result, the flow rate and the rate of descent of the interface gradually fall as the upper layer deepens and the colder lower layer drains from the room through the lower vent. Eventually, the flow ceases again when the positive buoyancy of the upper layer balances the negative buoyancy of the lower layer, but now the interface is arrested closer to the base of the room. This equilibrium is unstable to upward motion, and in time warm air from the upper layer displaces the exterior air in the stack. This causes the positive buoyancy to increase, and consequently the room drains upwards once more. In this way the pre-cooling makes the system oscillate between the upward and downward draining regimes, which ensue until the room is completely drained. The ventilation eventually stops altogether when the interface reaches the base of the room and the upper layer attains the temperature of the ambient. A simple model is developed to describe this transient oscillatory process and is compared with small-scale analogue experiments. This paper discusses the application of the model, and shows how pre-cooled draining may be employed appropriately to provide thermal comfort in an intermittently occupied room, when the exterior air is uncomfortably warm so that conventional flushing without pre-cooling may not be used effectively.     

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.     

Transient buoyancy-driven ventilation: Part 1. Modelling advection

The unsteady development of the vertical temperature profile in a ventilated space containing a heat source is modelled. The buoyant fluid released from the heat source is modelled as a turbulent buoyant plume, using a standard integral plume model with a fixed entrainment coefficient. Two types of natural ventilation flow are considered, with the flow driven entirely by the density contrast between the fluid inside and outside the space (stack effect). The ventilation types are (a) classic displacement ventilation, with outflow of warm air through upper openings and inflow of cool air through lower openings; and (b) doorway ventilation, with an exchange flow through the doorway. An improved version of the doorway exchange flow model is given as compared to previous studies. The boundaries of the space are considered to be perfectly insulating, so that heat is transported entirely by the fluid motion. The temporal stratification that develops within the space (outside the plume) is calculated using a modified filling-box model, with successive layers added to the top of the space over time. Laboratory experiments giving reduced-scale simulations of the flows were also conducted, where saline solution and fresh water are used to model fluid of different density. The developing density profiles in the laboratory experiments compare very well with the model predictions. The use of this type of model, capturing the main physical flow features, allows rapid and accurate calculations of transient stratification in ventilated spaces.     

Interzonal air and moisture transport through large horizontal openings in a full-scale two-story test-hut: Part 2 – CFD study

The aim of this paper is to study the air and moisture transport through a large horizontal opening in a full-scale two-story test-hut with mixed ventilation by means of computational fluid dynamics (CFD) simulations. CFD allows extending the experimental study presented in the companion paper [1] and overcoming some limitations of experimental data. More than 80 cases were simulated for conditions similar to those tested experimentally and for additional ventilation rates and temperature difference between the two rooms. CFD simulations were performed in Airpak and the indoor zero-equation turbulence model was used. The CFD model was extensively validated with the distributions of air speed, temperature and humidity ratio measured across the two rooms, as well as with the measured interzonal mass airflows through the horizontal opening. CFD simulation results show that temperature difference between the two rooms and ventilation rate strongly influence the interzonal mass airflows through the opening when the upper room is colder than the lower room, while warm convective air currents from the baseboard heater and from the moisture source placed in the lower room cause upward mass airflows when the upper room is warmer than the lower room. Finally, empirical relationships between the upward mass airflow and the temperature difference between the two rooms are developed.     

Natural ventilation of multiple storey buildings: The use of stacks for secondary ventilation

The natural ventilation of buildings may be enhanced by the use of stacks. As well as increasing the buoyancy pressure available to drive a flow, the stacks may also be used to drive ventilation in floors where there is little heat load. This is achieved by connecting the floor with a relatively low heat load to a floor with a higher heat load through a common stack. The warm air expelled from the warmer space into the stack thereby drives a flow through the floor with no heat load. This principle of ventilation has been adopted in the basement archive library of the new SSEES building at UCL. In this paper a series of laboratory experiments and supporting quantitative models are used to investigate such secondary ventilation of a low level floor driven by a heat source in a higher level floor. The magnitude of the secondary ventilation within the lower floor is shown to increase with the ratio of the size of the openings on the lower to the upper floor and also the height of the stack. The results also indicate that the secondary ventilation leads to a reduction in the magnitude of the ventilation through the upper floor, especially if the lower floor has a large inlet area.     

Assessment of pre-heating air through a double window system on different building location and weather condition

Double windows are a currently adopted construction system in Portuguese dwellings. Simple changes enable this construction system to pre-heat the ventilation air. The air coming from the outside circulates goes upward through the channel between windows where it is pre-heated before entering the building. Heat that escapes from inside through the inner window and solar radiation, heats up the air between the two windows. This paper presents the performance of such passive system and focuses on the design aspects of this system and its building integration in function of weather conditions. One type of building is used where the window is located on different facades, thus, different orientations. Four different weather conditions are chosen, from mild to cold winters. The methodology used is based on previous validated parametric studies. Results indicate that the ventilated double window system offers an alternative to cold natural ventilation in any cold region and any facade orientation. The colder the location, the higher the pre-heating of the incoming air. This study expects to help designers to conceive ventilated double windows duly adapted to local climate where natural ventilation is an important design consideration and where energy consumption must be reduced.     

Experimental work on a linked,dynamic and ventilated,wall component

In a conventional construction, the ventilation air enters the building through a combination of pathways, such as windows and vents, via cracks around external openings, joints between building materials, etc. By contrast, in a dynamic insulation construction, the ventilation air is drawn into the building through the insulation material of the building envelope. Structures with dynamic insulation incorporate porous insulating material in the envelope and air passes through the porous material.The aim of this paper is to present the experimental work carried out on a wall component that is a combination of dynamic insulation panel and a ventilated façade, tested under real weather conditions. The wall component consisted of two main sub-layers: the ventilated external envelope and the dynamic insulation (DI) sub-layer. The DI component consisted of layers of breathing materials that let the air enter the room at a reasonable pressure difference between the interior and exterior.The paper describes the experimental work carried out and the results drawn from the tests performed in the linked wall component under: (i) various differential pressure regimes between the interior and the exterior of the test cell and (ii) controlled and floating internal air temperature. In summary, the performance of the component was effective, since conduction losses were decreasing.     

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.     

The fluid mechanics of the natural ventilation of a narrow-cavity double-skin facade

This paper investigates the natural ventilation of a double-skin facade connected to a room in a multi-storey building. The room and the facade are connected to the exterior through vents at different levels. The room contains a horizontally distributed heat source analogous to occupants in an open-plan office or an underfloor heating system. The facade cavity contains a vertically distributed heat source analogous to a shading blind/louvers heated by solar radiation. These two sources of heat combine to provide buoyancy driving the ventilation. Two basic modes of facade operation are proposed and investigated. These two modes of operation should be alternated according to exterior climatic conditions. In colder seasons, the room draws air from the portion of the facade which extends one floor below the room, and solar radiation on the facade preheats supply air into the room. In warmer seasons, the room vents to the exterior through the portion of the facade which extends one floor above the room, and solar radiation on the facade enhances the ventilation and prevents overheating in the system. A quantitative model is developed to describe the fluid mechanics of the ventilation in these two modes of operation. The model is successfully tested with laboratory experiments. It shows how the height of the facade and the size of the openings can be adjusted to maximise the preheating of the room in colder seasons, and to prevent overheating in the room and the facade in warmer seasons. The model is used to explore the principles for design and control in different climatic and occupancy conditions.     

气循环玻璃幕墙应用技术

气循环玻璃幕墙作为一种双层的幕墙结构,是在普通明框或隐框的幕墙外再增加一层玻璃幕墙,通过幕墙通风设备的开关可使双层幕墙中间进入或逸出空气,开窗后房间可进行自然通风,幕墙中间的遮阳装置可减少气候对室内环境的影响,并且不妨碍玻璃幕墙的外观效果.结合工程实例,指出不同结构选型的气循环玻璃幕墙在实践应用中的技术参数和施工要点.     

Vertical profiles of temperature and contaminant concentration in rooms ventilated by displacement with heat loss through room envelopes

The purpose of this study is to examine the effect of heat loss through walls upon the gradients of temperature and contaminant concentration in room with displacement ventilation. It is known that conduction heat loss is governed by outside temperature, heat load inside the room, supply air temperature and overall heat transfer coefficient of walls. Experiments were conducted to measure the temperature gradient and the ventilation efficiency in the room ventilated by displacement ventilation with various combinations of heat load and temperature difference between supply air and outside air. In order to simulate the change of seasons, the supply air temperature was changed instead of the outside air temperature. The effect of supply air temperature and heat generation inside the room on the temperature gradient and the concentration of tracer gas were investigated through the experiments. As a result, it turned out that the higher the heat generation rate and the lower the supply temperature, the stronger the temperature stratification and the lower the concentration in the lower zone. Additionally, ventilation heat loss turned out to be a good index for assessing the concentration in the lower zone. Temperature differences of around 3 degrees C between supply air temperature and exhaust temperature are at least needed for displacement ventilation under the conditions of the experiment presented in this paper.     

Indoor Air Flow and Pollutant Removal in a Room with Task Ventilation

We studied the performance of a task ventilation system that permits occupants to control the flow rate and direction of air supplied to their work space through four floor-mounted supply grill. Air exited the room through a ceding-mounted return grill. To study air-flow patterns, we measured the age of air at multiple locations using a tracer gas stepup. To study the intra-room transport of tobacco smoke particles, cigarettes were smoked mechanically in one workstation and particle concentrations were measured at multiple locations. Our major findings were as follows: (1) deviations from uniform age of air, and uniform particle concentration, were generally less than 30 percent; (2)some supply air short-circuits to the return grill when directed toward the return with high velocity; (3) low supply velocities resulted in a floor-to-ceiling displacement flow; (4) directing the supply air toward the occupant, of partially toward the occupant, typically yielded an age of air at the occupant's breathing level that was 15 to 25 percent lower than the age at other breathing-level locations; (5) with low supply velocities und air directed toward the occupants, tobacco smoke particle concentrations in a ventilated non-smoking workstation were 50 percent of the chamber-average concentration.     

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.     

Roof pond cooling of buildings in hot arid climates

The weather in Baghdad, Iraq is hot dry in the summer while water is plentiful and cheap. These are conditions which encourage the use of evaporative cooling. A building with one space in it was used to test the effect of a roof pond which was ventilated mechanically for summer cooling. Thermal measurements were taken for the room in normal conditions without a pond, with a lone pond and no mechanical ventilation, and then finally with mechanically forced ventilation. The results showed a marked improvement in the space temperature with a significant reduction during the peak time outside temperatures at 3 O’clock reaching 6.0 °C between the room without the pond and with a ventilated one and 6.5° at 18:00 during peak inside temperatures. The study also showed that improvements would be better in real-life conditions where exterior wall area is less than the test room.     

Airflow and air temperature distribution in the occupied region of an underfloor ventilation system

Measurements were carried out in an office-type experimental room ventilated by a floor return-type underfloor ventilation system to investigate the distributions of airflow velocity and air temperature. A fan-powered floor air unit (FAU) with rectangular supply and return air outlets covered by straight-profile linear bar-type air diffusers was installed to deliver the conditioned air in the experimental room. Turbulence intensity and draught rate distributions inside the room were also calculated by using the measured data. From the experimental results, it is found that undesirable high air velocities and high draught rates were created within a small region near the supply outlet of the FAU. Temperature differences between different height levels were maintained within an acceptable comfort level under the tested supply air conditions and heat loads. The results indicated that the temperature stratification could be maintained at an acceptable comfort level by designing the supply air conditions properly. A clearance zone is suggested as a design consideration for locating the FAUs and occupants to avoid undesirable draught discomfort to the occupants.     

On the natural ventilation of two independently heated spaces connected by a low-level opening

We study the buoyancy-driven natural ventilation of two spaces which are connected to one another by a low-level opening, and each of which is connected to the exterior through a high-level vent. Each space is heated uniformly by an independent source, which provides buoyancy driving the ventilation. Using laboratory experiments, we show that these conditions lead to each space becoming well mixed at steady state. In this regime, a net flow from one space to the other is driven by the buoyancy created in the downstream space. Although it is possible in theory for the flow to develop in either direction, our new experiments and theoretical model show that, in reality, if the vents of the two spaces are at the same height, then the actual flow regime will depend primarily on the relative strength of the heat loads. If the two heat loads are sufficiently different, only the flow from the weakly heated space to the strongly heated one is stable. If the two heat loads are comparable, both modes are stable, leading to multiple flow regimes. The problem is generalised to show that, if the heights of the vents are equal, then the flow regime will depend on the relative height of the vents, as well as the relative strength of the heat loads. There is a range of combinations of vent heights and heat loads that still allow multiple flow regimes. We identify the limits of each regime and outline principles for control.     

Experimental study based on large-scale smoke propagation fire tests through a horizontal opening connecting two mechanically ventilated compartments

This work describes an experimental study of the flow through a horizontal opening (also referred to as a vent), applicable to specific situations typically encountered in nuclear installations. The configuration consisted of two rooms, which were mechanically ventilated and connected to each other by a horizontal opening, the fire being located in the lower room. The flow was governed by buoyancy due to the heat release from the fire, inertia resulting from the mechanical ventilation, and local momentum from the ceiling jet. Two flow regimes (bi-directional and uni-directional) were encountered depending on the fire power and the ventilation set-up. This study presents 17 large-scale fire tests, investigating the behaviour of the flow at the horizontal opening according to several fire scenario parameters: the fire heat release rate, the fire location, the ventilation configuration and the ventilation flow rate. This range of parameters enabled us to focus on different flow regimes, from pure natural convection (bi-directional) to forced convection (uni-directional). The new set of data obtained, based on detailed flow measurements, offers new insights for understanding the flow and developing sub-models to be used in zone codes.     

Modeling ventilation and radon in new Dutch dwellings

Indoor radon concentrations were estimated for various ventilation conditions, the differences being mainly related to the airtightness of the dwelling and the ventilation behavior of its occupants. The estimations were aimed at describing the variation in air change rates and radon concentrations to be expected in the representative newly built Dutch dwellings and identifying the most important parameters determining air change rate and indoor radon concentration. The model estimations were compared with measurements. Most of the air was predicted to enter the model dwelling through leaks in the building shell, independent of the ventilation conditions of the dwelling. Opening the air inlets was shown to be an efficient way to increase infiltration and thus to decrease radon concentration. The effect of increasing the mechanical ventilation rate was considerably less than opening the air inlets. The mechanical ventilation sets the lower limit to the air change rate of the dwelling, and is effective in reducing the radon concentration when natural infiltration is low. Opening inside doors proved to be effective in preventing peak concentrations in poorly ventilated rooms. As the airtightness of newly built dwellings is still being improved, higher radon concentrations are to be expected in the near future and the effect of occupant behavior on indoor radon concentrations is likely to increase. According to the model estimations soil-borne radon played a moderate role, which is in line with measurements.     

Simulation and performance enhancement of the air cooling system in the valve halls of a DC/AC power converter station

The present article reports on a ventilation system that uses impinging air jets to remove a portion of the heat generated in DC/AC converter towers. Each tower is a vertical stack of electronic components used in the power conversion. Airflow from the ventilation system enters the room through inlet ports located on the ground at two opposite sides of the DC/AC converter towers. The existing ventilation system circulates sufficient airflow in terms of the total heating load; however, elevated temperatures were reported within the towers due to poor air circulation. A numerical study has been conducted to investigate steady three-dimensional (3-D) turbulent mixed (combined free and forced) convection air cooling of vertical stacks of heat-generating blocks simulating typical towers in two valve halls of a DC/AC power converter station. The simulation results include the magnitudes of the net airflows for all the inter-block gaps, the maximum temperature in each gap, and the flow structure represented by streamlines at various locations of the 3-D domain. The location of the inlet, the inlet size and aspect ratio, and the location on the tower at which the airflow was aimed were varied parametrically to improve the ventilation relative to the existing design. These results demonstrate that, for fixed inlet mass flow rate, all the towers’ heat generating surfaces can be kept under 60°C via simple modifications of the variables used in the parametric study.     

A CFD-based test method for control of indoor environment and space ventilation

System dynamic simulation has been adopted to test and evaluate the local and supervisory control of air-conditioning systems for over twenty years, while the modeling of the space ventilation was usually simulated using perfect mixing models. However, the complete-mixing air model fails to consider the impact of non-uniform air temperature stratifications. This paper presents a CFD-based virtual test method for control and optimization of indoor environment by combining a ventilated room with a ventilation control system. The ventilated room and its dynamic ventilation control system are represented by a computational fluid dynamics (CFD) model and models of the temperature sensor, PID controller and actuator and VAV damper model respectively. The ventilation and its control system are programmed using the user defined function program and interfaced with the CFD model. A space temperature offset model is developed to improve the accuracy of temperature measurement and control at the occupied zone as a virtual sensor. Case studies show that the ventilation control models can interoperate with the CFD simulation of the space online which presents a new application approach of CFD simulation for testing and developing control and optimal control strategy before a system is constructed practically. The use of the virtual sensor can effectively compensate the effect of non-uniform stratification on the temperature control and improve system control reliability in a mechanical ventilated room.