Outdoor Airflow Research Articles

Analysis of wind-induced inflow and outflow through a single opening using LES & DES

In order to investigate induced flow properties through a single opening, large-eddy simulations (LES) are performed on ventilated air through the single opening of a room. In this flowfield, the outdoor air flows parallel to the wall and to the opening, and the room airflow is induced by the outdoor flow through the opening. Both the outdoor and indoor airflows are simultaneously simulated. Firstly, the dependency of the induced room airflow to the shape of the opening is investigated (by changing the shape of the opening). Secondly, the distributions of mean and turbulent variables at the boundary plane of the opening are investigated. It is clarified that the shape of the opening and the resultant mixing layer thickness developed along the opening boundary plane affect the characteristics of the air exchange rate and the feature of the room's airflow. In the case of a longer opening, the change in air exchange characteristics is apparent. Furthermore, detached-eddy simulation (DES) was performed on the same flowfields in order to examine the applicability of DES to building ventilation analysis. Two types of calculation (LES calculation and DES calculation with the same grid) were performed. It is concluded that DES over-estimates the sub-grid scale viscosity in the LES region, but the difference in room mean property between both cases is low.

Performance of a residential heat pump operating in the cooling mode with single faults imposed

The system behavior of a R410A residential unitary split heat pump operating in the cooling mode was investigated. Seven artificial faults were implemented: compressor/reversing valve leakage, improper outdoor air flow, improper indoor air flow, liquid line restriction, refrigerant undercharge, refrigerant overcharge, and presence of non-condensable gas in the refrigerant. This study monitored eight fault detection features and identified the most sensitive features for each fault. The effect of the various fault levels on energy efficiency ratio (EER) was also estimated. Since the studied system employed a thermostatic expansion valve (TXV) as an expansion device, it could adapt to some faults making the fault less detectable. The distinctiveness of the fault depended on the TXV status (fully open or not).

Energy saving strategies in air-conditioning for museums

In the museum environment a strict thermal-hygrometric control is necessary primarily for the correct artwork conservation and then for the visitor thermal comfort. Considering that the air-conditioning system has to operate constantly, suitable techniques permit to obtain useful energy savings, allowing, however, a good dynamic microclimatic control. In this paper a case study is presented about various strategies used to reduce energy requirements for HVAC systems in an exhibition room of a modern museum. Using the dynamic simulation code DOE 2.2 and typical climatic hourly data sets, the annual energy use for an all-air system has been calculated, as well as the savings obtainable using different techniques, such as dehumidification by adsorption (desiccant wheel – saving equal to 15% with respect to a base configuration), total energy recovery from the relief air (passive desiccant – 15%), outdoor airflow rate variation (demand control ventilation – 45%). Moreover, the correspondence has been analyzed between the energy request and the admitted variation of indoor temperature and relative humidity: changing the admitted indoor RH range from 50 ± 2% to 50 ± 10%, energy savings around 40% have been obtained. As regards the thermal-hygrometric performance, an optimal control of temperature has been guaranteed with all the configurations, while the best performance in RH control has been obtained with the desiccant system. Considering a simple payback analysis, if the artworks preserved in a museum are particularly sensitive to indoor humidity variation, a desiccant system should be properly used; on the contrary, when the indoor humidity control is not strongly needed, the use of a HVAC system with demand control ventilation is advisable, because of the lowest payback value. The system with total energy recovery presents intermediate features.

The life cycle costs of indoor climate systems in dwellings and offices taking into account system choice, airflow rate, health and productivity

The life cycle cost (LCC) of the heating, cooling and ventilation system was simulated for a typical cell office building, multi-family dwelling and detached house. Different ventilations systems were simulated resulting in different heating and cooling system designs. A health-related cost dependent on the airflow rate which was based on recent studies was added to the LCC for the dwellings as well as a productivity cost for the office building. An optimal airflow rate can be found depending on the assumed size of the health or productivity-related cost. The results for the office building indicate that a much higher outdoor supply airflow rate at presence, as well as a cooling system, is appropriate from a LCC perspective even if the influence on the human being still should be taken with precaution. With that higher airflow rate, the results show that there is a clear benefit with variable airflow rate ventilation systems. For dwellings, there is also a clear benefit from higher airflow rates even if the optimal airflow rate in the presented examples was not much higher than common requirements. Furthermore, it is not well known or defined how to valuate that possible influence. Despite this, this study is the demonstration of a method which can be used to optimize airflow rates in buildings if the influence and costs on human health related to the airflow rate is known.

Sick building syndrome in relation to air exchange rate, CO2, room temperature and relative air humidity in university computer classrooms: an experimental study

To study the effects of ventilation and temperature changes in computer classrooms on symptoms in students. Technical university students participated in a blinded study. Two classrooms had higher air exchange (4.1-5.2 ac/h); two others had lower (2.3-2.6 ac/h) air exchange. After 1 week, ventilation conditions were interchanged between the rooms. The students reported symptoms during the last hour, on a seven-step rating scale. Room temperature, relative air humidity (RH) carbon dioxide (CO(2)), PM10 and ultra-fine particles (UFP) were measured simultaneously (1 h). Illumination, air velocity, operative temperature, supply air temperature, formaldehyde, NO(2) and O(3) were measured. Multiple logistic regression was applied in cross-sectional analysis of the first answer (N = 355). Those participating twice (N = 121) were analysed longitudinally. Totally 31% were females, 2.9% smokers and 3.8% had asthma. Mean CO(2) was 993 ppm (674-1,450 ppm), temperature 22.7 degrees C (20-25 degrees C) and RH 24% (19-35%). Lower and higher air exchange rates corresponded to a personal outdoor airflow of 7 l/s*p and 10-13 L/s*P, respectively. Mean PM10 was 20 microg/m(3) at lower and 15 microg/m(3) at higher ventilation flow. Ocular, nasal and throat symptoms, breathlessness, headache and tiredness were significantly more common at higher CO(2) and temperature. After mutual adjustment, ocular (OR = 1.52 per 1 degrees C), nasal (OR = 1.62 per 1 degrees C) and throat symptoms (OR = 1.53 per 1 degrees C), headache (OR = 1.51 per 1 degrees C) and tiredness (OR = 1.54 per 1 degrees C) were significantly associated with temperature; headache was associated only with CO(2) (OR = 1.19 per 100 ppm CO(2)). Longitudinal analysis demonstrated that increased room temperature was related to tiredness (P < 0.05). Computer classrooms may have CO(2) above 1,000 ppm and temperatures above 22 degrees C. Increased temperature and CO(2) may affect mucosal membrane symptoms, headaches and tiredness. Room temperature was most important. CO(2) associations may partly be temperature effects.

Optimization of heat pump system in indoor swimming pool using particle swarm algorithm

When it comes to indoor swimming pool facilities, a large amount of energy is required to heat up low-temperature outdoor air before it is being introduced indoors to maintain indoor humidity. Since water is evaporated from the pool surface, the exhausted air contains more water and specific enthalpy. In response to this indoor air, heat pump is generally used in heat recovery for indoor swimming pools. To reduce the cost in energy consumption, this paper utilizes particle swarm algorithm to optimize the design of heat pump system. The optimized parameters include continuous parameters and discrete parameters. The former consists of outdoor air mass flow and heat conductance of heat exchangers; the latter comprises compressor type and boiler type. In a case study, life cycle energy cost is considered as an objective function. In this regard, the optimized outdoor air flow and the optimized design for heating system can be deduced by using particle swarm algorithm.

An Adaptive Demand-Controlled Ventilation Strategy with Zone Temperature Reset for Multi-Zone Air-Conditioning Systems

In air-conditioning systems serving multiple zones, conventional ventilation strategies often result in over-ventilation in some zones with under-ventilation in the other zones when the heating/cooling load and occupancy profiles of zones differ greatly. This article presents an adaptive demand-controlled ventilation (DCV) strategy for multi-zone air-conditioning systems, which employs real-time occupancy identification using the dynamic multi-zone ventilation equation and zone temperature set-point reset of critical zones. The strategy identifies the critical zones, and fully considers the outdoor air demand of critical zones and the unvitiated outdoor air in the re-circulated air from the other zones. The air temperature reset of critical zones results in the increase of their supply air fractions and therefore the amount of outdoor air delivered to the critical zones. Consequently the total outdoor airflow demand and the overall energy consumption are reduced while the ventilation of critical zones is satisfied. This adaptive DCV strategy is validated by comparing its energy and environmental performances with other typical ventilation strategies under various weather conditions.

Investigation of transient thermal environments

Human beings are commonly exposed to transient thermal environments in their daily life. But most of the studies on indoor thermal environment have been conducted under steady-state conditions. The aim of this paper is to summarize the investigation on human responses to transient thermal environment carried out by the indoor environment group at Tsinghua University, and to predict the possibility of using their research findings in practice. Human responses to transients have some special characteristics, which could be beneficial to environmental control. Air movement is especially effective at realizing a transient thermal environment, offsetting higher air temperature or operative temperature in warm climates. Based on the analysis of measured data, the characteristics of air movement outdoors are different from artificial air supply such as fans or air supply outlets, in the probability distribution of their velocities, turbulence intensity, and power spectrum. Based on subjective experiments, it is evident that artificial air movement, which is mainly simulated with outdoor airflow characteristics, has the highest occupant preference in warm conditions. Experimentally simulated air movement improves not only whole-body cooling, but also local cooling as from personal air supplies. Finally, it is important that introducing simulated natural air movement into the space in warm or hot conditions could significantly decrease the building's energy consumption.

Wind Tunnel Study for Estimating Outdoor Ventilation in a Dense Low-Rise Building Area

To accurately estimate the natural ventilation of outdoor spaces surrounded by low-rise buildings using a wind tunnel requires correct representation of the natural wind regime combined with appropriately scaled building models and testing method. Existing outdoor ventilation studies are largely based on wind speed and estimated air change rates. Wind speeds mainly influence: peoples’ comfort, safety in pedestrian areas, the heat transfer between outdoor surfaces and airflow, and evaporation from wet surfaces. Studies of air change rates in outdoor spaces (for regions below surrounding roof height) are mainly related to the dispersion and/or removal of pollutants, moisture and hot air. This paper considers the application of wind tunnel studies to estimate the ventilation of outdoor spaces. It also considers the role of CO2 as a tracer gas for such studies. It concludes that to accurately estimate the ventilation of outdoor areas by means of a wind tunnel analysis, the natural wind properties in the wind tunnel model must be correctly related to the local terrain category. Buildings themselves can be represented by low cost small scale models constructed from expanded polystyrene. The CO2 tracer gas method is a safe, low cost and efficient quantitative test method for estimating the ventilation of outdoor spaces using a wind tunnel.

The effect of perceived indoor air quality on productivity loss

This theoretical study reports on the impact of perceived indoor air quality for productivity loss in air-conditioned office buildings. A new derivation of productivity calculation model based on pollution loads and contaminant removal effectiveness is applied and the effect of the improved ventilation efficiency on productivity is estimated. The findings show that the proportion dissatisfied is a good predictor of productivity loss due to indoor air quality in different kinds of office work. It is possible to calculate the proportion dissatisfied from olf and decipol units. Productivity is possible to improve by increasing outdoor airflow rate, decreasing emissions and improving ventilation efficiency e.g. with displacement ventilation. In a case of one person per 10 m 2 (0.1 olf/m 2) and low-emitted material (0.1 olf/m 2), the total sensory pollution load is 0.2 olf/m 2. Normally, the minimum admissible outdoor airflow rate is 0.5–1.5 l/s per m 2 in office spaces. This means that 5–9% productivity loss should be accepted using the minimum airflow rate design method. With displacement ventilation, it is possible to improve indoor air quality in a manner that significantly increases productivity compared with traditional mixing system. The effect of the contaminant removal effectiveness on the productivity loss is about 0.5–2% between these systems using the same airflow rate.

Sensory pollution loads in six office buildings and a department store

Sensory pollution loads were measured in six non-smoking office buildings with mechanical ventilation without recirculation, and in a non-smoking department store with an air-conditioning system and recirculation. Untrained panels assessed the air quality on normal weekdays with occupants in the buildings, and in the case of office buildings, also on weekends without occupants present. On both occasions the ventilation system was in operation as on a normal working day. Outdoor airflow rate, air temperature, relative humidity and concentration of carbon dioxide were measured. The sensory pollution load from the building (without occupants) was found in offices to be 0.11 ± 0.09 olf/m 2 floor, which agrees well with the load recommended for low-polluting buildings in CEN CR 1752. This load is only half of the sensory pollution load found in previous investigations in offices and assembly halls where smoking was allowed. The load from building and merchandise in the department store was 0.15 olf/m 2 floor. A table is provided listing the mean sensory pollution load of the buildings measured in the present and in previous studies carried out in the period from 1988 to 2001 in different types of buildings in Europe. The table covers 120 buildings including offices and assembly halls (with and without previous smoking), schools, kindergartens and a department store.

Optimal and robust control of outdoor ventilation airflow rate for improving energy efficiency and IAQ

Combining demand-controlled ventilation (DCV) and economizer control can achieve adequate and even better indoor air quality with minimum cooling/heating energy consumption in buildings. The control instability during the transition processes between different control modes is among the major difficulties faced when combining DCV control with economizer control in applications. The practical energy benefit is another concern when using free cooling in the subtropical climate like Hong Kong. A robust control strategy, using “freezing”, gain scheduling, I-term reset and feedback transition control for different transition processes, is developed addressing the instability problems. The energy benefit of using economizer control is evaluated by over one year's comparison tests on two air-handling units in a building.

Measurement and simulation of indoor air quality and energy consumption in two Shanghai office buildings with variable air volume systems

Two modern office buildings in Shanghai with typical variable air volume (VAV) systems were selected for research. Four separate spaces on a standard office floor in each building facing different directions (north, south, east, and west) were selected for thorough site measurements of outdoor airflow rates and indoor air quality (concentrations of CO 2 and PM 10) during typical days of the four seasons (summer, autumn, winter, spring). Computer simulations and calculations were also done of outdoor airflow rates and CO 2 concentrations in the four-test spaces on an hourly basis for the entire year. In addition to the site measurements, monthly electrical consumption for the two buildings was recorded or estimated. Simulations and calculations were performed of the buildings’ energy consumption and energy cost using two different outdoor air control strategies of a typical VAV system as well as a fan coil unit (FCU) system. The site-recorded data, or estimated data, and simulation results are compared and analyzed. The study reveals that in a VAV system, the outdoor airflow rate distributed to each zone varies greatly, especially during part-load hours, making it difficult to always ensure sufficient outdoor air in each zone and avoid indoor air quality (IAQ) problems. However, this problem can be prevented by using appropriate outdoor air control strategies—e.g. a fixed high level total outdoor airflow rate.

Indirect evaporative cooling and economy cycle in summer air conditioning

The air-side economy cycle has a large application in warm and dry climates, where the simple increase in the amount of outside air supplied to the conditioned space can substantially reduce the cooling load. Although the dry-bulb economy cycle is the simplest implementation, requiring only a dry-bulb temperature comparison to operate the air flow rate regulation, the greatest potential in cost reduction is achieved by the wet-bulb economizer (WBE). Indirect evaporative cooling (IEC), based on energy recovery from the saturated exhaust airflow, is another technique to be applied in the reduction of a building cooling load. Preconditioning the outdoor airflow by IEC actually extends the applicability of the WBE. In such a way, specific synergies can be exploited when WBE and IEC are combined, even in humid climates. On the other hand, the largest benefits can be accomplished only by redefining the control strategy of the outside air flow rate. In the present paper, the outside air flow rate control strategies are described both for simple WBE and for preconditioned WBE cycles. Different regulation regions are defined on the psychrometric chart together with the relative control strategies. The incidence of these regions is evaluated for 14 different European and American climates. Finally, the seasonal cooling energy requirements are determined for the dehumidification by cooling process, which can be considered as mainly responsible for cooling energy costs in the considered localities. Traditional, preconditioned by heat recovery and IEC on the exhaust airflow, WBE and PWBE air conditioning cycles are compared. Copyright © 2003 John Wiley & Sons, Ltd.

Fault-tolerant control for outdoor ventilation air flow rate in buildings based on neural network

This paper describes a supervisory control scheme that adapts to the presence of the measurement faults in outdoor air flow rate control using sensor-based demand-controlled ventilation, maintains an adequate indoor air quality and minimizes the resulting increase in energy consumption. A strategy, which is based on neural network models, is employed to diagnose the measurement faults of outdoor and supply flow sensor, and accomplishes the fault-tolerant control of outdoor air flow when faults occur. The neural network models are trained using the data collected under various normal conditions. The residuals between the measurements of flow sensors and the outputs of the neural network models are used to diagnose the faults. When the fault of outdoor or supply air flow sensor occurs, the recovered estimate of outdoor or supply air flow rate obtained on the basis of the neural network models is used in the feedback control loop to regain the control of outdoor air flow. Tests using dynamic system simulation are conducted to validate the strategy. The control, IAQ and energy performances of the system under fault-tolerant control strategy in the presence of the faults in air flow sensor are also presented.

Influence of clearances on the energy performance of window-type air-conditioners at the same level outside residential buildings

Due to the hot and humid weather, the majority of residential units in Hong Kong are equipped with window-type air-conditioners. However, the provisions made in the building envelope design for their installation are often inadequate, particularly in the clearances for intake and disposal of outdoor air for condenser cooling. Consequently, the air-conditioners would consume an unnecessarily high amount of electricity and at the same time output less cooling. The paper describes a study on the outdoor airflow around air-conditioners in residential buildings and the related thermal phenomena. The CFD code FLUENT was employed to predict the airflow patterns in 20 installation configurations of window-type air-conditioners, which embrace different shapes of clearances around the air-conditioners that are on the same story in a building. The results reveal that insufficient clearance spaces could cause a significant increase in the temperature of the outdoor air entering the air-conditioners, which would also be accompanied by a reduction in the airflow rate. The increase in the electricity consumption of the air-conditioners could exceed 45% for the case where the air-conditioners were drawing in the discharged hot condenser air from other air conditioners located at an opposite wall for condenser cooling. The presence of a sidewall, however, could cause the discharged hot air to move quicker away from the air-conditioner leading to a lower entering air temperature to its condenser.

Simple and Cheap Air Change Rate Measurement Using CO2 Concentration Decays

Buildings are aired to evacuate indoor pollutants, in particular those produced by the occupants. CO2 is a good indicator of occupancy, as long as there is no other significant source. This is the case in most buildings. Cheap, portable analysers and loggers that allow easy recording of the CO2 concentration in a room or in the exhaust duct of a ventilation system are available on the market. The peak value of CO2 concentration during occupancy is an indicator of the minimum air flow rate per person. Analysis of the decays observed when the occupants leave the building provides the nominal time constant of the ventilated space, which is directly dependent on the outdoor air flow rate from the ventilation system and infiltration. Depending on the state of the ventilation system during the decay, this method provides either the total outdoor air flow rate provided by the system, or the infiltration rate. When combined with a simple pressure differential measurement, this method can also be used to check the air tightness of building envelopes. A user-friendly computer program helps with the interpretation of the records. This paper describes the method, its application to several rooms and buildings, and its validation by comparison with SF6 tracer gas measurements.

自動扉の開閉特性と外気侵入量に関する研究 : 第2報 各種扉の通行時有効開口面積特性

自動扉の開閉特性と外気侵入量に関する研究 : 第2報 各種扉の通行時有効開口面積特性

A critical review on the performance and design of combined cooled ceiling and displacement ventilation systems

This paper reviews the studies and design of cooled ceiling and displacement ventilation (CC/DV) systems in buildings. If properly designed, the combined CC/DV systems can provide better indoor air quality and thermal comfort level compared to the widely used variable air volume (VAV) mixing systems. The cooling load removed by DV is a key design parameter. A low DV load has a positive effect on thermal comfort due to a small vertical temperature gradient, yet also has a negative effect on indoor air quality due to the increased mixing of room air. The impact of the room height on the temperature and contaminant concentration profiles is negligible in the occupied zone. The CC/DV systems are more effective in removing active contaminants (as indicated by CO 2) than passive contaminants (e.g. VOCs). The condensation risk on the chilled ceiling panel is high because of the high humidity ratio in the region close to the panel. To prevent condensation on the panel, it is important to properly control the system for transient regimes, such as startup and shutdown periods, and to minimize infiltration of humid outdoor air. Whether a CC/DV system may or may not reduce energy consumption depends on the supply air temperature, outdoor airflow rate, and cooling load. Therefore, it is necessary to develop design guidelines for CC/DV systems for US buildings because the climate, building layout, and cooling load can be different from those studied elsewhere.

Sign of eye irritation in female hospital workers and the indoor environment.

The relationships between indoor environment factors and signs of eye irritation were studied in a sample of female personnel with (n = 112) and without (n = 64) mucous membrane symptoms working in 36 nursing departments at geriatric hospitals in the municipality of Trondheim, Norway. The indoor climate was characterised by high room temperature (interquartile range 23.0-23.7 degrees C), low relative air humidity (interquartile range 17-26%) and high outdoor airflow rate, as indicated by low indoor levels of carbon dioxide (interquartile range 490-650 ppm). An altered microbial flora was observed in nine of the departments, and Aspergillus fumigatus was found in three of these. Five of the hospitals were situated in urban parts of the municipality near roads with heavy traffic. In the initial analyses decreased tear-film stability (BUT) was observed in subjects working in departments with increased dust settlement rate (P = 0.03), in hospitals situated in urban areas with heavy traffic (P < 0.001), and in subjects working in departments with presence of A. fumigatus (P = 0.04). Increased conjunctival staining (CS) was related to alteration of the microbial flora. In the final multivariate analyses statistically significant relationships were observed between BUT and dust settlement rate and urban vicinity of workplace and between CS and alteration of the microbial flora.