Local Mean Age Of Air Research Articles

Theoretical and experimental investigation of impinging jet ventilation and comparison with wall displacement ventilation

This paper focuses on evaluating the performance of a new impinging jet ventilation system and compares its performance with a wall displacement ventilation system. Experimental data for an impinging jet in a room are presented and non-dimensional expressions for the decay of maximum velocity over the floor are derived. In addition, the ventilation efficiency, local mean age of air and other characteristic parameters were experimentally and numerically obtained for a mock-up classroom ventilated with the two systems. The internal heat loads from 25 person-simulators and lighting were used in the measurements and simulations to provide a severe test for the two types of ventilation systems. In addition to a large number of experimental data CFD simulations were used to study certain parameters in more detail. The results presented here are part of a larger research programme to develop alternative and efficient systems for room ventilation.

Effect of recirculation on the local mean age of air

When it is required to predict the local mean age of air, the calculations are often carried out assuming that there is no recirculation, that is that all of the supply air is fresh. However, local mean age values will be greater than in the fresh air case when some of the exhaust air is recirculated. This paper examines and discusses the effect of recirculation and shows that, for many systems, the increase in local mean age due to recirculation can be calculated by means of an elementary analysis that leads to a simple algebraic expression. The paper also shows how the zonal model may be adapted for the general case of multiple recirculation paths. Compatibility between the elementary approach and the general zonal method is demonstrated.

Effective depth of fresh air distribution in rooms with single-sided natural ventilation

This paper introduces the effective depth of fresh air distribution in rooms with single-sided natural ventilation. A numerical method for the determination of the effective depth is described. The numerical method is based on the CFD technique involving the governing equations for air flow and the transport equation for the local mean age of air. The renormalisation group k− ε model of turbulence is used with equations for the conservation of mass, momentum and energy to predict turbulent buoyancy-induced room air movement. The air flow through a large opening is derived from the Bernoulli theory. The predicted air flow pattern, air temperature and local mean age of air are used to determine the effective depth of fresh air distribution in a naturally ventilated room with a window opening for summer cooling. It is shown that the effective depth for thermal comfort may not coincide with that for air quality and in summer, the requirement for thermal comfort is the limiting factor to the effective room depth. The effects of window opening levels and room heat gains on the air flow rate and effective depth are investigated.

Local mean age of air: Predictive techniques compared

Local mean age, which is a very useful indicator of air change efficiency, may be predicted at design stage by post-processing the results of a computational fluid dynamics (CFD) analysis that has been used to establish the internal air movement characteristics. The local mean age may be determined by a number of procedures. In this paper the use of the zonal method, which is very efficient in terms of computer power, is investigated and compared with alternatives based on the age of air transportation equation and the measurement of contaminant concentration. It is concluded that the effect of turbulent diffusion on air distribution patterns is very slight, with the implication that the zonal method provides an extremely efficient means of assessing the air change efficiency within ventilated buildings. It is also concluded that the type and locations of objects within a space are as important as the method of introducing the air supply in determining the room airflow patterns and local air change efficiency.

Studies of ventilation efficiency in a protective isolation room by the use of a scale model.

To assess the ability of a protective isolation room ventilation system to reduce patient exposure to airborne infectious agents, using a small-scale model that permits cost-effective and unobtrusive study of relevant indices of performance. A one-half scale model of a protective isolation room at the University of Minnesota Hospital was constructed and equipped for tracer gas experiments to assess ventilation efficiency. Tracer gas (SF6) was injected into the model supply air. Tracer gas concentration was recorded over time and analyzed to determine local and room mean age of air. Age of air is a direct measurement of ventilation efficiency and can be used to predict patient exposure to contamination. Although for the room taken as a whole, ventilation efficiency was close to 50% (a value corresponding to perfect mixing), the experimental results for the local mean age of air indicate that some parts of the model were ventilated much better than others. Room air exchange rate is only one parameter useful in assessing ventilation in patient areas. Effective distribution of ventilation air also is critical to the control of airborne contamination. Areas of the room with poor ventilation would be expected to have higher concentrations of airborne infectious agents and other contaminants. Patient exposure can be minimized by placing the patient in well-ventilated areas of the room. Improved ventilation designs may reduce patient exposure further without increasing actual airflow rate.

Studies of Ventilation Efficiency in a Protective Isolation Room by the Use of a Scale Model

To assess the ability of a protective isolation room ventilation system to reduce patient exposure to airborne infectious agents, using a small-scale model that permits cost-effective and unobtrusive study of relevant indices of performance.A one-half scale model of a protective isolation room at the University of Minnesota Hospital was constructed and equipped for tracer gas experiments to assess ventilation efficiency.Tracer gas (SF6) was injected into the model supply air. Tracer gas concentration was recorded over time and analyzed to determine local and room mean age of air. Age of air is a direct measurement of ventilation efficiency and can be used to predict patient exposure to contamination.Although for the room taken as a whole, ventilation efficiency was close to 50% (a value corresponding to perfect mixing), the experimental results for the local mean age of air indicate that some parts of the model were ventilated much better than others.Room air exchange rate is only one parameter useful in assessing ventilation in patient areas. Effective distribution of ventilation air also is critical to the control of airborne contamination. Areas of the room with poor ventilation would be expected to have higher concentrations of airborne infectious agents and other contaminants. Patient exposure can be minimized by placing the patient in well-ventilated areas of the room. Improved ventilation designs may reduce patient exposure further without increasing actual airflow rate.

集中換気システムを設置した木造独立住宅の換気効率に関する実測調査 : 住宅の換気システムに関する研究 その1

Performance of a ventilation system used to be evaluated only by air change rate. But now it is possible to evaluate it by ventilation effectiveness, which is determined by such conditions as the distribution of tracer-gas concentration and an air flow in a room. This paper describes the results we got from the field measurement of air change effectiveness (i.e. local mean age of air and local air change index). The measurement was carried out on three wooden detached houses equipped with different kinds of ventilation system. The results are as follows: (1) The value of local mean age of air (τ_<P1>), based on the definition of AIVC and measured by the step-up tracer-gas method, is twice to fifth times as large as the value got through the step-down tracer-gas method, because of the infiltration of the houses. (2) The value of local mean age of air (τ_<P2>), based on the definition of Sandberg, is nearly the same through both the step-up and step-down methods. (3) The spatial distribution of local mean age of air (τ_P) and local air change effectiveness is very large when a circulating fan is not operating, but when a fan is working the distribution of τ_P and ε_P is relatively small.

Single-sided natural ventilation—How deep an office?†

Renewed interest is being shown in designing for natural ventilation, as a means of avoiding the need for air conditioning. This has come about as a result of concern about local and global environmental issues related to air conditioned buildings. On the local level there is, for example, the risk of health-related problems such as 'sick building syndrome'. Global issues concern the depletion of the ozone layer and the atmospheric greenhouse effect. It is generally acknowledged that, in operation, ozone depleting gases are released from refrigeration plant of most air conditioned buildings; they consume more primary energy, with an associated greater production of carbon dioxide. To provide adequate ventilation for cooling in summer, traditional guidance recommends cross-ventilation for naturally ventilated offices deeper than 6 m, but this is often not practicable in many modern designs for office buildings. There is little guidance on designing for single-sided ventilation, which may be an option in some cases. This report describes tracer gas measurements of the local mean age of air at different locations within an office room 10 m deep, ventilated through single-sided ventilation. These results show that fresh air is generally distributed over the whole depth, suggesting that if effectiveness of ventilation is the main criterion, natural ventilation may be incorporated into deep open-plan style rooms and wider buildings than is implied by the present limit. The adequacy of the total supply of single-sided ventilation to avoid overheating in summer was assessed using BRE and CIBSE design guidance calculations, and this indicated that comfortable temperatures can be maintained.