Air Distribution Methods Research Articles

The effects of mixing air distribution and heat load arrangement on the performance of ceiling radiant panels under cooling mode of operation

The cooling power of radiant panels can be effected by the arrangement of heat loads and by the room air distribution system. This impact can be important because often the cooling output is the critical factor for the design and usability of radiant panels. In this study, the impact of heat load arrangement and air distribution generated in a room by linear slot diffuser, radial multi-nozzle diffuser and radial swirl induction unit on the cooling power of radiant panels was compared. The impact on the thermal environment was also studied. Measurements were carried out without and with supply air in a test chamber equipped with two ceiling radiant panels and air distribution units flush with the radiant panels. The heat load was generated through the walls and with heated cylinders. The cooling power of the radiant panels was increased with the studied air distribution methods. The increase was from 5% to 17% depending on the air distribution method and the heat load arrangement. The most significant effect of the heat load arrangement occured when heat loads are located unevenly and their convection flow turns or weakens the supply air jet flushing the radiant panels.

Multi-mode ventilation: An efficient ventilation strategy for changeable scenarios and energy saving

The advanced air distribution methods such as displacement ventilation and personalized ventilation attempt to seek a balance between better indoor air environment and energy efficiency. However, multiple demand scenarios including variable locations of occupants and indoor sources may exist in daily operation of the ventilation system. Most of existing airflow patterns are single mode and cannot always be efficient when responding to changeable scenarios. In this paper, a novel ventilation strategy, i.e., multi-mode ventilation (MMV), is proposed to address multiple scenarios. MMV is designed by the combination of several single airflow patterns and aims to utilize the individual advantages of each airflow pattern in controlling specific scenarios. The energy saving potential of MMV is demonstrated by numerical studies of a meeting room. An indexed local cooling load (LCL) is adopted to evaluate the performance of airflow patterns. As a key parameter in LCL calculation, the required supply temperature to maintain the set temperature at the target zone is determined by computational fluid dynamics (CFD) method. The results show that each single airflow pattern can only be efficient for part of 8 scenarios. MMV performs better by combining single airflow patterns 1 and 2 regardless of the changes of the location and magnitude of the occupied zone as well as the location of the heat source. MMV achieves a reduction rate of up to 56.8% in cooling load compared with airflow pattern 4. MMV may provide a reference for real projects in controlling temperature, humidity and contaminant concentration under changeable scenarios.

Combined air heating and ventilation increases risk of personal exposure to airborne pollutants released at the floor level

Combined air heating and ventilation systems are often used in low energy buildings. However, operating these systems in the heating mode increases vertical air temperature gradient in rooms and can have negative effect on indoor air quality. The effectiveness of ventilation and personal exposure of the occupants to airborne pollutants generated at the floor level in rooms with the combined air heating and mixing ventilation was analyzed in this paper. The dispersion of pollutants was evaluated under three air distribution methods (in-ceiling four-way and one-way air supply, and high level wall grille air supply) and under three air change rates (1, 2, and 3ach). Each of these cases were tested at the supply air temperature of +20°C and +25°C by performing CFD simulations and experiments in the full-scale test chamber. The results of the experiments showed higher inhaled concentration of the contaminants under the heating mode. The inhaled-to-ambient pollutant concentration ratio varied in the range of 1.05–1.41 under the ventilation mode and it rose up to 1.14–1.78 under combined air heating and ventilation mode. CFD predictions confirmed that pollutants generated at the floor level are entrained into convective boundary layer of the occupant in cases when vertical air temperature gradient was present. The personal exposure to pollutants released at the floor level can increase significantly when a HVAC system runs in the combined air heating and ventilation mode; this effect will not necessarily dissolve with the increase of the air change rate.

Experimental study of airflow characteristics of stratum ventilation in a multi-occupant room with comparison to mixing ventilation and displacement ventilation.

The motivation of this study is stimulated by a lack of knowledge about the difference of airflow characteristics between a novel air distribution method [i.e., stratum ventilation (SV)] and conventional air distribution methods [i.e., mixing ventilation (MV) and displacement ventilation (DV)]. Detailed air velocity and temperature measurements were conducted in the occupied zone of a classroom with dimensions of 8.8 m (L) × 6.1 m (W) × 2.4 m (H). Turbulence intensity and power spectrum of velocity fluctuation were calculated using the measured data. Thermal comfort and cooling efficiency were also compared. The results show that in the occupied zone, the airflow characteristics among MV, DV, and SV are different. The turbulent airflow fluctuation is enhanced in this classroom with multiple thermal manikins due to thermal buoyancy and airflow mixing effect. Thermal comfort evaluations indicate that in comparison with MV and DV, a higher supply air temperature should be adopted for SV to achieve general thermal comfort with low draft risk. Comparison of the mean air temperatures in the occupied zone reveals that SV is of highest cooling efficiency, followed by DV and then MV. This study reports the unique profiles of flow, temperature, turbulence intensity, and power spectrum of stratum ventilation, which can have a number of implications for both knowledge and understanding of the flow characteristics in a stratum-ventilated room. With respect to the former, it expounds the fundamental characteristics of this air distribution method; and with respect to the latter, it reveals the mechanism of thermal comfort and energy saving under stratum ventilation.

Experimental study of the influence of a moving manikin on temperature profile and carbon dioxide distribution under three air distribution methods

The influence of occupants' movements is seldom considered in room air distribution studies. However, sedentary and walking occupants mix in many conditioned rooms, such as in an open plan office. The influence of movements on the room air distribution has been reported. In this paper, series of tests under three air distribution methods (stratum ventilation, displacement ventilation and mixing ventilation) with a moving manikin are conducted. The manikin is moving at speeds of 1.0 m/s and 0.5 m/s, along two moving routes respectively. Velocity, temperature and CO2 concentration are measured to determine the influence on air distribution. The results show that temperature profile and CO2 distribution are not influenced significantly by a short-time movement. When the movement lasts for a long time, the moving manikin produces a mixing effect and reduces the ventilation effectiveness. The influence under mixing ventilation is the smallest. The influence under stratum ventilation is smaller than that under displacement ventilation. Stratum ventilation has higher ventilation effectiveness at the occupant's head level than that of the other two air distribution methods. The results show the application potential of stratum ventilation under a condition with frequent movements.

An experimental study of the influence of a walking occupant on three air distribution methods

Most indoor air studies have not considered the influence of walking occupants. However, the occupants of many air conditioned rooms are a mixture of sedentary and walking occupants, such as in an opening plain office. In this paper, a series of tests are conducted under three air distribution methods (stratum ventilation, displacement ventilation and mixing ventilation) with a real walking occupant. The occupant walks at a speed of 1.5 m/s along two different walking routes. Velocity, temperature and CO2 concentration are measured to find the influence on air distribution. The results show that a short walk does not change the temperature or CO2 concentration profiles. When the walk lasts for a long time (30 min in this study), the walking occupant causes mixing effect. The influence under mixing ventilation is the smallest. The influence under stratum ventilation is smaller than that under displacement ventilation. The ventilation efficiency of stratum ventilation is still higher than that of mixing ventilation. Stratum ventilation and mixing ventilation recover from the influence more quickly than displacement ventilation. The results show the application potential of stratum ventilation with frequently walking occupants.

The impact of the air distribution method in ventilated rooms on the aerosol particle dispersion and removal: The experimental approach

The ventilation strategy, air distribution method and air change rate may affect building energy consumption significantly. However, this strategy is also related to contaminant dispersion, its removal efficiency and the risk of cross-infection in buildings. In this study, the effects of air distribution methods on aerosol particle behaviour in a ventilated room have been experimentally tested. Experiments were conducted in a full-scale test chamber with the source of contaminant (a nebulised solution of sodium chloride) positioned at the air supply and air exhaust sides. Displacement ventilation and mixing ventilation with one-way and four-way air supply through ceiling diffusers were tested at 1, 2, 3 and 4 air changes per hour. The concentration of particles was monitored within 10min after the injection using six optical particle counters located in one plane section of the room. Aerosol particle decay was used for calculating the age of the air and analysing the ventilation efficiency. Computational fluid dynamics (CFD) predictions were performed to determine the spatial particle dispersion in the room and were compared to the results of the experiment.Experimental results showed that at lower air change rates, one-way mixing ventilation directed particles towards air exhaust diffusers more efficiently, while four-way mixing ventilation enabled more particles to remain airborne. At higher air exchange rates (3 and 4ach), mixing ventilation with one-way air supply prevented aerosol particle transport to the opposite side of the room. The displacement air distribution appeared to be rather inefficient in the removal of particles from the chamber, which was reflected by the relatively high age of the air (of average 16.7s at 3–4ach) compared to the mixing ventilation (of average 9.9s at 3–4ach).

Modeling and Simulation of Combustion Chamber

The work process of oil fuel burner consists of atomization and combustion of oil. In this process, different atomization and air-distribution methods would affect the quality of combustion and then bring out problems of life-span of the burner, energy efficiency and environmental pollution. Therefore, in this paper, different air distribution devices and different sizes of nozzles are designed, and the numerical simulation software, Fluent 6.3, was employed to simulate the flow field of different conditions in combustor,. Through the simulation, the best work condition was achieved, which could help to provide optimization design parameters of the combustor.

Numerical Study of the Respiratory Aerosols Transportation in Ventilated Classroom

Ventilation is important for controlling the airborne transmission of infectious respiratory diseases, particularly in crowed and enclosed spaces, where exist higher risks of cross-infection. This paper studies the spatial concentration distribution and particle tracks of students talking continuously in a classroom with two air distribution methods, including mixing ventilation (MV) and displacement ventilation (DV) using CFD simulations. The classroom is occupied by 10 students with a seating arrangement of 5 rows and 2 columns. The objective of this study is to find out which ventilation mode is more effective in removing respiratory aerosols. The simulation results indicated that the displacement ventilation with the low air supply velocity magnitude and low turbulence could remove the respiratory aerosol droplets and minimize the risk of infectious effectively compared with mixing ventilation.

Spatial distribution of human respiratory droplet residuals and exposure risk for the co-occupant under different ventilation methods

Since the severe acute respiratory syndrome (SARS) epidemics in 2003, more attention is given to ventilation effects on airborne infection control. Questions remain on whether the three typical air distribution methods can have significant differences in reducing the infections between occupants in a space. This study adopts a novel modeling approach by including respiratory thermal manikins in the computational fluid dynamics domain to directly assess the inhalation dose of infectious droplets. Both the initial momentum of a coughing jet and the consequent droplet dispersion due to room airflow are simulated in detail. The preliminary results indicate that including the co-occupant in the modeling domain can present a more reliable exposure. The droplets movements are very complicated, and the whole exposure process can be roughly separated as two stages—a direct exposure at first and a later indirect exposure. The effects of the stratified air distribution systems on the co-occupant's inhalation of infectious droplets cannot easily be concluded. Although the inhaled dose is lowest under displacement ventilation for smaller droplets, under-floor air distribution can achieve the lowest inhalation for larger droplets.

A Method to Generate Effective Cooling Load Factors for Stratified Air Distribution Systems Using a Floor-Level Air Supply

A vertical temperature gradient is known to occur in an enclosure that is ventilated by stratified air distribution (STRAD) methods. Thermal stratification offers distinct energy-saving possibilities. For example, a portion of heat gains can be excluded when calculating the air supply rate and the air-conditioning load. The cooling load reduction of a heat source depends primarily on its vertical location and radiant and convective split. In this study, an effective cooling load factor (ECLF) was defined to calculate the effective heat gains, which can be used by design engineers for the determination of the reduced-supply airflow rate and reduced cooling coil loads for common- and split-return/exhaust configurations with STRAD. Numerical simulations used to obtain these factors for various internal heat sources in a small office were presented, and on-site measurements were carried out to validate the performance of computational fluid dynamics in predicting the thermal stratification in large spaces. A fully developed database of ECLFs will be very useful when designing such systems.

Measurements and predictions of the air distribution systems in high compute density (Internet) data centers

When equipment power density increases, a critical goal of a data center cooling system is to separate the equipment exhaust air from the equipment intake air in order to prevent the IT server from overheating. Cooling systems for data centers are primarily differentiated according to the way they distribute air. The six combinations of flooded and locally ducted air distribution make up the vast majority of all installations, except fully ducted air distribution methods. Once the air distribution system (ADS) is selected, there are other elements that must be integrated into the system design. In this research, the design parameters and IT environmental aspects of the cooling system were studied with a high heat density data center. CFD simulation analysis was carried out in order to compare the heat removal efficiencies of various air distribution systems. The IT environment of an actual operating data center is measured to validate a model for predicting the effect of different air distribution systems. A method for planning and design of the appropriate air distribution system is described. IT professionals versed in precision air distribution mechanisms, components, and configurations can work more effectively with mechanical engineers to ensure the specification and design of optimized cooling solutions.

Distribution of respiratory droplets in enclosed environments under different air distribution methods

The dispersion characteristics of respiratory droplets are important in controlling transmission of airborne diseases indoors. This study investigates the spatial concentration distribution and temporal evolution of exhaled and sneezed/coughed droplets within the range of 1.0 − 10.0μm in an office room with three air distribution methods, specifically mixing ventilation (MV), displacement ventilation (DV), and under-floor air distribution (UFAD). The diffusion, gravitational settling and deposition mechanism of particulate matter were accounted by using an Eulerian modeling approach with one-way coupling. The simulation results indicate that exhaled droplets up to 10μm in diameter from normal human respiration are uniformly distributed in MV. However, they become trapped in the breathing zone by thermal stratifications in DV and UFAD, resulting in a higher droplet concentration and an increased exposure risk to other room occupants. Sneezed/coughed droplets are more slowly diluted in DV/UFAD than in MV. Low air speed in the breathing zone in DV/UFAD can lead to prolonged human exposure to droplets in the breathing zone.

Numerical and experimental study of velocity and temperature characteristics in a ventilated enclosure with underfloor ventilation systems

Airflow and temperature distributions in an enclosure with heat sources ventilated by floor supply jets with floor or ceiling air exit vents were investigated using experimental and numerical approaches. These ventilation configurations represent the floor return or the top return underfloor ventilation systems found in real applications. Experiments and numerical simulations were performed on a full-sized environmental chamber. The results reveal that the temperature stratification in the enclosure highly depended on the thermal length scale of the floor supply jets. When the thermal length scale of the supply jet was >>1, temperature stratification was minor for all tested heat densities and air distribution methods. Significant vertical temperature gradients occurred when the jet thermal length scale was <<1. Changes in air distribution methods also became significant for temperature stratification at small supply jet thermal length scales. Temperature stratification also affected the terminal height of the momentum-dominant region of the vertical buoyant supply jets. The applicability of these results to underfloor ventilation design was also discussed. In designing underfloor ventilation systems, supply jet conditions and heat load density have to be considered to avoid thermal discomfort because of excessive temperature stratifications. This study demonstrated, by both numerical simulations and experiments, that thermal length scale can be used as a design indicator to predict thermal stratifications under a floor return and a top return underfloor ventilation setting.

Robustness of Air Distribution in Plenum-Based Ductless Ventilation Systems

This paper introduces a concept of robustness of an air distribution method, which is defined as being capable of meeting the ventilation requirements during varying operational conditions. The robustness performance may be particularly important when the system allows individual control of the supply air parameters. As a preliminary example, plenum–based (ductless) air distribution methods are studied using computational fluid dynamics. Among the four basic air distribution methods in plenum–based systems, it is found that the floor supply and ceiling return system does not always produce the conventional displacement ventilation system performance when the heat sources are not concentrated. The ceiling supply with ceiling return produces the best robustness performance over a wide range of supply velocities.

Influence of the floor-based obstructions on contaminant removal efficiency and effectiveness

Dimensioning of dilution ventilation is often made using the perfect mixing approximation, assuming uniform contaminant concentration throughout the room space. However, the contaminant removal efficiency and effectiveness of air conditioning system should be accounted for during design. The effectiveness is in this context used as a measure of the contaminant distribution uniformity within the occupied zone. Influence of an occupied zone obstruction level, air distribution method, air change rate, cooling load and contaminant source non-uniformity on the contaminant removal efficiency and occupied zone contaminant concentration uniformity were studied in scale model. The room air distribution method results in contaminant concentration non-uniformity inside the occupied zone. A method was developed to take this into account during the design of air distribution system. Contaminant supply non-uniformity was found to have great influence on the concentration non-uniformity with two tested air distribution methods.

Experimental validation of a computational fluid dynamics model for IAQ applications in ice rink arenas.

Many ice rink arenas have ice resurfacing equipment that uses fossil fuel as power. The combustion byproducts are a major source of contamination. Ventilation along with other pollution source control measures is the most widely applied strategy to lower the contaminant level below the threshold limit and maintain acceptable indoor air quality (IAQ). A computational fluid dynamics (CFD) model has been developed and used to predict the contaminant concentrations, air velocity, and air temperature distributions in ice rinks. The numerical results agree reasonably with the corresponding experimental data for both steady-state and transient conditions. The CFD model is a useful and inexpensive tool to investigate ventilation parameters, such as air distribution methods, ventilation effectiveness, air exchange rates, and various ventilation control strategies.

Classification for the room air conditioning strategies

This paper introduces a new strategy approach for the room air conditioning including classification and terminology. The basis of the classification is different aims or ideas of the temperature, gas, particle, humidity distributions and room air flow patterns that can be created within a room. A certain strategy can be applied by using different system combinations of room air distribution, exhaust, heating and cooling methods and their control. The realization of an ideal strategy is also dependent on the operating parameters and internal sources. Separating the ideal strategies from the practical room air conditioning solutions will help the evaluation of the present room air distribution methods in different operating conditions. It also creates a solid base for the development and promotion of new innovations in the field.

Beyond dollars and cents: how to select the appropriate service distribution method for your building

Reports on the implications for facilities managers attendant to selecting among various service distribution methods, in particular wire and air distribution methods. Details the performance capabilities of various distribution methods. Concludes that fully functional, efficient buildings are just as critical to financial performance as the salesforce, engineering or manufacturing.