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Indoor Environment and Well-Being
Published in Jacqueline A. Stagner, David S-K. Ting, Green Energy and Infrastructure, 2020
Lina Seduikyte, Egle Staniskiene, Zivile Stankeviciute, Indre Grazuleviciute-Vileniske, Laura Stasiuliene, Paris Fokaides, Tomas Sorius, Paris Fokaides
Two different mechanical ventilation systems were studied (mixing and displacement ventilation), and different CFD models with different boundary conditions were created. These two types of ventilation systems were selected as commonly used air distribution concepts. In a mixing ventilation system, air is supplied in an upper part of the room, that is, above the occupied zone where people are with high initial velocity (approx. 1 m/s). High initial velocity ensures that room air circulates and mixes. Air can be exhausted through the upper or lower part of the room. In displacement ventilation system, air is supplied directly to the occupied zone (at floor level) with low initial velocity (approx. 0.2 m/s) and a few degrees lower air temperature compared with the room temperature. In displacement ventilation convective flows from occupants and warm equipment, for example, computers are the driving force directing contaminated air towards the ceiling zone where exhaust is usually designed.
Local Thermal Discomfort
Published in Ken Parsons, Human Thermal Comfort, 2019
ISO 7730 (2005) provides the following equation for the percentage of dissatisfied due to temperature differences between head and feet (Δta,v) as PD = (100/(1 + exp(5.76 − 0.856Δta,v))), based on the research of Olesen et al. (1979). Although the experimental subjects were in thermal neutrality, it may be that any discomfort was caused by local air temperatures rather than air temperature differences. For experiments into comfort conditions for chilled ceiling and displacement ventilation environments, Loveday et al. (1998) found none or very little thermal discomfort over a wide range of temperature differences and humidity levels. Displacement ventilation is when cooled air (often accompanied by a chilled ceiling at a temperature above dew point to avoid condensation) is introduced to the floor by openings in the floor or from vertical ‘bins’ at floor level. People and machines (including computers) heat the air so that it rises and is ‘exhausted’ at the ceiling (so that it is breathed only once and any pollution is removed at the ceiling). In a field study in a modern office in the London city, displacement ventilation was used but people were complaining of headaches and drowsiness in the afternoon. On investigation, it was found that this was because of a build-up of carbon dioxide (from people) as workers at desks in the office had blocked the fresh air inlet ducts on the floor due to draught. Further research into vertical temperature differences is required as it is not clear if it is the difference that is causing discomfort or the local air temperature and air movement.
Room Air Distribution and Hybrid Secondary Systems
Published in T. Agami Reddy, Jan F. Kreider, Peter S. Curtiss, Ari Rabl, Heating and Cooling of Buildings, 2016
T. Agami Reddy, Jan F. Kreider, Peter S. Curtiss, Ari Rabl
Similar to UFAD systems, displacement ventilation only meets sensible loads. Space latent loads (along with conditioning the outdoor air) must be met by the supply of airstream conditioned in a separate air handler unit. Displacement ventilation systems aim to minimize mixing of supply air with room air, instead maintaining conditions in the occupied zone as close as possible to that of the conditioned supply air, leading to an improved ventilation effectiveness (see Section 3.5.4). ASHRAE 62.1 (2013) allows a reduction in the required minimum ventilation rate of 1.2, resulting in a 16% reduction in the outdoor air intake. Note that lights and other heat-generating equipment are usually located above the occupied space, and so the supply airstream does not have to meet these loads. As a result, the energy needed for conditioning the supply airstream is much less than that required for the fully mixed system This is a large energy saving in terms of avoided energy to condition this outdoor air as well as having to meet only some of the space heat loads. An important challenge to the designer for both systems is how to heat up the cold airstream leaving the cooling coil at 55°F (13°C) to 65°F (18°C) without wasting energy by reheat.
CFD modeling of room airflow effects on inactivation of aerosol SARS-CoV-2 by an upper-room ultraviolet germicidal irradiation (UVGI) system
Published in Science and Technology for the Built Environment, 2023
Youngbo Won, Donghyun Rim, Richard Mistrick, William Bahnfleth
Figure 2 shows spatial distributions of temperature and velocity for both mixing and displacement ventilation systems at a supply airflow rate of 6 h−1 (case 1 and case 15). With mixing ventilation (figures 2a and 2c), the air is supplied through a 4-way ceiling diffuser and moves along the ceiling and wall surfaces, circulating indoor air throughout the room. Due to the temperature difference between the occupant surface and ambient air, natural convection thermal plumes form around occupants. In displacement ventilation, air is supplied horizontally at floor height (behind the section view), which pushes the room air upward, resulting in vertical stratification of temperature. In such a case, a buoyancy-driven thermal plume is the major driving force for the air movement around occupants. In all cases, viral aerosols were released from the mouth of the infector (the occupant on the left in Figure 2) by a continuous air jet at an air speed of 2 m/s to simulate a talking (Gupta et al. 2010b).
Integrating IAQ control strategies to reduce the risk of asymptomatic SARS CoV-2 infections in classrooms and open plan offices
Published in Science and Technology for the Built Environment, 2020
Displacement ventilation is a better approach for fresh air delivery and reducing the concentration of pollutants emitted from occupants. It delivers the cool supply air at or close to the floor level and exhaust at or close to the ceiling level. The cool supply air spreads over the floor, is drawn by the upward natural convection flow around occupants to reach their breathing zones and brings the pollutants from exhaled air upward to the room exhausts located at or close to the ceiling level. A thermal stratification is formed in the lower part of the room to reduce the mixing and hence reduce the chance for cross contamination among the people. It has the potential to reduce the pollutant concentration in the occupied zone by a factor of 1.2 to 2 (Chen and Glicksman 2003).
Pollutant and heat removal of a small fire in a cinema hall by different ventilation systems
Published in Chemical Engineering Communications, 2023
Mostafa Ramezani, Amir Mohammad Jadidi, Roohollah Rafee
If the supplied air temperature is more than 27 °C, it has more negative effects on the thermal comfort of the people (Cheng et al. 2015). For the presence of people in the room, the airflow features are different between displacement ventilation (DV) and stratum ventilation (SV) (Cheng and Lin 2015). Displacement ventilation and underfloor air distribution systems have some beneficiaries like suitable thermal stratification, thermal comfort, ventilation efficiency, and reduced energy use in suitable climates (Rahmati et al. 2018).