Applied Energy xxx (2015) xxx–xxx
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Computational fluid dynamics (CFD) modelling of air flow field, mean age of air and CO2 distributions inside a bedroom with different heights of conditioned air supply outlet q Mao Ning a,b, Song Mengjie b,c, Chan Mingyin b, Pan Dongmei d,⇑, Deng Shiming b a
College of Pipeline and Civil Engineering, China University of Petroleum (East China), Qingdao, Shandong, China Department of Building Services Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong Special Administrative Region Guangdong Provincial Key Laboratory on Functional Soft Condensed Matter, School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China d School of Mechanical & Automotive Engineering, South China University of Technology, Guangzhou, Guangdong, China b c
h i g h l i g h t s An A/C system at five settings with different heights of supply outlet. Its ventilation performance was numerically evaluated. Air flow field, distributions of MAA and CO2 concentrations were studied. Air change efficiency was studied. Better performance was achieved with a lower supply outlet.
a r t i c l e
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Article history: Received 1 December 2014 Received in revised form 12 October 2015 Accepted 13 October 2015 Available online xxxx Keywords: Sleeping environments Mean age of air Air change efficiency CO2 concentration Height of supply air outlet CFD method
a b s t r a c t It was revealed in a previous related study that for an air conditioning (A/C) system used in a sleeping environment, the height of its supply air outlet would significantly affect its ventilation performance in terms of effectiveness. To further examine the underlying reasons of the differences in ventilation performance, the air flow field, distributions of mean age of air, air change efficiency and distributions of CO2 concentrations inside an experimental bedroom were numerically studied using CFD method under five different settings, where its supply air outlet was positioned at 5 different heights. The study results suggested that positing a supply air outlet at a lower level was effective in both saving energy and removing the exhaled CO2 in a breathing zone, and when a supply outlet was positioned at a higher level, less CO2 was removed from the region near the mouth of a sleeper. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction It was commonly acknowledged that the quality of sleep was mainly determined by the mental-physical factors of a sleeping person and the environmental factors in a bedroom. For decades, the influence of environmental factors on the quality of sleep, such as air temperature, air velocity and indoor air quality, have been
q This article is based on a short proceedings paper in Energy Procedia Volume 161 (2014). It has been substantially modified and extended, and has been subject to the normal peer review and revision process of the journal. This paper is included in the Special Issue of ICAE2014 edited by Prof. J Yan, Prof. DJ Lee, Prof. SK Chou, and Prof. U Desideri. ⇑ Corresponding author. Tel.: +852 2766 5989; fax: +852 2765 7198. E-mail address:
[email protected] (P. Dongmei).
gradually studied. Furthermore, it was revealed that carbon dioxide, an important index for indoor air quality, may be disruptive to normal sleep, since an increased CO2 level would result in arousal [1]. Residential A/C is widely used to provide occupants with a suitable indoor environment. A large number of numerical researches were carried out on indoor air quality in buildings, such as in kitchen, apartment and office. Zhou [2] carried out numerically study on the effect of the suction flow rate of range hood on CO2 distribution in a Korean apartment with kitchen. Liu et al. [3] studied the CO2 concentrations in a residential room with CFD method. The effects of supply air flow rate and thermal buoyancy were studied. It was found out that the reduction of indoor pollutant levels can be accomplished either by increasing the fresh air ratio, or by increasing filtered removal efficiency, or by increasing the
http://dx.doi.org/10.1016/j.apenergy.2015.10.096 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Ning M et al. Computational fluid dynamics (CFD) modelling of air flow field, mean age of air and CO2 distributions inside a bedroom with different heights of conditioned air supply outlet. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.10.096
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M. Ning et al. / Applied Energy xxx (2015) xxx–xxx
Nomenclature Symbols E g h keff P Qs T t toz ts tuz Tu V v
total energy (J) gravity acceleration (m/s2) enthalpy (J) effective conductivity (W/m k) pressure (Pa) supply air flow rate (l/s) air temperature (K) air temperature at a measurement position (°C) average air temperature in an occupied zone (°C) supply air temperature (°C) average air temperature in an unoccupied zone (°C) turbulence intensity (–) air velocity (m/s) air velocity at measurement position (m/s)
supplying airflow rate. Screbric et al. [4] conducted numerical study on the CO2 distribution around human body in a twoperson office. Lau and Chen [5] studied the indoor environment under a high-cooling load workshop with floor displacement ventilation system. The results from this study show that a workshop with floor-supply displacement ventilation can improve indoor air quality because the contaminant concentration in the breathing zone is lower than that of mixing system. Noh et al. [6] performed the experimental and numerical research on the indoor air quality (CO2 concentration) in a lecture room with a mixing ventilation system. It was found that when the ventilation rate is more than 800 m3/h, the CO2 concentration level is satisfied. Except for the studies on the indoor air quality during daytime, investigations were also carried out on the indoor air quality and ventilation during nighttime. Several studies have been carried out on the performance of A/C systems in sleeping environments [7–9]. Mao et al. [8] developed a simplified ductless bed-based task/ambient air conditioning (TAC) system so as to make it applicable to a real bedroom, and investigated its operating performance in terms of thermal control, ventilation effectiveness and energy saving. The results of this further study revealed that the different air conditioning (A/C) systems with different heights outlets have their own pros and cons in aspects of ventilation, thermal and energy saving performances, and for an A/C applied to a sleeping environment the height of its supply outlet would significantly affect its operating performances. And then a follow-up study to explore a suitable height of supply outlet for the A/C system to obtain the best performances considering the three aspects was carried out [9]. This study revealed that for an A/C system used in a sleeping environment, the height of its supply air outlet significantly affected its ventilation performance. However, the detailed indoor air flow and CO2 transportation were not investigated. Therefore, to further examine the underlying reasons for the differences in ventilation performance, the air flow field, distributions of mean age of air, air change efficiency and distributions of CO2 concentrations inside an experimental bedroom were numerically studied using CFD method under five settings, where its supply air outlet was placed at 5 different heights. The CFD study results are reported in this paper.
leff q
s seff Ui Ci
S/i RACE
se
i hs OACE
soe
hso i
effective dynamic viscosity (Pa s) air density (kg/m3) stress tensor (Pa) deviatoric stress tensor (Pa) arbitrary scalar (–) diffusion coefficient (–) source term of the scalar (–) Room Air Change Efficiency (%) local mean age of the air in the exhaust (–) room average age of air (–) Occupied Air Change Efficiency (%) local mean age of the air in the exhaust of the occupied zone (–) average age of air in the occupied zone (–)
supply air outlet (0.207 0.567 m) was placed at 1.103 m above the floor level to deliver the conditioned air to a cuboid (1.84 0.92 0.6 m), immediately above the bed with mattress, which was designated as an occupied zone in this study for the purpose of parameter measurement and results analysis, and return air inlet (0.374 0.420 m) at 0.322 m above the floor level, as shown in Fig. 2. The above experimental setup was established in an experimental chamber which was separated into two rooms, Room A and Room B. Room B was further separated into a larger space and a smaller space by a partition wall. The larger space was used as an experimental bedroom, measuring at 3.6 2.6 2.53 m. The smaller space was used as a simulated outdoor environment, where the two electrical heaters were placed. Therefore, conditioned air at a required air temperature, relative humidity and air flow rate can be supplied from Room A to the experimental bedroom through the supply air outlet. 2.2. Geometry model A geometry model was established for the experimental bedroom, as shown in Fig. 1. Its dimensions are shown in Fig. 2. To illustrate the impacts of different heights of supply air outlet on ventilation performance, the supply outlet was designed at five different heights: 800, 1100, 1400, 1700 and 2000 mm above the floor level, corresponding to five settings: H800, H1100, H1400, H1700 and H2000. More detailed description of the experimental
Sectional plane
Supply outlet
Window
2. Numerical study 2.1. Experimental setup An experimental setup was established for the experimental study and validation of the numerical study. In the A/C system, a
Bed with mattress
Thermal manikin
Return inlet
Fig. 1. A simulated experimental bedroom (H1100 setting).
Please cite this article in press as: Ning M et al. Computational fluid dynamics (CFD) modelling of air flow field, mean age of air and CO2 distributions inside a bedroom with different heights of conditioned air supply outlet. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.10.096
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M. Ning et al. / Applied Energy xxx (2015) xxx–xxx
Unoccupied zone
Supply outlet
1500
H2000 1840
1100
200
600
420
1770 320
H1100 H800
2530
800 1100 1400 1700 2000
H1400
1000
H1700
3620
2600
Occupied zone
Return inlet
Fig. 2. Schematics of five settings used in the numerical study.
accurate computational result, 10 layers of prism mesh were generated in wall-normal direction to guarantee y +
