{syc17, xfli}@mails.tsinghua.edu.cn
ABSTRACT
Measurement and simulation are two main methods to determine natural ventilation rates in a building. By contrast, the simulation method is widely used in engineering due to its simplicity and convenience. In the practical application of airflow simulation software such as CONTAMW, there exist many simplifications for the actual conditions of simulated buildings. This study focuses on three of the commonly used simplifications and analyses their effects on the simulation results. In addition, for rigorousness, tracer gas decay method is used to verify the reliability of CONTAMW simulation. The conclusions of this study can be used as guidance for airflow simulation software such as CONTAMW.
KEYWORDS
CONTAMW; natural ventilation rate; simplification;
simulation.
1 INTRODUCTION
Natural ventilation is an important way to improve indoor air quality and building energy performance [1]. To evaluate the effect of natural ventilation, it is necessary to determine the natural ventilation rate for a building.
Measurement and simulation are two main methods to determine natural ventilation rates in a building. By contrast, the simulation method is widely used in engineering due to its simplicity and convenience [2].
CONTAMW is a software developed by the National Institute of Standards and Technology (NIST) for simulating multi-zone airflow rate in buildings, which has been widely used for natural ventilation simulation [3].
In the practical application of CONTAMW, there exist many simplifications for the actual conditions of simulated buildings. This paper focuses on the following three simplifications:
a) Ignore the effects of indoor partitions;
b) Ignore the effects of building blockage in the wind flow direction;
c) The effective opening area of windows/doors is assumed as maximum openable area of windows/doors.
Natural ventilation rates are simulated and compared when the above simplifications are applied or not. The influence of these simplifications on simulation results is analyzed.
The most commonly used measurement method to determine natural ventilation rate is tracer gas decay method, which has been used in office buildings, residential buildings, and classrooms [4]. The test procedure of tracer gas decay method consists of the release, mixture, and monitor of suitable tracer gas. To verify the reliability of CONTAMW simulation, measurements of natural ventilation rate are conducted in a typical room of the simulated building using the tracer gas decay method. The measured results of tracer gas decay method and the simulated results of CONTAMW are compared with each other to validate the simulation.
This study aims to evaluate the effects of the above simplifications on simulation results and make guidance for the use of airflow simulation software such as CONTAMW.
2 METHODOLOGY 2.1 Simulation
The basic calculation principle of CONTAMW is the following equation, which is for airflow through large intentional openings [5]:
𝑄𝑄 = 𝐶𝐶$𝐴𝐴&2∆𝑝𝑝/ 𝜌𝜌 (1)
where 𝑄𝑄 is airflow rate, m3/h; 𝐶𝐶$ is discharge coefficient of opening, dimensionless; 𝐴𝐴 is effective opening area of window/door, m2; ∆𝑝𝑝 is pressure difference across opening, Pa; and 𝜌𝜌 is air density, kg/ m3.
The pressure difference across openings is caused by stack effect and wind pressure [5]. The pressure difference caused by stack effect is determined by indoor-outdoor temperature difference. While, the pressure difference caused by wind pressure is an input parameter, which is
generally obtained by simulating outdoor wind field using Computational Fluid Dynamics (CFD) software (such as FLUENT and PHOENICS). In this study, CFD software
—
PHOENICS
—
was used to simulate outdoor wind field and determine wind pressure across the window/door openings.The indoor temperatures were determined by iterative computation between indoor-outdoor temperature difference, natural ventilation rate and cooling load (only passive cooling by natural ventilation, air conditioning system off). Outdoor temperature set point is 20 ℃.
Then, natural ventilation rates were simulated using CONTAMW when the above three simplifications were applied or not. Results were compared to obtain the main conclusions.
2.2 Verification
Natural ventilation rates were measured in a typical room of the simulated building using tracer gas decay method.
Then, the measured results of tracer gas decay method and the simulated results of CONTAMW were compared with each other.
Carbon dioxide (CO2) was chosen as tracer gas because it is easily available. In the CO2 decay method, CO2 was injected into the test room and the decay of CO2 concentration was measured once a uniform concentration had been reached. The change of indoor CO2 concentration is expressed as
𝑙𝑙𝑙𝑙[𝐶𝐶12(𝜏𝜏) − 𝐶𝐶789(𝜏𝜏)] = −𝑁𝑁𝜏𝜏 + 𝑙𝑙𝑙𝑙[𝐶𝐶12(0) − 𝐶𝐶789(0)] (2) where 𝐶𝐶12 and 𝐶𝐶789 are indoor and outdoor CO2
concentrations, ppm; 𝑁𝑁 is air change rate, h-1; and 𝜏𝜏 is time, h. Based on Eq. (2), 𝑁𝑁 can be determined through the linear fitting between 𝑙𝑙𝑙𝑙[𝐶𝐶12(𝜏𝜏) − 𝐶𝐶789(𝜏𝜏)] and 𝜏𝜏.
Then, the natural ventilation rate of the test room is calculated using Eq. (3):
𝑄𝑄2>= 𝑁𝑁 × 𝑉𝑉 (3)
where 𝑄𝑄2> is natural ventilation rate, m3/h; and 𝑉𝑉 is room volume, m3.
3 RESULTS
3.1 Simulated Building
The simulated building is located in Beijing, China, which has four floors and dimension of 32.5 m×17.8 m×18.3 m (length×width×height). The layout of the third floor is shown in Figure 1. The maximum openable areas of exterior windows and doors on each floor are shown in Table 1. Natural ventilation rate in transition season (April and May) was simulated, during which time the outdoor temperature is lower than that indoor and natural ventilation is used for free cooling.
3.2 Effects of Indoor Partitions
Natural ventilation rate of each floor is simulated under actual building location situations and when the exterior windows and doors are opened to the maximum openable areas.
Figure 1. Layout of 3rd floor.
Orientation Type of window/
Table 1. Maximum openable areas of exterior windows and doors on each floor
CONTAMW models with or without indoor partitions are shown in Figures 2 and 3 (taking the first floor as an example). Changing the opening areas of inner doors on the partitions to fully open, half open and 1/4 open, the natural ventilation rate simulation results are shown in Figure 4 (air mixing rate: 100%). As shown in this figure, the influence of indoor partitioning on natural ventilation rate is related to the opening area of the partition. The smaller the opening area, the greater the flow resistance and the smaller the natural ventilation rate. When the opening area is large, for example, when the inner doors are opened at 90 degrees, the partition has little effect on natural ventilation rate.
Figure 2. CONTAMW model of 1st floor (without partition).
Figure 3. CONTAMW model of 1st floor (with partition).
Figure 4. Effects of indoor partitions on natural ventilation rates.
3.3 Effects of Building Blockage in the Wind flow Direction
Natural ventilation rate of each floor is simulated when there is no indoor partition and the exterior windows and doors are opened to the maximum openable areas.
Wind pressure over the building envelop is simulated using PHOENICS under actual building location situations and when removing other buildings blocked on the south side (the wind flow direction). Results are shown in Figures 5 and 6. As shown in these two figures, when removing the building blockage in the wind flow direction, the wind pressure over the building envelop significantly increases on every face.
The natural ventilation rate simulation results when the building is blocked or not are illustrated in Figure 7. The natural ventilation rate is significantly improved when there is no building blockage in the incoming flow direction. Specifically, the natural ventilation rate on each
floor is 8 – 36 % higher than that under actual building location situations. Notably, the lower the floor, the more the natural ventilation rate increases.
Figure 5. Wind pressure over building envelop when blocked by other buildings in the wind flow direction (grid size:
6m×6m×5m, KEMMK model).
Figure 6.Wind pressure over building envelop when not blocked by other buildings in the wind flow direction (grid size:
4m×5m×4m, KEMMK model).
Figure 7. Effects of building blockage in the wind flow direction on natural ventilation rates.
3.4 Effects of Effective Opening Area of Window/Door Natural ventilation rate of each floor is simulated under actual building location situations and without indoor partition.
Changing the opening areas of exterior windows and doors to maximum openable area and half the maximum openable area, the natural ventilation rate simulation results are compared in Figure 8. When reducing the effective opening areas of exterior windows and doors, the natural ventilation rate significantly decreases. In our case, when the effective opening area is reduced by 50 %, the natural ventilation rate of each floor decreases by 40 – 53
%.
0 5000 10000 15000 20000 25000
1st Floor 2nd Floor 3rd Floor 4th Floor Natural ventilation rate (m3/h)
No partition With partition (inner doors fully open) With partition (inner doors 1/2 open) With partition (inner doors 1/4 open)
1st Floor 2nd Floor 3rd Floor 4th Floor
Blocked 24089 16595 17609 15733
Non-Blocked 32757 22275 21821 17084
0 5000 10000 15000 20000 25000 30000 35000
Natural ventilation rate (m3/h)
Figure 8. Effects of effective opening area of window/door on natural ventilation rates.
3.5 Verification Results
The validation tests were conducted in the Conference Room on the 3rd floor (Figure 1). The CO2 decay and liner fitting of Test 1 is illustrated in Figure 9 as an example. The results of CO2 decay and CONTAMW simulation methods are compared in Table 2. The relative errors between the two methods are no more than 20 %, while the general simulation accuracy of natural ventilation is about 10 – 25
%, which indicates that our simulation method is reliable [6].
Figure 9. CO2 decay and liner fitting of Test 1.
Test number Test 1 Test 2 Air change rate by tracer gas
measurement (h-1) 1.86 1.81 Air change rate by CONTAMW
simulation (h-1) 1.50 1.56 Relative error 19.5 % 14.0 %
Table 2. Results of validation tests.
4 CONCLUSION
Simulation method is widely used in engineering to determine natural ventilation rates. However, in the
practical application of airflow simulation software such as CONTAMW, there exist many simplifications for the actual conditions of simulated buildings. This study focuses on three of the commonly used simplifications and analyses their effects on simulation results. In addition, for rigorousness, tracer gas decay method is used to verify the reliability of CONTAMW simulation. The relative error between results of tracer gas measurement and CONTAMW simulation is within 20%, which proves that our simulation is accurate and reliable.
According to this study, in the practical application of CONTAMW, it is necessary to consider the influence of indoor partition, building blockage in the wind flow direction, and effective opening area of windows or doors.
By simulation, the following conclusions are obtained:
a) The existence of indoor partition will decrease natural ventilation rate; the smaller the opening area on the partition, the greater the flow resistance and the smaller the natural ventilation rate.
b) The existence of building blockage in the wind flow direction will significantly decrease the wind pressure and thus decrease natural ventilation rate; the lower the floor, the greater the impact is.
c) The reduction of the effective opening area of exterior windows or doors will also decrease natural ventilation rate.
The above conclusions can be used as guidance for the use of airflow simulation software such as CONTAMW.
REFERENCES
1. Becker, R., I. Goldberger, and M. Paciuk, Improving energy performance of school buildings while ensuring indoor air quality ventilation. Building and Environment, 2007. 42(9): p. 3261-3276.
2. Tan, G. and L.R. Glicksman, Application of integrating multi-zone model with CFD simulation to natural ventilation prediction. Energy and Buildings, 2005.
37(10): p. 1049-1057.
3. Walton, G. and W. Dols, CONTAMW 2.4 user manual.
Gaithersburg, MD, USA, National Institute of Standards and Technology, 2008. 286.
4. ASTM, Standard Test Method for Determining Air Change in a Single Zone by Means of a Tracer Gas Dilution, ASTM Standard E741, American Society of Testing and Materials, Philadelphia. 2010.
5. ASHRAE, ASHRAE handbook—Fundamentals:
Ventilation and Infiltration. 2009: American Society of Heating, Refrigerating and Air Conditioning Engineers.
6. Jiang, Y. and Q. Chen, Study of natural ventilation in buildings by large eddy simulation. Journal of Wind engineering and industrial aerodynamics, 2001. 89(13):
p. 1155-1178.
Floor1st 2nd
Floor 3rd
Floor 4th
Floor
max openable area 24089 16595 17609 15733
1/2 max openable area 14449 7741 9297 7379 0