Experimental Measurement and 3D Numerical Simulation for the Characterisation of Building Infiltration

CHRISTOS A. TATATZIKIDIS, SOCRATES A. PIPERIDIS, DEMETRI G. BOURIS

Department of Mechanical Engineering University of Western Macedonia

Bakola and Sialvera, Kozani HELLAS

christostatatzikidis@gmail.com, socratespiperidis@gmail.com, dmpouris@uowm.gr

Abstract: - An experimental and numerical study of the air-tightness level of the building of the Department of Mechanical Engineering, University of Western Macedonia, Kozani, Greece was performed. Air permeability was measured with a Blower Door and a comparison of the results with European and International standards was performed. The Effective Leakage Area (ELA) of each room of the building was determined and the results were used as input data for the computational study. Openings equivalent to the ELA were included in the building geometry and a computational fluid dynamics methodology was used to perform a three dimensional simulation of the air flow inside and around the building. A comparison of the measured and the simulated infiltration rate was performed and the effect of the distribution of leakage area on the building shell has been examined.

Key-Words: -Building, Wind, Infiltration, Blower Door, CFD, Effective Leakage Area (E.L.A.)

1 Introduction Infiltration of air through building shells is a source of excess energy consumption through the increased need for HVAC energy that is induced. Quantifiable measurement of the infiltration rate is difficult as well as variable, depending on external wind pressure distribution, internal-external temperature differences, leakage area distribution etc. The purpose of the present research effort is the comparison between experimentally measured leakage area characterisation and that calculated through the implementation of a 3D CFD simulation. If the two approaches can be complementary then there is a possibility of developing an engineering tool to characterise a building in terms of infiltration. The experimental and numerical approaches were applied to the building of the Department of Mechanical Engineering, University of Western Macedonia, Kozani, Hellas (Figure 1).

Figure 1. Photograph of the building of the Department of

Mechanical Engineering, UOWM, Kozani, Hellas 2 Problem Formulation For the measurement of the airtightness it is important to achieve a pressure difference between the inner and outer environment of the building. This pressure difference can be achieved with the use of a Blower Door [1]. The apparatus is usually implanted in an outer

door of the building in order to create a pressure difference through pressurization or depressurization tests and allows measurement of the airtightness level. The relationship between the fan flow rate (Q) and the induced internal-external pressure difference (ΔP) is [1]:

(1) The values c and n were found from fitting the data (Q and ΔP) to:

(2) Test pressure differences usually range from 4 to 60-70 Pa. In situations where it is not possible to achieve at least a 50 Pa pressure difference, the air volume flow rate (Q) is calculated from (1) for a pressure difference of 50 Pa (Q50), where constants c and n have been calculated. The standard pressure differential used is 50 Pa. This is not in fact a very large pressure differential and corresponds to the pressure exerted by a column of water 5 mm high. Compared to the fact that buildings can withstand wind induced pressures of at least 500 Pa, this seems insignificant, but it is larger than wind induced pressure on a calm day, and by testing and quoting air leakage figures at 50 Pa (Q50), inaccuracies are reduced and repeatability is improved. Air Permeability is the air leakage rate and can be found from the equation:

(3) For Air Permeability calculation the area of the ground floor is always included in the envelope area of the building (ST) (according to ATTMA procedure) [2]. The Effective Leakage Area (E.L.A.) represents the accumulation of all cracks-openings from which air leakage exists on one surface. E.L.A. is calculated from the following equation [3],[4]:

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ISSN: 1790-5095 151 ISBN: 978-960-474-125-0

(4) The Effective Leakage Area is measured at a pressure difference between 4-10 Pa [4]. The correlation between air flow Q and pressure difference is assumed to follow eq. (1). If n=0.5 then E.L.A. and CD are assumed constant for the whole range of pressure differences. In the present study, E.L.A. was assumed to be constant for ΔP≠4 Pa. For simplicity, a constant value of CD=1 was also assumed since the actual geometry of the cracks is not known. The total air leakage openings were calculated at 4 Pa for each room and were subsequently used as equivalent openings in the three dimensional geometry of the building, for the computational part. 3 Problem Solution Measurements with INFILTEC Blower Door E3-A-DM4 took place in each room separately. The resolution is 0.1 Pa, the range is -750 Pa to 548 Pa, and the pressure accuracy is 1% of reading or 1 digit. During the tests, all electrical outlets and all non-external openings were sealed. In this way, an effort was made to minimize the air leakage towards the inner spaces of the building, so that only the existing leakages towards the outer environment would be measured. For the air permeability calculations, only the surfaces of the outer walls and the ground were included in the ground floor calculation of (ST). For two of the ground floor rooms, a part of the roof was also included due to the existence of an outer balcony of the first floor [2]. For the rooms of the first floor only the surfaces of the outer walls were included in the total calculation of (ST). Finally, for the rooms of the second floor, both the outer walls and the roof were included in the calculation of the outer surfaces. According to the ATTMA and CIBSE specified limits of Air Permeability referred to in Table 1, three rooms (101, 105 and 106) have values over the limit (accepted limit 10 m3/hm2) [5]. The room with the greater air permeability is room 105. This is due to the existence of a big crack-hole in the wall, through which a tube for the HVAC system passes and leads to the outer environment. This spot was not sealed but left uncontrolled during the test, because it was considered as an external opening in the shell and as a result it was obligatory to include this crack in the calculation. This observation led to the application of the thermographic test simultaneously with the pressurization tests in order to visualize the effect of infiltration through the cracks in the building shell, as will be presented subsequently. Rooms 101, 105, 106 are considered to be non airtight while the other 23 are considered to be airtight. Eleven of them are very airtight (rooms 1, 2, 3, 4, 5, 6, 7, 103, 203, 204, 207) with air permeability under 3 m3/hm2 (Figure 2). Total air permeability for the whole building was calculated equal to 6.6 m3/h m2. According to the

determined standards and limits in Great Britain (CIBSE) (Table 1) the building of the Department of Mechanical Engineering is proved to be airtight (air permeability limits: 3-9 m3/h m2).

Building Type Recommended Air

Permeability m3/hm2

Good

Practice Best

Practice

Irish Building regs

Air Permeability

m3/hm2

UK/NI Building regs

Air Permeability

m3/hm2

Offices, nat ventilation 7 3 - 10

Mixed Mode 5 2.5 - 10 Offices, A/C or

low energy 5 2 - 10 Superstores 5 1 - 10 Industrial 6 2 - 10

Dwellings (nat vent) 9 3 10 10

Dwellings 5 3 10 10

Table 1. Air permeability limits, good and best practice for different types of buildings (ATTMA, CIBSE)

Figure 2. Air Permeability per room

3.1 Thermographic Test Simultaneously with the blower door test, a thermographic test (AGEMA 550 thermal camera) was applied for room 105 of the first floor of the Department, where the largest value of air permeability was calculated [6]. A number of specific areas around the window were chosen as monitoring positions. In the presence of cracks, the depressurization would lead to infiltration of outdoor air that was expected to reveal cool spots during the thermographic visualization. Seven areas above and around the window were chosen to be studied (Fig. 3).

Figure 3. Visualization of seven chosen areas.

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The areas that were chosen are: 1. Frame of the window and places where there is

possible air leakage i.e. between the moving part and the frame (AR01, AR02, AR03, AR04, AR05).

2. An area next to the left side of the window (AR06). 3. Above the window, where there was an opening for

an air conditioning installation (AR07). From the processing of the data which were taken from the thermal imaging camera, the following temperature values were recorded at each monitoring position (Figure 4):

Figure 4. Temporal temperature variation for the seven chosen

areas of the window of room 105

A reduction of the temperatures at the chosen areas was observed. This is especially evident for the area above the window (AR07) where a 6,6 oC reduction of the temperature is observed indicating significant air leakage. This confirms the results of the airtightness test and explains the high values of air permeability shown in Figure 2. 3.2 Description of the computational study The computational study aims at determining permanent and uncontrolled air penetration from the openings-holes which represent the effective leakage areas (E.L.A) for each room, and comparison with the values found in the experimental study. For the present computational study the CFX commercial CFD software was used [7]. A steady state 3D RANS (Reynolds- Averaged Navier-Stokes) model was applied with the k-ε turbulence model on an unstructured grid. The flow was considered isothermal, thus neglecting the normally significant effect of thermal buoyancy forces. However, it is well established that the effects of pressure driving forces can be studied separately [8] so it is the wind effect that is studied here. 3.2.1 Geometry Creation For the creation of the geometry of the building floorplans were used to create 3D Autocad objects. Correction of modeling deficiencies were made for each floor (walls that were not in contact creating small surfaces-gaps) and simplifications on the geometry followed to make possible the creation of a

computational mesh (Figure 5). The next step was the placement of square openings (holes) in the appropriate walls-surfaces of each room. Their size (area) was determined by the respective measurements with the Blower Door where effective leakage area (ELA) for each room was calculated.

Figure 5. Three Dimensional visualization of the building The geometry file was saved in an ‘ACIS FILE’ format so that it could be imported to the preprocessor for mesh generation. After the geometry was imported, a cube was created surrounding the geometry of the building and defining the computational domain that represents the outer environment around the building. However due to the difference in scale between the holes and the building, mesh refinement resulted in excessive memory requirements in order to include the entire building and its environment. Therefore, it was decided to separate the problem into four (4) simulations. The geometry was divided into the surrounding area of the building (the building geometry was kept without the E.L.A. openings) and the three floors, which were considered separately. This resulted in four separate computational domains which were coupled through the calculated static pressure difference at the coordinates of the predefined holes in the building shell. The simulation of the air flow field in the surrounding area of the building was used for determining the pressure conditions at the holes and these were considered as the boundary conditions for each of the three simulations of the internal flow on each floor. It was assumed that the internal flows did not influence the external pressure distribution and that there was no flow between floors. 3.2.2 Mesh Generation The grid parameters for each of the four computational grids are as follows [7]:

• External Flow The computational domain around the building consists of a cube surrounding the building, having dimensions 341 m x 341 m x 40 m. The total number of the elements of the mesh of the surrounding area of the building is 1310594.

•Floor Mesh The grid on each floor depicts the interior of the building, which communicates with the environment through the E.L.A. holes. The mesh of the ground floor

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(Figure 6), 1st floor and 2nd floor consists of 2288064, 2002236, 1987061 respectively. The average grid cell size at the holes was 0.01 m (Figure 7) and for the rooms it was 0.2 - 1 m for all the three meshes created.

Figure 6. Visualization of the ground floor grid (~2.3·106

cells)

Figure 7. Detail of the grid near an ELA opening

3.2.3 Boundary conditions Boundary conditions are defined for each surface of each volume. In addition, each volume is determined as solid or fluid [7].

•Boundary conditions of the surrounding area All external surfaces of the cube and the building were designated as walls. The side of the cube that is to the North of the building has been defined as a flow entry plane and the south as an output, corresponding to a northern wind direction. The wind speed was set to be spatially uniform at 6 m/s (taking an extreme value for the region of Kozani, instead of an average).

•Boundary conditions of the floors All external surfaces of the floors and openings (holes) were defined as walls. The coordinates of the holes and the static pressure at the point of the holes were identified from the simulation of the surrounding area. The values of the static pressure were used as boundary conditions for the simulations of the floors (Table 2). Typical ΔP driving pressures were from -10 to 30 Pa, with negative values indicating flow from the inside to

the outside of the building envelope. 3.3 Numerical Simulation Results The results of the simulation of the air flow for every floor of the building are presented in Figure 8 and compared with the results of the measurements with the Blower Door.

Figure 8. Air flow volume per room according to the

measurements and computational study The simulation was performed on a 2.67 GHz, 3.00 GB RAM PC and the duration for each simulation was about 12 hours (2500-3000 iterations for each floor). Based on the results of air flow volume, air changes per hour (ACH) were calculated (Table 2) for each floor, taking into account the volume of each area measured.

Ground floor 1st floor 2nd floor ACH 0,054 0,234 0,297

Table 2. Air Change per Hour (A.C.H.)

Calculated values for air permeability show that the permeability of the 1st floor (16.34 m3/hm2) is greater than that of the 2nd floor (7.87 m3/hm2). This is the result of combining the values from Table 2 and the external surface area of each floor. The differences that appear in correlation to the measurements are encouraging given the complexity of the application and the accuracy in which geometry is represented. Assumptions have been made that may, in part, explain the difference between the numerical calculations and the measurements. First of all the distribution of the cracks has been ignored, representing them as a single hole for each room area. The position of the hole was selected based on the position of the largest actual openings (windows). Also, assuming that all internal openings (e.g. doors) are wide open brings on favorable conditions for the infiltration and transport of air through the building, resulting in an increase of air changes per hour (ACH) (no obstacles to the flow, like doors closed). In the simulation, the air will have to traverse the building by following paths that are determined by the internal openings and geometry. During the experimental measurements, the pressure difference measured in a room was specific, without taking into account the pressure difference in

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the rest of the building. Instead, the flow that occurs in the numerical simulation study is actually a result of the prevailing pressure differences throughout the building. Finally, and possibly most important, the discharge coefficient for all openings has been considered equal to unity. This is actually determined by flow Reynolds number and opening geometry and could have values as low as 0.4 with 0.6 corresponding to a sharp edged orifice. The distribution of pressure on the external surfaces depends on the external wind flow on the surfaces of the building envelope (Figure 9). Obviously, for different wind directions, this will change.

Figure 9. Visualization of the external wind flow field around

the building

From Figure 10 and Figure 11 it is observed that with the given wind direction and the placement of the holes on the building envelope, air enters the 1st floor of the building through 4 openings in contrast to 5 for the 2nd floor. The northern orientation of the openings on the 2nd floor leads to more infiltration (and therefore air changes per hour) in comparison to the first floor, even though air permeability has been found greater for the second floor. This shows that the orientation of the openings with respect to the wind direction is a decisive and important factor for infiltration and internal air flow in the building.

Figure 10. Visualization of the velocity at the 1st floor (red vectors indicate the direction of the flow at each opening)

A detail of the flow paths through the building is presented in Figure 12. From the previous discussion, it

is obvious that the orientation of the openings with respect to the external wind direction will strongly influence the level of infiltration. In fact, infiltration may be completely unexpected with regard to the measured values of air permeability or E.L.A..

Figure 11. Visualization of the flow lines at the 2nd floor

(entrance-exit of the flow, red vectors indicate the direction of the flow)

Figure 12. Visualization of the flow lines at the 1st floor

(room 105) (entrance-exit of the flow, red vectors indicate the direction of the flow)

For this reason, it won’t be possible to calculate the air changes per hour only from the measurements by taking into account an average difference of internal and external pressure. The pressure differences imposed at any point of the outer surface interact with the size of the opening or crack to determine the infiltrating air flow. It is proposed that a distribution of external surface pressures for a number of representative wind directions must be combined with the measured values of E.L.A. or air permeability in order to obtain a reliable infiltration characterization of a given building. 4 Conclusions The main purpose of the present study was the airtightness test and the three dimensional simulation of the air flow around and through the building with the use of computational fluid dynamics. The airtightness test was accomplished with the use of blower door and

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the three dimensional simulation of the flow with a the CFX commercial CFD software. During the experimental measurements with the blower door, the volume flow rate at each room was measured and the equivalent leakage area (E.L.A.) was calculated. Air permeability was found to be equal to 6.6 m3/hm2. Comparing this result with the limits (3-9 m3/hm2) of Great Britain for air permeability the tested building can be considered airtight. A thermographical test was conducted in order to monitor air leakage and it was found that there are significant leakage paths near HVAC openings and around the frames of the windows. From the numerical simulation of the external building surface pressure distribution due to windflow, it was proposed that infiltration indicators such as Air Changes per Hour (ACH) can not be calculated solely from measurements of air permeability. Many factors exist that influence infiltration such as building shape, distribution of openings on the building envelope, wind direction and the flow paths through the building. Air Permeability itself is simply an indication of the air tightness level of the outer shell and does not constitute absolute proof for the number of air changes. The distribution of pressure created by the wind and the geometry of the building could influence infiltration and reverse the conclusions arising from measurements of air permeability. References: [1] ASHRAE (2001), Fundamentals Handbook (SI), Chapter 26, Ventilation and Infiltration [2] ATTMA (2007), The Air Tightness Testing And Measurement Association, Technical Standard 1, Measuring Air Permeability Of Building Envelopes, 13/07/2007 [3] Brennan T., Persily A., Clarkin M., Emmerich S. (2007), Measuring airtightness at ASHRAE headquarters, ASHRAE Journal, September 2007. [4] CIBSE (2000), TM 23-Testing Buildings for air leakage, The technical note has been produced by the CIBSE Task Group on Mechanical Ventilation, October [5] Building Envelope Technologies: http://www.betechnologies.ie/building-air-leakage.html http://www.betechnologies.ie/airtightness-standards.html [6] Pearson C. (2002), “Thermal Imaging of Building Fabric, A best practice guide for continuous insulation” Technical Note TN 9/2002, BSRIA. [7] CFX-ANSYS, 2005

[8] Etheridge D. and Sandberg M. (1996) “Building Ventilation: Theory and Measurement”, John Wiley & Sons, Chichester

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