Measuring ventilation rate through naturally ventilated air openings by introducing heat flux

ARTICLE IN PRESS

Building and Environment 44 (2009) 27–33 www.elsevier.com/locate/buildenv

Measuring ventilation rate through naturally ventilated air openings by introducing heat flux Sezin Eren O¨zcan, Erik Vranken, Daniel Berckmans M3-BIORES (Measure, Model and Manage Bioresponses), Catholic University of Leuven, Kasteelpark Arenberg, 30, B-3001 Heverlee, Belgium Received 11 June 2007; received in revised form 22 January 2008; accepted 23 January 2008

Abstract Existing ventilation rate measuring techniques for natural ventilation do not provide an online quantification of ventilation rate through the building envelope. Therefore, a need for a practical, robust, and cheap direct measuring technique is obvious. This study investigates the possibility of using a heat source to trace ventilation rate through an air inlet/outlet. A rectangular heat source was introduced in the middle of the air inlet with a known ventilation rate, and afterwards, the 2D temperature field in the flow direction was recorded using an infrared camera. 2D temperature data obtained at different ventilation rates were used to design a sensor composed of 10 thermocouples attached to a circle or ellipse frame around a heat source. Maximum and average temperatures from these sensors were used to estimate the ventilation rate. If adequate heat was applied, it was possible to estimate the ventilation rate with 15% inaccuracy using temperature information from the arc. r 2008 Elsevier Ltd. All rights reserved. Keywords: Natural ventilation; Ventilation rate; Measurement technique; Thermal stratification; Infrared camera

1. Introduction The need for an accurate, robust, and easy measuring technique for ventilation rate through naturally ventilated buildings is apparent for online control of indoor air quality and better management of emissions from buildings [1]. Unfortunately, measurement of total ventilation rate through inlet or outlet is not straightforward. In most naturally ventilated buildings, ventilation openings are not well defined and the opening can be an inlet or outlet depending on the condition in time [2,3]. In addition, nonuniform flow along the opening limits the use of singlepoint measurement techniques generally applied in duct flow by anemometers. Therefore, direction and overall velocity through the opening(s) should be measured in order to calculate the total air flux. Researchers have been working for decades to understand the mechanisms of natural ventilation and for Corresponding author. Tel.: +32 16 32 17 26; fax: +32 16 32 14 80.

E-mail addresses: [email protected] (S.E. O¨zcan), [email protected] (D. Berckmans). 0360-1323/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2008.01.011

developing a suitable reference technique to quantify air movement [4–6]. Several methods were proposed and investigated, including analytical models based on driving forces [4] or on simple heat and/or mass balance [7], direct measuring techniques such as anemometers [8], simulations with computational fluid dynamics (CFD) and multizone modelling [9,10], indirect measuring techniques as pressure difference [11] and different tracer gas techniques [12–14]. However, none of these methods can provide a consistent accuracy better than 70% [15]. In general, the tracer gas technique is considered to be the best method among researchers. Although this technique is quite useful for research purposes, its applicability in practice is limited. In addition, its accuracy in naturally ventilated buildings should be verified with a reliable reference technique. Therefore, for practical use, a simpler, accurate and direct measuring technique is necessary. Temperature is already used to quantify air velocities with hot-wire and hot-film anemometers. These instruments create a local heat flux, and relate decay of temperature or heat needed to keep temperature at certain level to air velocity passing through the anemometer.

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This principle is normally used to measure point-like velocities at certain position. At a larger scale, temperature mapping of flow fields to estimate air velocity distribution was tested in earlier studies [16,17]. Infrared thermography was used in these and many other studies as a reference technique to measure temperature distribution in a 2D plane [16,19,20]. Flow rates through ducts can be estimated by calculating natural convection coefficient related to these temperature measurements. Hasegawa et al. [18] developed a thermography system for velocity measurements with the help of electrically heated wire grid system. Gartenberg and Roberts [16] used only one heated wire for flow field mapping, and to extract velocity profiles from that. In these studies, an empirical relation between air velocity and temperature was defined using Nusselt (Nu), Reynolds (Re), and Prandtl (Pr) numbers. Following equations are suggested for low and high Reynolds number flows. Nud ¼ 0:795Re0:384 d Pr1=3 Nud ¼ 0:911Re0:385 d Nu ¼ 0:683 Re0:466 Pr1=3 Nu ¼ 0:193Re0:618 Pr1=3 Nu ¼ 2 þ 0:6Re0:5 Pr1=3

Based on object diameter at 1oRedo35 Based on object diameter at 4oRedo40 For 40oReo4000

Suggested in [16] Suggested in [18]

Suggested in [21] For 4000oReo40000 Suggested in [21] For 2000oReo40000 Suggested in [22]

General form of relation between air velocity and heat transfer coefficient can be summarised as Nu ¼ c0 þ c1 Rem Pr1=3

(1)

where c0, c1 and m are constants depending on fluid velocity and flow condition. If Eq. (1) is opened  m   C p m 1=3 hD vD ¼ c0 þ c1 (2) k u k where h is convective heat transfer coefficient (W/m2 K), D is equivalent diameter (m), k is thermal conductivity (W/m K), v is velocity of air (m/s), u is kinematic viscosity (m2/s), Cp is specific heat capacity (J/g K), m is dynamic viscosity (Pa s), and c0 and c1 are constants. Consequently, velocity profiles can be obtained from thermal characteristics of the air. Assuming that air properties are constant at this temperature range h ¼ c00 þ c01 vm .

(3) 2

For a constant heat flux, q (W/m ), heat transfer coefficient can be expressed as h¼

q DT

(4)

where DT is the instantaneous temperature difference between the source and the measured point. Finally, DT 1 ¼ c000 þ c001 vm .

(5)

Since the value of m is around 0.5 [18,21,22], this value was taken for further calculations. One can easily realise that these relations are valid for air velocity. But, it can be claimed that since area of the opening is fixed, extraction of air velocity profile from the temperature profile will definitely provide the total air flux through that opening. Up till now, this relation has never been used to calculate the overall ventilation rate through air openings. This study aims to identify measurement accuracy of a temperature-based measurement technique, for ventilation rate to be used in naturally ventilated buildings in future. An innovative, low-cost, temperature-based measuring system to measure ventilation rate through naturally ventilated buildings was suggested. 2D dissipation of heat from a heat source at the middle of an air inlet is measured and is related to the ventilation rate of air through that opening. Besides, different conditions (flow direction, experimental uncertainties, etc.) were investigated to implement a robust technique that is independent of environmental disturbances. 2. Materials and methods Experiments were conducted at the outlet section of a standardised airflow rate calibration unit (Fig. 1) with a known flow rate. Since there is no reference technique yet defined for natural ventilation, a mechanically ventilated test rig with a standard ventilation rate measuring unit was used to test the working principle of the temperature-based method. The test installation was built in accordance with the German Standard DIN 1952 (reference airflow rate measurement with an orifice) [23], the Belgian Standard NBN 688 [24], and the British Standard BS848 [25]. Ventilation rate measurements were performed by a set of orifice plates with an accuracy of 10 m3/h in the range of 65–1500 m3/h. The repeatability of producing a certain flow rate, as calculated from 10 identical experiments, was found to be better than 3%. A more detailed description of the test installation and measuring procedure can be found in [26]. Test chamber of test rig was positioned in a closed room where the conditions were rather stable during the experiments. The outlet section of test installation was a square opening with dimensions of 0.6 m  0.6 m and 1.84 m above the floor. To visualise the flow pattern with the help of a thermal camera (Thermovisions 570), a heat source (Fig. 2) consisting of a resistance connected to metal fins was placed in the middle of the rectangular outlet opening. Dimensions of the heat source were 0.04  0.05  0.06 m3, thus it did not create a significant pressure drop along the flow path. The capacity of heat source was adjusted by changing the current to the resistances from 10 to 50 W.

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Disturbance Fan (Ø 0.5 m)

Paper sheet to visualise 2D temperature distribution (0.7 x 0.7 m)

Outlet Section (0.6 x 0.6 m) Flow Direction Thermal Camera Heat Source (0.04 x 0.05 x 0.06 m)

Fig. 1. Picture of test installation showing inlet installation with thermal camera and disturbance fan.

6 cm

1.2 cm

1 cm

1.5 cm

Fig. 2. Picture and drawing of heat source showing heating elements, aluminium fins, and metal support.

A detailed information on the 2D temperature distribution around the air outlet was obtained using a thermal camera. Since infrared images can only be taken from surfaces, a square frame (0.7 m  0.7 m) covered with thin paper was positioned in the direction of airflow. A circular fan (50 cm radius) was positioned 1 m above the air outlet, which creates a perpendicular disturbance flow against the horizontal flow direction to simulate the effect of wind disturbance on the ventilation opening. The operating speed of fan was adjusted such that 30 cm above the upper side of the air opening a constant air speed, measured by a uni-directional hot-wire anemometer, was achieved. Since external pressure variation was minimal, a stable disturbance was obtained during experiments. Steady-state conditions, with and without external disturbance, were created at two heating levels (30 and 50 W) in combination with 17 different ventilation rates (from 100 to 1500 m3/h) to provide information at all possible flow conditions (Table 1). For temperature difference, due to the difficulty of measuring heat source temperature with an infrared

camera, the nearest possible temperature was recorded as the heat source temperature. Temperature at a distance was formulated either maximum or average temperature on the above explained arcs. Experiments at same ventilation rate were repeated with uni-directional hot-wire anemometer positioned at 10 cm above the bottom of the opening. Single-point-velocity measurements were multiplied with the total area to obtain the total ventilation rate, and compared with set ventilation rates. These experiments provided comparison against the suggested heat dissipation method. Steady-state experiments require constant heating of heating element, which would increase electrical consumption and operational cost of the sensor. Additionally, lifetime of the heating elements should be considered for long-term measurements. For these reasons, intermittent measurements were taken by measuring the cooling dynamics of air on proposed arc sensors. Dynamic experiments were performed by turning off the heat source. After achieving steady-state conditions at a certain ventilation rate, heat source was closed and temperature

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Table 1 Experimental schedule at different ventilation rates performed with 30 W (a), and 50 W (b) heat source Ventilation rate (m3/h)

Application

No disturbance Disturbance—1 m/s Disturbance—1.5 m/s Dynamic Application

No disturbance Disturbance—1 m/s Disturbance—1.5 m/s Disturbance—2 m/s Dynamic

100

125

150

175

200

a, b a, b b b

a, b a, b b b

a, b a, b b b

a, b a, b b b

a, b a, b b b

Ventilation rate (m3/h) 250

300

350

400

450

500

600

700

800

900

1000

1500

a, b a, b

a, b a, b b

a, b a, b

a, b a, b b

a, b a, b

a, b a, b b a, b

a, b a, b b

a, b a, b

a, b a, b b

a, b a, b

a, b a, b b a, b b

a, b a, b

b

a, b b

b

a, b b

Fig. 3. Infrared camera images of heat plume in vertical plane at different ventilation rates ((a) 150 m3/h, (b) 300 m3/h, and (c) 1500 m3/h).

decay over time was recorded at every 10 s. On an average, total time for the dynamic experiments was 35 min. 3. Results and discussion When heat is introduced on the normal plane of an air inlet, a heat plume is created around the heat source. Fig. 3 shows three infrared images at a vertical plane where a heater is positioned at different ventilation rates. As ventilation rate increased, the buoyancy effect became less pronounced and inertial forces created horizontal and uniform flow pattern. For practical implementation, temperature readings were taken at 10 points on an imaginary supporting arc (Fig. 4). In future, only a number of thermocouples fixed on an arc-shaped frame can be used to estimate the ventilation rate instead of infrared thermography. Besides, during the experiments only uni-directional flow characteristics were investigated. In case of bi-directional flow, a fully circular arc could be used to measure the flow in both directions. Alternatively, instead of measuring at a certain distance temperature of the heater can be measured directly. An analytical relationship between the ventilation rate of air through the inlet (V) and temperature distribution at a certain distance from the heating element has already been

Fig. 4. A proposed circle-arc sensor with temperature measuring devices attached on.

derived as [18,21,22] V 0:5 ¼ c00 þ c01

1 T in  T i

(6)

where Tin is the temperature of the heat source, and Ti can be expressed by maximum or average temperature on an arc-shaped sensor. Different dimensions of circle and

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ellipse arc sections were taken. Optimum radius for circle arc was found to be 16 cm, while optimum minor radius for ellipse arc was 13 cm.

Two types of models were used for the relation between ventilation rate and average or maximum temperature measured on the arc V 0:5 ¼ c00;ave þ

Table 2 Summary of regression coefficient (R2) and percent standard error results for best possible circle and ellipse arc around air inlet at two different heating levels Model

Circle arc

Ellipse arc

30 W (%)

50 W (%)

30 W (%)

50 W (%)

Without disturbance (TinTmax)1 vs V0.5 (TinTave)1 vs V0.5

0.84/17 0.85/17

0.79/22 0.87/15

0.80/22 0.83/21

0.74/24 0.88/15

With disturbance (TinTmax)1 vs V0.5 (TinTave)1 vs V0.5

0.60/38 0.61/38

0.71/31 0.87/15

0.54/54 0.62/40

0.67/32 0.86/16

60

80 100 time (10 s)

1

Temperature Difference (°C)

0 -1 -2 -3 -4 -5 -6 -7 -8 -9

0

20

40

120

140

160

180

Fig. 5. Temperature decay of maximum temperature on arc with time.

(7)

c01;max . T in  T max

(8)

Table 2 summarises the results for steady-state experiments with and without disturbance. The model found for experiments without disturbance was applied to all experiments with different disturbance levels. Results showed that in all of the cases average temperature gave better correlation than maximum temperature. Moreover, the effect of heating level was negligible without disturbance, but more heating was required to model ventilation rate with disturbance. Therefore, average temperature on an ellipse or circulararc-shaped sensor was adequate to estimate ventilation rate at higher heating levels (50 W). As been described in the previous section, inlet ventilation rates were also calculated based on point-velocity measurements with the help of a uni-directional hot-wire anemometer. An inaccuracy of 15% was achieved at the inlet without disturbance. However, when the disturbance was introduced, inaccuracy raised to 31%. Therefore, the use of point measurements is misleading at non-uniform flow conditions, and the currently proposed method provided better results. Instead of constant heating, an on–off measurement system was also used. For these dynamic experiments, maximum and average temperature on the arc was drawn against time at every ventilation rate, as shown in Fig. 5. A first-order decay behaviour was observed for all ventilation rates. The decay rate was expressed in terms of decay constants at different ventilation rates as shown in Fig. 6, for circle and ellipse arc, respectively. A linear increasing trend was

8

6

y = 0.0039x

7

R2 = 0.8973

6

time constant (s-1)

7 time constant (s-1)

c01;ave T in  T ave

V 0:5 ¼ c00;max þ

8

5 4 3 2 1 0

31

y = 0.0045x R2 = 0.9204

5 4 3 2 1

0

500

1000 ventilation rate (m3/h)

1500

2000

0

0

500

1000 1500 ventilation rate (m3/h)

Fig. 6. Decay constant of temperature (s1) as a function of ventilation rate for maximum circle (a) and ellipse (b) arc temperature.

2000

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found with increasing ventilation rate (r2 ¼ 0.90) for circle arc. For ellipse arc, a better correlation was obtained (r2 ¼ 0.92). Therefore, measurement of decay constant for maximum arc temperature provides 26% and 25% inaccuracy for circle and ellipse arc, respectively, calculated from the difference between model estimate and actual ventilation rate. The same procedure was also followed for average temperature of circle and ellipse arcs (Fig. 7). In both cases, a quadratic relation was found between the decay constant and ventilation rate. In terms of average temperature, the shape of the arc had almost no effect, while both arcs provided rather high correlation (r2 ¼ 0.92 for circle arc and r2 ¼ 0.95 for ellipse arc). Both results corresponded to 24% and 18% inaccuracy, respectively. Although the correlation is rather better for average temperature, the range of decay constants for maximum temperature is larger and suggests a more robust model due to less sensitivity to measurement errors. Thermal dissipation produced by heat source is directly related to ventilation rate, as can be seen from the linear relation with maximum temperature on arcs. However, distribution of temperature as indicated by average temperature should have been affected by other factors such as thermal buoyancy that results from temperature difference between downstream temperature and temperature of the heated air. Therefore, for intermittent measurements use of maximum temperature is suggested. This study explains a preliminary attempt to quantify ventilation rate through a naturally ventilated opening. Further experiments and improvements are definitely needed to test the performance of the sensor at various conditions. Additional dynamic experiments are planned to examine the response of temperature measurements to the change in ventilation rate. Additionally, bi-directional flow characteristics representing horizontal component of external wind should be reproduced for further analysis. Different inlet types in terms of size and shape should also be studied.

10.5

Since none of the available techniques was reliable and accurate for the measuring of ventilation rate through naturally ventilated openings, a new system was developed to provide a simple tool for measuring the total air flux through openings. The basic principle of the new concept was based on an introduction of the heat source perpendicular to the direction of the flow opening, and measuring temperature decay very close to the heat source (0.13 m). Introducing a heat source at an air inlet can be used to estimate the ventilation rate through opening with a regression coefficient of 0.88 and standard error of 15%, even at non-uniform flow conditions created by an external fan above the inlet opening. The model calculates ventilation rate only by the two-parameter-model using average temperature on an arc-shaped sensor and temperature very close to the heat source. Instead of continuous exertion of heat through the heating element, the intermittent heating technique was tested where temperature decay rate was found after turning off the heater. In this case, inaccuracy increased to 25%; however, the model was simpler than the previous case for maximum temperature. Additionally, operational and maintenance costs of the proposed method decreased substantially. Compared to tracer gas experiments where reliability of the method is highly prone to mixing characteristics of gas in space, this new technique provides focusing more to

10.5

R2 = 0.9209

y = -2E-06x2 + 0.0014x + 9.719 R2 = 0.9456

10 time constant (s-1)

time constant (s-1)

4. Conclusions

y = -3E-06x2 + 0.0027x + 9.4771

10 9.5 9 8.5 8

Relying on the possibility of using heat source to estimate ventilation rate through an air opening, a line heat source covering whole inlet section is proposed for the following study. Therefore, the draw back of the current system due to point measurement will be minimised. Similarly, measuring heater temperature directly will reduce directional ambiguities caused by external disturbances on flow.

9.5 9 8.5

0

200

400

600

ventilation rate

800 (m3/h) 1

1000

1200

8

0

200

400

600

ventilation rate

800

1000

(m3/h)

Fig. 7. Decay constant of temperature (s ) as a function of ventilation rate for average circle (a) and ellipse (b) arc temperature.

1200

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inlets where air exchange basically takes place. Although average inaccuracy levels with the tracer gas technique seem to be similar, this direct method provides online measurements of ventilation rate in a continuous and less interfering way. Although 25% inaccuracy was achieved with current method even with non-uniform flow around the heater, studies should be performed to increase the robustness of the concept. For example, a line heat source will be introduced in a further study to trace the whole crosssection. Furthermore, different inlet configurations can be tested with computational fluid dynamics (CFD) software. Another important aspect to be tested is the effect of dynamic variation of ventilation rate on the proposed model. Further experiments will be conducted including dynamic experiments with ventilation rate and more turbulent flow conditions.

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