Assessment of Techniques for Measuring the Ventilation Rate, using an Experimental Building Section

J. agric. Engng Res. (2000) 76, 71}81 doi:10.1006/jaer.2000.0532, available online at http://www.idealibrary.com on

Assessment of Techniques for Measuring the Ventilation Rate, using an Experimental Building Section T.G.M. Demmers; L.R. Burgess; V.R. Phillips; J.A. Clark; C.M. Wathes  Silsoe Research Institute, Wrest Park, Silsoe, Bedford MK45 4HS, UK; e-mail of corresponding author: [email protected]  University of Nottingham, Sutton Bonington, Loughborough, Leics LE12 5RD, UK; e-mail: [email protected] (Received 10 July 1997; accepted in revised form 31 January 2000)

A full-scale, laboratory building section at Silsoe Research Institute was adapted to simulate cross-ventilating air#ow patterns representative of those that develop in naturally ventilated, intensive livestock buildings. In the fully controlled, steady-state environment of the building section, the distribution of aerial pollutants and/or tracers was shown to depend on source location and ventilation rate. No single location within the building showed a consistently representative concentration of tracer that would be suitable for calculating ventilation rate by the constant release tracer method. Estimates of ventilation rate using the constant release tracer method, based on the average concentration of groups of six sampling locations within the building section, ranged from 93 to 119% of the ventilation rate measured using calibrated fan-wheel anemometers placed in the ventilation ducts. The best estimates of ventilation rate were obtained when the tracer concentration was measured at the perimeter (in the outlets and inlets) of the building section*these ranged from 99 to 109% of the ventilation rate measured using the fan-wheel anemometers. Air#ow and pollutant distribution in the building section were predicted using computational #uid dynamics (CFD). However, these predictions were of limited help as a tool to select sampling locations within the building section, because the use of CFD requires the selection of parameters which entails veri"cation of predictions against experimental data. Visualization of the air#ow by smoke was simpler and more suitable for the present purpose.  2000 Silsoe Research Institute

and type of feed. The air#ow pattern is the most important factor in the distribution of a pollutant inside a building; it is in#uenced by building geometry and construction, size and distribution of ventilation inlets and outlets, heat production, and, particularly for naturally ventilated buildings, on the local wind environment. At a given ventilation rate, the source strength, which in turn depends on the #oor construction, waste storage system, etc., is the main factor determining the concentrations of aerial pollutants. The source location, i.e. point sources such as feeders, or area sources such as #oors, also a!ect the aerial distribution of the pollutant. Past research on air#ow patterns in livestock buildings has focussed mainly on the in#uence of obstructions (Randall & Battams, 1976) and on improving the design and control of the ventilation systems. Commonly used techniques visualize the #ow pattern with smoke or soap bubbles (Carpenter et al., 1972), or measure the air#ow

1. Introduction Environmental problems such as acid rain and the e!ects of greenhouse gases have increased the demand for knowledge of emission levels of aerial pollutants from livestock buildings in order to compile national inventories. Estimates of emission rate generally require the measurement of ventilation rate and pollutant concentration. The ventilation rate of mechanically ventilated buildings can be estimated using either fan-wheel anemometers or tracer techniques; for naturally ventilated buildings, it is far more di$cult to estimate ventilation rates. The distribution of aerial pollutants and tracer gases inside a livestock building, and therefore the estimate of pollutant concentrations and emission rates, depends on a number of factors such as the ventilation rate itself, the air#ow patterns inside the building, #oor construction, waste storage system, animal activity level 0021-8634/00/050071#11 $35.00/0

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directly using anemometers. This research has resulted in the design of inlet structures to in#uence inlet jet behaviour (Boon, 1978; Riskowski et al., 1993). A more analytical approach has been taken by Harral and Boon, (1997), who measured the air#ow pattern in an experimental cross-section of a piggery (using the building section at Silsoe Research Institute) and compared the measurements with a numerical model built using computational #uid dynamics (CFD) software. Predicted and measured air velocities were in good agreement although the turbulent kinetic energy was predicted well only in inlet areas. Measurements of the concentration distributions of gaseous and/or particulate pollutant levels in livestock buildings have been reported by several groups (Gorman & Barber, 1986; Smith et al., 1993; Ikeguchi, 1994). However, it is di$cult to compare the results of di!erent studies, owing to the di!erences in types of livestock housed, the husbandry methods, the building layout and the ventilation systems. Work by Boon et al. (1994) described numerical modelling of particulate pollutant concentrations in a livestock building, based on predictions of the tracks of individual particles by turbulent di!usion and mean streamline straining. Initial validation experiments showed good agreement between the model and measurements. The position of the source had a signi"cant e!ect on the resultant dust concentration. Although the work of Boon et al. (1994) provides basic knowledge needed for an understanding of the transport mechanisms involved in the dispersion of aerial pollutants in livestock buildings, further validation under a variety of conditions is required, as is extension of the model to include gaseous pollutants, and to obtain a model capable of predicting aerial pollutant concentrations in livestock buildings. No obvious technique is available to determine the best location for measuring the tracer and aerial pollutant concentrations to estimate, respectively, the ventilation rate and emission rate of a building. An experiment was therefore carried out under fully controlled conditions to investigate the in#uence of the location of the pollutant source and the ventilation rate on the distribution of a pollutant inside a building, and to seek to identify sampling locations within the building where a representative pollutant and/or tracer concentration existed in order to estimate the ventilation rate of, and hence the emission rate from, the building.

2. Options for measuring ventilation rate in livestock buildings The ventilation rate of mechanically ventilated livestock buildings is frequently measured using full-size,

fan-wheel anemometers placed in the ventilation ducts housing the extraction fans. An improved version of these fan-wheel anemometers (Berckmans et al., 1991) with an accuracy of $60 m/h over a #ow range of 200}5000 m/h is increasingly being used as a sensor in climate control systems. To minimize errors due to the e!ect of turbulence on fan rotation, it is preferable to place the fan-wheel anemometer upstream of the extraction fan and incorporate a #ow straightener between the two fans to protect the fan-wheel anemometer from the in#uence of vortices generated by the extractor fan. The ventilation duct should also be "tted with a bell mouth. Tracer methods are widely used to measure the ventilation rate and mixing characteristics of ventilated air spaces (Barber & Ogilvie, 1982; Bowes et al., 1993; Sandberg & Blomqvist, 1985; Sherman, 1990). Alternatively, the tracer ratio method is used to measure pollutant emissions from ventilated spaces directly, without the need to measure the ventilation rate. The theoretical basis for these methods is the equation for the mass balance between the tracer and air. Generally, a known #ow rate, or mass, of a tracer is introduced into a building and the ventilation rate is estimated from the resulting tracer concentration, using the equation of conservation of mass, in which the production and extraction rates must be equal. Allowance must also be made for short-term storage when the ventilation rate changes, and for natural sources or sinks of the tracer in the building. Strictly, tracer gas methods require complete mixing of the tracer within the ventilated space, such that the tracer concentration is uniform. When the internal volume is imperfectly mixed, the methods become less accurate, owing to variations in the measurement of internal concentration. However, when the ventilation rate is known, the variation can yield useful information on the extent of mixing (Barber & Ogilvie, 1984). Of the tracer methods available, i.e. tracer decay, constant tracer concentration and constant tracer injection, the last is most commonly used for continuous measurement of ventilation and emission rates in both mechanically and naturally ventilated animal houses (Scholtens et al., 1996; van't Ooster, 1993). The constant injection method avoids disturbing normal internal mixing, but it, like the other methods, is liable to unknown errors in the calculation of the ventilation rate (Demmers et al., 1998) should there be incomplete air mixing as is likely in livestock buildings. The accumulation term in the mass balance can be neglected in cases where changes in the ventilation rate are small (because changes in the internal concentration will then also be small) and in long-term measurements to determine an average emission rate. The ventilation rate can be calculated as a function of time from the tracer production and the internal tracer concentration

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after correction for the background concentration of tracer.

3. Materials and method The experiments were carried out at Silsoe Research Institute using an experimental building section which was designed as a highly controlled, full-scale simulation of a cross-section of a forced-ventilated fattening piggery (Carpenter et al., 1972). For this work, the ventilation of the section was changed to simulate that of a crosssection of a naturally ventilated livestock building (Fig. 1). The section was 11)73 m wide and 3)0 m deep with a 14)53 sloped ceiling, 3)34 m high to the ridge and 1)85 m to the eaves, and had similar insulation characteristics to a standard building. The section was surrounded by a temperature-controlled shell, allowing precise temperature conditions to be established and maintained. Air entered the section via one side wall duct and exhausted through a duct in the opposite side wall and through a ridge vent. The exhaust volume #ow was controlled such that 40% was extracted through the side vent and 60% through the ridge vent. In practice, this ratio will depend on building geometry, ventilator design, wind direction, and magnitude of wind/buoyancy driving forces, but the ratio selected for the tests re#ects a typical

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balance for naturally ventilated, intensive livestock buildings. Accurate, independent measurements of ventilation rate through the section were made with calibrated fanwheel anemometers, one placed in each vent duct, just upstream of the fans. Axial fans, controlled by feedback from each fan-wheel anemometer, were mounted in the inlet and outlet ducts. The side wall ducts were 0)45 m in diameter, and were mounted with the centre line 0)96 m above the #oor and 0)85 m from the rear wall, i.e. o! set from the mid-depth plane by 0)65 m (see Figs 2 and 3), which generated representative three-dimensional ventilating #ows. The ridge vent was a 0)30 m wide slot extending over the full depth of the building section. Electrical sheet heaters mounted 0)3 m above the #oor were used to simulate the sensible heat output at a realistic stocking density of livestock (350 W/m of occupied area). The heaters were aligned parallel to the front and back walls of the section so as to minimize obstruction to air#ow (Fig. 1). Nitrous oxide was chosen as the tracer gas, because it has a low background concentration and because of the availability of suitable analysers. The tracer gas was diluted with air (at least 200}1) to create a mixture of similar density to ambient air prior to being released into the building section (Fig. 1). The mixture was injected from either a point source or from a distributed source at a rate of 200 and 800 l/min, respectively. The point

Fig. 1. Layout of the experimental building section, showing the position of the sampling points, heaters and source locations in the mid-depth span-wise plane and the two transverse planes A}A and B}B (viewed from the inlet). The inlet and outlet locations are also shown: these were located 72% of the way towards the back wall

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Fig. 2. The airyow pattern measured in the mid-depth, span-wise plane (top) and in the two transverse planes A}A and B}B (bottom) of the building section with a low ventilation rate of six air changes per hour

Fig. 3. The airyow pattern measured in the mid-depth, span-wise plane (top) and in the two transverse planes A}A and B}B (bottom) of the building section with a high ventilation rate of 20 air changes per hour

source consisted of a uniformly perforated ball, 0)2 m in diameter, located in the centre plane of the building, 2)8 m from the ventilation inlet wall and 0)4 m above the #oor, just above the heaters. The distributed or area source consisted of two 10 m lengths of 1)2 m wide tubing, laid #at on the #oor. The sections of tubing beneath the heaters (see Fig. 1) was perforated to simulate an area release of ammonia emitted from the #oor (slats and slurry channel or straw bedding) of a livestock building. Uniformity of tracer release along the length of the perforated tubes was assured by the hole spacing in the tube and was checked by measuring the air speed above the holes. A moveable sampling mast was used to measure spatial variations in environmental conditions in the building section. The mast carried an ultrasonic anemometer (Gill), a sampling point for the measurement of nitrous oxide concentration, and a temperature and humidity sensor. It was traversed horizontally and vertically to measure the pattern of these variables in three vertical planes of the building section (see Fig. 1).

For each experiment, the ventilation rate, tracer gas concentrations, temperature and humidity were measured at the inlet and outlets, and airspeed and direction, tracer gas concentration, temperature and humidity were measured at 76 points distributed over the span-wise plane at the mid-depth (1)5 m) of the building section, and at 48 points distributed over two transverse planes at !2)5 and 1)25 m across the building section (see Fig. 1). All data were recorded as 5 min averages. For both the point source and the area source tracer release cases, an experiment was conducted at a low and a high ventilation rate of approximately 6 and 20 air changes per hour), respectively. A numerical three-dimensional simulation of the air#ow and pollutant concentration in the building section was prepared using the PHOENICS CFD code (Launder & Spalding, 1974) to prepare the data and solve the governing equations (Rosten & Spalding, 1987). The PHOENICS CFD code is based on the solution of the time-averaged Navier}Stokes equations coupled with the i}e turbulence model. This model simulates the

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after each run, and corrections made, instrumentation drift (up to 6 p.p.m. over 10 h) limited the accuracy of the ventilation estimates by the tracer method.

turbulent #ow e!ect and consists of two additional di!erential equations for the turbulent kinetic energy i and the dissipation rate e and considers the turbulence to be isotropic. The i}e model is valid only in fully turbulent #ows. The supply of nitrous oxide was controlled using a mass #ow controller (Hitec). The nitrous oxide concentrations at the two outlets and at the mast were measured using three infrared gas analysers (ADC Ltd), which were calibrated at zero and at 80% of full scale (where full scale was 50 p.p.m.) twice a day. The ambient nitrous oxide concentration at the inlet was measured at the start and end of the experiment. The air speed and direction in the building section were measured with an ultrasonic anemometer (Gill) mounted on the traverse mast. Temperature and relative humidity were measured using probes comprising a platinum-resistance thermometer and a capacitative humidity sensor (Rotronics P100). Air#ows through the inlet and outlet ducts were measured with calibrated full-size fan-wheel anemometers (Prototype Catholic University Leuven). All data were measured and stored using a Scorpio data logger (Schlumberger Ltd) and PC.

4.2. Air-ow patterns The air#ow patterns in the building section were measured under-steady state conditions using an ultrasonic anemometer mounted on an automatically controllable traverse unit. Measured velocities in the mid-depth spanwise plane and two transverse planes, A}A and B}B (Fig. 1), of the building section are plotted in Figs. 2 and 3 for the low and high ventilation rates, respectively. The measurements shown are those for the point source release, although the #ow patterns for the area source release case were very similar. The mean velocity vectors in all three directions over the 5 min measurement periods for all cases were always within the standard deviation measured for the point source case. The #ow pattern at the low ventilation rate (Fig. 2) was characterized by an inlet jet which dropped quickly to #oor level. The corrected Archimedes number (Randall and Boon, 1994), which characterizes the direction of the inlet jet, was 20, indicating that the air should remain horizontal after entry into the room. However, due to the low inlet jet velocity, the dimensionless jet momentum number (Barber et al., 1982) was 2)5;10\, clearly indicating unstable air circulation patterns, for which the Archimedes number is not valid. The highest velocity measured in the direction of the inlet jet at #oor level at the transverse plane B}B was 0)5 m/s, which is low compared with the theoretical inlet air speed of 1)2 m/s. The inlet jet appeared to disperse quickly. The main air#ow pattern was a circular #ow over the heaters at ground level, from the inlet wall towards the outlet wall. The return #ow was along the ceiling towards the ridge outlet, with some air #owing past the ridge outlet, which then mixed with the inlet air. An upward component of air velocity was apparent over the majority of the width of the building section. This was primarily due to buoyancy

4. Results 4.1. Stability of experimental set points Although each experiment took between three and "ve days to complete, the experimental conditions were very stable. The standard deviation from the mean ventilation rate, measured using the fan-wheel anemometers, was between 0.4 and 1% for the high and low ventilation rates, respectively. The tracer gas concentrations at the outlets did vary during the measurements, but the standard deviation was less than 3% of the mean in all cases (Table 1). The three infrared analysers used to measure the tracer concentration at the two outlets and the concentration within the building were prone to span drift. Although the instruments were calibrated before and

Table 1 Mean tracer concentration and SD measured during the experimental period at the outlets of the building section at 6 and 20 air changes per hour and two tracer release sources Tracer concentration, mg/m3 Six air changes per hour Location of sampling point Outlet left Outlet ridge

20 air changes per hour

Point source

Area source

Point source

Area source

29)6$0)9 33)4$0)9

29)9$0)6 30)9$0)5

25)4;1)4 31)2$0)7

30)0;0)7 38)8$0)6

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e!ects created by the heaters, although the e!ect was more pronounced for the area release of tracer gas than for the point release, suggesting that the tracer ejection velocity was contributing to the generation of upwards #ow. This upward #ow component over the heaters on the air inlet side contributed to a secondary clockwise eddy in this region of the building section. Air velocity measurements across the transverse planes of the building section showed the three-dimensional nature of the air#ow, which is consistent with the measurements made by Boon (1978) when the section was traditionally force-ventilated to achieve a jet ventilation system with the inlet jet at the ridge. This threedimensional air#ow is representative of real #ows and arises predominantly as a result of the side-wall inlet and outlet being positioned not at the mid-depth of the section but 72% of the way towards the back wall. The air velocities measured in the transverse planes in the present arrangement indicated that air #owed predominantly from the inlet towards the far outlet in front of the mid-depth span-wise plane, apart from a large eddy present at the front, inlet corner of the section, with the return #ow being mainly along the ceiling towards the rear wall. The strong upwards #ow near #oor level towards the back wall was probably caused by the physical obstruction of the electrical motors driving the moveable mast rather than being a result of buoyancy e!ects. The #ow pattern at the higher ventilation rate (Fig. 3), was similar to that at the lower ventilation rate, with the general air#ow being over the #oor towards the opposite wall then back along the ceiling to the ridge vent, with some air again by-passing the ridge vent and mixing with the inlet jet. The Archimedes number was 2, again indicating an inlet jet which would remain horizontal after entry into the building. The jet momentum number was 19;10\, indicating stable air circulation patterns. The approximately horizontal inlet jet dropped to the #oor much further into the building than it did with the low ventilation case. The highest measured air velocity in the building section was 1)0 m/s at #oor level in the spanwise plane, at a position 2)5 m from the inlet. Again, this was considerably lower than the mean inlet jet air velocity of 3)4 m/s. In contrast to the low ventilation rate case, the lowest air speeds (of 0)1 m/s) were measured in the centre of the building section. As in the low ventilation case, there was a signi"cant transverse component of the air velocity, especially in the inlet region of the section. However, in contrast to the low ventilation case, the main clockwise air#ow pattern was present at all mea-

surement locations, and the upward #ow component due to buoyancy was less apparent. The high upwards velocity measured near the #oor towards the rear wall of the section was again likely to have been a result of the presence of the moveable mast. The main features of the air#ow pattern described were con"rmed visually by smoke dispersion experiments (Fig. 4).

4.3. ¹racer concentration contours The tracer concentration contours were generated from the measured data using Unimap software and a linear interpolation method (UNIRAS AGS/UNIMAP 2000, 1991). The contours in the span-wise plane and the two transverse planes of the building section are consistent with the measured air#ow patterns. The contours show that the internal volume of the section was not well mixed for any combination of the experimental parameters. At the low ventilation rate (Fig. 5), using the point source, a steep vertical gradient of tracer concentration (82}106 mg/m above a height of 0)4 m) was evident close to the source (Fig. 5). In the other half of the building (transverse plane A}A) the concentration gradient, although less steep (76}106 mg/m above a height of 0)8 m), was still noticeable. The highest tracer concentrations were found in the centre of the building section rather than near the point source, as could be expected from the air#ow pattern. The tracer release plume from the point source was directed upwards and towards the front of the building section, and so is not readily apparent from the measurements in the span-wise plane close to the source. The lower concentrations at #oor level might have been caused by cleaner air from the inlet jet crossing the centre span-wise plane from the back to the front of the building section. At the high ventilation rate (Fig. 6), with the same point tracer source, the highest concentration ('106 mg/m) was measured in the area to the front of the building. As with the low ventilation rate, the tracer plume from the point source was directed from the middepth span-wise plane towards the front of the building section (Fig. 3). In contrast to the low ventilation rate, a strong transverse gradient in the tracer concentration is evident, owing to the strong transverse trend of the air#ow from the rear to the front in the inlet side of the building section: a tracer concentration gradient from 106 to 70 mg/m was measured over a horizontal

䉴 Fig. 4. Visualization of the yow pattern in the span-wise plane of the building section. The smoke particles were illuminated by a 10 cm wide vertical plane of light. The smoke was released in the air inlet duct (on the right-hand side). Photographs were taken 3, 6, 9 and 12 s after smoke entered the section. The ventilation rate was 20 air changes per hour

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to the ridge outlet. This pattern is consistent with the upwards #ow of air as measured with the sonic anemometer. The gradients in the tracer concentration were lower than with the point source, &35 mg/m versus '70 mg/m. At the high ventilation rate, the gradient was strongest in the outlet side of the section, just above the heaters, where the diluting e!ect of the inlet jet was low. The highest tracer concentration occurred at #oor level in the front corner on the outlet side of the section. As with the point source, high concentrations were also found at the intersection of the ceiling and the inlet side wall of the section, which is again consistent with the air#ow pattern.

Fig. 5. Tracer concentration contours in mg/m3 in the span-wise and transverse planes (viewed from the air inlet) of the building section for the point source tracer release and a low ventilation rate of six air changes per hour

distance of less than 0)5 m in the transverse plane B}B. Due to this transverse air#ow and the main circulatory air#ow pattern at the high ventilation rate, the highest tracer concentrations in the mid-depth span-wise plane were found near the ridge outlet and at the intersection of the ceiling and the inlet side wall of the section. The concentration contours measured when using the area source of tracer release were also strongly related to the air#ow pattern. For both the high and low ventilation rates, the highest concentrations arose close to the outlet side wall, where the air had passed over all of the source. At the low ventilation rate, there was a zone of high concentration stretching from the heaters in the inlet side

Fig. 6. Tracer concentration contours in mg/m3 in the span-wise and transverse planes (viewed from the air inlet) of the building section for the point source tracer release and a high ventilation rate of 20 air changes per hour

4.4. Predictions of air-ow patterns and tracer concentration contours Modelling the air#ow for the high ventilation case with a computational #uid dynamics program resulted in a well converged solution after 5000 iterations. To simplify the three dimensional model the heaters and point source for tracer release were assumed to form no restriction to the air#ow. The overall air#ow pattern was predicted well and showed the main circular air#ow pattern with high velocities along the #oor and lower velocities in the centre of the section. However, the predicted velocities tend to underestimate the measured velocities by a factor of approximately two. Most likely the values for turbulent kinetic energy i and dissipation rate e used in the model were incorrect, as was the assumption that heaters and other objects did not restrict the air#ow. Better estimates for i and e, based on measurements, were not available. Therefore, no further attempts were made to improve the model. The concentration contours, like the air#ow pattern, resembled the measured pro"le, but the concentrations levels were incorrect. The predicted tracer concentration in the mid-depth span-wise plane was also lower than the measured tracer values which was contrary to expectations that the lower modelled ventilation rate would produce higher concentrations in order to achieve a correct mass balance. However, as the solution for the concentration also converged after 7000 iterations, areas with higher concentration compared to actual value must exist outside the mid-depth span-wise plane to satisfy the mass balance condition. However, as no comparative measurements outside the mid-depth span-wise and transverse planes were available, this could not be veri"ed. 4.5.
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Table 2 Comparison of the ventilation rate in m3 /h of the building section estimated using the constant tracer release method with that obtained using fan-wheel anemometers. The set ventilation rates were 6 and 20 air changes per hour. Measurements are shown for tracer gas release from point and area sources. Where appropriate the standard error is given Set ventilation rate, air changes per hour 6 20

Source of tracer gas

Ventilation rate fan-wheel anemometers, m3 /h

Number of certainty

Point Area Point Area

700$0)76 699$0)77 1958$0)74 1954$0)48

79 81 124 82

Ventilation rate by the tracer method, % of fan-wheel anemometer results Location of sampling points Outlets 0)95 and 2 m height and !3)2, 0 and 3)2 m from centre 0)95 and 2 m height and !2)5, 0 and 1)25 m from centre 0)5 and 2 m height and !2)5, 0 and 1)25 m from centre

Six air changes per hour Point source Area source

20 air changes per hour Point source Area source

106$0)26

109$0)24

103$0)33

101$0)10

105

108

98

114

101

104

93

119

111

105

97

111

release method, compared with concurrent measurements using fan-wheel anemometers placed in the inlet and outlet ducts, which were regarded as the reference method. The calculated ventilation rate was estimated from concentration measurements made at selected locations which were corrected for the tracer concentration at the inlet, i.e. in the ambient air. The chosen sampling locations were at the outlets of the building section and in the mid-depth span-wise plane at approximately human head height (2 m) and at animal head height, either 0)5 or 0)95 m to re#ect di!erent animal species, at positions in the span-wise plane at !3)2, !2)5, 0, 1)25 and 3)2 m relative to the mid-span (Fig. 1). The estimated ventilation rates given in Table 2 are in good agreement. The greatest bias in the ventilation rate estimated from tracer measurements was 19% (area source, 20 air changes per hour, tracer measured at 0)95 and 2 m height and at !2)5, 0 and 1)25 m from the mid-span). In absolute terms, the bias varied from 8 to 370 m/h. The absolute error in the measurement of ventilation rate using a fanwheel anemometer (Berckmans et al., 1991) is 60 m/h. However, the large number of measurements at steadystate conditions has reduced the measurement error. The ventilation rate estimated from selected locations in the mid-depth span-wise plane of the building clearly depended on the location. The tracer source type (point or area) also had a signi"cant in#uence on the ventilation rate estimate, especially at the higher rate of 20/h. In contrast, the source type had no e!ect on the ventila-

tion estimate when the tracer concentration at the outlets was used.

5. Discussion The air#ows at the two ventilation rates represent internal #ow patterns dominated by an inlet jet and buoyancy e!ects, and so represent #ows of naturally ventilated buildings with discrete inlets (such as automatically controlled naturally ventilated buildings). The pattern at the low ventilation rate (six air changes per hour) showed strong buoyancy-driven upward #ows in the centre area of the building section and a secondary eddy near the inlet side wall. At the high ventilation rate (20 air changes per hour) there was a dominant re-circulation throughout the cross-section, without noticeable buoyancy e!ects. The observed tracer gas concentration contours were consistent with the observed air#ow patterns. It is abundantly clear that the volume is incompletely mixed, even though complete mixing is a theoretical requirement for the application of tracer methods. Incomplete mixing did not a!ect the ventilation rate estimate obtained using the tracer concentrations measured at the two outlets and the inlet of the building section. When compared with the rate measured using the fan-wheel anemometers placed in the inlet and outlet ducts, the estimate based on the average concentration

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T. G . M. D E M M E RS E¹ A ¸ .

di!erence between the outlets and inlet was within 3% at the high ventilation rate, and overestimated by no more than 9% at the low ventilation rate. The likely reasons for the larger bias at the low ventilation rate are the higher impact of concentration measurement inaccuracy on the ventilation rate estimate, and the lower accuracy of the fan-wheel anemometers at low #ow rates. Nevertheless, these errors of the constant release tracer method are well within the accepted error of 15% reported by Barber and Ogilvie (1982, 1984) and of 10}15% reported by van't Ooster (1993) from a theoretical study. In contrast, the estimates of ventilation rate based on concentrations measured at locations within the building are strongly a!ected by the spatial extent of the tracer source (point or area release). At the high ventilation rate, particularly with the area source, the ventilation rate is overestimated by 10}18% compared with the reference fan-wheel anemometer method. This discrepancy reveals the high sensitivity according to sampling point location, and is a consequence of the spatial variation of tracer concentrations. No obvious zones could be identi"ed within the building section that o!ered a representative concentration for all conditions and tracer sources tested. The large error in the ventilation rate estimate using internal sampling points is not acceptable, especially given the steady and well controlled conditions of this experiment. The constant tracer release technique with measurement of the concentration at the outlets on the perimeter of the building must therefore be preferred for &"eld' application. The practical problem with this for naturally ventilated buildings, however, is that ventilation openings do not act consistently as either outlets or inlets, but tend to #uctuate between the two depending on wind and thermal conditions. The CFD prediction of the #ow rate was based on parameters provided by previous experiments in the same building (Harral & Boon, 1997). The solution predicted by the CFD package converged satisfactorily. Even so, while the predicted air#ow pattern was realistic, the predicted air velocity was about one-half of the measured velocity. In addition, the predicted tracer concentrations within the building showed the correct gradients but the values were lower than those measured. Improvements to the model could be made by changing the parameters controlling turbulent kinetic energy i and the rate of dissipation of turbulent kinetic energy e. In any practical application, however, a CFD model would need to embrace features not present in the experiments such as pen walls, gates, ducts, and purlins which disturb internal air#ows; this would greatly increase the complexity of the model and the need to validate it against speci"c, relevant data (Christensen, 1992; Choi et al., 1990). New techniques are being developed to predict the distribution of pollutants within a building, such as indi-

vidual particle tracking, which show better results than did the CFD package used here (Boon et al., 1994). It was also considered that the simple approach of monitoring the air#ow pattern in the building by smoke dispersion patterns gave as useful an indication of the air#ow pattern for this present purpose as did the detailed air velocity measurements made with the sonic anemometer. For these reasons, it was concluded that it would not be particularly fruitful to attempt to develop and improve the CFD model further for this application. 6. Conclusions Variations in both the ventilation rate and the tracer release source a!ected the distribution of the tracer inside the building section. No one particular point within the building section showed a consistently representative tracer concentration suitable for the calculation of ventilation rate. Ventilation rate estimates using the constant release tracer method, each based on six sampling locations within the building section, ranged from 93 to 119% of the ventilation rate measured using fan-wheel anemometers placed in the ventilation ducts. The best estimates of ventilation rate were obtained when the tracer concentration was measured at the outlets and inlets of the building section: these ranged from 99 to 109% of the ventilation rate measured using the fanwheel anemometers. Therefore, to estimate the ventilation rate sampling of tracer gas concentration at the air inlets and air outlets of a building must be preferred to sampling within the building. Computational Fluid Dynamics models are of limited help as a tool to select sampling locations within a building because the predictions always need veri"cation against data. Smoke dispersion experiments are simpler and more suitable for the present purpose.

Acknowledgements We thank our colleagues Chris Boon for his help with modifying the ventilation system of the building section, Dave Matthews for his help in installing the equipment, and Lynn Short and Brian Harral for their assistance with the CFD analysis. The work was funded by the Ministry of Agriculture, Fisheries and Food under Contract OC9117/CSA2141. References Barber E M; Ogilvie J R (1982). Incomplete mixing in ventilated air spaces. Part I: theoretical considerations. Canadian Agricultural Engineering, 24, 25}29

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